Need to learn more about the ductwork for a kitchen exhaust system: its construction, code enforcement, and continued service? You have found yourself in the right spot. This blog post will dive deep into what you need to know.
KITCHEN EXHAUST SYSTEM – The Ductwork
When the hood filters are removed you will be able to see where the hood connects to the ductwork. The ductwork is used to draw air from the hood to the exhaust fan.
Ductwork systems will vary greatly all depending on each systems’ configuration. This configuration will vary based upon architectural design. The ductwork can be one of the most extensive complexities of the KITCHEN EXHAUST SYSTEM.
The biggest variable in a duct system is the length of the duct needed to connect the hood(s) to the exhaust fan(s).
The Duct – NFPA 96 Codes
In the NFPA 96 standard, Chapter 7 covers the codes regulating ductwork construction. There is a lot of history in the industry from conception to present. In this blog you will find a quick summary of what you must know to service these systems properly.
Ductwork Construction – Liquid Tight (NFPA 96 – code 7.5.2.1.)
All Type 1 ductwork by code shall be welded to be liquid tight.
Liquid Tight means that all grease particulates and exhaust fumes remain in the duct and do not extend to the structure. This is designed as a fire safety feature but is also very important for the efficiency of the Bare Metal Standard service process.
Figure 29 is a picture of the interior of a kitchen exhaust ductwork after Bare Metal Standard service process has removed all grease contaminates.
Type 2 ductwork does not have to be constructed to this “liquid tight” specification. Systems that fall in this category are bakery ovens, pizza ovens, dish washers, steam kettle hoods.
TYPE 1 Hood Systems
Adhere to all NFPA 96 codes to contain fire hazards (potential grease buildup). These systems are used to with appliances that produce grease, smoke, and exhaust:
Fryers
Broilers
Gas Range
Flat Top Grill Woks
TYPE 2 Hood Systems
These systems are not required to adhere to NFPA 96 construction specification because these exhaust system cover appliances that produce heat and steam, but no grease: Bakery Ovens
Pizza Ovens
Kettle Pots
Dish Washing Machines
Type 2 Systems may require additional wrapping to contain waste during the cleaning process, see Chapter X.
Ductwork – Access Panels (NFPA 96, codes 7.1.5 – 7.4.1.5)
Access Panels are to be installed every 12 ft. and at every change in direction. This access allows a Technician no more than six feet in each direction to access and service the kitchen exhaust system.
Using the classroom model duct system, the following 11 access doors (all green square on the duct) would need to be installed to have the system be code compliant (see Figure 30).
Access panels are NOT allowed to be on the bottom of the exhaust duct and are to be 1.5 inches
above the bottom edge which prevents grease or water from reaching the cut into the duct to create the access door. As in Figure 31, the access panel, circled in yellow is location in the middle of the duct.
Ductwork – Low Point in Ductwork (NFPA 96, code 7.1.4.3
Horizontal ductwork shall be constructed so that every foot of duct there is a ¼ inch slope back towards the hood.
This slope keeps all the grease in the system and water during the service process moving back towards the hood. If the ductwork is unable to be built in this manner a Low Point Drain will need to be installed.
In Figure 32, you can see a drain spout on the side of the ductwork circled in yellow, along with an access door. This drain at the low point of the ductwork allows an exit point for grease and wastewater during cleaning.