# Using compressed air to displace water

1. Mar 17, 2012

### cocopops

Not really sure how to word the title better!

Anyway, I'm looking at how I could design and do some basic flow calculations regarding compressed air.

The idea is to have a tank of highly compressed air piped to a tank of water. When the valve between them is opened, the compressed air rushes into the water tanks, forcing the water out of pipes at high velocity.

That's the plan anyway. Would this work, and how could I calculate how much air I need, the air pressure/compression ratio, flow rate of the water out of the tanks? Given I know how much water I want to force out.

Any help is appreciated since the only information I've been able to find so far is based on Super Soaker water guns...

Last edited: Mar 17, 2012
2. Mar 17, 2012

### Jobrag

Before you start get a copy of your local design codes for pressure vessels.

3. Mar 17, 2012

### cocopops

Thanks, I wasn't aware of these.

I'm not actually going to be making this system, it's just for a university project.

What I really need to know is what realistic compressed air pressures are? It's easy for me to just say "I'll make it 1000 atmospheres" but I doubt this is reasonable?

Once I know pressure at mass of the compressed air, I think I'll be able to do flow calculations. Would it just be a case of the water being at the same pressure as the compressed air when the valve opens? I'd also guess that this pressure will fall as the air escapes the tank, so how would I account for this?

Thanks!

4. Mar 18, 2012

### cocopops

Still really struggling to find any help on this problem. Any ideas would be appreciated.

5. Mar 19, 2012

### Jobrag

Go to your fluid mechanics text books and look for a worked solution of the problem of how long it takes to empty a tank open to atmosphere with an open valve at the bottom of the tank, and no flow into the tank.

6. Mar 19, 2012

### emailanmol

Hey,

It's a very good idea.

Have you heard of Pascals law of fluids.(valid for all fluids)
(you will also need to use gas equations which are pretty accurate)

And also applying Bernoulli''s equations you can find speed of efflux(but thats valid only for ideal fluids which may pose a problem).

However, by observing practical deviation from expected ideal values you can add some more practical value to your project.

Like you can even create a graph of deviation .

(Also using gas is a great idea)
Hope this helps.

7. Mar 28, 2012

### cocopops

Sorry for the delay in getting back, I've been very busy.

Jobrag, I think I know how to get rough values using Bernoulli's, this is what you meant, right? My issue is computing the changing pressure since the pressure will drop as there is a larger voume of air as water drains out. I hope that makes sense.

Any idea how I would work around this?

8. Mar 29, 2012

### Jobrag

Look at my last post and think about what happens in that situation.

9. Mar 29, 2012

### cocopops

My tank won't be open to the atmosphere though. Sorry if I'm missing something here, it's been a long day...

10. Mar 30, 2012

### Jobrag

What happens to the pressure head at the valve entrance as the tank empties???????