I am a process engineer new to a cryogenics unit. I am currently experiencing trouble with the refrigeration section of the unit. The cryo refrigerant system was installed in 1985 and originally used R-22 as the refrigerant. In 1997, the refrigerant was changed to refinery grade propylene (~70/30 propylene/propane) and subsequently changed to 99+% propylene. The system is driven by two (a booster and a high stage) Frick rotary compressors (RWB II - 316 and RWB II - 399). These compressors use a moveable slide valve and slide stop to control the load on the system. The booster compressor discharges to the high stage, which discharges into an oil separator/surge reservoir. From this drum, the refrigerant goes through a refrigerant condenser, which uses cooling water on the utility side. The refrigerant then goes through a "rock" filter/drier before discharging into a high pressure receiver, which essentially acts as a surge vessel. The refrigerant then exchanges heat with (i.e. is cooled by) the cryo recovered liquid leaving the unit before going to the economizer. As the refrigerant enters the economizer, its pressure is dropped across the economizer level controller. The economizer has a pressure controller that takes vapor back to the high stage suction. Liquid leaves the economizers and goes to the low pressure receiver where it again expanded across the low pressure receiver level controller. The liquid in the low pressure receiver goes to the quadraplex exchanger by thermosiphon. In the quadraplex the refrigerant cools the inlet process gas for the cryo and vaporizes back to the low pressure receiver. Vapor from the low pressure receiver is then sent back to the booster compressor to begin the refrigerant cycle again.

The problem I am seeing is that there doesn't appear to be room to run the booster compressor, which is limiting recovery in the cryo. When the booster is started the high stage hits a high discharge pressure limit and can't load up, which leads to a high discharge pressure on the booster machine. This high pressure then cause the booster to hit a high amp limit because it can't get enough load to "satisfy" the machine, which is observed as high vibration.

A similar problem occurred last summer and the high discharge limit was raised. After that change the system worked fine, but this summer it doesn't seem to be enough.

My thought is that the refrigerant condenser is fouled or under performing, so we aren't able to get adequate cooling to condense the additional refrigerant circulation when both machines are on. Still, I am wondering if there are any other likely problems that I might be overlooking.

Its probably a reach, but the one thing I am aware of that is different than normal is that the refrigerant (liquid) levels in the economizer and low pressure receiver are higher than usual (but still <50%). I don't suspect any liquid carryover to the compressors. Could too much refrigerant in the system lead to this problem?


(Sorry for the long post, but I appreciate any help or guidance.)