Clarify the time it takes to fill an air cylinder 2

[homebrewdude]

I have a cylinder that needs to be pressurized with air.

My standard setup has a 50psi air hose direct into the cylinder. This gives my standard "X" fill time.

My modification will re-route the hose, using adapters and fittings.

Assuming I don't reduce the cross sectional area of the hose smaller then the existing hose, will the fill time remain the same?

As example.

My existing hose is 1/2" ID

My new design with have the 1/2" hose, then a section of pipe with 3/4" back down to 1/2" pipe where it enters the cylinder.

Would this effect the fill time? Since it is still restricted by the 1/2" area?

 

[Montemayor]

ppgregar:

I don't think you'll be able to notice the difference - if you can measure any. You don't state the size of your air receiver nor the amount of resistance you're introducing, so I can't state this with 100% certainty - but I suspect you're talking about something in the order of less than 1,000 scf (standard cubic feet) and minimal new resistance.

Initially, when the receiver is practically void, you will establish choke (sonic) flow during the transfill. True, the friction and other effects due to pipe or hose diameter will affect the pressure drop, but this will be indistinguishable at the relative high pressures and with choke flow - assuming that your modification doesn't introduce significant friction and resistance to flow. The fill rate will be differentially decreasing as the driving force (delta P) decreases due to the receiver being filled differentially. However, in my experience, the % of the choke flow time is so much greater than the last portion of the fill cycle that, for practical purposes, you can consider that the choke flow rate (which is constant) will predominate. Since the choke flow rate is the same practically for all cases, the time to fill will be the same.

I have used this technique probably millions of times during the era when I operated and oversaw air separation plants overseas. We produced Oxygen & Nitrogen and sold the products by filling nominal 250 scf HP cylinders through the use of manifolds. We also filled tube trailers. That's how and why I came across the subject and looked at it seriously in order to size and optimize fill operations. I was never able to find a serious flaw in how we estimated our fill time.
Hope this answers your query.

Art Montemayor
Spring, TX

 

[zdas04]

The only thing that Art left out of his excellent post is that if your new configuration is significantly longer than the old one you could run into the situation where the pipe friction eats up your dP so fast that you don't have choked flow into the cylinder - that would slow your fill time down.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[homebrewdude]

if it matters....

This is a piston application.

As the air enters the cylinder, it pushes a piston out.

The original configuration is 2 inches.
I will be changing this to around 6 inches.