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The science of commercial kitchen ventilation includes both exhausting air as well as providing replacement air within the cooking area.
Whether a restaurant is a small free-standing site or a large institutional kitchen, managing and balancing airflow is a complex issue.
It is a challenge to properly ventilate commercial kitchens, as they require moving large volumes of air through ductwork and equipment placement in very restricted spaces.
Overall design, construction, installation coordination, and maintenance are required to get optimum performance and an energy-efficient air balance from the system.
SMACNA technical manuals provide the information and drawings to illustrate the elements of construction and installation of commercial kitchen exhaust hoods.
The information is intended to encourage standardization in installations and to call attention to the appropriate segregation of responsibilities of those involved with food service design and installation.
We can help your restaurant achieve optimum performance and energy efficiency in commercial kitchen ventilation systems. This information is applicable to new construction and, in many instances, retrofit construction. These guidelines can assist kitchen designers, mechanical engineers, food service operators, property managers, and maintenance personnel. For more comprehensive design information, see the Kitchen Ventilation Chapter in the ASHRAE Handbook on HVAC Applications.
An effective commercial kitchen ventilation (CKV) system requires balance - air balance that is. And as the designer, installer or operator of the kitchen ventilation system, you may be the first person called upon to perform your own "balancing act" when the exhaust hood doesn't work. Unlike a cooking appliance, which can be isolated for troubleshooting, the exhaust hood is only one component of the kitchen ventilation system. To further complicate things, the CKV system is a subsystem of the overall building heating, ventilating and air-conditioning (HVAC) system. Fortunately, there is no magic to the relationship between an exhaust hood and its requirement for replacement or makeup air (MUA). The physics are simple: air that exits the building (through exhaust hoods and fans) must be replaced with outside air that enters the building (intentionally or otherwise). The essence of air balance: AIR IN = AIR OUT
If the replacement air doesn't come in, that means it doesn't go out the exhaust hood and problems begin. Not only will the building pressure become too negative, the hood may not capture and contain (C&C) cooking effluents due to reduced exhaust flow. We have all experienced the "can't-open-the-door" syndrome because the exhaust fan is sucking too hard on the inside of the restaurant. The mechanical design may call for 8000 cubic feet per minute (cfm) of air to be exhausted through the hood. But if only 6000 cfm of outdoor air is able to squeeze in through closed dampers on rooftop units and undesirable pathways in the building envelope, then only 6000 cfm is available to be exhausted through the hood. The exhaust fan creates more suction (negative pressure) in an unsuccessful attempt to pull more air through the hood.
There is no piece of equipment that generates more controversy within the food service equipment supply and design community than the exhaust hood in all its styles and makeup air combinations. The idea that by not installing a dedicated makeup air supply, the operator is going to save money (in both first cost and operating cost) is short sighted. It may be okay if, by design, all of the makeup air can be provided through the rooftop HVAC units (this strategy has been adopted successfully by several leading quick-service restaurant chains). However, in full-service and institutional kitchens with larger exhaust requirements, it may not be practical (or energy efficient) to supply 100% of the replacement (makeup) air through the building HVAC system.
The solution is to specify an independent makeup air supply. But, once dedicated MUA has been added to the system, the challenge becomes introducing this air into the kitchen without disrupting the ability of the hood to capture and/or without causing discomfort for the kitchen staff. Kitchens are not large and dumping 7000 cfm of MUA, for example, in front of a cook line does not go as smoothly in practice as it does on the air balance schedule! Not only can makeup air velocities impact the ability of the hood to capture and contain cooking effluent, locally supplied makeup air that is too cold or too hot can create an uncomfortable working environment. This design guide presents strategies that can minimize the impact that the makeup air introduction will have on hood performance and energy consumption.
A typical kitchen ventilation system includes an exhaust hood or canopy, ductwork, fan system, and a means of providing adequate make-up air. The entire system must constitute a fire-safe assembly within the building.
Exhaust hoods and canopies capture heat and contaminants in the air by means of filters, extraction baffles (cartridges), and water mist systems. There are many style variations of hoods with canopy styles; a large box with an open bottom being the most common. Styles selection is based on the type of oven and the expected contaminants to be removed.
While there are several styles of hoods, all fall within two major categories:
* Type I hoods carry a listing label and are manufactured and installed according to the manufacturer's and listing agencies' requirements. They are designed to handle grease and include a number of integrated components within the hood.
* Type II hoods are used in the collection of steam, vapor, heat, and odors - but not grease. The two sub-classifications of Type II hoods are condensate and heat/fume.
Hot air rises! An exhaust fan in the ceiling could easily remove the heat produced by cooking equipment. But mix in smoke, volatile organic compounds, grease particles and vapor from cooking, a means to capture and contain the effluent is needed to avoid health and fire hazards. While an exhaust hood serves that purpose, the key question is always: what is the appropriate exhaust rate? The answer always depends on the type (and use) of the cooking equipment under the hood, the style and geometry of the hood itself, and how the makeup air (conditioned or otherwise) is introduced into the kitchen.
Cooking appliances are categorized as light-, medium-, heavy-, and extra heavy-duty, depending on the strength of the thermal plume and the quantity of grease and smoke produced. The strength of the thermal plume is a major factor in determining the exhaust rate. By their nature, these thermal plumes are very turbulent and different cooking processes have different surge characteristics. For example, the plume from hamburger cooking is strongest when flipping the burgers. Ovens and pressure fryers may have very little plume until they are opened to remove food product. Open flame, non-thermostatically controlled appliances, such as underfired broilers and open top ranges, exhibit strong steady plumes. Thermostatically controlled appliances, such as griddles and fryers have weaker plumes that fluctuate in sequence with thermostat cycling (particularly gas-fired equipment). As the plume rises by natural convection, it is captured by the hood and removed by the suction of the exhaust fan. Air in the proximity of the appliances and hood moves in to replace it. This replacement air, which originates as outside air, is referred to as makeup air.
The design exhaust rate also depends on the hood style and design features. Wall-mounted canopy hoods, island (single or double) canopy hoods, and proximity (backshelf, pass-over, or eyebrow) hoods all have different capture areas and are mounted at different heights relative to the cooking equipment. Generally, a single-island canopy hood requires more exhaust than a wall-mounted hood, and a wall-mounted hood requires more exhaust than a proximity hood. The performance of a double-island canopy tends to emulate the performance of two backto- back wall-canopy hoods, although the lack of a physical barrier between the two hood sections makes the configuration more susceptible to cross drafts.
Lastly, the layout of the HVAC and MUA distribution points can affect hood performance. These can be sources that disrupt thermal plumes and hinder capture and containment. Location of delivery doors, service doors, pass-through openings and drive-through windows can also be sources of cross drafts. Safety factors are typically applied to the design exhaust rate to compensate for the effect that undesired air movement within the kitchen has on hood performance.
The phrase "hood capture and containment" is defined in ASTM F-1704 Standard Test Method for the Performance of Commercial Kitchen Ventilation Systems as "the ability of the hood to capture and contain grease-laden cooking vapors, convective heat and other products of cooking processes." Hood capture refers to these products entering the hood reservoir from the area under the hood, while containment refers to these products staying in the hood reservoir and not spilling out into the adjacent space. The phrase "minimum capture and containment" is defined as "the conditions of hood operation in which minimum exhaust flow rates are just sufficient to capture and contain the products generated by the appliance in idle or heavy-load cooking conditions, and at any intermediate prescribed load condition." The abbreviation "C&C" refers to the "minimum capture and containment" flow rate as defined in ASTM F-1704.
Air that is removed from the kitchen through an exhaust hood must be replaced with an equal volume of makeup air through one or more of the following pathways:
Transfer air (e.g., from the dining room)
Displacement diffusers (floor or wall mounted)
Ceiling diffusers with louvers (2-way, 3-way, 4-way)
Slot diffusers (ceiling)
Ceiling diffusers with perforated face
Integrated hood plenum including:
1. Short circuit (internal supply)
2. Air curtain supply
3. Front face supply
4. Perforated perimeter supply
5. Backwall supply (rear discharge)
6. Combinations of the above
Cross drafts have a detrimental affect on all hood/appliance combinations. Cross-drafts adversely affect island canopy hoods more than wall mounted canopy hoods. A fan in a kitchen, especially pointing at the cooking area, severely degrades hood performance and may make capture impossible. Cross drafts can also be developed when the makeup air system is not working correctly, causing air to be pulled from open drive-through or pass-through windows or doors.
Side (or end) panels (as represented in Figure 12) permit a reduced exhaust rate in most cases, as they direct the replacement airflow to the front of the equipment. They are a relatively inexpensive way to improve capture and containment and reduce the total exhaust rate. In fact, one of the greatest benefits of end panels is to mitigate the negative effect of cross drafts. It is important to know that partial side panels can provide almost the same benefit as full panels. Although tending to defy its definition as an “island” canopy, end panels can improve the performance of a double-island or single-island canopy hood.
An increase in overhang should improve the ability of the hood to capture, although for unlisted hoods this may mean an increase in the code-required exhaust rate. Larger overhangs are recommended for appliances that create plume surges, such as convection and combination ovens, steamers and pressure fryers.
In order for the exhaust system to work properly, make-up air is required to replace air equal to the amount removed. Make-up air can be provided via an independent system or in combination with the building's HVAC system.
To better understand why a restaurant kitchen exhaust hood needs to be designed and constructed in a very specific manner, the principles behind air movement must be understood. Buildings are required to adhere to indoor air quality regulations and, depending upon the jurisdiction, sometimes exhaust air quality regulations. The food service industry must meet higher air quality regulations than standard building exhausts due to the type of contaminated air produced by cooking food.
Exhaust air is the starting point in restaurant kitchen ventilation design. Exhaust air is the air that is contaminated by smoke and grease-laden vapor (aerosols) created by the cooking source. This air must be removed from the building in a manner that complies with local codes and ordinances.
Make-up or, supply air must be provided in approximately equal amounts to replace the kitchen air being exhausted. Typically, outside air is supplied through a designed make-up air system. Most health codes require that an amount of fresh outside air be included in any replacement air calculation to assist in indoor air quality requirements.
Commercial kitchen exhaust fans are a vital part of your ventilation system. They remove odors and improve indoor air quality. Commercial kitchen fans also remove moisture, which can increase the level of humidity. High humidity can cause mold, mildew and bacteria growth which can ultimately result in major health code violations.
A Heat Recovery Ventilator (HRV) also exhausts moisture and odors. An HRV is a self-contained ventilation system that provides balanced air intake and exhaust. Like a central exhaust fan, it can be connected to several rooms by ducting.
Exhaust ductwork provides the means to transfer contaminated air, cooking heat and grease vapors from the hood to the fan.
* Ducts accumulate combustible grease and should be constructed from 16- steel or 18-gage stainless steel as per code requirements.
* The ducts must me securely supported by non-combustible duct bracing and supports designed to carry the gravity and seismic loads as per code requirements, no fasteners should penetrate the duct.
* The duct is often run inside a shaft enclosure and that enclosure is typically constructed of gypsum board, plaster, concrete, or ceramic tiles and must be an approved continuous fire-rated enclosure.
Exhaust fans move the heat and contaminated air out of the building. All exhaust fan components must be accessible or have removable access panels for cleaning and inspection and must be designed to contain and drain any excess grease. There are three major types of exhaust fans:
* Up-blast fans are typically aluminum centrifugals that are designed for roof mounting directly on top of the exhaust stack.
* Utility fans are normally roof-mounted with the inlet and outlet 90 degrees from each other and are typically used where high-static pressure losses exist.
* Inline fans are typically located in the interior duct and are used where exterior fan mounting is impractical.
Wall-mounted canopy hoods function effectively with a lower exhaust flow rate than the single-island hoods. Island canopy hoods are more sensitive to MUA supply and cross drafts than wall mounted canopy hoods. Engineered proximity hoods may exhibit the lowest capture and containment flow rates. In some cases, a proximity hood performs the same job as a wall-mounted canopy hood at one-third the exhaust rate.
Interior angles close to, or at, the capture edge of the hood improve C&C performance, allowing reduced exhaust by directing effluent back towards the filters. Hoods designed with these better geometric features require as much as 20% less exhaust rate compared to hoods identical in size and shape without these features. Capture and containment performance may also be enhanced with active “low-flow, high-velocity air jets” along the perimeter of the hood.
Variable Speed Fans and Idle Conditions
Appliances idle much of the day. Using two-speed or variable exhaust flow rates to allow reductions in exhaust (and makeup) while appliances are idling would minimize operating costs. NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations) was recently amended to allow minimum exhaust duct velocity as low as 500 fpm (at the exhaust collar and ductwork). Typical design values of 1500 to 1800 fpm at the exhaust collar are still recommended for normal cooking conditions. This code change will facilitate the application of variable speed systems.
The exhaust ventilation system can be a major energy user in a commercial kitchen – but it doesn’t need to be in temperate climates like California. Mild climates, such as San Diego, may require no heating or cooling. Some facilities may cool replacement air to improve kitchen comfort. Combined heating and cooling costs for MUA range from $0.00 to $0.60 per cfm in California climates, assuming 16 hours per day for 360 days per year. California climates are mild compared to other areas in North America so heating and mechanical cooling of MUA often is not necessary. Evaporative cooling can be very effective in desert climates.
The strategy used to introduce replacement (makeup) air can significantly impact hood performance and should be a key factor in the design of kitchen ventilation systems. Makeup air introduced close to the hood's capture zone may create local air velocities and turbulence that result in periodic or sustained failures in thermal plume capture and containment. Furthermore, the more makeup air supplied (expressed as a percentage of the total replacement air requirement), the more dramatic the negative effect.
The following design suggestions can improve the energy efficiency and performance of commercial kitchen ventilation systems:
* Group appliances according to effluent production and associated ventilation requirements. Specify different ventilation rates for hoods or hood sections over the different duty classification of appliances. Where practical, place heavy-duty appliances such as charbroilers in the center of a hood section, rather than at the end.
* Use UL Listed proximity type hoods where applicable.
* Hood construction details (such as interior angles and flanges along the edge) or high-velocity jets can promote capture and containment at lower exhaust rates.
* Install side and/or back panels on canopy hoods to increase effectiveness and reduce heat gain.
* Integrate the kitchen ventilation with the building HVAC system (i.e., use dining room outdoor air as makeup air for the hood).
* Maximize transfer air/minimize direct makeup air.
* Do not use short-circuit hoods (Figure 14). Use caution with air-curtain designs.
* Avoid 4-way or slot ceiling diffusers in the kitchen, especially near hoods.
* Diversify makeup air pathways (use combination of backwall supply, perforated perimeter supply, face supply, displacement diffusers, etc.).
* Minimize MUA velocity near the hood; it should be less than 75 fpm.
* Use direct-fired MUA heating if heating is necessary.
* Consider variable or 2-speed exhaust fan control for operations with high diversity of appliances and/or schedule of use.
* Provide air balance requirements to avoid over- or under-supply of MUA.
* Require building air balancing and system commissioning as part of the construction requirements.