What are Refrigeration and Air-Conditioning Compressor Heaters Used For

Figure 1: Compressor crankcase heater.

The compressor heater (oil heater, crankcase heater, compressor heating belt) is a part of the refrigeration compressor. Although this part has nothing to do with the performance parameters of the compressor, it plays a key role in the normal start of the compressor.

Usually, the compressor manufacturer will remind in the instruction manual that the oil heater should be started first before starting the compressor for the first time or after a long-term shutdown. But usually this does not attract people's attention, and even many operators do not connect the oil heater to the electrical system, which causes many unnecessary failures.

Why is an oil heater needed? Let's first understand what will happen due to improper starting of the compressor.

1. Problems Caused by Improperly Starting the Compressor

When the refrigeration system stops running for a period of time (a few days or even longer), if the oil heater is not started to preheat the crankcase before starting the compressor, and the compressor is started directly, it can be observed through the oil sight glass that the oil in the crankcase boils. All you can see through the oil sight glass is foam, and the oil surface disappears. At the same time, the compressor often makes a rattling sound—a liquid slugging occurs.

Why does the refrigeration oil in the crankcase boil—turn into foam—and finally disappear? Below we discuss two aspects, the solubility of refrigerant and oil and the migration of refrigerant, to explain the main reasons for this phenomenon.

Figure 2: Crankcase heater.

Solubility of Refrigerant in Oil

Except for centrifugal compressors, the refrigerant of other compressors is in contact with compressor oil. The solubility of various working fluids in oils is different, mainly divided into completely miscible, partially soluble and insoluble or slightly soluble. Solubility is related to temperature, so the partial solubility and complete miscibility mentioned above can be mutually transformed.

Refrigerant solubility in oil is considered to be a crucial issue in determining system characteristics and machine life.

After the compressor stops, the high and low pressures of the entire refrigeration system tend to be in balance, the high-pressure gas in the system flows towards the low-pressure area, and the crankcase pressure rises. Due to the solubility of refrigerant in oil, after the refrigerant gas reaches the crankcase, the refrigerant gas in contact with the oil surface begins to dissolve.
At this time, the pressure on the oil surface decreases, and the surrounding refrigerant will quickly replenish the dissolved part to maintain the pressure. After this repeatedly, a large amount of refrigerant is dissolved in the oil, which is the reason for the phenomenon of "refrigerant migration".

Take R22 as an example: at higher temperatures, R22 liquid is infinitely miscible with oil. When the temperature drops below a certain critical value, stratification begins to occur. The lower layer is mainly a large amount of refrigerant and a small amount of oil, and the middle is a homogeneous mixture of the two. The upper layer is mainly a large amount of oil and a small amount of refrigerant.

Figure 3: Critical curves of refrigerant solubility in oil.

Refrigerant Migration

Therefore, after the compressor stops for a period of time, the oil level can be seen through the oil sight glass to rise, which is the result of refrigerant migration.

So how can we remove the refrigerant dissolved in the oil and ensure the normal start of the refrigeration compressor? At this time, the oil heater plays a vital role.

Refrigeration migration is a slow process, the longer the compressor is down, the more refrigerant is dissolved in the oil. When the compressor starts, the pressure in the crankcase drops, and the refrigerant dissolved in the oil will evaporate rapidly, causing the oil to boil violently.
Due to the saponification (foaming) of oil caused by boiling, a large amount of foam enters the cylinder through the air passage and becomes liquid in the cylinder. When the liquid flows, the momentum is greater than that of the gas, and the impact force generated is also greater than that of the gas. The impact force in the suction valve plate generated by the sucked two-phase gas flow is not only large but also high in frequency, and can easily cause liquid slugging.

Refrigerant migration not only easily causes liquid slugging in the compressor, but also dilutes the oil. Although the diluted oil is transported to various friction surfaces, after the viscosity is reduced, a protective oil film with sufficient thickness cannot be formed, which will cause wear over time.
The more dissolved refrigerant, the more the viscosity of the oil will be reduced. Not only will not enough oil film be formed on the friction surface, but also the original oil film will be washed away and reduce the effect.
If there is no sufficient friction surface, the temperature will rise rapidly, and the oil will carbonize and coke after exceeding the specified upper limit, which will eventually lead to failures such as shaft holding and cylinder jamming of the compressor.

Moreover, when the compressor is started and the oil pump sucks oil mixed with a large amount of refrigerant, the refrigerant quickly turns into gas at the inlet of the oil pump, and blocks the inlet of the oil pump, causing a normal oil circuit system not to be established. At this time, the oil pressure will drop and the protection action of the oil pressure differential controller will affect the normal operation of the compressor until the percentage concentration of the refrigerant dissolved in the crankcase oil is reduced to a level that can be recognized by the oil pump and the oil pressure differential controller.

Figure 4: Oil sight glasses.

2. The Role and Importance of Crankcase Heaters

The oil heater that the compressor equips with can largely avoid the problems of liquid slugging and lubricating oil failure caused by refrigerant migration, but it is ignored by many users.

After the compressor stops for a short time (such as at night), keeping the crankcase heater energized can effectively prevent refrigerant migration. If it is not used for a long time (such as a winter), heat the oil for several or more than ten hours before starting up to evaporate most of the refrigerant in the oil.

After the crankcase heater is energized, many air bubbles will be found on the oil surface through the oil sight glass, and the oil level will continue to drop. This is the phenomenon caused by the evaporation of the refrigerant in the oil. Therefore, starting the oil heater before starting the compressor can effectively avoid the occurrence of liquid slugging, eliminate the failure of the compressor, and prolong the service life of the compressor.

Figure 5: Compressor heating belt.

3. How to Use the Crankcase Heater Correctly

But if the oil heater cannot be used correctly, the failure phenomenon cannot be avoided, so how to use the oil heater correctly?

(1) Without special circumstances, the compressor can only be started after the crankcase heater of the compressor is energized for at least 4 hours or when there is no bubble or a small number of bubbles seen through the oil sight glass during the power-on process.

However, when the ambient temperature is extreme or the suction line is exposed to the external environment, the external cooling load and the heating value of the heater balance each other, and refrigerant migration will still occur. In this case, the ambient temperature of the compressor can be increased by adding an auxiliary heat source, thereby avoiding the occurrence of refrigerant migration.

(2) Use the oil heater at an appropriate voltage. When the voltage is too high, it will cause excessive current to make the heater burn out. Do not use the oil heater at a voltage exceeding its rated standard.

(3) The oil heater is not in the heating state during the whole refrigeration process. The correct working process is that when the unit is in the shutdown state, the oil heater is started, and when the unit is in the startup state, the oil heater is disconnected.

Figure 6: Installation of crankcase heater diagram.

To sum up, the setting of the oil heater is crucial not only to the compressor, but also to the normal operation of the entire refrigeration system, and the users should also pay attention to the correct use method. When the compressor suffers from liquid slugging, shaft holding, cylinder jamming, or valve plate breakage, first check whether the oil heater is used correctly. Instead of solving the problem from the root cause, simply repairing the compressor may cause the failure again.

4. Conclusion

The use of crankcase heaters in refrigeration compressors and air-conditioning compressors can keep the lubricating oil of the compressors at a certain temperature, thereby reducing the proportion of refrigerant dissolved in the oil and preventing the mixture of oil and refrigerant from becoming too viscous when it is cold to make the starting resistance of the compressor larger.

This method is generally adopted for large-scale refrigeration units to protect the compressor, but for small-scale refrigeration units, due to the small amount of refrigerant in the refrigeration system and the small pressure difference between high and low pressure, this method is generally not adopted.

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