When designing a kitchen ventilation system, there are a number of factors that should be considered, such as project budget, building layout, surrounding buildings, system maintenance, operational cost, fire risk from grease buildup, and more.

It’s important to take these into account as well as consider the specific needs of the K-12 school. As more schools move to healthier meals, the kitchen layout and functionality must be taken into account as well as the quantity of the food being prepared. Keeping the cooking equipment properly cleaned and well maintained is essential to maintaining a safe and sanitary cooking environment. This same level of care should be taken for the kitchen ventilation system as it will extend the life of the system while also minimizing unanticipated downtime.

The core components of a kitchen ventilation system include the kitchen hood, grease-rated ductwork, and exhaust fan. Once the equipment has been selected and a layout has been determined, the next step will be selecting a kitchen hood and determining the exhaust cfm.

 

Kitchen hood selection

There are two types of commercial kitchen hood classifications, type 1 and type 2. Type 1 hoods are designed to contain and capture heat, grease, and effluent, which is given off in the cooking process. Type 2 hoods are used over non-grease applications, where heat and condensation are being generated.

One common byproduct of the cooking process is grease. How and where grease is captured in your kitchen ventilation system will play a big role in upkeep and ongoing maintenance costs. The first line of defense in the kitchen filtration system is the hood. There are a number of mechanical filters that can be chosen that vary in efficiency and pressure drop. Generally speaking, the more efficient the filter the higher the pressure drop.

  • Baffle filters remove approximately 28 percent of grease at 8 microns with a pressure drop of 0.55 inches wg; 

  • Centrifugal filters are slightly better and will remove approximately 69 percent of grease at 8 microns with a pressure drop of 0.75 inches wg; and 

  • A multistage filter utilizes a centrifugal filter as the first stage of filtration, and the second-stage filter is packed with beads to capture the remaining grease particles. This two-stage filter removes nearly 100 percent of grease at 8 microns with a pressure drop of 1.2 inches wg. 

The more grease that is captured by the filters the less grease that will migrate downstream to the ductwork, exhaust fan, and ultimately be discharged into the ambient air. 

 

Exhaust Rate

Determining the exhaust cfm is a very important part of the design. If the exhaust cfm is too low, contaminated air will be generated faster than the exhaust system can take it away, resulting in spill over. Conversely, oversizing the exhaust cfm will result in a non-value-added expense in the form of wasted energy.

There are several methods used to determine the exhaust rate. For non-UL-listed hoods, the International Mechanical Code outlines a cfm value per linear foot based on the categorization of the cooking appliance. A categorization of light, medium, heavy, and extra heavy-duty will be assigned. Once the cfm values have been determined for each piece of equipment, the largest cfm rate will then be applied over the full length of the hood.

A UL-710-listed kitchen hood will go through a number of different tests pertaining to the capture and containment of smoke and flare ups. When determining the exhaust cfm, this method will factor in the updraft velocity in feet per minute of each cooking appliance and will also apply a hood factor based on the mounting conditions and associated accessories. The resulting exhaust cfm is typically lower than that of a non-UL-listed hood.

Once the exhaust cfm has been determined, the volume of makeup air can be calculated. The purpose of makeup air is to bring outdoor air into the kitchen to replace the air being exhausted. Makeup air is typically tempered to within 10º of the kitchen space temperature.

To help minimize cooking odors from migrating into other rooms, the makeup air will be slightly less than that of the exhaust cfm to keep the kitchen slightly negative as compared to the space next door.

 

Energy Savings

As you might imagine, the operational cost and energy consumption of the kitchen equipment and ventilation system can be high compared to other systems in the school.

Over the course of the day, the cooking load will vary based on the food being prepared and the pieces of equipment in use. If your kitchen ventilation system is set up to match this load, significant savings can be had. A variable volume hood system will automatically adjust the exhaust fan cfm to match the heat and contaminants being generated. As a result, the makeup-air fan will also adjust to track the exhaust fan cfm. This reduction in total system cfm will save a lot of energy while also extending the life of the equipment.

 

Duct System Selection

When selecting a grease-rated duct system, there are a number of factors that can influence whether the duct system should be field-fabricated or be a prefabricated factory-build, UL-listed system. Variables like ceiling space constraints, duct run, construction timeline, and project budget should all be considered.

 

A field-fabricated, non-UL-listed duct system will require field welding and field-applied, fire-rated insulation. Factors to consider:

  • Installation time to hang, weld, and test;

  • Duct slope of 0.25-inch per foot toward the hood. For horizontal runs longer than 75 feet, the slope shall be 1 inch per foot;

  • Upfront material costs are typically lower but require more time and money to install; and

  • Requires skilled tradesmen to weld the duct connections.

 

Prefabricated factory-build UL 1978-listed grease duct system with a UL 2221 classification:

  • Manufactured in a controlled environment;

  • Premanufactured pieces ship to the job site and can be assembled on-site quicker than traditional systems;

  • Have the advantage of reducing the clearance to combustibles, and these systems also allow for non-welded joint connections;

  • A zero-clearance to combustibles can be achieved with a UL-2221 classification, which can also serve as a two-hour fire rating; and

  • Many UL-listed systems are able to achieve low or no slope depending on the duct run and application. The allowable slope varies based on the manufacturer.

 

Grease-rated Fan Considerations

UL 762 kitchen exhaust fans are designed to handle grease-laden air and elevated temperatures. The fan will also be equipped with a grease drain port and hinged base for serviceability. Grease collected at the fan will collect in the grease trap and needs to be cleaned out periodically. There are a number of fan styles available, such as upblast, inline, and utility set, depending on what works best for the application.

A static pressure calculation should also be done, which takes into account the pressure drops from the filters, duct run, and elbows. System effects and duct velocity should also be considered.

Armed with the style of fan, volume of air being exhausted, and the static pressure calculation, a grease-rated exhaust fan can be selected.

 

Grease Management

Grease not captured by the kitchen hood’s grease filters will collect and accumulate in the ductwork. Managing grease accumulation is an important part of keeping your kitchen exhaust system working properly while also minimizing the fire risk.

The amount of grease in your system will vary based on how often the kitchen is being used and the type of food being prepared. Thus, it is recommended to monitor grease buildup as a part of your routine maintenance schedule.

 

Grease and Odor Reduction Technologies

There are a number of grease- and odor-reduction technologies that can also be utilized if the requirements of your project warrant it.

For UVC hoods, UV lights are generally mounted behind the kitchen filters but within the hood cavity. Ultraviolet lights used in commercial kitchens are designed to produce short-wavelength UVC rays to kill and inactivate bacteria and microorganisms. They also break down grease and odor particles into smaller molecular chains. Depending on the output of the UVC lamp, they can be ozone producing or non-ozone producing.

Duct-mounted ozone generators are another technology that can be used to reduce grease buildup and odor-related complaints. Ozone produced via corona discharge is created by passing air over a high-voltage electrical field, and the resulting electrical discharge produces ozone very efficiently.

Regarding installation and placement of the ozone generator, a small volume of kitchen/plenum air is filtered and pulled through the ozone generator and into the main exhaust duct system via a bleed air connection near the top of the kitchen hood. 

Ozone being more reactive than atmospheric oxygen accelerates the naturally occurring process of oxidation but in a much shorter timeframe. The longer the duct run the more time the ozone will have to react with the grease and odor particles breaking them down into water vapor, carbon dioxide, and dry minerals.

Both technologies result in less grease buildup in the ductwork, which lowers duct cleaning costs and reduces fire risk to the building.

 

Conclusion

As you can see, a number of considerations must be taken into account when designing a kitchen ventilation system for use in a K-12 school. Considerations like project budget, lifespan of the building, and many other factors will influence the ultimate outcome. 

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