Installing kitchen exhaust ducts in a building can be very challenging. With limited ceiling heights, long duct runs and the code requirement to be sloped back to the hood; one can face major hurdles. In this 2 part series Evolve Mechanical Solutions will present the problem in the 1st blog and then the solution in the 2nd blog.
The picture below is of an NFPA-96 kitchen exhaust duct that is welded liquid tight. Its job is to remove products of combustion from the cooking process to its termination point: the kitchen exhaust fan on the roof. Its secondary function is that in the event of a fire it removes the smoke and flames away from the kitchen and out of the building.
During normal operation and over time grease will build up within the kitchen exhaust duct. In the event of a fire flames could be pulled into the grease duct igniting any grease that has been collecting. NFPA-96 explicitly states that there can be no dips or traps in the kitchen exhaust duct and that it must slope back to the hood to drain.
NFPA -96 States:
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7.1.4 All ducts shall be installed with a minimum 2 percent slope on horizontal runs up to 22.86 m (75 ft) and a minimum 8 percent slope on horizontal runs greater than 22.86 m (75 ft).
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7.1.4.4 All ducts shall be installed without forming dips or traps.
