Crankcase Pressure Regulators

June 4, 2004
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A crankcase pressure regulator (CPR) is a common accessory added to many low-temperature refrigeration applications, such as walk-in and reach-in freezers. They are designed to prevent the compressor's motor from overloading when its crankcase pressure rises above its designed working pressure. On many low-temperature applications, this can occur during or after a defrost cycle, or after a normal shutdown period. The CPR is an outlet pressure regulator and will not allow the crankcase pressure to rise above a predetermined level.

Typically the CPR is installed in the suction line between the compressor and the evaporator. Normally, there are no other components installed downstream, between the outlet of the CPR and the compressor. This is to ensure that the outlet of the CPR senses the true crankcase pressure of the compressor.

This type of regulator is not recommended for a system that also uses a maximum operating pressure (MOP) expansion valve. The use of both of these valves on the same system may cause longer pull-down times. This, however, may be overcome if the pressure settings of both valves are sufficiently spread apart.

When using a CPR on a system that is also using a discharge bypass valve for capacity control, the pressure setting of the CPR must be higher than that of the bypass valve.

The typical CPR is a close-on-rise regulator. The outlet pressure (crankcase pressure) of the valve is the closing force within it. The opposing opening force within the valve is an interior spring. These two forces oppose each other to regulate the port size within the valve.

The interior spring is adjustable to apply a maximum opening force. If the pressure applied at the outlet of the valve (crankcase pressure) is above the spring force, the valve will close down. The inlet pressure of the CPR is applied equally to both the underside of its bellows and the top of its valve seat disc, canceling out the effect of the inlet pressure.

The selection of a CPR is based on five system considerations:

1. Refrigerant.

2. Refrigeration capacity.

3. Design suction pressure.

4. Maximum crankcase pressure recommended by the compressor manufacturer.

5. Pressure drop across the valve at design load conditions.

These valves should not be selected based on the size of the suction line, since a manufacturer may use the same line size for many different capacities.

The adjustment of a CPR should be done on start-up, when the evaporator pressure is normally at its highest level. Some valves will have a pressure tap on the inlet port in order to allow a technician to measure the pressure there. When adjusting these valves, the pressure setting should be low enough to protect the compressor, but not so high as to penalize the system's capacity or pull down time. Always refer to the manufacturer's instructions when adjusting these valves.

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

Publication date: 06/07/2004

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