unloaders and oil press.

[aaron crimmins]

can anyone tell me why when a compressor unloads the oil pressure falls? i can watch the oil pressure on compressors and as it fully unloads the oil pressure instantly goes down? these are suction cut unloading heads. i also notice when they unload the oil foams. this is not a problem i am having with any particular unit, just see it all the time. thanks for the info.

[Gibson]

What's the application? Carlysle compressor on air conditioning? Remember the compressor is doing less work when it unloads... Some cylinders become useless... or useful depending on how you look at the unloading situation!

[Robearbam]

From my experience on a 100 ton R-12 system, oil pressure is directly related to suction pressure. The oil pressure gauge fluctuates with suction pressure, this is normal. Net pressure (Oil pressure minus the suction pressure) is what you should be concerned with. I know on a 12 system my oil pressure must be maintained at 35 to 40 PSI net. This design pressure seems to be what a radial or recip is comfortable with.
Foaming ... don't you just love it! I have to maintain just the right head pressure to prevent this. When ambient changes and cooling tower water gets a few degrees below what it runs at on a hot day ... Mr foaming returns. Hope this helps

[Brian]

the reason the oil pressure drops off is because the loaders drop out which are actuated by the oil pressure, this causes a drop in pressure because they open up,
then the foaming appears because the expansion valve cannot react quick enough to the change in load, so it slugges liquid back.