Short Answer:

A crankcase heater is an electrical heating device used in refrigeration and air-conditioning compressors to keep the compressor oil warm during periods when the system is not operating. It prevents refrigerant from condensing and mixing with the oil in the crankcase. This helps in avoiding damage to the compressor during start-up.

By maintaining the temperature of the oil above the refrigerant condensation point, the crankcase heater ensures smooth lubrication, reduces wear, and improves the life of the compressor. It is usually installed at the bottom of the compressor crankcase and operates automatically when the compressor is off.

Detailed Explanation:

Crankcase Heater

A crankcase heater is an important accessory used in refrigeration and air-conditioning systems. Its main function is to keep the compressor crankcase warm when the system is not running. During the off-cycle, the refrigerant in the system can migrate to the compressor and condense inside the crankcase oil due to low temperature. When the compressor starts again, this refrigerant boils rapidly, causing foaming of oil and loss of lubrication. This can severely damage the compressor.

To prevent this, a crankcase heater is installed. It provides a small amount of heat continuously or automatically during the off period to maintain the oil temperature slightly above the evaporating temperature of the refrigerant. This prevents the refrigerant from condensing in the crankcase and keeps the compressor ready for safe operation at all times.

Construction of Crankcase Heater

The crankcase heater is generally a small electrical resistance heater made of materials such as nichrome wire or other heating elements. It is designed to operate on standard electrical supply (230V AC or 110V AC) depending on the equipment. The heater may be of various forms depending on the type of compressor and manufacturer. Common designs include:

Belt Type Heater:
This is a flexible heater strap wrapped around the lower portion of the compressor crankcase. It provides uniform heating and is easy to install or replace.
Insertion Type Heater:
This type is inserted directly into an opening or well provided in the compressor crankcase. It heats the oil from inside and is commonly used in large compressors.
Base Plate Type Heater:
In this design, the heater is fixed at the base of the compressor. It transfers heat through conduction from the bottom surface.

Each type is designed to maintain the oil at a temperature high enough to prevent refrigerant condensation without overheating or degrading the oil.

Working of Crankcase Heater

The working principle of a crankcase heater is simple. When the refrigeration or air-conditioning system is turned off, the compressor remains stationary, and pressure inside the system becomes equal. At this time, refrigerant tends to migrate to the coldest part of the system, which is usually the compressor crankcase. Inside the crankcase, the refrigerant vapor condenses and mixes with the lubricating oil.

When the system starts again, the sudden pressure drop causes the dissolved refrigerant to vaporize violently, creating foam and forcing oil out of bearings. This results in poor lubrication, mechanical wear, and even compressor failure.

The crankcase heater prevents this by maintaining a slightly higher temperature (5–10°C above ambient) in the crankcase. The continuous gentle heating keeps the oil warm, discourages refrigerant migration, and ensures the oil remains pure and effective.

In modern systems, the crankcase heater may be automatically controlled by relays or thermostats. It switches on when the compressor stops and turns off when the compressor starts, ensuring efficient energy use.

Functions and Advantages of Crankcase Heater

Prevents Refrigerant Migration:
During the off-cycle, refrigerant vapor tends to move toward the crankcase. The heater keeps the crankcase warm, preventing condensation inside it.
Ensures Proper Lubrication:
By keeping oil and refrigerant separate, the heater maintains oil viscosity and ensures the moving parts of the compressor are well-lubricated.
Prevents Oil Foaming:
When refrigerant mixes with oil and the compressor starts, the sudden expansion of refrigerant can cause foaming. The heater eliminates this problem.
Reduces Mechanical Wear:
Proper lubrication minimizes friction and wear between the compressor’s mechanical parts, increasing compressor life.
Improves Compressor Starting:
A warm crankcase helps the compressor start easily and reduces stress on electrical components.

Location and Installation

The crankcase heater is installed either around the bottom of the compressor shell or inside the crankcase, depending on the type. It should always be positioned in such a way that heat spreads evenly across the oil. The heater must be connected to a reliable power source and, if necessary, controlled through relays linked to the main control circuit of the compressor.

Before charging refrigerant into a new system, the crankcase heater should be switched on for several hours to drive out any refrigerant vapors that may have entered the oil during installation or storage.

It is also essential to ensure that the heater is properly insulated to avoid electrical hazards and unnecessary heat loss.

Applications

Crankcase heaters are widely used in:

Large commercial and industrial refrigeration systems
Air-conditioning units, especially in low ambient temperatures
Systems that frequently cycle on and off
Compressors installed outdoors or in cold environments

They are particularly important for reciprocating and scroll compressors, which are more sensitive to refrigerant migration.

Conclusion

A crankcase heater is a simple but essential device that protects the compressor from damage caused by refrigerant migration and oil dilution. By keeping the oil warm and pure, it ensures proper lubrication, prevents foaming, and extends the compressor’s life. Without a crankcase heater, the reliability and efficiency of a refrigeration or air-conditioning system can be seriously affected. Thus, its use is a vital safety and performance feature in all modern refrigeration systems.