The hottest location in a refrigeration system is at the discharge port of the valve plate inside the compressor head. It is difficult for a technician to measure the exact temperature at this location. However, measuring the temperature of the discharge line approximately 6 inches from the compressor outlet will give a good indication of the temperature at the discharge port.

There will be approximately a 50 degree to 75 degree F drop in temperature between the discharge port and 6 inches out on the discharge line. If a temperature of 175 degrees is measured on the discharge line, it would indicate an approximate temperature at the compressor discharge port of between 225 degrees and 250 degrees.

For a reciprocating compressor to operate properly, the temperature of the discharge port should never rise above 300 degrees. This means the discharge line temperature should never exceed 225 degrees (300 degrees minus 75 degrees). At temperatures between 300 degrees and 320 degrees inside the compressor, the refrigeration oil will start to lose its ability to lubricate. This will cause premature wear of the compressor’s cylinder and piston rings. At temperatures above 350 degrees, the oil will start to break down, causing accelerated wear and allowing contaminants to enter the system. Once this occurs, compressor failure will soon follow.

Possible Causes

High compression ratios are the result of either lower than normal suction pressures or higher than normal discharge pressures. Changes in suction pressure will affect the compression ratio more rapidly than changes in the discharge pressure. For this reason, it is important to keep the suction pressure at its highest possible value.

Causes of low suction pressure can include incorrect sizing of components, misadjusted or defective metering devices (TXVs), loss of refrigerant charge, plugged driers or strainers, and excessive suction line pressure drop.

Although not as sensitive to change as the suction pressure, the discharge pressure can still greatly affect the compression ratio. Keeping the discharge pressure within normal operating conditions is still important.

Causes of high discharge pressure can include dirty condensing coils, undersized discharge line, a blockage or recirculation of condenser air, erratic condenser fan operation, refrigerant overcharge, noncondensibles in the system, and an undersized condenser.

Many larger refrigeration systems with electronic controllers will automatically monitor the temperature of the discharge line and will send out an alarm to alert the owner or service contractor of a potential problem. Regardless of how the discharge line temperature is monitored, it should always be checked as part of any service to a refrigeration system.

A technician should also check the discharge line temperature within 6 inches from the outlet of the compressor. Any temperature above 225 degrees should be investigated and the cause found and repaired.

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

Publication date: 09/01/2003