Book - School of Science and Technology

394 Air-conditioningPrimaryair2 Pipeconnections(not shown)DamperclosedDamperopenSingle coilDrain pan(a) (Shown in metal case) (b)Watercontrolvalve2 PipeconnectionsPrimaryairSinglecoilHeating andcooling coilsPrimaryairWater controlvalves(c)(Shown in timber case)(d)4Pipe connectionsand drainFigure 14.21 Alternative control arrangements for induction unitsthe return air from the room is drawn either through the coils for heating or cooling, orthe coils are bypassed to a greater or lesser extent. In the type shown in Figure 14.21 (c),control is by variation of water flow: increase in water flow is required to lower the airtemperature during the cooling cycle, and increase in flowis required to raise the airtemperature in the heating cycle. There is thus required some means of change-over ofthermostat operation according to whether the winter or summer cycle is required.An alternative method, using the so-called non-change-over system, avoids this problemby always circulating cool water through the coils of the induction units and varying theprimary air temperature according to weather only. Thus, throughout the year, theheating or cooling potential of the primary air is adjusted to suit that component ofdemand imposed upon the system, or zone of the system, by orientation, by outsidetemperature or by wind effect. Any other variant ± solar radiation, heat from lighting oroccupancy ± will necessarily produce a local heat gain and the sensible cooling needed tooffset this will be provided by the capacity of the unit coils under local control. In so far as