Maximum Discharge Temperature

[dallan]

What factors are used to establish the maximum allowable discharge temperature?

Do the compressor manufacturers issue guidelines?

Up until now I have used 100'C as an absolute maximum, which probablly will not be too far away as an upper limit, but are some compressors more likely to fail at a lower temperature than others.

[chemi-cool]

Hi Dallan.

On every compressor nameplate you will see the maximum working pressure on discharge and suction.
It is written as high side and low side.

Chemi

[Dan]

Marc, I kept my eye out for synthetic oils raising allowable discharge tempertures over the years since the need for them arose with the hfc's. What I recall seeing in the literature is that the oil will break down basically at the same tempertures we were taught with mineral oils, which puzzled me. I am speaking of POE's, specifically.

It is news to me that it is now published that a concern for refrigerant decomposition has taken precedence over a concern for lubrication decomposition.

I find it hard to fathom the implications.

[dallan]

I managed to find some old Copeland info on their Discus compressors which gives a maximum discharge temperature of 107'C.

From a predictive failure point of view what can we expect once the max discharge temperature is exceeded. Will we get mechanical damage due solely to the temperature, damage due to the oil breaking down or a combination of both.

With liquid injection cooling on LT compressors be it a demand cooling module or equivalent what temperature are they trying to maintain in the head, with the demand cooling module a cut in temperature of 105'C seems familiar.

Thanks

David

[Dan]

Dan, found a Danfoss instructions pdf doc that mentions 130 and 135°C. Unfortunately, unlike the paper manual I have here, it doesn't mention refrigerant or oil

I don't know what to think now.

[Andy]

Hi
My opinion would be if the discharge temp is over 105 deg c you have a problem with the long term viability of the compressor. Either the circuit design was incorrect (two stage compression) or the compressor has mechanical damage. It could also be short of gas, but the design of the controls and the system should protect the compressor before the HT cut-out see this.

Me I don't like discharge temp above 90 deg c, but sometimes you have to accept higher, especially on NH3 recips, better using an oil cooler screw on these applications

Increase the temperature and reduce the compressor life, remember the idea oil temperature falls between 45 and 75 deg c, why would we want to see the oil up at near 100 deg c

Kind Regards. Andy

[dallan]

Digging through some books I found a rule of thumb for calculating discharge temperature. It seems not too bad, maybe the results are a little high.

t2=2.5(tc-te) with the temperatures expressed in K.

David