air lines and valve flow rate

[merrill77]

I'm considering putting a solenoid-actuated valve on my air tank, so I can turn that on/off remotely from the shop. My lines leak a little, so I'd prefer to only have the compressor run when I'm actually using it. The flow specification for the valve says "cv 7". The valve has 3/4" fittings which, I think, will be correct for my compressor. Anyone know what effect that flow spec will have in effective flow rate? IIRC, my compressor is rated for ~10 CFM at 135 psi - I do not foresee every using more than one tool at a time, but do hope to try my hand at spraying with an HVLP system.

TIA!
Chris

 

[CDPeters]

Chris -

The CV rating refers to liquid service. It is the number of gallons of water the valve will pass with a pressure differential of 1 psi.

With a 3/4 NPT fittings, my guess is that this valve should be plenty for your needs. Maybe Ethan will pipe in with a much more technical evaluation :gar-Bi.
C.

 

[Alan in Little Washington]

Chris,

I've researched this issue on and off for a few months now and haven't been able to find a simple answer, or at least one I can wrap my brain around without getting a headache!

The best so far is the example for "Gas Applications" on page L27 of this pdf. The valve in the sample problem has a flow factor (Cv) of .58 with a flow of 500 SCFH (8 CFM) at 35 psi and a pressure drop of 5 psi.

Unless I am missing something, looking at the equation, Cv varies directly with Q (flow) so with the same flow, temp, pressure, and pressure drop, a valve with a 7 Cv should flow 12 times as much in that situation, although I think the main purpose is to calculate the pressure drop.

Using 10 CFM (600 CFH) flow, 55 deg. 135 psi inlet, assuming a 100 psi outlet I get a required Cv of .84. So either a Cv of 7 is way more than enough, a decimal is missing and it should be .7, or I'm doing something else wrong.

Come on, somebody with a little Fluid Dynamics background, please weigh in!

I'm considering putting a solenoid-actuated valve on my air tank, so I can turn that on/off remotely from the shop. My lines leak a little, so I'd prefer to only have the compressor run when I'm actually using it. The flow specification for the valve says "cv 7". The valve has 3/4" fittings which, I think, will be correct for my compressor. Anyone know what effect that flow spec will have in effective flow rate? IIRC, my compressor is rated for ~10 CFM at 135 psi - I do not foresee every using more than one tool at a time, but do hope to try my hand at spraying with an HVLP system.

TIA!
Chris

 

[scsmith42]

Unless that 3/4" valve has some type of small orfice inside it, it will flow way in excess of your needs - probably 10X if not more.

 

[Bryan S]

Come on, somebody with a little Fluid Dynamics background, please weigh in!

While I would love to speak eloquently on the subject, and impress you with a highly knowledgable technical explination, but that was a long time ago, so I will not try. :no:

It is true that the valve is a restiction in the piping, but in your case, I expect that will be negligeble. I agree with Scott that the valve will flow far more cfm than you will need. Remember you have 2 other restrictions, the capacity of the compressor pump itself and the 3/8, or even 1/4 hose from your piping to the tool. The hose is a far greater restriction than the 3/4 inch valve and you should not have any trouble at all.

 

[Gofor]

As long as the opening on the solenoid is as large or larger than the inside diameter of the air hose connected to the tool, you should not have a problem. Most air flow problems occur from using too small of an air hose. If using a 1/4 id hose, you want it to be short (preferably less than 12') for most conventional tools. If going to HVLP, you want to use a minimum 3/8" id hose with the hi-volume air fittings on the hose and gun (the hi-volume quick disconnects have almost a 3/8" id where the normal ones are about 1/4").

A 50' length of 1/4" air hose can drastically reduce the volume of air delivered to the tool. Although not noticed with short duration air tools like impact wrenches, etc, it will definitely starve a spray gun or pneumatic sander.

Go

 

[merrill77]

Thanks for the responses! Now I just need to figure out how to control it from my shop (30' away). Perhaps I'll look into one of those X10 remote control units.

 

[CDPeters]

Chris -

Beware the unit you posted requires 1.5 amps at 12 volts DC for the coil. That's a fairly hefty wall-wart to supply that much current. The wall wart would have to plug in to the X10.

C.

 

[DaveD]

Didn't I notice that they have the same valves with 120V solenoids?

 

[merrill77]

Beware the unit you posted requires 1.5 amps at 12 volts DC for the coil. That's a fairly hefty wall-wart to supply that much current. The wall wart would have to plug in to the X10.

I had no trouble finding 12V 2A converters, such as this one. Most of the X10 Modules are rated to handle 100W light bulbs - this adapter would draw less than 24W. Am I missing something?

 

[Alan in Little Washington]

Wire an outlet to your shop lighting circuit for the 12V transformer so the valve will be closed whenever the lights are off and open when you are in the shop with the lights on.

That is what I had planned to do, but with my numerous leaks, the piping would bleed down which is ok for my second story shop, but I wouldn't be able to use my garage level drop.