Control devices are fitted into the system to ensure maximum efficiency and to protect components from damage.
A thermostat is used to sense the temperature of the evaporator fins and help ensure that they do not drop below 1°C. If the temperature of the fins were to drop to 0°C or less, moisture condensing out of the air would freeze on the unit and block the air flow through the evaporator.
When the fins of the evaporator drop to 1°C the thermostat contacts open. This breaks the electrical circuit to the compressor clutch and stops the compressor from circulating refrigerant.
When the evaporator temperature rises, usually to about 4°C the contacts close and the electrical circuit to the compressor clutch is restored.
The fixed orifice tube cannot alter the amount of refrigerant entering the evaporator. Therefore as extra cooling is required and the heat load on the system increases, the compressor cycles on for longer periods.
As the heat load decreases, the compressor cycles off for longer.
This cycling adjusts the system to the heat load.
System protection against high pressures is provided by a high-pressure switch mounted in the high pressure line between the condenser and the orifice tube.
High temperatures and pressures may develop if the condenser is unable to transfer its heat effectively to the outside air. This may be due to blockage of the condenser fins or an inoperative electric fan.
If this occurs, the high-pressure switch open-circuits the compressor clutch and prevents the compressor from circulating refrigerant.
If a leak develops and the refrigerant is low in charge, a low pressure switch on the low pressure side of the system turns the compressor off when the system pressure falls below a set value.
As lubricating oil is carried around the system by the refrigerant this prevents damage to the compressor due to lack of lubricating oil.