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  1. #1

    Kitchen Exhaust CFM issue

    Recently, I reworked a Hood Duct exhaust system for a customer. The unlisted 15' Hood provided many constraints and obsticles to properly size the duct. We were able to fit a 8" x 22'' trunkline on top of the Hood with 2 Exhaust risers centered on each side. The ductwork 90'd off the right side of trunkline( to the rear of the Hood), then transitioned to 16" x 16" (with 2 offsets inorder to get through the wall and obsticles) out the building 10' to an Outside Wall curb positioned at 45 degrees. The system passed inspection but needs to draw more CFMs as the Kitchen is very Hot and smokey because of the Radiant Charbroiler.
    Also installed was a Front PSP Make Up Air system.
    The Upblast fan is 2Hp 4750 CFMs.
    I'm thinking about changing the 90 to a radius corner and ending the Trunkline at the end of the Leftside riser( currently the trunkline extends 8-10" beyond the Riser).
    Any Suggestions??

  2. #2
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    Depending on the appliance layout you may be able to insert blanks in place of filters to increase velocity over the broiler, etc. Any dead space like the ends that may not actually cover an appliance? Just be sure you aren't covering and fusible links of the fire suppression system.

    Is your make up air at a much higher CFM than the exhaust? If the make up is over powered you won't get the rolling effect under the hood and it will actually blow the heat and smoke into the kitchen. Have also seen hoods that were installed too high that would have the same result. Wishing I had kept all those NFPA books now lol Been out of the fire suppression field too long to remember all the codes and tricks.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  3. #3
    Join Date
    Jun 2005
    Location
    Winston-Salem NC
    Posts
    1,133
    I just spent a couple of hours examining a hood system of sorts.
    At least there are two hoods, over the grill/oven and grill/fryer/some middle-eastern death trap grill that has standing pilots and no safety controls.
    Hood one, is connected to a piece of what appears to be insulated duct work, and then wanders off into the recesses of the ceilings and walls never to be seen again. It does have fire suppression heads installed both in hood and in several other places I can see before I lose it.

    The other hood has a make up air duct made right on top of the hood itself, feeding 4 20x20 inch filter grills. Have no idea how many cfms it is producing, as the blower is located about 18 feet up in the air, over top 2 drop in ceilings on in kitchen about 9 feet high and then another at 9-10 feet. Once we figured out where the breaker for it was located (5th floor, in a closet off the fire escape, restaurant and hood are located on 1st floor) we got it to run, and it helped out immensely with the heat build up.

    The owner wants to have a contractor come out and hook the hood which goes to an actual exhaust, (by way of several 90s and vents out a sidewall on the 4th floor) to the other one, so that exhaust duct can carry both.

    I basically verified that both hoods could be connected, as far as physical access goes, and that about half the ceiling will have to be removed and the restaurant will have to be shut down for at least three days.

    Now to find a local contractor to do the work, as I am not touching an exhaust hood modification in an historical building, downtown, with the monstrosity it currently has. I am always willing to back away and have the people who do it for a living handle it. My only experience with exhaust hoods are installing them according to the engineer's drawing and blueprints, and fixing/replacing the fan motors.

  4. #4
    Join Date
    Aug 2011
    Posts
    568
    when it comes to exhaust hoods in restaurants, i leave that stuff to guys that specialize in it, to me it's almost a separate trade.imo. any way

    GO N.Y. YANKEESSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS!

  5. #5
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Kinda curious as to the hood riser to trunk connections, sounds like a good opportunity to have big loses. Was the volume ever measured?

  6. #6
    Join Date
    May 2007
    Location
    Dry as a bone Tucson
    Posts
    4,207
    Quote Originally Posted by RedSoxFan22 View Post
    Recently, I reworked a Hood Duct exhaust system for a customer. The unlisted 15' Hood provided many constraints and obsticles to properly size the duct. We were able to fit a 8" x 22'' trunkline on top of the Hood with 2 Exhaust risers centered on each side. The ductwork 90'd off the right side of trunkline( to the rear of the Hood), then transitioned to 16" x 16" (with 2 offsets inorder to get through the wall and obsticles) out the building 10' to an Outside Wall curb positioned at 45 degrees. The system passed inspection but needs to draw more CFMs as the Kitchen is very Hot and smokey because of the Radiant Charbroiler.
    Also installed was a Front PSP Make Up Air system.
    The Upblast fan is 2Hp 4750 CFMs.
    I'm thinking about changing the 90 to a radius corner and ending the Trunkline at the end of the Leftside riser( currently the trunkline extends 8-10" beyond the Riser).
    Any Suggestions??
    Why the trunk?
    Were there no risers?



    A flat horizontal plenum is not a good thing. Grease will settle out and it could create a fire hazard.
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  7. #7
    Join Date
    Jun 2011
    Location
    Hammond,La.
    Posts
    1,176
    The horizontals aren't a problem if they have the required clean out access and are maintained. The IF part is the problem.0
    I have yellow and red tagged many systems for that very reason in the past.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  8. #8
    Join Date
    Feb 2010
    Location
    Cal
    Posts
    1,596
    Quote Originally Posted by MM#7 View Post
    GO N.Y. YANKEESSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS!
    I think there should be a bunch of Y's and only one S...but I am indifferent when it comes to bbbbbbbaseball teams.

  9. #9
    The Hood originally exhausted out the back side of the hood, into a 200 year old New England chimney up three stories. The problem wes the duct was old fashioned galvanized heating duct with pourous seams. Fire Department wanted to shut down the kitchen.
    Above the Hood we had 8" of clearance, the large chimney behind the Hood and the "offices" above the Hood lead us to one route - off to the side and out the side of the building.
    The Hood passed including a smoke bomb test but barely at best.
    The kitchen Director did not want to change the Hood.
    My best solution was to remove the "unlisted" Hood and install a backshelf allowing me to use larger duct above the Hood.
    The riser are 2"

  10. #10
    Join Date
    May 2000
    Location
    Monmouth Junction-NJ-USA
    Posts
    5,996
    2" riser????? What size is the hood. All ducts are to be continous weld and water tight.
    If you really know how it works, you have an execellent chance of fixin' er up!

    Tomorrow is promised to no one...

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