Why Must Refrigerant Be Compressed in Air Conditioners?

[haynewp]

I have been trying to learn how home air conditioners work since I had some problems yesterday with my split system. I was reading here:

http://www.energyquest.ca.gov/how_it_works/air_conditioner.html

This is a stupid question and basic mechanical engineering, but there is something about the process that seems counter-intuitive to me. I understand that the evaporation part is essential to removing heat from your house. But when compressing the refrigerant, you are adding heat into the system which has to be removed anyway in the condenser.

My question is; must the gas be compressed in order for it to condense into a liquid so that the evaporation can occur? Or is this just a necessary part of the pumping part of the process?

 

[Averagesupernova]

It must be compressed to make it warmer than the surroundings. This way the surroundings can absorb the heat out of the condenser and ideally make the condeser the same temp as the surroundings. This heat in the condenser does not come free. It also does not come solely from the inefficiency of the compressor (friction). Ok, so now we have (ideally) the refrigerant the same temp as the surroundings and in a compressed state. When the pressure is lowered on the refrigerant it aborbs heat (becomes cool). Since it was already cooled to the temp of the outside air (in your case), it has to get cooler.
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As for the requirement to be compressed into a liquid: Virtually any thing can be used as a refrigerant. However some substances are much more suited than others. It is a natural property of all things to heat when compressed and cool when the pressure is reduced. So while it is not a requirement to condense into a liquid for this property to be apparent, it is most efficient.

 

[Mech_Engineer]

I understand that the evaporation part is essential to removing heat from your house. But when compressing the refrigerant, you are adding heat into the system which has to be removed anyway in the condenser.

Well, the problem with this statement is that when the working fluid is being compressed, you're not really adding heat, the temperature of the fluid is going up because its specific volume is decreasing; "adding heat" implies that energy is being added to the system to raise the temperature. By compressing the fluid and raising its temperature, you can remove heat through an exterior radiator (outside the fridge, outside the house, etc.) to bring the fluid in it's new state to ambient temperature. This way, when the fluid goes through an expansion, it becomes relatively cold (colder than ambient), and can absorb thermal energy from the area being cooled, such as a freezer compartment or air-conditioned room. In this way, energy can be removed from a cold area and dissipated in a warm area.

 

[DaveC426913]

Yes, an air conditioner produces its own heat. It needs to move the heat elsewhere - that is both heat from the room and heat of its operation (including heat from compression/condensation).

 

[haynewp]

Well, the problem with this statement is that when the working fluid is being compressed, you're not really adding heat, the temperature of the fluid is going up because its specific volume is decreasing; "adding heat" implies that energy is being added to the system to raise the temperature. By compressing the fluid and raising its temperature, you can remove heat through an exterior radiator (outside the fridge, outside the house, etc.) to bring the fluid in it's new state to ambient temperature. This way, when the fluid goes through an expansion, it becomes relatively cold (colder than ambient), and can absorb thermal energy from the area being cooled, such as a freezer compartment or air-conditioned room. In this way, energy can be removed from a cold area and dissipated in a warm area.

I see now, I was thinking of it as adding energy to the fluid as a result of the work done during the compression process.

 

[Mech_Engineer]

I see now, I was thinking of it as adding energy to the fluid as a result of the work done during the compression process.

A small amount of heat may be added to the fluid because the compressor will not be 100% efficient, but for simple problems this is usually disregarded.

It also depends on how the compressor is modeled, such as isentropic vs. isothermal. There will usually be an efficiency rating for the compressor that you use in your calculations.

 

[quark]

Contrary to the above suggestion, entire work done by the compressor (leaving only part of it) is being added to the refrigerant, as you understood correctly, and finally rejected to the sink (generally,atmosphere). This is very essence of a refrigeation or heat pump cycle.

As to your original question, the refrigerant is compressed to rise its condensation temperature, so that phase change takes place at ambient temperature.

Any method that will increase condensation temperature of the refrigerant does add heat (vapor absorption or vapor compression) and is to be discharged into the atmosphere, ultimately.

 

[russ_watters]

Nice catch (3 weeks later...). In anything, whether a thermodynamic cycle or the lights in your home, all of the energy used ends up as heat.

 

[DaveC426913]

I was running our AC the other night, and I wondered about how quickly it is able to cool air. Specifically, how fast does the temp drop and rise again as the coolant circulates?

I guess what I'm looking for is, in a standard household AC:
If I injected some hypothetical traceable object into the system:
How long (in seconds) would it take to complete one circuit?
How far (in metres) would it travel to complete one circuit?
And I guess, one more: Of that circuit, how many metres are devoted directly to the compression process and the expansion process? (Ideally, the sum of the two would be 100%, right?)