Crankcase heater
Encyclopedia
A crankcase heater is an electrical component in a compressor
in an air-conditioning system, heat pump
system, or a chiller
system. The crankcase heater is normally ON all the time even when the unit is not running, though temperature sensors and set points may be used to turn it off when not needed. A crankcase heater's sole purpose is to prevent refrigerant migration and mixing with crankcase oil, when the unit is off, and to prevent condensation of refrigerant in the crankcase of a compressor. The crankcase heater is designed to keep the refrigerant at a temperature higher than the coldest part of the system. A crankcase heater has the same electrical symbol as a resistor
because it converts electricity directly into heat via electrical resistance
. The resistance in the heater coil determines the heat it will produce when voltage is applied.
starts to turn.
migration from occurring. Refrigerant vapor always migrates to the coldest part of the system (the compressor). The refrigerant migrates to the compressor crankcase where it is attracted to the compressor oil. This refrigerant vapor condenses and will return to a liquid in the off cycle. On the next start of the compressor, the oil is in a watery state and washes the bearings out, which leads to locked-up, frozen or totally burnt-out compressors. The temperature is sensed at the compressor, indoor coil, and outdoors. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, then the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures, the crankcase heater is de-energized.
Crankcase heat is common in air conditioning (cooling) systems where R-22 has been used for many years. The home owner often shuts off the disconnect to the outdoor condensing unit for the winter, leaving the compressor without crankcase heat. If the home owner then starts the unit without some time for the heat to boil the refrigerant out the oil, damage will likely occur to the compressor. On start-up, the crankcase pressure reduces as soon as the compressor starts to turn. The refrigerant will boil and turn the oil to foam. The oil and refrigerant (some of the refrigerant may be in liquid state) will be pumped out of compressor. Valve and bearing damage may occur and compressor may be operated with a limited oil charge until it returns to the crankcase from the evaporator.
Refrigeration compressors typically operate all seasons and will not have this seasonal shutdown, but if they are shut down for a period of time, the preceding situation will occur, so it is likely that crankcase heat will become popular.
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
in an air-conditioning system, heat pump
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
system, or a chiller
Chiller
A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required.-Use in air conditioning:...
system. The crankcase heater is normally ON all the time even when the unit is not running, though temperature sensors and set points may be used to turn it off when not needed. A crankcase heater's sole purpose is to prevent refrigerant migration and mixing with crankcase oil, when the unit is off, and to prevent condensation of refrigerant in the crankcase of a compressor. The crankcase heater is designed to keep the refrigerant at a temperature higher than the coldest part of the system. A crankcase heater has the same electrical symbol as a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
because it converts electricity directly into heat via electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
. The resistance in the heater coil determines the heat it will produce when voltage is applied.
Installation
Some manufacturers of air conditioning equipment install crankcase heaters either in the compressor (insertion type), or mount them externally around the base of the compressor (bellyband type). The two wires of the heater are wired directly to the input side of the contact from the main power supply, and power is always supplied to the heater. Caution should be used since the heater can become very hot and cause second degree burns as well as cause electrical danger due to live circuits.Start-up
Most manufacturers of air conditioning require that the heater needs to be energized for 24 hours before start-up of the system. The compressor must have crankcase heat before start-up or damage will be done to the compressor. The crankcase pressure of the compressor always drops when the compressorGas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
starts to turn.
Operation
While the system is running, there is enough heat being generated by the compressor running to prevent refrigerantRefrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
migration from occurring. Refrigerant vapor always migrates to the coldest part of the system (the compressor). The refrigerant migrates to the compressor crankcase where it is attracted to the compressor oil. This refrigerant vapor condenses and will return to a liquid in the off cycle. On the next start of the compressor, the oil is in a watery state and washes the bearings out, which leads to locked-up, frozen or totally burnt-out compressors. The temperature is sensed at the compressor, indoor coil, and outdoors. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, then the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures, the crankcase heater is de-energized.
Crankcase heat is common in air conditioning (cooling) systems where R-22 has been used for many years. The home owner often shuts off the disconnect to the outdoor condensing unit for the winter, leaving the compressor without crankcase heat. If the home owner then starts the unit without some time for the heat to boil the refrigerant out the oil, damage will likely occur to the compressor. On start-up, the crankcase pressure reduces as soon as the compressor starts to turn. The refrigerant will boil and turn the oil to foam. The oil and refrigerant (some of the refrigerant may be in liquid state) will be pumped out of compressor. Valve and bearing damage may occur and compressor may be operated with a limited oil charge until it returns to the crankcase from the evaporator.
Refrigeration compressors typically operate all seasons and will not have this seasonal shutdown, but if they are shut down for a period of time, the preceding situation will occur, so it is likely that crankcase heat will become popular.