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reggie
21-07-2002, 11:13 AM
After reading many posts on screw superheat tolerances has anyone got a rule of thumb for a normal recip compressor superheat rating to provide optimum performance? I dont mean the 7-10c on the low side for cooling the motor.
The actual work done on the discharge side/top of condenser and what temperature range are we looking for at the disharge pipe.

Andy
21-07-2002, 12:02 PM
Hi, Reggie,
Discharge temp is a great big rock that we all can perish on. Even the guys working on the gear can't agree. Discharge temp, or what is though to be aceptable, varies with aplication, refrigerant and quite frankly system designer.
I have worked on 404a supermarket packs where the discharge was as low as 45c and I have worked on pumped NH3 systems where discharge temps exceeded 100c all day, every day.
Screws are used on low temp single stage duty instead of recips precisely because discharge temps can be limited by employing oil cooling which takes heat from the compressor and dumps it out of the refrigeration circuit (water cooled, second circuit in the condenser or a dry air cooler/fluid cooler).
Call me picky but I like a compressor running with a discharge temp of around 65-70c, this allows in a recip your oil to run at say 55c without the need for addtional oil cooling. This is fine and well on high or medium temp applications, with nearly any of the common refrigerants (R12-134a, R22-407c or NH3) but not on low temp, hence the advent of R502 and all it's more recent replacements. All a low temperture refrigerant is designed to do is to increase effeciency at it's operating conditions, or lower the discharge temps.
So what I any realy saying is that discharge temps should not rise with a lower evaporation, if we select the correct refrigerant. Well not always, sometimes we cannot select a refrigerant with all the properties we would like, so we compromise and the discharge temp goes up and my likes go out the window. I supose one case would be NH3, it is a very effecient refrigerant, but discharge temps are high even on single stage high temp aplications, so we add an imtermediate stage (or two) and oil cooling and we throw in a synthetic oil for good measure and still we are lucky to be running at a discharge temp of 95c.
Why do we limit the discharge temp anyway? Well we are using a vapour compression cycle and at the heart of this cycle is a compressor, this compressor uses oil to lubricate it, oil basically works best at certain temperature again 55c comes to mind, this optimum operating temperature varies with the oil employed, hence a change to synthetic oil allows use to run at a higher discharge temp. My advise to systems designers would be stick to the lower discharge temps, don't push the boat out, a good quality highly developed oil does not make-up for a poorly designed application.
Regards. Andy.

Steve25
12-09-2002, 05:34 AM
Manufactures spec thier own ratings, Carlyle "E" body's have a built in sensor in the head, limits temp to around 280 deg F 6" from the head, 325 in the head. The spec books will have a graph showing apox discharge temp based on suction press, temp, discharge press and ref type. Ask a local wholesaler for the charts.

Steve

superheat
12-09-2002, 01:41 PM
Some sources says 250F other say 225F max discharge temp for recip compressor. The temp is influenced by suction superheat and compression ratio. (Ideal Gas Law). Air cooled Semihermetics run lower discharge temp because the suction gas does much less motor cooling (air cooled). The superheated gas at the suction valve is heated some more when it cools the motor. You would think amp draw and quantity of gas would have an effect. That is taken into account in compression ratio in a properly charged properly running system. If the amp draw is higher than normal for that situation, there is something wrong with the compressor and the discharge temp will be higher. A low charge has less gas cooling the motor and higher superheat, so you get higher discharge temp. The high discharge temp is what kills the motor in a cronicly undercharged system.
Discharge temp can be controlled most easily by suction superheat. Reduce suction superheat and discharge temp will fall.

Gary
12-09-2002, 05:41 PM
I agree with Andy. Cooking the oil is what kills the compressors. Discharge temperature is important as a rough indicator of oil temperature. It is a symptom and nothing more. Oil temperature is what really matters.