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Ductwork 90's

zepfan
zepfan Member Posts: 319
Does anyone know if there is rule of thumb of how much equivalent length of ductwork a standard 26 gauge 6" or 8" round sheet metal 90 takes up. I know there is a formula to find the pressure loss, but could find nothing conceding the total feet to be accounted for. Any assistance would be much appreciated. Thanks to all

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 11,126
    http://rileysales.com/sites/default/files/assets/air_flow_dynamics_and_duct_sizing_reference_guide.pdf

    The attached Southwark chart says 30 feet of pipe = 1 90 deg elbow
    zepfan
  • zepfan
    zepfan Member Posts: 319
    Thank you so much. This helps a lot ,take care.
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    So , ... the 90 used as a take - off is given a lesser value than one used in a run . That conflicts with anything I have read in the past. I have always used 10 for one used in a run and 35 for one used as a take-off .
    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

  • JUGHNE
    JUGHNE Member Posts: 9,263
    Some of the pressboard slide ruler air duct calculators have that info on the back. I have one from "Tappan" when it was it's own company, not a name purchased by Nordyne.
    There are several versions of these with different info on the back and inside.

    A 90 in a branch (group 6) shows 10,
    A 90 take off (Group 1-plenums) has a square by round straight at 10. A straight round off the plenum (top collar) is 35.
    Group 3 with square to round top take off is 40.

    Seems to be many methods.....I just go for the main duct slightly oversized and get the feeling if a branch is too far with too many 90's then one size larger for that round branch run.
    I suppose this is too much "old school" but seems to have worked out for many years. FWIW
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 11,126
    I agree with @JUGHNE a lot of conflicting information. I do more with pipe than I do with ductwork but it's the same issue.

    Every book or chart you pick up to convert pipe fittings to feet of pipe has different footages for the various fittings. who do you believe??

    I just did one last month that had a pump too small on a 500 ton chiller. Needed 1500 gpm which the original pump had but it wouldn't produce enough head.

    This was all 8" Victaulic fittings. I calculated it with Victaulics fitting information. I came up with 66ft of head. They ordered it for 70 ft. TG it worked and we got enough flow.(expensive mistake).

    Generally if flow is within 10% over or under the results will not be disasterious . As long as you add "something" for each fitting it seems to work out.

    Wild guesses don't work
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    Too bad larger base mounted pumps don't have an extra speed tap on the motor , lol.

    The duct calculator sizes and friction are based on 100 feet. After 100 feet ( equivalent ) there is a correction for the friction baseline ( and why 35 feet for a 90 in a branch runout may be excessive ). I don't do anything with sheet metal any more but I do see more duct calculators than I do TEL correction charts.
    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller