Double duct cooling load

Whether load diversity may be taken advantage of in estimating the required refrigeration capacity depends on which one of two systems is used.

The most common form of system (Figure 8.10(a)) has the cooler coil in the cold duct and the heater battery in the hot duct on the discharge side of the fan. The advantage is that no air is first cooled and then reheated. If a higher supply air temperature than for summer design conditions is required (say ts rather than tc), some hot air is mixed with cold air. No wasteful reheat is involved. Ultimately, all the air would be from the hot duct and none from the cold duct. The disadvantage of the system is that the moisture content of the supply air varies. With a supply state S, for the same moisture pick-up in the room, a condition R’ rather than R is maintained, as shown in Figure 8.10(fe).

The system is seldom used because it is expensive in capital cost, in running cost and in its use of building space.

Posted in Air Conditioning Engineering


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