Atmospheric dampness and ventilation

• Breathing, bathing, clothes washing, kitchens bathrooms all generate moisture. Water vapour produced by these means is carried by warm air.

• When the air cools the water vapour is deposited as water droplets - especially against colder surfaces.

• Fungal spores, ever present in the atmosphere, will germinate on damp surfaces and gather together as black or grey ‘mould’ on damp surfaces. Thus giving rise to the typical condensation damp appearance.

• Damp air and lack of ventilation is also a health hazard. Air borne fungal spores from mould can cause or exacerbate asthma.

• Decorations are spoilt and rented properties are particularly blighted by this problem as tenants often dry clothes on radiators causing considerable extra air borne water vapour.

• A pressurised air ventilation system is now one of the leading cures for condensation.

• This system works by drawing fresh dry air into the property which then gently forces out the stale damp air from within. Thus the air within is constantly dry.

Ventilation

• As buildings become better insulated and more airtight in response to the drive for better thermal performance and reduced carbon dioxide emissions, their proper ventilation becomes more and more of an issue…

• Heat loss resulting from uncontrolled ventilation can become a major proportion of a building’s total heating demand and,

• Air leakage through the fabric cannot be relied upon to provide enough background ventilation to keep our buildings healthy.

Types of ventilation

Extract ventilation

• Designed to remove water vapour and defined pollutants that are generated locally, usually in large quantities from rooms in the domestic setting.

• In dwellings, this means wet rooms, i.e. kitchens, bathrooms, utility rooms and sanitary accommodation (WCs).

• In other environments, water vapour and pollutants might arise from production processes or office equipment, such as photocopiers.

Whole-building ventilation

• Designed to provide what is known as background ventilation. Ensures that the building is provided with fresh air from the outside as well as allowing for the removal of internally generated low levels of vapour and pollution caused by occupation or emanation from construction materials.

• It should be capable of being reduced or closed completely when the building is unoccupied, no significant amount of moisture is being generated and no polluting equipment is running.

• Typically includes trickle vents in windows and background ventilators through walls, but also can be provided by means of passive stack ventilation or as part of a complete mechanical system.

• Designed to provide fast removal of high concentrations of pollutants, such as those that occur when decorating or as a result of accidents (e.g. burning food), or if a room has been unoccupied for a period with the whole-building ventilation closed down. It can also be used to improve thermal comfort in summer.

• Purge ventilation is usually provided by opening windows or switching on local extract fans if provided.

Purge ventilation

Types of Natural Ventilation

Stack

• Driven by temperature differences between the hot air in the occupied space and the cooler external air. Warm air rises upwards in the building and exits through one or more high level openings. The air displaced from the building causes cooler fresh air to be drawn into the building through low-level openings, such as windows and vents.

• Enhanced by increasing the height of the ventilation stack or by providing a small fan in the extract path for hot still days.

Roof Mounted

• The driving force is the pressure difference between the inlet and extract segments of the ventilator.

• The terminal can be in a variety of shapes, although the most common are square and circular. The inside of the terminal is divided into four separate chambers, which act as the main inlet and extract ducts.

Trickle Vents

• The use of trickle ventilators in buildings is appropriate providing that the occupants are educated as to their correct use.

• Many natural ventilation strategies operate automatic controls during the summer months but rely on manual control of vents during winter. As the larger vents are likely to remain shut, the provision of a separate ventilation path in winter is required.

Actuators

• Powered window actuators enable multiple windows and other ventilation openings to be controlled as part of an automatic control strategy, such as a building management system.

• Manual control is also possible by means of push-button switches located in close proximity to the windows or using remote control devices, for example hand-held infrared controllers.

Mechanical Ventilations

Extract ventilation

• Generally used in environments where air becomes directly contaminated by a particular activity or process. The main benefit of mechanical extract over natural ventilation is a constant and predictable ventilation rate. For many applications this is an essential requirement; the use of natural ventilation would not be appropriate.

Examples of such systems:• - Commercial kitchens• - Toilets and bathrooms• - Underground car parks• - Factories or industrial buildings• - Localised industrial extraction

Roof-mounted warm air units

• Primarily used in high volume industrial and commercial buildings with a large floor to ceiling space. The units provide a mixture of fresh and re-circulated air that can be warmed in the winter to provide space heating.

• Mechanical extract and supply is also an option.

Supply and extract systems

• Comprises a central air handling unit (AHU), typically containing separate supply and extract fans, an air filter and/or a heating coil supplied with hot water from the building boiler system.

• To save energy in cold weather, some systems are designed to recirculate a portion of the warm extracted air back into the supply. This cuts down the amount of outside air that needs to be heated.

• A heat recovery device can also be incorporated into the AHU. This takes heat from the exhaust air and transfers it back into the supply air without mixing the two air streams. This is important in some industrial and medical applications where contaminated extract air must not be reintroduced into the space.

Heat recovery• Used to transfer heat from the extract air to the supply air during the heating

seasons.

• Utilse heat that would otherwise be rejected to waste, resulting in lower running costs and potentially reduced plant capacities. The most common types of heat recovery devices are:

• - Thermal wheels• - Cross-flow plate heat-exchangers• - Run-around coils.

• As electricity is about three times more polluting than mains natural gas, the heat recovery unit will need to recover three units of heat to one unit of electricity to justify its inclusion.

Air filtration

• Many different types of particulate air filters have been developed, and each has its own applications, limitations, advantages and disadvantages.

• The three main categories of filters are primary filters, secondary filters and high efficiency filters.

• Average arrestance is the amount of dust retained as a percentage of the total load (a ratio).

Primary filters

• Primary filters (Grade G1 to G4) have an average arrestance ranging from 50 – 90%. Primary filters can arrest particles down to 5 um but have a minimal effect on particles below 1 μm.

• These filters can be permanent and cleanable (made of foam or metal mesh) or disposable (made of a glass or synthetic media).

• Primary filters are mainly used to protect ventilation and air conditioning equipment or as pre-filters to extend the operating life of fine filters installed further downstream.

Secondary filters

• Secondary filters (classes F5 to F9) have an average efficiency ranging from 40 – 95%. Average efficiency is defined at the weighted average of the efficiencies for the different specified dust loading levels up to final pressure drop.

• Secondary filters are effective in trapping pollen, bacteria and dust.

• Secondary filters can be of the extended media bag type, pocket type or a rigid bag.

High efficiency filters

• High efficiency particulate air (HEPA) filters are defined as having an efficiency of 85 – 99·995% (H10-H14). The filters are made of pleated glass fibre paper.

• Typical applications are in clean rooms, medical rooms and nuclear applications.

• Even higher efficiency filters are also available. Referred to as ultra low penetration air (ULPA) filters, these filters have an efficiency ranging from 99·9995 – 99·999995 (U15-U17).

Re-usable filters

• Re-usable filters are often used in kitchens to trap grease and prevent it building up in ductwork.

• Metal filters are non-flammable, heat conducting and can act as flamearresters.

• Mesh filters are coarse panel filters made of several layers of wire mesh that can be coated with oil to improve the dust collection and holding ability.

• Both types are cleaned in warm soapy water and re-coated with oil if required.

Electrostatic filters

• Can be highly efficient in filtering out particulate contaminants like smoke, dust and pollen.

• Thin wires suspended in the air stream are charged with a high positive voltage. The positive ions created in the ionising field become attached to particles which then pass into the collecting plate section. This section consists of a series of parallel plates, with alternative plates positively charged and the other plates at ground potential. The electric field forces the charged particles on to the oppositely charged plates.

• The retention of particles on the collector platesis by a combination of electrical and intermolecular adhesion.

Activated carbon filters

• Activated carbon is highly porous and can be used to absorb volatile chemicals. Practical applications can include the removal of volatile organic compounds and odours from an air stream.

• Activated carbon is usually combined with conventional filter techniques to provide filtration for larger particulates. Activated carbon filters are usually used in combination with secondary filters to prevent the pores becoming blocked with particulate matter.

Ultraviolet irradiation systems

• Can be used to neutralise micro-organisms as they are sensitive to the effects of invisible light at certain wavelengths (the DNA structures of the micro-organisms are damaged and are not able to replicate).

• Ultraviolet irradiation has been used in hospital applications based on the following options:

• Portable forced air re-circulation equipment utilising both UVGI and HEPA filtration to trap and kill pathogens by introducing an air current to provide room air mixing.

Photocatalytic filters• Can be used to remove odours and micro-organisms.

• This technology is based on titanium oxide. An ultra-violet light source is used to initiate a photocatalytic process resulting in the formation of hydrogen peroxide and hydroxyl.

• These two substances possess powerful oxidising properties. Through mutual interaction they are able to decompose odorous substances into carbon dioxide and water.

• The filter can also remove bacteria and deactivate viruses.