When troubleshooting commercial refrigeration systems, many times it is necessary to measure the discharge pressure of a system and determine if the pressure is normal, higher than normal, or lower than normal.

In order to make this determination, the correct operating discharge pressure must be known. For systems utilizing an air-cooled condenser, this can be determined by measuring the system's suction pressure and the temperature of the air entering the condenser.

Below are general procedures for determining the correct operating discharge pressure of a commercial refrigeration system utilizing an air-cooled condenser:

Step 1: Measure the dry-bulb temperature of the air entering the condenser. (We will refer to this as the EAT - Entering Air Temperature).

Step 2: Measure the system's operating suction pressure.

Figure 1. Evaporator temperature in degrees F.
Step 3:Using a pressure/temperature (P/T) chart, convert the suction pressure to its equivalent saturation temperature.

Step 4: Determine the system's application: low temperature, medium temperature, or high temperature. A low temperature system is one that operates at an evaporating temperature of 0 degrees F and below.

A medium temperature system is one that operates at an evaporating temperature of between 0 degrees F and 25 degrees. A high-temperature system is one that operates at an evaporating temperature of between 25 degrees and 45 degrees.

Step 5: Using the chart above, determine the appropriate Temperature Rise (TR). The TR is the difference between the EAT and the condensing temperature of the refrigerant in the condenser. (See Figure 1.)

Step 6: The condensing temperature (CT) can then easily be determined by adding the EAT and the TR. CT = EAT + TR.

Step 7: Once the condensing temperature is known, its equivalent saturation pressure can be determined by using a P/T chart. This will be the correct operating discharge pressure of the system.

Let's try an example.

Suppose we are working on a walk-in cooler using R-134a refrigerant with an operating suction pressure of 18.4 psig suction pressure and 76 degree air temperature entering the condenser.

Step 1: The EAT in our example is 76 degrees.

Step 2: The system's operating suction pressure is 18.4 psig.

Step 3: Using a P/T chart we convert our suction pressure of 18.4 psig to a saturation temperature of 20 degrees.

Step 4: We determine the system's application to be medium temperature.

Step 5: Using the chart we determine the appropriate TR to be 32 degrees.

Step 6: The CT of our system will be 108 degrees. (108 degrees = 76 degrees + 32 degrees)

Step 7: Again using a P/T chart, we can conclude that our operating discharge pressure should be 142.8 psig.

Using this procedure should enable you to better determine if the discharge pressure of a commercial refrigeration system is normal, higher than normal, or lower than normal.

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

Publication date: 10/03/2005