Von Linde is credited with launching the age of modern industrial refrigeration in 1877, when he patented a machine that harnessed the cooling action of expanding gases and used it to manufacture artificial ice. Others had experimented for decades with this concept, called vapor-compression refrigeration, but von Linde was a leader in commercializing the idea. His ice maker used ammonia as its refrigerant. Allowed to rapidly expand, the ammonia vaporized into its gaseous phase and generated coolness. The gas then was compressed at high pressure and directed into a condenser, where it regained its liquid state and gave up its heat.

Linde also mixed mineral oil with the ammonia, so the fluid provided lubrication to the compressors moving parts and corrosion resistance to the rest of the system (evaporator, valves, condenser and piping) with each pass through the circuit. Ever since, refrigerant and oil have cycled together through most refrigeration units, going from the cool side to the warm side and back again.

Whats changed in the intervening years, says Wolfgang Bock of Fuchs Europe Schmierstoffe in Mannheim, Germany, is the array of refrigerants that are used, the variety and complexity of todays compressors, and the highly specialized nature of refrigeration oils. Being able to handle the extreme temperatures on both sides of the cooling-and-heating circuit is just the first of many hurdles the oil will face. Each lubricant formulation also must be tested for its compatibility with the intended refrigerant, of which there are literally hundreds.

Bock, a Fuchs product manager for industrial oils, spoke last month at the 18th International Colloquium Tribology at the Technische Akademie Esslingen in Ostfildern, Germany. To serve this market, he told the conference, you need to know all about refrigerants, compressor types and the new refrigerants coming to the market – which are not always easy to handle when it comes to lubrication.

High Expectations

Refrigeration oils have very high requirements with regards to raw materials, production and quality controls, Bock noted. The oils must have extremely low moisture content for example (less than 50 parts per million), and special manufacturing equipment, filling machines and packaging materials are needed to maintain their purity. Also, Bock said, the lifetime expectation for refrigeration units is anticipated to be 20 to 30 years, and the oil needs to be guaranteed in the application.

Polyol ester (POE) refrigeration oils are widely used in air condition systems for vehicles, including buses, which rely on piston compressors. The shipping industry typically uses scroll compressors to chill individual shipping containers, and these also take POE based refrigeration oils. Foodstuff applications, such as beer manufacturing and dairy plants, normally use ammonia refrigerant and much larger piston compressors; here the lubricant must handle freezing temperatures as low as -30 degrees C (-22 F), so synthetic polyalphaolefin (PAO) lubricants are ideal, said Bock. And most recently, commercial supermarkets such as Europes Aldi chain have begun using innovative units from Carrier-Linde with CO2 as the refrigerant, presenting a fresh opening for lubricants based on esters and polyalkylene glycols (PAG).

For many decades, refrigeration oils were based largely on naphthenic base oil (as many still are) and refrigerants themselves were chlorofluorocarbons (CFCs) such as R-12 or Freon, made by Dupont and others.

Then the big discussion started about 20 years ago around chlorine, due to its contribution to ozone depletion, Bock said. Their harmful chlorine content sparked an international ban on CFC refrigerants, and their manufacture ended around 1995. This led to the introduction of other fluorinated refrigerants, which demanded polyol ester based lubricants, Bock said.

Initially, refrigeration equipment manufacturers turned to hydrochlorofluorocarbons (HCFCs), which did not deplete ozone quite so much as CFCs, and then to hydrofluorocarbons (HFCs). The tide is turning again, however, with a new emphasis on refrigerants global warming potential.

The Next Wave

Now were seeing the phase-out of HFC fluorinated refrigerants too, said Bock. Were seeing more use of environmentally acceptable natural refrigerants like R-744, which is CO2, and R-717 (ammonia).

These changes have reignited the search for another generation of lubricants, one that is compatible with both the new refrigerants and the revamped systems that use them.

One of the first to switch away from HFCs is vehicle air conditioning systems. This market, Bock said, was dominated until now by Japanese lubricant manufacturers, but today faces an immediate phase-out of fluorinated refrigerants, which opens it up to new options. What will replace HFC? The two principal contenders are CO2 and a hydrofluoro olefin called R-1234yf. The latter has been commercialized as HFO-1234yf by a joint venture between Dupont and Honeywell.

HFO-1234yf is a propane derivative, or 2,3,3,3-Tetrafluoropropene. Fuchs has developed two types of its Reniso brand lubricants for these systems – one based on PAO, one on PAG. Bock noted that all HFO-1234yf refrigeration oils must be stabilized with specific additives to assure thermo-chemical stability, anti-wear and metal surface protection. Without this, PAG lubricants may have hydrolysis issues, where they absorb water and lead to corrosion throughout the entire refrigeration circuit.

Almost all current vehicle air conditioning equipment can use the new HFO-1234yf refrigerant because it is compatible with the same seals and components that were designed for the current refrigerant, R134a. With the new lubricant, they should be good to go. But if car air conditioners want to adopt CO2, Bock said, this will require wholesale replacement of the equipment, much higher pressures, and another look at the lubricants.

Mixing It Up

Air conditioning for vehicles is just the tip of the iceberg, as other refrigeration systems face similar mandates. There are many types of compressors too, and each design needs an optimal mix of refrigerant and oil, Bock said. The small hermetic compressors found in home refrigerators are not like the axial/radial piston compressors used to cool a bus, or the scroll compressors in commercial refrigeration systems, or the room-size screw compressors favored for large-scale industrial chillers. Many equipment manufacturers are pondering what refrigerant to use, which oil, and in what proportion the two should be mixed for each application.

Currently, to assure a proposed lubricant is fully miscible with the selected refrigerant, the two are mixed, heated and cooled according to the DIN 51514 standard test. Then the blend is checked to see if it remains intact, or if there is phase separation. Any phase separation is a negative because it means you have oil separating out from the refrigerant, Bock commented. You also need to know if the refrigerant may dissolve into the oil and create a viscosity reduction.

Another tool that helps OEMs gauge the miscibility of their selected refrigerant and the proposed concentration of oil is a pressure-viscosity-temperature diagram. Theyll also look at the lubricants elastomer compatibility, antiwear and corrosion performance, and foaming behavior.

Future Trends

Two of the rising stars in commercial equipment now are ammonia and CO2. CO2 requires high compression, which generates enough heat to expose the oil to temperatures in the range of 140 to 160 C (284 to 320 F). So you must test the lubricant at 220 degrees C for two weeks at pressures of 50 bar of CO2, Bock said.

Following that drill, the oil is measured for changes in its viscosity and chemical structures. Based on these thermal stability tests, the best lubricants for CO2 refrigerants appear to be polyol esters, but PAG and PAO are also showing promise, Bock indicated. Performance of mineral oil with CO2 has been poor, though. Key issues for formulators include how to deal with dissolved CO2 in the oil and selecting the right antiwear additives, Fuchs has found.

Finally, Lindes old standby refrigerant, ammonia (NH3), has never left the field and is one area where naphthenic base oils continue to hold their own, along with PAO. NH3 is gaining ground again, especially as a refrigerant for large-scale and commercial applications, thanks to its benign environmental footprint. However, this refrigerant usually is limited to open equipment, like large open reciprocating piston compressors or open screw compressors, which is served by an oil pump or centrifugal lubrication. It cannot be used with sealed compressors because the ammonia will attack the electric motor elements, Bock explained.

PAO is best in these open compressors with ammonia, he added, but you first need to check the fluid for seal compatibility, or else in very short time you will have all of the oil come out of the system. Pure PAO has a shrinking effect on some seal materials, so you must optimize the whole system.