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The air conditioning system of a building is essentially a refrigeration system. These systems use a compressor to circulate refrigerant and a lubricant is essential to the function of a compressor.
An air conditioning system is based on the ideal gas law and the direct relationship between the pressure gas is under, and its temperature.
In an air conditioning system, the compressor functions as a pump that moves refrigerant. The refrigerant departs the compressor as a pressurized gas and enters a condenser, where it is pressurized into becoming a liquid. The liquid refrigerant then flows through a pipe until it gets to a metering device, often called a thermal expansion valve. The liquid then drops down to an opening in the line where it experiences a large drop in pressure. The pressure drop causes the refrigerant’s temperature to drop as well.
The cooling of air occurs in a part of the system known as the evaporator. The temperature of the air moving over the evaporator is greater than the temperature of the refrigerant inside, and this temperature difference causes a heat transfer from the moving air to the refrigerant in the evaporator. The refrigerant is then pumped to the condenser where the heat is removed. The compressor drives this entire circulation of the refrigerant.
Lubricants have multiple purposes in the compressor of an air conditioning system. Naturally, they must be capable of lubricating moving parts. In some A/C systems, the lubricant also acts as a cooling fluid, in addition to being a sealant.
Compressor lubricants tend to be a unique mixture of additives and foundational oils. This blend provides essential lubricating qualities while still being suitable for use with a refrigerant, as incompatibility could have devastating results for the system.
Compressor lubricants are often synthetic and specially formulated to provide a longer service life than natural lubricants. Despite compatibility with a refrigerant being the most important issue for these lubricants, there are other issues to consider. For example, moisture can be very damaging to some man-made base oils, as moisture can react with some oils oil to create acids, alter the viscosity, and hinder the oil’s lubricating qualities. This can result in untimely compressor failure and poor system cooling.
Essential Qualities of Air Conditioning Lubricants
When determining the proper lubricant to use for a compressor in an air conditioning system, it is important to consider essential qualities such as viscosity, chemical stability, dielectric strength, and moisture.
Viscosity is a measure of how thick a fluid is and its capacity to withstand shearing stress. Normally, larger compressors have bigger gaps that require thicker oil for effective lubrication.
An oil's viscosity is assessed by placing a sample on standardized testing equipment at a fixed temperature and allowing the oil to run through a specifically sized hole. With thicker oil flowing more slowly than thinner oil, the length of time it takes for a sample to flow through the test hole is documented to ascertain the viscosity number, which is written in SSU (Saybolt Seconds Universal) or the metric unit poise.
It is possible for the compressor lubricant to react with refrigerant. Highly reactive oils can react to produce unwanted residue, carbon, and acids that can impact the performance and lifespan of the compressor.
Since compressor lubricant is generally in contact with the motor, it must be a good insulator, also described as having strong dielectric strength.
Lubricants are prone to incorporating moisture and when water becomes incorporated in a refrigerant, ice can form when the temperature falls under 0 degrees Celsius, causing serious problems in a compressor's expansion valve. Moisture can also affect the dielectric strength of the lubricant, as water is a good conductor of electricity.
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