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Factors to consider when selecting a refrigeration compressor oil

Refrigeration Compressor Lubricants min

This guide offers information about refrigeration compressors and the oil required to provide effective lubrication. When selecting the appropriate refrigeration compressor oil, it is essential to consider the equipment and refrigerant being used. Without the correct one, your compressor will break down quickly.

Refrigeration compressors

Compressors are of pivotal importance to HVAC, industrial cooling, and the food and beverages industry, where quality and safety of produce depend on the correct temperature control. In industrial plants, compressor systems are among the most critical machines and it is therefore important to obtain regular oil samples to monitor the health of both lubricant and machine.

Compressors are highly sensitive components which need to be lubricated to guarantee a long service life The three types of compressors used with refrigerants are rotary, reciprocating, and centrifugal. With all three, check the lubricant’s base oil, additives and viscosity grade. The lubricating properties must be compatible with the refrigerant, to avoid potential damage. When a refrigeration compressor requires a rebuild or upgrade, the suitability of the oil-cooling system will depend on the compressor size, your cooling needs and your budget.

The core functions of compressor lubricants

It is important to understand the needs of the compressor and how lubricants flow when selecting the proper base oil and additive properties. Refrigeration oil typically serves FOUR purposes: lubrication, sealing, cooling and energy regulation.

Lubrication

Refrigeration oil is used for compressor lubrication, in order to minimise friction and wear and tear of the compressor operation, thereby extending its service life.

Sealing

Refrigeration oil plays a sealing role in the compressor to prevent refrigerant leakage.

Cooling

When the moving parts of the compressor are lubricated, the refrigerant oil extracts the working process heat. This enables the moving parts to maintain a lower temperature, and this in turn improves the efficiency of the compressor.

Energy regulation

Using the pressure of the refrigerant oil also provides an energy regulating mechanism for the refrigeration compressor.

Which lubricants are used in refrigeration systems?

Polyolester is the most common synthetic lubricant, used with hydrofluorocarbon refrigerants such as R134a. Most compressor lubricants are synthetic, enabling a longer service life better than mineral-based fluids. Polyalphaolefin is a refrigeration lubricant offering both thermal and chemical stability. It offers good temperature viscosity but its miscibility with refrigerants is low. It can also shrink seals, a problem often solved by mixing with alkylbenzenes. Polyalkyleneglycols are refrigeration lubricants with a high viscosity index and optimal thermal stability, although also hygroscopic and soluble with ammonia.

Three oil-cooling methods

Three common oil-cooling techniques are used in industrial refrigeration compressors.

Liquid injection oil-cooling is a more cost-effective upfront investment but can also result in compressor failure and technical issues if not checked or adequately maintained. This option is not ideal for smaller compressors or those with lower heat loads.
Water or glycol-cooled oil cooling is an energy-effective and cost-efficient way of reducing energy consumption and controlling oil temperature and can be adapted to different applications and compressor sizes, whilst also reducing the risk of refrigerant contamination. It is, though, more expensive than liquid injection, and needs to be located near accessible bodies of water. It also requires treatment to stop corrosion and bacteria.

A third option is thermosyphon oil cooling, an energy-efficient solution for managing compressor oil heat levels. Because thermosyphon systems rely on natural movement they consume less energy than pump-dependent systems. They need less maintenance and mechanical repair. However, installation can be made more difficult by the orientation and positioning of the heat exchanger and compressor, and its reliability can be affected by variations in temperature and environmental conditions.

Other considerations

Substituting one oil-cooling system for another is a major investment. Perform a comprehensive annual review of your compressor’s functionality. Inspection audits will spot mistakes and oversights in compressor maintenance, so it is important to check relief valves, microprocessors, transducers, oil pumps, temperatures, pipe stress and the motor itself.
Be aware that moisture contamination is bad for some synthetic base oils that are hydrolytically unstable. Moisture reacts with the base oil to form acids, affect viscosity and undermine the oil’s lubricating properties. This can result in premature compressor failure in addition to improper system cooling.

Effective oil cooling is vital to the effectiveness and longevity of refrigeration compressors. Whilst regular servicing is important, a system designed with an appropriate oil-cooling technique can extend a compressor’s life by thousands of hours. Each method has its own set of pros and cons, which is why it is crucial to consider your application and its operational needs. For further advice, contact Oil Store for refrigeration compressor lubricants today.

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