Refrigeration lubricants

Introduction

May it be a frozen fast food snack like pizza, or frozen meat like chicken wings, fish or veg items such as minced spinach, green pea or corn, we normally buy it from supermarket refrigeration section. Most of the items are ready to eat, some seasoning may enhance the taste and presentation. We also find their use on a small scale in medical industry where hospitals use them for blood storage, chemist shops for storing infusion medicines and solutions, houses for storing eatables and bakeries for storing cakes and pastries. Refrigeration systems are usually used to delay deterioration and decomposition of consumables.

The term refrigeration means cooling a space or substance and/or maintaining its temperature below the ambient temperature. A refrigeration unit accomplishes this task through a cycle of compression and expansion of refrigerant along with manipulating its pressure.

A refrigeration system consists of four basic elements:
• Compressor – Converts low-temperature and low-pressure gas into high-pressure, high-temperature gas.
• Condenser – Liquifies refrigerant gas
• Expansion device – Creates pressure drop resulting in temperature reduction
• Evaporator – Serves as a heat exchanger.
• Refrigerant – A cooling liquid which keeps recirculating in a loop, cooling down the internal components.

Since mechanical components are involved in the refrigeration process, lubrication becomes necessary for smooth working of these parts. Few key components of compressor such as bearings, gears and seals require lubrication for achieving optimum performance and longevity. Apart from reducing friction and wear of compressor parts, the lubricant also acts as a sealant restricting leakage of gas, a dampening agent reducing noise of working internal components, a cooler removing heat generated at the time of compression and a cleaning liquid flushing wear metal debris or any external contaminant out of the system.

Techniques used for lubricating compressor

Depending upon the compressor type, different techniques are used for lubrication. Splash lubrication method is used for pistons and scroll type compressors whereas screw type compressors employs pressure lubrication method. Spray and drip lubrication systems are also being used for certain compressors.

Characteristics of refrigeration lubricants

Not any lubricant will do the job, the refrigeration lubricant should have following properties before being considered for use:

1. Good lubricity
2. Good low temperature flow properties
3. Good solubility and miscibility with refrigerant
4. Good electrical insulation
5. Good compatibility with materials
6. Low volatility
7. Low toxicity
8. Low neutralization number
9. Low flash and fire point

Types of refrigerants used in refrigeration systems

1. CFC: CFC refers to the chemical composition of the refrigerant. Chlorofluorocarbon indicates that the refrigerant is comprised of Chlorine, Fluorine, and Carbon. These are very harmful to ozone layer. Examples: R11, R12, R13, R13b1, R14, R113, R114 and R115.
2. HCFC: Hydrochlorofluorocarbon indicates that the refrigerant is comprised of Hydrogen, Chlorine, Fluorine, and Carbon. These refrigerants are slightly harmful to the ozone layer. Examples: R21, R22 and R123.
3. HFC: Hydrofluorocarbon indicates that the refrigerant is comprised of Hydrogen, Fluorine, and Carbon. These refrigerants are not harmful to the ozone layer. Examples: R134a (used in car air-conditioning systems) and R152a.
4. Ammonia (R717 (NH3): Although ammonia is environmentally friendly, pure ammonia gas is highly toxic to people. Ammonia is used extensively in large industrial refrigeration plants.
5. HFO: HFOs are organic compounds composed of hydrogen, fluorine and carbon. The are not harmful for ozone layer and have low Global Warming Potential and so offer a more environmentally friendly alternative to CFCs, HCFCs and HFCs.

Types of refrigeration lubricants used in refrigeration systems

1. Mineral oils: Naphtenic oils have a low pour point, which make them more suitable at lower temperatures. They are particularly suited for use with CFC, HCFC refrigerants and ammonia.
2. Alkylbenzenes (AB): Alkyl benzene oil types were the first synthetic oils to be used in the refrigeration industry. They have a good thermal and chemical stability. The miscibility with refrigerant is high. AB oils are specially recommended for refrigerants like R-22 and HCFC blends. They are completely soluble with mineral oils and PAO.
3. Polyalphaolefin (PAO): The miscibility with refrigerant is low, hence it is used in applications where miscibility is not a concern. PAO can cause seals to shrink, which can be solved by mixing PAO with AB. PAO oils can be used in refrigeration systems working in extreme conditions with R-22 or ammonia thanks to their low pour points and excellent thermal stability.
4. Poly alkylene glycol oils (PAG): These refrigeration oils have a high viscosity index and thermal stability. However, PAG is hygroscopic, which means it can attract water. PAG oils are mainly used in copper-less car air conditioning systems using R-134a, because of their tendency to absorb water and their incompatibility with copper.
5. Polyol ester oils (POE): These constitute the second generation of lubricants. POE oils are excellent lubricants, less hygroscopic than PAG and chemically more stable than PAG when in the presence of water. This refrigeration oil is the most common type of synthetic lubricant being used with HFC refrigerants, like R134a.

Importance of compatibility of refrigerant and lubricant

In most other refrigeration systems, the blending of the refrigerant fluid and the lubricant is inevitable, so it’s important to select a lubricant with the right miscibility and solubility characteristics. In systems not equipped with oil-separation capability, the lubricant carried over from the compressor into the evaporator must be sufficiently miscible with the refrigerant at the evaporator temperature so that the refrigerant fluid–lubricant blend remains in one phase after expansion in the evaporator and at a sufficiently low viscosity to travel through to the compressor. If the lubricant separates in the evaporator due to poor miscibility with the refrigerant fluid, or the blend viscosity is high, fluid is likely to get trapped in the evaporator and adversely affect the system’s cooling capacity and efficiency.

Compatibility of refrigerants/oil with material

Elastomeric seals swell by varying amounts in the presence of different types of oil, thus there must be good compatibility between the selected oil and any elastomeric materials.  It is necessary to perform compatibility tests because plastics of the same type have different molecular mass, structure of polymers, different plasticizers, and temperature and other factors can decrease plastics resistance to the influence of refrigerants.

Additives for refrigeration lubricant

Whether to use additives or not is still an issue. Instances such as decreased lubricant stability, sludge formation and deposits on surface suggests non-additive based refrigeration lubricants to be better. On the other hand, some suggests their use to boost antiwear protection, metal passivation, acid scavenging and defoaming.

Importance of oil separator in refrigeration cycle

Oil in the evaporator tubing can act as an insulator and reduce the efficiency by about 14%. The oil separator will be in the discharge line close to the compressor so that excess oil will be prevented from going through the system. The trapped oil stays in oil separator temporarily and then it is returned to the compressor sump.

References

1. Refrigeration Lubricants: Transitioning to new refrigerants by Dr. Neil Canter STLE TLT December 2009
2. Refrigerants and refrigerant oil, https://www.slideserve.com/hide/refrigerants-and-refrigerant-oil
3. Compressor lubrication, https://www.swep.net/refrigerant-handbook/3.-compressors/3.3-compressor-lubrication/
4. Refrigeration Lubricants, https://www.sangroup.co.za/images/pdf/Refrigeration_oil_types.pdf
5. Which oil types are used for refrigeration systems? https://www.q8oils.com/energy/refrigeration-compressor-oils/
6. Parts Of A Refrigerator And It’s Functions, https://www.reviews.in/parts-of-a-refrigerator-and-its-functions.html
7. Major Elements of Refrigeration System and their Functions, https://1cold.com/blogs/major-elements-refrigeration-system-functions/
8. The 4 Main Refrigeration Cycle Components, https://www.superradiatorcoils.com/blog/4-main-refrigeration-cycle-components
9. Selecting the right lubricant for your refrigeration system, https://club.mobilindustrial.com/lube_talk/b/tip_of_the_week/posts/selecting-the-right-lubricant-for-your-refrigeration-system
10. Introduction of 5 lubrication methods of air compressor, https://www.handeair.com/info/introduction-of-5-lubrication-methods-of-air-c-36979447.html
11. Nu-Calgon Product Bulletin – Refrigeration Oils, https://www.yumpu.com/en/document/read/12034730/emkarate-refrigeration-oil-nu-calgon
12. Selecting, Installing Oil Separators, https://www.achrnews.com/articles/91004-selecting-installing-oil-separators
13. Abedrabbo, F., Soriano, D., Madariaga, A. et al. Experimental evaluation and surface integrity analysis of cryogenic coolants approaches in the cylindrical plunge grinding. Sci Rep 11, 20952 (2021). https://doi.org/10.1038/s41598-021-00225-6

Images 

1. Refrigeration oil comparison Next generation lubricants and their lubricants by Jen Shiang Tsaih, Patech Fine Chemicals Co. Ltd. Slide 48/62
2. Compatibility of refrigerants with plastics, elastomers and refrigerants. 
3. Which oil is compatible with each refrigerant fluid? By Embraco
4. https://www.facebook.com/1867842833542196/posts/oil-separator-work-in-refrigeration-systemthe-function-of-an-oil-separator-is-to/2836477420012061/