Duct choice helps ensure restaurant stays clean, comfortable
Smoke and grease buildup in a commercial food service kitchen is bad for business. Unfortunately, many North American restaurant and food service kitchens may not realize they are experiencing ventilation issues.
Typically, this common HVAC construction problem stems from installation errors resulting in excessive velocity from the kitchen’s makeup air or HVAC supply air, which causes turbulence and disruption of the cooking hood’s designed airflow exhaust.
Consequently, cooking smoke spills out from under the hood and deposits grease on walls and surfaces, instead of being drawn up into the capture hood and out the flue. While not seen, the cooking heat may not be exhausted either, which leads to kitchen temperature challenges. In addition, cooking odors can waft into the dining area and affect patrons, some of which may not return to a facility that bombards them at their dining tables with smoke and smells.
There are several reasons for kitchen cooking hood spillover.
Kitchen exhaust and supply air are commonly installed by separate contractors. While owners may focus and invest heavily in capture hood technology, how or where airflow enters the space and how it impacts hood capture, employee comfort or food quality is often forgotten.
Restaurants many times outgrow their original kitchen design by adding more cooking hoods and/or heat-producing appliances that skew the original ventilation specifications. Adding more cooking hoods in a remodel or update can also impinge the ability of the supply air to distribute evenly.
Poorly balanced ventilation systems — either by design or installation oversight — can change a kitchen’s critical air pressure from negative to positive. They should be operating under a negative pressure; however, once installed or remodeled, they can be inadvertently skewed to a positive pressure, which impedes the cooking hood’s ability to exhaust properly. An example is a cooking hood exhausting 3,000 cubic feet per minute in a kitchen that’s supplied with 2,500 cfm of makeup air and a kitchen HVAC system supplying 1,000 cfm into the kitchen. Consequently, the kitchen’s negative pressure turns positive as the extra 500 to 1,000 cfm of supply air pushes the kitchen odors, grease and heat into the dining area.
One of the San Francisco Bay area’s popular restaurants, Bridges Restaurant & Bar in Danville, California, had similar challenges with its dual-exhaust cooking hood. Smoke and food odors wafted into portions of the 3,300-square-foot dining area, raising complaints from patrons seated at tables near the kitchen.
The 24-year-old, independently owned Bridges is popularly known in the San Francisco area for its casual yet elegant dining and also as the famous location for the late actor Robin Williams’ comedic multiple costume changes in the 1993 Hollywood movie, “Mrs. Doubtfire.”
Because of its upscale image and popularity, the vexing dining area cooking odors were bad for business and affected the overall guest experience, according to Kevin Gin, executive chef and one of the owners of the restaurant.
The situation grew bad enough for management to consider replacing its HVAC system with a new $20,000 rooftop makeup-air system. Fortunately for the restaurant, consultants pinpointed the problem as the HVAC system’s supply air, not the mechanical equipment itself.
Besides dining room infiltration, drafts from the existing HVAC system were drawing smoke and grease from the hood, which was then settling onto the metal air diffuser, walls, ceilings and other kitchen surfaces requiring more than five hours per week in extra maintenance.
Another problem: Drafty air over the food preparation area was prematurely cooling prepared hot entrees while employees in other kitchen areas were uncomfortably hot.
A large retrofit HVAC construction project seemed imminent until an arranged meeting between Gin and an HVAC ventilation product manager attending a 25th anniversary dinner for the nearby Food Service Technology Center in San Ramon, California.
A representative from Peosta, Iowa, ductwork manufacturer DuctSox Corp. recommended a custom textile air-dispersion system designed specifically for kitchens. The ceiling-mounted device disperses airflow at approximately 70 feet per minute, uniformly across the front of the kitchen exhaust hood. The fabric diffuser is an easy-to-install replacement for metal air distribution diffusers that commonly cause kitchen exhaust hood overflow.
Instead of a 5,000-cfm makeup-air equipment replacement, only the kitchen’s lone conventional 3-by-3-foot supply box and four metal diffusers were switched out. Located over the food prep line to distribute air throughout the 1,000-square-foot kitchen, the diffusers were replaced with two 8-foot-long, 28-inch-diameter D-shaped KitchenSox fabric diffusers.
While the metal diffusers had an estimated 500-foot-per-minute airflow disrupting the cooking hood’s capture work, the air was gently and evenly dispersed through the fabric into the entire kitchen area at a significantly slower 85-fpm rate. Without the previous high-velocity airflow and subsequent turbulence, the dual exhaust cooking hood now efficiently performs up to its design standards and captures smoke, grease particulates and cooking odors without overflow.
CulinAire Systems of El Dorado Hills, California, a food service ventilation and sheet metal works contractor specializing in demand ventilation controls and engineered kitchen ventilation equipment, fabricated the sheet metal plenum and metal/fabric adaptors that supply the 16 linear feet of fabric duct. The fabric duct system and metal accessories were installed by CulinAire overnight without disruption to kitchen operations.
The original installation used DuctSox’s first KitchenSox fabric, the RX200, which is a white, engineered polyester-woven fabric designed for air dispersion in environments such as laboratories. The RX200 features an anti-microbial agent and is a highly permeable fabric that allows air to easily flow through.
DuctSox has recently created a second KitchenSox airflow model, the DT200. Bridges served as a test site for the DT200 and installed it in-house in approximately 20 minutes. Gin said he prefers the DT200 for its reduced maintenance and increased employee air comfort.
The new DT200 features thousands of micro perforations that deliver air with similar performance characteristics of the porous fabric, but doesn’t accumulate particulates on its interior surface. Any particulates missed by the HVAC system’s filter most likely will flow through the perforations, rather than function as a secondary filter and accumulate particulates on the interior surface of the tighter woven RX200.
The newer model silver-colored fabric matches commercial kitchen stainless steel appliances. The silver color was made to show less dirt than white fabric. Both models can be commercially washed.
Gin said the original fabric dispersed air very uniformly and surpassed his expectations, but the new perforated model’s slight velocity increase provides better employee air comfort, but without disrupting cooking hood smoke capture or prematurely cooling entrees.
Either fabric diffuser model mixes better with ambient air than conventional metal four-way radial or laminar diffusers, which don’t work particularly well in commercial kitchens because of their inherent high velocities and diffusion characteristics. The radial diffuser projects air out and it sinks quickly to the floor without mixing and tempering all the surrounding ambient air. The laminar diffuser, which typically has a flat face directed downward, generates a column of air with an even more dramatic descent to the floor. The quick air descent of both diffuser types tends to pull air away from the cooking hood’s drawing area.
Comparatively, fabric diffusers disperse the air more slowly with a wider pattern that mixes with ambient air more efficiently. This generates a minimal turbulence of 60- to 70-fpm at the lip of the hood, allowing it to draft as the design intended. Today, there are several major restaurant chains — representing hundreds of locations — that have experienced kitchen ventilation challenges similar to Bridges. Already, many chains have implemented fabric diffusers for their kitchens, and many more are currently considering it.
Bridges’ HVAC ventilation challenges were solved completely with fabric diffusers that cost less than 10 percent of the approximate $20,000 for a new makeup-air system. The energy savings and reduced maintenance costs offer a payback of less than one year on the air-diffusion system.
“This is really a common problem,” Gin said of the situation Bridges faced. “I think many restaurants suffer from and don’t realize there’s a simple, inexpensive solution. We once thought replacing the HVAC system would improve our kitchen ventilation challenges, but we now realize it probably wouldn’t have remedied what was actually a ventilation problem that fabric ductwork solved.”
While improved kitchen cleanliness from KitchenSox have eliminated the need for daily cleaning, Gin does plan annual cleaning since the fabric diffuser can be quickly disassembled and laundered by kitchen staffers, a process that would require days to complete by an outside HVAC market contractor with a metal system.
Other restaurants with ventilation challenges should first look to an HVAC construction mechanical contractor for test and balancing. Ductwork dampers, variable frequency drives (if any) on fan motors and other adjustments might help contractors balance the system. If balancing doesn’t alleviate the problem, then replacing metal diffusers with fabric might be the best and most economical solution. Replacing the HVAC unit generally will not remedy a problem caused by a dysfunctional air distribution system in the kitchen.