screw compressor high discharge temp

[petrochemical87]

we have an oil flooded screw compressor for propylene refrigeration service that trips frequently on high discharge temperature(78 deg c, ) (usually when the plant is running at turndown)what changes do u guys think can be made to lower the discharge temperatures(lowering suction pressure,increasing economizer duty etc)

 

[petrochemical87]

I have also noticed that propylene coming to the compressor from the evaporators is sometimes highly superheated at the suction conditions as per the mollier diagram
what could be the possible reason? is this the correct forum or should i be posting somewhere else?

 

[zdas04]

That high temp trip must be based on propylene properties, because it doesn't have anything to do with the compressor properties--it is 4C lower than the lowest maximum INLET temperature that I've ever seen for a screw (and 30C lower than the lowest max discharge temp I've seen). If you are using mineral oil (doubtful) in the screw, then the outlet temperature I recommend is 96C to 102C. If you are using synthetic oil, it is fine up to at least 120C.

Anyway, to lower the discharge temperature why in the world would you consider LOWERING inlet pressure? Oil Flooded screw compressor discharge temperature is complicated, but the source of the heat is heat of compression, which is a function of compression ratios. If you want to lower the discharge temperature, one technique is to raise the suction pressure. Another is to increase the flow rate of oil. A third is to lower the inlet temperature of the oil. Finally you can sometimes lower the discharge pressure (e.g., when you have a discharge pressure controller).


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[zdas04]

You might take a look at thread391-294813 to find a pretty good discussion of how discharge temperature can be determined. I plan to turn that thread into an FAQ, but haven't done it yet.

David

 

[Compositepro]

Petrochemical87, a word of advice; This is a professional forum so please do not use text speak, and use capitalization.

 

[dcasto]

Go back to the control room and ask the operators a few questions about what they change during "turndown". specifically about Chiller level. A lot of times they will lower the chiller level to control outlet temps better. This allows for superheating the C3=. In any case, you will find that the operators will lead you into the right direction that Zdas pointed out.

 

[petrochemical87]

Thanks for the reply dcasto.I shall keep that in mind Compositepro.