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Thread: Static location

  1. #53
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    Quote Originally Posted by hvacker View Post
    Reading through the posts here I remembered in a T&B book showing the fan to duct transitions that affect fan performance. These are often overlooked because they kind of sit in the background.
    Abrupt transitions too close to the outlet.
    Changes in direction opposite fan rotation ( fan rotates CW but the transition goes CCW).
    Side inlet to fan.
    And the most common one usually in residential seen so often it looks normal are branches too close to the fan outlet. Seems most resi installs have a short plenum from the unit to the trunk line going to a Tee or elbow usually with no vanes where the fan discharge runs (slams) into the top of the duct.
    i agree the fan effects are probably the most critical single component transitions in any duct system... get them REALLY wrong and you can add like .1iinch easily.

    whats annoying is some air handlers will dump from a blower outlet into a much larger equipment flange... so you have to either cut out the old flange, or put duct on the existing flange and use like... 20 gauge sheet metal to form a proper 3 side asym. diverging transition inside that duct.

    if you look at the ASHRAE duct fitting database, it makes these transitions so easy its not even funny.

    ocassionally i fall back to the SMACNA docs for static regain and negative coefficients, but you can do 90% of all designs and probably 99% of residential designs with the ASHRAE duct fitting database.

  2. #54
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    queequeg152 Thanks for the ASHRAE information on duct fitting. Is that in the Systems book? I've got to check that out.
    Knowing the information is out there. next problem is how to change the industry. With resi installs I hear an echo saying "That's how we've always done it."
    Over the years on this forum there is a bunch more attention to static pressure than in past years. At least it seems that way. I also think that a lot of understanding about static and it's relationship to velocity is missing. Two sides of the same coin.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  3. #55
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    Quote Originally Posted by queequeg152 View Post
    design build is honestly the only way to go when you need a project done correctly. and this is comeing from a guy working 90% design bid build jobs.

    client wants to save like 90 grand so they go with some no name company that is trenching and backfillling storm sewer and pretending that they didnt know they were supposed to notify us for a site visit... contractors are just the worst man... no offense to the good ones on this forum... but god damn we spend HOURS writing specs for a reason, and DAYS on detailed drawings...
    But isn't design build a lot more fun?
    I had a job for a Government laboratory where they wanted a control system for a laser facility. I asked the GC where the print was. He told me there wasn't one. I heard my head say to myself, COOL!
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  4. #56
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    i think me and you might have different different definitions for design build verses design bid build?

    in a design build contract, the client and engineer will get together with a contractor BEFORE the design phase. we engineers then get to work with the contractor from the get go all the way down to the pre construction meetings and site meeting.

    believe it or not...most engineers dont know anything about how crap actually gets built, so working with a contractor from the get go can save immense amounts of time becasue the contractor will come back to us and say hey... can we substitute these valves for these valves from this other company that is 30% cheaper? or can we change the lifting lugs on this tank to this configuration because of the company crane? etc etc.

    design bid build is basically we design everything as we would normally, then throw out the contract docs and drawings to like 10 contractors for bid... then HOPEFULLY we get to reccomend a winner to the client, but most often the client will just pick the cheepest guy... and this is where crap starts. the cheap guys are always crappy.

  5. #57
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    Thread Starter
    Update

    5' of straight round 20" duct installed on inlet side.
    11% drop in amps
    Inlet static still at -0.35
    Supply static up to 0.71
    20 x 20 return at 1090fpm by hotwire reading average so about 3000 cfm.
    2300 cfm supply DT, plus 300 by flow hood. All duct was sealed altho I can still feel a bit of leakage.
    Static at 16 x 16 is 0.22 where 2300 cfm was read, so 0.71 then 0.4 after heating coil then .22 after cooling coil.
    Total flow hood reading was 2000.
    I can't believe there is that much leakage still. Makes me question the new hood. No good location for a DT to make a multiplier for hood.



    On the a.c. side, I was able to open the txv 2.5 turns. Installed a Pressure tap on the equalizer line and found a 10 psi pressure drop from coil to comp. I didn't check piping but it's 1 1/8 for 80 ft for 7.5 ton.
    Anyhow it is what it is and the piping won't be changed. 20 years old.

    This system used to use a lot of OSA so that's why the heating coil. I think it can go, and the room baseboards do the heating but we may put a 12" bypass around the ht coil and just let the coil temper the air a bit. needs to work for 3 more years so limited spending allowed. This will happen next week and I guess we'll see.

    Sent from my SM-G930V using Tapatalk

  6. #58
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    The thing a flow hood does best is to proportionally distribute the air. It is however vulnerable to slot diffusers and non deflected airflow such as air outlets that are not diffused and blow straight out. For undiffused air outlet measurement take a reading then rotate the hood and take another. Your amp draw is encouraging because it indicates an improvement in system effect. The return airflow measured is probably the most accurate because you usually have a better velocity profile. It would be good to check the fan airflows against all other fan design data. If I recall correctly the return flow is pretty close to design.

  7. #59
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    Yes Wayne. It was cool how the deafening noise changed to an actual fan noise with the ducting change. Cfm really improved. I really think I have more air than the hood shows too. The dx coil sits off center in the ductwork and I think the bypass is going to help, as it will blow into the slower airflow side of it and should improve its performance.

    Thanks again for your help!
    Your comments are always wanted.

    Sent from my SM-G930V using Tapatalk

  8. #60
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    The difference between the return CFM and supply isn't abnormal. Differences as much as 20% can be expected but isn't a true indication of duct leakage but does give you some idea of what you are dealing with. If I understood correctly your indicated leakage was about 400 CFM which isn't bad. If you wrote down each traverse reading you can plot the velocity profile of the supply and return. This will give you a good idea of the accuracy of both traverses.
    If you really need more air it looks like you can get it. The owner should be happy with saving some energy and you probably made a friend. Good job of hanging in there.

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  10. #61
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    I brought up the fan curve for this fan. According to the curve you need a 5 horsepower motor to get 3600 CFM at 3 inches static and a fan speed a little over 3200 RPM. At 3000 CFM the curve indicates about 2.2 inches static. You might want to look into this.

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