The Professor: Systematically Troubleshoot an Evaporator
As the article pointed out, refrigerant migration can cause the compressor’s crankcase to lose its oil, thus circulating the oil throughout the refrigeration system. This oil in circulation usually gets caught in the evaporator and can cause an oil-logged evaporator.
TROUBLESHOOTINGSystematic troubleshooting using a system check sheet is still the best method for the conscientious service technician to pinpoint hard-to-find system problems. Here we will explore how evaporators can become oil logged, and include symptoms with explanations for a system with an oil-logged evaporator. This refrigeration system is a low-temperature system using R-134a. It has a thermostatic expansion valve (TEV) for the metering device and a receiver at the condenser outlet.
Listed below are ways an evaporator can become oil logged:
• A flooded compressor circulating oil at start up.
• Too much oil in the system.
• System not piped correctly (no oil traps or piping too large).
• Liquid migration during off cycle, causing crankcase oil foaming on start ups.
• TXV out of adjustment (too little superheat - flooded compressor).
• Not enough defrost periods for low-temperature system.
• Wrong oil viscosity.
OIL IN THE SYSTEMOil in a refrigeration system has many functions:
• Deadens noise.
• Transfers heat (cools).
• Reduces friction.
• Minimizes mechanical wear.
• Seals valves (prevents blow-by in valves and other mechanical parts).
Oil usually logs in the evaporator because it is the coldest component with the largest tubes, thus the slowest refrigerant velocity. Oil logged in the evaporator coats the inner wall of the coil and reduces the heat transfer through the walls. This will cause a loss of capacity and poor performance. The compressor is robbed of some of its crankcase oil and run with a lower-than-normal oil level. This may score or ruin mechanical parts in the compressor.
Too high of a viscosity oil will also be hard to return from an evaporator and will surely cause oil logging. Usually, heat from the defrost heaters will warm and thin the oil in the evaporator, so it can be returned to the compressor once the compressor starts up. This will happen only if the right viscosity (thickness) of oil is used.
If a suction line is oversized, the refrigerant velocity will be decreased. This will prevent the oil from moving through the suction line to the compressor’s crankcase. Remember, it is the refrigerant velocity that will move the oil through the refrigeration system’s piping.
Figure 1 shows a system check sheet for an oil-logged evaporator. Pressures and temperatures will vary depending on the severity of the logging.
• Noisy compressor.
• Low oil level in sight glass on compressor’s crankcase.
• TXV having a hard time controlling superheat (hunting).
• Low evaporator and compressor superheat.
• Warmer-than-normal box temperatures with loss of capacity.
Noisy compressor: Compressor may be noisy because of lack of oil. Metallic sounds may be heard from lack of lubrication, or parts out of tolerance from excessive wear. Oil is a sound deadener as well as a lubricant.
Low oil level in sight glass: Because a lot of the oil is in the evaporator, the crankcase will be low on oil. In fact, the entire system’s components excluding the compressor may have too much oil. This would cause a low oil level in the compressor’s crankcase sight-glass. Many times a compressor that is flooding with refrigerant will turn into an oil pumper. The crankcase will be foaming from the liquid refrigerant flashing in it. Small oil droplets entrained in the oil will be pumped through the compressor. This will oil log many components in the system.
The velocity of the refrigerant traveling through the lines and P-traps will try to return the oil from the system to the crankcase. Even an oil separator in the compressor’s discharge line may have a hard time keeping up with excess oil in circulation. However, oil will continue to get into the system if the compressor-flooding situation is not remedied.
TXV having a hard time controlling superheat: The TXV can also see too much oil passing through it. The evaporator’s tailpipe will be oil logged and the inside of the tubes will be coated with oil. The remote bulb of the TXV at the evaporator outlet will have a hard time sensing a true evaporator outlet temperature because of the reduced heat transfer through the line. The TXV will hunt and keep trying to find itself. A constant superheat will not be maintained.
The TXV remote bulb may sense a warmer-than-normal temperature from the oil insulating the inside of the line. This could make the TXV run a low superheat, and flood or slug the compressor with refrigerant. Often the sight glass in the liquid line will be discolored, with a yellowish or brown tint from refrigerant and oil flowing through it.
Technicians may confuse this low superheat reading with an overcharge of refrigerant. However, an overcharge will give high head pressures and high condenser subcooling readings. TXV systems usually can tolerate a bit of an overcharge and still hold a good evaporator superheat if set properly. However, once head pressures get too high, the TXV will soon overfeed the evaporator and show low superheat.
Low compressor superheat: Because the TXV may be running low superheat, this can cause the compressor (total) superheat to run lower.
Warmer-than-normal box temperatures with capacity losses: Because of the reduced heat transfer in both the condenser and evaporator from the excess oil coating the inner tubing, capacity will be decreased. The compressor will run longer, trying to maintain a desired box temperature. Evaporator temperatures and pressures may run low because of the reduced heat transfer, a result of the oil insulating the evaporator tubes. This will cause reduced mass flow rates and low evaporator pressures.
Service technicians must recognize the symptoms caused by excessive oil circulating in a refrigeration system. The service check sheet can be your No. 1 tool to help you recognize this hard-to-detect problem.
Publication date: 07/05/2010