# How to size motor for compressor

1. Jul 21, 2009

### martian212

HI - I'm looking for some help. I'm trying to design a low flow single piston reciprocating compressor. Here is what I have:

-Piston bore diameter- 1.50 in
-stroke- 0.100"
-should be able to go up to 30 psig
average pressure will be 20psig
-I am looking for the air flow to be 4.00 lpm at a back pressure of 20 psig
-I believe the rpm of the motor should be about 2700 rpm

How do I calculate what torque will be required at a back pressure of 20 psig and obtain a flow of about 4 LPM.

I would really appreciate the help.

Thanks!

2. Jul 21, 2009

### martian212

One more thing-I plan to use a brushless dc motor for this design

3. Jul 25, 2009

### martian212

4. Jul 25, 2009

### Bob S

You are not going to get much compression with a piston that has a stroke of only 0.100". Please check all dimensions of your compressor.

5. Jul 26, 2009

### martian212

All the dimensions are correct. The stroke is so small because I do not need that much flow.

6. Jul 26, 2009

### ank_gl

You can work that out roughly by energy method. Power = ρgQH = Tω.

7. Jul 26, 2009

### Bob S

You are asking for 30 psig. This is 15 psia to 45 psia, a 3:1 compression. I doubt that you can get this with a 0.100" stroke, when you consider the whole volume that is getting compressed. Are you planning to use 1-way check valves, or is the compressor an overhead valve machine?

8. Jul 27, 2009

### martian212

True, I may have to up-stroke. I am planinning to use a flapper which is similar to a one way check valve. Any thoughts on what my required torque would be? Or how to calculate it?

9. Jul 27, 2009

### Bob S

Don't you have TWO flapper check valves, one on the input and the other on the output? The total compression stroke you have is 0.177 cubic inches with a 0.1" stroke. The TOTAL compressed volume between flapper chsck valves has to be about 33% (or less)of 0.177 cubic inches. How close (in mils) will the piston be to the top of the compression chamber at the top of the stroke? 10 mils? How sacred is your 1.5" piston diameter?