which would require more energy, to decompress air like a vacuum or compress air?
How would you be able to decompress air to an absolute vacuum? And compress air up to what pressure?
"decompress air like a vacuum"
is a meaningless statement.
Yes, you'll need to be much more specific about what you are talking about. Do you have a specific scenario in mind?
Im assuming its a similar thing to pumping losses in an engine.]
i.e is it harder to compress air trapped in a cylinder, or to pull against a vacuum.
That's the way I read it. In reality, it is tougher to pull a vacuum because those systems are much more prone to leaking.
See, the problem here is that I read the same thing that Chris said and reach exactly the opposite conclusion! We need more info about what the OP is getting at. Anyway...
I wasn't thinking the OP really meant energy because in either case, energy could be infinite depending on what you are asking. Ie, since you can't generate a perfect vacuum, you would expend an infinite amount of energy in the attempt. At the same time, compressing air requires more and more energy as well and the OP didn't specify parameters. Here's what I thought of when I read the OP:
Consider a piston/cylinder arrangement, such as a syringe, 1/2" cross sectional area, 4" long. The syringe is half full of air at atmospheric pressure and is closed. How much energy is required to pull or push the piston 1" in either direction? Roughly:
Pulling on the syringe and moving it 1" would yield a minimum internal pressure of 7.4 psi, force of 3.7 lb, and work of 1.8 inch-pounds. In fact, the maximum possible force you could generate with such a syringe in "vacuum pump mode" is 7.4 lb, which limits the amount of work requried to move the piston. Not so in "compressor mode":
Working as a compressor, pushing it 1" yields a pressure of 14.7 psi, a force of 7.4 lb, and requires a work of 3.7 in-lb.
hi i know what you mean, whilst i was with edwards vcauum when we used to un the vacuum pump and test the current drawn ,,oncea hard vacuum is reached say.005 mbar vacuum the level of current amps to hold this pressure is qyuite low in compairiosn to an industrial compressor..but its not really relative unless you wre talking absolute compression and absolute vacuum as there are way too many variables
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