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  1. #1
    danny x Guest

    Make up Air Help

    Not being and expert in make up air I have a question. Hibachi steak house which only uses the kitchen cooking equipment on occasion.

    I have a hood that is 17 X 4 Long. The ventilator is a FX24BFT. The hood has 4 rectangular openings(close to 7X10) inside the front which lead up to two round take offs(9) which lead up to a 30 X 30 fresh air opening. When the ventilator kicks on it creates a negative pressure condition in the kitchen pulling open the doors. Can I add a make up air unit to this hood? If so, how do I size it? How do I know the right ventilator for this hood was selected?

    Dan

  2. #2
    Join Date
    Nov 2005
    Location
    New York
    Posts
    177
    IF the FAI intake is mounted on the roof. It's probably best to transition the duct into a MAU. Or any supply air fan to interlock with the kitchen exhaust fan. CFM of the fan should be equal to the KEF CFM. Maybe a little bit over.

    If the hood was manufactured by a company that makes specialty equipment the intake collars were probably sized correctly. Is it a long duct run to the outside?

  3. #3
    danny x Guest
    Each 9” take off runs with flex about 4 feet, ties into a transitional trunk then turns up to the 30 X 30. The take offs are welded which leads me to belief they were designed with the hood however the fresh air was only figured to have an elbow with bird screen on the roof. The exhaust fan is not the same size as the curb which was cut into the roof originally which gives me the feeling it was replaced at some time and perhaps oversized which is why the fresh air with out a fan unit is insufficient This exhaust fan can move between 2500-10500 CFM so my concern is, how much CFM should the MAU be rated for? If I select a unit that with a 15” wheel which can move between 2000-8000 CFM(found at http://www.empirehood.com/RTP/rtp_performance_data.htm), how do I know the hood is designed properly to handle the air change?

    Thanks for the help,

    Dan

  4. #4
    Join Date
    Aug 2004
    Location
    S.E. North Carolina Coast
    Posts
    124
    If I recall this correctley an island hood open on all four sides requires 150 cfm per sq.ft.
    A hood against a wall require 100 cfm per sq.ft.( open on three sides ).
    Using your size : 17 x 4 = 68 sq. ft. x 100 cfm = 6800 cfm.
    The exhaust velocity needs to be 1500 fpm.

    Make-Up Air (supply) needs to be approx. 80% at the hood.
    Using your size hood, 6800 cfm x 80% = 5440 cfm

    The buiding (kitchen) pressure needs to be neutral or slightly positive.
    With your hood, that means 6800 cfm - 5440 cfm = 1360 cfm of fresh air is still required to be supplied to building. Generally advised to be supplied thru economizers from Roof Top Gas Pak or A/C.

    This should help you pick the correct sized Make-Up Air Ventilator. Then you must size the duct based on the static. The Mfg. of the ventilator will be able to help you size this up.

  5. #5
    Join Date
    Jun 2006
    Location
    right, here! in the heartland of the homeland!
    Posts
    737

    If i remember right , i had that problem after a couple of

    power hood exhausts were added, and so i compensated with the yacs or rtu's
    the only time i had a prob was when it came to winter, so i then compensated and left the doors pefectly closed the units closer to the doors i compenstated for this and kept the kitchen in a .04 neg for the odors
    and the fresh air was always there, the gas on the hot water demand for the chafing dishes etc , was a relighting prob, until i compensated with the gas pressure and pilot light adjustment,,,
    ashrae use to have kitchens set for 20/15/8 thats occupancy per person/cfm per person/ L/S -person telaire systems inc., IAQ
    it all depends on how your air is set up and where on your ac systems or pacs or rtu's too, and how much they can move, i had four 80 tonners, they didnt have enough space for the air i could move, or enough heat load to give me a run for the money!
    I usually had to readjust the air , because they had guys that had passwords to make adjustments to make everyone miserable ,and themselves dangerous!
    Changed a 40 hp copeland or two because of that same reasoning.. that was fun

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by mardoman View Post
    If I recall this correctley an island hood open on all four sides requires 150 cfm per sq.ft.
    A hood against a wall require 100 cfm per sq.ft.( open on three sides ).
    Using your size : 17 x 4 = 68 sq. ft. x 100 cfm = 6800 cfm.
    The exhaust velocity needs to be 1500 fpm.

    Make-Up Air (supply) needs to be approx. 80% at the hood.
    Using your size hood, 6800 cfm x 80% = 5440 cfm

    The buiding (kitchen) pressure needs to be neutral or slightly positive.
    With your hood, that means 6800 cfm - 5440 cfm = 1360 cfm of fresh air is still required to be supplied to building. Generally advised to be supplied thru economizers from Roof Top Gas Pak or A/C.

    This should help you pick the correct sized Make-Up Air Ventilator. Then you must size the duct based on the static. The Mfg. of the ventilator will be able to help you size this up.
    Those face velocities sound about right for non listed hoods. The listed ones will have lower airflow usually.

    A good reference on kitchen ventilation is
    http://www.greenheck.com/products/ki...tion_guide.php
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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