# Vacuum vs. Compression: Which Requires More Energy?

• violin_writer
In summary, the conversation discusses the energy requirements for decompressing air to a vacuum and compressing air to a certain pressure. The participants also mention the difficulty of pulling a vacuum and the potential for leakage in vacuum systems. The comparison between the two processes is dependent on specific parameters and variables.
violin_writer
I wander...
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.

FredGarvin said:
That's the way I read it...
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...
In reality, it is tougher to pull a vacuum because those systems are much more prone to leaking.
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

Last edited by a moderator:

## 1. What is the difference between vacuum and compression?

Vacuum refers to a space that is devoid of matter, while compression refers to the process of reducing the volume of a gas or substance by applying pressure.

## 2. Which requires more energy, creating a vacuum or compressing a substance?

This ultimately depends on the specific situation, as both processes can require varying amounts of energy. However, in general, compressing a substance usually requires more energy as it involves actively pushing against the natural state of the substance.

## 3. Can vacuum and compression be used interchangeably?

No, vacuum and compression are two distinct processes that cannot be used interchangeably. While they both involve manipulating the state of matter, they have different effects and uses.

## 4. What is the purpose of creating a vacuum or compressing a substance?

Creating a vacuum is often used in industrial and scientific processes to remove air or other gases from a space, while compression is used for a variety of purposes such as storage, transportation, and chemical reactions.

## 5. How does energy play a role in vacuum and compression?

In both vacuum and compression, energy is required to manipulate the state of matter. In vacuum, energy is needed to remove the air or gas from a space, while compression requires energy to apply pressure and change the volume of a substance.

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