The use of by-passed air instead of reheat
It is clearly wasteful to cool and dehumidify air and then to cancel some of the sensible cooling capacity to deal with a reduced sensible heat gain in a room. One alternative approach, that avoids some reheating, is to by-pass a portion of the recirculated air around the cooler coil. Figure 8.5 shows the plant arrangement and the psychrometry. If a state S were required for supply to a room it could be achieved by cooling and dehumidifying all the air from a mixture state M to an off-coil state W, and then reheating this to state S. Alternatively, without the use of wasteful reheat, part of the recirculated air could be diverted around the cooler coil, the remaining part mixing with fresh air and forming a new off-coil state, W’. State W then mixes with state R to give the desired state S.
Fig. 8.5 Plant arrangement and psychrometry for the case of recirculated air by-passing a cooler coil, ignoring the temperature rise across the supply air fan for simplicity of illustration.
The actual performance is more complicated than this. When some of the recirculated air is by-passing the cooler coil there is less recirculated air available to mix with the fresh air so the mixture state, M is not the same as the mixture state M. Further, when air is bypassing the cooler coil, less air is flowing through it and this affects performance (see sections 8.6, 10.4 and 10.7).
Although the method has been considered in the past it has not proved popular, for several reasons. The ducting arrangement round the air handling unit to accommodate the by-passed airflow is a complication in plantroom layout and occupies too much room. The capital cost is consequently greater than for other methods. Commissioning the correct airflow proportions may sometimes not be easy. In any case, a reheater is needed for winter operation. Other techniques, such as the use of face and by-pass dampers, have given better results.
Posted in Engineering Fifth Edition