Low-Pressure Controls, Refrigerant Blends

Low-pressure controls are used on many medium-temperature refrigeration systems to control case temperature. In order for this type of control to work properly, both its cut-in and differential pressure settings must be set correctly.

The control's cut in setting must be set to cycle the compressor back on at a pressure that will ensure adequate defrosting of the evaporator, and at a case temperature that is not too high for the product being refrigerated. For some applications, this may be at a saturation pressure corresponding to a 38°F saturation temperature.

Since refrigerant blends have a varying saturation temperature corresponding to a specific saturation pressure, a service technician must choose which saturation pressure/temperature relationship to use. When setting the cut-in pressure of a low-pressure control, the dew point temperature of a refrigerant blend should be used. This will cycle the compressor on at an appropriate coil/case temperature.

Once the cut-in pressure has been set, the control's differential setting must be set. The differential setting is used to cycle the compressor off at the lowest designed case temperature. The differential setting represents the difference between the cut-in and the cut-out pressure of the pressure control. For example, if the cut-in is set at 34 psig and you want the compressor to cycle off at 24 psig, the differential setting would need to be set at 10 psig (34 psig – 24 psig = 10 psig).

The appropriate cut-out pressure when working with a refrigerant blend must reflect the average saturation temperature within the evaporator minus any pressure drop through the suction line.

For example, if an average coil temperature of 25° is desired and the refrigerant blend enters the evaporator at a saturation temperature of 21° and is at 29° before the refrigerant becomes superheat, the cut-out pressure would then be set corresponding to the dew point temperature of the higher saturation temperature. This example would be 29° minus any pressure drop through the suction line.

If R-401A were the refrigerant used in our example and a pressure drop of 2 psig was assumed across the suction line, then the actual cut-out pressure would be 26 psig minus the 2 psig, which would equal a cut-out of 24 psig. If the cut-in value is set at 34 psig, then the differential would need to be set at 10 psig.

Calculating the average saturation temperature required and the pressure drop across a suction line may be difficult at times. Here's an alternate method that can be used to set the cut-in and differential pressure setting of a low-pressure control:

  • First set the cut-in value of the low-pressure control to the appropriate value for the refrigerant to be used.

  • Then initially set the differential of the low-pressure control to an extremely high value (a value the suction pressure should never reach under normal operating conditions; 30 psig should suffice).

  • Allow the system to run while monitoring the box temperature.

  • Once the box reaches its lowest desirable temperature, slowly adjust the differential counter-clockwise until the compressor cycles off.

    Using this procedure will ensure the differential settings have been properly set and the system will cycle properly.

    Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or joe@coldtronics.com.

    Publication date: 05/01/2006

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