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