What is Ventilation?

Ventilation can be simply described as air circulation. This is the extraction of stale, overheated and contaminated air, and the supply and distribution of fresh air in amounts necessary to provide healthy and comfortable conditions for the occupants of the space being ventilated.

As applied to homes, offices and workshops, ventilation also means the creation of an environment that stimulates the worker to higher efficiency.

Although natural ventilation is often relied upon to dissipate the heat from factory and office buildings, its effects are uncertain, unreliable and difficult to control. It may be satisfactory in some cases, but fans have become an essential part of good ventilating systems for the following reasons:

• They operate irrespective of internal temperature and external winds.

• They can be more easily and accurately controlled.

• They can often be used for either extract or intake, and therefore cater for a wider variety of winter and summer conditions more easily.

• On extract much smaller inlet openings are necessary in building structures for air replacement, due to the greater suction pressure provided by a fan.

• On intake they give positive air movement for relief from radiant heat, can incorporate filters for use in dusty atmospheres, and heaters if required during cold weather to augment the normal heating system of the building.

Natural ventilation, with open windows in summer, may suffice for the living rooms and bedrooms in our homes, where there is plenty of space per person and no generation of steam or cooking fumes. However natural ventilation is unpredictable, and will fail altogether in unfavourable conditions of wind and weather. In many areas within a building mechanical ventilation, powered by fans, is a practical - and often a legal - requirement. The rate of ventilation, measured in litres of air per second, must be sufficient to satisfy the following three requirements:

• Sufficient air movement throughout the room or building to prevent the formation of pockets of stale air.

• Sufficient fresh air supply and foul air exhaust to limit the level of air pollution from all sources in the building, including humidity.

• Reduction of air temperature, within the limits set by the climate, by the removal of heat generated within the building or supplied by the sun.

What is Air?

Air is a gas mixture composed mainly of oxygen and nitrogen with small percentages of carbon dioxide and water vapour (moisture) in varying amounts. Dust particles and bacteria area also present, and may have to be dealt with at some point in the ventilation system, depending on the nature of the job, as also would odours given off by people, animals or vegetable matter. The air in an occupied space will gradually become less pure due to the bodily functions of the occupants and the sort of work they are doing.

Carbon dioxide and water vapour from the lungs, organic impurities from the body, smoking, fumes, gases and dust from industrial processes all tend to increase the temperature, humidity, dust and odours and to reduce the percentage of oxygen in the air to make it less comfortable to live with. Volatile organic compounds (VOC's) are given off from materials used in building construction and furnishings. Formaldehyde from carpets, upholstery and certain wall cavity insulation. Solvents from paints can include Toluene and Xylene, both considered carcinogenic. In the interests of health, hygiene and efficiency, some dilution of the impurities, and removal of undesirable heat and moisture must be carried out by ventilation.

Composition of Ambient Air at Sea Level and 70 °F/ 21 °C

For most humans the daily oxygen demand is about 300-800 litres.

This corresponds to about 0,750 – 1,5 kg of oxygen, or 1500 - 4000 litres of air.

Not all oxygen is extracted from the air inhaled with each breath. Only about 4 % is absorbed; exhaled air contains 16 % residual oxygen. This surplus is necessary to maintain the oxygen gradient to which the human lung has adapted. Human beings are capable of living in air containing 12-60 % oxygen. An atmosphere that is more than 60 % oxygen is toxic; air with less than 12 % oxygen will not maintain human life.

In the lung oxygen is exchanged against carbon dioxide (CO2). If inhaled air already contains CO2 the breathing ratio has to be increased to purge CO2 at the rate at which it is produced. A safe level fir indoor living is considered to be 0,1 %. Excessive carbon Dioxide triggers increased breathing and eventually narcosis. High CO2 levels can accumulate in deep wells and in wine cellars because CO2 is heavier than air.

Exhaled air is almost fully saturated with moisture at the body temperature of 37 ºC regardless of the breathing rate. The water exchange between air and the human body differs from that of oxygen and CO2 in two fundamental ways. First, water is not only exchanged in the lung. But is also released via the skin; second, the water release does not serve a chemical purpose, but supplements conductive, convective, and radiative heat transfer as necessary to ensure disposal of metabolic waste heat. If inhaled air is already moist, water transfer in the lung is simply reduced. For room air at 21 ºC and 50 %

humidity, the water transfer amounts to about 0,4 kg/day per person. In a room with 25 % humidity, 0,7 kg of water will be exhaled. This is only a fraction of the water that is released by perspiration through the skin. Typical vaporisation losses are 1,5 to 4,5 kg/day.

This water adds to the moisture already present in air. If several people share a small room, the water flux constitutes a significant burden on the capacity of the indoor air. For example, three sedentary people watching TV in a living room will generate about 130g of water per hour. During an exciting show, however, the release can easily triple. Since a 5 x 5 x 2,5 m living room at 20 ºC and 50 % humidity can absorb only 34 g of water, air in tightly sealed room will reach 100 % humidity in about 15 minutes and water will condense on the coolest surface, usually the floor, windows, or walls. In order to maintain a stable room humidity of, say, 70 %, a ventilation rate of 40-85 m3/h must be maintained. This corresponds to about one air change per hour.

Almost all water enters the body as liquid. Whether water is excreted as liquid or as vapour is determined by physical needs. As far as water flux through the skin and lung is concerned, the best comfort condition is achieved if the air humidity is about 40-60 %, regardless of temperature. This level provides for continuous removal of metabolic products via respiration and perspiration.

Air also has Weight

1 m³ of air weights approximately 1,2 kg. Therefore, energy has to be used to move it through the atmosphere, to push it along ducting, to turn it around corners, and to squeeze it through grilles and filters. The energy required to do this work increases as the cube of the air velocity, whilst the resistance of the particular ventilation system increases as the square of the air velocity. For instance, if ducting in a system is reduced from 400 x 400 mm to 300 x 300 mm with the same volume of air. The resistance of the ducting to airflow would be increased more than 3 times. This subject will be expanded later when duct resistance is discussed.

Ducting, Air Velocity and Resistance

Ducting

A duct is a tube through which air may be moved from one location to another. Air is a fluid (like water but much less dense) but the difference is that air (a gas) can be compressed bur water (a liquid) cannot.

Ducting can take many forms and can be made from various materials. The most common for larger section duct is sheet metal, which can be made into square, rectangular, circular and oval section. Square and rectangular ductwork is usually custom built to a certain size and is relatively simple to manufacture, install and join. Most circular ducting is 'spirally wound' (i.e. a narrow strip or metal is formed into an airtight spiral).

Plastics can also be used for ducting, but because of the high cost of either moulding or extruding the section, is limited to smaller size up to 150 mm diameter and small, square and rectangular sections.

Flexible ducting, using a flexible sheath around a wire helix is very popular in the range 100 to 400 mm diameter.

When it comes to the lowest resistance of moving air, a circular dust is the best, as it has the greatest cross sectional area and a minimum 'skin surface'. An oval is acceptable to a maximum width to height ratio of 2 to 1 (the aspect ratio). Square and rectangular section (the same aspect ratio applies) allows air turbulence in the four corners and has a greater 'skin surface'.

Friction between the internal surface of the duct and the adjacent moving air, has the effect of slowing the air. The rougher the internal surface of the dust the greater the friction. For example, an unlined brick built chimney is a duct for the smoke and fumes from an open fire, but has a far greater friction factor (50 times greater than spirally wound metal duct). Flexible ducting also has a high friction factor, about double that of spirally wound. Air does not like being squeezed or allowed to expand suddenly, of being forced round corners and through grilles. All these have the effect of resistance to the airflow.

Air Velocity

Air can only be heard when it is moving. Still air is quiet. The faster the air moves the louder the noise. Writers describe the wind 'moaning gently', 'whistling round a building' or as a 'howling gale'. When air changes direction the noise level is increased. What we are actually hearing is caused by fluctuation in the pressure of the air.

A fan is a means of moving air and will create noise, which will be aggravated by an obstruction in the airflow. The fan itself may also cause mechanical and electrical noise and create vibration, but these are all subjects for a later section - The important factor is to keep the speed of the air (its velocity) as low as possible. This is usually achieved by having a large size fan moving the same volume of air through a larger aperture or duct, keeping the velocity low.

There will always be a conflict of interests here. It may be that there is not room for a larger fan or duct, there may be financial reasons for using a smaller (cheaper) fan. The actual installation will also govern fan selection. A quiet library will require lower noise levels

(hence air velocities) than an engineering workshop. Another consideration for air velocity is the proximity of moving air to people, not only from noise aspect, but also because of 'draughts'. A draught is created either the air velocity is too high, or the temperature of the air is too low.

The advantage of mechanical ventilation over natural ventilation is that it is controllable, thus the velocity can be governed and the temperature varied if required. A natural air velocity between 0,15 and 0,5 m/s is usually acceptable. Mechanical air movement up to 3 m/s can be acceptable provided people are not directly in the airflow.

When air is moved down a duct, all the duct characteristics will have an effect on the air velocity. These include changes of direction, section and terminations.

Resistance

Continuing the air/water analogy, in a domestic water system the pipes supplying the bath taps are of a larger diameter than those supplying the hand basin. This is so that water under the same pressure can pass a greater volume and fill the bath more quickly.

An air duct can also be likened to a motorway. If duct size is reduced and the air pressure remains the same, the air flow is reduced. On a motorway, if the width is reduced (three lane coned down into two) the traffic slows down as the lanes merge. Theoretically, if the traffic was to increase its speed, the same volume could flow in two lanes instead of three. To increase the velocity of air in a duct requires a considerable increase in power required, and also increases the noise.

There are three things we all are familiar with, that follow similar laws of physics. Electricity in a wire, water in a pipe and air in a duct. Most people appreciate that the wire supplying power to a kettle has to be "fatter" than the wire to a table lamp. Water pipes are mentioned above, but for some reason, people think you can shove as much air as you like down a small duct.

System Resistance

The resistance of a ventilating system is caused by:

• The loss of energy at the point of entry of the air due to a sudden increase in air velocity from practically zero to the velocity along the duct. Keep air velocity at entry around 2,5 m/s or less.

• The friction between the air and the inside surface of the duct. Keep duct velocities low - about 2,5 m/s for general use.

• Changes of cross-sectional area of duct, where there are expansions and contractions, or changes of shape (say from square to oblong section). Expansions, contractions, and changes of size or shape should be made by gradual taper sections, not abruptly.

• Changes of direction, such as bends and Tee-junctions are large wasters of energy. Changes of direction should be by easy bends and well-rounded corners, not by sharp elbows, unless fitted with guide vanes (expensive).

• Auxiliary items, such as grilles, louvers, filters, heaters. These items should be large enough to keep air velocities through them down to a reasonable level, consistent with the velocity in the main duct.

NOTE: the resistance of any system of ducts, grilles, filters, etc, is proportional to the square of the air velocity through it - keep velocities down by using recommended duct, grille and filter size.

Temperature

A good ventilation system is designed to keep the temperature and humidity at comfortable levels, to create enough air movement of acceptable velocity and direction to produce a feeling of freshness and comfort, and to maintain the purity of the air reducing the concentrations of dust, bacteria, odours and carbon dioxide to desirable levels for health and hygiene. The main factors then to be considered for satisfactory comfort conditions are:

• Temperature

• Humidity

• Air Movement

• Purity

The first three factors are to large extent interdependent, as at normal room temperature say 20 ºC, an increase in humidity increases one's sense of warmth, whereas at an outside winter temperature of say 2 ºC a similar increase in humidity makes one feel cooler, and an increase in air movement about the body in either case would give one a sense of a reduced air temperature.

If the temperature drops below 18 ºC or if it is draughty, people do not feel comfortable at home, office workers are not fully productive, and the frequency of influenza increases significantly. Likewise, if it is stuffy or smoky, or if the temperature and humidity are high, we feel sluggish. In fact, to function properly we require an environment whose physical, chemical and biological properties are narrowly defined. Since nature does not provide us with such a climate, we must create an artificial indoor habitat which better fits our needs.

The indoor environment is determined by the outdoor climate, building design, building management, and the actions of a building's occupants.

A sedentary or slowly walking person has a metabolic efficiency of about 5 %. A trained weight lifter can deliver about 20 %. Thus, the metabolic efficiency of a human being in converting chemical energy into mechanical energy is about the same as that of a car engine. Both can only function if a steady negative temperature gradient is maintained toward their environment so that waste energy can be discarded. The human energy balance has been thoroughly studied during the last 200 years and it is now known that a sedentary adult dissipates heat at a fairly constant rate of about 50-80 W. This corresponds to the heat emitted by a normal incandescent light bulb.

Clothing

The heat transfer is strongly influenced by the insulation value of clothing. A nude person with a fully exposed skin surface is most comfortable at an air temperature of 31 ºC. Heavily clothed people prefer an air temperature between 16 and 24 ºC.

Medical doctors have recognised for several hundred years that vanity can make people insensitive to their thermal needs. Thus, most business people invariably wear a three-piece suit, closed collar and tie. Moreover, the suit is often made of synthetic fibre cloth, which is impermeable to moisture but an excellent conductor of heat. Thus, the wearer freezes in winter in the same clothing that acts as a sweat suit in summer. Ideally,

fabrics should provide for moisture transmission, moisture absorption and heat insulation. It is little short of a wonder that workers in an office can peacefully coexist at any one chosen temperature. Natural vegetable fibres, such as cotton or linen are ideal for hot climate clothing. Man-made fibres worn close to the skin are uncomfortable in these conditions.

Heat Dissipation

If a person performs physical work, the body produces waste heat in an amount proportionate to the physical load.

If the body temperature of a working person is to remain the same as that of a sedentary person, heat transfer to the environment must be increased. This may be achieved by regulating sweating, or by modifying the properties of the environment. Sweating is highly efficient because the heat of vapourisation of water is 540 cal/g.

Thermal Comfort

As mentioned above, conditions are very rarely ideal for heat transfer from the body, and the perception of comfort changes constantly as a person's needs change. Obviously, a hard-working athlete's perception of the comfort of a heated arena will differ from that of his sedentary audience. Hot dry air is comfortable because it enhances evaporation of perspiration; hot moist air is uncomfortable because the moisture gradient is insufficient for the air to absorb perspiration as it forms and accumulates on the skin. Air movement can offset the unpleasant effect of partly saturated moist air because of the increased volume of air that touches the skin.

The human body is capable of surviving very large air temperature gradients for considerable time. Some saunas are kept at 90 °C and the earth provides temperature extremes reaching from -62 °C in the Yukon to 56 °C in Death Valley, California.

In contrast, the human body is extremely sensitive toward internal heat accumulation and therefore requires continuous heat dissipation.

The human body copes with this constant challenge by a variety of straight-forward and instinctive actions and reactions. These involve a large number of complex physical, biological and chemical mechanisms that defy simple scientific characterisation. In fact, these interactions are so complex that apparently simple daily observations -the sensation of sudden cold that is experienced on stepping out of a cold shower into warm air, for example -provoke lengthy (and inconclusive) discussions among experts.

Typical time budgets for family members disclose that on the average, everybody spends at least 20 or more hours each day indoors. Add to this travelling time (train, bus, car) and it may be that many people spend less than 30 minutes per 24 hours outside. The main exceptions being people who play (and watch) outdoor sports and dog-walkers. It is incongruous that smokers may now have more fresh air, as many buildings ban smoking indoors.

The overwhelming majority of the current housing stock was built before 1970 when energy was freely available. At that time, it made sense to save on insulation and capital investment because heating and cooling buildings cost relatively little.

During the last 10 years the cost of energy has increased by almost a factor of 10. The UK Government, private industry and the commercial sector have all developed plans for

better designed buildings and for retrofitting the current building stock with better insulation and other energy conservation devices.

For example, it is easier to reduce heating costs by cutting the influx of fresh air than by installing insulation. As a result, many people currently live and work in tightly sealed structures in which 100 % of the air is recirculated.

This has led to complains of eye irritation, headaches, dizziness, fever, nausea, sleepiness and poor concentration from every segment of the population and every type of building, as well as from the passengers and crew aboard aeroplanes because airlines can achieve a 1 % fuel savings by reducing ventilation.

The Temperature of an occupied space is increased as heat is passed to it from:

• The structure, i.e., walls, windows and roof, which are heated by the sun or warmer outside air

• Electrical or other heat-producing equipment

• The body heat given off by the occupants

For bodily comfort the heat produced by the body, which may normally vary between 120 and 440 watts depending on the amount of activity, must be dissipated to preserve the inner body temperature at about 36.9°C, and this is done naturally by heat loss from the skin. If the air surrounding the body is cool, heat loss is rapid, but if very warm air surround the body it may gain heat from the air, and this additional heat must be dispersed as well as that generated by the body. The processes used by the body for regulating its temperature are:

• Radiation

• Convection

• Evaporation

Humidity

Humidity is the condition of the atmosphere in relation to the water vapour it contains and is a fairly complicated subject to deal with fully, but a few brief notes will help you to understand enough of the subject for our purpose.

Water vapour is always present in the air in varying amounts, the amount that the air can hold depending on its temperature, the higher the temperature the more water vapour it can hold. The dew-point is the temperature at which air containing a certain amount of water vapour becomes saturated; any further reduction in temperature would result in condensation.

The Role of Humidity

Because of the temperature gradient, humidity decreases rapidly with altitude. The extremely low humidity at 12,000 m is responsible for the uncomfortably low humidity in commercial aircraft. The ground-level outdoor humidity is constantly changing. During the day, water vapourises from forests, fields and lawns at about 1 mm/day, about the same rate as from lakes. As moist air rises to cooler air levels, clouds are formed. Also, there is always a humidity gradient between sunny and shadowy spots on the ground. Even a slight wind is effective in transferring moisture to a cooler spot, where condensation can occur. Traces of atmospheric dust or other matter, such as the leaves of some plant species, are capable of inducing from saturated air condensation that then drips to the ground and waters the roots.

During the diurnal cycle, the water content of air increases while the sun shines. During the night the temperature drops and dew forms and recycles moisture to the soil. In coastal areas, the humidity can approach 100 % at night on a regular basis.

Indoor climate

The indoor climate differs in several fundamental ways from the outdoor climate, because air inside of buildings is confined in a comparatively small volume. In fact, it is often insufficient to maintain human moisture and pollutant effusion below a noticeable level. Furthermore, indoor air is not part of the biological and climatic air cycles which purify ambient atmospheric air and disperse pollutants. As a result, the quality of indoor air undergoes tremendous variations in a short time. This is reflected in indoor humidity trends. The moisture content of a closed room increases rapidly because each occupant continuously adds moisture to the air in the form of perspiration and with every breath day and night at a rate of several litres of water per day. This water readily condenses on cold walls and windows, but since there is no soil or surface to absorb it, every effort must be made to replace the water-laden air by ventilation.

Thus, if buildings are not carefully ventilated, the indoor habitat becomes saturated with moisture, causing not only air quality problems, but eventually structural damage.

Air Movement

An individual feels comfortable when metabolic heat is dissipated at the rate at which it is produced. Extensive experimentation has shown that for an average, sedentary, lightly clothed person this occurs most readily when the air in a standard room has a temperature of 24,5 °C, a relative humidity of 40 %, and an air velocity of 0,25 m/sec. According to thermal comfort standards currently valid, 80% of all adults dressed for winter indoor conditions find temperatures acceptable between 20-23,5 °C, a relative humidity of 30-60 % and the air velocity at 0,15-0,25 m/sec. Acceptable summer indoor temperature is between 20-26,5 °C. Very lightly clothed people prefer a temperature between 26-29 °C. The summer temperature can be increased to 28,5 °C if the air velocity is increased to 0,8 m/sec.

Moreover, two people will often choose a difference approach for deriving comfort from a given situation. For example, for a tourist from Northern Europe, comfort on a tropical island might mean a day of basking, minimally clad in the sun on a beach, whereas for a native it would more likely mean resting, fully dressed, in the shade of a tree. The tourist enjoys the very high rate of water transport through the skin without the wet feeling that a similar perspiration rate would produce in clothed state in still air at home.

Air movement enhances heat transfer between air and the human body and accelerates cooling of the human body. Exposure to wind has the same effect as lowering the temperature of still air. Thus, both processes cause the same sensation.

Air Movement then is essential for bodily comfort as it helps the body to dissipate heat gained from all sources by increasing the effects of convection and evaporation. Air movement in an occupied space gives a feeling of freshness by lowering the skin temperature, and the more varied the air currents in velocity and direction the better the effect. With room air temperatures between 18 °C and 21 °C, a comfortable but stimulating environment is created by a range of air movement velocities of 0,15 to 0,4 m/s. A draught is created when the temperature of the moving air is too low and/or the velocity too high. Desk fans do no change the temperature of the air, but by providing a flow of air across exposed skin, especially the face, a feeling of freshness is created.

Purity

Life Support

In order to maintain life, the human body must be kept at a steady temperature of 37 °C. Careful measurements on astronauts have shown that the normal internal base temperature varies by less than 0,3 °C among different people.

Since the human body does not register temperature, wind velocity and humidity readings, per se, the question arises how the body registers and regulates comfort. The answer to this question is not known. In fact, we do not even know where human internal temperature sensors are located nor how they function.

All five senses are active in the indoor habitat and have to be satisfied within a narrow range between deprivation and over stimulation or pain. Judgement of indoor air quality is, as we have indicated, highly subjective and related to many factors. At the dinner table, for example, food odours are as important as taste. In aeroplanes, however, food orders are generally perceived as inappropriate.

Temperature and Touch

Temperature is usually associated with touch, although tactile contact is not necessary for the perception of radiative heat. Touch receptors are distributed over the entire body, but their density varies. The sense of touch is poorly developed in many people and operates on a very muted level in schoolchildren, who are notoriously insensitive to heat the cold.

On a cold day, for example, the radiation from a fireplace is perceived as comfortable, but on a hot day the same stimulus causes discomfort. Likewise, on a cold day the consumption of ice water or ice cream causes the blood to flow in such a way as to counteract the chill to the internal organs, whereas on a hot day it sends a stop signal to the skin glands responsible for regulatory sweating. Conversely, when hot tea or coffee is consumed, the internal sensors initiate sweating even on a cold day. This response is not paradoxical, since the function of the temperature sense is to maintain an adequate gradient, or to adjust the gradient, or heat flux, rather than to maintain a constant skin temperature.

Very little is yet understood about this amazing sense, which must keep the temperature of the core of the body stable within + 0,5 °C. Cases of partial deficiency, common with the other senses, are not observed, because any error of this sense would induce fatal changes in metabolism.

Sight

The eye is the most thoroughly schooled sense. Thus, visual perception often overrides that of other senses. For example, we tend to equate sunshine with warmth, even if we are exposed to a chilly wind or air conditioner. Although the physical nature of light is well known and light can be accurately analysed, we still know relatively little about the way the eye and brain translate frequencies into colours.

Hearing

A healthy young ear can recognise sound vibrations over a frequency range from

about 16 to 40,000 Hz, a range of 12 octaves. In addition, the ear can voluntarily discriminate and de-emphasise background noise, such as the gushing or the blood in the skull, the hum of an air conditioner or ventilator in a concert hall, or traffic noises. Further more, it can analyse a sound into individual musical instruments or voices in a group. Yet on an oscilloscope a sound signal appears as a simple bleep. It is not known how the brain deciphers it, but again psychologists claim that people function best if they experience a wide range of sound stimuli.

Many features in buildings are designed to reduce noise and echo effects. If properly chosen and placed some, such as carpets and drapes, can enhance indoor air quality by serving as buffers for humidity; other merely collect dust and create suspended particulates.

Smell and Taste

Odour and taste are chemical senses, activated by contact between osmogenic molecules and geminal nerves.

The human nose is extremely sensitive and can discriminate among literally hundreds of different molecules; it can sharply distinguish between certain chemical functional groups such as acids, aldehydes, esters and ketones, as well as among the various substituents of benzene and other aromatic compounds. Smell also differs from the other senses in that it is strongly time dependent: steady odours become less obvious. Thus, people in hospital rooms rapidly become indifferent to odours that are so strong they repel visitors. This characteristic, which enables people to live with their own internal and external body odours, deprives them of a defence against toxic gases that accumulate slowly.

Summary

Although still very poorly understood, the human senses constitute a vital link between our environment, our comfort and our behaviour. One reason for this is that our senses serve life-support needs that cannot be expressed in simple physical parameters. They respond to differences, concentrations and gradients of quantities which change periodically. Furthermore, an individual's perception of comfort changes as a function of body functions. Thus, stimuli that are perceived as pleasant at one time can be almost intolerable at others. Consider, for example, the gourmet who relishes raw onions at dinner, but finds their odour repulsive at the breakfast table; the executive who needs bright lights at work, but favours a dimly lit room in which to read a novel at home; or teenagers who thrive on loud music but are prevented from sleeping by the hum of a fan or air conditioner.

Obviously, human beings require variety and an evenly lit office with a steady noise level and constant temperature and humidity would be stultifying. Yet the environmental engineer, architect and ventilation engineer have to find a range of stimuli appropriate for all employees, whether they perform physical or mental work and whether they smoke or not.

Problems in homes can also involve formaldehyde, microbes, radioactive radon and pesticides, but the major indoor problems are moisture, combustion gases, especially carbon monoxide, nitric oxide and smoke from tobacco smoke, kitchen stoves, heating ranges and wood stoves and fires.

Monitoring of Air Quality

Normally, building occupants have to rely on their five senses and such signs as odour, dry throat and burning eyes to detect indoor air pollution. Unfortunately, our senses

are better at judging comfort than at judging the threshold of toxicity. They are reliable for only a few molecules, such as ozone, nitric oxide or formaldehyde, whereas the more dangerous carbon monoxide, particulates, asbestos, and sulphates are undetectable far beyond safe limits.

Air Quality Control

Most indoor air pollution problems can be lessened or solved by increased air mixing by ventilation, by eliminating indoor sources and adjusting our activities, or by cleaning recirculated air. There is no sure or easy way to find the best mix. The foremost consideration in all buildings is to provide adequate transfer of oxygen and metabolic products. In residential buildings, the main problem is humidity control. A family of four releases about 20 l of water from perspiration, respiration and household activities per day. This volume requires 1300 m3 of air at 75 °F/ 23 °C and 50 % relative humidity (RH).

In reality, however, the water is produced during the two short mealtime and bathing peaks; instead of vanishing slowly, it migrates to cold spots where it condenses and remains stubbornly hidden in the form of moisture in the building materials, while most of the indoor air rapidly becomes normal or dry.

Condensation is a very serious problem if buildings designed for freely flowing air are suddenly retrofitted with insulation or vapour barriers to reduce heating costs. This condensation threatens the health of both the basic building structure and the occupants. The only adequate solution is to 'seal' the building and to provide intentional forced or natural ventilation at a rate adequate to mix air fully and remove excess moisture. Furthermore, buildings that rely on mechanical ventilation should not rely on uncontrolled infiltration, but should provide either natural cross-draught ventilation, or forced air circulation or both. Further, it is vital that such buildings have an appropriately placed air intake through which air may be admitted, either continually or in batches, as desired. In any case, natural ventilation should always be provided.

Basic Needs

In order to stay alive, a sedentary person has to inhale about 10,000 breaths of air each day. This adds up to about 10-20 m3/day of air and about 0,7 kg of oxygen, which is used to fuel metabolism. The air should be fresh and clean because the human respiratory system is a very sensitive and efficient transmitter of gases, benign or poisonous and of fine dust.

This air is exhaled at 37 °C and 100 % humidity. We further need to dissipate waste heat at a rate of 80 W in order that the heat content of the body remains constant and its core temperature be maintained at 37 + 0,3 °C. This stability is achieved partly by perspiration, which contributes about 2 litres of water to indoor air. The exchange depends on activity.

Furthermore, each day we need to dissipate about 0,5kg of carbon dioxide and traces of some 200 chemicals; the latter are responsible for normal body odours. A lightly clothed sedentary person feels most comfortable at 24,5 °C, 40 % humidity and an air flow of about 0,25 m/sec, but individual comfort differs considerably, depending on activity level. If the temperature is not suitable, the body can adjust its level of heat dissipation by such defence mechanisms as increased metabolic activity and sweating or shivering. Comfort also depends on cultural expectations. The following table shows the temperature proposed by the war restoration board in England in 1945. In 1931, 60 % of residences had unheated outdoor

privies to keep odours away from the house. Today, a pleasant year-round temperature of about 24 °C is expected in all residences. Obviously, life-styles in industrialised nations are changing rapidly. The average life expectancy in the United Kingdom has increased from 38 years in 1850 to 47 years in 1900 and 75 years today.

Radiation, Convection and Evaporation

Body Heat Balance

Why does anyone feel limp in hot surroundings? Why is a breeze refreshing? These sensations relate very much to the efforts the body has to make to maintain its temperature. The less the effort the more comfortable we feel. Man is a warm blooded animal and must maintain the temperature of his vital organs within a few degrees of 37 °C throughout life.

The human body, fuelled by the food we eat, continuously produces heat, associated with chemical change and muscular activity. This metabolic rate of heat generation can exceed 1 kW with maximum exertion. The following table gives an idea of the range; individuals will vary with age, weight and other personal characteristics.

The body has not much heat storage capacity because we cannot allow the greater part of our substance to get significantly hotter or cooler without distress. Apart from minor temporary deviations we must dissipate heat just as fast as it is generated - i.e. at the metabolic rate. There are three processes by which the body loses heat: convection; radiation; evaporation.

Convection and Radiation

A layer of cool air in contact with warm skin or clothing will pick up heat, and as its temperature rises its density will fall. The lighter air now rises up away from the body, taking heat with it, and is replaced by fresh, cool air which continues the process. This is natural convection.

Even the slightest air movement around the body will increase the rate at which warm air is replaced by cool, and thus increase the heat loss.

If the air temperature is on the high side the extra movement will be felt as a pleasant breeze, and can be increased with advantage by the use of ceiling fans or circulating fans. If, on the other hand, the temperature is normal or low, movement will be felt as a draught and should be kept below the limit of perception - about 0,25 m/s - for maximum comfort.

Like all matter, the body transmits heat by radiation and receives heat by the same path. If all the surfaces surrounding us were at the same temperature as our skin there would be no net gain or loss of heat by radiation. In practice, of course, the walls and most of the surfaces are cooler and radiation carries part of the necessary heat loss. Radiant heaters have high temperatures over small areas, contributing a net heat gain to the body. As radiant heat, like light, travels in straight lines, only the ‘illuminated’ side of the body will be heated, but, in the absence of excessive draught, the blood stream will distribute the heat satisfactorily over the body.

Within the comfort zone of external air conditions, convection and radiation losses account for about 75 % of the metabolic heat at rest, or with mild activity. When the temperature rises above or falls below the comfort zone an automatic reaction known as vasomotor regulation comes into play, with the object of preserving both internal temperature and heat loss unchanged. The tissue layers under the skin contain a network of veins which can be enlarged or contracted under the control of the nervous system. In a warm environment, a copious flow of blood is allowed through these vessels, bringing the skin temperature up towards a maximum of about 35 °C, so as to maintain its temperature excess over the surroundings. In a cold environment the veins are constricted, reducing the thermal conductivity of the tissue layer so that the skin cools. The blood flow along legs and arms is also modified, allowing further cooling along their lengths to keep down the heat loss. Ultimately, hands and feet may be only a few degrees above the air temperature - even to the extent of allowing frost-bite of fingers and toes rather than loss of temperature at a vital centre.

Evaporation

At temperatures above about 30 °C the system just described is unable to secure the necessary heat loss, even at rest. Indeed, above 35 °C convection and radiation cease to be losses and become heat gains making the body’s task even harder. Reaction to this situation is known as evaporative regulation. It consists in the automatic activation of the sweat glands over an area of skin proportional to the corrective effort required.

The transformation of sweat into water vapour absorbs energy, just as the boiling of water does. This energy is taken from the wetted skin surface in a form known as the latent heat of evaporation.

The cooling effect is powerful: the worker in a hot industrial environment may easily produce and evaporate one kilogram of sweat each hour, which will remove heat at the rate of 680 watts. The water and salt lost by the body have to be replaced, and it is natural that such workers should be copious drinkers.

Apart from the sweat mechanism, evaporation of the water vapour in the exhaled breath carries away heat at a minimum rate of around 20 watts. Note that the metabolic rate cannot be reduced, though we may increase it if we are cold by voluntary activity - walking briskly, stamping the feet, swinging the arms, etc. Shivering is an automatic reaction with the same purpose.

In a still atmosphere the air next to the skin and trapped in the clothing becomes almost saturated, and its capacity to absorb and carry away moisture is severely limited. The sweat produced stays wet on the skin and the body’s effort to give off heat is retarded. The surplus sweat which drips off or is wiped away is virtually useless for heat removal.

Currents of air flowing round the body correct this situation. Saturated air is replaced by fresh and evaporation is helped in exactly the same manner as heat removal is helped by forced convection.

Relative Humidity

The Relative Humidity (RH) is the amount of water vapour in the air at any particular temperature compared with the maximum amount that it will hold at that temperature, i.e., when saturated, and is stated as a percentage, e. g., air at 10 °C and 100 % RH (i.e., saturated) if heated to 21 °C would then be at 50 % RH because air at 21 °C is capable of holding twice as much moisture as air at 10 °C. Conversely warm air at 21 °C and 50 % RH, if cooled down to 10 °C, say by coming into contact with cold water pipes or window panes, would be at saturation point and any further reduction in temperature would result in condensation of water vapour on the cold surfaces. High relative humidity increases the feeling of warmth in a high temperature, and the feeling of coolness in a low temperature. Low relative humidity, or excessive dryness of the air also produces uncomfortable effects, such as dry nose and throat.

What is a fan?

A fan is simply a machine for moving air and other gases by means of a rotating impeller using centrifugal or propeller action, or both. There are four main types of fan used for general ventilation work: centrifugal, propeller, mixed flow and axial flow. Sub-division of these main types need not concern us here, but a few brief notes will help you to recognise them and to know what they are doing.

Centrifugal fans

A centrifugal fan has an impeller with a number of blades around the periphery, and the impeller rotates in a scroll or volute shaped casing, and it is this casing which identifies the centrifugal fan. As the impeller rotates, air is thrown from the blade tips centrifugally into the volute shaped casing (snail shell) and out through the discharge opening, and at the same time more air is drawn into the ‘eye’ of the impeller through a central inlet opening in the side of the casing, thus creating a continuous flow of air through the fan impeller and casing.

The volute shape of the casing helps to transform some of the velocity pressure of the air leaving the impeller into useful static pressure to overcome resistance to airflow in the ducting system to which the fan is connected. In normal ventilation work, a centrifugal fan would be used for static pressures (system resistances) up to about 750 Pa(=N/m²). A point to note is that the air flow through a centrifugal fan cannot be reversed.

In-Line Centrifugal fans

Instead of using a volute casing to collect the swirling air from a centrifugal impeller, it may be allowed to spin forwards into a concentric annular casing. Guide vanes will then convert the swirl velocity pressure into fan static pressure, and an outlet duct can be fitted in line with the inlet duct as for axial fans. Performance tends to be somewhat inferior to the corresponding volute model, and the chief advantage is avoidance of the transverse bulk and right angle direction change associated with a standard centrifugal fan.

Propeller fans

A propeller fan usually has a curved sheet metal-bladed impeller fitted to the motor spindle, the motor being mounted on a ring for wall fixing, or in a short length of duct for duct fixing. The air is drawn into the impeller in a fairly smooth pattern from all directions, and discharged in a direction approximately parallel to the axis of the fan, but with a helical twist.

Its main use is for moving large volumes of air against low system resistances, say up to 65 Pa, and is very popular in ventilation work in diameters from 300 mm to 900 mm, due to the robust construction and comparatively low price. However, where appearances matter, such as in offices, shops and hotels,this type is not usually acceptable. It can give reversed air flow at reduced volume andpressure by reversing the direction of rotation. The power required to drive this type of impeller continues to increase as the resistance to airflow increases (i.e., an overloading power characteristic), and the motor must have sufficient power to deal with the heaviest load possible at the designed fan speed to prevent it being overloaded.

Mixed Flow fans

A mixed flow fan combines the characteristics of the large volume of air moved by the propeller fan, (axial flow intake), and the higher pressure of the centrifugal fan, (radial flow discharge). This type of fan with its peripheral centrifugal discharge, fits in very well for roofmounting, say over an exhaust duct system serving a tall block of offices or flats. The airflow through the fan cannot be reversed in direction. It will operate against static pressures up to about 750 Pa and has a non-overloading power characteristic.

Axial Flow fans

An axial flow fan is a development of the propeller fan, but is more efficient (70% - 80%) due mainly to the aerofoil section blades and finer clearances between the impeller blade tips and the cylindrical fan casing. It has a non-overloading power characteristic which enables the correct motor horsepower to be used for any particular fan or application without causing a burn-out due to overload.

It is less bulky than a centrifugal fan for the same output and has the advantage of straight-through airflow, but for static pressures higher than about 250 Pa its higher running speed tends to make it noisier than the centrifugal fan unless special precautions are taken. To increase its performance against higher resistances two or more impellers can be used, forming a multi-stage fan, usually with guide vanes or contra-rotating impellers so that the air leaves the last impeller in an axial direction without helical twist, thereby increasing considerably the possible maximum pressure available. The airflow through the non-guide vane fan can easily be reversed by reversing the direction of the rotation of the impeller, but as the aerofoil section of the blades would then be running back to front, i.e., with the trailing edge of the aerofoil leading, the fan output would be reduced by 25 % or more.

There are, however, special reversible fans available which give equal volumes in either direction, by arranging alternate blades on the impeller to face in opposite directions, i.e., one correctly fitted for extract and the next for intake, or by having flat blades. These inevitably reduce the output by 15 % or more below that of the normal arrangements. Another point worth noting in axial fan application is that concerning hot or moist fumes. For such cases the centrifugal fan can have the advantage of a motor outside the airstream, so that a standard motor can be used.

To overcome this disadvantage in the case of the axial flow fan, it can be made with a bifurcated casing, so that a motor with slightly extended shaft can be mounted outside the airstream, which passes on either side of the motor through the special casing.

Fan Performance

While the selection of a ventilation unit from the output tables to perform under free air conditions is a simple matter, it is useful to know the rudiments of fan performance against some resistance to airflow, such as ducting and filters, or even sufficient free area for the passage of replacement air into a room from which the unit is extracting.

Characteristic Curve

Any particular fan design has its own characteristic curve, which is a graph made by plotting a number of test points showing volume delivered against different resistances. Volumes are measured in cubic metres per second (m³/s), which for the sake of convenience in calculations in our class of work is converted to cubic metres per hour (m³/h, and

pressures are measured in Pascals -Pa).

Pressure

The total pressure produced by a fan is made up of the static pressure, that is the useful working pressure available for overcoming the resistance of a ventilating system and velocity pressure, which is the pressure due to

How fan works

Propeller and Axial fans

Blade angle affects volume and pressure, the greater the angle, the greater the volume moved, but lower pressure. Velocity at tip is greater than at root.

Centrifugal fan types

Paddle Blade (Radial blade)

Simple and robust, good for conveying dusts, self-cleaning, bulky, not very efficient – 45 %.

Forward Curve (Multi-Vane)

High outlet velocity and good volume, fairly quiet, general ventilation duties, quite efficient – 70 %. Sizes 75 mm to 1,8 m maximum pressure about 750 Pa.

Backward Curve

Can have aerofoil blades, high speed and pressures, non-overloading, high efficiency

– 85 % Sizes 38 mm (15") to 3,6 m (12') max. pressure about 1500 Pa for average ventilation jobs. Much higher for "specials".

Characteristic curves

Fan curves are published to enable users to calculate the amount of air the fan will move under different conditions. In its simplest form, the curve is based on vertical and horizontal scales worked in volume of air moved against resistance, m3/s and Pa. Sometimes m3/h or l/s are used to indicate volumes. In imperial days it would have been ft3/min against inches waterguage. Conversion from SI to Imperial units has already been discussed. The curves that manufacturers publish in their catalogues, and computer selection programs should be to British Standards 848 parts 1 & 2, which deal with Performance Flow and Noise measurement respectively.

The layout of the curves will be similar, irrespective of the fan type, Axial, Mixed flow, Centrifugal, but the horizontal and vertical scales will probably vary, as indeed they will for differing impeller diameters of similar fans. The curves we will look at have another variation; the angle (pitch) of the impeller. Some form of cased axial flow fans, i.e., those built within a length of circular duct, have the option of variable angle blades. This ploy is to enable the actual fan performance to match the performance required more closely.

Figure 1 shows a curve for an axial fan. This is obtained by collecting a series of data points from the instruments on the test rig. The starting point is to measure the airflow at zero pressure, sometimes referred to as Free Inlet & Discharge or FID. The test rig is then adjusted to increase the pressure that the fan has to work against and to measure the airflow at each point. It is normal for only the “stable” part of the fan performance curve to be published. This curve is shown “smoothed”. The actual test points will not necessarily form a smooth curve, but these slight variations are omitted.

Figure 2 shows a “family” of curves for an Axial flow variable pitch impeller fan. The

volume of air moved decreases with the angle of the impeller. The speed of rotation remains theoretically constant.

Figure 3 shows Fan Dynamic Pressure or Velocity Pressure. This is calculated as

follows:

2Velocity×ρ×5,0=essurePrDynamic Where p is the density of the air or gas in Kg/m³ and Velocity is in m/sec. This is obtained by:

)m(Area)s/m(Volume

2

3

This is important to understand because if the volume flows around or through an obstruction, such as a bend or damper, resistance will have to be overcome. A measure of this resistance is characterised by 'k' factors.

Figure 4 shows Figure 1 reconstructed with the Static Pressure replaced by Total Pressure. The latter is the arithmetic addition of Static Pressure and Dynamic Pressure.

Unfortunately one of the recommendations of the proposed replacement British Standard, is to publish performance data using the term Fan Pressure which is Total Pressure - as always, this is likely to cause confusion for years!

Figure 5 shows the efficiency contour lines. The efficiency is calculated using the test

data by the following equation:

)

1000×)Watts(PowerShaftActual)s/m(FlowVolume×)Pa(essurePrTotal

=)Total(EfficiencyFan3

The illustration shows that for a fan there is a ‘best’ efficiency point. In practice, there is only a small chance of operating at the duty point which meets this. The reason is that most manufacturers design impellers to operate over a wide range of flows and pressures, they are not designed for a specific duty point. However, there are manufacturers which do design impellers and fans for specific conditions including operating at high efficiency value - this is particularly important with fans absorbing large amounts of electrical

energy. This may seem relatively unimportant for small fans, but where multiple fans are required for ventilation systems in buildings, a minimum efficiency value is sometimes included in the Specification by the Specifier or Architect in order to keep electrical supply requirements to a minimum.

Figure 6 shows several features from earlier slides and is a practical example. The required duty point is plotted, volume of air against pressure, but is seen to be beyond the inherent fan speed characteristic curve. If the system curve is calculated as discussed earlier, and plotted, where the fan and system curves bisect each other is the actual duty point. It may be that an adjustable pitch Axial fan can be used with a slightly high blade pitch angle, which will give nearer the actual performance required.

Fan Laws

Fans of the same basic design and proportions operate theoretically in accordance

with certain fan laws. In practise, these laws do not apply exactly because of design considerations and manufacturing tolerances, but they are useful in estimating approximate outputs of similar fans of different diameters and speeds as applied to normal ventilation work, and can be summarised as follows: 1. Volume of air flow varies as (fan diameter)³ and as rpm 2. Pressure developed varies as (fan diameter)² and as (rpm)² 3. Power absorbed by the fan varies as (fan diameter)5 and as (rpm)3

It is important to note, however, that these Laws apply to the same point of operation on the fan characteristic. They cannot be used to predict other points on the fan's curve. These laws are most often used to calculate change in flow rate, pressure, and power of a fan when the size, rotational speed or gas density is changed. Therefore, in the following Laws the suffice "1" has been used for initial known values and the suffice "2" for the changed values and the resulting calculated value when:

Q = volume flow rate P = pressure (total, static or dynamic) p = gas density n = fan rotational speed D = impeller diameter W = impeller power

1. VOLUME of air flow varies as the (fan diameter)³ and as the rpm or: 2.

)()( 3

rpmOldrpmNew

diameterimpellerOlddiameterimpellerNewVolumeOldVolumeNew ××=

or:

)nn

(×)DD

(×Q=Q1

23

1

212

3. PRESSURE developed varies as the (fan diameter)² and as the (rpm)² or:

22 )()(PrPrrpmOldrpmNew

diameterimpellerOlddiameterimpellerNewessureOldessureNew ××=

or:

)nn

(×)DD

(×P=P 2

1

22

1

212

4. POWER ABSORBED by the fan varies (fan diameter)5 and as (rpm)³ or:

35 )rpmoldrpmnew

(×)diameterfanolddiameterfannew

(×PowerOld=PowerNew

or:

)nn

(×)DD

(×W=W 3

1

25

1

212

Pressure and power calculation should also take gas density into account.

)ρρ

(or)pressureoldpressurenew

(1

2

But if this or any other variable is unchanged they can be omitted from the equation: for example if the fan diameter is constant, only speed variation applies:

)nn

(×Q=Q1

212

)nn

(×P=P 2

1

212

)nn

(×W=W 3

1

212

A simple example of the application of the fan law dealing with volume can be shown using Vent-Axia data. (TX Window model).

FAN LAW VOLUME varies as (fan diameter)³ and as rpm e.g., a 300 mm Ø fan running at 1190 rpm delivers 1415 m³/h. What will 190 mm Ø fan running at 1290 rpm deliver? VOLUME of air flow varies as the (fan diameter)³ and as the rpm or:

)()( 3

rpmOldrpmNew

diameterimpellerOlddiameterimpellerNewVolumeOldVolumeNew ××=

or:

)nn

(×)DD

(×Q=Q1

23

1

212

or:

084,1×254,0×1415=)11901290(×)

mm300mm190

(×Q=1415 31

h/m389=Q 32

This slight variation from our quoted output for the size is negligible from the

practical point of view and will be due to small differences in the similarity of the two units being compared, and to the "rounding off" of test figures. System Resistance Laws

The resistance of a ventilating system is caused by:

a. the loss of energy at the point of entry of the air due to a sudden increase in air velocity from practically zero to the velocity along the duct.

b. the friction between the air and the side surface of the duct.

c. changes of cross-sectional area of the duct, where there are expansions and contractions, or changes of shape (say from square to oblong section). Expansions, contractions and changes of size or shape should be made by gradual taper

ds and well-rounded corners, not by sharp elbows, unless fitted with guide vanes.

s a given volume of air rough the system will vary as the (volume flow rate)² i.e. P °C Q².

at four times the original pressure! ND EIGHT TIMES THE FAN MOTOR POWER!

points for plotting the system resistance curve may e derived from the following formula:

sections, not abruptly, ideally 15 ° included in angle. d. changes of direction, such as bends and Tee-junctions are large wasters of energy.

Changes of direction should be by easy ben

The loss of pressure due to all of these sources, known as the system resistance, is for practical purposes proportional to the square of the velocity at the point of loss. Therefore, for a fixed system, it may be said that the pressure required to pasth

Therefore, if it is required to double the air flow through a system, the fan must be capable of providing twice the volume flow rateA

If a specified duty requirement does not exactly match the available fan performance, it is advisable to superimpose a system resistance curve onto the fan performance curve to confirm the final anticipated duty. Datab

)QQ

(×P=P 2

1

212

eat e procedure until there are enough points to plot the curve - three will usually suffice).

P1, Q1 = Specified system pressure and volume flow. P2, Q2 = New values of pressure and volume flow to be plotted.

(Simply choose a new value for Q2 and calculate the corresponding new value for P2. Repth Square Law

we can say that Resistance varies as the square of the volume. The equation then becomes:

Resistance Varies as the Square of the Velocity. As velocity varies directly as volume,

2)(tanRetanReVolumeOldVolumeNewcesisOldcesisNew ×=

Simple energy recovery

With energy costs increasing, more and more houses are being insulated to conserve heat.

Draught proofing, double glazing, high levels of insulation and intermittent thermostatically controlled heating systems are sensible ways of conserving heat and energy. However, the air quality in such sealed dwellings can rapidly fall to an unacceptable level, leading to health hazards through contaminants in the air.

Moist and polluted air, if allowed to remain leads to condensation. This results in the problems of streaming windows, peeling wallpaper, damp clothing and bedding and eventually unsightly mould growth leading to permanent damage of the building fabric.

Mechanical extract fans, with sophisticated sensing systems are often the most cost effective way of controlling levels of indoor contaminants. In order for these systems to work efficiently, a similar amount of air to that extracted must be allowed into the building. A convenient air entry point is not always available leading to reduced ventilation levels.

The Solution

Heat Recovery ventilation units solve these problems in three ways:

• Firstly, the extraction of pollutants and simultaneous intake of fresh air balances the air movement.

• Secondly, the heat energy (up to 75 %) of the extracted air that otherwise could be wasted is transferred to the incoming air. The heat exchanger associated within this maintains separate extract and intake air flows and uses no energy. For a room temperature of 20 °C, the intake air will be prewarmed to about 15 °C from the outside temperature of 0 °C.

• Thirdly, heat recovery ventilation will dry the excessive airborne moisture. A cubic metre of air weighs over 1 kilogram and in a bathroom with a temperature of 20 °C and 80 % Relative Humidity this includes up to 14 grams of water, waiting to condense. At 0 °C, with 80 % Relative Humidity, the outside air contains just 4 grams of water. So, for every cubic metre of air exchanged the moisture content is reduced by 10 grams per cubic metre using a heat recovery ventilation unit.

The Benefits

Heat Recovery ventilation units provide a warmer, drier, more comfortable home with uncontrolled condensation eliminated. There is also a financial benefit, especially now that VAT has been applied to fuel.

The microscopic droppings of the house dust mite can cause asthma, rhinitis, bronchial and other allergic problems. Heat recovery ventilation can reduce the Relative Humidity to below 70 % which inhibits the ideal living and breeding conditions of the house dust mite.

The long term benefit is global. Domestic housing uses a very large percentage of the UK’s total energy production so the whole world will benefit from the energy savings gained

from using heat recovery equipment, in turn this will reduce the potential of global warming and the greenhouse effect.

Controlled heat recovery meets the demands of today’s life style, and sets the trend for the future of ventilation.

Ventilation, Condensation and Heat Recovery

The Buildings Regulations Document F (Document K in Scotland); outlines the specified rates for ventilation in the home - kitchens, utility rooms, bathrooms, shower rooms and toilets. The thinking behind Document F is twofold.

Firstly, the requirement is for a supply of fresh air in the building. Secondly, the pollutants including dust, cooking and toilet odours, tobacco smoke and moisture must be removed.

Modern houses are well insulated, properly heated and now Document F is in force, ventilated. However, in a modern house with an energy rating of 8 out of 10 there is a significant heat loss involved in extracting warm air from the building and allowing cooler air into it.

Building Regulations Document L

The Building Regulations, Document L is concerned with ‘Conservation of fuel and power’, and will be linked with Document F. Heat recovery is just one of the measures introduced to meet Document L.

The Alternative

The alternative to controlled heat recovery ventilation is known as passive stack ventilation. This is comparable to the wasteful air movement of Victorian houses or even stone-age mud huts with a hole in the roof to let out the fire smoke. Controlled heat recovery ventilation however can be finely tuned to control temperature, humidity and air quality levels. Passive stack ventilation relies on outdoor conditions, including wind speed and direction.

It is estimated that uncontrolled passive stack ventilation could, at best, waste up to 30 % more heat than controlled heat recovery ventilation.

Air Replacement

Provision for Air Replacement

While in many cases the normal crevices around doors and windows are sufficient for this purpose, it is often necessary and advisable to make special provision for replacement fresh air to be brought into the room through grilles of a suitable size and design fitted in doors or walls to ensure draughtfree ventilation, and the minimum of restriction to extract fans.

Special provision for air replacement should be considered if:

(a) windows and doors are draught proofed

(b) the location of fans is such that satisfactory coverage of the space by cross-ventilation cannotbe made with air pulled in from the available doors and windows

(c) smoke from solid fuel fires, fumes from gas fires or boilers are pulled back down the flue into the room.

Minimum Free Areas

If special provision has to be made, then the minimum free areas to allow for air should be based on 1,300 cm² for every 1,500 m³/h performance.

Note that the free area of a grille or louvered panel is the open area through which air can pass freely, and may be as low as 20 % of the face area for fly-proof gauze, or as high as 90 % for eggcrate type grille. The above minimum figures give an air velocity through the grille of between 2,6 to 3,3 m/s and would cause a 10 % drop in fan output; they should be doubled or trebled if:

(a) more comfortable air velocities are required in the room

(b) maximum fan output is required

Location of Inlets

When considering air replacement, the location of suitable air intake points is as important as the location of the extract units, because the distribution of the air flow through the space being ventilated can give success or failure to the scheme. The main points are:

1. Aim for full cross-ventilation of the space.

2. Eliminate “dead” spots by preventing short-circuiting of air flow straight from inlets

to extract units without “sweeping” the room.

3. Locate units at high level, and inlet grilles usually just above head level to avoid uncomfortable draughts to the occupants. The natural upward convection currents and the secondary entrained air movements caused by the “jet” of air, more or less horizontal, from the inlet grilles will give sufficient gentle air turbulence around breathing level to maintain a feeling of freshness. It is, however, sometimes convenient to locate the inlet grilles in the walls behind existing radiators, in which case the incoming air can be warmed slightly in cold weather.

4. Use sufficient well-spaced inlet grilles to keep incoming air velocities below 1,5 m/s if possible (that is double the figures given earlier).

5. If the room is very wide, say over 25 m, it may be necessary to extract centrally and bring in replacement air along each side. If central extraction is difficult to arrange, an alternative method of ensuring a reasonable velocity of air movement over the full width of the space and prevention of a near-stagnant area down the centre, is to use intake units along one side, extract units along the other, and a number of ceiling sweep fans as air circulators, fixed centrally between 3,0 m and 7 m above floor level to any convenient structural beams or roof truss members. This method is usually considerably cheaper and less unsightly than a central extrac duct, has proved very acceptable and successful in large stores, particularly basement, and open plan offices.

Flued Appliances

Smokey Flues

While dealing with the minimum free areas for air replacement, a not uncommon problem is that of the ‘smoky fire’ and flue gases pulled down the chimney of a gas-fired boiler when the extract units are switched on in a kitchen or public house bar.

It should be pointed out that the National Gas Boards insist on permanent air inlets in rooms in which gas boilers are fitted to ensure a sufficient supply of air for combustion, the free area of the inlet being twice the cross-sectional area of the flue pipe. For a 127 mm diameter flue pipe the free area of this inlet would be 252 cm² to which should be added the free area necessary for the extract unit.

The following table gives the minimum free area for grilles to suit various combinations of flue pipe and fan diameters:

The same method can be used in any room with exhaust units fitted if there is a complaint of ineffectivenesss of the system. An extractor fan is not a vacuum pump and will only work if there is an adequate air supply to match the volume extracted. If this replacement air is not available, the fan will work at a reduced rate or possibly even run stalled, i.e., without actually moving any air.

However most fans are powerful enough to overcome the natural updraught in an open-flued combustion appliance and pull the product of combustion back into the room. This can lead to carbon monoxide poisoning which is fatal. The Building Regulations Approved Document F1 (1995 Edition) addresses this.

Interaction of mechanical extract ventilation and open-flued combustion appliances

Mechanical extract ventilation can cause the spillage of flue gases from open-flued combustion appliances in dwellings whether or not the fan(s) or extract air terminals and combustion appliances are located in the same room. Such spillage of flue gases is dangerous and in dwellings where it is proposed to install open-flued appliances and mechanical extract ventilation the appliance needs to be able to operate safely whether or not the fan is running. For example with:

(a) gas appliances, where the appliance and the fan are located in the kitchen, the maximum recommended extract rate is 20 litres/second. A spillage test as described in BS5440 : Part 1, Clause 4.3.2.3 should be carried out whether or not the appliance and the fan are in the same room. Where a fan causes an appliance in a different room to spill, the extract rate may be reduced to cure the problem. (Further advice is contained in BRE Information Paper 21/92).

(b) oil-fired appliances, installed in compliance with Technical Information Note T1/112, which can be obtained from: Oil Firing Technical Association for the Petroleum Industry (OFTEC), Century House, 100 High Street, Banstead, Surrey, SM7 2NN. For further advice, contact OFTEC.

(c) solid fuel appliances, mechanical extract ventilation should not be provided in the same room. For further advice contact HETAS (Heating Equipment Testing and Approval Scheme), PO Box 37, Bishop’s Cleeve, Gloucestershire, GL52 4TB.

Open-flued appliances take their combustion air from the room or space in which they are installed and so contribute to the extract ventilation when in operation. They can also be arranged to provide adequate extract ventilation when not firing. For instance no additional extract ventilation would be necessary to satisfy the requirement if:

(a) the solid fuel open-flued appliance is a primary source of heating, cooking or hot water production.

or

(b) the open-flued appliance has a flue with a free area at least equivalent to a 125 mm diameter duct and the appliance’s combustion air inlet and dilution air inlet are permanently open, i.e., there is a path with no control dampers which could block the flow or the ventilation path can be left open when the appliance is not in use.

Heat Removal

Ventilation on the basis of heat removal from rooms where some considerable temperature drop is a main factor, needs special consideration. The amount of heat produced in the room must be estimated from:

• Body heat of occupants

• Electrical apparatus

• Other heat-producing processes

Solar heat gain through structure •

The volume of air required for ventilation can then be assessed by using the following formula:

( ) ( )( )CtΔ

kWPhmV ×=

3000/3

The Δt (°C) is the difference between outside shade temperature and the slightly higher required inside temperature.

e.g., If the total heat gain is say 11,75 kW per hour, the outside shade temperature 21 °C and the maximum inside temperature required 27 °C then the volume of air required is:

( )hmV /58756

300075,11 3=×

=

Note that a small reduction in the temperature difference makes a considerable increase in the volume of air required. For instance, if in the example above the temperature difference is reduced by say 2 °C, i.e., required inside temperature 25 °C, then the volume of air required would be increased to:

( )hmV /88124

300075,11 3=×

=

a 50 % increase in air volume.

Always extract from as high as possible to prevent the collection of heated air under the roof or ceiling from extending downwards to near breathing level. Fans at or near the ridge in such cases are ideal, with air replacement from side windows above the head level. This method assists the natural tendency for warmer air to rise, and the quicker this ‘fug’ is removed from high level the better will be the conditions at breathing level.

Note that with ventilation alone, the temperature inside a room cannot be reduced to the outside shade temperature - it will always be a few degrees above. However, it is worth considering pulling the replacement air from the "cold" side of the building - the north facing (north of the equator).

It should be borne in mind particularly when dealing with general purpose single-storey factories that there are two separate ventilation problems, one for summer and one for winter. A high rate of ventilation is required in summer to deal with the build-up of heat from solar radiation, production processes and the workers themselves, whereas in winter a very low ACH is required, simply to prevent vitiation of the air and to remove odours and water vapour.

This variation in requirements may range from 6-15 ACH in summer and 2 ACH in winter, and is a good case for installing a larger number of well-distributed smaller (300 mm) fans with speed control, rather than one or two large fans (600 to 900 mm).

Radiant Heat

Note that radiant heat from hot surfaces, such as cooking vessels, steam presses, pipes etc., does not warm the surrounding air, but only solid bodies in the path of the radiation. The extraction of air therefore will not reduce or remove the radiant heat, but it will create air movement which would give relief by removing convected heat. To reduce radiated heat the hot surfaces should be insulated.

Heat Input

There are occasions when a ventilation scheme must include some form of heating for the replacement air to prevent the temperature of the air in the room from being lowered to an uncomfortable level.

Unless the air replacement openings can be located behind existing radiators, or the incoming air can enter from a room already warmed, the most convenient method is to use an intake fan in conjunction with a heater battery, either electric, hot water or steam, the required heat output being calculated by a similar formula as given for Heat Removal, using the known factors of air volume supplied and the temperature rise required.

e.g. A ventilating unit supplying 1530 m³/h of air at 0 °C which is to be warmed to 19 °C before being discharged into the room. What will be the required output of the heater battery?

The formula is now transposed to read:

( ) ( ) ( )3000

3 CthmVkWP Δ×=

( )kWP 7,93000

191530=

×=

Electrical heater batteries are made with the elements built into a sheet steel flanged short duct complete with safety thermal cut-out, ready to fix to a duct or wall, the elements being of the 'black heat' type. Hot water and steam heater batteries are not so flexible in application as the electrical type, particularly for the smaller outputs such as used in the above example, and the selection of a battery for a particular duty should be left to the manufacturers. The additional resistance to air flow of the heater battery must also be taken into account; this will depend on its type, size and design.

Patterns of Airflow

Approach Velocities

The pattern of the airflow as it enters an Axial flow fan is different from the pattern as it leaves. Assuming no interference from the layout of the building, or louvres on the fan, the air entry pattern is hemispherical (shaped like half a globe or sphere). By contract the pattern of airflow as it leaves an Axial flow fan is an almost parallel sided jet, actually expanding slightly at an included angle of 15 °C.

The velocity of the approach air increases as it nears the fan. There is little likelihood of a draught problem on the approach side. As a guide, if the air passing through a 620 mm diameter fan is at 10 m/s. The air velocity at 2 m from the fan is under 1 m/s. The approach velocity can be calculated by dividing the air volume (in m³/s) by the surface area of half a sphere, multiplied by the distance from the fan in m².

( ) ( )( ) ( )mdmA

smVsmw×

=

2

// 2

3

The formula for the surface area of a sphere is 4πr² , so for half a sphere 2πr²

( ) ( )22

3

284,6//

mssmVsmw××

=

For a 300mm diameter fan moving 0,43 m³/s (1560 m³/h), the approach velocity at 1 m from the fan is:

( )smw /068,011284,6

43,022 =

××=

Against the actual velocity through the fan:

( )smAperture /06,6071,043,0

==

Translated into domestic terms an average 150 mm impeller diameter fan in a kitchen should have a minimum performance of 60 l/s (0,06 m³/s) to meet Building Regulations Document F1. The approach velocity of this fan at 1 m is:

( )sm /01,0284,606,0

≈

The velocity of hot air rising from a domestic cooker can be as high as 0,3 m/s, thirty times faster than the air going towards the fan. The hot air would therefore, rise to ceiling level before being drawn slowly towards the fan. There could be some spillage before the hot air is entrained towards the fan. Does this mean the recommended fan performance in F1 is inadequate? If a fan is drawing air from outside into the building, (to pressurise it) the approach velocity is of importance only if there is a source of pollution nearby. For example,

the outlet of a fuel burning appliance. For this reason another of the Building Regulations, (Document J) recommends at least 600 mm between the flue outlet and an air inlet to the building.

As the hot boiler flue gases rise, it could be suggested that an air inlet is not sited vertically above an outlet and not less than 600 mm either side of the vertical line.

By contract, the airflow on the discharge side of an axial fan follows a different pattern, expanding at an included angle of 15 °C. This means that its velocity is maintained over a far greater distance. Taking the same 620 mm diameter fan with a through velocity of 10 m/s, this velocity in still air at 2 m, is still air 6,5 m/s at 5 m and just under 4 m/s at 7,8 m from the fan. If these velocities are on the outside of an exhaust fan, they could cause problems with proximity of adjacent buildings. If the fan is pressurising the building, (blowing air into it), the possibility of draught discomfort inside the building is high. There are inherent design problems to ensure that the mechanical airflow into and out from the building do not suffer from conflicting airflow patterns. Air tries to flow in a straight line, when turned it tries to continue in the new straight line. It does not bounce. This aspect is discussed more fully in the section on ducting.

The same airflow patterns apply to grilles and louvres. The extract grilles from a room should be the simple eggcrate type. This allows the hemispherical approach with resultant low approach velocities. The same grille on the intake airflow to the room would give an uncomfortable high velocity downwards air flow. Inlet grilles should have deflector vanes to direct the airflow parallel to the ceiling, ensuring a mixing of air before circulating into the room volume.

Example 1 What is average velocity 1m from a size 12 unit moving 1560 m³/h?

( )smV /43,036001560 3==

( ) ( min/5,13/068,01284,6

43,02 ftsmw ==

×= )

Example 2 In a domestic kitchen with a 150 mm window model, what will be the average velocity at a cooker 1 m away?

( )smV /06,03600216 3==

( )smw /001,01284,3

06,02 =

×=

Now the velocity of hot air rising from a domestic cooker can be as much as 0,33 m/s. This is 30 times as fast as the air going towards the unit at this point. The hot air would therefore, rise to ceiling level before being slowly drawn towards the unit. As long as the unit is big enough to give the correct air change rate in the kitchen, it does not matter if the hot air from the cooker is not drawn directly towards the unit, but goes first up to the ceiling, but is a 150 mm fan big enough, even though it conforms to Building Regulations?

Average velocity at any point 'P' :

( )2222

2

3

284,62

42

/mr

Vm

V

mAsmVw

××=

××=

×=

π

When 'P' is at 1D. ave. vel. = 12,5% of entry velocity 2D. ave. vel. = 3% of entry velocity 3D. ave. vel. = 1,4% of entry velocity

Actual velocities reduced on centreline to: 1D - 10% (= dotted line area) 2D - 2.5%

3D - 1%

Wind Pressure and Flow Around Buildings

Wind has pressure due to its velocity. This pressure is calculated from the equation Vp = 0,6xV², where Vp is the velocity pressure in Pascals (Pa) and V = velocity, in m/s 0,6 is a constant, derived from the density of the air, at 20 °C and average RH. A fresh gale blowing at Beaufort scale force 8 (80 km/h or 48 mp/h) will exert a pressure of 296 Pascals on a building.

As the air passes over and round a building, the pressures are positive on the windward side and negative on the lee or sheltered side. It is often the negative pressures that suck roofs off buildings and windows out of walls. When wind is stopped, its velocity pressure is converted into static pressure.

The effect of wind on simple domestic extractor fans can be considerable. Wind speeds above 25 km/h can stall a fan on the windward side of a building and cause “free wheeling” on the lee side. If a fan is in the wall or window of a dwelling that is recessed into the building, forming a balcony or well, the trapped positive pressure will find the only route available to balance the pressure, this could be through the fan. Even the most efficient backdraught shutter will not prevent air pressure equalisation.

A mushroom shaped cowl will afford better protection than a louvred grille, but even this will allow air transfer in a positive pressure area.

Wind has pressure due to its velocity. This pressure is calculated from the equation:

Vp = 0,6xV² where Vp = Velocity pressure in Pascals (Pa) V = Velocity in m/s

Duration of Average Wind Speeds

Wind speeds are connected to a standard height of 10 m (33 ft) above ground level. Figures can vary widely from different parts of the British Isles.

Wind Flow Around Buildings

If there are no obstructions, wind normally flows in a very turbulent state, but in

generally straight direction in spite of veering and backing. When an obstruction like a building is encountered, the flow pattern is markedly changed.

Large towns rarely have wind gusts exceeding 110 km/h (Vp=560 Pa)(70 mph).

Most coastal areas can have gusts of 145 km/h (Vp = 970 Pa) (90 mph)

Some Scottish areas can have gusts exceeding 160 km/h (Vp = 1185 Pa) (100 mph)

The maximum wind force exerted on a building can be considerable.

Example: For a building with 15 m (50') wide × 6 m (20') high wall exposed to 160km/h gusts in an exposed Scottish coast area.

)/(8,12080665,9

)(1185 mkgPaForceWind ==

Actual force transmitted to building will be between 50 % and 80 % of theoretical maximum, depending on surface roughness of the building.

( ) (tonstonnesingrceOnBuildMaxTotalFo 2,87,81000

%80120615== )×××

=

Variation of Wind Velocity Due To Height Above Ground Level

In cities, the large number of buildings, some of them quite tall, have the effect of slowing down the wind. This causes a big difference between low and high level wind velocities. In open country there is far less difference between low and high level wind velocities.

When stopped, wind velocity is converted into static pressure.

Positive Pressure Ventilation

An alternative method of ventilation which has found favour with many Local Authorities and Housing Associations is Positive Pressure Ventilation. This is an extension of the intake ventilation concept (as heat recovery is a version of a combined system). This rate of ventilation is very low, half an air change per hour (0,5 ach) and is designed to be continuous. Sensors within the unit change the actual rate according to the ambient air temperature.

The concept of this method of ventilation is that outside air is usually drier than the inside air. BS5925, 1991 Section 4.5 (Control of Internal Humidity) cites “The contribution made by ventilation is to lower the moisture content of the internal air by dilution with the outside air which normally has a lower moisture content”. The amount of moisture that air can hold is dependent on the air temperature, the hotter the air, the more moisture it can hold, and vice versa.

Relative Humidity (RH) is the ratio between the moisture content of air at a certain temperature, and the maximum moisture it could contain at that temperature, expressed as a percentage.

Saturation of the air takes place at 100 % RH, i.e. the air contains as much moisture as it can hold. This element and other similar expressions (dew point, condensation, etc.) are detailed in the section on Condensation.

Positive Pressure Ventilation introduces outside dryer air into the dwelling where it is mixed with the internal air, lowering the total moisture content, and gently removing the moist air, either through natural leakage or via specified devices that open and close automatically depending on temperature, moisture content or both.

The most recent development in this type of unit incorporates provision for external sensor control when the air movement rate is increased to cope with higher moisture production periods, and provision for drying out new and substantially refurbished properties for a timed and self-cancelling 14 day period. This again is highlighted in BS5925 Section 4.5.

“It should be noted that in newly constructed buildings large quantities of moisture are released from the fabric as the building dries out.Consideration should be given during this drying out period to the question of whether additional ventilation should be provided”. The amount of water used in the construction of a traditionally built 3-bedroom house can be as high as 4 tonnes.

The air at ceiling level is usually up to 8 °C hotter than at lower levels, as hot air naturally rises. This can sometimes be very visible in rooms where heavy smoking occurs with insufficient ventilation. The hot air rises to ceiling level, creating a barrier for the cigarette smoke, which visibly hangs some distance below ceiling level, forming a band of smog at about head height. In the domestic environment positive pressure ventilation mixes this wasted heat at high level, with the dryer outside air and circulates it, firstly across the room at ceiling level, then to the rest of the room volume, without causing cold draughts.

As dry air costs less to heat than moist air, and the room heat is de-stratified, this form of ventilation can reduce heating bills, despite bringing in a small amount of outside air which may be below the internal air temperature at certain times of the year.

As the fans are designed for continuous operation, other than two or three months during high summer when condensation is not a problem; most units incorporate latest technology, low voltage, brushless DC motors.

These motors are much more efficient than usual mains voltage AC motors; up to 80% against mains motor 20 %. The reduction in running expenses is considerable. The actual cost of using this type of ventilation equates to about a penny per day throughout the year. During many months of the year, the outside air is tempered by a higher loft temperature (where the unit is sited). This, again, can effect a reduction in heating bills. To avoid problems with external and internal dust, efficient filters are incorporated, usually to EU5 or higher filtration levels.

This method of ventilation together with thermostatic radiator valves can reduce heating costs quite considerably for most of the year. Versions of this type of unit for use in dwellings without lofts (flats, maisonettes, etc.) are also available. These units are usually wall mounted and have a smaller air volume performance commensurate with the smaller dwelling size anticipated.

Which ventilation system is used can be a matter of personal choice, either by the occupier, owner, specifier or even contractor. However, a logical choice is often more dependent on the actual size and layout of the dwelling and, once again, the lifestyle of the occupier.

So called “dry” occupation, where the people are out for most of the day, results in two peaks of moisture generation. This is normally between 0630 and 0800 when they shower and dress for the day and again between 1830 and 2000 when evening meals are prepared, laundry is done and maybe showers are taken again before an evening out. It is usual for central heating to be used only during these two peak periods. In colder weather, this could mean that the building does not reach a high temperature before the heating switches off, depending on the thermal mass and insulation.

Unit ventilation, with sensor operation and manual override, may be most suitable for this type of lifestyle. The sensor will ensure that any residual moisture is exhausted while the building is unoccupied.

A combination occupation where the breadwinner leaves and returns as above, leaving the partner at home, say to care for children or an elderly relative, requires a different ventilation system. Provided that the home has some form of central heating that stays on during the day, either individual room or whole house heat recovery units can offer a condensation solution. Permanent trickle ventilation will cope with the more general and continuous moisture generation during the day and sensors in the kitchen and bathroom will boost the unit to a higher speed for any short duration required. The energy recovered from the exhausted air will mean a relatively short payback time, typically eighteen months or so.

For households where the occupants are indoors most of the day, morning and evening peaks are not as evident. Positive pressure ventilation can offer a simple solution for this lifestyle as the unit is running continuously. This prevents moisture build-up and condensation occurrence, but this can still switch to a higher speed should it be necessary.

For larger dwellings and families, a combination of continuous heat recovery or positive pressure, plus individual multipoint or sensor operated unit extractor fans in kitchen and bathrooms, will cover all eventualities.

Compact positive pressure fans are an ideal and economical means of ventilation for smaller flats,especially if the occupier is a single parent or elderly.

The economy made in reducing heating costs by redistributing and circulating existing heating, together with the low cost installation of a single unit and only one penetration of the building fabric, has found favour with owners and occupiers alike.

Ducting Systems

Flexible Ducting

Using the simple velocity method of calculations to determine the resistance to airflow, it is generally acknowledged that flexible ductwork offers approximately twice the resistance of a smooth bore duct, even when fully stretched. Flexible duct is manufactured around a wire helix. The flexible sheath will adopt a smaller diameter than the helix, reducing the cross sectional area. A duct with a normal diameter of 100 mm will reduce to an actual diameter of around 80 mm when not fully stretched and at a bend. A sharp 90 ° bend in flexible duct has up to three times the resistance of a smooth bore duct. The reason for these higher resistance values is due to turbulence in the duct caused by the constantly varying surface.

Flexible duct should be supported when in false or suspended ceiling applications to avoid the socalled ‘Loch Ness Monster’ effect of a series of humps. Each of these humps can have the resistance of two 90 ° bends.

The main advantage of flexible ducting over rigid is its “flexibility”! The cost per metre for straight runs is higher than rigid, but these are no expensive bends - of any angle.

This can also be the potential downfall of flexible duct, because it may lead to lazy workmanship, and higher than calculated resistance. In an ideal world, flexible duct would only be used with rigid straight duct to reduce the cost of specially made bends, but as we all know, the world is not an ideal place. Therefore, err on the side of caution when calculating the resistance of a flexible duct system. Keep the diameter of the duct on the high side, at least 1,25 times the impeller diameter for simple, small diameter Axial fans and no less than the diameter of the inlet/discharge spigot for centrifugal and in-line centrifugal and mixed flow fans. The general rule of thumb for calculating the appropriate discharge, a free area of 1,5 times the duct cross-section is equally important, and should also apply to any inlet grilles.

Metal Duct (Square and Spiral Wound)

The velocity method calculations for spiral wound duct are included in the tables in the System Calculator. Square duct should be based on an equivalent cross sectional area basis.

It is not within the remit of this course to discuss specially made ductwork which is based on static regain calculations. This is the realm of the consulting engineer. There are several external courses organised by such as the Mid Career College that cover more complex duct calculations.

Design of a Simple Ducting System

It is very important that ventilation systems comply with any Fire Regulations, Building Regulations, Codes of Practice, etc., relevant to the installation and the components

being used.

As much information as possible must be obtained from the customer or other sources regarding the application of these regulations to the building and/or area to be ventilated before attempting to design a system.

A simple procedure for designing a ducted system is as follows:

a. Calculate the Room Volume to be ventilated Width x Length x Height = m³ (cubic metres)

b. Calculate the Air Volume requirement by multiplying the Room Volume by the Air Change Rate per hour (see Table 1) = m³/h.

c. Decide on the best position for the intake and the extract outlets to the atmosphere and the best route for duct runs. The design should provide good air distribution in the room, whilst keeping the duct layout as simple as possible.

d. Determine each section of main and branch duct, the size and shape of each grille and duct bend.

There are several ways of approaching designing and sizing ducted systems. The simplest is the velocity method, which involves selecting main and branch air velocities as in Table 1, used in conjunction with trial calculations.

e. Sizing the duct(s). A calculation is necessary to establish a duct size, which will

provide the Air velocity which equates most closely to the velocities required, see Table 1.

Substitute the Air volume for the room (m³/h) you have previously calculated, and the velocity (from Table 1) in the equation below.

3600)/()/()(sec

32

×=

smVelocityhmVolumemareationcrossDuct

Select the next size up duct dia. from (Table 2) and calculate the air velocity in the duct in the equation below:

3600)()/()/( 2

3

×=

mAreahmVolumesmVelocity

)()/()/( 2

3

mAreasmVolumesmVelocity =

If the resultant velocity is too high then the duct diameter is too small, the system is likely to be noisy and it is unlikely that there is a fan to suit.

If the resultant velocity is lower than recommended, the system will be extremely quiet but the ducts oversized and the overall cost may rise unnecessarily.

The Cross Section area in m² for Circular Ducting is as follows:

f. In order to proceed to the next stage you need the following information:

1. The preferred duct diameter (calculation e) 2. The air velocity m/s (calculation e) 3. A list of duct items including duct length and the number of bends. Select the

duct diameter and component resistance chart and list the resistance in Pa (Pascals) against each item shown.

Add up all of the component resistances for the Total System Static Resistance. Using the manufacturers information, select a fan with the same duct diameter as the system, ensuring that the fan produces a slightly higher performance than required, as a speed controller can be used to reduce the fan's performance to the correct level.

Grilles and Louvres

When holes have been made in a wall, partition, door or duct to allow the passage of air to or from a ventilated space; grilles, registers (i.e. grilles fitted with adjustable dampers), and louvred panels are used to cover these holes. Apart from providing a neat and attractive finish to the installation, grilles and registers are used to give directional and volume control to the flow of air, and louvred panels fitted externally are designed to prevent the entry of rain.

Types of Grilles & Louvres

There are numerous types of designs of grille, such as expanded and extruded metal, pressed sheet steel in various patterns, pressed steel louvres, double or chevron louvres for light proof installations, fixed and adjustable louvres, and egg-crate patterns in anodised aluminium and plastics. They are made in a wide range of sizes and various finishes, and the final choice is usually ased on appearance, size and price, although the ability to allow ransfer and/or deflection of air is the prime objective.

Free Area

A very important factor to be taken into consideration when selecting these items for a particular duty, and this cannot be stressed to highly, is their FREE AREA. That is the open area of the grille through which are can pass freely, and this may be between 40 % and 90 % of the face area. When investigating complaints of unsatisfactory operation of an installation, we find that the majority of these are due to the grilles used in walls or ducting to allow the movement of air to or from the room, having insufficient free area for the purpose, and therefore causing too high a resistance to airflow. The free areas of grilles, and in some cases their resistance to airflow, will be found in manufacturers' catalogues.

A guide to some of the types of grilles available:

Approximate free areas of grille types: Egg Crate up to 90 % Pressed steel up to 75 % Fixed Louvre 30-70 % Adjustable blade 45-65 % Double deflection up to 60 % Chevron louvre up to 65 % Weather louvre 40-55 %

Filters

Dust has been described as matter in the wrong place, and the object of air filters is to prevent as much dust as possible from entering spaces where it is not wanted by separating it from the air that is carrying it.

These dusts have to be separated from the air because of their effect on the health of persons occupying a room to which air is being supplied. Their effect on the operating efficiency of machines or appliances (e.g. computers), and on the appearance of valuable stock (jewellery); their nuisance potentialities (discharged dust spreading over adjoining goods or property); their economic value (collection for sale or re-use).

Types of Air Cleaners

The types of air cleaning equipment is common use are settling chambers, cyclones, filter panels of various types, electrostatic precipitators, carbon filters and air sterilisers for (for bacteria). The selection of equipment suitable for a particular job depends on the degree of fineness of the dust (particle size), the required efficiency of collection (the percentage of the finer particles to be removed), and the amount of dust to be collected.

Dust Sizes

The size of dust particles is measured in microns (1 micron = 1/1000 of millimetre, approx. 1,25000 of an inch). It is obvious therefore, that the smaller the particle to be collected, the finer must be the mesh of the filter, and therefore, for a given size of filter panel, the greater will be the resistance to airflow, and consequently, also the power required to push air through it.

With the comparatively low pressures and motor power of small axial fans (quiet and economical running), we must use fairly large filtering areas for the size of fan, and must limit the size of particle collected and the efficiency of collection to about 99 % of dust down to 5 microns, which is ideal for normal ventilation work. Another advantage using larger filter panels is that they will give useful and effective service for a longer period before becoming loaded due to the larger area through which the dust-laden air can pass.

A rough idea of particle sizes are as follows: Smoke up to 1 micron Bacteria 1 to 10 microns Dust 1 to 1000 microns and over Spores, moulds 3 to 50 microns Pollens 9 to 80 microns Fog 1 to 60 microns

Specialist ethnic cooking (Chinese Woks, etc.,) will require special treatment. The temperatures are very high and the volatile oils used will need special grease eliminators. Fans should be rated for a minimum 100 °C ambient air temperature.

Fish and chip cabinet fryers usually have ducted extract provided by the cabinet manufacturers. The air/oil mixture can be an explosive mixture and require appropriately rated fan motors.

Canopies and Hoods

The removal of fumes and steam from cooking and industrial processes should be done as near to the source as possible. Warm fumes and steam rise quickly and spread over a comparatively large area of the kitchen and must be 'picked up' and removed quickly.

To deal with this sort of local problem by increasing the general ventilation rate of the room is not always economical or convenient due to the large volume of air extraction necessary to reduce the spread of the fumes. In such cases a canopy, or hood, would be fitted directly above the equipment and overlapping it by up to 300 mm all round to collect the fumes. The canopies and fans should be of sufficiently large capacity to 'hold' and carry away the fumes without undue spillage from the mouth of the canopy.

To achieve this, the velocity of the air through the open area between the canopy and the equipment must be sufficiently high to draw in fumes near the edge of the equipment against the eddying effects of local draughts which could be caused by the movement of people around the equipment.

Air Volume

Where the items of cooking equipment to be placed under a canopy are known, the total of the volumes of air required for each piece of equipment will constitute the extract volume to be provided by the canopy extract fan/s. (See Table 1 for volume of air required for cooking equipment). Where the equipment is not known, the formula shown below can be used. This formula uses the base area of the canopy, rather than the open perimeter area used in earlier formulae, and more closely matches the volume of hot air rising from the cooking equipment. The volumes obtained by this formula should be regarded as minimums and no harm will result if they are increased by 50 %.

Vol. (m³/s) = L (metres) x W (metres) x K where K = 0,25 for domestic

0,30 for Light Commercial 0,40 for Commercial and Light Industrial 0,50 for Heavy Commercial and Industrial (Welding, etc.)

(The factor K represents the face velocity (m/s) of the airflow at the canopy)

Plastic flexible ducting should not be used to extract from kitchen canopies, as it is very difficult to clear and would constitute a fire hazard. Steel ductwork should be used, with adequate access panels for cleaning. In special cases, flexible metal ducting could be used, but only where it is short enough to be easily dismantled for cleaning or replacement.

Example

A canteen kitchen (equivalent to a light commercial kitchen) is to have a canopy 3 m x 1,25 m covering cooking equipment not yet specified.

L = 3 m W = 1,25 m K = 0,3 Air req. (m³/s) = 3 × 1,25 × 0,3 = 1,125 m³/s (4050m³/h) Other points to consider:

a) Minimum height from floor to underside of canopy 2 m b) Air replacement based on 75-85 % of extracted air c) Temperature of replacement air must not be below 10 °C when coming into

contact with cooked food d) Maximum duct velocity 6 m/sec

Efficient ventilation is an important factor in kitchen design. It must effectively remove cooking fumes and odours and the products of combustion given off by gas cooking apparatus, and ensure an adequate supply of comparatively fresh replacement air without creating uncomfortable draughts.

The main points when preparing a scheme are: 1) Give an adequate air flow. Use a minimum ventilation rate of 25 ACH for

commercial kitchens, increasing these figures as necessary to deal with higher than average loading and cooking equipment. When calculating the amount of air necessary to give the selected ACH it is usual to base the volume of the kitchen on a height of 3 m. This will automatically compensate for different ceiling or roof heights by increasing the ventilation rate for a low ceiling, and reducing it for a high ceiling.

2) Specific Volumes for Cooking Equipment. Current practice for commercial

kitchen ventilation extends the guidelines for sizing ventilation schemes. Whilst retaining the minimum of 20-30 ACH, specific quantities of air to be provided for each piece of cooking apparatus are now available. Therefore, when the details of the equipment are known, a more accurate assessment of the air volume required can be made. These requirements can result in substantially higher rates of extraction than the minimum rates, and will take much of the uncertainty out of deciding by how much the minimum must be exceeded. The volumes can be used for determining both general extraction and canopy extraction requirements.

Alternatively, calculations can be based on the number of meals prepared per hour, multiplied by 10-15, to give an extract volume in litres per second. This method highlights the different requirements between, for example, an expensive restaurant with one sitting per table per evening, to steak bars with around 3-4 sittings/table/evening, to pizza restaurants (semi fast food) with 5-6 and a burger bar. The amount of cooking, hence air movement required, increases the faster the food.

3) Locate extract units as high as possible and as near the source of the fumes as

convenient. Hot moist fumes from cooking operations rise fast to ceiling level, and unless they are removed quickly from that level they will spread over ceiling, walls and windows depositing the moisture content and grease as it condenses on the cooler surfaces. Roof lights and lantern lights are sometimes an ideal location for extract units in a commercial kitchen as they are usually over some cooking equipment at or near the centre of the kitchen, and it is a simple matter to fit roof fans in the glazing. If due to some obstruction it is not possible to site the unit at high level directly above the cooker, then keep it at high level and move it a foot or two to one side. This is better than putting the unit immediately above the cooker but only half way up the wall, as the velocity of the steam and fumes would carry them past the unit to ceiling level where they would spread horizontally and hang about for some time before cooling sufficiently to drop to the level of the extract point. This is a common fault in domestic kitchens, the low siting of the unit sometimes allowing cooking fumes to float through the top of a doorway before they can sink low enough to be extracted by the fan.

4) Use canopies over 'heavy' cooking equipment, particularly in commercial kitchens,

to collect and 'hold' the fumes at source. Estimate the total volume of air required for the kitchen, subtract the volume required for the canopy, then allow units over the wash-up and food preparation sinks to make up the difference. Canopy grease filters are necessary to remove the bulk of the oil and fat droplets from the air before it passes along ducting and through extract fans.

5) Ensure ample air replacement openings, well distributed to eliminate local

draughts and to spread the supply of fresh air. Some air replacement from adjoining rooms is not a disadvantage as the flow of air through the doorways will reduce the possibility of fumes from the kitchen passing through to these adjoining rooms.

Extract units should be switched on as soon as any cooking apparatus is in use to prevent a build-up of hot fumes, and should be left running for 20 to 30 minutes after cooking is finished to clear away any residual fumes and hot air convected from the cooker surfaces.

Check that sufficient free area for air replacement has been provided for fuel burning equipment, including gas cookers.

CIBSE Guide B2 states that: A supply of fresh air is required for: a) Human respiration b) Removal of pollutants (odours, cigarette smoke, moisture) c) Combustion appliances d) Thermal comfort e) Smoke clearance (fire)

In a commercial kitchen, the principal objective is to enable the occupants to 'pursue their working activities in comfort'. "If you cannot stand the heat, get out of the kitchen", has no foundation. It is a legal requirement to provide correct working conditions for staff. It is also a legal requirement to provide healthy conditions for patrons.

The Heating and Ventilating Contractors Association has recently published DW/171 Standard for Kitchen Ventilation Systems (ISBN 0-903783-29-0). It is recommended that this publication is referred to for all aspects of Commercial Kitchen ventilation, canopies, filtration and ducting.

Types of Systems

We have seen that good ventilation must remove vitiated, contaminated and overheated air and replace it with fresher air at such a rate and in such a manner as to provide comfortable and healthy conditions. When dealing with a particular ventilation problem, the basic decisions to be made are:

1. Type of System required i.e., extract, intake, or a combination of both. 2. Ventilation Rate (ACH) necessary for the conditions. 3. Provision for air replacement. Will existing openings be sufficient, or are

special arrangements necessary?

Extract System

An Extract system is designed to remove foul air, usually at high level unless the fumes are heavier than air, when extraction would take place near floor level. This extraction creates an area of low pressure around the fan causing the fresher replacement air to flow into the space through doors or windows, or through air intake grilles suitably spaced.

This is by far the simplest, most common, economical and effective system for normal ventilation work. It cannot be used if filtered air is required in the room, as any leakage through cracks around doors and windows, or through doors when opened, would be into the room, and it would not be possible to filter all incoming air at these points.

Intake System

An intake, or plenum system blows in fresh air, which mixes with the air already in the room and forces its way out to atmosphere through any available openings. Careful location and speed control of intake fans and evenly distributed air outlet grilles are necessary to prevent draughty conditions, even in warm weather; incoming air may need filtering and/or warming, adding to the expense and difficulties of location of the equipment. Note that if filtered air is required an intake system is essential, and the room would be under a slight positive pressure, so that any leakage of air would be outwards from the room.

Combined System

A combined system using both extract and intake doubles the cost of a scheme, but is more effective than extract only in large offices, say over 15 m wide, as controlled mechanical intake can be used to give positive gentle air movement in warm weather to create a feeling of freshness. It also reduces the number and size of openings required in the structure for replacement fresh air.

Pubs, Clubs and Discos The Problems:

o Tobacco smoke o Body odour o Stale air

Solutions: PUBS AND CLUBS - Air change rate per hour 12 minimum Draughts, dust, traffic fumes and noise usually result in windows and doors being kept closed as much as possible. The simple form of controlled mechanical ventilation provided by extract fans is all that is needed to remove unwanted air, smoke and odours; and to bring in fresher, cooler air to give more comfortable conditions. In those buildings where food and drink are prepared and consumed, good ventilation is essential to hygiene. Normally, a ventilation installation for this type of area is straightforward. A number of window, wall or roof units may be used to give the required ventilation rate. The units should be well distributed at high level and located to give cross ventilation from suitable air replacement points. These can be doors and windows or inlet louvres at the opposite ends of rooms from the units. The louvres should deflect incoming air upwards to minimise draughts. Where there are false or suspended ceilings, in-line fans and ductwork can be concealed, with grilles or registers sited to be most effective. Particular attention should be paid to remove the risk of passive smoking, especially for bar staff. Position air replacement sources over the bar area, to create an air movement away from the bar towards the extract points. Place pool tables, dart boards, fruit machines, etc., close to extract points to remove tobacco smoke at source and prevent drift. No cords or controllers should be accessible to the public. Single or multi-unit controllers will be necessary to cope with widely varying conditions, and should be located where they may be controlled by the staff.

Reversible fans allow air replacement from the most suitable source and give a simple installation. Dedicated intake systems may require filtration and heating to prevent unwanted draughts. Ensure adequate air replacement is available for any fuel burning appliances, open fires and log effect gas fires, when all fans are on extract. TOILET ACCOMMODATION Legal minimum requirements must be regarded as just that. Recommended rates are 10 ACH, as the area must be classified as unsupervised. Building regulations ask for 6 l/s per WC, but in gents' areas the urinal area can be the main problem, treat each individual urinal, or "standing space" for trough urinals as half a WC and calculate accordingly. DISCOS - Air change rate per hour 12 minimum The same problems exist as in Pubs and Clubs, with the addition of body heat from the occupants, which require a high rate of air change. Where noise transfer to the outside is sensitive in discotheques, attenuation can be fitted to either individual fans or duct systems to reduce noise break-out to acceptable levels. Particular attention should be paid to air replacement to ensure that the dancers are not subjected to draughts. Up to 1kW per person can be generated by enthusiastic and energetic dancing, so heating provision is not usually necessary. Special effect lighting can also create a problem. Refer to the Section on heat removal. Ceiling sweep fans create air movement, but do not contribute to actual ventilation levels. For safety, the impeller should be at least 3m above floor level. Do not site sweep fans below bright lights as the passage of the blades at certain RPM's can create unwanted stroboscopic effects. Single of multi-unit controllers will be required to cope with widely varying conditions and should be placed under the control of the staff.

Arrangement of fans and ducts to create cross flow ventilation where only one external wall is available. Ensure that the scheme complies with relevant local authority by-laws and fire regulations. SIZE AND NUMBER OF VENTILATION UNITS Calculate the volume of the room (height × width × length) and multiply by ACH. This equals the volume required per hour. Choose the size and number of units from the ouput table to ensure that this minimum volume is achieved.

Shops and Stores The Problems - General:

o Heat o Lack of air movement o Tobacco smoke o Condensation o Body odours

The Problems - Shop Windows:

o Heat o Condensation o Deterioration of stock

Solutions: SHOPS/STORES - Air change rate per hour 8 minimum The wide variation in size and particular purpose of these buildings makes very little difference to the basic method treatment: namely, extract at high level and draught-free air replacement at low or medium level, speed control being essential. The air flow through the premises can be from front to back, or back to front if the traffic fumes from the front are a nuisance; or from low level to ceiling or roof. A comparatively small shop with no outlet at the rear, can be dealt with by extracting at one side of the front of the premises and intaking at the other side, both at high level. Alternatively, if the shop is more than 9 metres long, the best results are obtained by fitting a duct from the front intake point to the rear of the shop, with suitable grille(s).In this way, the fresh air is delivered to the back of the shop, which normally needs more air movement. It is then drawn forward to the extract point, thus 'sweeping' the space with cooler, fresher air. Larger shops and supermarkets usually can be dealt with by using a simple arrangement of extract units at high level in walls or roof. Air replacement is via open doors and/or intake units.

SPECIFIC PROBLEM AREAS e.g. Lighting and refrigerators In some premises, discomfort can be caused by heat build up from intense lighting and warm air discharged into the shop from refrigerated cabinets. In such cases, 15 air changes per hour or more may well be necessary, together with 24 hours thermostatic control. DISPLAY WINDOWS Adequate ventilation of the shop itself will in may cases be all that is necessary to control window condensation. Where a condensation problem exists which is limited to the display space only, specific ventilation will be required, possibly to a rate of up to 15 air changes per hour. Heat gain from lighting can also cause drying out and discolouration of stock. Use one 230 mm extract unit for every 2 kW of lighting. (1 kW in difficult cases). Condensation should always be extracted to atmosphere, with air filtered replacement from atmosphere at low level. For heat problems, air replacement may be taken from the shop or atmosphere at low level, with discharge to atmosphere or back into the shop during colder periods. SIZE AND NUMBER OF VENT-AXIA UNITS Calculate the volume of the room (height × width × length) and multiply by ACH. This equals the volume required per hour. For display windows see text. Choose the size and number of units from the output table to ensure that this minimum volume is achieved. Controllers can be used to boost the output of the units during warmer spells. Ensure that the scheme complies with relevant local authority by-laws and fire regulations.

Squash Courts_Gymnasiums The Problems - Squash Courts:

o Condensation in Winter o Heat build-up in Summer o Body odours

The Problems - Gymnasiums:

o Body odour o Moisture o Stale air

Solutions: SQUASH COURTS : Air change rate per hour 4 minimum Condensation is a widespread problem in squash courts, control being of prime importance to prevent deterioration of the playing surfaces and dangerous floor conditions. This is often aggravated by inadequate heating and poor insulation and is particularly troublesome during the first few months, whilst the building materials are drying out. Correct ventilation removes moisture laden air, making it easier for damp surfaces to dry out. Better breathing conditions are created by reducing the pollutants in the air, and reducing solar heat gain. For detailed advice on heating and insulation refer to the Squash Racquets Association. Where quiet conditions are required, units should be sized to run on low speed.

SINGLE COURT Ventilation units should be installed to extract at high level above the viewing gallery. Air replacement should be via inlets at low level in the front play wall and side walls. MULTIPLE COURTS There is usually ample air replacement at the viewing gallery ends. Therefore, provision

should be made for extraction at high level in the front play wall ends, either in the wall itself or in the roof above. Ventilation openings should be protected by grilles e.g. eggcrate, to stop damage caused by squash balls. CHAMPIONSHIP COURTS These usually have large audience areas and will require higher rates of air movement, to cater for both players and spectators. GYMNASIUMS: Air change rate per hour 6 minimum. The above rate of extraction is required to remove body odours and stale air. Care should be taken to ensure that air replacement does not create draughts i.e. air replacement grilles should be situated at the opposite ends of the areas to the units, and deflect incoming air upwards. Consider heat recovery units to reduce heating bills. SIZE AND NUMBER OF VENTILATION UNITS Calculate the volume of the room (height x width x length) and multiply by ACH. This equals the volume required per hour. Choose the size and number of units from the output table to ensure that this minimum volume is achieved.

Cafes, Restaurants, Canteens The Problems:

o Smoke o Condensation o Unwanted smells

Solutions: CAFES : Air change rate per hour 10-12 RESTAURANTS AND CANTEENS : Air change rate per hour 8-12 In many cafes and restaurants, particularly small ones, the problem in the public area is aggravated by inadequate ventilation of the kitchen. If the eating area leads straight into the kitchen, the latter should be ventilated before attempting any further ventilation of the eating area. The airflow through the premises can be from front to back. Fit units at high level in windows, walls or through the roof. It may be necessary to duct through the kitchen for access to atmosphere at the rear of the premises.

A comparatively small cafe with no outlet at the rear can be dealt with by extracting from one side of the front of the premises and intaking at the other side, both at high level. Alternatively, if the cafe is more than 9 metres long, the best results are obtained by fitting a duct from the front intake point, to the rear of the eating area, with suitable grille(s) or ventilation unit(s). In this way the fresh air is delivered to the back of the area, which normally needs more air movement. The air is then drawn forward to the extract point, thus 'sweeping' the space with cooler, fresher air. Larger cafes and restaurants can usually be dealt with by using a simple arrangement of extract units at high level in walls or roofs, air replacement coming from open doors and/or intake units. Particular attention should be paid to avoid tobacco smoke drift from smoking to non-smoking areas.

The simple approach of siting smokers on the periphery of the room, close to extract points may mean nosmoking tables are in the centre of the room, away from windows and any surrounding interesting scenery.

SIZE AND NUMBER OF VENTILATION UNITS Calculate the volume of the room (height × width × length) and multiply by ACH. This equals the volume required per hour. Choose the size and number of units from the output table to ensure that this minimum volume is achieved. Ensure that the scheme complies with relevant local authority by-laws and fire regulations. Controllers will enable the ventilation rate to be adjusted to suit changing indoor and outdoor conditions.

Hairdressers, Beauty Salons The Problems:

o Heat o Condensation o Fumes o Body Odours o Tobacco Smoke

Solutions: HAIRDRESSERS/BEAUTY SALONS: Air change rate per hour 10-15 Conditions in these premises often become unpleasant. There is usually considerable heat build up from hair dryers and fumes from permanent waving solutions, hair fixative sprays and general beauty preparations. Doors and windows are usually kept closed because of draughts. This lack of air movement also encourages the build up of moist air and formation of condensation on cold surfaces such as the front windows.

Effective controlled ventilation is necessary to remove excessive heat and moisture for comfortable conditions; and more importantly, hair fixative lacquer which can be injurious to health. For normal conditions, use the air change rate given. For salons containing a large number of hair dryers in a comparatively small area, the higher of the rates specified should be taken as a minimum.

HAIRDRESSERS, BEAUTY SALONS : Fit ventilation units to extract from the back of the salon, ducted if necessary to atmosphere through rooms at the rear. Use window or wall units, or roof units if there are convenient roof lights. Then, position them so that air movement is well distributed through the salon. Where there is no access at the rear or side of the premises, extract via a duct running forward to discharge at the front. TOILETS : Should be ventilated to meet statutory levels, normally 3-6 ACH. CIBSE suggest standby fans may be required. Rooms without openable windows require run-on timers. STAFF ROOMS : Smoking may be allowed in a Staff Room. Ventilation at the rate of 8-16 l/s/person is recommended, possibly extract and intake. The Staff Room may be classified as a kitchen (domestic type) if simple cooking or tea-making facilities are present. In which case 60 l/s will be required. STOCK ROOM : Ventilation to clear residual odours at 10 ACH should suffice. There may be a washing machine and/or tumble dryer for towels etc. in which case domestic utility room rates apply (30 l/s). AIR REPLACEMENT: The washing and drying of hair makes customers especially susceptible to draughts. Take care to provide adequate air replacement, positioned to ensure that clients and staff are not exposed to discomfort. Air may be introduced at high level and deflected upwards towards the ceiling, or brought into an ante-room where it can mix with room air before entering the salon. Air inlets should be closeable to suit ambient conditions and sited to ensure the replacement air is free of traffic fumes, etc. Suspended ceilings allow fans and ducts to be concealed, with grilles and registers placed to be most effective. SIZE AND NUMBER OF VENTILATION UNITS: Calculate the volume of the room (height x width x length) and multiply by ACH. This equals the volume required per hour. Choose the size and number of units from the output table to ensure that this minimum volume is achieved. Ensure that the scheme complies with relevant local authority by-laws and fire regulations. Controllers will enable the ventilation rate to be adjusted to suit changing indoor and outdoor conditions.

Darkrooms,_X-Rays,_Radiography, Optical The Problems:

o Heat o Condensation o Fumes o Body Odours o Tobacco Smoke

Solutions: DARKROOMS : Air change rate per hour 10-15 Because of the necessity to exclude light, all natural ventilation is also excluded. In general, darkrooms should be ventilated at a rate of not less than 10 air changes per hour. Where a print glazer, dryer or other heat producing equipment is in operation, 15 air changes per hour should be given. Darkroom models are designed to provide extract ventilation without letting in light. They may be installed through blacked out windows, roofs, and through walls. They normally consist of a fan with an external roof cowl, together with an extra internal matt black cowl. Internal darkrooms may be ventilated to outside atmosphere using ducting and accessories. In general, units should be sited as close as possible to the local source of fumes, such as developing baths. Some chemicals are heavier than air and benefit from an extract point between bench and head height.

Adequate air replacement is necessary to ensure correct functioning of the unit and to avoid air being drawn in from the wrong sources (i.e. "through-the-wall" continuous developing machines), with possible degradation of print quality. The air replacement source should be as far away as possible from the unit and should be light-tight. Non-vision grilles, black finish, are available as ventilation accessories. Because darkroom conditions vary, controllers should be included to enable the ventilation rate to be adjusted. If dust is a problem, suitable filters can be fitted to ducted air replacement systems or internal transfer grilles. Ensure that the scheme complies with relevant local authority by-laws and fire regulations. Size and Number of Ventlation Units : Calculate the volume of the room (height x width x length) and multiply by ACH. This equals the minimum volume required per hour. Choose the size and number of units from the output table to ensure that this minimum volume is achieved.

Lecture Theatres,_Assembly Rooms, Conference Rooms The Problems:

o Tobacco Smoke o Body Heat o Noise o Body Odours

Solutions: LECTURE THEATRES / ASSEMBLY ROOMS: Air change rate per hour 5-8 Assembly rooms are normally fairly spacious. Many people may be present and smoking is generally allowed. Lecture theatres, however, tend to be smaller in relation to the number of people present but smoking maybe less prevalent. Therefore, in both cases the same air change rate may be sufficient. Check that a minimum volume of 28 m³/h (8 l/s) per person is allowed for. Units should be sited at high level at the opposite end to the main source of air replacement, usually the doors. Where doors are normally closed during lectures, air replacement should be provided via grilles or louvres, mounted at high level in the wall and deflecting incoming air upwards. Units should be sized to run on low speed where quieter operating conditions are required. Site controllers where they can be operated by the management.

CONFERENCE ROOMS : Air change rate per hour 8-12 Here the problem is smoking and to a lesser degree body heat. Conference rooms are normally much smaller than assembly rooms and need a higher rate of extraction. Check that the 28 m³/h (8 l/s) minimum per person is provided. Units should be sited at high level in windows or walls, at the opposite end to the air replacement source - usually the doors or windows or air replacement grilles in the wall. The units should be sized to run at low speed using controllers.

SANITARY ACCOMMODATION - Follow CIBSE or Building Regulation requirements and allow 6-10 ACH or 6 l/s per WC. Avoid the risk of "cross talk" in toilet suites, especially ducted systems. Standby fans may be required. Ensure that the scheme complies with relevant local authority by-laws and fire regulations. Size and Number of Ventilation Units Calculate the volume of the room (height x width x length) and multiply by ach. this equals the volume required per hour. choose the size and number of units from the output table to ensure that this minimum volume is achieved.

Offices The Problems:

o Tobacco Smoke o Body Odours o Heat and fumes from office equipment o Solar heat gain

Solutions: OFFICES: Air change rate per hour 6 minimum Smokey, airless offices increase fatigue with consequent loss of concentration, efficiency and encourage the spread of infections. As occupants are sedentary, draughts from open windows are a nuisance; the windows are kept closed and mechanical ventilation is a necessity. Single Offices Ensure that the unit is sited as high as possible and opposite to the main source of air replacement (usually the door), but try to avoid placing the unit right behind the occupant - this may create a draught. If the door is normally closed, fit air replacement grilles in or near the door. An air intake facility is useful for warmer periods. Site the controller within easy reach of the occupant. Larger Offices These are treated in the same way as small offices, except that more units are required and air replacement has to be treated with greater care. The use of a number of units at high level enables a well distributed airflow to be achieved; fit air replacement louvres at high level to deflect incoming air to ceiling upwards. Controllers can either be centralised, or individuals can be given control over their own microenvironment.

INTERNAL OFFICES: Air change rate per hour 6 minimum. These areas may be ventilated by means of units mounted in the wall or ceiling, and ducted via adjoining areas or ceiling voids to atmosphere. Where noise or fumes exist, a further ducted input system may be required. Where dust is a problem, suitable filters can be incorporated. PHOTOCOPIER ROOMS/MACHINE OFFICES: Air change rate per hour 15 minimum These areas are generally complicated by a heat problem and persistent dust and fumes which can present health hazards. Local "spot" extract ventilation will prevent pollution becoming widespread. Ensure that the scheme complies with local authority by-laws and fire regulations. SANITARY ACCOMMODATION - W.C. areas should be ventilated at 3 ACH or 6 l/s/WC to meet Document F1 requirements. These figures should be regarded as a minimum. Where toilet suites occur, care must be taken to avoid cross talk especially via ductwork. Special provision should be made for disabled W.C.'s: a minimum of 10 ACH should be allowed. Size and Number of Ventilation Units Calculate the volume of the room (height x width x length) and multiply by ACH. This equals the minimum volume required per hour. In busy offices, the rate should be increased to 10 air changes per hour and where a solar heat gain problem exists, 15 air changes per hour minimum. Choose the size and number of units from the output table to ensure that this minimum volume is achieved.

Changing Rooms/Showers Changing Room Problems:

o Body Odours o Condensation

Shower Problems:

o Steam o Condensation

Solutions: CHANGING ROOMS: Air change rate per hour 6 minimum The problems present in changing rooms are normally body odours and stale air. Units should be mounted at the opposite end to the entrance door and at high level. As the doors are usually shut, provision should be made for draught-free air replacement. This can be achieved by fitting nonvision grilles in or near the doorways to the lobby, which allow reasonable air transfer but prevent voyeurism.

To meet Building Regulations Document F1, shower areas require 15 litres/second/shower or bath. An area with 6 shower head will therefore need 90 l/s or 324 m³/h. Mains voltage units should be out of reach of persons using a fixed bath or shower. Window mounting fans can be fixed within the depth of the wall and be protected by a suitable grille or louvre. SHOWER AREA: Air change rate per hour 15-20 Where changing rooms are linked with showers, condensation from the shower can be a problem in the changing area. This can be relieved when the shower area is properly ventilated. The amount of steam given off from a hot shower is considerable and would require a high rate of extraction to remove it quickly. On the other hand, a large amount of air moving through the shower room over naked bodies could cause discomfort and a compromise is necessary.

Calculate for the above air change rates, using speed control to reduce for cooler weather. Extract at high level and if special provision for air replacement is necessary, use high level grilles or windows to reduce the possibility of causing draughts at body level. Overrun timers may be used to ventilate the shower for a period of up to 35 minutes after use.

Heat recovery units will reduce cold draughts and quickly lower internal humidity levels. Toilet suites require 6 l/s per W.C. This should be regarded as a minimum in unsupervised areas, and be doubled for disabled W.C.'s . Size and Number of Ventilation Units Calculate the volume of the room (height x width x length) and multiply by ACH. This equals the volume required per hour. Choose the size and number of units from the output table to ensure that this minimum volume is achieved.