Piston Or Scroll Compressors
To the chagrin of many compressor-manufacturer engineers, I have been a long-time advocate of scroll compressors, and I wrote one of our industry’s first major articles about how they work back in the late 1980s. Why do I like them so well?
Having served as an industry technical instructor for several years, I have taught many classes about how piston compressors work, and of the ways that they can be damaged. And at the time, as the requirements for higher-efficiency systems were being given greater importance by the U.S. government, I could see that the changes which had to be made to get higher efficiencies out of piston compressors would make an already vulnerable product more prone to damage. And once I understood how they work, I could see that scroll compressors would provide both higher efficiencies and greater reliability.
The problem with piston compressors is something called “clearance.” That’s the little gap between the top of a compressor piston when it is at its top position, and the valves and valve plate. In order to get the most efficiency out of this type of a compressor, the clearance has to be minimal, because you pay to compress all the gas, and when some is left in the cylinder to re-expand, you must pay to compress it again.
On the other hand, when clearance is reduced, there is less space for any oil and/or liquid refrigerant that may naturally be present in the cylinder after startup, or when the system is in a refrigerant flood-back condition. And since the laws of physics teach us that you can’t compress a liquid, something is going to fail when there is more liquid than clearance.
However, scroll compressors are not as affected by the presence of liquid, since they just move refrigerant forward in gulps from a low-pressure condition to a high-pressure condition. So there is no piston hitting on liquid between it and the valve plate. However, even scroll compressors can be damaged when too much liquid refrigerant is returning, since this reduces the viscosity of the compressor oil.
Then why did it take so long to bring scroll compressors into the HVACR industry? Because the strict tolerances for precisely manufacturing the scroll plates were once impossible to meet - thank God for computers and automation!
Anyhow, scroll compressors have certainly lived up to their billing, for since they have come into common use, mechanical compressor failures have been greatly reduced. And they have provided impressive efficiency improvements, for when many manufacturers had to raise their system efficiencies to 10-SEER a few years ago, most found that the only change they had to make was to replace their piston compressors with scrolls.
Of course, at every AHR Expo I am collared by some engineer from a compressor manufacturer who wants to tell me that piston compressors are no longer a thing of the past, and that there have been great improvements in their design. Maybe, so I’ll keep an open mind and report it to you if I find this is true.