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  1. #1
    Join Date
    Jul 2009
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    Conowingo, MD
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    Maintaining Static of a Commercial Pool Dome (Affinity Laws)

    This is my first experience with a pool dome. We have a new customer with a Commercial Pool dome. It's over 30yrs old, and many of the mechanical pressure relief dampers no longer work. It's sort of a mess. Anyway customer says main unit never had any issues maintaining 1.25" until this year. Now the backup is constantly kicking in, and it must be the motor is what they insist. The motor is fine. It pulls 17.5A with 203V at the static pictured. It's 3yrs old, but not by us. The wheels maybe dropping a few a CFM due to age, but if you can't maintain static It's going somewhere! They have all pressure relief dampers closed. I honestly think they have leaks in the dome they refuse to address.

    I know how it is with fans and blowers. My question is about affinity laws for static pressure with this motor. I noticed that if you open a door of the dome Static pressure goes down, the amps go up. Is that equivalent to no work? Shouldn't it work harder too bring the dome back up? What I am asking is could a leak in the dome cause higher amps, and if the dome was tight and sealed, and pressure reliefs working as they should amps would stay down due to correct design load on motor.

    I have also never thought of 100% outside air with those laws. I have always thought of them as having a return ducted in.

    Motor pulley is 7"
    Shaft Pully 14"
    Distance apart is 36.5"

    None of that has been changed from what I know of, and only thing I know that has been changed are motors, and 4yrs ago they got a new dome. Customer states they have no design specs, so who knows. I just want to better understand the affinity laws of this type setup, so I can confirm my Job is correct. We don't do the dome at all!

    Any help is much appreciated! Thanks guys!
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  2. #2
    Join Date
    Feb 2016
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    The fan laws apply no matter the application. It takes more energy to move air than to develop static pressure. If you open the doors and let the static pressure go down the fan current draw will increase without the fan speed changing. You do need to determine if you have dome leaks that are causing the problem or if the fan has a problem.
    I would try a smoke bomb.
    No man can be both ignorant and free.
    Thomas Jefferson

  3. #3
    Join Date
    Nov 2011
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    Fascinating...Ive never even heard of such a thing!! What a great idea!

    If I were tossed into that situation, the first thing I would do is walk the outside structure, and check every single joint that touches the ground and any penetrations (doors, etc.). Find any leaks, use toilet paper, soap bubbles, etc. to find all the leaks I could.

    I assume the Dwyer is a controller that is supposed to control the relief dampers based on pressure? Is it pneumatic? Electric? Or both?

    Basically fan laws are fan laws. Doesnt matter if its outside air or not. Regardless, you need to stop any hemorrhages before you can address the mechanical side. I used to deal with pressurized electrical and chemical safe rooms. If those rooms werent 100% sealed, there was no way you could maintain the space pressure.

  4. #4
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    Jul 2009
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    Conowingo, MD
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    Thread Starter
    Thank you for the explanation. I kinda figured that, but wanted confirmation to be sure since I was in a unfamiliar application I have never seen before.

  5. #5
    Join Date
    Jul 2009
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    Conowingo, MD
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    Thread Starter
    It's all electrical that has been manually shut due to inability to hold pressure. That is exactly how I was thinking of it. If it's 100% sealed the reliefs would be a must to relieve pressure. The dome has leaks, and I just wanted confirmation on the fan laws to adress the issue. He insist on a new motor which we are installing for him, but we have told him multiple times that is not the issue.

    I like the smoke bomb idea. I may suggest that to customer.

  6. #6
    Join Date
    Jul 2018
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    Wisconsin
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    How does that work in winter? Wouldn't it be raining in there? How is the air heated?

    Sent from my SM-G960U using Tapatalk

  7. #7
    Join Date
    Jul 2018
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    Wisconsin
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    Could you run the fans with a VFD. Or are the relief dampers engineered as part of purging/exhausting the chlorine pool air?

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  8. #8
    Join Date
    Sep 2018
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    I think an important distinction isnt made here by other responses.

    Are fan laws applicable? Of course they are, if you want to answer questions about how the system will respond the changing fan speed.

    But then everyone starts mentioning closing up leaks. Fan laws wont answer those questions however.

    Heres what I mean:

    Fan laws will tell you that, if you change nothing about the dome, and assuming you make 1.1 static now and want to make 1.25 then you need to increase wheel speed by a factor of (1.25/1.1)^1/2 and the motor BHP will rise by (1.25/1.1)^(3/2).

    However, if as other suggest, you want to know what happens if you seal up the dome, then you need the fan curve. Since your changing the system, fan laws really arent applicable anymore.

  9. #9
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    Nov 2011
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    In a positive pressure space, the amount of loss (or gain) due to leaks is a HUGE issue. The pressure differential is so small, any tiny hole can change everything.

  10. #10
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    Dec 2006
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    Vancouver, B.C.
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    Have you completed the basic maintenance on the system yet? Checked for plugged filters, dirty blower wheel, worn pulleys and belt? Made sure there are no volume dampers that have decided to close themselves?

    Im not sure what components your system has, Im not familiar with this type of equipment either but without knowing that the system is operating correctly then its too early to start troubleshoot for a problem.


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  11. #11
    Join Date
    Sep 2007
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    edmonds wa
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    Looks like you have some more amps you can use. I would verify clean blower wheels and worn pulleys and belts first , then up the blower speed using a VP type pulley and then fixed size if the pressure comes up.
    UA Local 32 retired as of Jan 2020

  12. #12
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    Aug 2002
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    Southold, NY
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    Quote Originally Posted by buford View Post
    Looks like you have some more amps you can use. I would verify clean blower wheels and worn pulleys and belts first , then up the blower speed using a VP type pulley and then fixed size if the pressure comes up.
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  13. #13
    Join Date
    Mar 2015
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    Medford, N.Y.
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    What is the Mfrg Pressure Spec for the dome??? That photo of the dome does not seem to be "under deflated" due to not enough CFM/pressure.
    Was that access panel open when you took your readings?
    How are the press relief dampers controlled at the moment?

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