Each year, grease fires result in over $100 million dollars in direct property damage within U.S. commercial kitchens. ¹ The National Restaurant Association forecasts that restaurant-industry sales for the estimated 878,000 restaurant locations across the U.S. will reach a record $440.1 billion in 2004, equivalent to 4 percent of U.S. gross domestic product. ² That's big business. In addition to fire, kitchen grease-induced damage primarily includes roof damage and the annoyance of greasy film on neighboring property. The properly designed kitchen ventilation system provides safety, comfort, effluent control, and cleanliness and does so in a manner that is as energy efficient as practical.

This article will examine the ins and outs of grease removal in kitchen exhaust systems. Grease control in commercial kitchen exhaust has seen some significant changes in recent years. The changes are a primarily a result of research, code revisions, and improvements in grease filtration and extraction technology.

THE GREASE REMOVAL SYSTEM
The basic commercial kitchen grease exhaust system is composed of three components: the Type I hood (with integral grease filtration), ductwork, and the fan. A control interlock exists in this system that assures sufficient make-up air (roughly 90 percent of exhaust volume) when the exhaust system is in operation.

THE HOOD
At the heart of the kitchen exhaust system is the Type I hood, equipped with grease filtration and extraction device(s). Section 507.11 of the 2000 International Mechanical Code (IMC) specifies minimum distances between the lowest edge of a grease filter and the cooking or heating surface for Type I hoods. The hood exists to capture the plume of heated air rising from the hot cooking surface. Most of the effluent (gaseous, liquid, and solid contaminants) released from the food and the heat source is entrained in this plume. Capture and containment of the plume is affected by draft. Therefore, placing 4-way diffusers near the hood is to be avoided. As a general rule, make-up air velocity near the hood should be less than 75 fpm. Effluent consists of grease particles, grease vapor, heat, odor, moisture, and VOCs. Hoods must be Underwriters Laboratory (UL) tested and listed (UL 710, Standard for Safety for Exhaust Hoods for Commercial Cooking Equipment) and grease filters must also be separately tested and listed (UL 1046, Grease Filters for Exhaust Ducts).

FILTERS
Historically, grease filtration and extraction devices have been, in essence, fire-protection devices. Three main types of grease filters and extractors in use today include the baffle filter, water wash filter, and dry-cartridge (or removable) filter. Removing submicron grease particles and grease vapor, which make up a substantial portion of grease effluent, has historically been a shortcoming of these filters. New baffle filters have recently been developed that have larger surface areas than their predecessors. These serve to condense vapor and moisture and increase centrifugal separation to significantly improve efficiency. Filters also now exist which use a filter media to intercept grease particles. Ultra-violet (UV) treatment is also being incorporated into new hood design. UV light breaks down grease molecules into smaller harmless compounds of carbon dioxide and water vapor, which are carried out with the exhaust airflow. The UV treatment process does create ozone, which, depending on whom you speak to, may or may not be a problem. Multi-stage filters are now on the market that employ a combination of the above advances and afford high efficiencies. These new, improved filtration systems increase fire safety, reduce odors at the fan discharge, and promote environmental values while decreasing operating costs due to the reduction of grease deposit in the ductwork and fan system in addition to reduced maintenance/cleaning cost.

Several other technologies are available and can be applied to additionally treat the exhaust stack effluent. These particulate removal units are typically located downstream of the primary filtration devices located in the exhaust hood. They include:

• Electrostatic precipitator ( ionization).
• Water cooling/cleaning units.
• Disposable pleated or bag filters.
• Activated carbon filters.
• Oxidizing pellet beds.
• Incineration.
• Catalytic conversion.

These technologies are not specifically addressed or approved in or by the codes, though they are receiving individual approval by local jurisdictions on a case-by-case basis. ³ The 2003 ASHRAE Handbook, HVAC Applications, provides a good summary of each particulate removal unit along with some qualifications and concerns about its use. The primary benefits of this additional level of grease exhaust treatment is discharge mitigation to the surrounding environment and reduction of system energy use, primarily through application of heat recovery devices. However, these devices are costly to install and maintain.

DUCTWORK
The grease exhaust ductwork needs to be designed and constructed in accordance with building codes and National Fire Prevention Association Standard 96, Standards for Ventilation Control and Fire Protection of Commercial Cooking Operations (NFPA 96). The ductwork should be constructed of at least 16-gage steel or 18-gage stainless steel. Duct systems should be constructed to provide adequate access for cleaning and be grease (and liquid) tight via external welds or brazes. If at all possible, grease exhaust ducts should be routed vertically to the roof with minimal offsets and turns to minimize pressure losses. Sidewall grease duct venting is possible in some instances but local codes and code official interpretations should be confirmed prior to construction. Grease hood/duct static pressure, including grease filter loading, is typically between 1.5 to 2.0 w.g., but will vary based on velocity. Dampers are disallowed in the exhaust system per NFPA 96. UL-listed prefabricated duct systems are also available and in use,per UL 1978, Grease Ducts. They provide the benefit of reduced clearance to combustibles, with the normal clearance being 18 in. Ductwork should pitch back to the hood and to a grease collection container. Foilencapsulated, duct-wrap, fiber insulation is also available and offers the benefits of zero clearance to combustibles, eliminating the need for a rated enclosure around the ductwork.