Mechanical Ventilation Systems Are Frequently Applied to Commercial Buildings

7/29/2019 Mechanical Ventilation Systems Are Frequently Applied to Commercial Buildings

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Mechanical ventilation systems are frequently applied to commercial buildings,

workshops, factories, etc., where the air change requirements are defined for health and

welfare provision. There are three categories of system:-

1. Natural inlet and mechanical extract

2. Mechanical inlet and natural extract

3. Mechanical inlet and mechanical extract

The capital cost of installing mechanical systems is greater than natural systems of

air movement, but whether using one or more fans, system design provides for more

reliable air change and air movement. Some noise will be apparent from the fan and air

turbulence in ducting. This can be reduced by fitting sound attenuators and splitters.

Internal sanitary accommodation must be provided with a shunt duct to prevent smoke or

smells passing between rooms. In public buildings, duplicated fans with automatic

changeover are also required in event of failure of the duty fan.


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Basement car parks require at least 6 air changes per hour and at exits and

ramps where queuing occurs, local ventilation of at least 10 air changes per

hour. Duplicate fans should be provided with a fan failure automatic change over.


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Fan assisted ventilation systems supplying external air to habitable rooms

must have a facility to pre-heat the air. They must also have control over the amount

of air extracted, otherwise there will be excessive heat loss. A mechanical inlet and

mechanical extract system can be used to regulate and balance supply and emission

of air by designing the duct size and fan rating specifically for the situation. Air may

be extracted through specially made light fittings. These permit the heat enhanced

air to be recirculated back to the heating unit. This not only provides a simple form of

energy recovery, but also improves the light output by about 10%. With any form of

recirculated air ventilation system, the ratio of fresh to recirculated air should be

at least 1:3. i.e. min. 25% fresh, max. 75% recirculated. In large buildings where

smoking is not permitted, such as a theatre, a downward air distribution system may

be used. This provides a uniform supply of warm filtered air. Ductwork in all systems

should be insulated to prevent heat losses from processed air and to prevent surface

condensation.


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TYPES OF VENTILATION SYSTEMS

1. Natural Inlet And Mechanical Extract

A mechanical extract system will function irrespective of wind and

temperature difference and will be positive in action. Since the air to be extracted

from the space must be replaced and the means provided for admission of outside

air will present some resistance to flow, leakage inward from surrounding spaces is

more likely than leakage outward. In consequence, escape of steam, fumes and

noxious vapours generated within the ventilated space is less likely than would be

the case if reliance were placed upon natural extract alone. A difficulty arises

however in providing a satisfactory means of admitting the bulk of the air required to

balance the extract volume and of heating it in winter. Fresh air inlets behind ranges

of gilled tube or some other form of natural convector as shown below need regular

cleaning and have only a limited application.


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For summer use only in a small building, infiltration via the type of window

ventilator ('trickle' or 'night') referred before may suffice. In some circumstances,

replacement air may be drawn from another part of the building to serve a dual

purpose; a good example being extract ventilation from a small kitchen which, by

drawing air through serving hatches, serves to dilute the spread of cooking smells

to the associated canteen. For industrial applications, the natural roof ventilator

shown earlier might be replaced by some form of fan-powered extract unit as shown

below. Units of this type are made in a variety of patterns with mounting

arrangements and weathering covers to suit most types of roof construction, pitched

and flat.


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2. Mechanical Inlet And Natural Extract

It is typified, for application to an office building, by the arrangement shown in

Figure below. As may be seen, an air supply from some central source is ducted

through a corridor-ceiling void to individual rooms; the vitiated air escapes there

from, through a low-level register, to the corridor proper and thence to outside. It is

now unlikely that a fire authority would permit the construction of such a system

since it has the potential to endanger the atmosphere of an escape route. A single

room, perhaps an office, might have an outside air supply from some form of

reversible fan unit as described earlier but this hardly falls within the context of the

present heading. The ventilation of boiler houses is a special case as a result of the

air consumed in combustion. The volume required may be estimated.


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3. Mechanical Inlet And Mechanical Extract

This final combination is that which must be noted as not only having the

widest application but also as providing the greatest challenge since the air

distribution, in terms of quantity, pressure and temperature, is wholly in the hands of

the designer. It may be applied to all manner of spaces and is greatly to be preferred

to any of the compromise systems discussed earlier. In application, the ratio

between the air-volume duties of the inlet and extract systems must be selected with

care in order to suit the particular application.


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Mechanical Ventilation Systems Are Frequently Applied to Commercial Buildings

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