An Appetite For Detail (November 1999)
The primary focus of kitchen ventilation is the removal of heat, smoke, steam, and odors which are considered contaminants. A hood is used to capture and contain the contaminant flow which is cleaned by the filters then transported to the outdoors by the duct and fan system.
In order for the kitchen ventilation to be effective, all the design issues must be evaluated into one inclusive system. This total engineering concept is in contrast to just connecting ducts to hoods selected by others, and reviewing a fire protection system also selected by others. The ventilation rates for odor control, the arrangement of air outlets for comfort cooling, and makeup air for the hoods must be designed to complement the hood selection. The hood controls and any hood damper operation must be considered in both the operational and fire modes.
Two Types Of HoodsConsideration of the following issues is essential to evaluate and coordinate a successful commercial kitchen ventilation system.
The model codes categorize commercial kitchen hoods as either Type I or Type II hoods.
A Type I hood collects and removes grease and smoke. Therefore, a Type I hood must be equipped with UL-tested and -certified grease filters or grease extractors (UL Standard 1046). Three varieties of this type are:
- Water wash extraction. Grease is extracted by separating the grease particles from the airstream as the air passes through a series of baffles at a high speed, causing the grease to be thrown out of the air- stream by centrifugal force. The grease is washed out of removal gutters by the daily wash cycle.
- Removable extractor. Grease is extracted by separating the grease particles from the airstream as the air passes through the cartridge. This device is also called a high-velocity cartridge filter. Filters can be removed for cleaning.
- Baffle filter. Grease is extracted by centrifugal force through an aluminum, steel, or stainless steel baffle path. Filters are removable for cleaning.
On the other hand, Type II hoods collect and remove steam, vapor, heat, or odor. Type II hoods have two subcategories: one type is a canopy to remove condensate from the steam, and the other is a canopy to remove heat or odor.
Each variety of the Type I hood is available in a canopy style. The water wash extractor and baffle filter types are also available in a backshelf style. A Type II hood is often just a canopy without a filter. If a filter is provided, it is the baffle type. Each type and style of hood has a cost and an air-quantity relationship that must be considered. The following are general cost-benefit relationships.
- The lower the hood cost, the higher the exhaust air required.
- The lower the hood cost, the lower the grease-extraction efficiency.
- The lower the hood cost, the lower the fan static requirements.
The hvac designer must evaluate the cost-benefit relationship of the hood that may have been selected by the architect or kitchen consultant. The hood with the lowest first cost may not offer the lowest overall system cost because of the requirements for greater exhaust and makeup air quantities.
Construction SpecsType I duct construction consists of at least 16-ga black steel or 18-ga stainless steel. Slope the duct 0.25 in./ft toward the hood or an approved grease reservoir. For ducts longer than 75 ft, the slope shall be at least 1 in./ft. Provide cleanout doors in the side or top of the duct at least every 12 ft and at the top and bottom of the riser.
Type II duct construction consists of rigid metal ductwork in accordance with normal SMACNA standards. Slope is not required, although it should be considered in ducts that exhaust steam or moisture such as dish and pan washers.
The air velocity for Type I ducts should not be less than 1,500 ft/min and should be evaluated for duct static requirements and noise if more than 2,500 ft/min. Duct exhaust collars are sized to provide a velocity of 1,800 ft/min. Coordinate the location of the hood duct collar so it does not interfere with the structure of the building. The exhaust duct should run as straight as possible from the hood to the fan. If you are not sure of the exact conflict location, the hood may be ordered with the duct collars shipped loose so they can be installed in the field.
What's New in Duct Construction?A duct enclosure must be provided for a Type I duct that penetrates a fire-rated ceiling, wall, or floor. This enclosure must extend from the point of penetration to the exterior of the building. Duct enclosures shall be made of at least 1-hr fire-resistive construction in all buildings and 2-hr fire-resistive construction in Types I and II fire-resistive buildings. Enclosures are to be vented to the exterior.
The vent and chimney industry has developed a ceramic-fiber insulated, sealed, liquid-tight, prefabricated grease duct system. The system allows the duct to be installed without a rated chase.
The duct-cleaning groups have worked toward the power-washing of grease ducts with revolving brushes and detergents. The ductwork should be designed to pitch to a low point and have sewer drain access. The top of the stack should be accessible for the power wash equipment.
An Air Of NecessityMost code-calculated air quantity formulas are designed to exhaust enough air, no matter what equipment is located below the hood. Codes tend to use prescriptive formulas for ease of use and enforcement. Using this design approach often over-exhausts the kitchen. This results in excess system capacity requirements and energy usage.
While the model codes use prescriptive formulas, they have also taken a step in the right direction by categorizing the Type I and Type II hoods. However, it is the cooking equipment producing the contaminant that must be exhausted. A hot cooking surface creates thermal air currents which carry the steam, heat, and smoke up into the hood to be captured. The filters remove the particle contaminants before the exhaust air is ducted to the fan for release into the atmosphere.
Hood manufacturers, the Gas Research Institute (GRI), and ASHRAE have tested and are developing standards for heat release volumes for various pieces of cooking equipment located under hoods. The exhaust air quantity is relative to the cooking surface temperature. The exhaust air is to keep the filter surface temperature below 200°F, and it attempts to cool the vapors so they can be separated at the filter bank. The heat release volumes have also been translated into cubic feet per minute (cfm) per linear ft of hood (Table 1). Most code authorities will accept an "engineered" hood volume design if the engineering data used is placed on plans and the hood is "UL Listed."
Table 1 is only a general guide. Refer to hood manufacturers for their specific design criteria which, as mentioned, should be shown on the plans for each specific project.
What's In A Hood?The exhaust effluent captured by the kitchen hood is comprised of grease, water gases, and aromatic hydrocarbons.
The main purpose of the hood filter is to prevent flame penetration into the exhaust ductwork. A secondary feature of the filter is to condense, collect, and drain away the particles and vapor that can be collected at the filter unit. The exhaust airflow quantity is also relative to keeping the filter bank surface temperature below 200°F. This temperature fosters condensing at the filters and prevents grease from baking onto the surfaces. The smoke particles and other gaseous emissions will be exhausted out the stack.
The following is a list of the various technologies available to control the stack effluent discharge:
- Electrostatic precipitators;
- Water cooling/cleaning units;
- Bag filters;
- Carbon filters;
- Oxidizing beds;
- Incineration; or
- Catalytic conversion.
However, all of these devices are expensive to install, expensive to maintain, and often fail because of contamination beyond their capacity.
A simple approach is to select the proper cooling exhaust flow, select a high-velocity extractor filter in the hood, and design a high-velocity vertical discharge stack.
The concept of a high-velocity vertical stack (2,050 to 3,000 fpm) is to discharge and dilute the stack effluent and avoid recirculation of the contaminants either into nearby intakes or occupied areas. The design parameters for this stack are the same as for a laboratory exhaust fan stack.
Food Preparation OdorThe most commonly used ventilation rate to control food preparation odors is 12 to 15 air changes/hr. Compare the hood exhaust volumes with the kitchen ventilation rate to verify that at least 12 air changes/hr have been provided for the kitchen area.
The exhaust discharge from a Type I grease hood is required to be at least 40 in. above the roof line, with the outlet at least 10 ft from the supply air intake. The fan most often selected to meet this criterion is an "upblast power roof ventilator" (PRV). This type of fan has a volume capacity limit of about 10,000 cfm and a static pressure limit of 2 to 2.50 in. wg. However, the PRV vendors are raising the fan static limits to 5 in. wg.
A centrifugal fan has greater volume and static pressure capability. A utility set arrangement with a backward inclined type of wheel can be specified. Since this type of fan must be mounted to have the discharge 40 in. above the roof, it requires a mounting stand. A scroll drain and access door should be provided for cleaning. The utility set also adapts well to a high-velocity discharge stack to disperse the smoke particles and odors. The fans used for grease exhaust must have the UL 762 listing.
A hybrid centrifugal type of upblast fan has been developed for kitchen exhaust use by laying the fan horizontal and directing the discharge upward. This type of fan has capacity of up to 14,000 cfm with 5 in. wg.
The static pressure requirements for hood design vary, depending on the hood filter style and cfm/linear ft of hood. Table 2 is only a general guide. Please refer to actual manufacturer's data for final selection.
Makeup AirThe makeup air for the hoods should come primarily from the kitchen area, with enough air drawn from adjacent spaces to maintain a 10% odor-control airflow into the kitchen. The kitchen makeup air can be supplied to the kitchen space (side wall or ceiling) or directly to the hood. The direct hood makeup can be in the hood face. An air balance table should be used on the documents in order to define the intake and exhaust air volumes.
The short circuit or compensating hood was developed to provide the makeup air quantities required by prescriptive codes. Any exhaust air required above the amount needed to capture the thermal-borne contaminants is just for the sake of meeting a prescriptive code-required air volume. The design engineer should avoid this hood concept because it is used only to satisfy a code-required air volume. Instead, offer an engineered system with documentation on the plans or a UL listed hood design.
Ventilation And A/cMost codes are adopting ASHRAE Standard 62-1989 regarding outdoor air requirements for ventilation. This standard set 15 cfm/person for the kitchen area and 20 cfm/person for the dining areas.
A potential rooftop AHU capacity limitation must be closely examined. For example: A 5-ton unit supplies 2,000 cfm, of which 300 cfm (15%) is outside air. If this air is used in the dining area, the unit satisfies 15 occupants at the code-required 20 cfm/person. However, the ventilation and makeup air capacity of standard rooftop units is limited to 10% to 15% outside air in the maximum modes of winter (-20°F) and summer (above 80°F). The ventilation and conditioning capacity of the AHU may fall short of the ventilation standards required for the space.
As for air conditioning, it is not economically sound to provide total air conditioning capacity to satisfy the calculated kitchen cooling load. Reasonable kitchen air conditioning design achieves a balance of spot (workstation) cooling, occupant ventilation, and hood makeup air capacity. Hot cooking-line cooling is a spot cooling application because most of the heat load is a radiant heat effect on the cook.
Care should be taken with the spot cooling design to avoid objectionable drafts on the back of the cook's neck and air patterns to disrupt the thermal plume under the hood. Consideration must be given to where the tempered air hood makeup should be introduced in a manner that does not diminish the air conditioning supplied in the kitchen area.
Economizers, in theory, can reduce the cost of mechanical air conditioning by using 100% outside air when outside temperature and humidity are suitable for indoor use. However, the economizers packaged for rooftop units in the 5- to 15-ton range are crude, high-maintenance items. In order to keep operation of the kitchen ventilation system in balance and easy to operate, forego any potential saving in air conditioning costs by using two-position outdoor air dampers. The dampers open when the unit starts and close when the unit is off.
The tempered makeup air unit for the kitchen should have an adjustable discharge-air thermostat which can be reset to the lowest acceptable temperature for the kitchen staff. Tempered makeup air should be heated to at least 50°F in the colder climates.
Fire ProtectionAll hoods require fire protection. The various types of protection systems are as follows:
- Wet chemical;
- Wet mist.
Wet chemical is the system of choice for most projects. It is the best system to use in a non-sprinkled building. If the building is sprinkled, consider the wet mist cycle which can be connected to the sprinkler system. However, the market tends to provide the wet chemical system for the hoods and the wet sprinkler for the ducts.
The design engineer should negotiate for the entirely wet sprinkler system in a fully sprinklered building because of the dual control and alarm problems when the wet chemical is mixed with the sprinkler system. A wet chemical system requires a manual activation location as well as the automatic release. A sprinkler-type system does not require manual activation.
The designer must coordinate the hood fire cycle with the other type of hood protection selected. Water wash extraction hoods have a fire cycle that is actuated by thermostatic devices. The hood may have fire dampers on the makeup air and exhaust air duct connections. See Figure 1 for examples of supply air fire damper locations that are available and must be defined on the plans. Also, the operation of the dampers and fans must be defined on the plans.
Table 3 designates typical damper and fan operations with the fire protection system selected.
The gas fuel valve (mechanical or electrical solenoid) and/or the electrical power trip for equipment located under the hood must also be coordinated with the operation of the fire protection system. In addition, the fuel and electrical trip systems require reset controls which the designer must specify and locate. The exhaust ducts also require sprinkler heads as noted in NFPA 3-9.3 through 3-9.7. The hood can be specified to come prepiped so that it can accept the selected types of fire protection. If the hoods are field piped, all piping below the filters must be stainless steel or chrome plated, and the piping penetrations to the hood must be grease-tight as per NFPA 96.
SummaryAll of these design issues must be evaluated to have a coordinated kitchen ventilation system. Exhaust air quantities vary considerably when the exhaust air volume is selected using heat release values rather than selecting a hood volume from a prescriptive code formula. Most code authorities will recognize the lower hood exhaust volumes if the equipment below the hood is identified and the appropriate engineering data is placed on the plans.
The makeup air system should be designed to complement the hood selection. This may require makeup to the hood or to the kitchen space. Dampers are required for air supplies to hoods and the control of these dampers and exhaust dampers must be coordinated with the fire protection system selected. Fan selection is a function of the air volume and the system static pressure required. Therefore, the fan selection is directly related to the hood selection.
A washdown hood often has a fire-cycle control as part of the control panel. The plans should show how this control works in conjunction with the hood fans and duct fire protection system selected. A good design rule to follow is that if the designer can explain how the total system will operate, the system will more than likely operate correctly. However, if the designer cannot explain the system, the system has little chance of operating correctly. Once again, all the component interrelations of the kitchen ventilation system must be considered and defined on the plans. ES