Many owners of household appliances do not even think about what is going on inside the metal “pot” located at the back of the refrigerator while the unit is working quietly and properly. However, when there is a knock, a hum, or the device stops freezing, the question arises about the internal mechanics, where oil in the refrigerator compressor plays a critical role. This is not just an auxiliary fluid, but the “blood” of the entire system, ensuring the durability of the rubbing parts.
Inside the sealed housing, a complex thermodynamic process takes place, requiring ideal synchronization of all elements. Piston group, shafts and bearings operate under high pressure and at significant temperature changes, which creates extreme operating conditions. It is the special oil that takes on the function of protecting the metal from rapid wear, corrosion and overheating, allowing the engine to operate for years without human intervention.
If you hear strange sounds or notice a decrease in cooling efficiency, you cannot ignore the problem, as this may be a direct consequence of degradation of lubricants. Understanding why and how this fluid works will help you notice signs of malfunction in time and make the right decision about repair or maintenance. Mixing oils of different chemical bases (for example, mineral and synthetic) is strictly prohibited, as this leads to irreversible chemical separation and failure of the compressor.
The main functions of the lubricating fluid in system
The main task of any lubricant is to create the thinnest film between moving parts, which prevents their direct contact. In a refrigerator compressor, oil serves not only as a separator of rubbing surfaces, but also as a sealer of the gaps between the piston and the cylinder walls. Without this seal, it would be impossible to create the necessary pressure for the circulation of the refrigerant through the circuit.
In addition to mechanical protection, the liquid is involved in removing heat from heating components. As the gas is compressed, the temperature inside the chamber rises sharply, and the circulating lubricant helps conduct excess heat to the compressor housing, which in turn dissipates it into the environment. This prevents local overheating and deformation of metal parts.
⚠️ Attention: An attempt to start the compressor without oil or with a critically low level will lead to scuffing of the cylinders and jamming of the shaft in a matter of seconds.
Also, the lubricant protects the insides of the unit from corrosion, enveloping metal surfaces and displacing moisture. In a system where there may be a microscopic amount of water or acids formed during the breakdown of freon, anti-corrosion properties become a determining factor in service life.
Types of oils and their chemical composition
The choice of lubricant directly depends on the type of refrigerant used, since they must be chemically compatible. Historically, mineral oils were used for older refrigerators powered by Freon R12. They have excellent lubricating properties, but are poorly soluble in modern refrigerants.
With the advent of environmentally friendly freons, such as R134a and R600a, the industry switched to synthetic compounds. The most common are polyester (POE) and polyvinylbenzene (PVB) oils. They are highly hygroscopic, that is, they actively absorb moisture from the air, which requires special care during storage and replacement.
Here are the main types of oils used in modern technology:
- 🛢️ Mineral - traditional compositions for R12, characterized by stability, but incompatible with new freons.
- 💧 Polyester (POE) - synthetics for R134a and R407c, with high dissolving ability.
- 🧪 Polyvinylbenzene (PVB) - an alternative to POE, often used in household refrigerators on R600a.
- ⚗️ Alkylbenzene (AB) - semi-synthetic, compatible with some types of mineral oils and new refrigerants.
Incorrect selection of the type of liquid can lead to the fact that it stops returning to the compressor crankcase, leaving rubbing parts without protection. Viscosity is also selected individually for a specific engine model and its operating conditions.
Oil circulation and interaction with refrigerant
One of the key features of the operation of a refrigeration unit is that some of the oil inevitably enters the refrigeration circuit along with the flow of gaseous and liquid freon. This is not a defect, but is provided for by the design. For normal operation of the system the oil must circulate throughout the entire circuit along with the refrigerant, returning back to the compressor.
If the lubricant is too thick or poorly soluble in this type of freon, it will begin to settle in the evaporator or condenser, clogging thin tubes capillaries. This creates a so-called “oil plug,” which disrupts heat transfer and increases discharge pressure. As a result, the compressor operates with overload, and the cold in the chamber disappears.
In modern systems, especially on isobutane (R600a), the amount of oil in the system is strictly dosed and amounts to only a few grams. Excess of lubricant is just as dangerous as its lack, since excess liquid can cause water hammer in the piston group cylinder during startup.
Problems: foaming, oxidation and waste
During operation, the properties of lubricants inevitably change. One of the common problems is foaming oil. This happens when the refrigerant dissolves in the oil at low pressure, and with a sharp drop in pressure (at the moment of startup) it begins to violently release in the form of foam. Foam, unlike liquid, does not create a protective film, which leads to dry friction.
Another enemy is oxidation and sludge formation. When overheated or moisture enters the system, the oil changes its chemical composition, darkens and turns into a thick, resinous substance. This slag clogs filter driers and capillary tubes, reducing the efficiency of the entire system.
Main signs of lubricant degradation:
- 🌑 Discoloration to dark brown or black.
- 📉 The appearance of an acidic odor when the system is opened.
- 📉 A decrease in viscosity, the oil becomes like water.
- 📉 The presence of solid particles or carbon deposits on the filter.
⚠️ Attention: If during repairs you find that the oil in the compressor has turned black and smells burnt, simply replacing the fluid will not help - the system is contaminated with decay products and requires thorough flushing.
Diagnostics of condition and oil change
Checking the level and condition of the oil is usually carried out only during professional repairs, since the system is sealed. However, indirect signs, such as constant overheating of the compressor housing or a change in the nature of the operating sound, may indicate problems with lubrication. For accurate diagnosis, technicians use an analysis of the acidity and viscosity of the sample taken.
Changing the oil at home is possible, but requires a vacuum pump, a pressure gauge station and a full understanding of the system. The process includes removing the old freon, opening the system, flushing the circuit and filling with new oil of a strictly defined brand and volume.
☑️ Procedure for changing the oil
The system must have a strictly defined balance. Below is a table of approximate correspondence between oils and refrigerants:
| Refrigerant type | Recommended oil type | Chemical base | Features |
|---|---|---|---|
| R12 | Mineral Oil | Mineral | Not hygroscopic, incompatible with R134a |
| R134a | POE / PAG | Polyester | Strongly absorbs moisture, requires vacuuming |
| R600a | POE / Mineral (special) | Synthetics/Mineral | A small amount is used, fire hazard |
| R407c | POE | Polyester | Complex mixture, requires a complete oil change |
Possible Is it possible to mix oils from different manufacturers?
It is theoretically possible to mix oils of the same chemical type (for example, two different POE) from different manufacturers, but it is not advisable. Different additive packages may react. It is better to completely remove the old fluid before filling with a new one.
Prevention and expert recommendations
To extend the life of the compressor and preserve the properties of the oil, you should avoid frequent turning on and off of the refrigerator. Short cycles prevent the oil from warming up and any moisture that has entered the system from evaporating, leading to emulsification. It is also important to ensure good ventilation of the rear wall of the unit in order to prevent critical overheating.
When transporting the refrigerator, you must follow the rules: units with piston compressors are not recommended to be transported lying down, as oil can flow into the circuit, from where it will be difficult to remove. If transportation in a horizontal position is unavoidable, after installation, turn on the device no earlier than 12-24 hours.
Regular defrosting (for No Frost models this is not required, but for “crying” systems are important) and maintaining the cleanliness of the condenser at the back of the refrigerator also indirectly affects the operating temperature of the oil. A clean heat exchanger allows the system to operate normally without overload.
Frequently asked questions (FAQ)
How much oil should be in a refrigerator compressor?
The volume depends on the model, but in household refrigerators it usually ranges from 150 to 300 ml. The exact quantity is always indicated in the technical documentation or on the compressor nameplate. Excess oil is just as harmful as too little.
What oil to use if the brand is not indicated in the instructions?
If the exact brand is unknown, you need to focus on the type of refrigerant. For R12 - mineral, for R134a and R600a - synthetic (POE or PVB). As a last resort, you can use universal synthetic oils for refrigeration compressors, but it is better to consult with a specialist.
Why does the compressor hum after changing the oil?
The hum may occur because the new oil has not yet been distributed throughout the system, or the wrong type of viscosity has been filled. It is also possible that during replacement, air entered the system, which must be removed by vacuuming.
Is it possible to change the oil without replacing freon?
No, this is technically impossible in a sealed system. To access the oil, you need to open the circuit, which inevitably leads to a refrigerant leak. After the procedure, the system is evacuated and refilled with a new portion of freon.