Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Wall mounted propeller fans have been used in the extraction mode since the very earliest days of mechanical fans. Refer­ence to Chapter 1 shows that these were popular from the 1880s. Usually positioned in the gable ends of buildings, they are still a feature of many older industrial buildings.

Following the 2nd World War, special roof extract units were de­veloped. These largely superseded the “Heath Robinson" ar­rangements designed by ventilation specialists who incorpo­rated standard propeller fans into fabricated upstands and roof cowls.

Figure 21.6 Direct drive centrifugal unit with side discharge

figure 21.6 direct drive centrifugal unit with side discharge
Over the years, the application of roof extract fans has ex­tended and they are now also used for air supply. Various op­tions have become available and these include:

• Direct or belt drive

• Side or vertical discharge

• Propeller, mixed flow or centrifugal impeller options

Examples of these are shown in Figures 21.2 to 21.10 in cross-section.

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilationIn the first generation of out-of-town stores, roof extract fans were the most important method of ventilation. Air curtains or rotary doors were used to restrict the ingress of cold air. The high customer occupancy gave rise to considerable heat gains such that the natural buoyancy of the air assisted the extract process.

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

SHAPE \* MERGEFORMAT Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.7 Vee belt driven mixed flow unit with vertical discharge, running Figure 21.2 Direct drive mixed flow unit with vertical discharge and standby motors

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.8 Vee belt driven centrifugal unit with vertical discharge, running and standby motors

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.9 Vee belt driven mixed flow unit with side discharge, running and standby motors

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.10 Vee belt driven centrifugal unit with side discharge, running and standby motors

Single storey factories are also ideal for this kind of ventilation and they are widely used in agricultural buildings such a poultry and pig houses.

Indoor air quality has received increasing attention over the last few years (see Section 21.1). Indications are that this will re­quire greater quantities of airto be extracted, contrary to the ob­ligations of a green energy policy.

Powered versus “natural” ventilation

The difference between the performance of fan-powered roof ventilators and “natural” roof ventilation is not always recog­nised. (Perhaps the latter should be more correctly described as passive ventilators.)

The effectiveness of passive units is dependent on weather conditions and is contrary to ventilation requirements. Air movement through a natural cowl occurs because warm inte­rior air rises and will pass through any roof outlet. The rate of air movement depends on the temperature difference between the inside air and the air immediately above the roof. It is also af­fected by the velocity of the wind which may increase its movement. (Figure 21.11).

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.11 The effect of natural ventilation in cold windy weather

In hot weather, when the sun raises the roof temperature to a point where the temperature is as great or greater than that of the interior air — there will be little or no air movement through the natural cowl. Thus a natural roof ventilator is most effective in cold windy weather, but is least effective in hot windless weather when ventilation is most required. (Figure 21.12).

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.12 The effect of natural ventilation in hot sunny weather

Comparative tests

Tests were carried out in the 1970s within the Northern Hemi­sphere, with one natural and one fan powered roof ventilator over a period of 6 months from December to June. Both had a similar open area when operating, and uniformly, both were mounted within an identical open area whilst the tests were car­ried out. The units were mounted at 8.8 m above the floor level on the south facing slope of a north light roof covering a factory building of 991 m3 with an external doorway measuring 4.25 m x 4 m wide.

Measurements of the air movement through the units were taken daily. Over the 6 month period the average air extraction rate of the natural ventilator was only 10% of the fan-powered unit. Moreover, the extraction rate of the natural unit varied con­siderably with the weather conditions and was very much influ­enced by the opening and shutting of doors. When the extrac­tion rate of the fan-powered unit with the doors closed was taken as 100%, the following extract rates were measured, an average being taken over a number of tests under varying weather conditions.

Door Closed

Door Open

Fan powered roof unit



Natural roof ventilator



In the case of natural ventilators that open vertically to permit maximum release, these units will admit rain unless closed in wet weather. A“rain detector” apparatus may be needed to give warning for the closure of the ventilators when the rain begins. No such requirement is needed for fan-powered roof units.

The justification for mechanical ventilation

The fan-powered roof extract unit gives:

• A specified rate of extraction at all times irrespective of weather changes or conditions.

• It ensures the required number of air changes

• The ventilation can always be controlled

• The actual aperture is of greater size for natural ventilation than that for a powered system.

Two important advantages are to be gained by using them as the fan unit for ducted systems:

• The location of fans on the roof eliminates the need for plant room space.

• Flexibility is gained in designing extract systems. Varying extract requirements in different parts of the building can be handled expeditiously by roof ventilators of different sizes and speed.

The affordable alternative

Over the years, the word ventilation has been veiled with a cloak of secrecy usually associated with black arts. However, it is in the reach of everyone to design simple systems without re­quiring specialist skills. In this Section are a few simple guide­lines and a calculation example is now given:

A) Usually for general ventilation requirements, such things as heat dissipation from lathes, milling machines and drills need not be considered. However, furnaces, ovens or any other large powered machine should have such values considered.

B) The first criteria to establish is the volume of the building. It can be seen from the example in Figure 21.15, the only di­mension which may cause difficulty is the height. We can see that we only have a height of 4 metres, to the under­side of the eaves. To save some geometric calculation, the best idea is to add an extra metre to the height as a contin­gency and call it 5 metres.

28 m

Fan pressure development

Mixed flow and centrifugal roof ventilators develop static pres­sures ranging from 100 Pa to 1000 Pa according to size and fan speed. This fact makes them suitable not only for free-inlet ex­traction from areas immediately below the roof but also, for use with ducted extract systems. (Figure 21.13 and Figure 21.14).

| Milling area


Drilling area

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.15 Roof extract unit extraction example

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.13 Extraction from more that one floor of a two or multi-storey build­ing

подпись: figure 21.13 extraction from more that one floor of a two or multi-storey building
Thus the volume is determined by:

Length (L) x width (W) x height (H) = volume

Dimensions are — 28 m x 12 m x 5 m = 1680 m3


Figure 21.14 Local extraction e. g. from over vats

подпись: figure 21.14 local extraction e.g. from over vatsThe next item to recognise is the air change rate. Over the years the data in Table 21.1 has become a suggested ref­erence for air change rates.


Air changes per hour


Air changes per hour

Assembly halls


Furnace rooms








Hospital wards




Hospital treatment rooms


Figure 21.16 Siting of low level inlets

подпись: figure 21.16 siting of low level inlets Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.17 Mounting of roof extract option

подпись: figure 21.17 mounting of roof extract option Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation


подпись: inlet


Air changes per hour


Air changes per hour



Kitchens for restaurants


Boiler houses





8- 12







A Churches

1 — 10


1 -6

♦ Cinemas

6- 10

Paint shops





1 -2



♦ Restaurants


Dance halls

8- 12

Storage areas

1 -2

Domestic kitchens


Swimming baths




♦ Theatres

6- 10

Engine rooms






Note: ♦ General requirements are 28 m3/h (8 I/s) per person

Minimum in public places, this figure increases if smok­ing is allowed.

* Dependent on height of building and number of per­sons.

Table 21.1 Recommended air changes per hour

From Table 21.1 it can be seen that for a general workshop the rate varies between 6 and 10 air changes per hour. The most convenient rate to calculate on is 8, midway through the range. (Later on, we will see that when selecting the fans, the actual air change rate given will vary, usually upward).

D) The formula for obtaining the volume flow of air is: Ventilation rate (m3/sec) =

Volume of building (m3) xair exchanges per hour 36ФФ

Our example:

Volume of building = 1680 m3

Air changes per hour = 8

3600 is a constant 1680×8

= 3.73 mJ/s


Again the best rule of thumb is the area of such inlets should equal twice the area of the extract fan.


A 500 mm diameter fan has an area of 0.2 m2, so the air inlets have an area of at least 0.4 m2.

An important point to remember is to site inlets sensitively so that they do not cause draughts to the personnel in the building. Usually the inlets need to be situated at low level to allow for cross ventilation (see Figure 21.16). The higher the inlets are mounted, then higher levels of air stagnation (no air movement) may occur, at low levels.

J) The mounting of the roof extract units should followthe ex­amples in Figures 21.17 and 21.18. Achoice can be made

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation


подпись: 9) Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation


подпись: h)

We know that we need a fan or fans to be able to pro­duce/at least 3.73 m3/s to achieve 8 air changes per hour.

The next step is to choose the correct type of fan

For industrial purposes there are three basic types:-

• Roof mounted

• Wall mounted

• Duct mounted

Both wall and roof mounted options offer the best degree of flexibility in terms of positioning and controllability. One should avoid choosing a single fan to provide the ventila­tion, due to the inflexibility and the problem it would cause in leaving areas of the building without any air movement.

When roof extract units are considered, then the rough guide is to use one unit per 10 metres of length of building. (The same is true when considering wall mounted units). However, if very high air change rates are required then the 10 metre rule may need to be modified.

Before considering where to position the extract fans, careful consideration is needed to ensure that the air inlets to the building are also taken into account. Normally these are not powered.

End elevation Figure 21.18 Mounting of roof extract option

To situate the units on one side of the roof as near to the apex as possible (Figure 21.17) or one can alternate as in (Figure 21.18), remembering that the units need to be as close as possible to the apex of the roof.

K) The wall mounted fans should be located on one wall; as high as possible, with the inlets set in the opposing wall give in a cross-ventilation system (see Figure 21.19).

Sizing the fans

Wall mounted

The unit needs to be complete and fully assembled with a wall plate, and motor compliance guard.

Using the earlier rule of one fan per 10 m, then at about 30 m length, 3 fans should suffice. From a typical standard range cat­alogue, we need to be looking for a volume per fan of about 1.25 m3/s. The best choice would be a 450 mm diameter fan running at 1370 rpm, which for one particular manufacturer gives 1.71 m3/s.

Do not worry too much about the extra flowrate; you will simply achieve 10 air changes per hour. By using closely matched speed controllers the flowrate can easily be reduced.

Roof mounted

Using roof extract units, the number necessary would be the same as the wall mounted requirement, e. g. three units being sufficient for the project.

Once again 450 mm units running at 1370 rpm for this manufac­turer each give 1.38 m3/s of volume flow. Again this is more than the 1.25 m3/s per fan necessary to give 8 changes of air per hour. Then, by using the same speed controllers as the wall mounted system, a reduction in flowrate can easily be achieved.

So, by following these few simple steps, affordable ventilation is possible.


Cowl and base

The cowl and base should be manufactured from stainless steel, aluminium or cold pressed glass reinforced polyester resin to ensure long life and resistance to the severest weather conditions. If the cowls are made from such a resin, then they should contain an inbuilt ultra violet stabiliser to ensure that they do not fade during intensive sunlight. The method of con­struction described will ensure a material which is strong, light weight and which offers excellent properties against atmo­spheric corrosion. All fasteners and fittings should be of stainless steel.


Electric motors should have the following features:

• Totally enclosed, single or three-phase AC as appropriate

• Rated for continuous running in ambient temperatures up to 50°C

• Squirrel cage induction type for direct on-line starting

• All should have Class F insulation

• Pre-lubricated with high quality grease — re-lubricate after

30,0 hrs or 5 years

• Should have excellent speed control characteristics, capa­ble of regulation to 20-30% of full speed

• Ratings to comply with BS 5000 Part 99 and IEC 60034-1

• Protection to IP55 — IEC 60034-5

• Overheat protection as standard on all single phase motors. To be available as a cost option on others

Flameproof and/or two speed motors may be necessary for some applications according to zone and also the variation in summer/winter duties.


Fan support arms should be of mild steel resiliency attached to the base.

Mounting positions

• The roof extract units should be designed to operate effi­ciently when mounted horizontally or on a pitched roof up to an angle of 30°.

• Purlin boxes, soaker sheets, and direct mounting sheets for most popular roof profiles should be available for all sizes of units.

• Curb mounts — it is normal for the building contractor to fabri­cate this item.

Anti-backdraught shutters

Aluminium anti-backdraught shutters should be fitted as stan­dard. The use of these shutters does not reduce the fan unit performance and will improve weathering under extreme condi­tions.

Shutter features:

• On side discharge units the shutters are opened by in­creased airflow and closed on flow reduction.

• On vertical discharge units the shutters are opened by in­creased airflow and closed by stainless steel springs.

• Synthetic buffers should be fitted to the units to ensure quiet operation.


The following ancillaries may be necessary according to the ap­plication:

Inlet guards — Closes off the aperture within the ceiling.

Bird guards — Prevents entry of foreign objects via the dis­charge aperture.

Security bars — Manufactured with double row of steel bards for added protection.

Motorised dampers — Heat loss a critical factor — mount into ceiling aperture. Manufactured in aluminium, opposed multi-leaf blades.

Acoustic curbs — To meet critical noise criteria, splitter type specially developed to fit the units.

Speed controllers — Solid state electronic or auto-trans­former.

Input units

It is not always desirable to have low-level inlets set in walls, es­pecially where a building is full of equipment. Extract ventilation without a power supply will mean that the pressure within the building will be below atmospheric. To reduce leakage at doors and other openings, input units mounted on the roof may there­fore be a solution to the problem. These units are similar to the extract fans, with of course, a reversal in the airflowdirection.

Where the building is not full of heat generating equipment and where there are occupants whose environment needs to be heated then air heaters are often added. There may also be the need for this supply air to be cleaned to maintain an acceptable indoor air quality. In such cases, various types of filler are added.

The major cause of deaths at a fire is from the hot toxic smoke, rather than from the fire itself. Control and essential removal of this smoke from a building is therefore a vital component in any fire protection scheme. As our knowledge of the behaviour of fire increases, the traditional methods of exhausting the fire smoke, via natural venting have proved inadequate and sys­tems using more positive and readily controlled fan-powered units are now frequently favoured.

Unlike normal ventilation systems, extraction rates for fire smoke venting have little to do with the size of the room. The amount of smoke produced depends largely on the size of the fire. As the smoke plume rises, surrounding cool air is entrained into the plume and becomes so well mixed with the hot smoky products of combustion as to form an inseparable component of the smoke. See Figure 21.20.


Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Figure 21.20 Smoke production

The quantity of smoke produced by a fire will depend on three factors:

• The perimeter of the fire.

• The temperature of the flames in the plume.

• The effective height of the column of hot gases above the


At flame temperatures of around 800°C and ambient air tem­peratures around 17°C (density 1.22 kg/m3), the production of smoke from a fire can be obtained from the simple expression:

M = 0.19 P Y 15 Equ21.3


M = mass of smoke produced (kg/s)

P = perimeter of the fire (m)

Y = height of the smoke layer (m)

The temperature of the smoke can be calculated using the for­mula:

0= 5s Equ21.4



0 = temperature of the smoke above ambient (°C)

Qs = heat carried by the smoke (kW)

M = mass of smoke production (kg/s)

Early smoke venting systems were designed around single storey low ceiling factory buildings. Here, the height through which the smoke rises is small. Hence, the smoke remains hot enough so that, when combined with a relatively deep smoke layer, it provides the buoyancy required to force it through the natural smoke vents provided.

In larger and more complicated buildings and especially for atri­ums and shopping malls, there is often insufficient buoyancy.

During the critical early stages of a fire, when people are escap­ing, the smoke may be too cool to form a stable layer.

The relative merits of natural ventilation and fan powered venti­lation are summarised in Table 21.2.





1) Lightweight {if aluminium)

2) Self-regulating

3) Easy to retrofit or reuse

4) Operate at high temperatures

5) Units in non-fire zones can provide replacement air

1) Easily affected by wind

2) Require large areas of inlet

3) Large openings on roof

4) “Cool" smoke a problem

5) Material distortion

6) Not acceptable to all approving authorities


1) Guaranteed exhaust rate

2) Few smaller openings in roof

3) Can handle “Cool" smoke

4) Small area of inlet required

5) Can be used with ducting

6) Can be sited away from risk area

7) Will provide normal ventilation for building (2 speed)

1) Weight can cause a problem

2) Electrical supply and wiring

3) Retrofit not always possible

4) Expensive if high temperature (above 400°C)

Table 21.2 Merits of natural and fan-powered ventilation Extractor fan requirements

The requirements for an extractor fan in a fire smoke venting system can be listed as follows:

A) To extract the hot smoky gases for a sufficient period of time to enable occupants to escape from the building.

B) To keep the building free of smoke for long enough to as­sist the firemen in locating the seat of the fire (will usually do this whilst performing requirement a) above.

C) If possible, to assist in clearing the residue smoke from the building, after the fire has been extinguished.

D) To provide the normal ventilation requirements of the building.

E) To extract the cold smoke during the early critical stages of a fire.

Because of the dilution which takes place, the smoke tempera­ture is much lowerthan might be anticipated. It is often however extremely toxic. Nevertheless it is necessary to design the me­chanical and electrical parts to withstand temperatures which, as far as the UK London fire authorities are concerned, reach 300°C, to allow sufficient time for building evacuation.

And also for the fire to be fought, this temperature has to be withstood during operation for one hour. Requirements in other countries differ. In all cases it is essential that the electrical sup­ply is from an independently protected source, connected to the fan motor by fire resistant cable such as Pyrotenax.

It might be thought that “plastic” roof cowls would be unsuitable for such fans. This however is not the case and especially with vertical discharge propeller units the hot gases are kept away from the fibreglass reinforced plastic and the units continue to function for the requisite time. Atypical system is shown in Fig­ure 21.21 for which the design parameters were a sprinkled building with a design fire size of 3 m x 3 m with a heat output of 5 MW.


The use of extraction fans has continued to increase over the last few years despite their apparent simplicity. It is recognised that they have many advantages where a free floor area is de­sired and where the use of the building may change. Contrary to natural ventilation, they are independent of the weather and the

K] IS]



Curtain [g] [x]






Sales area (2 zone«)

[X] * Smoke extract units {4 off — 9 m3/sec each) CS — Smoke extract units (4 off -14.7 m3/sec each) / — "Open sky” roof louvre — "Open sky" wall louvre

Design parameters

Sprinklered building

Design fire size ~3mx3mx5 MW


M [XI IX! ™ ™

подпись: warehouse
m [xi ix! ™ ™
Figure 21.5 Direct drive mixed flow unit with side discharge Extract ventilation

Sales area 5 m

подпись: sales area 5 m

Height of clear layer Smoke extract rates per zone

Maximim smoke temp Fan specification

Units selected

подпись: height of clear layer smoke extract rates per zone
maximim smoke temp fan specification
units selected
Warehouse 8 m

36 m3/sec 58.6 m3/sec

203QC 109°C

Category H. T. 300/0.5 Category H. T. 105/5 (300 °C for 1/2 hour) (150 °C for 5 hours)

Figure 21.21 Typical smoke extract system

Ambient air conditions. The rate of extract can be tailored to suit the building usage and the number of openings in the roof or walls can be reduced. Modern units, incorporating mixed flow or centrifugal impellers, can overcome the resistance of ducting systems and filters to ensure good indoor air quality.

Where there is a desire to reduce the power used to a minimum, then hybrid systems should be considered. These would use natural or passive ventilation when ambient conditions favour it, but powered ventilation would be available when the outside air was warm and there was an absence of wind. It would also be of use for smoke ventilation, if the units were appropriately rated.

Ventilation was also seen as a means of controlling volatile or­ganic compounds, radon emissions from brickwork in granite and similar areas, body odours etc. Whole house ventilation has therefore become almost the “norm” in all new accommo­dation in these countries and this has included the roof spaces, attics, etc.

Introduction of the new part F Building Reg­ulations

Part F (Ventilation) of the Building Regulations for England & Wales is currently under review by the UK Government. This is as a result of changes to Part L which includes minimising un­controlled air leakage through the building envelope, which, al­though contributing to the ventilation, can result in draughts as well as wasted energy. However, sealing up the building can mean that there is not adequate ventilation, and the new Part F due to take effect from January 1 st 2006 will deal with designing adequate ventilation systems to maintain good indoor air qual­ity, without relying on air leakage into the building. The UK Gov­ernment’s approach may be summed up as “build tight and ventilate right”.

With the introduction of the new part F and similar legislation in other European countries there will undoubtedly be a similar trend to whole house ventilation in the UK. Essentially the strat­egy has been outlined in BRE Digest 398, which is a best value approach already common in flats in the UK.

Essentially the concept is to provide simultaneous low rate ex­traction from at least kitchens, bathrooms, shower rooms, utility rooms and WCs and this is ducted to a central extract fan. Ex­tract grilles are positioned in these various rooms as appropri­ate but should be as close as practicable to the actual source of water vapour. The extracted air should then be discharged out­side the building via a single duct terminating in an outlet duct or cowl. The more sophisticated systems therefore incorporate a heat exchanger to minimise the heat losses to atmosphere.

Air tightness of dwellings

Posted in Fans Ventilation A Practical Guide

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