Rotary screw air compressor oils must maintain their ability to lubricate, cool, seal, clean, and protect compressor internals across a wide range of ambient and operating temperatures. From freezing cold start-ups to scalding hot full load operation, these variations in temperature cause both temporary and permanent changes to the oil’s physical, chemical, and performance properties. Understanding the effects these varying temperatures can have on the oil and compressor is crucial for maximizing reliability and reducing maintenance costs of your rotary screw air compressors. Here’s a breakdown of what you need to know.
Temperature & Oxidation / Service Life
High Discharge Temperatures:
- Problem: Oxidation rates double and oil life is cut in half for every 18°F increase above the lubricant’s rated temperature. For example, the typical compressor oil rated for 8,000 hours at 200°F will oxidize and degrade twice as fast, and can be expected to last only half as long, or 4,000 hours, if operating at 218°F.
- Effects: Oxidation of compressor oils produces acids, increases the oil’s Total Acid Number (TAN), reduce its pH, and accelerate corrosion. Oxidation also depletes the oil’s protective oxidation and corrosion inhibiting additives, which accelerates other harmful chemical reactions that can increase viscosity and produce varnish and sludge in many oils, including synthetic hydrocarbons (SHCs) and polyalphaolefins (PAOs). These solid deposits can foul oil coolers, restrict oil flow to bearings, cause temperatures to increase, and reduce the useful life of the compressor’s oil, bearings, separators, and oil filters.
- Solution: Monitor compressor discharge temperatures, keep oil coolers clean, and maintain oil temperatures within recommended limits (< 200°F). Perform routine oil analysis to monitor oil TAN, pH, and viscosity. Use a polyglycol (PAG) based synthetic oil with high Viscosity Index (VI), that will not form varnish or sludge, and will keep the compressor running clean and cool. For added protection, reduce oil oxidation rates, control acid buildup, and dramatically increase oil life by continuously removing the oil’s acids using a compressor oil purifier (COP) with ion exchange technology, and periodically replenish the oil’s additives.
Temperature & Viscosity Changes
High Temperatures:
- Problem: As operating temperatures increase, the oil’s viscosity, or resistance to flow, naturally decreases.
- Effects: When oil temperatures get too high and its viscosity drops too low, the oil’s film thickness and strength can become insufficient to properly lubricate and protect compressor bearings. Without adequate viscosity and lubrication, metal to metal contact of bearing components can occur and result in excessive wear, reduced air-end life, or even catastrophic failure.
- Solution: Ensure the compressor is filled with the proper viscosity oil (typically 38 – 46 cSt @ 40C) and operates within its recommended temperature range. Choose a fluid with a high VI and strong oxidation resistance to maintain proper lubrication and fluid life at elevated temperatures. Monitor oil viscosity with routine oil analysis. When viscosity is out of its recommended normal range, investigate for contamination (varnish, sludge, wrong oil added).
Cold Temperatures:
- Problem: Starting compressors in freezing cold temperatures can cause lubricant viscosity to increase dramatically. For example, a typical 46 cSt viscosity fluid might thicken to 1000-2000 cSt in extreme cold temperatures depending on its Viscosity Index (VI). This can be especially problematic for compressors located in cold climates that operate with Auto Start/Stop.
- Effects: Cold, thick, high viscosity oils struggle to flow and circulate, which can starve compressor bearings from lubrication and cause the compressor to quickly overheat and shut down. If oil viscosity is too high at start-up, it can dramatically increase the starting torque and electrical draw, and risk tripping circuits and sheering couplings or even the drive shaft.
- Solution: Protect the air compressor from freezing temperatures and/or use sump heaters with insulation to keep the oil warm when not in use. On air-cooled compressors, regulate the fan’s air flow and temperature by using a variable speed fan and/or hot air recirculation. Use a compressor lubricant with good low-temperature fluidity and high VI (> 170), like those formulated with synthetic PAG base stocks. Higher VI lubricant viscosities change less with temperature variations and perform better in extreme cold and hot conditions.
Temperature & Water Accumulation
Low Discharge Temperatures:
- Problem: Air compressors are designed to operate hot. Operating a compressor at too low a discharge temperature can cause water to build up in the lubricant and settle to the bottom of the sump when not in use. Low temperatures can be caused by excessive part load or unload operation, a faulty thermal valve, or excessive cooling. This can occur with air-cooled compressors operating in sub-freezing temperatures or water-cooled compressors supplied with too cold, or too much, cooling water.
- Effects: Excessive water in a lubricant can cause a variety of problems ranging from accelerated additive depletion and oil oxidation, which leads to wear and corrosion, and ultimately reduces oil, separator, and bearing life. Upon start up any water that has settled out in a cold sump will slug the bearings to wash away the lubricant, reduce lubrication, and increase wear. As a result, many compressor OEMs recommend users manually drain the water off the bottom of the sump before each cold start up.
The maximum amount of water a lubricant can hold or tolerate before it becomes problematic is unique to each formulation and primarily depends on the lubricant’s base stocks. Polyglycol (PAG) based oils can normally tolerate up to 8,000 ppm of water in solution before destructive “free water” is formed. Whereas synthetic hydrocarbons (SHCs) and PAOs can typically tolerate less than 500 ppm of water in solution before free water is formed.
- Solution: Discharge temperatures should be regularly monitored to ensure the compressor is running hot enough to keep the water, ingested as humidity in the air, in vapor form. Rotary screw air compressors need thermal valves to force them to run at elevated temperatures to keep water from condensing in the oil at elevated pressures. For this to occur, the oil’s discharge temperature should be kept at least 100°F above the ambient air temperature. Discharge temperatures lower than this are often the result of excessive cooling from a faulty cooling system, a leaky discharge check valve when the compressor is not running, or the compressor frequently runs unloaded, or at part load for extended periods of time.
Monitor the oil’s water content with routine oil analysis. When your oil’s water level is above its recommended limit, first check to see that the oil sample is being taken properly, from a hot flowing source, and not from a cool compressor at the bottom of the sump tank where water naturally accumulates. Next, confirm that the lab knows the specific water alarm limit for your particular compressor oil. Without this information, the lab can erroneously flag your oil’s water content when it’s perfectly normal, and mistakenly recommend costly replacement or dehydration.
Finally, if excessive water content is confirmed, the most likely cause is related to the oil’s operating temperature and cooling system should be investigated to identify the source of the water. Simply replacing the water contaminated oil with new oil is not a solution. Without addressing the problem at its source, the water problems will quickly return to the new oil.
Key Takeaways
- Monitor Temperatures: Ambient and compressor operating temperatures need to be regularly monitored and maintained within acceptable limits to avoid the negative effects temperature can have on the physical, chemical, and performance properties of rotary screw air compressor oils. Failure to keep temperatures within their limits will cause temporary and permanent changes to the oil that negatively affect the reliability and maintenance and repair costs of your compressors.
- Perform Routine Oil Analysis: Regularly monitoring the oil’s viscosity, water content, TAN, pH, and additive levels will alert compressor users to problems often related to the oil’s operating temperature. Understanding the cause-and-effect relationship between oil temperature and the oil’s properties, performance, and service life, enables compressor users to identify, correct, and avoid future problems.
- Choose the Proper Oil: Use high VI lubricants for better performance in extreme temperatures. Use water tolerant, non-varnishing polyglycol (PAG) compressor oils in high temperature and high humidity environments. Older compressors, using a 46 cSt PAO or SHC oil with a history of running hot (> 200°F), would benefit from using a lower viscosity 38 cSt PAG oil to improve cooling and lower temperatures.
- Be Proactive – Purify & Replenish: The greatest impact temperature has on compressor oils is the dramatic reduction in oil life, caused by accelerated oxidation rates, acid production, and additive depletion. However, by proactively removing the acids using a Compressor Oil Purifier, and periodically replenishing the oil’s protective additives, users can easily mitigate the effects of temperature, retard oxidation rates, increase protection and reliability, and dramatically increase oil life.