Today's Most Serious Field Problem

November 6, 2006
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The routing of compressor return gas temperature as it enters the suction screen of the compressor and flows through passages in the motor barrel and around the motor windings.
Compressor overheating is still today's most serious field problem. It is caused by high compression ratios that are a combination of high condensing (head) pressures and low evaporating (suction) pressure.

Compression ratio is the high-side absolute pressure divided by the low-side absolute pressure. If a system were running with a head pressure of 235 psig and a suction pressure of 10 psig, the compression ratio would be:

(235 + 15) Ã· (10 + 15) = 10:1
or
250 Ã· 25 = 10:1

A compression ratio of 10:1 means that the high-side pressure is 10 times greater than the low-side pressure. The higher the compression ratio, the higher the heat of compression and the hotter the discharge temperature.

The discharge temperature is the temperature measured coming out of the compressor on the discharge line. Once the discharge temperature reaches 250°F, the compressor is in danger of overheating the valve plate and valves. At 275°, failure will occur in a matter of a short time.

Most oils will start to break down at 350°. When they do, they create acids that can cause sludge and eat away at metal parts in the compressor.

As mentioned earlier, causes of high discharge temperatures are high head pressure, low suction pressure, or a combination of the both.

Common causes of high head pressure include:

• Dirty condenser.
• Air in the system.
• Too small of a condenser.
• Overcharge of refrigerant.
• High ambient temperature.
• Recirculated air over condenser.

Common causes of low suction pressure include:

• Iced evaporator coil.
• Dirty evaporator coil.
• End of the cycle.
• Defrosts set wrong.
• Suction line pressure drop.
• Dirty liquid line filter-drier.
• Dirty suction line filter or drier.
• Undercharge.
• TXV set wrong (starving).

Compressor return gas flow passages around the piston's cylinders for cooling.

DON'T FORGET SUPERHEAT

Another cause of compressor overheating is too high superheat coming back to the compressor. The total superheat coming back to the compressor has a factor on the compressor's discharge temperature. The higher the compressor superheat, the higher the compressor's discharge temperature.

You measure the temperature of the suction line entering the compressor and the suction pressure at that point, and convert it to a saturation temperature; the difference between the two is the compressor superheat.

Compressor superheat is often referred to as total superheat because it consists of suction line and evaporator superheat. Refrigerant-cooled compressors rely on their return gas to be cool enough to cool their motor windings and cylinder walls.

Compressors should have at least 20° of compressor superheat to ensure that when the TXV hunts, no liquid will come back to the compressor. However, some compressor manufacturers require that the compressor return gas temperature should not be hotter than 65°, or the gas will not be dense enough for motor winding and cylinder cooling.

Publication date: 11/06/2006

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