Sizing Guide for Restaurant Grease & Exhaust Hoods
Posted: Friday, July 28, 2006
by John Stack
ACityDiscount
Please note that this is a reference guide and should not be viewed as a substitute for National or Local Fire Code. Also you must have a Licensed HVAC or Mechanical Contractor to do all of the installation and permitting.
A Commercial, Restaurant, Kitchen, Grease, Exhaust, Type-1, Vent, Hood is essentially comprised of 3 main components: The Shell of the Hood , the Baffled, Grease Filters , and the Trough at the bottom of the Filters. You have to then produce negative pressure or a vacuum to ensure that all of the Heat, Smoke, Soot, and/or Grease-ladened Vapors are expelled from the Facilities. An Upblast, Grease Fan that is on the exterior of the Facilities, accomplishes this. An additional Fan may be required as dictated by the amount of exhaust or by local Code.
To determine the correct size Grease, Exhaust Hood to install, National Fire Code (NFC) dictates that the Hood should be 6 inches larger on all sides compared to the equipment that is under it. Please consult your local Building Code Office to see if they require more overhang. For example if you have a 36 inch wide, 6-eye Range, 24 inch Char Broiler, 36 inch Flat Grill (Griddle), and 2- Gas Fryers (average 16 inches wide), you would add the total widths to come up with 128 inches. With the additional 12 inches (may vary) for the overhang of the Hood, this comes up to 140 inches or 11 feet, 8 inches. Rounding up to the nearest foot, you would therefore need a 12-foot Hood. You should also keep in mind what the future may dictate in terms of additional or larger items. You may outgrow a 24 inch Broiler and swap it out with a 36 inch or larger model and thus, would need the Hood to be larger. Also keep in mind that certain Code dictates certain spacing betweens items.
Sizing an Upblast, Exhaust Fan:
The Hood Manufacturer dictates the Volume of Airflow (Cubic Feet per Minute or CFM ) for their Hoods depending on the equipment under the Hood and whether the Hood is against a Wall or in the middle of the Room (Island Hood). The Manufacturer derives the airflow from parameters set out by National Fire Code (NFC). NFC dictates that, depending on the Cooking Equipment the Volume of Airflow can vary depending on low, medium, high, or very high heat cooking. Essentially, 10 feet of Open Flame, Wok Cookers will need significantly more Exhaust than the standard equipment of a Range, Griddle, Fryer, etc. As an example we can use the aforementioned set up referenced in the "Sizing a Hood" Section, the 12' Hood with the equipment listed would need an average of 4000 CFM.
Determining if you need a Make-up or Supply Air Fan
Make-up Air (MUA) Fans blow air into the Facilities to compensate (make-up) for the Air being pulled out by the Exhaust Fan . For some it may be a Local Code issue as to whether they need a MUA Fan. For others, it may be a matter of convenience and/or efficiency. The general size and rule of thumb for MUA is a 6' Hood or larger or 2000 CFM or more of Exhaust flow. If your Conditioned Air, whether heated or cooled, is getting pulled out of your Facilities at a rate of 2000 CFM or greater it will cost you allot more money in the long run than the initial cost of a MUA Fan. You could even find it rather hard to open the Door or the Door gets slammed shut by the large amount of negative pressure being produced by the Exhaust Fan. To ensure that all of the Smoke, Grease, or Odors get pulled out of the Kitchen, it is necessary to have a little negative pressure. The general rule of thumb is about 85% of Exhaust. Therefore the proper MUA Fan for 4000 CFM of Exhaust would be about 3400 CFM.
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