When warm air passes across the evaporator fins, the cooling action causes water vapor in the air to condense and adhere to the fins in the form of water droplets.
If the temperature of the fins drops to O°C [or 32°F] or below, ice will form on the fins, restrict the air flow and prevent heat being transferred to the refrigerant.
A thermostat is used to monitor the temperature of the evaporator fins and control system operation within set limits.
The bellows type has a capillary tube that is placed about 25 mm [or 1 inch] into the evaporator core so that it contacts the fins.
The capillary tube and bellows are sealed and filled with a temperature sensitive fluid which expands and contracts with temperature changes. This increases or decreases the pressure in the bellows.
The bellows is positioned to open and close a set of contacts which form part of the electrical circuit to the compressor clutch.
Springs connected between the bellows contact frame and the thermostat housing try to keep the contacts open. Pressure in the bellows tries to force the contacts closed.
When the temperature at the evaporator fins falls below the minimum setting, the fluid in the capillary tube contracts, reducing the size of the bellows.
The springs mounted on the pivoted frame move the frame to open the contacts and break the electrical supply to the compressor clutch.
The compressor stops circulating refrigerant around the system and the temperature of the evaporator fins begins to rise.
This increased temperature is applied to the fluid in the capillary tube causing an increase in pressure at the bellows.
The bellows pushes on the pivoted frame against the spring force and closes the contact points to complete the circuit to the compressor clutch once more.
Circulation of refrigerant is resumed and system operation re-commences. The thermostat usually cycles the system between a minimum of 2°C [or 35.6°F] and a maximum of 8°C [or 46.4°F] to maintain the evaporator at the required temperature.