Read e-book online Automatic Controls for Heating and Air Conditioning. PDF

By K. M. Letherman, N. S. Billington

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In order to meet changes in cooling or heating loads on a room, an alternative to supplying air at constant flow rate and variable temperature is to supply at constant temperature and variable flow rate. Reduction in load will result in reduced running costs by saving on fan power, provided that the variation in flow rate is not achieved by using simple dampers to throttle the flow. Special solid-state units are available for modulating the motor speed, and inlet vanes on centrifugal fans or variable-pitch blades on axial fans can be employed.

This avoids fuel waste due to overheating when, for example, solar gains are sufficient to offset heat losses in some rooms. In this case a bypass between boiler flow and return may be required. 2). The control systems described above, except for the thermostatic radiator valves and outside compensators, can also be applied to warm-air systems. Individual room control in such systems may be obtained manually with adjustable dampers in each room or automatically with thermostats controlling motorised dampers.

There is a bifilar winding of platinum wire over the surface, so that electrical current can travel between the wires only across the surface of the LiCĀ£ crystals. There is a temperature sensor "inside the glass tube and the element takes up a temperature which is related, but not equal, to the dew-point temperature of the surrounding air. The bifilar windings are connected to an alternating voltage supply of about 50 V. This allows the surface of the element to maintain itself in equilibrium with the surrounding air.

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