ACHRNEWS

Oil-Logged Evaporators

February 6, 2002
Systematic troubleshooting using a system check sheet is still the best method for the conscientious service technician to pinpoint hard-to-find system problems. This article explores how evaporators can become oil logged, and includes symptoms with explanations of a system with an oil-logged evaporator.

This low-temperature, R-134a refrigeration unit has a thermostatic expansion valve (TXV) for the metering device and a receiver at the condenser outlet.

Oil usually logs in the evaporator because it is the coldest component with the largest tubes, thus it has the slowest refrigerant velocity.

Here are some ways an evaporator can become oil logged:

  • A flooded compressor is circulating oil.
  • Too much oil is in the system.
  • The system is not piped correctly (there are no oil traps or the piping is too large).
  • There is liquid migration during the off cycle.
  • TXV is out of adjustment (too little superheat-flooding).
  • There aren’t enough defrosts for low-temperature machines.
  • The wrong type or viscosity of oil was used.
Oil logged in the evaporator will coat the inner wall of the coil and reduce heat transfer through the walls. This will cause a loss of capacity and poor performance.

The compressor will be robbed of some of its crankcase oil and will run with a lower-than-normal oil level. This may score or ruin mechanical parts in the compressor. Table 1 is a system check sheet for an oil-logged evaporator. Pressures and temperatures will vary depending on the severity of the oil logging.

SYMPTOMS

Some symptoms of an oil-logged evaporator include:
  • Noisy compressor;
  • Low oil level in the sight glass on the compressor’s crankcase;
  • TXV has a hard time controlling superheat (hunting);
  • Low evaporator and compressor superheat; and
  • Warmer-than-normal box temperatures with loss of capacity.
Here is a more-detailed look at those symptoms.

Noisy compressor: The compressor may be noisy because of the lack of oil. Metallic sounds may be heard due to the lack of lubrication or because parts are out of tolerance from excessive wear. Oil is a sound deadener as well as a lubricant.

Low oil level in compressor’s sight glass: Because a lot of the oil is in the evaporator, the crankcase will be low on oil. In fact, all of the 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 foam 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. Oil will continue to get into the system if the flooding is not remedied.

TXV has a hard time controlling superheat: The TXV will 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.

As a result, 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 yellow or brown tint from refrigerant and oil flowing through it.

Low compressor superheat: If the TXV is running low superheat, this will cause the compressor superheat to run lower.

Warmer-than-normal box temperatures with capacity losses: Because of the reduced heat transfer in both the condenser and evaporator (caused by 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 from the oil insulating the evaporator tubes. This will cause reduced mass flow rates.

Service technicians must be able to recognize the symptoms brought forth by excessive oil circulating in a refrigeration system. The service check sheet is the number-one tool for helping technicians recognize that they have this hard-to-detect problem.

Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and author of the book, Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at tomczykj@tucker-usa.com (e-mail).

Publication date: 02/11/2002