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  1. #1

    Default Air compressor piping

    I just purchased a Kobalt 60 gallon compressor and I have a question on the recommended pipe size to run in the garage. The tools I am looking to use are spray gun, D/A sander, impact/ratchet wrenches. From the factory the tank has a 2"x3/8" reducer which I was thinking of changing that to a 2"X3/4" then run some 3/4" copper pipe. The overall run will be around 40-60 ft around the room with 3 drops, depending on the elevation I am guessing drop lengths of 5ft or so.

    What do yoy guys run or recommend?

  2. #2
    Join Date
    Jan 2008
    Location
    East Coast
    Posts
    20

    Default

    3/4" copper is my suggestion. 1/2" is adequate for what you described but the reduced air speed in the larger pipe will increase the time to cool thereby dropping more moisture.

    Scott

  3. #3
    Join Date
    Feb 2008
    Posts
    5

    Default

    I read somewere that copper and pvc have a tendency to seperate under pressure and they suggestd that you use cast iron pipe! it was on another post somewhere on here. I"ll trie to find it if I can its realy interesting.

  4. #4
    Join Date
    Jan 2008
    Location
    Colorado
    Posts
    673

    Thumbs up

    Visegrip:
    Ridgid PVC is NOT gas pressure rated. When a water pipe splits open there is only a spray of water as the water is not compressed to many times its normal volume. There is little explosive action. Compresed air expands and the explosive action of the expanding gasses can throw shrapnel everywhere.

    mxtras:
    There are thousands of articles readily accessable about the dangers of using PVC for compressed air. To summarize all of these articles: Don't use PVC for compressed air systems.

    EDITED: I retract my approval of PVC. :embarrassement:
    Last edited by Craig in Denver; 02-28-2008 at 10:35 PM.
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  5. #5
    Join Date
    Dec 2007
    Posts
    157

    Default 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.

    http://books.google.com/books?id=H5w...l=en#PPA293,M1

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

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

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  6. #6
    Join Date
    Dec 2007
    Location
    Oklahoma
    Posts
    711

    Default

    Quote Originally Posted by Handy560 View Post
    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.

    http://books.google.com/books?id=H5w...l=en#PPA293,M1

    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.

  7. #7
    Join Date
    Mar 2006
    Location
    Queens NY
    Posts
    1,547

    Default

    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.
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  8. #8
    Join Date
    Dec 2007
    Posts
    157

    Red face 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
    Last edited by Handy560; 03-19-2008 at 12:35 PM. Reason: cause I cAn'T tYpE
    John

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  9. #9
    Join Date
    Aug 2006
    Location
    NY
    Posts
    38

    Default

    I would go 3/4 because you will get more volume (CFM) to your tools. Black pipe would be the best, but very time consuming to run. I have PVC in my shop even though they say you should not use it. I haven't had a problem with it yet.

  10. #10

    Default

    If we want these forums to stay clean an neat .
    We should moderate ourselves and try to post in the correct forum.
    This should maybe be in the Disussion Forum.

    Never use pvc.
    Under pressure if you hit it with a hammer or a object. it will shatter with sharp projectiles and cause very serious injury or death.

    Copper or black iron
    there is a special plastic or similar type pipe for air ,but I cant rember the name right now.

    Rich

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