The discharge temperature of a compressor can usually tell the servicing technician what is happening inside an a/c or refrigeration system.  The
discharge of the compressor is the hottest part of the system.  It's temperature can be read by placing an insulated temperature sensor (ones that are shaped to fit piping work better) on the discharge line about two inches from the compressor.  The back side of the discharge valve is actually the
hottest part of the system but since it can't be accessed, the discharge line is the next best spot to read the temperature.  Because the temperature
"describes" what's happening inside the compressor, monitor it carefully.

In general, the biggest reasons for high discharge temperature are:
- High condensing pressures
- High system superheat
- Low suction pressures
- High compression ratios

High condensing pressures are usually caused by dirty or undersized condenser coils, inoperative condenser fans, lack of or too hot water supply (water cooled systems), overcharging, or non-condensable in the system.  Higher
condensing temperatures cause higher condensing pressures.  In turn, the compressor has to work harder and generate more heat of compression in
compressing the suction pressure to the higher condensing pressures.

High system superheat is typically a result of an evaporator being starved of refrigerant.  Starving is caused by a TXV or cap. tube underfeeding, plugged
liquid line filter-drier, or any other form of refrigerant restriction.  The compressor discharge temperature reflects the latent heat absorbed in the
evaporator, evaporator superheat, suction line superheat, heat of compression, and compressor motor-generated heat.  All of this heat is accumulated at the compressor discharge and must be removed.

Low suction pressures are generally caused by system undercharge, TXV or cap. tube underfeed, faulty evaporator fan, plugged suction line filter-drier or compressor inlet screen, frosted evap. coils, low or no evaporator load, or
any form of undesired restriction in the suction line.  Here again, the compressor must work harder, therefore, higher heat of compression is
generated in compressing a low suction pressure to a suitable condensing pressure.

High compression ratios can be caused by either low suction pressure, high head pressure, or a combination of the two.  The higher the compression ratio, the higher the discharge temperature will be at the compressor.  This is due to heat of compression generated when the gasses are compressed through a greater pressure range.

The high limit temperature is always about 250 degrees F, but 225 degrees F is a more favorable temperature.  Should the discharge temperature exceed 250 degrees F, the system will likely begin to fail.  Failure will be in worn rings, acid formation, and oil breakdown.  An important thing to remember is that if the discharge line is 250 degrees F, the actual discharge valve is approximately 75 degrees F hotter, meaning that the valve itself may be 325
degrees F.  Most refrigeration oil will begin to break down and vaporize at 350 degrees F.  In that condition, serious overheating problems result.

  Overheating is the most serious compressor problem in today's a/c and refrigeration systems' compressors.  Simple measures can and should be taken to make minimal the chances of compressor overheating.  Routine THOROUGH preventative maintenance is the best.  If one detrimental condition of the system is overlooked or ignored, it's effect can quickly snowball into increasing numbers of more serious, costly, reputation-fouling problems.

So, keep a close eye on that compressor discharge temperature!!

Mark

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