Hot deck-cold deck systems
Certain types of packaged air handling plant offer the facility of providing multiple zone control of temperature without the use of multiple reheaters. In some respects there is a similarity here with double-duct systems.
The type of plant is illustrated in Figure 8.9(a). Air is filtered and drawn through a fan which blows the air over a pair of coils, one heating and one cooling, arranged in parallel.
The downstream part of the plant is split by a horizontal dividing plate into two chambers, an upper ‘hot deck’ and a lower ‘cold deck’. The outlets from these decks are dampered, and a number of ducts may accept air from the outlet. Any particular duct outlet can then have air at a controlled temperature by accepting the appropriate proportions of air from the two decks, through the motorised dampers. The particular damper group for one zone would be thermostatically controlled.
Figure 8.9(b) illustrates the psychrometry, the temperature rise due to fan power etc., being ignored. Air comes off the hot deck at state H and off the cold deck at state C. The two airstreams fed to zone 1 are dampered automatically and mix to form state St. This is the correct state to give condition Rt in zone 1. Similarly H and C mix to give state S2 for supply to zone 2, maintaining state R2 therein. It is seen that differences in relative humidity are possible between zones.
As with sequence heating, there is no direct cancellation of cooling, hence account can be taken of cooling load diversity in assessing refrigeration load.
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