Because the compressor's discharge temperature is superheated, a pressure-temperature relationship does not exist and it must be read directly on the discharge line by some sort of temperature-measuring device.
The compressor's discharge temperature should be measured about 1 to 2 inches away from the compressor on the discharge line. This discharge temperature should never exceed 225°F. Carbonization and oil breakdown can occur if compressor discharge temperatures exceed 225°.
The three causes for high discharge temperatures are:
HIGH CONDENSING TEMPERATUREThere are many potential causes of high compressor discharge temperatures. A high condensing temperature is one of them.
When the condensing temperature is high, the compressor must compress the refrigerant from the low-side (evaporating) pressure to an elevated high-side (condensing) pressure. This added work done by the compressor would raise the heat of compression. Thus, the compressor's discharge temperature will be higher.
Remember, condensing temperature is the temperature the refrigerant is as it changes from a vapor to a liquid in the condenser. There is a pressure-temperature relationship with the condensing temperature because of the phase change. A gauge reading on the high side of the system is all that is needed to find the condensing temperature. Convert this pressure to a temperature using a pressure-temperature chart. This is the condensing temperature.
However, there are many causes for high condensing temperatures, which will also cause high discharge temperatures; high condensing temperatures cause high compressor discharge temperatures. Listed below are causes for high condensing temperatures:
LOW EVAPORATOR PRESSURES AND TEMPSLow evaporator pressures also may cause a high compressor discharge temperature. When evaporator pressures are low, the compressor must compress refrigerant from this lower evaporator pressure to the condensing temperature. This added work of compression would make the heat of compression higher. Thus, the compressor's discharge temperature will be higher.
Remember, evaporator temperatures are the temperature of the refrigerant as it changes from a liquid to a vapor in the evaporator. There is a pressure-temperature relationship with the evaporating temperature because of the phase change.
A gauge reading on the low side of the refrigeration system is all that is needed to find the evaporating temperature. Convert this pressure to a temperature using a pressure-temperature chart. This will be the evaporating temperature.
However, there are many causes for low evaporating pressures and temperatures, which will also cause high compressor discharge temperatures, since low evaporating pressures cause high compressor discharge temperatures. Listed below are causes of low evaporator pressures:
HIGH COMPRESSION RATIOSHigh compression ratios are a result of high condensing pressures or low evaporator pressures, or both. So, any time there are high condensing pressures or low evaporator pressures, or both, there will be high compression ratios. And, anytime there is a high compression ratio either from high condensing or low evaporator pressures, or both, there will be more work added to the compression stroke of the compressor. This will cause the heat of compression to increase and the compressor will have a higher discharge temperature.
Compression ratio is defined as the absolute discharge pressure divided by the absolute suction pressure. Discharge pressure and condensing pressure are one in the same and will be used interchangeably throughout this article. The same holds true for suction pressure and evaporating pressure.
Compression ratio = Absolute discharge pressure Ã· Absolute suction pressure
A compression ratio of 8 to 1 (expressed as 8:1) simply means that the discharge pressure is 8 times the magnitude of the suction pressure.
Again, a compression ratio of 12.3:1 simply indicates to the technician that the "absolute" or true discharge pressure is 12.3 times as great as the absolute suction pressure.
VOLUMETRIC EFFICIENCYThe volumetric efficiency is expressed as a percentage from 0 percent to 100 percent. Volumetric efficiency is defined as the ratio of the actual volume of the refrigerant gas pumped by the compressor to the volume displaced by the compressor pistons. A high volumetric efficiency means that more of the piston's cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases. The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder with each down stroke of the piston, and thus more refrigerant will be circulated with each revolution of the crankshaft.
The system will now have better capacity and a higher efficiency. So, the lower the discharge pressure, the less re-expansion of discharge gases to suction pressure. Also, the higher the suction pressure, the less re-expansion of discharge gases, because of the discharge gases experiencing less re-expansion to the higher suction pressure and the suction valve will open sooner.
The compressor's volumetric efficiency depends mainly on system pressures. In fact, the farther apart the discharge pressure's magnitude is from the suction pressure's magnitude, the lower the volumetric efficiency is because of more re-expansion of discharge gases to the suction pressure before the suction valve opens. Compression ratio is the ratio that measures how many times greater the discharge pressure is than the suction pressure; in other words, their relative magnitudes. Remember, a compression ratio of 10:1 indicates that the discharge pressure is 10 times as great as the suction pressure, and a certain amount of re-expansion of vapors will occur in the cylinder before new suction gases will enter.
This is why lower compression ratios will cause higher volumetric efficiencies and lower discharge temperatures. So, keep those compression ratios as low as possible by keeping condensing pressures low and evaporator pressures high.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at firstname.lastname@example.org.
Publication date: 02/06/2006