The code that governs this system is the International Mechanical Code (IMC) - 2009, Sections 506.1 through 506.5.5.; Commercial Kitchen Hood Ventilation System Ducts and Exhaust Equipment. The Commercial Kitchen Hoods Code requirements are defined in the IMC-2009, Section 507.
STACK MATERIAL CHOICES
The duct material serving the Type I grease ducts shall be constructed of not less than 16 gauge steel or not less than 18 gauge stainless steel. All seams, joints, and penetrations shall have a liquid-tight continuous external weld. The duct must slope 1/4-in. per foot toward the hood or an approved grease reservoir. For horizontal duct runs longer than 75 ft, the slope shall be at least 1 in./ft. Clean-out panels are to be provided in the side or top of the duct at least every 12 ft and at each change of direction (Figure 1). Listed and labeled factory-built commercial kitchen grease ducts use stainless steel liners and double-wall insulated construction. These units have reduced clearance to combustible ratings and have allowable non-welded joint connections.
The welded metal ducts may be round or rectangular and must maintain a minimum of 18-in. from combustible materials. Fire-rated insulation material can be applied to the exterior of the ducts to reduce the minimum distance to combustibles. Ducts running vertically in an enclosed fire-rated shaft must have a vented curb at the roof level to relieve any heat build up within the shaft enclosure.
The vertical duct systems should be evaluated for both economic and visual considerations. A multi-story vertical stack that rises more than two stories within the facility should consider using a listed and labeled factory-built assembly. The horizontal run to the vertical stack within the kitchen ceiling can be a welded steel field insulated type. The material costs for the factory-built units may be higher then a completely welded steel/insulated type; however, the installation labor cost is less. Any vertical stack on the exterior of a building should also consider a factory-built assembly because the stack will look like a chimney.
DUCT SIZING CONSIDERATIONS
The model building codes, IMC and NFPA Standard 96, set the minimum air velocity at 500 fpm. The maximum air velocities are selected to limit velocity noise in the duct riser and at the terminal discharge. The industry norm is 2,500 fpm. The hood manufacturer sizes the duct collar connections near 1,600 fpm; therefore, most duct risers are sized to operate between 1,500 and 1,800 fpm. The system discharge velocity of 2,500 fpm will be discussed later in this article. The lower limit of 500 fpm is to allow the use of variable-speed exhaust systems and reuse of older systems that were designed for larger cfm exhaust rates than are needed today.
A kitchen may have more than one hood that can be connected to a single exhaust stack/fan system. The hoods must be on the same floor, in the same room, or in adjoining rooms. The multi-hood grease duct cannot serve solid fire-burning appliances. The hood branch ducts must be designed using the pressure drop method to keep each branch the same duct resistance from the hood collar to the main stack riser. However, hood manufactures and code officials are now allowing a slide damper at the hood’s duct collar as a method to balance the airflow in the branch duct runs.
The duct stack does operate in a hot condition and may become a chimney in case of a grease fire in the duct, therefore, heat gain to the kitchen area and clearance to combustibles must be part of the system’s design. Also any vertical shaft enclosure must be rated in accordance with the building code. A 1-hr-rated shaft enclose must be used if the shaft penetrates less than three stories. A 2-hr-rated shaft enclosure must be used for shaft penetrations more than three stories. Most codes require the minimum of 18 in. to combustibles. The distance to combustibles may be reduced by the application of external duct wrap or applied material to the duct stack. The duct insulation material information should be in accordance with NFPA Standard 96. The double-wall listed grease ducts are to be in accordance with UL Standard 1978. In a multi-story building a high temperature sprinkler head can be installed in the warm area at the top of the stack to help extinguish any fire in the stack.
In spite of grease duct filters, some grease does pass though the hood’s filter bank and does accumulate within the Type I grease duct. In the old days, this grease was manually scrapped and washed out of the duct’s interior. This still is done in some simple one-story grease ducts. The kitchen grease duct cleaning industry today uses power washing techniques and a cable camera to verify the conditions of the duct before and after the cleaning. In the old days, the grease duct construction was designed to prevent grease from leaking from the duct. However, with the use of power wash water systems, today the grease duct has to be liquid tight.
LIQUID TIGHT DUCT TESTING
The duct must be tested before insulation is applied to the exterior of the duct.
A leakage test shall be performed to determine that all welded joints and seams are liquid tight. Duct sections may be pretested prior to complete installation of the stack system. However, the total stack assembly must be tested again after final installation. The current leakage test options are as follows:
Light test. The light test is done by passing a 100W light bulb through the entire section of ductwork to be tested. The lamp shall be open to emit light in all directions. No light shall be visible through any exterior surface.
Air pressure test. The duct shall be sealed at both ends and pressurized to a minimum pressure of 1.0-in. wc column and hold this pressure for a minimum of 20 min (Figure 2). Duct sections may be pretested in the field or in the shop. The final test must be in the complete field installed stack.
Water test. The test uses a power washer operating a minimum pressure of 1,500 psi. The water is applied to all joints to be tested. No water applied to the duct interior shall be visible on any exterior surface during the test.
DISCOVERED DUCT LEAKAGE PROBLEMS
Grease ducts have been around the commercial kitchen industry for a long time, and they have been cleaned for many years. However, with the use of power washing it has been discovered that some of the older duct stacks do leak water at welded seams and at the gasketed access panels. This moisture leakage is causing mold and mildew problems in shaft spaces and the suspended ceilings area above the kitchen area. It has been discovered that the duct access panels gaskets are fire rated, but not necessarily liquid tight. Also after many years of removing an access panel to clean the duct, the gasket material has shown wear and damage. System choices to be negotiated with code officials may include constructing the code-required access panels to the ductwork, but do not cut the opening into the duct (if access is required at some future time, the hole can be cut through for access); and contacting access panel vendors to see if they have developed a fire-rated and seal tight gasket for their panels.
In today’s world of legal concerns, it is recommended to conduct an air pressure test on existing grease stacks and repair any leakage problems discovered. Maintain a record of this work to demonstrate that a “standard of care” has been maintained.
EXHAUST FAN CHOICES
The exhaust fan provides air movement to transfer cooking effluent captured by the hood to the outdoors. The majority of these fans use a centrifugal backward-inclined blade wheel, with the motor out of the airstream, and are constructed to be non-rustable. The fans must be UL Standard 762 rated. Roof-mounted exhaust fans must discharge the exhaust 42-in. above the roof surface to satisfy the NFPA 96 requirement designed to keep the grease off the roof surface. The following types of exhaust fans are used in the commercial kitchen ventilation industry:
Power roof ventilator (PRV). These fans use an upblast discharge and are designed to be mounted on a roof curb at the exhaust duct outlet. The fan assembly must contain a grease drain, a grease collection device, and a hinged roof curb to have access for cleaning on the fan and the duct. These fans can also be wall mounted to blow away from the building. The upblast PRV does have some shortcomings. The static pressure is normally limited to the 2.0 to 2.5-in. wc range. This becomes a problem when there is a long run of exhaust duct and/or when the hood contains filters that have a greater resistance than a simple baffle type. Cross-wind recirculation is also a problem. The average discharge velocity from PRV units is 1,000 fpm. Therefore, with a 15 mph crosswind, the discharge plume will stay at the 42-in. discharge level above the roof and may be introduced into a nearby rooftop A/C unit (RTU) (Figure 3).
Centrifugal fan. These fans are also known as a utility set. The motor and drive is enclosed in a weather cover if the fan is mounted on a roof curb. The fan can be mounted indoors with a duct run to the outdoors. The discharge can be vertical or turned to discharge horizontally. The fans also must have a grease drain, a grease collection device, and a blower access panel to be able to clean the fan housing/wheel assembly. The fans can be steel, but aluminum and stainless steel are available to prevent rust damage.
Tubular centrifugal. These fans are also known as inline fans. These fans are generally steel; however, stainless steel units are available. The fans are installed within the building and must have a grease tight gasketed joint at the ductwork’s connections. The lowest part of the fan must have a grease drain to a container, and the fan housing must have an access panel to be able to clean the fan wheel and housing.
ROOF FAN OUTLETS
The fan outlet should be directed away from any outdoor air intakes. Some codes specify a minimum vertical and horizontal distance to air intakes. Grease should be prevented from dripping on the roof surface. Most grease drippings at the fan discharge are caused by exhaust flow below the designed cfm. The vaporized grease is not cooled at the filter and not centrifugally separated at the filter because of the low airflow.
Consider the high-velocity vertical stack discharge for utility sets and inline fans as shown in Figure 5 and 6. Rain, cold, down drafting air, and snow are tenant problems that some discharge dampers can solve. Codes generally do not allow dampers in Type I grease ducts, but a horizontal centrifugal fan outlet dampers is sometimes acceptable to prevent cold air from flowing down the stack when the fan system is off. A lightweight aluminum butterfly damper within a wind band on the vertical discharge of an inline fan will keep rain, snow, and cold air from flowing down the stack when the fan system is off. Figure 4 shows a fan discharge with a high velocity stack head fitting that removes the exhaust away from the roof area. These vertical fan discharge stacks can also be roof mounted with the fans indoors. The stacks can be constructed of round schedule 10 stainless steel piping material that provides a “good” outlet look for nearby neighbors’ view.
SIDEWALL FAN OULETS
The exterior wall louvers should not direct the exhaust flow down to the street level. A plenum should be constructed before the louver to capture any dripping grease and any exterior rain water. Provide a grease drain cup and a cleaning access panel. Again if grease dripping is a problem, verify that the exhaust system’s cfm rate is where it is supposed to be. Low cfm rates will cause grease dripping from the sidewall discharge for the same reasons described in roof fan outlets.
Cooking odors that passersby can smell outside a facility are always objectionable. If the discharge is near to a high-rise living facility with exterior balconies, the problem must be solved. The first step is to provide a higher-efficiency grease removal filter at the hood. However, be aware that these units will require a higher static fan pressure. A water wash filter system is the next step; again, beware of insufficient airflow through the unit’s baffle path. Other devices such as activated charcoal, oxidizing bed filters, deodorizing agents, ultraviolet destruction, and catalytic conversion can be used downstream of the hood filters.
A system that is used in industrial laboratory exhaust systems for similar protection of humans can also be used to control odor complaints. The solution is outdoor air dilution into a high-velocity stack discharge stack or fan system (Figure 7). A good dilution rate rule of thumb is to introduce 20% of outdoor air from the roof area. Example: design cfm for a 10-ft kitchen medium duty wall mounted canopy hood is 3,000 cfm. Then add 20% more dilution air (600 cfm) and design the fan for 3,600 cfm with a vertical stack discharging at 2,500 fpm velocity.
There is much more to designing a commercial kitchen ventilation stack than just picking a duct size and running the duct to the outdoors. The duct leakage problem that has arisen from the use of power washing must be addressed, and the codes are requiring leak testing. The options for fans and discharge outlets give the system some good, better, and best choices. The codes are changing and designers must be prepared to with Authority Having Jurisdiction (AHJ) on the topics of non liquid-tight access panels and the use of discharge dampers. For neighborhood odor potential complaints, consider using dilution for the solution to eliminate cooking smell objections from the neighbors. ES