Why simpler, smaller systems often deliver better reliability, safety, and efficiency.

Nick Chopper hoped that oil wasn’t going out of fashion

The Herrick Compressor Conference at Purdue University has been held every second year since 1972 and at the meeting in July this year it was clear that there is as much to learn as there ever was, if not more. To celebrate the 50th anniversary the plenary address on the last day of the conference was in the form of a panel discussion, moderated by the conference chair, Professor Eckhard Groll, and with three representatives from some of the largest and most successful compressor manufacturing companies in the world.

One topic divided the room into two distinct camps. The question was whether there was any prospect of developing compressors for mainstream refrigeration applications that had no need for lubricating oil. I confess that I am firmly in the (oil)less is more camp. I had never given this much thought until I started working with small, hermetic, oil-free centrifugal compressors in about 2005 and I realized that I had to recalibrate my thinking about what “good” looked like. In industrial refrigeration, draining oil from the evaporators in an ammonia system is one of the most hazardous things we ask our technicians to do, sometimes on a weekly, if not daily, basis.

The compressor guys, not surprisingly, took a different view. Oil provides several useful functions in refrigeration compressors. It lubricates bearings, shaft seals and all other contact surfaces, preventing premature wear and minimizing frictional losses that would otherwise impair the machine efficiency. It also provides a sealing function that reduces internal leakage of compressed gas back to suction, enabling machines to maintain a high degree of efficiency without excessively tight manufacturing tolerances that could make production expensive, if not impossible. In some types of machines, the oil also provides a valuable cooling function, again to preserve efficient operation and minimize losses, and finally, because it is there it is often used as a hydraulic fluid to actuate capacity regulators and other controls.

The question is therefore not “How do we design an oil free compressor?” but rather “How do we do this at the same cost of production and with the same reliability and efficiency as we have at present?” This question extends beyond the compressor: many of the other control components in a refrigeration system, particularly pilot operated control valves, rely on a spot of oil to keep them running smoothly.

Oil has been a recurring theme at the Herrick conferences for many years, but it is as relevant now as it ever was because new materials, new production techniques and new compressor types have rekindled the desire to go oil-free. It seems that the only people advocating oil in compressors at the conference were the compressor guys and the oil salesmen.

However, they have a point. The main theme I picked up from the conference in general was that efficiency is as important as ever. Higher cost of production can perhaps be justified in terms of savings in operating expense, although this has always been a difficult argument to win. On the other hand, if the elimination of oil means that we would sacrifice a point or two in efficiency terms, then I suspect that now is not a good time to raise the topic and we will be oily for some time to come.