Welding Discussion Forums

Home » Resources » Communities » Welding Discussion Forums

1. Senior Member
Join Date
Dec 2007
Location
Oklahoma
Posts
743
Originally Posted by Handy560
Short restrictions in air supply piping do not act as predictably as one may think. The smaller nipple may not play a role under most circumstances. The air is compressed and then finds the pressure/volume on the other side of the restriction until the point that the "choke flow" ratio is reached.

Choke flow calculations are fairly deep into calculus and I certainly am not an expert. What it boils down to is that the pressurre on the downstream side of the restriction would need to be almost half of the supply side before there would be any restrictive effect. I do not think you will experience any problem unless you plan on running multiple high air volume consumption tools at the same time, like a DA and a plasma cutter or impact. You just would not normally use the air fast enough for that to act as a restriction.

That is why most smaller compressors only have a 3/8 outlet.

Of course, to gain the greatest volume and delivery regardless of how quickly the air is consumed, opt for the largest fittings. Easier than trying to do the calculus

I just wanted to point out that some minor restriction may be irrelavant based on the actual use of the system.

Thanks,
Incorrect. Once a line is restricted the system cannot pass more air than what can pass through the restriction. Period. No calculus needed.
In this case the 3/8 orifice may or may not impact this gentleman's useage but it will restrict the system's potential.
Griff

2. Senior Member
Join Date
Dec 2007
Posts
157

Hmmm?

Well, then really we agree.

The restriction can ultimately limit the amount of air volume.

The fitting may or may not be a factor depending on the use.

The easiest way to avoid being marginal is not to use restrictive fittings.

The fitting may limit the systems potential.

But my point was that in a compressed air system the physics are more complicated and a small restriction may be inconsequential to his use.

The restriction only comes into play when you can use enough volume to reach the point of choke flow.

This is a link to an engineering book on hydraulic and compressed air piping systems. The pertinent section is between pages 293 and again on 297 where this book mark is.

Note the section on "choke flow" and the related calculations.

This is not as simple as a restrictor in a shower head.

Makes for boring reading but if someone is interested there it is...

3. Senior Member
Join Date
Dec 2007
Location
Oklahoma
Posts
743
Originally Posted by Handy560
Well, then really we agree.

The restriction can ultimately limit the amount of air volume.

The fitting may or may not be a factor depending on the use.

The easiest way to avoid being marginal is not to use restrictive fittings.

The fitting may limit the systems potential.

But my point was that in a compressed air system the physics are more complicated and a small restriction may be inconsequential to his use.

The restriction only comes into play when you can use enough volume to reach the point of choke flow.

This is a link to an engineering book on hydraulic and compressed air piping systems. The pertinent section is between pages 293 and again on 297 where this book mark is.

Note the section on "choke flow" and the related calculations.

This is not as simple as a restrictor in a shower head.

Makes for boring reading but if someone is interested there it is...
I was not referring to a shower head.

4. Senior Member
Join Date
Mar 2006
Location
Queens NY
Posts
1,547
It is my understanding that the restriction is a function of diameter and length. especially where gases are concerned. I'm no expert but i'm pretty sure if you plumbed with the same diameter tubing as the restriction you would see a great drop in airflow. I have an engineer friend who has dealt with gas flow extensively, i believe he is the one who told me this but the conversation probably happend 10+ years ago. I'l run it by him.

5. Senior Member
Join Date
Dec 2007
Posts
157

Sorry, no offense

In this case the issue is one of a fitting creating an orifice as the restriction; a fitting on the 60 gallon tank. If you were to reduce the entire length of the remaining supply pipe you would restrict the volume based basically on the same flow rate calculation which would be a function of the pressure 1, restriction size or pipe size, pipe resistance per length (rough/smooth) and the pressure 2 on the end of the pipe, i.e. atmosphere, other back pressure or vacuum. Each fitting, elbow, tee, etc. has a calculated resistance which is added as a function of feet of resistance. There are even more variables, such as heat loss, temperature of the gas, moisture etc.

My recollection, and it has been some years is that increasing pressure1, the source, will continue to increase the velocity and volume of gas until the speed of the gas becomes sonic, at which point you reached choke flow.

In other words when the P2 (your pipe/hoses system) reaches approximately half of P1 (your tank/compressor) the air moves so quickly across the orifice or restriction that it goes from subsonic to sonic and at that point no additional volume of air can pass.

Keep in mind that when the pneumatic system is in use these variables are not constants, the compressor kicks on, other users use air etc. The other less apparent variable is the size of the system after the restriction. When the system is static, both sides of the restriction reach equilibrium. As soon as you begin to use the air, the volume on the user side decreases reducing the pressure on that side and the air from the source rushes in (through the restriction) to replace the void seeking equilibrium. As long as the ratio of P2 pressure to P1 pressure is above the choke flow point and the air moving across the restriction is subsonic, the volume to the user/device remains the same. If the compressor doesn't kick on eventually the pressure will dwindle, if the compressor is running and it constantly can provide P1 pressure sufficient to maintain the volume the flow remains the same. You can see that there would be many possible variables based on your specific use and number of users. If the system then has a large volume (lots of large piping or a tank after the restriction) when the air begins being used the proportion of volume depleted is less (slower pressure drop) on the user side of the restriction so the chance of P2 falling quickly enough to choke the flow is diminished.

I only wanted to raise this point because it is easily misunderstood, not to be offensive. My point about the shower head is that there are different dynamics in a fluid system when you add a restriction because of the compressibility of gases.

My physics is limited to a high school class and it was mannnnnny years ago and this is my best recollection. I would be interested in someone who is an engineer to chiming in. Maybe i'm off base... OK, shoot me!

Thanks

6. Senior Member
Join Date
Mar 2006
Location
Queens NY
Posts
1,547
That explanation sounds more along the lines that i remember. One of the factors that affects the gas flowing through the orifice is the fact that it is compressible, which fluids aren't. The velocity will increase as the air moves through the orifice too then slow again as it enters the larger pipe. I cannot explain it as well as above but both of these factors affect why it works differently for air.

7. Senior Member
Join Date
Jan 2008
Location
VA
Posts
298
Most home shops only have one air using device running at a time. size for the largest device. If you find that the tool works well when you first hit the trigger or valve and then very quickly drops off in power or flow you have a choke point in the system. a choke point that is close to the tool will make the tool sluggish one that is far from the tool will make it start out with power but drop off. For home shops 1/2 copper pipe feeds plenty of flow at standard pressures over standard distances. My 24X 30 shop uses a remote compressor and in shop regulator and desiccant based air dryer. All the piping is in 1/2 and my impact gun hits hard without drop off and the media blast cabinet works well.

Put you system together as best as you can and see if it works well. If not trouble shoot and correct.

8. Just make a hand coil and run it acroos the floor Or run it all in hydraulic hose....

There are lots of ways to run an air line, At work its all in galvinized, my shop I have it ran in hydraulic hose to 2 different reels. honestly in my dads shop he has CPVC and its ran in the ceiling, out of the work place. It can't be hit by anything up there. and Its hooked to several different air hose reels mounted in the ceiling through out the shop. May not be the "proper" way but has been there forever. I made him up a 1/2" hydraulic hose to run from the compressor up to the ceiling, then its CPVC all the way to the first hose reel

The plastic airbrake line might be an idea

Page 5 of 5 First 12345

Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

Warning: Function split() is deprecated in /mnt/stor3-wc1-dfw1/357822/357839/www.millerwelds.com/web/content/lib/footer.inc.php on line 79

1