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57
cooling may be provided from packaged direct expansion refrigeration units
within each zone; manually operated internal or external shading blinds; passive
solar architecture that may include thermal storage walls; chilled-water beams
or flat panels may be installed at high level within offices to provide limited
cooling (the chilled water in the beams and panels is maintained at a tempera-
ture that is above the room air dew-point so that there is no condensation on the
exposed surfaces or within the ceilings, avoiding dehumidification and control
of the zone relative humidity); chilled beams and panels provide no ventilation air.
Assisted natural ventilation - a development of natural ventilation, as
above, where applicable. Mechanically operated ventilation louvres and exhaust
air fans improve the control of air flow through the occupied spaces; the incom-
ing outside air may be cooled with a water spray evaporative cooler in hot, dry
climates; evaporative cooling is a low-cost means of cooling the incoming out-
side air, which is exhausted by either natural openings of doors and windows,
or by exhaust air fans in confined zones; the incoming outdoor air may be
cooled through a specific temperature range, say 10 K, to provide limited cool-
ing by means of direct expansion or chilled water refrigeration plant.
Mechanical ventilation which only passes outside air through the zone.
This usually applies to moderate climates such as the United Kingdom where
minimal cooling is required; in climates where the outside air temperature ex-
ceeds 30 °C d.b. the flow rate of outside air is likely to be insufficient to pro-
vide enough cooling for zone temperature control in an air-conditioning system;
where it is possible to locate the exhaust air duct alongside the incoming out-
side air duct, within the ceiling space or a plant room, an air-to-air fiat-plate
heat exchanger is used to transfer heat between the incoming and outgoing air
streams; the outgoing exhaust air is already at the correct zone temperature, of
around 22 °C d.b. (it cannot be recycled as it is vitiated with carbon dioxide,
odours and atmospheric pollutants); heat transfer works throughout the year (in
winter, the incoming outdoor air is preheated by up to 10 K from the outgoing
exhaust air; in summer, the incoming outdoor air is precooled by up to 10 K
from the outgoing exhaust air); heat transfer efficiency is around 55%; a similar
heat transfer can be obtained from a recuperative heat wheel that transfers heat
from the outgoing exhaust air to the incoming outside air, generating up to a 10
K temperature change in the outside air stream; the heat transfer medium of the
wheel may be strips of mylar film.
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