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General Science => General Science => Topic started by: ibmace on 17/03/2010 16:35:16

Title: How cool does expansion of compressed air get
Post by: ibmace on 17/03/2010 16:35:16

Hi All,

I understand that air heats up when its compressed - like when a tire is being pumped up with air the pump become quite hot. Similarly if the compressed air is released and allowed to expand it cools down.

Therefore if I were to use a small electric pneumatic pump to compress air into a cylinder then remove the heat generated by the compression using some means and later if I release the compressed air out via the cylinder's nozzle the cylinder/air must get pretty cold right?

My question is this. Why do air conditioners use CFC and such gases to create low temperatures. Instead why don't they simply compress the air thats to be cooled, release the unwanted heat some place else and then expand the same air to create low temperatures within the space to be cooled.

If I were to try this out how cool can I expect the air to get provided I do manage to get the air compressed enough. If its not a good idea kindly explain. Thanks.

Title: How cool does expansion of compressed air get
Post by: Geezer on 17/03/2010 18:00:29

I think this will work, but only on a rather small scale. To cool down the air in an entire house is going to require a very large volume of compressed air. As you point out, the heat has to be removed from the air after it is compressed, so it has to be stored in a pressure vessel while the heat is removed.

You can get some idea of the volume required if you think about simply mixing the expanding air with the air that is already in the house. It's going to need a lot more than you can get even from quite a large air compressor.

One way around this would be to actually liquefy the air. The liquid air will take in a lot of heat when it returns to a gas. However, it's not so easy to liquefy air. The process requires refrigeration and compression.

That is why conventional AC systems use refrigerants like CFC. When the refrigerant changes state from liquid to gas, it takes in a huge amount of heat, so a relatively small volume of refrigerant can transfer a lot of heat energy.

EDIT:

It struck me there is another problem with the compressed air approach. The energy used to compress the air does two things. It heats the air up and it also compresses it (strangely enough!). The heat energy is dissipated, but some of the energy is stored in the cooled compressed air. (It's analogous to winding up a spring.)

When the compressed air is allowed to expand it does absorb heat, but the energy stored in the compressed air is doing no useful work, so that energy might simply be wasted. However, I suppose you could do something useful with it like assisting the compressor. The expanding air might also do work in a Venturi which can produce some very low temperatures.

Title: How cool does expansion of compressed air get
Post by: ibmace on 18/03/2010 12:10:41

Hi Geezer,

Thanks a lot for explaining things out so very clearly. As you have pointed out to me, I now understand that normal air conditioners do a better job at cooling huge quantities of air with relatively less quantity of CFC. I also understand the shortcomings of trying to cool air directly without using CFC. Thanks again : )

Title: How cool does expansion of compressed air get
Post by: Geezer on 18/03/2010 17:15:00

You are welcome Ibmace. I thought it was an interesting question.

Someone else will probably provide more information or ideas on this one.