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Thread: Static location

  1. #40
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    How did it go?

    I was thinking it wouldn't be too difficult to make a couple of simple devices to prove your point.

    Like maybe showing negative pressure on a supply duct immediately after a 90 if measured on the inside radius. I know, not the same thing, but it is quite the visual that may get the grey matter churning.

    Or take some square duct to simulate what you've got, then show the difference with several pipe diameters of properly sized round.


    Quote Originally Posted by icy78 View Post
    Update. I get to pitch my case for duct change tomorrow. Ho ho!
    Wish me luck!

  2. #41
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    Well this is cool. After showing the pics of vena contracta, and describing how air cannot make that second 90 degree turn into the fan vanes, and the resulting turbulence and unmeasurables, I suggested cutting out the square duct on the return side and installing a 5ft piece of 20" spiral with 11-15 degree transition into the spiral. Then we'll check amps and bhp and cfm. If that shows the result I expect , then they'll pretty it up AND leave the exsisting fan. If it doesn't work, it will really only be a small loss of labor because the duct has to be removed anyhow for the new fan, which has been rush ordered. If the old one works o k., my boss says no worries cause we'll use the rush order fan somewhere. I got him really interested if the duct change will work. Pretty cool of him to go with it I think. I did put a lot if effort into researching in that nasty attic lol! which he recognized. 🖒


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  4. #42
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    Congratulations,
    You are a better salesman than I am.
    Will the new duct go straight into the fan?

  5. #43
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    Quote Originally Posted by WAYNE3298 View Post
    Congratulations,
    You are a better salesman than I am.
    Will the new duct go straight into the fan?
    Yes, should be 5' I hope. He said the sales catalog said 2 diameters minimum, but I pointed out the IOM says 3 plus the transition. They'll what they can. It'll be a while probably but I'll post back on the results. Here's hoping!

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  6. #44
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    Don't be surprised if you also have to adjust and/or change sheaves to get design airflow. With the present install that wasn't possible and if installed per design in my opinion the fan size is also suspect. Sounds like you are going to find out. I agree with you on straight run length. AMCA also requires 3 duct diameters.

  7. #45
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    Almost forgot to ask how was the new fan was sized?

  8. #46
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    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.
    Give me a relay with big enough contacts, and I'll run the world!

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

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  10. #47
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    Quote Originally Posted by WAYNE3298 View Post
    Almost forgot to ask how was the new fan was sized?
    I hate to say this but it looks like a shot from the hip. This is what I overheard....so we have 2100cfm at 3/4" static. A fan capable of 3000 at 2.5" should do it.
    It's the; "SHOULD do it" part that gets me. It would also be a different style fan. Don't know the type.

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  11. #48
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    Quote Originally Posted by icy78 View Post
    I hate to say this but it looks like a shot from the hip. This is what I overheard....so we have 2100cfm at 3/4" static. A fan capable of 3000 at 2.5" should do it.
    It's the; "SHOULD do it" part that gets me. It would also be a different style fan. Don't know the type.

    Sent from my SM-G930V using Tapatalk

    Isn't easier to give the information to the fan supplier? Let them cypher the information.
    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

  12. #49
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    Quote Originally Posted by hvacker View Post
    Isn't easier to give the information to the fan supplier? Let them cypher the information.
    I would agree. Design build company tho . Bit of ego involved.

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  14. #50
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    Agree someone is shooting from the hip and will probably shoot themselves in the foot. From the fan data you included if the existing fan was bucking a total static of 3/4" it should be delivering about 3,400 CFM. Not trying to be mean but mother nature doesn't recognize or respect ego. System effect is preventing accurate inlet static pressure measurement but the fan RPM and pitot traverse indicate it is significant.

  15. #51
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    Given your conditions the fan supplier can't do you any good.
    By the fan laws if you really had 3/4" total static to deliver 2,100 CFM you would only need 1.53" total static to deliver 3,000 CFM.
    If you increased the static from 3/4" to 2.5" the airflow would increase from 2100 CFM to 3834 CFM.
    The existing fan data indicates it is borderline class II and could have been specified class I or II. I would have specified it class II to allow increasing the speed if needed. If it is class II you have a lot to work with. If it is not class II you can't speed it up.
    All you really know about the fan is RPM, current draw and total airflow and that system effect is reducing performance. Your boss would be ahead to let you correct the install and then decide what to do if anything more is needed. The only other way I know to size the fan and know you are in the ballpark is to calculate friction losses in the entire system based on eliminating the inlet system effect.

  16. #52
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    Quote Originally Posted by icy78 View Post
    I would agree. Design build company tho . Bit of ego involved.

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    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...

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