Temperature difference between evap & cond

[timbeau]

Hi,

I would like to know the typical temperature differences between the medium in the chiller part of an air conditioner and the hot part (that sits outside). To lose heat in hot countries I am guessing that it has to be pretty hot, and to chill hot air the cold part must be pretty cold!

Anyone ideas/typical examples or links?

Thanks
Tim

 

[Latexman]

Tim,

Based on my experience in the Southern U.S.A. with residential air conditioners (R-12), the evaporator runs about 35-45 F (don't want to build up ice or run under a vacuum) and the condenser runs about 110 - 130 F.


Good luck,
Latexman

 

[timbeau]

Thanks Latexman.

I am looking to find out the extremes possible in a compressor driven condensor-evap mechanism and am guessing that the medium/gas used would be a factor.

I have heard of 'cryocoolers' and I am guessing they may also produce alot of heat, so do these produce wider temperature gradients?

Thanks in advance

 

[ClydeMule]

Timbeau,


The definite answer is... it depends.

The temperature difference you are concered with is the difference between evaporating and condensing temperatures. Compressors are split up into application ranges, which means the compressor is optimized for a certain combination of evap/cond. temps.

High Temp/Air Conditioning 45/130
Medium Temp: 20/120
Low Temp 0/105
Ultra Low Temp -20/105
Cyrogenics Lower than everything above

These are not hard and fast rules, and there are ranges in between. There compressors that are "extended medium temp."

What is happening is that for every evaporating and condensing temp there is a corresponding saturation pressure. The difference in pressure from evaporating to condensing is called the compression ratio. This is one of the major factors in compressor selection. The compressor required to move 1 ton in an air conditioner not only has a smaller motor, but the compression mechanism is built totally different than one required to move 1 ton in a freezer.

The freezer has a much higher compression ratio.

The refrigerant selction also is important. A scroll compressor using R404a can accomodate slightly higher compression ratios than the equivalent compressor using R22.

Operating a compressor outside it's application range will kill the compressor. It sounds paradoxical, but one of the biggest compressor killers in low temp applications is compressor overheating.

Low temperature applications also cost more per BTU than high temp, because the compressor is physically larger, plus there are devices required to keep the compressor and its oil from over heating. Liquid injection and oil coolers are some examples.

All of this applies to single stage systems, which means all of the compression is done in one stroke. Multi stage systems are often used in ultra low or cryogenics, because of the high compression ratios. Basically the total compression ratio is divided up in different cylinders or compressors. Another way of doing this is a cascade sytem.

But this is getting way too long, and cryogenics is out of my knowledge base.

Good Luck

 

[VSSriram]

Two other factors play a big role in the condensing temperature/pressures.

1. Whether it's an air-cooled or water-cooled condenser.
2. Ambient dry bulb temperature for an air-cooled system and ambient wet bulb temperature coupled with cooling tower characteristics for a water-cooled condenser.

The 105 and 130 indicated by ClydeMule may not apply to many countries where the ambient temperatures are much higher.

The corresponding pressures for various refrigerants may be different as well, for the same temperatures. For Example, R410A is a high pressure refrigerant.

As ClydeMule has rightly indicated at the beginning of his reply - It Depends.

 

[pennpoint]

timbeau
Oh yes! and then there's cascade refrigeration, or if your really brave try cocktails of refrigerants that evap. at differant temperatures/pressures which takes it down into the range of -100F or more and condense at temps <+100F. Very exotic indeed.
The one thing you must understand: The air outdoors still must be lower than the designed condensing temperature of the refrigerant in use. That is to say if the design condensing temp is 120F. The outdoor air should be 110F or lower so capacity of the system will not be significantly reduced.

Using your brain, like a mass transit, it can take you anywhere?

Regards
pennpoint

 

[IRstuff]

Cryocoolers of the Stirling-cycle variety use helium as the coolant and are only capable of providing tens of watts of cooling capacity.

Don't forget that that the power requirements are proportional to the temperature difference generated, so cryocoolers must operate in a high vacuum to eliminate convection losses and use mirrored surfaces within the vacuum dewar to minimize radiative losses.

TTFN