Cooling in sequence with heating
When it is necessary to control only temperature in the conditioned space, reheat can be avoided by arranging that the heater battery is used only when the cooler coil is off. Figure 8.8(a) illustrates the control operation: Ra and Rb are three-port motorised mixing valves which are controlled from a room thermostat C, in sequence. As the room temperature falls, C progressively reduces the capacity of the cooler coil, the by-pass port of Ra opening until eventually no chilled water flows through the coil. Upon further fall in room temperature, C progressively increases the capacity of the heater, the by-pass port of Rb closing. Thus, the two work in sequence, and since the heater is never on when the cooler is also on, none of the cooling capacity is ever wasted by cancellation with reheat.
The effect of a reduction in sensible heat gain, while the outside state remains constant, is shown in Figure 8.8(b). If a supply temperature ts> is required instead of ts, then, presuming the latent gains to be unchanged, the relative humidity in the room will rise, state R’ being maintained.
If objectionable increases in humidity are to be prevented, a high limit humidistat is necessary. This overrides the action of C, partly closing the by-pass port of Ra for the purpose of doing some latent cooling. The temperature fall which would result is prevented by C partly closing the by-pass port of Rb. Thus, under these circumstances, the heater is behaving as a reheater rather than as a sequence heater.
Fig. 8.8 The plant layout and psychrometry for cooling in sequence with heating. To simplify the illustration of the psychrometry it is assumed that 100 per cent fresh air is handled. The system works equally well when recirculated and fresh air are mixed, which is usually the case.
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