Most reach-in coolers use a temperature control that does not directly measure the case’s air temperature but rather its evaporator coil temperature. This style of temperature control provides both adequate control of the case temperature and a means to defrost the evaporator coil.
This control style works because there is a definite relationship between the evaporator coil temperature and its inlet air temperature. When the system is running, most evaporators will have a 10°-15°F temperature difference between their coil temperature and their inlet air temperature. That being the case, it is easy to control the case temperature by monitoring the coil temperature. When the coil reaches a specific temperature, it will correspond to a specific case temperature. For example, if the desired case temperature is 35°, and the evaporator has a 10° temperature difference, when the evaporator coil reaches 25°, it will correspond to a 35° case temperature.
By sensing coil temperature, you also ensure the evaporator coil defrosts each time the control cycles the compressor off. When the compressor cycles off, the evaporator fan motor will continue to operate, and any frost that has developed on the coil of the evaporator will begin to dissipate. The coil temperature will then begin to approach the case’s temperature, since the compressor is not operating. The cut-in setting on this type of controller is set at a fixed temperature that will cycle the compressor back on at a high enough coil temperature to ensure the evaporator is completely defrosted, and the case temperature does not reach an unacceptable high temperature (usually around 38°). The actual cut-in temperature can vary between manufacturers and case designs. Always check with the manufacturer of the cooler to find the correct cut-in value of the controller used.
When replacing this type of controller, it’s usually best to obtain the original component from the case manufacturer. This will ensure the cut-in and cut-out temperature range is appropriate for the design. However, there are universal temperature controllers available that seem to work fine for many applications. These replacement temperature controllers are available with a constant cut-in setting of 38°-41° and varying cut-out ranges. Also, when replacing this type of controller, make sure the sensing element is properly secured to the evaporator. Most evaporators will have a well the sensing element can be inserted into. Make sure the sensing element is fully inserted to ensure it will sense the proper coil temperature. Replacing these types of temperature controllers isn’t difficult as long as the right controller is used. Using the wrong temperature controller will lead to problems.
Troubleshooting these controls can be a little tricky at times. A system with a slightly low refrigerant charge can cause the case to run longer than normal and may drop the case temperature too low. At first glance it would seem the temperature control is the cause, but the coil temperature where the sensing element is located may be sensing a slightly warmer temperature than normal, due to the loss of refrigerant. This causes the system to run longer, leading to a drop in the case’s temperature.
Publication date: 12/4/2017