The low side of a refrigeration system usually houses the evaporator, suction line, and any accessory components located within the suction line. Accessory components can include evaporator pressure regulator valves, suction filters and driers, crankcase pressure regulator valves, and suction accumulators.
The suction inlet to the compressor often contains a screen or filter, which can become obstructed. Even though it seems that the majority of refrigeration system restrictions occur in the high side of the system, service technicians must be aware of evaporator and suction line restrictions and their symptoms when strategically troubleshooting a system.
Oil-logging is often a major cause of low-side restrictions. There are many ways in which evaporator and suction lines inside tubing can become oil-logged, including:
- Wrong type or viscosity of oil.
- Too much oil in the system.
- Thermostatic expansion valve (TXV) out of adjustment (too little superheat causing a refrigerant-flooded compressor).
- Not enough defrosts for low-temperature application machines.
- System not piped correctly (no oil traps or piping too large).
- A liquid-refrigerant flooded compressor circulating oil at startup.
- Liquid refrigerant migration during the off-cycle, causing crankcase oil foaming on startups.
MIGRATION AND FLOODING
Refrigerant migration and system flooding conditions are often two of the reasons why system lubricant or oil escapes the compressor’s crankcase and enters the system. Refrigerant migration deals with refrigerant migrating back to the compressor’s crankcase during the off-cycle. This migration is due to a pressure difference between the oil in the compressor’s crankcase and the refrigerant. 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, thus circulating the oil throughout the refrigeration system. This oil usually gets caught in the evaporator and can cause an oil-logged evaporator. This is actually a restriction in the low side and will cause low suction pressures.
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 can 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. Again, excessive oil in the system will get caught in the evaporator and cause an oil-logged evaporator and suction line.
Compressors can become noisy due to the lack of lubricating oil, which has left the crankcase and ended up in the low side of the system. Metallic sounds may be heard from lack of lubrication or parts out of tolerance from excessive wear, because oil is a sound deadener as well as a lubricant. If 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, which would cause a low oil level in the compressor’s crankcase sight glass.
Many times, a compressor that is flooded with refrigerant will turn into an oil pumper. The crankcase will foam 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. However, oil will continue to get into the system if the flooding situation is not remedied.
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, there is no such thing as a perfect system in the real world. Inevitably, the compressor’s lubricating oil will escape the compressor’s crankcase and enter the system’s tubing. Hopefully, this escaped oil will eventually 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 and the piping arrangements that brings the oil back to the compressor’s crankcase. That is why it is so important to have accurate line and coil sizes to support the correct refrigerant velocity for proper oil return. Just as important are appropriately sloped lines and correct location of P-traps.
METERING DEVICE PROBLEMS
The TXV may also see too much oil passing through it. The evaporator 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, which could make the TXV run a low superheat and flood or slug the compressor.
Often the sight glass in the liquid line will be discolored with a yellowish or brown tint from refrigerant and oil flowing through it. Because the TXV may be running low superheat, this will cause the compressor superheat to run lower. 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.
Oil usually logs in the evaporator because it is the coldest component with the largest tubes and thus has the slowest refrigerant flow rate. Oil in the evaporator will coat the inner wall of the coil and reduce the heat transfer through the walls, causing 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. Some compressors have an oil sight glass for visual inspection of the oil level in the compressor’s crankcase.
If the viscosity (thickness) is too high, oil will have a hard time returning 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 of oil is used. Also, 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.
Being aware of the symptoms of evaporator and suction line restrictions can make it easier for service technicians to pinpoint problems when strategically troubleshooting a system.
Publication date: 5/6/2019