Since the purpose of a refrigeration or air-conditioning system is to reduce or maintain temperature, a thermostat is usually fitted to stop the equipment or reduce its capacity when the required condition is reached. The following types are in use:

1. Movement of a bimetallic element

2. Expansion of a fluid

3. Vapour pressure of a volatile fluid

The above produce a mechanical effect which can be used directly to operate an electric switch or modulate the pressure of an air jet (pneumatic system).

4. Electric resistance

5. Electronic — various types

These last two produce an electric signal which must be measured and ampli­fied to operate the controlled device.

In order to prevent excessive cycling, a differential of say 2 K should be incorporated.

Posted in Refrigeration and Air Conditioning