Building Services Engineering 5th Edition Handbook

148 Ventilation and air conditioning
Variable volume controller
or reheater coil
Access panel
Extract
air duct
Window
Supply air
Airflow diffuser
Supply duct
Grille
Extract duct to
recirculate heated air
5.11 Single-duct all-air installation in a false ceiling.
building. A terminal heater coil under the control of a temperature sensor within the room can
be employed to provide individual room conditions.
A variable air volume (VAV) system has either an air volume control damper or a centrifugal
fan in the terminal unit to control the quantity of air flowing into the room in response to signals
from a room air temperature sensor. Air is sent to the terminal units at a constant temperature
by the single-duct central plant, according to external weather conditions. A reducing demand
for heating or cooling detected by the room sensor causes the damper to throttle the air supply
or the fan to reduce speed until either the room temperature stabilizes or the minimum air flow
setting is reached.
Air flow from the diffuser is often blown across the ceiling to avoid directing jets at the
occupants. As a result of the Coanda effect the air stream forms a boundary layer along the ceiling
and entrains room air to produce thorough mixing and temperature stabilization before it reaches
the occupied part of the room. When the VAV unit reduces air flow, there may be insufficient
velocity to maintain the boundary layer, and in summer cool air can dump or drop from the ceiling
onto the occupants, resulting in complaints of cool draughts.
Dual duct
In order to provide for wide-ranging demands for heating and cooling in multi-room buildings,
the dual-duct system, as shown in Fig. 5.12, is used. Air flow in the two supply ducts may, of
necessity, be at a high velocity (10–20 m/s) to fit into service ducts of limited size. Air turbulence
and fan noise are prevented from entering the conditioned room by an acoustic silencer.
In summer, the hot duct will be for mixed fresh and recirculated air, while the cold duct is
for cooled and dehumidified air. The two streams are mixed in variable proportions by dampers
controlled from a room air temperature detector. During winter, the cold duct will contain the
untreated mixed air and the air in the hot duct will be raised in temperature in the plant room.
The system is used for comfort air conditioning as it does not provide close humidity control. It
reacts quickly to changes in demand for heating or cooling when, for example, there is a large
influx of people or a rapid increase in solar gain.
Induction
Induction is a less costly alternative to the all-air single- and dual-duct systems for multi-room
applications. The central air-conditioning plant handles only fresh air, perhaps only 25% of the
supply air quantity for an equivalent single-duct system. All the humidity control, and also some