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Want Fries With That?

When cooking up a new kitchen design, key ingredients to consider are the proper ventilation and conditioning of the space. Balancing the heat output of the kitchen equipment, managing odor migration to other spaces, preventing smoke roll-out at hoods, and maintaining a comfortable working environment requires a carefully designed mechanical system.

The heat output from the fryers, griddles, ranges, ovens, and steamers in a typical full service kitchen is substantial. The primary method of removing this heat is via exhaust hoods. By exhausting air in large volumes, exhaust hoods not only capture fumes and control cooking odors, but they also help control the heat released into the kitchen. A hood typically exhausts about 300 cubic feet per minute (CFM) per linear foot of hood, so exhaust for a full service kitchen with multiple cooking lines and hoods can be as much as 10,000 CFM or more.

All of this exhaust comes with a price. Building codes require that make-up air be provided to hoods. Make-up air is fresh outside air brought into a space to balance out the exhaust so that the inside building pressure does not become excessively negative when the kitchen is operating. Make-up air cannot be delivered as raw outside (cold winter or hot summer) air, as building codes require that make-up air be tempered. Further, it must be distributed so as to not decrease the comfort of the kitchen, or create smoke roll-out at the hood.

“Smoke roll-out is prevented by properly managing the distribution and delivery methods of make-up air and other air conditioning in the kitchen.”

Smoke roll-out occurs when drafts – either from air diffusers placed too close to the hoods or from strong air currents (opening of receiving area doors for instance) – cause smoke, cooking vapors, and fumes to literally roll out from underneath the hood. Smoke roll-out is not only dangerous to cooking staff but can push odors and grease throughout the kitchen and into adjacent areas. Smoke roll-out is prevented by properly managing the distribution and delivery methods of make-up air and other air conditioning in the kitchen. One effective and proven approach for controlling smoke roll-out, especially in smaller kitchens with numerous hoods, is the use of compensating hoods. These hoods have integral supply plenums and integral duct collars so that make-up air is ducted directly into the hoods. However, in larger, more complex kitchens, compensating hoods may not be the most energy efficient solution when it comes to keeping the kitchen cool.

The cooling requirements of kitchens are approximately three times those of common office environments. In large kitchens with ample ancillary support spaces, or in large display-cooking kitchens that face a seating area, the need for air conditioning and make-up air can be combined by using the make-up air to ventilate and condition both the kitchen and seating area. In cases like these, the make-up air requirements become integrated into the whole-building HVAC system, which is both cost and energy efficient.

The style of hood and the conditioning approach taken depend on the desires of the Owner and food service consultant, as well as the HVAC requirements of the kitchen and surrounding spaces. The style of hood should always be reviewed together by the food service consultant and HVAC engineer so that ventilating needs of the kitchen can be balanced and coordinated with its conditioning needs. Without this coordination, it is easy to ‘order up’ an overheated kitchen with a side of wasted energy.

– Julie K. Good, PE, LEED AP