However, some of the most common refrigerants are thousands of times more polluting than cars, which receive far more focus as sources of greenhouse gas. For example, according to the California Air Resources Board, just 1 pound of R-22—the most commonly used refrigerant today—is nearly as potent as one ton of carbon dioxide. R-404A, a common replacement for R-22, is doubly potent. One small 30-pound tank of R-404A, for example, holds the warming equivalent of the carbon dioxide emitted by driving 14 cars for a year.

These refrigerants aren’t released into the atmosphere during operation the way cars emit carbon dioxide as they run, but they are released when units are improperly installed or disposed of, or if they malfunction during operation. This means that higher-efficiency refrigerants with a lower global warming potential are increasingly important.

More efficient air conditioning can be achieved in a few ways, and refrigeration and air conditioning manufacturers are taking major steps to replace those high-GWP refrigerants. But doing so comes with additional complexity and new implications for the proper operation of reliable refrigeration technology. The use of high-performance fluids and lubricants within these applications is important. This article explores a few reasons why.

Chemical Interactivity

Lubricants for refrigeration applications, like most other applications, are composed of base stocks and additives. But due to the low ISO viscosity grades required in refrigeration applications, there is no thickener or polymer added to increase the viscosity. The most common base stocks used for refrigeration applications are polyol esters in the ISO VG range of 10 to 220, and the most common grades are ISO VG 32 and 68.

Any lubricant formulation must be compatible with the refrigerant used in the application. Generally, miscibility, or the lubricant’s ability to mix together with the refrigerant to form a single-phase solution, is one of the most important considerations.

Additionally, the solubility of the lubricant in the refrigerant must be balanced for the system in use. If the refrigerant is too soluble in the lubricant, this will cause viscosity reduction and could result in wear on the moving parts of the compressor. If the refrigerant is insoluble in the lubricant, this could cause lubricant carryover into the system, resulting in efficiency reduction as well as other issues.

New, low-GWP refrigerants have a wide range of polarities, and as a result, there is a variety of base stocks that could be used. These include mineral oil, polyalkylene glycol, polyol ester and alkylbenzene.

The types of additives typically used in refrigerants are antioxidants, extreme pressure and antiwear additives, corrosion inhibitors, defoamers and acid scavengers. With these additives included, some low-GWP refrigerant formulations are inherently less stable, and compatible new lubricants must be able to interact with these new chemistries and provide the added benefit of stabilizing the refrigerant chemistry.

What follows is a breakdown of some of today’s refrigerants, some proposed replacements and several compatibility considerations with different lubricant formulations.

R-410A. While the chlorofluorocarbon R-22 had traditionally been used with mineral oil lubricants, it has largely been replaced by the less ozone-depleting hydrofluorocarbon blend R-410A (a blend of R-32 and R-125), which is more polar and requires the use of a polar lubricant like POE. At the moment, there is no simple, direct replacement for R-410A. In order to reduce the global warming potential of the refrigerant, many newly proposed formulations are at least mildly flammable. For example, R-32 or blends that contain R-32, like R-452B or R-454B, are mildly flammable (classified by the American Society of Heating, Refrigerating and Air-conditioning Engineers as A2L), polar and miscible with POE.

R-134a. This refrigerant was used as a replacement for the older, ozone-depleting R-12. The nonpolar R-12 refrigerant is miscible with mineral oil lubricants, while more polar R-134a is miscible with POE. As with R-22, there is no simple replacement for R-134a, and likewise, refrigerant formulators are forced to use mildly flammable refrigerant solutions in many cases to reduce the global warming potential. Several replacement candidates have emerged as a result. The hydrofluoroolefins R-1234ze(E) and R-1234yf, and blends that contain them like R-513A, are all slightly flammable, polar and miscible with POE. Polyvinyl ether and PAG are currently being considered for use.

R-404A. The HFC refrigerant mixture of R-404A was used as a replacement for the older, ozone-depleting CFC mixture R-502 (blend of R-115 and R-22). The nonpolar R-502 is miscible with typical mineral oil lubricants, while R-404A, which is a mixture of several HFCs (R-125, R-134a and R-143a), is more polar and miscible with POE lubricant. Once again, reducing global warming potential also moved toward increased flammability, so a few replacement candidates have emerged. Applicable, nonflammable refrigerants include R-448A or R-449A, but these don’t offer much reduction of global warming potential. Mildly flammable refrigerants like R-454C or R-455A have a lower global warming potential. In all cases, compositions of the refrigerants vary by adjusting the ratios of the HFC and HFO, and these polar mixtures are miscible in POE, PVE and PAG.

The takeaway? As OEMs begin to utilize new refrigerants, there is not an immediate, one-size-fits-all solution for compatibility between high-performing lubrication and low-GWP refrigerants. Considering the refrigeration and air conditioning industry at large has not yet settled upon a few common low-GWP refrigerants, there is a complex balance that must be struck.

In the short term, it’s likely that a variety of different refrigerant formulations will be available. And that means modern lubricants must be able to work in harmony with a variety of new refrigerants, available in versatile formulations that deliver the right performance characteristics and enable ongoing reliability of air conditioning and refrigeration systems.

Efficient Protection

Air conditioners and refrigerators utilize a variety of internal components to operate, including compressors and other moving parts. Just like an engine or industrial equipment, those moving parts need to be lubricated for protection and reliable operation.

For example, there are several types of compressors that are commonly found in refrigeration technology that must be properly lubricated.

• Reciprocating compressors. Moving parts include the piston, connecting rods and bearings. These systems typically use splash lubrication to provide lubricant to the crankshaft, connecting rod and piston. In splash lubrication, a revolving part dips into a reservoir of lubricant, then slings it onto other parts of the machine.
• Screw compressors. Moving parts include screws and bearings. Splash lubrication is used in small systems, and injection lubrication—which delivers a precise amount of oil directly to the ball and roller bearings—is used in larger systems.
• Scroll compressors. Scrolls and bearings are lubricated via splash lubrication. In these systems, it is critical to lubricate the crank shaft and thrust bearings.
• Rotary or rotary vane. Here a rotor, vane, shaft and bearings are all lubricated using splash lubrication. In these systems, it is critical to lubricate the rotor, vane tips and thrust bearings.
• Centrifugal. Impeller, shaft and bearings must be lubricated. These systems are generally larger, requiring injection lubrication to adequately protect the journal and thrust bearings.

When it comes to practical protection and performance, the lubricant plays a dual role. First, a high-performing lubricant will eliminate friction in the application, enabling the compressor and other parts to move more efficiently and to generate more cool air more quickly for the application.

Second, the lubricant’s ability to protect each of these important parts ensures greater longevity for the entire piece of equipment, lengthening its service life and time needed before repair or replacement. These qualities are especially important given that the emission of high-GWP refrigerant gases typically occurs as a result of improper installation or service practices.

It also speaks to the importance of highly protective lubricants in all applications—high-GWP refrigerants are being phased out, but they won’t disappear overnight. It’s incumbent upon the lubricants industry to offer the right protection today and in the future.

As refrigerant and air conditioning OEMs and fluid suppliers look to minimize the environmental impact of their systems, it is essential to work with a lubricant supplier that can meet a range of changing performance needs and requirements. And as lubricant suppliers look to meet and deliver what their customers need, identifying the right chemistry solutions is equally important.

Highly efficient refrigeration and air conditioning technologies will be increasingly important in a changing world. For the lubricants industry, it’s incumbent upon us to enable the broad-scale use of low-GWP refrigerants.

Michael Costello, Ph.D., is strategic research and innovation director at CPI Fluid Engineering, a division of Lubrizol Corp. He possesses technical expertise in chemistry and lubricant formulation, as well as product development programs for industrial and refrigeration compressors. Contact him at mike.costello@lubrizol.com or (989) 698-1149.