NFPA Cooks Up Revisions to Commercial Kitchen Ventilation Standard

This May at NFPA’s World Safety Conference and Exposition in Salt Lake City, members will vote on proposed changes to more than 30 technical documents, among them, NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations. The standard covers equipment for dampers, cooking, exhaust, fire-extinguishment and grease control/ removal.

Amendments to this particular standard are important because they could open the door for significant whole-building energy savings in facilities such as hospitals, universities and hotels, where food service is not the core mission.

One change, already part of the standard as an errata, but scheduled to become an official change in May, deals with the minimum air velocity through a grease exhaust duct. The 1998 Edition listed a minimum velocity of 1,500 ft. per min. (fpm), a mandate that Robert Utech, general manager of kitchen ventilation systems with Greenheck, Schofield, Wis., says has been on the books since the 1930s—and no one quite knows why. Utech notes the number may be related to another standard at the time, which required the carrying velocity of air in coal-burning furnaces to be 1,500 fpm. This number, he said, was possibly carried over to grease exhaust applications.

Whatever the reason, the number was reduced, thanks to an ASHRAE-supported study conducted by Dr. Tom Kuehn at the University of Minnesota during 1999-2000 (ASHRAE Research Project 1033-RP). Kuehn’s study revealed that high duct velocities increased grease accumulation in a duct. Lowering the velocity, however, reduced accumulation and kept grease particles moving. “You don’t want grease accumulating in the duct,” Utech explained. “That’s fuel. So, you want it to drain.”

As a result of Kuehn’s research, the 2001 Edition changed the requirement to 1,200 fpm. However, in January 2002, NFPA 96 corrected the number with Errata 96-01-01 to 500 fpm—one-third the original number.

So why wasn’t this issue addressed until recently? Utech explained that for a long time, slowing fans down was simply not practical, and variable-frequency drives were very expensive. However, prices have come down in recent years, allowing commercial kitchen facilities to adapt from running exhaust fans at 1,500 fpm all the time to only at peak times, and reducing the speed to 500 fpm when the kitchen isn’t running at full capacity.

“When you crack the door open [on an oven],” Utech said, “steam comes out. That’s when you need maximum ventilation—not when the door is closed. You’re ultimately trying to match the exhaust fan’s speed with what’s going on in the kitchen.”

There are several advantages to being able to control the exhaust velocity. First, a lower grease accumulation rate reduces the risk of fire, as well as the frequency of duct cleaning. Retrofit possibilities and increased design flexibility (including properly sized fans) are other pluses. A lower fan speed also reduces the noise in a kitchen and makes for a quieter, more comfortable work environment.

But perhaps the most important benefit is energy savings. With the fan running at full speed only during peak times, less air is being displaced, and therefore, less air needs to be replaced. “The whole issue here is not to save money on the exhaust fan voltage,” Utech noted. “It’s really about reducing the cost of the supply air. For every cubic foot of air you take out, you have to put one back in.”

Other NFPA 96 changes being reviewed in May:

• Chapter 15, Down Draft Appliance Ventilation Systems now contains requirements for down draft ventilation systems for appliances used in processes that produce smoke or grease-laden vapors, typically, tabletop or demonstration cooking areas (think Benihana). More and more of these systems are being installed, but they are not currently addressed in the standard.

• There is now a clarification of the requirements for horizontal duct installations. A new section has been added that requires at least one 508-mm x 508-mm (20-in. x 20-in.) opening for personnel entry, as none of the current grease duct standards cover the capability of the grease duct supports to carry the load of the cleaning technician.

• A new reference for the listing of factory-built grease duct enclosures requires them to be listed in accordance with UL 2221, Tests of Fire Resistive Duct Enclosure Assemblies. UL 2221 evaluates the fire-resistance capabilities of both factory-built and duct-wrapped grease duct systems. The NFPA 96 Technical Committee approved this revision unanimously in 2003.

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