In most refrigeration and air conditioning systems, refrigerant will want to travel, or migrate, to a place where the pressure is the lowest. During the off-cycle or especially during a long shutdown, the compressor’s crankcase is usually the location that has the lowest pressure. Refrigerant migration happens because fluids travel from a place of higher pressure to a place of lower pressure. In fact, refrigerant migration is defined as refrigerant, either liquid or vapor, traveling to the compressor’s suction line or crankcase during the compressor’s off-cycle.
The crankcase usually has a lower pressure than the evaporator because of the oil it contains. Oil has a very low vapor pressure, and refrigerant will flow to it whether the refrigerant is in vapor or liquid form. If the oil did not have a very low vapor pressure, it would vaporize every time a low pressure exists in the crankcase, or a vacuum was pulled on it.
If the crankcase has a heater, the vapor will be forced away from the crankcase and end up in the suction line. This refrigerant may condense in the suction line and cause slugging in the compressor’s cylinders on start-up. This can often happen if the compressor is located outdoors in a cold ambient, as this will amplify the lower vapor pressure area and also help condense the vapor to liquid.
The crankcase heater helps keep the oil in the crankcase free of refrigerant from refrigerant migration. Even if the compressor does not slug a mixture of refrigerant/oil foam, the amp draw will be very high from the high density saturated vapors being pulled into the cylinders from the crankcase.
Because refrigeration migration can occur with refrigerant vapor, the migration can occur uphill or downhill. Once the refrigerant vapor reaches the crankcase, it will be absorbed and condense in the oil. Refrigerant and oil have a strong attraction for one another and mix very well. Since liquid refrigerant is heavier than oil, the liquid refrigerant will be on the bottom of the oil in the crankcase.
The amount of refrigerant absorbed in the oil depends on the temperature of the oil and the pressure in the crankcase. The lower the temperature and the higher the pressure, the more refrigerant will be absorbed. On short off-cycles, the migrated refrigerant does not have a chance to settle under the oil but does still mix with the oil in the crankcase. When the compressor does turn on, the sudden pressure drop on the crankcase containing liquid refrigerant and oil will cause the refrigerant in the oil to flash to a vapor. This is called a flooded start, and it causes violent foaming in the crankcase. The oil level in the crankcase will then drop, and mechanical parts — especially bearings — will be scored from inadequate lubrication.
The foaming action of the oil and refrigerant can also restrictions in the oil passages and cause pressure to build. The crankcase pressure will rise, and the mixture of refrigerant and oil foam can now be forced through compressor passages and around piston rings and be pumped by the compressor. Not only does this situation cause loss of oil from the crankcase to the system, but it can also cause a mild form of slugging in the compressor’s cylinders. High compressor current draw, which will lead to motor overheating usually, follows. Also, broken or warped valves can occur as a result of overheating and/or slugging.
Once the valves are broken or warped, the amp draw will be low due to pressure leakages from the high side of the system to the low side of the system. The only sure remedy to refrigerant migration is through the use of an automatic pump-down cycle.
Consider this scenario. A service technician arrives at a store to troubleshoot a problem with a glass door reach-in freezer. The store owner explains that there has been a gradual increase in case temperatures of a glass door reach-in freezer from 0°F to 15°F over the last three weeks. Customers have been complaining of a warm product load, and the store owner also lets the technician know that the condensing unit has a 100% run time in trying to keep the product partially cold.
The technician enters the basement and notices that the condensing unit is a 3-hp, R-404A, semi-hermitic reciprocating compressor with a forced air, standard efficiency condenser. The system also has a thermostatic expansion valve (TXV) and a receiver. The temperature in the basement is 70°F.
The technician installs gauges on the low and high side of the compressor and takes an amp reading. The pressure on the low side reads 33.5 psig (0°F), and the high side reads 174 psig (80°F). The amp reading was 4 amps under rated load amps (RLA) on the name plate.
The technician then puts a temperature probe on the condenser and evaporator outlets in order to get a condenser subcooling and an evaporator superheat reading respectively. The condenser outlet temperature reads 71°F, and the evaporator outlet temperature reads 14°F. This would give the system 9°F of condenser subcooling:
|(Condensing temperature)||- (Condensing outlet temperature)||= Condenser subcooling|
|80°F||- 71°F||= 9°F|
Evaporator superheat calculations revealed that there is 14°F of evaporator superheat:
|(Evaporator outlet temperature)||- (Evaporator temperature)||= Evaporator superheat|
|14°F||- 0°F||= 14°F|
|Evaporating temperature||0°F (33.5 psig)|
|Condensing temperature||80°F (174 psig)|
The technician realizes the unit cannot be low on refrigerant because of the 9°F of condenser subcooling, which signifies that there is liquid in the condenser. Also, evaporator superheat is usually much higher than 14°F on systems that are low on charge.
After thinking a moment, the technician wonders why the condensing temperature is only 10°F hotter than the ambient or surrounding temperature in the basement. This is an indication that the condenser is not rejecting very much heat from the system to the basement air. The difference in temperature between the condenser temperature and the ambient air is often called the “condensing split,” and it should run from 25° to 30°F on standard efficiency condensers under normal heat loads. The technician also realizes that this system actually has low head pressure with higher-than-normal suction pressure.
Analysis and Remedy
The technician sits down and analyzes the system check again. A dirty condenser would give a high condensing temperature and pressure, while a dirty evaporator would give a low evaporating temperature and pressure. So, it cannot be either of those two. If the liquid line or metering device were restricted, the evaporating temperature and pressure would be low, but it is high. The technician wonders what would cause a high evaporating temperature (pressure), low condensing temperature (pressure), and low amp draw.
The technician then realizes that the compressor’s valves or piston rings could be worn and leaking. This would cause leakage of pressure between the high and low side of the system as the pistons reciprocated up and down, causing a lower condensing pressure with a higher evaporating pressure. The amp draw also would be low because of this pressure leakage within the cylinders and/or valves.
The technician pumps down the compressor and examines the valves and valve plate. Sure enough, the valves are not seating properly and are warped. A new set of valves with gaskets for the valve plate and head are installed, and the compressor is evacuated and put into commission. After about two hours of running time, the system pulls down to 0°F. A new system check is taken and everything seems to be running right:
|Evaporating temperature||-16°F (20 psig)|
|Condensing temperature||96°F (222 psig)|
Both the condensing pressure and evaporating pressure are normal. Because the freezer is often shut down for times, the service technician blames the valve problem on refrigerant migration causing wet compression over time. The technician recommends to the store owner that it would be advantageous to have an automatic pump-down system installed on the system to combat the migration problem.