Little by little, a compressor’s lubricating oil will eventually escape the compressor’s crankcase and enter the system’s tubing. Hopefully, this escaped oil will slowly find its way through the condenser, receiver, liquid line, metering device, evaporator, and suction line and end up back in the compressor’s crankcase. It is the velocity of the refrigerant traveling through the system, as well as the piping arrangement, that brings the oil back to the compressor’s crankcase. That is why it is so important to have the proper line and coil sizes to support the correct refrigerant velocity for proper oil return.
Just as important for proper oil return to the compressor are properly sloped lines and proper location of P-traps. Ideally, the perfect situation for the oil in a refrigeration or air conditioning system would be for it to stay inside the compressor’s crankcase to lubricate the compressor’s moving parts. However, because of ever-changing heat loads on the system and varying system conditions caused from refrigerant undercharges or overcharges, inoperative valves, faulty or misadjusted metering devices, dirty condensers, dirty evaporators, fan motor failures, or plugged filter driers, this is unlikely to happen in the real world.
Escaping Crankcase Oil
Refrigerant migration and system flooding are often two of the reasons why the system lubricant or oil escape the compressor’s crankcase and enter the system’s tubing.
Refrigerant flooding refers to liquid refrigerant entering the compressor’s crankcase during the on or running cycle. Flooding can cause flashing of the oil in the compressor’s crankcase because of the liquid refrigerant boiling under the oil and also cause excessive pressures in the crankcase. This phenomenon can also cause the compressor to lose its oil and circulate it throughout the refrigeration system. Excessive oil in the system will again get caught in the evaporator and cause an oil-logged evaporator.
Refrigerant migration involves refrigerant migrating back to the compressor’s crankcase during the off-cycle. Migration is an off-cycle phenomenon and is due to a pressure difference between the oil in the compressor’s crankcase and the refrigerant itself. Oil has a very low vapor pressure and will attract the refrigerant in both the vapor and liquid states. Refrigerant migration can cause the compressor’s crankcase to lose its oil at start-up, 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. As can be imagined, there are several problems that can occur with an oil-logged evaporator, as well as when the compressor’s crankcase low on oil.
Some of the ways in which an evaporator’s inside tubing can become oil logged include:
- Not enough defrost periods for low temperature application systems;
- System not piped correctly (no oil traps or piping too large);
- A liquid refrigerant flooded compressor circulating oil at start-up;
- Liquid refrigerant migration during the off-cycle causing crankcase oil foaming on start-ups;
- Wrong type or viscosity of oil;
- Too much oil in the system; and
- Thermostatic expansion valve (TXV) out of adjustment (too little superheat causing a refrigerant-flooded compressor).
The oil in a refrigeration or air conditioning system has many functions, including mechanical lubrication (which minimizes mechanical wear), noise deadening and heat transfer, reducing friction, and valve sealing, which 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 will coat the inner wall of the coil and reduce the heat transfer through the walls, which will cause a loss of capacity and poor performance. The compressor will be robbed of some of its crankcase oil and run with a lower than normal oil level, which may score or ruin mechanical parts in the compressor. Many larger capacity compressors have an oil sight glass for visual inspection of the oil level in the compressor’s crankcase. If the viscosity of the oil is too high, it will also be hard to return from an evaporator and will surely cause oil logging. Usually the 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.
Proper Suction Line Sizing
It is of utmost importance for the designer of the refrigeration or air conditioning system to properly size the suction line. If a suction line is oversized, the refrigerant velocity will be decreased, which 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.
The refrigeration or air conditioning service technician must also make sure that the right size (diameter and length) suction line be installed back into the system if one is to be replaced for damage or for other service reasons. An undersized suction line can cause restriction to refrigerant flow and give low suction pressures with higher compression ratios.
Listed below is a system check for a severely oil-logged evaporator. The system is an R-134a system incorporating a TXV as a metering device. Pressures and temperatures will vary depending on the severity of the oil logging. This type of service check is the number one tool in helping technicians recognize that they have this hard-to-detect problem.
|Compressor discharge temperature||190°F|
|Condenser outlet temperature||78°F|
|Evaporator outlet temperature||-18°F|
|Compressor inlet temperature||-13°F|
|Surrounding ambient temperature||75°F|
|Refrigerated box temperature||10°F|
|Low-side (evaporating) pressure/temperature||3.5 inches Hg/(-20°F)|
|High-side (condensing) pressure/temperature||104 psig/(90°F)|
- Low evaporator and compressor superheat;
- Warmer-than-normal box temperatures with loss of capacity and lower-than-normal suction pressure;
- Low oil level in sight glass on compressor's crankcase;
- TXV having a hard time controlling superheat (hunting); and
- Noisy compressor.
Low compressor superheat: Because the TXV may be running low superheat, this will 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 from the oil insulating the evaporator tubes. This will cause reduced mass flow rates and low evaporator pressures.
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, 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, and 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, but 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 will see too much oil passing through it. The evaporator’s tailpipe will be oil logged and the inside of the tubes 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, and 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 of refrigerant will give high head pressures and high condenser subcooling readings. TXV systems can usually tolerate a bit of an overcharge and still hold a good evaporator superheat if set properly. However, once the head pressures get too high, the TXV will soon overfeed the evaporator and show low superheat.
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. Remember, oil is a sound deadener as well as a lubricant.
As can be seen, it is important to design refrigeration and air conditioning systems with proper oil return to the compressor. Otherwise, oil will coat the inner walls of the evaporator, reducing heat transfer and causing a loss of capacity and poor performance.