Commercial refrigerator thermostats measure and regulate the temperature in large commercial refrigerators. They do this by opening and closing an electric current that turns the refrigerator's compressor on and off. When the compressor is on, the temperature inside the refrigerator drops; when it is off, the temperature inside the refrigerator rises toward the ambient external room temperature. By turning the compressor on and off at the correct times, a thermostat causes the compressor to maintain a specific temperature inside the refrigerator.
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Air-sensing thermostats comprise the majority of thermostats in commercial refrigerators. The thermostats measure the return air in the refrigerator, which refers the average temperature in the refrigerator; it is distinguished from the supply air. Air-sensing thermostats use a straight, liquid- or gas-filled capillary attached to a bulb whose pressure varies with the temperature, opening and closing an electric current to turn the refrigeration unit on and off. As with evaporator coil sensing thermostats, the sensor is located inside the evaporator housing, in a tube at the front of the evaporator.
Evaporator Coil Sensing
An evaporator-sensing thermostat measures the temperature of the refrigerator's evaporator coil, rather than that of the air. This can be helpful in cases where the temperature in the refrigerator suddenly rises and causes the compressor to overcompensate so that frost forms on the evaporator coil. Air-sensing thermostats would not register the frost build-up and would therefore allow the overcompensating compressor to continue operating, but a sensor measuring the temperature of the evaporator coil itself would deactivate the compressor until the frost on the evaporator coils dissipates. The capillary tube sensor in evaporator coil-sensing thermostats coils into a spiral next to the evaporator.
Pressure control thermostats differ from air-sensing and evaporator coil-sensing thermostats in that they are located outside the evaporator housing near the compressor. Commercial refrigerator units use a refrigerant fluid to remove heat from inside the refrigerator, lowering the temperature. The refrigerant fluid circulates between the compressor and the evaporator at a specific pressure, and the pressure correlates to the resulting temperature inside the refrigerator. Low-pressure control thermostats measure the pressure of the refrigerant to indirectly measure and control the temperature in the refrigerator.
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