Oil carry over Problem in Refrigeration Unit

[imranmd]

I am facing oil carry over problem in the Air refrigeration unit installed at N2 plant. We blocked 2 tubes in the Air cooler (evaporator) as these were leaked. The Oil Separator was opened and inspected the float. It was perfectly allright. 68kg refrigerant R134a charged with Clavus 68 Oil. The system perfomance was immaculate after flushing and recharging, but we are facing problem of oil carry over 3L oil to be charged in every week. There is oil recovery line starts from bottom of the Evaporator having Sight Glass goes to the Compressor suction via a small heat exchanger where heat exchanged by high temperature of delivery line of the Compressor. There is one more Sight Glass after that heat exchanger. There is no fluid flowing through these sight glasses. The system is equipped with Hot gas bypass valve also. After adjusting all the working parameters, expansion valve, the oil is not recovering from the system. Can you help me in finding the problem? Can we use Shell Clavus 68 with R134a gas, as we have been using it since last 6 years without any abnormality?

[monkey spanners]

Hi Imranmd

Shell clavus oil is a mineral oil, it is mot mixable with R134a/hfc refrigerants. You need to be using a ester (polyolester) oil such as ICI Emkarate for R134a. I don't know if shell make a ester oil.
Beware, when changeing to ester oil it will flush back the mineral oil to the compressor so you will need to keep an eye on oil levels for a while. Idealy the mineral oil concentration needs to be below about 3%. You can buy testing kits to check this. I don't know why the system worked before on mineral oil but doesn't now, maybe something to do with the bypassed evaporator circuits?
Also ester oil absorbs moisture from the air really quickly so you need to keep the time the oil is exposed to the air to a minimum, even more so than with mineral oil. When changing the oil, a new drier core is a good precation.

Jon