Vent Axia Vent Guide

7/31/2019 Vent Axia Vent Guide

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VENT AXIA VENT GUIDE

Fan curves are published to enable users to calculate the amount of air the fan will move under differentconditions. 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. Inimperial days it would have been ft 3/min against inches waterguage. Conversion from SI to Imperialunits has already been discussed. The curves that manufacturers publish in their catalogues, and computerselection programs should be to British Standards 848 parts 1 & 2, which deal with Performance Flowand Noise measurement respectively.

The layout of the curves will be similar, irrespective of the fan type, Axial, Mixed flow, Centrifugal, butthe horizontal and vertical scales will probably vary, as indeed they will for differing impeller diametersof similar fans. The curves we will look at have another variation; the angle (pitch) of the impeller. Someform of cased axial flow fans, i.e., those built within a length of circular duct, have the option of variableangle 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 theinstruments on the test rig. The starting point is to measure the airflow at zero pressure, sometimesreferred to as Free Inlet & Discharge or FID. The test rig is then adjusted to increase the pressure that thefan has to work against and to measure the airflow at each point. It is normal for only the stable part ofthe fan performance curve to be published. This curve is shown smoothed. The actual test points willnot necessarily form a smooth curve, but these slight variations are omitted.

Figure 1. Axial Fan Performance Curve

Figure 2 shows a family of curves for an Axial flow variable pitch impeller fan. The volume of airmoved decreases with the angle of the impeller. The speed of rotation remains theoretically constant.


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Figure 2. Variation in Blade Pitch Angle

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

Dynamic Pressure = 0.5 p Velocity

Wherep is the density of the air or gas in Kg/m and Velocity is in m/sec.This is obtained by Volume m/sec

Area m

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 3. Fan Dynamic Pressure

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


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

Figure 4. Total Pressure Curve

Figure 5 shows the efficiency contour lines. The efficiency is calculated using the test data by thefollowing equation:

Fan Efficiency (Total)= Total Pressure (Pa) Volume Flow (m/sec)Actual shaft power (Watts) 1000

Figure 5. Efficiency Curve

The illustration shows that for a fan there is a best efficiency point. In practice, there is only a smallchance of operating at the duty point which meets this. The reason is that most manufacturers designimpellers 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 conditionsincluding operating at high efficiency value - this is particularly important with fans absorbing largeamounts of electrical energy.


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This may seem relatively unimportant for small fans, but where multiple fans are required for ventilationsystems in buildings, a minimum efficiency value is sometimes included in the Specification by theSpecifier 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 isplotted, volume of air against pressure, but is seen to be beyond the inherent fan speed characteristiccurve. 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 aslightly high blade pitch angle, which will give nearer the actual performance required.

Figure 6. Volumetric Flow Rate


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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 manufacturingtolerances, but they are useful in estimating approximate outputs of similar fans of different diameters andspeeds as applied to normal ventilation work, and can be summarised as follows:

a.Volume of air flow varies as (fan diameter) and as rpmb.Pressure developed varies as (fan diameter) and as (rpm)c.Power absorbed by the fan varies as (fan diameter)5 and as (rpm)

It is important to note, however, that these Laws apply to the same point of operation on the fancharacteristic. 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 thesize, rotational speed or gas density is changed. Therefore, in the following Laws the suffice "I" has beenused for initial known values and the suffice "2" for the changed values and the resulting calculated value

when:

Q = volume flow rateP = pressure (total, static or dynamic)

p = gas densityn = fan rotational speedD = impeller diameterW = impeller power

1. VOLUME of air flow varies as the (fan diameter) and as the rpm

or: New Volume = Old Volume new impeller diameter

new rpm

(old impeller diameter ) (old rpm )

or: Q2 = Q D2

n2(D1) (

n1)

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

or: New Pressure = Old Pressure new impeller diameter

new rpm(

old impeller diameter)

(

old rpm

)

or: P2 = P1 D2

n2

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

or: New Power = Old Power new fan diameter5

new rpm(

old fan diameter)

(

old rpm

)

or: W2 = W1 D25

n2(D1

) (n1

)


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

(new pressure) orp2(old pressure) p1

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

or Q2 = Q1

n2

(n1

)

P2 = P1

n2

(n1

)

W2 = W1

n2

(n1

)

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

FAN LAW VOLUME varies as (fan diameter) and as rpm e.g., A 300mm fan running at 1190 rpmdelivers 1415m/h. What will 190mm fan running at 1290 rpm deliver?

1. VOLUME of air flow varies as the (fan diameter) and as the rpm

or: New Volume = Old Volume new impeller diameter

new rpm(

old impeller diameter

)

(old rpm

)

or: Q2 = Q D2

n2(D1

)(

n1)

or: 1415 190mm

1290(

300mm)

(

1190) = Q2

or: 1415 6859000 1290

27000000 1190

= Q2

1415 0.254 1.084 = 389 (Published Figure 395)

This slight variation from our quoted output for the size is negligible from the practical point of view andwill be due to small differences in the similarity of the two units being compared, and to the "roundingoff" of test figures.


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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 velocityfrom 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 andcontractions, 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, 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 bends and well-rounded corners, not by sharpelbows, unless fitted with guide vanes.

The loss of pressure due to all of these sources, known as the system resistance, is for practical purposesproportional to the square of the velocity at the point of loss. Therefore, for a fixed system, it may be saidthat the pressure required to pass a given volume of air through the system will vary as the (volume flow

rate) i.e. P C Q.

Therefore, if it is required to double the air flow through a system, the fan must be capable of providingtwice the volume flow rate at four times the original pressure! AND EIGHT TIMES THE FAN

MOTOR POWER!

If a specified duty requirement does not exactly match the available fan performance, it is advisable tosuperimpose a system resistance curve onto the fan performance curve to confirm the final anticipatedduty. Data points for plotting the system resistance curve may be derived from the following formula:

P2 = P1 (Q2/Q1)

2

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. Repeat theprocedure until there are enough points to plot the curve - three will usually suffice).

Square Law

Resistance Varies as the Square of the Velocity P C Q

As velocity varies directly as volume, we can say that Resistance varies as the square of the volume. Theequation then becomes:

NEW RESISTANCE =NEW

VOLUME OLD RESISTANCE

OLD VOLUME


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Units of Measurement of Noise

The noisiness of a sound source could be expressed in terms of its Sound Power (Watts). However, therange of values found in practice (from a soft whisper at 0.000000001W to a Saturn rocket at40,000,000W) make this impractical. Noise is therefore expressed by its Sound Power Level, a ratiowhich logarithmically compares its Sound Power with a reference power, the Picowatt (10-12 Watt).

The unit ofSound Power Level (Lw) is the Decibel (dB) re 10-12 Watts.

Figure 1 clearly shows how the logarithmic scale compresses the unacceptably wide range of possibleSound Powerto Sound Power Levels having a practical range of 30-200dB.

Sound Power (Watts) Sound Power Level (dB) Source

25 to 40,000,000

195 Saturn Rocket

100,000 170 Ram Jet

10,000 160 TurboJet engine 3200kg thrust

1,000 150 4 propeller airliner

100 140

10 130 75 piece orchestra

1 120 Large chipping hammer

0.1 110 Blaring radio

0.01 100 Car on motorway

0.001 90Axial ventilating fan (2500m/h)

0.0001 80 Voice - shouting

0.00001 70 Voice - conversational level

0.000001 60

0.0000001 50

0.00000001 40

0.000000001 30 Voice - very soft whisper

Frequency

The unit of measurement of frequency is the Hertz (Hz). This has generally replaced the older cycle persecond (cps or c/s).

To better describe the intensity and quality of a noise, the frequency scale is split into bands and a SoundPower Level quoted for each. Octave Bands are generally used for this purpose (bands of frequency inwhich the upper limit of frequency is twice that of the lower limit), and are specified in terms of theirmid-band frequencies. These would commonly be 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.

Sound Pressure Level (Lp)


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The Sound Power Level of a fan can be compared to the Power Output of a heater in that both aremeasure of the energy being fed into the environment surrounding them. Neither, however, will tell us theeffect that will be experienced by a human being in that surrounding space. In the case of the heater, thetemperature that we feel is dependent on the surroundings. The effects of distance, volume of space,absorbing and reflecting surfaces, other heat sources, etc., will combine to determine the resultingTemperature at any point.

In a similar way, the Sound Pressure Level that we hear is determined by the surroundings and theacoustic engineer must take this into account when specifying noise levels.

Sound Pressure Level (LP) is also measured on a logarithmic scale but the unit is the Decibel (dB) re 2x 10-5 Pa

In-duct, Open - inlet/outlet, Spherical radiation, Hemispherical radiation.

A fan manufacturer can publish noise information for various installation conditions.

It is important to consider the use or installation conditions of a fan when checking a manufacturers noise

data. The information given may not always be appropriate and it is as well to check. As is the case withSound Power Levels, Sound Pressure Levels must be quoted or each Octave Bandif a complete picture ofthe effect of the noise on the human ear is required.

Sound Pressure

(Pascals)

Sound Pressure

Level (dB)Typical Environments

200.0 140 30mm from military aircraft at take-off

63.0 130 Pneumatic chipping and riveting (operator'sposition)

20.0 120 Boiler shop (maximum levels)

6.3 110 Automatic punch press (operator's position)

2.0 100 Automatic lathe shop

0.63 90 Construction site - pneumatic drilling

0.2 80 Kerbside of busy street

0.063 70 Loud radio (in average domestic room)

0.02 60 Restaurant

0.0063 50 Conversational speech at 1m

0.002 40 Whispered conversation at 2m

0.00063 30

0.0002 20 Background in TV and recording studios

0.0002 0 Normal threshold of hearing

THE ENGINEER MUST CLEARLY DISTINGUISH AND UNDERSTAND THE DIFFERENCE

BETWEEN SOUND POWER LEVEL AND SOUND PRESSURE LEVEL. HE MUST ALSO APPRECIATE

THAT dB RE 10-12 WATTS AND dB RE 2x10-5 Pa ARE DIFFERENT UNITS WITH NO ABSOLUTE

FORMULAE CONNECTING THEM.

It is impossible to measure directly the Sound Power Level of a fan. However, the manufacturer cancalculate this level after measuring the Sound Pressure Levels in each octave band with the fan workingin an accepted acoustic test rig.


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What he cannot do is unequivocally state what Sound Pressure Levels will result from the use of the fan.This can only be done if details of the way the fan is to be used, together with details of the environment itis serving, are known and a detailed acoustic analysis is carried out.

A disadvantage of the Decibel scale is that values cannot be added or subtracted using the normalarithmetic rules. Decibel values must be converted back into absolute units of power (Watts), when theycan be added or subtracted directly before re-converting back into ecibels. However, this tedious process

can be avoided by using the following simple but approximate method. Column 2 shows how much mustbe added to the higher of two sound powerlevels to obtain the equivalent combined level, when the dBdifference between the two levels is shown in Column 1.

Difference between the two levels dB Quantity to be added to the higher level

0 3

1 2.5

2 2

3 2

4 1.55 1

5 1

6 1

7 1

8 0.5

9 0.5

10 or more 0

As an example, if one fan has a sound level of 50dB, and another similar fan is added, (the sound level isdoubled). The combined sound level of the two fans will be 53dB. If two more fans are added, i.e. thesound source is doubled again, the resultant sound level from all four fans will be 56dB.

If the first fan had a sound level of 50dB and another, larger fan with a sound level of 55dB was added,the difference between the two is 5dB, so 1dB is added to the higher figure, giving a combined level of56dB.

To estimate the effect of distance from a sound source the rule of thumb calculation is very simple. Soundnormally weakens by 6dB each time the distance from the sound source is doubled. i.e. a sound level

measured as 60dB at 1 metre will be 54dB at 2 metres and 48 at 4 metres.

Smaller fan sound levels are sometimes measured at 1m, whereas the industry standard is becomingstandardised at 3m, for comparison purposes. The difference between 1m and 3m is therefore 9dB.(41dB@3m) and 50dB@1m are in effect the same).

dBA (dBB, dBC)

This is a single number, weighted sound index. It is obtained by subtracting different values from each ofthe frequency bands in an attempt to approximate the response characteristics of the human ear. Theresultant values are then added together to obtain a single number sound level. However, too much

information is lost in this process to allow this one figure to be of any use for calculation.


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Unfortunately, the majority of fan manufacturers indicate the noisiness of their products by SoundPressure Levels expressed in dBA (or dBC). These figures refer to the Sound Pressure Levels whichwould be experienced by an observer at a distance of 3m (or occasionally 1m or 3 x fan diameters) fromthe fan if both were suspended in an infinitely large and empty volume (technically called Free Field). Tosay the least, this is a very unlikely set of conditions and these dB values must only be used to comparethe noisiness of similar types of fan!

It cannot be stressed too often, that on no account must the engineer be tempted to assume that the

Sound Pressure Levels quoted in manufacturers catalogues will in anyway be similar to those

achieved in practice. Depending on circumstances, they can be substantially exceeded or reduced.

Thought of objectively, using dBA to describe the noisiness of a fan is as absurd as stating the output of aheater in terms of the temperature it would produce three metres from its face if it were suspended inspace! It will be better for all involved when only Sound Power Level information is used to specify fannoise. In the meantime, engineers must always be careful to check how noise information is expressed.

Noise Criteria Curves (NC)

It is obvious that the combination of a single figure index such as dBA, with more information on theshape of the frequency content would be useful. Noise Criteria curves (NC) were evolved to meet thisneed.

NC curves consist of a family of octave band spectra, with each curve marked with its own NC ratingnumber. The octave band spectrum of the noise being analysed is plotted on the same grid and the NCrating of that noise corresponds to the highest NC curve touched by the noise spectrum.

Figure 3 shows a set of NC curves together with a table indicating recommended levels for variousenvironments. The spectrum of a noise with a NC rating of 35 is also shown on the grid.


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NC ratings are particularly suitable for selecting and assessing suitable background noise levels forventilating and air conditioning systems.

It is pointless specifying Sound Pressure Levels or NR ratings to a fan manufacturer unlessadequate information about how and where the fans are to be used is also provided. In view of theconsiderable amount of work involved in calculating Sound Pressure Levels and NR criteria, engineerswill be well advised to check whether a particular manufacturer includes this analysis work in his service

or whether acoustic engineers must be employed.

EnvironmentNR

criterion

Concert halls, opera halls, studios for sound reproduction, live theatres (-500 seats) 20

Bedrooms in private homes, live theatres (+500 seats), cathedrals and large churches,television studios, large conference and lecture rooms (50 people)

25

Living rooms in private homes, board rooms, top management offices, conference and lecturerooms (20-50 people), multi-purpose halls, churches (medium and small, libraries, bedroomsin hotels, etc., banqueting rooms, operating theatres, cinemas, hospital private rooms, large

courtrooms

30

Public rooms in hotels, etc., ballrooms, hospital open wards, middle management and smalloffices, small conference and lecture rooms (20 people), school classrooms, small courtrooms,museums, libraries, banking halls, small restaurants, cocktail bars, quality shops

35

Toilets and washrooms, large open offices, drawing offices, reception areas (offices), halls,corridors, lobbies in hotels, hospitals, etc., laboratories, recreation rooms, post offices, largerestaurants, bars and night clubs, department stores, shops, gymnasia

40

Kitchens in hotels, hospitals, etc., laundry rooms, computer rooms, accounting machinerooms, cafeterias, canteens, supermarkets, swimming pools, covered garages in hotels, offices,etc., bowling alleys

45

NR50 and above

NR50 will generally regarded as very noisy by sedentary workers but most of the classifications listedunder NR45 could just accept NR50. Higher noise levels than NR50 will be justified in certainmanufacturing areas: such cases must be judged on their own merits.

Recommended NC levels for various environments

EnvironmentRange of NC levels

likely to be acceptable

Factories (heavy engineering) 55 - 75Factories (Light engineering) 45 - 65

Kitchens 40 - 50

Swimming baths & sports areas 35 - 50

Department stores & shops 35 - 45

Restaurants, bars, cafeterias & canteens 35 - 45

Mechanised offices 40 - 50

General offices 35 - 45

Private offices, libraries, courtrooms & schoolrooms 30 - 35

Homes, bedrooms 25 - 35Hospital wards & operating theatres 25 - 35

Cinemas 30 - 35


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Theatres, assembly halls & churches 25 - 30

Concert & opera halls 20 - 25

Broadcasting & recording studios 15 - 20

Noise from Fan Systems

The noise resulting from a fan system can be caused in several ways and can enter an area by more thanone route.

A The noise energy generated by a fan will be red into the duct system, forced to pass along its length,and a proportion will enter the area being served by the system. The balance of original noise energynot reaching the ventilated area will be absorbed by the system.

B Noise will also be generated by the airflow as it is obstructed and turned in its passage along the ductsystem. Sound energy will therefore, be introduced into the system at bends, dampers, heater batteries,etc., and again a proportion will pass into the ventilated area. And noise produced as the airflow passesthrough the terminal devices themselves will be radiated directly into the ventilated area.

C Some of the noise absorbed by the duct system is in fact lost through the duct walls and can be a

nuisance in those areas through which the ducting passes. This break-out noise can even reach theventilated area itself. Airborne noise from a plant room, and vibration energy from the fan, can also betransmitted through walls, floors and ceilings into adjacent areas.

If the Sound Power of a fan is too large, and no other selection is possible, attenuation must be introducedimmediately after the fan to prevent unacceptable noise passing along the system.

Attenuation can be provided by lining the ducts with absorptive material or by insertive proprietaryattenuator units. Duct lining, especially at bends, is extremely satisfactory providing sufficient length ofduct is available and thick enough lining is used.

System Design

The engineer should remember that noise generation within an air distribution system is caused byaerodynamic turbulence. If, therefore, he conforms to the codes of recommended design practice, payingspecial attention to those areas where turbulence is like, both aerodynamic and acoustic efficiencies willimprove. (An example of the correlation between aerodynamic and acoustic efficiency has already beengiven).

FIGURE 7 shows designs which will cause turbulence (and hence noise) and how they can be improved.


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Care must be taken not to exceed recommended outlet velocities at terminal devices because even smallexcesses over the recommended levels can cause appreciable increases in noise.

When the airflow generated noise is unacceptably high engineers should always first attempt to reduce theair velocities (airflow generated noise is proportional to the sixth power of the velocity!). In practice,airflow generated noise can be ignored when velocities are below 7.5m/s in the main duct and 3m/s inbranch ducts.

If this is not possible then secondary attenuators, inserted as near to the outlet as possible, must be used toabsorb the unacceptable noise energy. FIGURE 7 shows one example of the use of an attenuator.

Obviously attenuators cannot be fitted after terminal devices and therefore engineers must rely absolutelyon the selection of the correct outlet itself; perhaps oversizing it from the aerodynamic point of view, inan effort to avoid noise generation at a point where it cannot be subsequently eliminated.

WARNINGNoise, not produced by the ventilating system, can enter through the ductwork and subsequently be anuisance in the ventilated area. This situation can be thought of as break-in and must be treated in asimilar manner to break-out. In the same way outside noise (e.g., aircraft noise) can enter the systemthrough the external louvre and must be attenuated if unacceptable.

1. It may be acceptable to reduce the airflow through the system. A 20% reduction in airflow will reducethe noise level by approximately 5 dB.

2. The system may be aerodynamically re-balanced so that duct branches responsible for feedingexcessive amounts of noise into the area have their airflow (and hence their noise energy carryingcapacity) reduced. However, care must be taken that the dampers etc., used to re-balance do not cause


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significant noise increase or alternatively as in 5 below additional silencing may be required.

3. Attenuation, either in the form of duct lining or proprietary units, may be introduced into the system.

4. Terminal devices may be exchanged for models which produce less noise, or for larger size deviceswhere, because of the resulting lower air velocity, noise generation is reduced.

5. Dampers, and other adjusting and balancing devices, may have to be relocated away from outlets, andsecondary attenuation inserted between their new positions and the outlets.

6. Quiet fans may have to be installed in place of the original units.

If none of these measures reduce the noise level sufficiently, then it may well prove necessary to attemptto alter the acoustic characteristics of the area being served in an effort to absorb more of the soundenergy discharging from the fan system. However, depending on circumstances the reduction achieved bythis method may be somewhat limited. An acoustic engineer should be consulted if such a stage isreached.

However, everyone must be aware (especially clients!) that reducing noise levels below reasonable valuesis incredibly expensive. In fact, the total cost of a system increases exponentially as the NR rating islowered. For this reason, the target noise level resulting from a fan system must be chosen with care after

studying the guides and the conditions special to the contract.

Remember - there is no justification in paying for a fan system which would generate a background levelof only NR25, if the noise from an adjacent road would make NR35 more appropriate!


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Flexible Ducting

Using the simple velocity method of calculations to determine the resistance to airflow, it is generallyacknowledged 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 willadopt a smaller diameter than the helix, reducing the cross sectional area. A duct with a normal diameterof 100mm will reduce to an actual diameter of around 80mm when not fully stretched and at a bend. Asharp 90 bend in flexible duct has up to three times the resistance of a smooth bore duct. The reason forthese 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 socalledLoch Ness Monster effect of a series of humps. Each of these humps can have the resistance of two 90bends.

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, andhigher than calculated resistance. In an ideal world, flexible duct would only be used with rigid straightduct 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 thediameter of the duct on the high side, at least 1.25 times the impeller diameter for simple, small diameterAxial fans and no less than the diameter of the inlet/discharge spigot for centrifugal and in-linecentrifugal and mixed flow fans. The general rule of thumb for calculating the appropriate discharge, afree area of 1.5 times the duct cross-section is equally important, and should also apply to any inletgrilles.

Metal Duct (Square and Spiral Wound)

The velocity method calculations for spiral wound duct are included in the tables in the SystemCalculator. 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 regaincalculations. This is the realm of the consulting engineer. There are several external courses organised bysuch as the Mid Career College that cover more complex duct calculations.


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

Types of Grilles & LouvresThere are numerous types of designs of grille, such as expanded and extruded metal, pressed sheet steel invarious patterns, pressed steel louvres, double or chevron louvres for light proof installations, fixed andadjustable louvres, and egg-crate patterns in anodised aluminium and plastics. They are made in a widerange 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 AreaA 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 throughwhich are can pass freely, and this may be between 40% and 90% of the face area. When investigatingcomplaints of unsatisfactory operation of an installation, we find that the majority of these are due to thegrilles used in walls or ducting to allow the movement of air to or from the room, having insufficient freearea for the purpose, and therefore causing too high a resistance to airflow. The free areas of grilles, andin some cases their resistance to airflow, will be found in manufacturers' catalogues.

A guide to some of the types of grilles available:

Size of Grille (mm) to give approximate free area cm% free area ofgrille 325 500 650 1000 1300 1600

90 200 200 250 250 300 250 350 350 400 400 450 40075 250 200 300 250 300 300 400 350 450 400 500 45065 250 200 300 250 350 300 400 400 450 450 500 50060 250 250 300 300 350 300 450 400 500 450 550 50055 250 250 350 300 350 350 450 350 500 500 550 55050 300 250 350 300 400 350 450 450 550 500 600 550

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%


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

These dusts have to be separated from the air because of their effect on the health of persons occupying aroom 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 orre-use).

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

dust to be collected.

Dust SizesThe size of dust particles is measured in microns (1 micron = 1/1000 of millimetre, approx. 1.25000 of aninch). It is obvious therefore, that the smaller the particle to be collected, the finer must be the mesh of thefilter, and therefore, for a given size of filter panel, the greater will be the resistance to airflow, andconsequently, also the power required to push air through it.

With the comparatively low pressures and motor power of small axial fans (quiet and economicalrunning), we must use fairly large filtering areas for the size of fan, and must limit the size of particlecollected 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 andeffective service for a longer period before becoming loaded due to the larger area through which thedust-laden air can pass.

A rough idea of particle sizes are as follows:

Smoke up to 1 micronBacteria 1 to 10 micronsDust 1 to 1000 microns and overSpores, moulds 3 to 50 micronsPollens 9 to 80 microns

Fog 1 to 60 microns

Specialist ethnic cooking (Chinese Woks, etc.,) will require special treatment. The temperatures are veryhigh and the volatile oils used will need special grease eliminators. Fans should be rated for a minimum100C ambient air temperature.

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


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The removal of fumes and steam from cooking and industrial processes should be done as near to thesource as possible. Warm fumes and steam rise quickly and spread over a comparatively large area of thekitchen 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 alwayseconomical or convenient due to the large volume of air extraction necessary to reduce the spread of thefumes. In such cases a canopy, or hood, would be fitted directly above the equipment and overlapping itby up to 300 mm all round to collect the fumes. The canopies and fans should be of sufficiently largecapacity 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 mustbe sufficiently high to draw in fumes near the edge of the equipment against the eddying effects of localdraughts 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 ofair required for each piece of equipment will constitute the extract volume to be provided by the canopyextract fan/s. (See Table 1 for volume of air required for cooking equipment). Where the equipment is notknown, the formula shown below can be used. This formula uses the base area of the canopy, rather thanthe open perimeter area used in earlier formulae, and more closely matches the volume of hot air risingfrom the cooking equipment. The volumes obtained by this formula should be regarded as minimums andno harm will result if they are increased by 50%.

Vol. m/s = L.metres W.metres K, where K =

0.25 for domestic0.30 for Light Commercial0.40 for Commercial and Light Industrial0.50 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 clearand would constitute a fire hazard. Steel ductwork should be used, with adequate access panels forcleaning. In special cases, flexible metal ducting could be used, but only where it is short enough to beeasily 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 coveringcooking equipment not yet specified.

L = 3mW = 1.25mK = 0.3

Air req. m/s = 3 1.25 0.3 = 1.125 m/s (4050m/h)

Other points to consider:

a) Minimum height from floor to underside of canopy 2m


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b) Air replacement based on 75-85% of extracted airc) Temperature of replacement air must not be below 10oC when coming into contact with cooked foodd) Maximum duct velocity 6m/sec

Commercial Kitchens

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 adequatesupply 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 cookingequipment. When calculating the amount of air necessary to give the selected ACH it is usual to basethe volume of the kitchen on a height of 3m. This will automatically compensate for different ceilingor 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 airvolume required can be made. These requirements can result in substantially higher rates ofextraction than the minimum rates, and will take much of the uncertainty out of deciding by howmuch the minimum must be exceeded. The volumes can be used for determining both generalextraction and canopy extraction requirements.

Table 1: Kitchen Cooking Equipment Volume RequirementsApparatus m/h m/s l/s ft/min

Cookers per m 1080 0.3 300 640Pastry ovens 1080 0.3 300 640Fish fryers 1620 0.45 450 950Grills 900-1080 0.25-0.3 250-300 530-640Steak grills 1620 0.45 450 950Salamanders 1620 0.45 450 950Boiling pans (140-180 litre) 1080 0.3 300 640Steamers 1080 0.3 300 640Sink (sterilising) 900 0.25 250 530Bain Marie 720 0.2 200 420Tea sets 540-900 0.15-0.25 150-250 320-530

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 requirementsbetween, for example, an expensive restaurant with one sitting per table per evening, to steak barswith 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 moistfumes from cooking operations rise fast to ceiling level, and unless they are removed quickly fromthat level they will spread over ceiling, walls and windows depositing the moisture content andgrease as it condenses on the cooler surfaces. Roof lights and lantern lights are sometimes an ideallocation for extract units in a commercial kitchen as they are usually over some cooking equipmentat 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 itat high level and move it a foot or two to one side. This is better than putting the unit immediatelyabove the cooker but only half way up the wall, as the velocity of the steam and fumes would carrythem past the unit to ceiling level where they would spread horizontally and hang about for some


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time before cooling sufficiently to drop to the level of the extract point. This is a common fault indomestic kitchens, the low siting of the unit sometimes allowing cooking fumes to float through thetop 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 thevolume required for the canopy, then allow units over the wash-up and food preparation sinks tomake 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 flowof air through the doorways will reduce the possibility of fumes from the kitchen passing through tothese adjoining rooms.

Extract units should be switched on as soon as any cooking apparatus is in use to prevent a build-upof hot fumes, and should be left running for 20 to 30 minutes after cooking is finished to clear awayany 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 respirationb) Removal of pollutants (odours, cigarette smoke, moisture)c) Combustion appliancesd) Thermal comforte) Smoke clearance (fire)

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

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


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We have seen that good ventilation must remove vitiated, contaminated and overheated air and replace itwith 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 thanair, when extraction would take place near floor level. This extraction creates an area of low pressurearound the fan causing the fresher replacement air to flow into the space through doors or windows, orthrough 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 andwindows, or through doors when opened, would be into the room, and it would not be possible to filter allincoming 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 forcesits way out to atmosphere through any available openings. Careful location and speed control of intakefans and evenly distributed air outlet grilles are necessary to prevent draughty conditions, even in warmweather; incoming air may need filtering and/or warming, adding to the expense and difficulties oflocation of the equipment. Note that if filtered air is required an intake system is essential, and the roomwould be under a slight positive pressure, so that any leakage of air would be outwards from the room.


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Combined System

A combined system using both extract and intake doubles the cost of a scheme, but is more effective thanextract only in large offices, say over 15m 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 numberand size of openings required in the structure for replacement fresh air.

Vent Axia Vent Guide

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