Mechanical Ventilation Systems Are Frequently Applied to Commercial Buildings
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Transcript of 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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