The Professor: Crankcase Heaters
Coping With Refrigerant Migration
Migration of refrigerant to the compressor’s crankcase during an off cycle is a serious problem. Significant compressor damage can occur if the problem is not remedied. If refrigerant migration does occur, an immediate drop in crankcase pressure will occur when the compressor starts. This will cause the refrigerant and oil mixture to explode violently and foam up through the cavities of the compressor and cause broken discharge valves and reeds.
The only sure remedy for compressor migration is an automatic pump down system. One must clear all refrigerant (liquid and vapor) from the evaporator and suction line before every off cycle. Automatic pump down is accomplished with a thermostat controlling a liquid-line solenoid in combination with a low-pressure controller terminating the on cycle once the evaporator and suction lines are void of any refrigerant. This will ensure that there is no refrigerant in the evaporator or suction line to migrate towards the compressor.
The crankcase heater is a resistance heater that is strapped on, inserted, or connected in some way to the compressor’s crankcase. The crankcase heater is designed to help combat refrigerant migration. The function of the crankcase heater is to hold the oil in the compressor’s crankcase at a temperature higher than the coldest part of the system. Refrigerant entering the crankcase will then be vaporized and driven back into the suction line.
However, the next on cycle could draw this refrigerant from the suction line and liquid slugging of the compressor may occur. Again, crankcase heaters do help in combating migration but do not prevent slugging at startup or liquid flood back once the compressor is running.
Crankcase heaters can be energized continuously (Figure 1) or during the on cycle (Figure 2). However, in order to avoid carbonizing of the oil from excessive heat, the wattage inputs of the crankcase heaters must be limited. In ambient temperatures approaching 0°F, or when exposed to cold winds, the crankcase heater may be overpowered and refrigerant migration to the compressor’s crankcase may still occur.
In one approach for dealing with this, there is a pump down of the evaporator and suction line before each off cycle, and, at the same time, energizing a crankcase heater during the off cycle. The crankcase heater is a safety precaution in case the liquid-line solenoid leaks refrigerant. The heater will prevent any refrigerant from getting to the crankcase and causing oil flash at startup. However, it will not prevent slugging or flooding of liquid refrigerant from the suction line or evaporator at startups. Another approach employs both a crankcase heater and a properly sized suction-line accumulator to protect the compressor from liquid returning to the compressor.
It is often thought that a crankcase heater will prevent migration. Crankcase heaters will keep the compressor’s crankcase warm and prevent refrigerant migration to the compressor’s oil. However, condensed migrated refrigerant will be driven from the compressor and will sit in the suction line near the compressor waiting for the next on cycle. If excessive liquid refrigerant has been driven to the suction line, severe liquid slugging may occur during startups. Frequently, compressor damage, such as broken valves and damaged pistons, will occur. Crankcase heaters can be effective in combating migration, but they will not remedy slugging at startups from liquid flood back, unless used in conjunction with a properly sized suction
Publication date: 3/31/2014