One diagnostic reading that techs seldom take or look at is the Discharge Line Temperature. The discharge temperature of a compressor can usually tell the servicing technician what is happening inside an A/C system.
The discharge of the compressor is the hottest part of the system. This temperature can be read by placing an insulated temperature sensor (or thermocouple) on the discharge line about two to four inches from the compressor. Be very careful when doing this since the discharge line could be anywhere from 185 degrees F to 275 degrees F. 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.
High condensing pressures
- High system superheat
- Low suction pressures
- High compression ratios.
High condensing pressures are usually caused by dirty condenser coils, inoperative condenser fans, bad fan capacitors, overcharging, or non-condensable in the system. Higher condensing temperatures cause higher condensing pressures. In turn, the compressor has to work harder and generates more heat of compression in compressing the suction pressure to the higher condensing pressures. This also causes high amperage draws on the compressor.
High system superheat is typically a result of an evaporator being starved of refrigerant. Starving is caused by a TXV or orifice under-feeding, plugged liquid line filter-drier, kinked or restricted liquid line or any other form of refrigerant restriction. Since we are “starving” the coil, the refrigerant picks up more “heat” in the evaporator. 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. Also keep in mind that too much air flow across the evaporator will cause high saturated suction temperature. The increase load on the coil transfers too much heat to the refrigerant. More refrigerant is vaporized, elevating the temperature & pressure. The hotter refrigerant entering the compressor needs to be removed. The condenser requires more of the condenser surface to reject the heat. More condenser is needed, less room for sub-cooling. This equates to a lower sub-cooled refrigerant and it becomes a vicious cycle until the compressor fails.
Low suction pressures are generally caused by system undercharged, under feeding TXV or metering device , faulty evaporator fan, not enough air flow, plugged suction line filter-drier or compressor inlet screen, frosted evaporator coils, low or no evaporator load, or any form of undesired restriction in the suction line (improper line size). Here again, the compressor can only pump what it can see so it 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. By having one or the other pressure out of normal range, the compression ratio in the compressor changes.
The high limit temperature is always a maximum of 250 degrees F, but 225 degrees F is a more favorable high end 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 and “carbonization” of the oil. 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. Copeland compressors state that the discharge temperature should never be over 225 and an average discharge temperature should be around 180 to 200 degrees F. on a normally operating system.
By adding this diagnostic measurement to your “arsenal” of diagnostics, you can really see what is going on with a system. Again, I emphasize that most of the causes of high discharge temperature are still external to the refrigerant itself (either under or over-charged or non-condensables). Dirty coils, filters, not enough air flow, too much air flow, etc, can all lead to high discharge temperatures.
I strongly encourage you to make use of this diagnostic measurement along with the rest of your diagnostic temperatures and pressures. By using all the tools and filling in the A/C – HP Critical Reading diagnostic form, finding the true problem will be a lot easier.
(Thanks to Ed Schmidt (JCI) for making the suggestion and for some of the information in this posting).