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Grease Exhaust Duct Friction

Grease Exhaust Duct Friction

(OP)
I am looking for friction information on grease-laden welded black iron ductwork for a kitchen hood exhaust. I need to have a certain static at the hood collar and am trying to size up the roof fan. Something tells me that most people just use the galvanized ductulator slide rule for this type of application, but it that acceptable? If not, what is typically used to size grease exhaust ductwork and calculate required fan static?

RE: Grease Exhaust Duct Friction

Design you exhaust system in accordance to NFPA 91, a standard for exhaust system including kitchen hood exhaust. Once you have determine your system CFM based on air velocity in the duct per NFPA 91, then you should be able to calculate by hand, friction losses and determine fan size based on fan characteristic curve. A few things to keep in mind are 1)the capture velocity of the fumes over the cooking appliances 2) if energy conservation is important particularly in the cooling season, consider a push pull system 3) many communities in the US require stainless steel for the kitchen hood; for exhaust ducts, check with your community building inspector.

RE: Grease Exhaust Duct Friction

(OP)
Thank you, but I have been unable to locate and good friction information for greasy black iron duct. Any ideas?

RE: Grease Exhaust Duct Friction

As an uninformed guess, a friction factor or % blockage would depend on indoor air quality and particulate- if there is only grease to stick to it, roughness ~ drop size. If someone is ventilating a deep fryer in shared space with for the sake of discussion, a stable, filter or duct loading could be a big problem.

RE: Grease Exhaust Duct Friction

Bron y r Aur,
Why are you making a grease duct out of Black iron , I have always made them out of 14guage welded galvanized sheet.
If you are relying on the grease in the duct to protect the iron , it does not work, there is always some moisture and the ducting rusts. So at the very least you will need to paint the duct, then calculate your friction for a painted surface.
B.E.

You are judged not by what you know, but by what you can do.

RE: Grease Exhaust Duct Friction

(OP)
NFPA-96 calls for welded carbon steel or stainless ducts. Never heard of galvanized being used for kitchen exhaust.

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