Varnishing is another problem that has plagued many users of rotary screw and vane compressors since their development. Compressor lubricants are subjected to very harsh operating conditions due to the internal temperature spikes within airends. (Your bulk oil temperature gauge may read 180 degrees F, but temperatures in the airend can be much higher in certain spots.  Mechanical shearing of complex molecules used in oil additive packages, and the constant mixing with contaminants in the inlet air all contribute to the breakdown of your compressor lubricant. 

 A well known rule of thumb tells us that  “for every 20 degrees above the ‘normal’ operating temperature a compressor must run, the lubricant change intervals should be cut in half.”  The photographs that follow show a Sullair airend that had failed due to severe varnishing. (Please note, we have seen this type of failure in all brands of airends. It is a problem in application and maintenance, not in design.) In the foreground of the picture on the left showing the varnish buildup on the input shaft and gear assembly, note the solid clump of varnished oil standing on end. Varnish had built up a half inch thick in several places, totally plugging some oil injection ports.


Could this problem have been avoided? Certainly!  At the risk of sounding simplistic…

Trust your nose! When checking or adding lubricant as recommended in the owners manual, take time to sniff the cork so to speak. Even in early stages of buildup, varnishing begins to give the compressor oil a burnt smell. If your lubricant smells burnt, change it. Be careful to electrically isolate and relieve all pressure from the compressor before checking the oil or removing the filler plug… or your cork may pop!  


For those who are not the hands on, smelly- touchy- feely mechanic types, we would highly recommend regular oil analysis. Not only will it indicate the high acid number and increased viscosity associated with a varnish problem before it becomes severe, but it can also point out several other less easily diagnosed problems. More about that later. 

Of major concern to us in the remanufacturing business is the effect this varnishing has also had on the other components of the compressor package. It is well accepted that even small amounts of varnish left in the oil system will accelerate more varnish formation in the future. All components coming in contact with compressor oil would need thorough cleaning, including the oil cooler, sump, oil lines, thermal valve, oil filter assembly, oil stop valve or oil pump where applicable, and so on. A devarnishing agent may also be added to the fresh oil at startup. However, be prepared with more fresh lubricant and filters, since any varnish deposits left in the oil system will break loose and collect in the filters causing them to plug up and begin the cycle over again.