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Cold Air Distribution
There are several systems available for cooling a space. One of these, an "all air"
system which heats and cools air centrally and then distributes conditioned air to
various building spaces by means of ductwork can be modified to distribute colder air.
Description of System
In conventional "all air" systems, the temperature of the air supplied to an occupied
space is usually between 50ºF and 60 ºF (depending on the type of space, age
of the mechanical systems and type of air handling equipment). Assuming that the space
temperature is maintained at 75ºF, the temperature difference available to cool
the space is 20ºF (Dt), based upon a 55ºF supply air temperature (i.e.
75ºF minus 55ºF).
"Cold air distribution" systems are generally defined as systems where the supply
air temperature is between 40ºF and 45ºF. For the above example, assuming a
supply temperature of 45ºF, the temperature difference available is now 30ºF
(75ºF minus 45ºF). The result is that the same cooling effect can be achieved
by a quantity of supply air that is reduced by a third.
The benefits of a cold air distribution type of system include:
Reduced duct size
Reduced fan horsepower for air handling systems
Special consideration must be given to condensation, humidification, fresh air
requirements and air movement issues when the use of a cold air distribution system is
considered.
It is sometimes possible to obtain a rebate when refrigeration storage, which reduces
daytime electrical demands is used. Refrigeration storage is an ideal match for low
temperature supply since the storage is often in the form of ice, an ideal cooling source
for "cold air distribution".
Cold air temperature distribution is not common, but the potential savings in space
is considerable and with the development of careful design solutions worth considering.
Theory of Cooling
A space is cooled by a combination of the available temperature of the air supplied
to a space, the required space temperature, as well as by quantity of air supplied to
the space. This is represented by a simple formula:
Cooling Effect = CFM x Dt x K
It is important to understand that the supply air temperature discussed here is the
temperature of air supplied to the room, not the temperature of the air leaving the air
conditioning unit cooling coil. The temperature leaving the cooling coil is less, to
allow for fan heat gains (where the fan is located after the cooling coil) and duct
heat gains (since the air surrounding the ductwork is usually at a higher temperature).
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