Evolve Mechanical Solutions blogged about a deficient kitchen exhaust duct a few months back. In this 2nd part of this series we will demonstrate the solution to this common issue facing grease duct installations.
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:
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).
220.127.116.11 All ducts shall be installed without forming dips or traps.
In this case the contractor dipped down and up thus creating a trap for grease to pool creating a potential fire hazard.
The solution was to drain the grease duct at its lowest point by welding black iron piping to it. This piping is then sloped back and tied into a grease interceptor thus successfully removing the effluent in a safe manner as per NFPA-96.
NFPA -96 States:
18.104.22.168* Drains shall be provided at low points in horizontal ducts (grease ducts).
22.214.171.124.1 Where provided, drains shall be continuously welded to the exhaust duct or listed grease duct drains, in accordance with the terms of the listing and the manufacturer’s installation manual.
The James Hicks P.Eng
Evolve Mechanical Solutions