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MUA question

Discussion in 'Commercial' started by icy78, Oct 20, 2017.

  1. icy78

    icy78 New Member

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    523
    Condemned a 40 year old MUA today for a restaurant. Multiple breaches in the heatex.
    The existing unit does not supply enough air, (negative building) and out of interest, I'm wondering how smart people, would go about sizing the new mua.
    One could measure all the exhausts and go that route, but lets say that can't be done. Take a static on the building and calculate somehow?
    There's got to be a way to do that , correct.? Any literature someone could point me too?
    My part will probably just be commissioning. I'm just interested in the sizing process. (At my workplace there will be a good bit of guess work)

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  2. Chuck

    Chuck SSP

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    Add up the exhaust fan cfm and use that for a start
     
  3. icy78

    icy78 New Member

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    Well yes, but I'm looking for another way. Imagine a room. Equal pressure with outside. Calculate the amount of air to pressurize that room to 0.2".
    Seems like @flange would do that stuff, maybe others? Idk, I just find it interesting.

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  4. flange

    flange Act like you care and do SOMETHING!!

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    ashrae has complex equations for this type of thing. essentially you need to know a few things....volume of room, volume of mechanical exhaust, and room leakage rate(via doors and windows, cracks, etc.). when we do this, we make a few assumptions based upon the type of construction, door seals etc for example, an average door will lose about 300 cfm. door seals and gaskets can lower this to 200 or less. here is a decent paper on the topic. http://airflowdirection.com/ASHRAE Original Reprint.pdf
     
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  5. LemonWizard

    LemonWizard Blinky Lights Wizard

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    You probably don't need to actually measure the cfm of the mechanical exhaust. Manufacturer data on how much air the exhaust fans are designed to move is probably sufficient. That's all an engineer would use when designing a new building. Plus the other information @flange mentioned.
     
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  6. icy78

    icy78 New Member

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    Pretty interesting article. I've sometimes wondered about the door opening/direction effect for critical rooms and it's discussed in what you posted.
    Thanks for the post and link.

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  7. Chuck

    Chuck SSP

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    If you're looking for quick and easy and accurate enough, then just look up the exhaust fan numbers and add up the cfm. If you're trying to calculate it based on building pressure you will have to know the exhaust cfm anyway. If you're looking for a more complicated way I can't help, I try to find the simplest way that's close enough for the real world ;-)
     
  8. Chuck

    Chuck SSP

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    If all exhaust fans are on and no make up air and you have a negative in the building of say .15" then what? That alone is changing the cfm of each exhaust fan based on its individual fan curve. If you blow some amount of make up air into the building that raises the pressure up in the building and then the exhaust fans are moving more air and there is less leakage infiltrating the building. So you would need to recalculate and buy a new mua unit and try again

    It's a moving target and it won't be possible to calculate cfm based only on building pressure.
     
  9. Zman

    Zman Hanging by a Thread

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    Interesting article...but what's a "Barrier Room for Nude Nice". This must be some secret world of Rodents we don't know about. :D
     
  10. icy78

    icy78 New Member

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    I hear you Chuck and mostly I agree, but it's hard to imagine there's not an engineering formula out there for my hypothetical situation.
    I'm sure I'll be sent out to measure.
    I'll probably velgrid the exhaust hoods and flowhood the other exhausts. Close enough.
    I still have the MUA supplying it's air so that helps. I'll measure that too.


    I have a couple balancer friends who may be able to help too. If I get anything I'll post it.



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  11. mike1020

    mike1020 entitled millennial

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    At Big Orange we would just figure out what the makeup air unit is and install an identically sized makeup air unit lol
     
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  12. iron

    iron we men are wretched things

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    Hood manufactures will tell you that make up air should be over 70 percent of exhaust but in the really real world its more like 47 percent.
    Calculate the outside air on your units at 10 percent. Some will say as high as 27 percent and some cities now have code at 32 percent. Ignore them. Set your outside air at 10 percent. Set your exhaust according to the hood manufactures spec and add outside air to get to the building positive the engineer calls for. Do not send an rfi to the building engineer, you will be tied up in their messiah complex forever. do as I say, 10 percent outside air on the units, then make up air to get positive building from hood exhaust.
     
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  13. icy78

    icy78 New Member

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    Thanks iron.
    Thinking about your comment of setting the exhaust.....got me thinking that hell! what are the chances of the present exhaust being correct? Little.
    So it would behoove to measure and set the hood exhausts correctly first, even tho they've been operating for years like this. If they are high, then great, can be slowed down. MAYBE all four hoods are high and can be adjusted and thus save them on their MUA requirements .
    Guess I'll wear a balancing hat for a day. Could really save the customer some $ and look competent . Good way to keep them!


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  14. mike1020

    mike1020 entitled millennial

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    Is it a kitchen?

    What's under the hoods?
     
  15. icy78

    icy78 New Member

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    Ill have to check. I know capture velocities will vary based on use. Hopefully the nameplate is still on them, but otherwise typical capture velocities will suffice I reckon.
    I'm in kitchens very little, but I'm the only one with balancing experience at our shop so I'll probably have to do it.

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  16. icy78

    icy78 New Member

    Messages:
    523
    Ill have to check. I know capture velocities will vary based on use. Hopefully the nameplate is still on them, but otherwise typical capture velocities will suffice I reckon.
    I'm in kitchens very little, but I'm the only one with balancing experience at our shop so I'll probably have to do it.

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  17. icy78

    icy78 New Member

    Messages:
    523
    Ended up measuring 2000 cfm in and 6300 out! Building was at 0.2" negative at times. And that's with the hoods drawing about 1/2 what they should. Ouch. Up to the bosses now.


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  18. Daryl_Dixon

    Daryl_Dixon New Member

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    3,780
    I have a MUA question.

    Is it MUA or is it MAU?

    :D
     
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  19. knave

    knave Undeterred

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    8,193
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    MAP
    Make A Pair
     
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  20. Rather B Riding

    Rather B Riding Behind bars

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    Interesting thread. We had a restaurant that had a wood fired grill. 3 exhaust only 2 MUA. One of our techs out on a 95° day closed the econo min position on 2 of the 3 RTUs. There was a negative pressure on the building to the point customer was complaining about smoke in the restaurant. We played with damper positions till we got it right. The 2- 25 ton units were open to around 20% to balance the building.