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58
Mechanical ventilation with recirculated room air. The maximum quantity
of conditioned room air is recirculated to save energy use at the heating and re-
frigeration plant; the outside air motorized dampers are modulated from closed
to fully open to control the zone air temperature without the use of the me-
chanical refrigeration plant for as long a time as possible (this provides low cost
cooling to the building); when the refrigeration plant has to be used, the outside
and exhaust air motorized dampers are moved to their minimum outside air po-
sitions, often around 10% open, allowing the maximum use of recirculated
room air; a range of ducted air-conditioning systems are in use, including single
duct, dual duct, induction units, fan coil units and variable air volume systems.
The single-duct system works in the following way. Some of the air ex-
tracted from the room is exhausted to the atmosphere and as much as possible is
recirculated to reduce running costs of heating and cooling plants. Incoming
fresh air is filtered and mixed with that recirculated; it is then heated by a low-,
medium- or high-pressure hot-water or steam finned pipe heat exchanger or an
electric resistance element. The heated air is supplied through ducts to the
room. The hot-water flow rate is controlled by a duct-mounted temperature de-
tector in the extract air, which samples room conditions. The electrical signal
from the temperature detector is received by the automatic control box and cor-
rective action is taken to increase or reduce water flow rate at the electrically
driven motorized valve at the heater battery.
During summer operation, chilled-water from the refrigeration plant is
circulated through the cooling coil and room temperature is controlled similarly.
A temperature detector in the fresh air duct will vary the set value of the
extract duct air temperature - higher in summer, lower in winter - to minimize
energy costs. A low-limit temperature detector will override the other controls,
if necessary, to avoid injection of cold air to the room.
The building is slightly pressurized by extracting only about 95% of the
supply air volume, allowing some conditioned air to leak outwards or exfiltrate.
Energy savings are maximized by recirculating as much of the condi-
tioned room air as possible. Room air recirculation with economy-cycle motor-
ized dampers can, sometimes, be retrofitted to existing systems as an energy
conservation measure. In mild climates, such as in the United Kingdom full
outside air systems are also used. These have no recirculation air ducts; either a
fiat-plate heat exchanger or run-around pipe coils can be installed to preheat
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