Kitchen Exhaust Hoods
Purpose & Theory of Operation
Commercial and institutional cooking establishments employ exhaust removal systems to provide ventilation of the workspace, thus supporting the kitchen operation with the removal of heat, grease laden vapor, smoke, contaminants, and cooking by-product.
The primary device utilized in the exhaust system for the purpose of capturing the plume produced by the cooking appliances, containing fire, and reducing the contaminants entering the exhaust system is the exhaust hood.
As cooking equipment produces heat, grease laden vapor, and smoke, the routing of these by-products to the outside of the building must be conducted in a manner to effectively minimize the hazard, as well as the effect on the surrounding areas with respect to the removal of conditioned air from the building.
The heating of cooking appliances generally results in a natural upward draft of air, which rises from the cooking battery. At the point whereby food is being heated and cooked, the updraft or plume, also known as effluent, becomes entrained into the air stream rising from the cooking surface. This by-product will be produced at varying rates based on the surface temperature of the cooking appliances, and the type of foods being prepared. With cooking processes ranging from low to high temperatures, and vast differences in the grease content of the foods being prepared, the need exists to provide adequate ventilation of the workspace, contaminant removal, and fire prevention based on the severity of the cooking operation.
Some applications for hoods may involve only the removal of vapor, and heat from the workspace, such as in the ventilating of cooking appliances which do not produce grease laden vapor, steam tables, and ware washing, and will have less stringent construction requirements from the hoods used in grease production type cooking.
The purpose of this chapter is to identify the various types of hoods, hood components, hood construction, applications, airflows, maintenance, and the codes associated with the application of hoods.
Non-Grease Versus Grease Type Hoods
Two basic types of hoods are commonly utilized for ventilation purposes within commercial and institutional kitchen facilities, with the main factor of their division being weather they are designed to ventilate grease laden vapor, or simply heat and vapor. These are divided into type 1, (commonly referred to as class 1), constructed and designed for the venting of grease laden vapor, and type 2, (or class 2), for the venting of heat, vapor, and odor only.
Class 2 Hoods
Application of class 2 hoods will generally be limited within kitchen facilities to non-cooking areas, such as above the dishwasher, or would be utilized in cooking areas where non-grease cooking, or re-heating of foods is being performed, when approved by the local Authority Having Jurisdiction.
It should be noted that a class 1 hood, constructed for handling of grease vapor, can be used in a class 2 cooking application, since the construction requirements are much more stringent than that of class 2 however, class 2 constructed hoods cannot be used for any application where grease vapor is being produced.
Class 2 exhaust hoods may be a simple, non-welded, light gauge (18-22 gauge), sheet metal box, hung above the non-grease vapor producing appliance, and the application often does not include any provisions for fire extinguishing systems, or filtering of the exhaust air. Additionally, the class 2 hoods almost never bear a Listing by a third party such as Underwriters Laboratories, and are considered to be the least expensive type of hood to purchase and install.
Commonly associated with the employment of class 2 hoods would be light gauge sheet metal exhaust ductwork (or non-welded stove-pipe), and non-grease type blowers, which could pose a hazard if used in grease applications because of the flammability of the by-product grease. Some previously approved cooking appliance applications of class 2 hoods (and duct systems) have included pizza ovens, box steamers, soup kettles, baking ovens, and steam tables.
Application and use of class 2 constructed exhaust systems for cooking equipment normally would require approval by the local Authority Having Jurisdiction.
Class 1 Exhaust Hoods
Cooking applications which produce grease-laden vapor require the use of class 1 hoods, which will be divided into two main groups; Listed exhaust hoods, and unlisted hoods.
Listed exhaust hoods have been tested for performance by a third party Listing Authority, such as Underwriters Laboratories, for their performance in capturing smoke and vapor at varying temperatures, ability to withstand fire conditions, and electrical components survivability in the application.
Listed hoods are sub-divided into two groups: Hoods with fire dampers, and hoods without fire dampers. Fire dampers are not installed in any unlisted exhaust hood assembly or exhaust ducts, and generally will be limited to either fuse link activation, or would be thermostatically activated to allow closure in a fire condition.
Grease Removal Devices
Listed and unlisted exhaust hoods, which are used in cooking applications, will include grease removal devices to reduce the grease volume of the exhaust airstream prior to the exhaust air entering the exhaust duct. Grease buildup in the exhaust ductwork is considered a fire hazard since the deposited by-product remains flammable. With the accelerated air velocities within the exhaust ductwork induced by the blower, potential risks escalate with the buildup of flammable byproduct. Failure to remove this byproduct at the source of the grease removal device within the hood, will result in migration of the flammable load throughout the exhaust duct system as associated with the daily re-heating of the appliances, and the blower operation.
Grease removal devices are normally located within the exhaust hood, and may be either removable for cleaning, or may be a fixed component within the exhaust hood for automatic wash down, such as a water wash type hood. Removable type filters are normally arranged within a filter rack, are located just below the exhaust duct collar for the hood, and would have a metal grease collection container, which does not exceed one gallon.
Early primary grease removal devices utilized within exhaust hoods for cooking were commonly mesh type filters.
Utilizing a matrix of layers of stainless steel, aluminum, or steel housed within a frame, mesh filters provided grease extraction based upon the principle of impingement.
The National Fire Protection Standard 96 no longer allows the use of mesh filters in cooking applications as the primary grease removal device due to their ability to store large amounts of cooking by-product if not regularly removed, and cleaned. Other undesirable characteristics of the mesh filters in cooking applications is the inability of the filter to continuously drain, the reduction of airflow while the filter is loaded, and the changing resistance offered by the filter between the clean and soiled conditions.
Most common filter hoods today utilize baffle type filters, which operate based upon the principle of centrifugal grease extraction. These filters are configured with a series of overlapping baffles, which force the grease laden exhaust air to make several changes in direction within the grease filter. The grease is dismissed form the air stream by centrifugal force, held within the filter interior, and accumulation of the grease aerosol particles which begin to liquefy continuously drain from the filter to a grease drip tray, or trough, and then drain into a metal container which does not exceed one gallon.
Baffle filters operate with the lowest resistance offered by any grease removal device, which would normally be between .50" and .75" static pressure, and the average air velocity at the face of the filter would normally be between 150 feet per minute, and 400 feet per minute, both depending upon the hood airflow in volume.
Baffle filters are Listed by Underwriters Laboratories Standard 1046, which tests their ability to perform in the application, and in fire conditions. Several advantages exist with baffle filters compared to mesh filters, such as the ability to continuously drain during use, the resistance gained is minimal when the filter is soiled, and because of the changing air velocities in front of and behind the filter, they assist in decreasing flame spread into the plenum behind the filters, and thus into the duct.
NFPA Standard 96 contains several requirements regarding use and construction for baffle grease filters in hoods, such as distances between the cooking appliances and the grease removal devices, they shall be listed, constructed of steel or listed equivalent, shall be of rigid construction that will not distort or crush under normal operating conditions, shall be arranged so that the exhaust air goes through the filters, must be accessible for removal and cleaning, installed at not less than 45 degrees from the horizontal.
Extractor Cartridge Type Filters
Extractor cartridges operate on the same centrifugal extraction as baffle type filters, however their construction varies greatly compared to baffle filters.
The extractor cartridges operate with greater resistance since the exhaust air is pulled through one opening on the extractor, which will vary in height from 11/2" to 4", instead of a series of openings across the face of the filter. These are used only in hoods designed to accommodate the extractor cartridge and are not considered to be interchangeable with standard baffle filters.
Cartridge hoods operate with higher static pressure and intake velocity than standard baffle filters, with the velocity at the intake slot ranging from 750 feet per minute to 1200 feet per minute, and the resistance normally between 1" and 1.75" static pressure depending on the design and airflow of the hood.
When positioned within the hood, cartridges will create an opening at the intake slot similar to that of a water wash hood, however much like baffle filters, they require manual removal, cleaning, and placement back into the hood for maintenance.
Research on methods of increasing the efficiency of the primary grease removal devices is now being conducted by several firms, and these items are expected to become into the market sometime in the near future.
Most designs incorporate a secondary filter media bed, which would be located behind the primary grease removal device in the hood. These media beds would contain beads of ceramic or other nonflammable strata to broaden the spectrum of aerosols able to be arrested by the primary grease removal device itself.
Future applications of these products will be entering into the marketplace after final development, research, and listing by recognized firms. Retrofit applications, required maintenance, and effect on resistance remain unknown at this time.
Fixed Baffle Hoods
Fixed baffle hoods, also known as water wash hoods, incorporate a series of fixed baffles on the interior of the hood, normally behind face plates or inspection doors. Similar to the extractor cartridge type hood, the air is centrifugally cleaned as the exhaust air stream is forced to make several changes in direction around the baffles, depositing the grease onto the baffles on the interior of the hood.
This area, sometimes referred to as the grease extraction chamber, would normally incorporate a series of wash nozzles which would release hot water and detergent each day after the blower is shut down to remove the daily deposited grease load, thus washing the by-product from the interior of the hood and out of the fire zone above the appliances, and down a drainage field which would normally be equipped with a grease trap.
One advantage of this arrangement is to assist in the elimination of human error associated with the regular removal, cleaning, and replacement of the primary grease removal devices as required for fire prevention purposes.
As these system incorporate a control system of electrical, plumbing, and detergent dispensing systems, a broad array of configurations exist today, based upon the number of manufacturers offering these products.
Automation of the control assemblies to farther decrease human error, large detergent supply containers to minimize required refilling, periodic wipe down of the exposed intake slot area, and periodic operational checks and preventive maintenance on the equipment remain valuable to help insure safe operation consistent with the manufacturer's recommendations.
Fixed baffle hoods operate with airflows and resistance similar to that of the aforementioned cartridge type hood however, due to the ability of the manufacturers to segment, or vary the airflow along the length of the hood to reflect the actual net exhaust air as demanded by the cooking appliance lineup, these hoods can operate with offerings of significant long term energy savings. Because of the required control packages, construction complexity, and the material required to construct the hoods, fixed baffle hoods have the highest initial costs of purchase.
Options for fixed baffle hoods include such features as cold water mist which operates with the exhaust blower to increase grease removal efficiency or to offer a spark arrestor for application involving solid fuel appliance venting.
Fuse link or thermostatically operated fire dampers are also regularly specified, which operate in conjunction with the wash system to combat fire conditions. Exhaust blower shut down, closure of the fire damper, and activation of the wash cycle normally would constitute a fire cycle, and is often viewed as an advantage over standard filter hoods, which rely on only the fire extinguishing system for protection of the grease removal devices and the plenum, and the duct.
Additional designs of fixed baffle hoods incorporate a cold water feed to maintain a supply of water within the bottom of the extraction chamber of the hood, and the water is cyclonically agitated by the movement of the exhaust air through the hood interior, to create a scrubber or water bath. Hot water wash with detergent would also flush the extraction chamber area at the close of the cooking day after blower shut down.
Various Types Of Hoods
Several configurations of exhaust hoods exist which allow the user to choose the most practical layout for the application, and the cooking appliances being vented. The most widely applied configuration of exhaust hoods are listed below along with a brief description of the respective layout:
Likely the most common exhaust hood applied today, the canopy hood is positioned above the cooking equipment and may be either wall mounted, or freestanding, island configuration to cover a variety of cooking appliances. Grouping of several of the canopy type hoods together in a freestanding manner, back-to-back, allows for two lines of cooking appliances to be vented at varying airflows to combine the hotter production appliances and the cooler (less demanding) appliances utilized for food preparation. This configuration is considered a double island canopy arrangement.
Canopy hoods normally will be sized based upon the underlying cooking appliances employed, plus an overhang at the ends and the front edge of 6" to 12" assisting in capture, and to allow for the thermal expansion of the air as related to the operation of the appliances.
Filter hoods, cartridge hoods, and water wash type hoods may all be produced in the canopy hood configuration, and the materials used in their construction would normally be either stainless steel, or galvanized steel, may be listed with, or without fire dampers or unlisted.
Back Shelf and Passover Hoods
Unlike canopy hoods, the back shelf and passover type hood are normally applied where low type counter or floor model cooking appliances are configured in a short order fashion, and may be positioned against a wall, or built into a counter to facilitate serving of food directly over the top of the hood.
These hoods will typically not overhang the front of the cooking appliances and therefore, depends upon the exhaust airflow itself, and not the physical sizing of the hood to provide capture and containment of the exhaust plume rising from the appliances. Some cooking appliances of high temperature operation may not be suited for this type of hood due to either the ability of the updraft from the appliances to out run the capture velocity of the exhaust air, or due to challenges with required clearances from open burning appliances to the grease removal devices.
Construction of the back shelf and passover hoods is normally of stainless steel, they are listed for the application, with or without a fire damper. These hoods may also be of the filter type, cartridge type, or water wash.
Eyebrow Type Hoods
Normally applied above and in front of baker type ovens, and rotary ovens, the eyebrow hood provides a small canopy above the opening of the appliance whereby effluent exiting from the opened appliance rises into the canopy.
Additionally, the heat flue of the appliance may be piped through the wall of the canopy, to facilitate venting of the combustion gasses into the interior of the canopy for evacuation by the exhaust blower.
Eyebrow hoods may be of filter, high velocity cartridge, or water wash design, may be listed with, or without fire damper, or may be unlisted.
Exhaust Hood Air Flow Rates
Many model code authorities have applied the required exhaust air flow volume based upon mathematical formulas related to the physical size of the exhaust hood, or the opening between the exhaust hood and the appliances. As differences in the cooking appliances, and cooking technique have developed, often the net exhaust required utilizing these formulas has been found to be excessive and wasteful in many cases.
At the point that energy costs began to escalate, several entities have sought improved methods to assign airflow rates based upon the cooking appliance type, and the nature of their heating in an effort to reduce the cost of operating the kitchen exhaust system.
Many of the exhaust hood manufacturers re-thought this process by calculating the natural updraft of the cooking appliances at operating temperature, using a multiplier to add safety margin, and calculated the net exhaust required for the cooking appliance line.
The result of this assignment of required exhaust airflow based upon fuel type and calculated square feet of the appliances offered adequate capture and considerable savings from an operating standpoint because the volume of the exhaust was effectively reduced.
Additional reducing of the net exhaust was obtained by some manufacturers, through segmenting of the exhaust volume along the length of the exhaust hood. This allows for the higher temperature appliances to have greater exhaust volume, and reduction of the exhaust volume above the lower temperature appliances.
Calculating the cost of operating the exhaust hood includes consideration of the cost of evacuated conditioned air from the kitchen and surrounding spaces, cost of the delivery, and possible tempering of the make up air. If the net exhaust volume reduces, so will the size of the exhaust and make up air blowers, and the duct sizes for each respectively.
The American Society of Heating Refrigerating and Air Conditioning Engineers (ASHRAE) subsequently devised a minimum exhaust flow rate chart for both listed and unlisted hoods. This allows the cooking equipment to be divided into four groups as follows:
Minimum Exhaust Flow Rate in Cubic Feet Per Minute per Lineal Foot
Type of Hood Light Duty Medium Duty Heavy Duty Extra-Heavy Duty
Wall Canopy Unlisted 200 300 400 550
Wall Canopy Listed 150-200 200-300 200-400 350+
Unlisted 400 500 600 700
Listed 250-300 300-400 300-600 550+
Double Island (per side)
Unlisted 250 300 400 550
Listed 150-200 200-300 250-400 500+
Unlisted 250 250 N/A N/A
Listed 150-250 150-250 N/A N/A
Unlisted 300 300 400 N/A
Listed 100-200 200-300 300-400 N/A
The above chart illustrates the advantage of utilizing Listed hoods, since significant savings of the net exhaust may be realized.
Make Up Air
Make up air is brought into the kitchen through a dedicated system, or the HVAC system, and is designed to supply outside air to replace the exhaust being removed from the kitchen through the hood system.
Make up air may be supplied through a series of ceiling mounted diffusers, or registers away from the exhaust hood, or it may be integral with the hood, and distributed in a variety of configurations which differ from one another in both methodology and characteristics.
As a general rule, kitchen areas where the hood(s) are located must remain slightly negative in pressure compared to the surrounding areas in order for the hood to provide proper capture and containment of the plume produced by the cooking appliances.
In order to establish ample relief of the negative pressure created by operating the exhaust blower, and to prevent the removal of large volumes of conditioned air within the building, most hood manufacturers recommend that make up air be supplied at the rate of 75% to 80% of the exhaust volume.
As the make up air is normally supplied in the kitchen area where the exhaust hood is located, the given volume able to be delivered must not upset the natural rising of the effluent off of the cooking appliances, or smoke loss from the hood will result as the exhaust plume is washed away from the confines of the exhaust hood.
Several different methods of delivering make up air (which is integral with the hood) exist. These delivery methods may effect the volume of make up air able to be delivered, therefore the chosen method of delivery may have an impact on the operating efficiency of the exhaust hood.
The most common types of integral make up air distribution used by hood manufacturers are listed below along with some of the operating parameters:
Make up air being supplied through the front of the exhaust hood, normally through perforated stainless steel panels. Make up air is delivered into the room, at low velocity, so that the hood exhaust may minimize the trajectory of the introduced make up air into the room. This type of distribution may allow up to 80% to 85% make up air be supplied, depending upon design.
Fire-actuated dampers may be require by NFPA standard 96, on face discharge make up air if the supply air duct inlet, or the outlet penetrates the continuously welded shell of the hood assembly.
Make up air being supplied at the bottom of the front of the exhaust hood, normally through registers or perforated panels. As the discharge area is relatively small, higher velocities are common, which may decrease the volume of make up air able to be delivered without upsetting the plume rising off the cooking appliances. Make up air may require tempering in cooler climates due to personnel discomfort, as the make up air is delivered from the lower edge of the exhaust hood.
Most downward discharge make up air arrangements allow only 60% make up air to be delivered, since dumping larger quantities in this manner could upset the plume by siphoning off the rising effluent.
If downward discharge make up air distribution outlet, or the inlet penetrates the welded shell of the hood, a fire actuated damper is required by NFPA standard 96.
Make up air being delivered into the inside of the canopy, normally toward the exhaust intake, as to prevent the make up air from entering into the kitchen area.
Many drawbacks exist with this manner of distribution since the area within the hood canopy is needed for the storage of the plume of heat, grease vapor and smoke. Increasing the volume of make up above the 50% to 60% range will often result in the make up air velocity remaining higher than the face velocity of the filters, whereby cooking effluent is washed from the face of the filter.
When the make up air is internally supplied to an exhaust hood, the employment of a fire- actuated damper is required by NFPA standard 96.
Integral make up air distribution may incorporate multiple points for discharge to allow a combination of the above listed configurations. This may be beneficial depending upon such factors as conditioning of the kitchen, climate of the location, and design quantities specified.
Exhaust Hood Construction
Local model code authorities, and the National Fire Protection Association Standard 96 normally dictate the construction requirements for kitchen exhaust hood assemblies.
Listing authorities, such as Underwriters Laboratories provides evaluation of hood assemblies offered by the manufacturers of exhaust hoods for survivability in the application, electrical components incorporated in the assemblies, and structural integrity in fire conditions. Hood assemblies meeting or exceeding the testing requirements, and the construction requirements of the evaluating authority are subsequently listed.
After the initial listing of the product, periodic evaluation of manufactured products remains ongoing by the listing authority at the point of the manufacturing. Additionally, field evaluation of listed products, which may have been altered during their installation or use, is also provided by the listing authority to determine compliance with the terms of the equipment listing.
Highlights of the National Fire Protection Standard 96 requirements for construction of hood assemblies designed to handle grease-laden vapor are listed below:
1). Shall be constructed and supported of steel, not less than 18 Gauge, or stainless steel, not less than 20 Gauge.
2). Seams, joints, and penetrations of the hood enclosure that direct and capture grease laden vapor and exhaust gasses shall have a liquidtight continuous external weld to the hoods lowermost outer perimeter.
3). Penetrations of the hood shall be permitted to be sealed by devices that are listed for such use, provided the structural integrity of the assembly is not degraded.
4). Listed exhaust hoods with or without fire dampers shall be permitted to be constructed of materials required by the listing.
5). Hoods shall be sized and configured to allow for the capture and removal of grease-laden vapors.
6). A fire-actuated damper shall be installed in the supply air plenum at each point where a supply air duct inlet or a supply air outlet penetrates the continuously welded shell of the assembly.
7). Fire damper actuation devices shall have a maximum temperature rating of 286 degrees F.
8). Listed hood assemblies shall be installed in accordance with the terms of their listing and the manufacturers instructions.
For complete listings of NFPA 96 requirements concerning construction of, and operation of exhaust hoods, refer to the current standard 96, or http://www.nfpa.org.
|Created on 2004-01-21 11:15:51 by Robert4Powerwash.com
Updated on 2004-01-21 12:44:11 by Robert4Powerwash.com