Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 149
Hot supply duct
Mixing box
Flexible ducts
Access panel
Diffuser
Cold supply duct
Recirculation
duct
Supply air
Supply air
Mixing dampers
Cold air duct
Hot air duct
Mixing box with
sound attenuator
Damper motor
5.12 Dual-duct installation in a false ceiling and detail of the mixing box.
of the heating and cooling for the building, is achieved by conditioning the fresh air intake in the
plant room.
Primary fresh air is injected through nozzles into the induction unit in each room. These units
may be in the floor, in the ceiling void or under the windowsill. Because of the high-velocity
jets, the local atmospheric pressure within the unit is lowered and air is induced into it from the
room. The induced air may enter at three or four times the volume flow rate of primary air, and
it flows through a finned pipe bank and dust filter before mixing with primary air and being
supplied to the room.
The secondary air flow rate can be manually adjusted using a damper. Either hot or chilled
water is passed through the room coil depending upon demand. A two-, three- or four-pipe
distribution system will be used. The two-pipe system requires a change-over date from heating
to cooling plant operation, but a three-way valve can blend hot and chilled water from
the three-pipe arrangement. The third alternative has separate hot- and chilled-water pipe coils
and pipework.
The extract ductwork and fan removes 90% of the primary air supply and exhausts it to the
atmosphere. All recirculation is kept within the room and this greatly reduces duct costs and
service duct space requirements. Figure 5.13 shows a typical installation in an office.
Fan coil units
Heating and cooling loads that prove to be too great for induction units can be dealt with by
separate fan and coil units fitted into the false ceiling of each room or building module. Better
air filtration can be achieved than with the induction unit. A removable access hatch below the
unit is required to facilitate motor and filter maintenance.
Care is taken to match the fan-generated noise to the required acoustic environment. As
with the other systems, the extracted air can be taken through ventilated luminaires to remove
the lighting heat output at source and avoid overheating the room. The supply and extract