Kitchen Ventilation Exhaust & Make-Up Air Design for

Commercial Hoods

Dr. Ali Hammoud BAU-2015

Commercial Kitchen Ventilation (CKV)

• The primary goal of an CKV system is to remove the cooking effluent and appliance generated heat from the kitchen while maintaining a comfortable, safe and energy optimized environment .

Design Considerations

• Fire safety and prevention

• Heat

• Humidity

• Performance

• Comfort

• Energy

CKV - Key Trends and the Future

1. Engineered hoods, robust performance at lowest exhaust (CFM)

2. Demand control ventilation becoming standard for many applications

3. Improved kitchen and building comfort

4. Fire prevention and safety

5. Building integration, computer monitoring, and system controls

Integrated Design Approach

• Codes & Standards

• Determining Exhaust Rates

• Makeup Air

• Exhaust Fans

• Kitchen Exhaust Duct

• Grease Emissions

• Fire Suppression Systems

Codes and standards used

• NFPA #96

• International Mechanical Code

• ASHRAE Standard 154

TYPES OF COMMERCIAL KITCHEN VENTILATION HOODS

Two Types of Hoods:

• TYPE I hoods are used over cooking equipment producing heat and grease laden effluent. These hoods require a fully welded ducting system.

• TYPE II hoods are used over non-grease producing cooking equipment exhausting heat and condensation.

TYPE I hoods

• Wall mounted canopy:

The wall canopy hood is used when the cooking equipment is placed against a wall Hoods that are used against a wall have a tendency to capture and contain the effluent using less airflow than in an island type application.

• Single Island Canopy: A single island hood is used over one row of cooking equipment placed where no walls exist. Single island hoods can be seen from all directions, therefore, have four finished sides. This type of hood is more susceptible to cross drafts, spillage, and is dependent only on the thermal updraft of heat from the cooking equipment. These hoods should be sized larger and use more airflow than a wall canopy hood with the same cooking battery. The single island hood must overhang the cooking equipment by a minimum of 6 inches on all four sides of the hood.

• Double Island Canopy:

A double island hood is placed over two rows of cooking equipment placed back to back. This configuration is made up of two wall canopy hoods placed back to back, thus creating four finished sides.

• Proximity Hoods (Back shelf):

Proximity hoods are TYPE I hoods that are shorter in height and depth than a typical canopy hood. The name “Proximity” or “Backshelf” refers to the close location of the hood with respect to the cooking equipment.

A note of caution: Although a well-engineered proximity hood can be applied with success at very low exhaust rates (e.g., 150 cfm per linear foot over medium-duty equipment), this same style of hood (if specified without performance data and/or in accordance with maximum height and setback permit-ted by code) may fail to effectively capture and contain the cooking effluent at exhaust rates of 300 cfm/ft or more.

• Pass Over Hoods:

The pass-over hood configuration is used over counter-height equipment where a pass-over capability is required. That is, prepared food is passed over from the cooking surfaces to the serving side.

• Eyebrow hoods:

Eyebrow style hoods are mounted directly to ovens and dishwashers to catch effluents. This hood type can be designed to operate only when appliance doors are opened or at certain points in the cycle.

TYPE II hoods

• Oven Hood:

The oven hood is an exhaust only canopy hood with an exhaust duct collar for the removal of heat and vapor. These hoods are the simplest of all hoods and are usually placed over ovens or small appliances only producing heat and odor. For complete capture and containment, overhangs should be measured with the oven door open.

• Condensate Hood:

The condensate hood is an exhaust only canopy hood with U-shaped gutters to capture and direct condensate to a drain. It also has an exhaust duct collar for heat, moisture, and odor-ridden air to exit. Condensate hoods are usually found mounted over dishwashers.

Capture and Containment (C&C)

The entire effluent plume is to be captured by the exhaust system and evacuated from the building.

(spillage)

Side Panels and Overhang

• Side Panels

The figure illustrate partial and full side panels. Side panels permit a reduced exhaust rate in most cases, as all of the replacement air is drawn across the front of the equipment, which improves containment of the effluent plume generated by the hot equipment. They are a relatively inexpensive way to improve C&C and reduce the total exhaust rate. Another benefit of end panels is to mitigate the negative effect that cross drafts can have on hood performance. It is important to know that partial side panels can provide almost the same benefit as full panels.

• Overhang An increase in overhang should improve the ability of a canopy hood to capture because of the increased distance between the plume and hood edges. This may be accomplished by pushing the appliances as far back under a canopy hood as practical and/or by increasing the side length. Although this improves C&C performance. Larger overhangs are recommended for appliances that create plume surges, such as convection and combination ovens, steamers and pressure fryers. (Size: 300mm at min)

Spillage with 150 mm

Of Front Overhang With 450 mm Of Front Overhang

SUPPLY AND MAKE-UP AIR(MUA)

The design of the make-up air system will have the single largest affect on hood performance. Supply air is defined as air that is brought into the space, but makeup air is dedicated to “making-up” the air being exhausted. Make-up air is brought into the kitchen at approximately an equal rate to the air being exhausted by the kitchen hood. This means that 100% of the air being exhausted must be made up. This can be accomplished through one supply type, transfer air, or multiple sources. A slight negative pressure is desirable in the kitchen with respect to the dining room to keep odors out of the dining area. The key to designing a system is to introduce make-up air in the most economical way without affecting the capture and containment of the hood.

Replacement air quantity shall be adequate to prevent negative pressures in the commercial cooking area(s) from exceeding 4.98 Pa (0.02 in. water column). ”NFPA 96 sec 8.3.1”. …”International Mechanical

code”.

• Tempered or Untempered?

Air that is heated or conditioned before it is brought in from the outdoors is called tempered air. If the goal is to make the kitchen comfortable, then utilize tempered air. If the goal is low cost, then use untempered air. Both tempered and untempered can be introduced, however, selecting the proper supply types will affect comfort and economic efficiency. Once this decision has been made a type of make-up air system can be selected, but always keep two things in mind. When tempering the air, use a source that will distribute the air throughout the kitchen to increase employee comfort. When using untempered air, use a source that will keep the air near the hood so it can be exhausted quickly without mixing in the space causing discomfort and increased heating/cooling loads.

• Supply Options: Make-up air can be introduced through the hood with an integrated supply plenum or an external supply plenum. The shaded region represents the volume of the hood. Increasing the volume allows more smoke and heat to be held in the hood until it can be exhausted. This is important over cooking equipment that produces a great deal of heat and smoke, such as a char-broiler. External supply plenums are usually less expensive and can be retrofitted to most exhaust only hoods.

• Exhaust Only Hood with Non-Directional Ceiling Diffusers:

This system will work best when bringing tempered air into the kitchen or can be used in climates where outside air closely matches desired indoor conditions. An exhaust only hood has no make-up air entering the room through the hood. This system is the least complex and in most cases works the best, however, may not be the most economical. The amount of exhausted air must be made up, therefore non-directional perforated ceiling diffusers and/or transfer air would be used to make-up 100% of the air. The most important thing to remember is to place many non directional perforated diffusers throughout the room to keep air velocities low and uniform. Uneven air distribution will cause drafts in the kitchen causing capture and containment to suffer

• Face Supply:

Located on the front of the hood ,face discharge is designed to throw make-up air across the room. Use face supply when tempered air is brought in through MUA into a tempered kitchen or when the MUA and kitchen are untempered because mixing will occur with the air in the space. Registers can be used for larger kitchens with longer throws, but perforated face panels are recommended for lower air velocities, which will minimize drafts in the kitchen. The maximum supply rate is 250 cfm/ft. through perforated panels under ideal conditions. For optimum performance design to recommended values of 150 cfm/ft. Face supply should not be used when a wall, another hood, menu board, or other object is less than 6 feet from the face.

The problem with bringing hot untempered air into an air conditioned room can be seen in trhe Figure . Hot air will not fall into the room and cycle back out through the hood, rather the hot air will hug the ceiling because it is more buoyant. If humidity is present in the hot make-up air, it will condense on the metal ceiling diffuser when it mixes with the air-conditioned air brought through it. Most of the hot air along the ceiling will be taken in at a return grill by the roof top unit (RTU) and conditioned before it is introduced back into the room, thus totally defeating the purpose of bringing in untempered make-up air.

• Integrated Air Curtain:

The hood integrated air curtain discharges air at the bottomfront edge of the hood and directs air downward. If spot cooling for the cooking personnel is desired, use tempered air. This type of hood can also be used to keep untempered air near the hood, although employee comfort will suffer. The maximum supply rate is 125 cfm/ft. For optimum performance design to recommended values of 65 cfm/ft.

• External Air Supply Plenum

The external air supply plenums provide spot cooling when using tempered air, but can also keep untempered air near the hood, which will save on heating/cooling loads. There are advantages over the integrated air curtain. Mounted 14-20 inches above the bottom edge of the hood or flush with drop ceiling, external air supply plenums can supply airflow at a maximum rate of 180 cfm/ft. For optimum performance, design to the recommended rate of 110 cfm/ft. In addition, external plenums can be attached to the face or ends of an exhaust only hood to create a curtain of air on all exposed sides of the hood, thus increasing the volume of air brought in at the hood.

In the above Figure ,notice the pocket of low pressure caused by the air flowing from the external air supply plenum. When velocities are too great, there is enough pressure differential to cause the hood to spill heat and contaminate. This effect can be observed on external and integrated air curtains, however, integrated air curtains are more susceptible to it due to the location of discharge.

• Combination Hood: Combination hoods are a combination of face supply and air curtain supply and are better suited for cooler climates where outside air can be used to cool the kitchen. More make-up air can be brought through a combination hood than a face or air curtain alone, but the same limits exist for each part of the plenum, maximum 250 cfm/ft. from the face and maximum 125 cfm/ft. from the air curtain. Perforated panels should always be used to reduce air velocities and eliminate spillage from the hood. Supply rates should be designed to recommended values of 150 cfm/ft. through the face and 65 cfm/ft. through the curtain for optimum performance.

An exhaust only hood with a variable supply plenum can be used instead of a combination hood which will increase maximum supply rates (see external air curtain, face supply) and not take up valuable capture and containment volume.

• Back Supply Plenum:

An effective way to introduce untempered make-up air into the kitchen is from the rear of the hood through a back supply plenum. These plenums are also ideal for heating air during the colder months since hot air will rise from its low discharge position. This plenum is mounted 31.25 inches above the finished floor and directs airflow through perforated panels behind and below the cooking equipment without affecting capture and containment, cooking surface temperature, or pilot lights. Back supply plenums are able to supply a maximum of 250 cfm/ft. For optimum performance design to the recommended rate of 145 cfm/ft.

• Multiple Sources:

The figure shown depicts two scenarios. The picture on the left shows air brought in through one side of the room while the picture on the right shows air brought in evenly throughout the room. To accomplish even airflow, use any one of the hood supply types along with multiple non-directional ceiling diffusers, or transfer air from another room. The amount of air to each diffuser decreases with an increase in number of diffusers, thus lowering air velocities. Various types of diffusers can be used, but non-directional perforated panel diffusers work best.

• Roof Top Units (RTU’s):

In many places where comfort is the main goal, a roof top unit will be used to supply the make-up air . These units condition the space while only taking in some outside air. The example shows that each RTU is providing 1000 cfm, but removing 800 cfm for a net of 200 cfm per RTU. Thus, the three RTU’s are providing a total of 600 cfm. RTU’s that are set to run in this situation should be in the “ON” mode instead of the “AUTO” mode.

• Integrated Makeup Air:

Duty Classification

Light Duty Appliances (ASHRAE Standard 154 & IMC):

• Gas and electric ovens (including standard, bake, roasting, revolving, retherm, convection, combination convection/steamer, conveyor, deck or deck-style pizza, and pastry) .

• Electric and gas steam-jacketed kettles

• Electric and gas compartment steamers (both pressure and atmospheric) .

• Electric and gas cheesemelters.

• Electric and gas rethermalizers .

Medium Duty Appliances (ASHRAE Standard 154 & IMC):

• Electric discrete element ranges (with or without oven)

• Electric and gas hot-top ranges

• Electric and gas griddles

• Electric and gas double-sided griddles

• Electric and gas fryers (including open deep-fat fryers, donut fryers, kettle fryers, and pressure fryers)

• Electric and gas pasta cookers

• Electric and gas conveyor (pizza) ovens

• Electric and gas tilting skillets/braising pans

• Electric and gas rotisseries

• Heavy Duty Appliances (ASHRAE Standard 154 & IMC) :

• Electric and gas underfired broilers

• Electric and gas chain (conveyor) broilers

• Gas open-burner ranges (with or without oven)

• Electric and gas wok ranges

• Electric and gas overfired (upright) broilers

• Salamanders

Extra-Heavy Duty Appliances (ASHRAE Standard 154 & IMC):

• Appliances using solid fuel such as wood, charcoal, briquettes, and mesquite to provide all or part of the heat source for cooking.

Estimation of Exhaust Flow Rates

• Listed Equipment : Equipment and materials which, following evaluation and acceptance by a qualified testing agency, are placed on a list of certification. The listing shows that the equipment and materials comply with accepted national standards, which have been approved or evaluated for conformity with approved or national standards.

• Minimum CFM for Unlisted Hoods (per 2012 International Mechanical Code):

• Based on ASHRAE Standard 154:

Velocity Limitation in duct

…”International Mechanical code”

…”NFPA 96”

System Losses (Static Pressure)

• Depends on:

– Type and design of the hood. –Grease removal devices (Filters). – size of duct connections – Flow rate.

Filters

Grease Filters:(NFPA sec:6.2.3): 6.2.3.1 Grease filters shall be listed. 6.2.3.2 Grease filters shall be constructed of noncombustible material. 6.2.3.3 Grease filters shall be of rigid construction that will not distort or crush under normal operation, handling, and cleaning conditions. 6.2.3.4 Grease filters shall be arranged so that all exhaust air passes through the grease filters. 6.2.3.5 Grease filters shall be easily accessible for removal. 6.2.3.6 Grease filters shall be installed at an angle not less than 45 degrees from the horizontal.

• Filter pictures:

• NUMBER OF FILTERS REQUIRED:

The minimum required number of filters for a particular hood can be calculated by dividing the total volume of air to be exhausted, in CFM, by the optimum operating velocity of the filter, in FPM usually 300 to 400FPM. This number is then divided by the actual square footage of the standard size filter (excluding the frame). The resulting figure represents the minimum number of filters required to efficiently remove the grease from the exhausted air.

• Example: Assume an exhaust hood with a minimum required airflow of 4200 CFM. Baffle type filters with a nominal size of 16" x 20", have an actual filtering surface of 14" x 18". (Nominal size minus the frame equals the actual filtering area.) Calculate the number of filters required, considering an optimum operating face velocity of 300 FPM across the filter.

solution: Filter Area Needed = Volume of exhaust air / Permissible face velocity

Filter Area Needed = 4200 / 300 = 14 sq-ft

Actual filter surface area (minus frame) = 14” x 18” = 252 sq- in or dividing by 144 (conversion factor for sq-ft to sq-in)

Actual filter surface area = 1.75 sq-ft

Number of filters required = Filter area needed / actual fitter surface area = 14 / 1.75 = 8 filters. Therefore, in this example, 8 filters would be required to provide adequate removal of the grease. Any space in the hood not occupied by a filter should be blanked off with sheet metal. As much as possible, the blanks should be divided equally between the filters. This will ensure optimum performance and will equalize the air velocity over the entire length of the hood opening.

Case Study

Preliminary Specifications and Constraints: (i) The working fluid will be an exhaust gas mixture from the appliances. (ii) The building has one story and a fire-rated roof-ceiling assembly. (iii) NFPA Standard 96 requires the use of an exhaust system in this type of kitchen application. (iv) The International Mechanical Code and ASHRAE Standard 154 should be consulted. (v) A wall-mounted canopy type hood, ductwork, and an exhaust fan will be required.

• Detailed Design: Objective: To design a duct system to remove odors, latent heat, and combustion by-products. The size and material of the duct will be determined. The fan and hood will be selected.

Data Given or Known: (i) An architectural drawing has been provided, complete with dimensions.

(ii) The types and sizes of the appliances have been provided by the client.

(iii) A 14 ft wall space has been allocated for the appliances that will be located under the hood.

Assumptions/Limitations/Constraints: (i) The friction losses in the ductwork should be about 0.1 in. of water per 100 ft of ductwork to mitigate noise and vibration. In this case, the fan will be sized after the ductwork system has been designed.

(ii) Circular ducts will be used. This will facilitate installation and reduce pressure loss and noise.

(iii) Type I exhaust ducts and hoods are those that are installed where cooking appliances produce grease or smoke, such as with fryers and oven ranges. According to Section 506.3.1.1 of the International Mechanical Code, for Type I exhaust ducts, steel of not less than No. 16 gage (0.0575 in. thick) or stainless steel of not less than No. 18 gage (0.044 in. thick) must be used to construct the duct system. Since excessive amounts of water will likely not be present to induce corrosion or other chemical reactions, No. 16 galvanized steel shall be used as the duct material. This is permitted by Sections 7.5.1.1 and A.7.5.1 of NFPA Standard 96. (iv) Where appropriate, 90◦ elbows will possess long radii. This will reduce the minor loss equivalent lengths.

(v) The exit from the duct system to the fan will be a bellmouth to reduce frictional losses.

(vi) Assume that the kitchen is an industrial setting.

(vii) Section 8.2.1.1 ofNFPAStandard96 and Section 506.3.4 of the International Mechanical Code require that the exhaust gas velocity in the ductwork be at least 500 fpm. In order to maintain a low-noise, low-vibration ductwork system, the gas velocity should not exceed 2200 fpm in the main duct and 1800 fpm in the branch duct. Therefore, a target maximum velocity of 1800 fpm will be chosen for the duct system for a conservative design approach. A note to Section 5.3.1 of ASHRAE Standard 154 also suggests general design velocities up to 1800 fpm for commercial exhaust systems for cooking operations. (viii) Additional pressure losses will occur when the exhaust gas enters the duct from the hood, passes through the grease filters, and is ejected from the fan against external prevailing winds. Assume that the losses at the entrance of the duct are similar to that encountered in a bellmouth entrance (Le/D = 12). The losses across the filters will be assumed to be the same as those across 30/30 filters (0.30 in. wg). The losses due to wind currents may be between 0.1 and 0.5 in. wg. For a conservative design and equipment sizing, assume the loss to be 0.5 in. wg.

• Sketch: A complete architectural drawing was provided. A sketch of the ductwork is shown in the drawing.

• Analysis: The ductwork and fan will need to be sized. Reference will be made to the sketch of the ductwork.

• Size of the Hood:

Section 507.12 of the International Mechanical Code and Section 4.3 of ASHRAE Standard 154 provide clear guidance regarding the minimum size of the exhaust hood relative to the size of the appliances. These codes require at least 6 in. of end overhang and 12 in. of front overhang for wall-mounted canopy hoods. Therefore, for this design problem and for the appliances selected, the minimum size of the hood should be 14 ft 7 in. long and 3 ft 7 in. deep to cover all the appliances. Section 507.15 of the International Mechanical Code requires that each exhaust outlet on a hood service no more than a 12 ft section of hood. Therefore, two exhaust outlets will be required in this hood.

A box exhaust canopy from American Hood Systems, Inc. will be chosen. A drawing of the custom-made hood in plan and elevation views is shown. The depth of this hood will be 3 ft 7 in., which meets the minimum requirement established by the code. The length of the hood will be specified as 15 ft, which satisfies Section 507.15 of the International Mechanical Code. The features of the hoods include compliance with NFPA Standard 96 and 16 and 18 gage steel construction. The baffle-type filters are each 20 in. wide and the stainless steel cup tray traverses the entire length of the hood.

• Flow Rates in the Sections of the Ductwork:

The volume flow rates of the gases that need to be exhausted through the system are usually specified by international or municipal codes. Section 8.2.2.1 of NFPA Standard 96 states that the exhaust hood and system should move a sufficient volume of gas to ensure capture and removal of grease-laden cooking vapors. However, Section 507.13 of the International Mechanical Code provides specific guidelines on the capacity (volume flow rate) of hoods for appliances that are defined as heavy duty, medium duty, or light duty. Type I wall-mounted canopy hoods should exhaust at least 400 cfm per linear ft of hood for heavy-duty appliances and 300 cfm per linear ft of hood for medium-duty appliances.

The same code section requires that the minimum exhaust flow rate required by the heaviest duty appliance covered by the hood should apply to the entire hood. Therefore, 500 cfm per linear ft of hood will be used to design the exhaust duct system and select the fan. Therefore, for the hood in question, the exhaust gas volume flow rate will be:

For each section of the ductwork system, the volume flow rates are: Section a-b: 3750 cfm (for each side of the symmetrical system), Section b-c: 7500 cfm.

• Sizing the Circular Duct:

The flow rate through each section of the ductwork system is known. It was assumed that the pressure loss in the system will be guided by a value of 0.1 in. wg per 100 ft of ductwork and the target maximum velocity will be 1800 fpm. The approximate diameter of the circular ducts will be found by using the flow rates, pressure losses, and the appropriate friction loss chart for round, straight galvanized steel ducts.

7500 CFM

3750 CFM

1550 fpm

1500 fpm

0.14

Thus,

Section a-b: 22 in. diameter, 1500 fpm, 0.14 in. wg per 100 ft.

Section b-c: 30 in. diameter, 1550 fpm, 0.10 in. wg per 100 ft. Note that the velocities of the exhaust gas in each section of the ductwork system are less than 1800 fpm, as required for a low-velocity, low-noise, low-vibration duct system. It is likely that the horizontal portion of Section a-b will be 30 in. to accommodate installation and attachment to the 30 in. duct of Section b-c.

• Total External Static Pressure Required from the Fan:

The friction loss for each section can be used to estimate the pressure loss in the duct. The equivalent length (straight duct + equivalent lengths of the fittings) of the longest branch should be used. The longest branch is section a-b-c. For section a-b: Le,a−b = Lstraight + Lent + Ltee,branch The exhaust outlet is located about 3 ft from the edge of the hood to facilitate centering the outlets. Therefore, the horizontal length of straight duct is approximately 4 ft. Therefore, Lstraight = 4 ft.

Lent

𝐷=12

Lent=12×D=12× 22 in=264 in/12=22ft.

To find L tee,branch: L tee

𝐷=40

Ltee=40×D=40× 22 in=880 in/12=73ft Le,a−b = 4 ft + 22 ft + 73 ft Le,a−b = 99 ft.

• It should be noted that the use of a 90◦ bend instead of a tee fitting would have reduced the losses significantly. If a 5-piece or 3-piece 90◦ bend was chosen, the radius of curvature would need to be 1.5 times the duct diameter. For a 22 in. diameter duct, the radius of curvature would need to be 33 in. For the 30 in. duct, it would need to be 45 in. Given that a 30 in. diameter duct may be used for the horizontal portion of section a-b, the 90◦ bend would likely not be feasible or easy to install given the 40 in. space that is available between the top of the hood and the finished ceiling (see final drawing). Therefore, the tee fitting might be the most likely fitting that would be installed.

For section b-c: Le,b−c = Lstraight + Ltee,branch. Section 4.2.1 of NFPA Standard 96 requires a minimum 18 in. clearance from the fan to the roof if the roof were made of combustible material. To facilitate installation, a 2 ft clearance will be provided, which will be added to the 3 ft length over which section b-c penetrates the attic space for connection with section a-b. Therefore, the length of section b-c is approximately 5 ft. L straight = 5 ft. L tee

𝐷=40

Ltee=40×D=40× 30 in=1200 in/12=100ft. Le,b−c = 5 ft + 100 ft

Le,b−c = 105 ft.

The total static pressure required from the fan is: Pfan = Pduct + Pfilter + Pwind = (0.244 + 0.30 + 0.5) in. wg Pfan = 1.1 in. wg.

A Greenheck tubular centrifugal belt drive roof upblast fan will be selected for this application. The specifications are marked in the catalog sheet. The fan should be able to move 7500 cfm of exhaust gas over 1.1 in. wg of external static pressure.

The fan that can produce 1.5 in. wg of external static pressure will meet the design requirements. From the selection table for the Greenheck TCBRU-2-22 line of fans, a 1542 rpm speed and a 4.64 hp motor is selected.

Thanks For listening