5 replies to this topic

[USR]

Dear Forum members,
I have been working with oil flooded screw compressor(uses propylene refrigerant).Two issues i want to discuss with you all:
1.As it is oil flooded,we have a safety interlock for the compressor.When the oil/gas differential pressure reaches low 2.5kg/cm[2] the compressor trips.This is occurring frequently even after maintaining a positive healthy diff pressure of 3.5kg/cm[2] the diff pressure drops suddenly and the compressor trips.Is this some kind of logic error.
2.If i run the compressor at minimum load can i run the refrigeration cycle effectively by throttling the joule Thomson valve which is infact in the expansion stage of the refrigeration cycle.

[meetyourmaker]

USR,
If there is a filter in the lube oil circuit then you should check whether it is clean. If the differential pressure suddenly drops then it could be a fault with the pressure transmitter. I am not sure i understand the second part of your question. Are you having problems running the compressor at minimum load?

[Zauberberg]

There is a minimum differential pressure that must be maintained in order to ensure oil-flooding of the compressor. Why this pressure differential fluctuates is something that needs to be investigated, and in my opinion - very soon. Compressor manufacturer should be deeply involved in this subject.

As for the second question: load of the compressor refers to circulation of the refrigerant, the way I understood it from your post. It has nothing to do with the J-T valve. The valve is there just to control the level in the refrigerant evaporator/chiller, and its position normally depends on the circulation rate of the refrigerant. If your system is tuned properly, it will self-adjust, including the valve - less demand for refrigeration will result in less % of valve opening. As long as the compressor runs within its operating envelope, there should be nothing to worry about.

[USR]

Dear Zauberberg,
I probably didn't communicate well.My second question is somewhat related to my first question.Due to high ambient conditions at my place condenser capacity has come down which has forced us to run the compressor at no loads(also because of sudden drop in oil/gas differential pressure).So if the compressor couldn't be loaded anymore can throttling the level control valve or the JT valve can aid in some amount of refrigeration.

[pavanayi]

USR,

your second question is still unclear. What I understood from your question was whether the refrigeration would improve if you reduced the refrigerant pressure in your evaporator, from what it normally is, thereby having a higher delta T between your process side and refrigerant side? Please correct me if that is not the case.

What that would mean is then a higher pressure ratio for the compressor to achieve to get the refrigerant back to its discharge pressure, to be condensed against current cooling media. It might be possible depending on the capacity of the installed compressor (the manufacturer is the best party fo advice).


There might be other options available, it depends on the configuration of the refrigeration loop.

[meetyourmaker]

USR,
As zauberburg points out the LC valve maintains level in the evaporator and its opening simply depends on refrigerant flow(compressor loading). Lowering the compressor suction pressure will increase the evaporator duty but will also result in a higher discharge temperature.Also loading the compressor may be required to lower suction pressure.
The cooling water temperature must be a lot higher then the design value since you are forced to operate the compressor at zero loading.