Hi!

I'm not an oil expert. One of our partners has APV Hall screw compressors. This is the type with one horizontal screw and two vertical star type rotor. As they told us the compressor oil cooling method is quite rare: oil somehow mixing directly with ammonia, and being separated after that. Oil viscosity icreasing to double of starting value within weeks.

Any idea or experience? Could it be physical change or result of chemical reaction?

I am no oil expert either and no experience. Just an uninformed swag

1 - Chemical as you suggest. Similar to the effects of mixing different oils which can cause changes like particulates forming from interaction of additives.

2 - Physical - possibly when the ammonia separates it somehow takes the lighter (shorter chain) oil molecules with it and leaves behind longer molecules?

Either way it sounds like a tough situation to tolerate. I would be all over the equipment OEM and also your oil supplier asking for help. To motivate your oil supplier, mention you are considering switching to another brand of oil to address the problem ;-)

Also you might try the forums at noria.com