Have a r22 low temp rack with oil cooler on each compressor. What is the minimum oil temp allowed
What kind of compressors do you have?
Since you say each compressor has its own oil cooler, I'm thinking you have Copeland's and you are talking about the oil cooler that sits on top of each compressor and the body cooling fan blows down on it?
If that's the case, I wouldn't worry about it too much.
Now if you have a screw compressor rack with some sort of oil mixing valve or mechanical oil cooler, this is another story, and why it's important you tell us what kind of compressors you have.
Also, what kind of oil do you have? the colder oil is, the thicker it gets & more resistant to flow. So that's important too.
Well, if you wanted to control oil temperature. I would cycle off the cooler with a thermostat on the oil line leaving the exchanger and set it for a 100* minimum exiting oil temperature. 100* will maintain rated viscosity. This is MUCH more important on screw compressors.
As JP mentions, AB oil flashes to a vapor around 310*, so more importantly, make sure your Compressor discharge line is operating below 225*. The internal cylinder walls operate typically 75* above discharge line temperature.