Going With the Flow in a Cafeteria Kitchen
Refrigeration is a high-profile part of any commercial kitchen with reach-in refrigerators and freezers as well as ice machines, not to mention the occasional walk-ins. Refrigeration technicians may complain that such equipment isn’t operating properly because the equipment may be too close to a deep fryer or other heat-producing device, not to mention problems when there is improper ventilation in the kitchen.
The fact of the matter is, an energy-efficient kitchen is truly a balancing act with ventilation a high priority. How innovative ventilation played a big part of creating an optimized kitchen is the story at a high school in Elk Grove Village, Ill.
As the story goes, a consulting engineer used a fabric-based diffuser to solve a seemingly impossible cafeteria kitchen ventilation challenge and along the way may have discovered a solution for the age-old problem of foodservice area airflow and exhaust imbalances.
THE CHALLENGEElk Grove High School (EGHS) had been gradually adding dishwashers, stove/draft hoods and other exhaust-requiring appliances over several years. This increased the kitchen area’s exhaust to approximately 20,000-cfm without adding to the 12,000-cfm make-up supply air - resulting in a negative air pressure. Without enough supply air to draw from, the added exhaust systems began drawing air from nearby rooms, including the shop department’s automobile repair learning center. Consequently, the exhaust systems were drawing vehicle emissions through the nearby hallways and into the kitchen.
Township High School District 214 hired CS2 Design Group, an Elk Grove Village-based consulting engineering firm that specializes in mechanical and electrical engineering design for educational institutions. CS2’s Steven Schafer, P.E., LEED® AP and Peter Kaczor, mechanical engineer, discovered the negative pressure during a study that also revealed little retrofit flexibility in the space above the 1,500-square-foot kitchen’s ceiling.
Above the kitchen, the 16-inch high bar joists were spaced approximately 3 feet on center with only 4 inches between the bottom of the joists and the suspended ceiling. Even with proper air balance in the space, the existing conventional 2 by 4-foot metal ceiling diffusers presented too much draft and turbulence for exhaust hood performance.
Typically, engineers would have specified critical environment metal diffusers to provide an acceptable turbulence and velocity of makeup air that is slow enough for the cooking draft hoods and other exhaust vents to draw air properly. Unfortunately the metal diffuser profiles available to the HVAC industry surpassed the available 4-inch space between the ceiling and joists. Lowering the existing 9-foot-high ceiling to accommodate these special diffusers would have raised the project cost significantly in rerouting utility piping, reducing heights of draft hoods, and other construction costs.
THE SOLUTIONAs an alternative, Kyle Schultz, engineering manager, Air Products Equipment, suggested fabric-based diffusers typically used in laboratory settings. LabSox™ is a series of laboratory air dispersion solutions by DuctSox®. Schultz said the product’s low profile and airflow performance were perfect for the EGHS kitchen.
“We think fabric lab diffusers might be a good application for our future foodservice-area air diffusion specification because of the low air velocity they provide,” said Schafer. “One hidden benefit with fabric diffusers is the ability to be taken down and laundered, which is a big sanitation advantage in a food prep area.”
The LabSox All-Fabric Surround Flow diffuser has a flat fabric back panel with a flexible fabric connection for connections within the restricted space between the joists and suspended ceiling.
“The fabric diffusers went into the ceiling very nicely and took about one man-hour each of installation time,” said Lenny Hart, foreman, Air-Con Refrigeration & Heating, Waukegan, Ill., which was the installing contractor. “The clearances and space constraints above the ceiling were really difficult for running the ductwork through the joists, so the flexibility and the low profile of the product were a big help.”
CS2 also specified a Kees 100 percent makeup air rooftop unit, packaged with a 92 percent efficient modulating gas valve and variable-frequency drive (VFD) for improved efficiency over the aged and undersized original unit it replaced. If only one kitchen hood exhaust is employed, operation is ramped down to save energy. The unit is monitored and controlled by the school district with controls integrated by ITG Solutions. Andover Controls manufactured the control system.
Makeup air flows through LabSox’s RX-200 fabric at a velocity of 70-95 fpm. The end result is a slower, more even airflow distribution that allows the kitchen exhaust hoods and other exhaust outlets to perform correctly, according to officials.
CS2’s design specification also included the reuse of three 2 by 2-foot roof penetrations, while also adding two more in order to route new ductwork drops down to the ceiling space. New plenum boxes in the truss area distribute air via a total of 34 10-inch-round supply ducts that serve each of the six 2 by 2-foot and eight 2 by 4-foot fabric diffusers with 400- and 850 cfm. respectively. The remaining 12 10-inch-round ducts serve six other 2 by 2-foot nonfabric diffusers located away from the exhaust hoods.
Laboratory and foodservice exhaust hoods are very similar in that both need nonturbulent and consistent inlet air velocities, according to Schafer. Therefore, Schafer said fabric lab diffusers might serve a performance purpose in CS2 Design Group’s future foodservice kitchen designs.
Aside from performance, the new system creates no drafts, which contributes to improved indoor air comfort and IAQ for kitchen employees, not to mention improved operation of equipment.
“When you’re dealing with the quantity of air we had on this project, this type of diffuser design is a good solution for retrofits and new construction,” said Schafer.
For more information, visit www.ductsox.com.
Publication date: 03/08/2010