The operation of a household refrigerator is a complex physical a process in which every element plays a critical role, with the compressor at the center. Many equipment owners don’t even think about the fact that inside this black metal tank, in addition to the refrigerant, there is a special liquid that ensures the stability of the entire system. It is compressor oil responsible for the durability of rubbing parts, allowing the piston group to function for years without major repairs.
If you have ever heard a characteristic knock or hum when starting the refrigerator, know that this may be the first signal of lubrication problems. The lack of high-quality lubrication leads to instant overheating and jamming of the mechanism, after which restoration becomes economically impractical. In this article, we will analyze in detail the physical and chemical properties of this liquid, and also answer the question of why you can’t just add the first technical oil you come across.
Understanding the principles of operation of the lubrication system will help you avoid costly mistakes when servicing or attempting self-repair. Refrigerant and oil in modern systems are inextricably linked, circulating together along the circuit, which imposes special requirements on them compatibility. Next, we will look at the types of oils and their effect on the efficiency of your refrigerator.
The main functions of the lubricating fluid in the system
The main task of the oil is to create a stable film between the moving metal parts of the compressor. When the engine starts, the friction between the piston and cylinder is at its maximum, and it is the lubrication that prevents dry friction, which would lead to rapid wear. Without this layer of metal, they would simply “weld” together in a matter of seconds of operation at high speeds.
In addition, the liquid performs the function of removing heat from rubbing pairs. When gas is compressed, the temperature in the combustion chamber rises sharply, and the circulating oil helps remove excess heat, preventing thermal deformation of parts. This is especially important for hermetic compressors where there is no possibility of external cooling by a fan.
⚠️ Attention: An attempt to start the compressor without oil or with a critically low level of oil is guaranteed to lead to jamming of the shaft and burning of the motor windings.
Also, the lubricant creates an additional seal between the piston rings and the walls cylinder. This prevents compressed refrigerant from leaking back into the suction chamber, providing the necessary pressure drop for efficient operation refrigeration cycle. If the oil viscosity is chosen incorrectly, the tightness of the friction pair is broken and performance drops.
Finally, the oil protects internal surfaces from corrosion. Since a minimum amount of moisture is always present in the system, anti-corrosion additives in the liquid neutralize oxidative processes. This keeps the internal channels clean and prevents the formation of sludge that could clog the capillary tube.
Types of oils: mineral versus synthetic
In the history of the development of refrigeration technology, various types of lubricants were used, the choice of which depended on the refrigerant used. Traditionally, older models running on R12 freon were used mineral oils. They are produced by deep purification of oil and do an excellent job in systems with low environmental requirements.
With the advent of more environmentally friendly refrigerants, such as R134a and R600a, mineral oils have ceased to be effective due to poor solubility in them. They have been replaced by synthetic compounds, in particular polyester oils (POE). They are highly hygroscopic, that is, they actively absorb moisture from the air, which requires special care during storage and replacement.
There is also a class of semi-synthetic oils (AB —alkylbenzene), which are often used as a transition option or for retrofitting (replacing the type of refrigerant). They are less hygroscopic than polyester, but provide better compatibility with modern refrigerants than mineral analogues.
Mixing different types of oils is strictly prohibited, as this can lead to sedimentation or the formation of acids that destroy the insulation of the windings. If the instructions for your Bosch refrigerator or Liebherr type indicate the POE type, you cannot fill it with mineral oil under any circumstances.
Circulation of oil and refrigerant in the circuit
Unlike car engines, where the oil is in the crankcase and does not leave it, in a refrigeration compressor the lubricant constantly circulates along with the gas. When the piston operates, part of the oil in the form of a fine suspension is carried away by the refrigerant flow into the condenser and further along the entire circuit. This is a normal operating process provided by engineers.
For the system to operate stably, the oil must have a certain solubility refrigerant content. If the oil is too viscous or does not dissolve, it will begin to accumulate in the evaporator, forming an “oil plug”. This will sharply reduce heat transfer, and the refrigerator will stop freezing, although the compressor will work continuously.
To return the oil back to the compressor crankcase, special loops and slopes are provided in the horizontal sections of the pipeline. The speed of gas movement is calculated so as to carry along with it the oil flowing down the walls of the pipes. Violation of the geometry of the pipeline during installation can lead to oil remaining in the radiator and not returning to the mechanism.
| Oil type | Compatible refrigerant | Hygroscopicity | Application |
|---|---|---|---|
| Mineral | R12, R22 (partially) | Low | Old models of refrigerators |
| Alkylbenzene (AB) | R12, R134a, R406A | Average | Retrofitting, transitional models |
| Polyester (POE) | R134a, R407C, R410A, R600a | High | Modern household refrigerators |
| Polyvinyl ester (PVE) | R410A, R32 | High | Industrial and new household systems |
What will happen if the oil does not return to the compressor?
If the oil gets stuck in the evaporator, the compressor will start to run dry. This will lead to a sharp increase in housing temperature, noise and eventual jamming of the piston group after several hours or days of operation.
Signs of the need for replacement or topping up
The need for intervention in the lubrication system can be determined by a number of indirect signs, since it is impossible to visually check the level in a hermetic compressor. The first alarm bell is a change in the nature of the operation: the appearance of a metallic knock, clanging or excessively loud hum when starting and stopping.
It is also worth paying attention to the temperature of the compressor housing. If it heats up to temperatures at which it is impossible to hold your hand (more than 80-90°C), this may indicate increased friction due to degradation of the oil properties. Normal operating temperature is usually lower, although it depends on the model and load.
Another sign may be the appearance of oil on the external elements of the refrigerator, for example, under it or on the rear grille. This indicates depressurization of the system, through which not only refrigerant flows, but also lubricant. In such a situation, simple topping up will not help - you need to search for leaks and evacuate the system.
If you are carrying out repairs related to replacing a compressor or filter-drier, a complete oil change in the circuit is a mandatory procedure. Old oil contains decay products, acid and moisture, which will instantly damage the new unit.
☑️ Diagnosis of lubrication problems
Technology for changing oil in a compressor
The process of changing oil is a complex technical operation that requires special equipment, such as a vacuum pump and a pressure gauge station. It is impossible to simply “drain and refill” new oil into a working system, since there is pressure and refrigerant there. First, it is necessary to completely remove the old gas into the atmosphere or into a special receiver.
Then the compressor is sealed off from the mains, turned over and the waste is drained. It is important to let it sit to free up the internal cavities as much as possible. After this, fresh oil is poured into the system strictly in the volume specified in the technical documentation (usually 300-400 ml for household models). The exact volume is critical: excess oil will cause water hammer, and too little will cause overheating.
The next step is installing a new filter-drier and evacuating the system. Vacuuming necessary to remove air and moisture that could get inside during replacement. Polyester oils absorb moisture from the air in a matter of minutes, so all operations must be carried out quickly and carefully.
⚠️ Attention: Never leave the compressor pipe or a new can of oil open for longer than 10-15 minutes. Moisture trapped in polyester oil turns into acid, which corrodes copper and motor windings.
After assembling the circuit and checking the tightness with helium or nitrogen, the system is charged with refrigerant. Only after this can a test run be carried out and the consumption currents of the electric motor monitored.
Frequent errors during maintenance
The most common mistake made by home repairers is the use of automotive or transformer oils liquids. These substances have a completely different chemical composition and are not intended to work in tandem with freon. They do not have the necessary solubility and temperature stability, which will lead to rapid failure of the equipment.
Another mistake is ignoring the replacement of the filter-drier. When the system is opened, the old filter is already saturated with moisture and will not be able to protect the new oil from oxidation. Trying to save money on a filter that costs a few dollars can lead to a compressor breakdown costing hundreds of dollars.
Some try to add oil directly through the process pipe without vacuuming, using a syringe. This leads to airing of the system. Air trapped in the circuit causes an increase in condensation pressure, an increase in temperature and the formation of ice plugs in the capillary.
It is also dangerous to mix oils from different brands, even of the same type. Various additive packages can react chemically, forming a solid deposit that will clog the thinnest passages of the capillary tube or expansion valve.
Can air conditioning oil be used in a refrigerator?
In most cases, yes, if the types of refrigerants are the same (for example, R134a). However, the viscosity of oils for air conditioners may differ, which will affect the return of oil to the compressor. It is better to use specialized compounds.
Questions and answers (FAQ)
How often do you need to change the oil in the refrigerator?
In a working hermetic compressor, the oil is designed for the entire service life of the equipment, that is, 10-15 years or more. No scheduled replacement required. Intervention is only necessary during repairs (replacing a compressor, eliminating leaks) or if an oil analysis shows the presence of acid and moisture.
Is it possible to replace mineral oil with synthetic oil?
It is impossible without special flushing of the system. Synthetic oils (POE) may be incompatible with mineral residues, resulting in the formation of an emulsion. The transition is possible only after thoroughly purging the circuit with nitrogen and replacing all rubber seals with compatible ones.
Why did the oil in the compressor turn black?
Darkening of the oil indicates overheating of the compressor or moisture entering the system. Resins and acids are formed as a result of thermal decomposition or chemical reaction. Such oil loses its lubricating properties and requires immediate replacement along with the filter-drier.
How much oil is in the refrigerator compressor?
The volume depends on the power and model of the compressor. For most household single-compressor refrigerators, the volume ranges from 300 to 450 grams. Exact information can be found on the nameplate of the compressor itself or in the manufacturer’s service documentation.
What happens if you pour too much oil?
Excess oil will cause the liquid fraction to begin to enter the condenser and evaporator, occupying useful volume and worsening heat transfer. In addition, water hammer is possible - incompressible fluid entering the cylinder during startup, which will physically destroy the valves or piston.