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  1. #1
    Mud's Avatar
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    Default How do you size air lines?

    The air lines in my shop are mostly 3/4" pipe. They were set up for a 5hp compressor and were plenty large. Then I upgraded to a 10hp and added a bunch more machines. Now I'm putting in a 15 hp and rebuilding the 10hp for a backup. The longest run inside the shop is about 100 ft. I could connect the main lines into a loop, so that any single drop would have air from both directions once it left the initial 15 or 20 feet of supply line. The blasting cabinet is the largest draw and it's very close to the compressor so I can run a large line easily to it. The next largest draw is a Cincinnati Cinturn that draws 23 cfm all by itself, it takes all the overdriven 5hp pump can make alone, and it sits right in the middle of the 100 ft long shop.

    So what are the rules for pipe sizing? Does having too small of a line aggravate the creation of condensation? If I have to go larger I can most easily run a large pipe right down the middle and connect it in several locations to the existing 3/4 pipe. The shop is so crowded it would be a nightmare to pull out what's there and replumb it.

    Any help? Suggestions? Rumors? Theories? Bar BS? I know how the lines should be run with the tees facing up and the lines sloped to deal with condensation, they are not like that now.

  2. #2
    sfriedberg is offline Titanium
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    Here are a couple of places to start. I would worry much more about too-small pipe restricting flow and causing pressure loss than aggravating condensation problems.

    Chart of flow for pressure versus pipe size
    Estimating Pressure Drop

  3. #3
    PeteM is online now Diamond
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    The two easy fixes you've already noted should do it with respect to pressure. Run a larger pipe direct to the sandblaster. Then, create a loop around the shop.

  4. #4
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    bosleyjr is offline Diamond
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    The engineering way would be to figure your maximum flow for each machine, and compute the drop. This site:

    Pressure Loss in Compressed Air Pipe Lines

    Has presssure drops. All of them stop at about 10% of nominal pressure. Select a value (say 3 or 5%) of your set pressure, and use that as a guide.

    Loops are a bit more difficult, but if you assume the (3 or 5%) pressure drop, calculate the length from the air tank (or the air set), and assume each leg gets 1/2 the flow you can calculate a diameter.

    In the end, the cost differential between using 3/4 for everything and using 3/8, 1/2, 5/8, and 3/4 as need may not make up for the benefits of standardization. You might just start by assuming everything is 1/2 inch, then checking drop for each piece of equipment. Any drop over 5%, go to 1 inch. This way you get most of the benefits of fewer pipe size, with minimum numbers of ells, tees, etc. to contend with.

  5. #5
    cash's Avatar
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    A loop is the best way to do it. Then you do not have an end where water can sit.

    The larger your pipes are then they act as a holding tank.

    Our back up air compressor which is 15 h.p. will maintain 100 psi in our shop. some of our air lines are 3" diameter.

    We have 3 filtration units for our grinders that use a tone of air when they cycle.. If we are on our back up 15 h.p. compressor we will drop to 70 psi then slowly build bak up to 100 psi.

    We have over 50 machines in our shop.

    I would think you should be fine.

    Our main compressor is a 25 h.p. unit.

    We have found alot of this is trial and error as well. This is how we sized the 15 h.p. back up.

    Our entire system is several hundered feet. Some of it is 1" and the rest is 3". This is just additions over time, a span of 40 years.

  6. #6
    surplusjohn is online now Diamond
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    if you have a set up that is particuarly sensitive to pressure drops, or that uses large bursts of air, you can add a surge tank at the point of use. this tank can either be open to the system or can have a check valve so that it does not loose pressure back into the system. if you do this make sure and the tank is isolated, make sure you have the proper valves that allow you to empty the tank, since it would not be self emptying.

  7. #7
    gbent's Avatar
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    Run a 2" line down the center of your shop and tie it to your existing lines at the far end. This gives you the loop and gives you big air capacity for the machines that really need it while maintaining the shop air connections that are adequate.

  8. #8
    Tonytn36 is online now Diamond
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    Our facility is set up in loops of 100' x 150' for compressed air, there's an 8" loop around the perimeter of the area, with a cross 8" at the 75' mark. 3" parallels run the 75' between the cross and end loops every 25' and this is where the drops are taken from.

    We use 3/4" drops to the point of the machines, irregardless of the need of the machine system as long as it's 3/4" or less. This is for standardization and provides some additional volume.

  9. #9
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    steamboat54 is offline Plastic
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    The Curtis air compressor website has a chart of pressure/cfm/pipe size/length as a pdf download on their FAQ page

  10. #10
    Mud's Avatar
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    Quote Originally Posted by steamboat54 View Post
    The Curtis air compressor website has a chart of pressure/cfm/pipe size/length as a pdf download on their FAQ page
    That made it easy. Looks like I'm safe now, a few mods will make it more than enough. Only 3 machine tools are using air now, the rest is for blow guns and air tools etc. Thanks for all the advice.

  11. #11
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    I was just going to add that in my experience in a large automotive plant we never ran anything larger than 3/4" drops. Not any turbine or large constant-flow venturi machinery but plenty of large air cylinders that were constantly cycling.

    I think the looped (double fed) setup is best unless you have a lot of money to go towards large headers.

  12. #12
    jkilroy is offline Diamond
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    My system has five legs that all connect to the compressor and then cross connect into multiple loops, works well, and I haven't had to put up big iron or, who can afford it, copper.

  13. #13
    AmickRacing is offline Plastic
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    I would think for most home or small shop applications 3/4 or 1" pipe would be enough of a header. By going up to 3" pipe you would gain some capacity, but I can't imagine it'd be enough to justify the extra cost and hassle of running large pipe. Going for a 50' run, the difference between 3/4" and 3" pipe is around 15 gallons. Considering the cost difference, that's an expensive 15 gallons.

    I like the loop idea, but if you can't do that, perhaps add a second tank to your system now, perhaps even adding the 2nd tank on the other end of the header?

    I'm thinking by up-sizing your compressor like you are doing, you'll really be helping out the compressor efficiency and moisture problem by lowering run times and lowering the air temp since the compressor won't have it's tongue hanging out all the time.

    I'm far from a compressor guy, but I'd tie both tanks together into the air lines, set the backup pressure switch just a bit lower than the main compressor, so if it goes down, or falls behind the other one can step in and give it a hand.

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