# The Naked Scientists Forum

### Author Topic: Does hot air require more work to compress than cold air?  (Read 12619 times)

#### Nabo0o

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• Posts: 4
##### Does hot air require more work to compress than cold air?
« on: 09/01/2010 18:05:07 »
Since this area is partly about engineering, I figured this would be the right place to ask this question.
So, presuming the pressure is equal in both instances, does a certain amount of air require more work to be compressed than another but colder amount of air? Or is it perhaps the opposite?

Since I couldn't find any reasonable answer on the net for this question, even by using google, I figured you could give a satisfying answer.

Thanks!

#### somewakko

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• Posts: 6
##### Does hot air require more work to compress than cold air?
« Reply #1 on: 03/05/2010 02:23:38 »
Sorry I'm just reading this now. It really depends on the apparatus you are using, and your specific environment.

I'm assuming this is a scuba air compressor or a garage air compressor. In those cases the volume of the air is greatly reduced, and the pressure and temperature go way up during the compression.  Afterwords, the compressed air cools back to room temperature slowly, and as it does, the pressure drops (during cooling the volume remains constant). If you compress a scuba tank to 3000 psi it might rise to 100 degrees fahrenheit, then if it cools to 70 degrees the pressure will drop to 2839 psi.

If you started with air that was 10 degrees hotter, the compressed tank would simply be about 10 degrees hotter. Ideally the work to compress would be about the same.

But the engineering forum was the right place to ask this question, because you don't care about "ideals". Compressors have much higher internal temperatures than the air they output. The big battle with a compressor is keeping it cool. The cooler you keep it, the more efficiently it works. So dropping the input air temperature is a great start.

But this is where it really depends on your environment and your specific application. If you are choosing between compressing 85 degree outside air versus 70 degree air-conditioned inside air, you probably want to pick the outside air. Why? Compressing the inside air makes your A/C work harder because compressing lots of air means pulling in lots of hot outside air to your building. This is also a bigger load on the compressor because it needs to suck air in through the seals in your doors and windows. If you compress outside air, there is a limitless volume of available air with virtually no resistance to get at it.

If you are choosing between compressing 42 degree outside air versus 70 degree indoor air, you definitely want to choose the outside air.

#### Nabo0o

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• Posts: 4
##### Does hot air require more work to compress than cold air?
« Reply #2 on: 23/05/2010 17:39:47 »
Hey thanks for responding, it doesn't matter if its a little late. I did also ask approximately the same question on physicsforum.com and they gave a little different answer. You can see it here: newbielink:http://www.physicsforums.com/showthread.php?t=369289 [nonactive]

So what they say is that it takes more energy the hotter air we compress, even if its the same pressure going in. Hmmm, maybe I misunderstood that answer.

But so, is that your opinion too if you look at it from the perspective of the other tread, or is the temperature truly irrelevant to the work needed in compression? Thanks!

#### doppler1

• Sr. Member
• Posts: 156
• Bazinga
##### Does hot air require more work to compress than cold air?
« Reply #3 on: 27/05/2010 13:42:43 »
I must agree that the hotter the air is, the more work is required to compress it. Just from a practical point of view, hot air has bigger spaces between each particle than cold air so just to move the particles closer together already requires more work as the cold air is already in a more compressed state than the hot air....or at least that it how it seems to me.

#### Nabo0o

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• Posts: 4
##### Does hot air require more work to compress than cold air?
« Reply #4 on: 27/05/2010 15:00:25 »
What I'm thinking is that pressure is almost an artificial attribute created by us.
What's really the point is how much matter (air molecules) there is and how energetic (hot) they are. Those two things again creates pressure.

But is it really easier to compress many molecules of a less energetic state than a few molecules in a high energetic state?
Of course there is probably a formula for this.....

I'm just interested in getting this straightened out throughly, as it is of critical importance to a project of mine :D

Julian
« Last Edit: 27/05/2010 15:16:24 by Nabo0o »

#### FuzzyUK

• Sr. Member
• Posts: 206
##### Does hot air require more work to compress than cold air?
« Reply #5 on: 28/05/2010 11:48:45 »
So, I should pump my bicycle tyres up on a cold day, eh?

#### The Naked Scientists Forum

##### Does hot air require more work to compress than cold air?
« Reply #5 on: 28/05/2010 11:48:45 »