100% Outside Air Gas/Electric Packaged Unit
100% Outside Air Gas/Electric Packaged Unit
(OP)
I'm investigating the operation of an existing 30-ton 100% outside air package unit (nominal 12,000 cfm). Our design conditions are around 105F. I was told that if we try to cool 12,000 cfm to 55F we can ruin our compressors. Our options are therefore to either lower the cfm or increase the supply temperature. Can you please explain how this could create a situation where we would reuin our compressors? Thanks.





RE: 100% Outside Air Gas/Electric Packaged Unit
I think some of the concern is that the temperature of the suction gas may not provide enough cooling for the motor windings(if this is a hermetic compressor) and the oil temperature could get very high. Also consider that you are going to be trying to condense the refrigerant with this same 105 degree air. The head pressures are going to be extreme. Disreguarding any latent heat which is likely the best you could do is about 77 degrees but your suction temperatures are still going to be 85-90 degrees at best this will still put your discharge temperatures way up there, threatening oil break down.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: 100% Outside Air Gas/Electric Packaged Unit
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: 100% Outside Air Gas/Electric Packaged Unit
RE: 100% Outside Air Gas/Electric Packaged Unit
RE: 100% Outside Air Gas/Electric Packaged Unit
Just buttin' into your thread.
Regards
RE: 100% Outside Air Gas/Electric Packaged Unit
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: 100% Outside Air Gas/Electric Packaged Unit
Man it's bad when at 2:00am you wake up and realize that you (me) made a large error in posting a response to your (EEJaime's) post. If you use an evap cooler to pre-cool the air in the above scenairio you will still not be able to achieve the desired 55 degrees. You might lower the entering air temperature enough to avoid damage to the compressor due to high discharge temperatures, but 30 tons is 30 tons.
Evaporative cooling is an adiabotic process were your changing temperature of a substance but you have not change in the enthalpy, heat is heat.
Well that being said, now I can get some sleep tonight, G-D I love this sight!!!
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: 100% Outside Air Gas/Electric Packaged Unit
Also, if/when conditions allow, the compressors can stay off and the pre-cooler can do all the work.
If anyone can corroborate or dispute this please do so as we are considering this option.
I love this site too, you all are very helpful.
RE: 100% Outside Air Gas/Electric Packaged Unit
I'm thinking that you can cool exhaust air using the evaporative cooler then run that exhaust air through an air to air heat exchanger to pre-cool the make up air. I would look at several different options and plot them out on a psychrometric chart or use a program if you have one. With out being too bold I have a FAQ posted on the HVAC forum on psychrometrics if your interested.
Back to the unit, I might, with out looking at the numbers too close lean toward a run around coil as it can function as a pre-cooler and pre-heater. But too be sure I would do an analysis on all options. The key is keeping the latent load down and the discharge temperature of the refrigeration system low as well.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: 100% Outside Air Gas/Electric Packaged Unit
Other tack would be to reduce conditioned air makeup. Can't untreated OA be used for direct exhaust makeup similar to kitchen hoods with integral direct OA makeup up to about 70% and the makeup only requiring heating to 60°F, no cooling?