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Commercial Kitchen Exhaust - Overview

By Tim Beatty | Territory Manager | Air Quality Engineering, Inc.

Restaurants and other commercial kitchens exhaust the grease, smoke and other harmful contaminants that they produce through a source capture system that in most instances consists of: A hood over the kitchen appliances, a run of grease duct, and an exhaust fan (blower). For a variety of reasons (code, ordinance, nuisance) there may be a desire to filter this exhaust before it is allowed to contaminate the outdoor air. There are in-duct solutions that can be installed in between the hood and the exhaust fan that will filter this exhaust and remove the contaminant from the air stream before it is exhausted. Air Quality Engineering offers such solutions.

Summary of the Individual parts of the Commercial Kitchen Exhaust Source Capture System

1. Kitchen Exhaust Hoods

The hood is the first line of capture in the Kitchen Exhaust System. Hoods are located directly over the kitchen appliances that they are servicing and are designed to funnel contaminated air into the hood where it can then be transported through the exhaust system. Standard exhaust hoods are made from stainless steel with a thickness of between 16 and 18 gauge. This steel is welded seamlessly together in order to prevent harmful gases from escaping.

Type I hoods are installed over appliances that produce smoke and /or grease. Type II hoods are installed over appliances that produce only heat and/or moisture. For our purposes we will focus only on Type I hoods, Type II hoods do not have contaminant that needs to be filtered. Filters are utilized within Type I hoods (hood filters) to capture a portion of the grease and other contaminants. Efficiencies vary among these filters but they are typically only marginally efficient and intended to trap large particulate.

Basic Varieties of Type I Kitchen Exhaust Hoods – There are many sub types in each of these categories but below are the two main hood types for Kitchen Exhaust.

  • Wall Canopy Hoods – This is the most common Type I hood. This hood is wall mounted on one side directly over the appliances and it provides the maximum efficiency through lower exhaust rates (velocities).
  • Island Exhaust Hoods – The island style Type I hood meets the challenges of open floor plan cooking applications. These are commonly utilized where there is tableside cooking (hibachi type) applications. Higher exhaust rates (velocities) are associated with this type of exhaust hood.

The cubic feet per minute (CFM) of air traveling through the hood is a very large factor in determining the size of the air cleaner, odor module and exhaust fan that will be located downstream. It is very important that the CFM be accurately calculated. Estimating and/or “ball-parking” this figure can have disastrous results, including the potential for complete failure of the exhaust system.

2. Kitchen Exhaust Grease Duct

A grease duct is a duct that is connected to the Kitchen Exhaust hood and is specifically designed to vent grease, smoke and other harmful contaminants from commercial cooking equipment to the outside of a building. Grease ducts are regulated both in terms of their construction and maintenance, forming part of the building’s passive fire protection system. Even the cleaning schedule is typically dictated by the local fire codes and evidence of compliance must be kept on file by the owner.

Grease laden vapors are hot to begin with. As the vapors cool down, the grease settles on colder items. It is thus important for occupational safety and health as well as compliance with local fire codes to vent such vapors outside the kitchen and outside the building where the kitchen is located. Grease, of course, is not only slippery, but also highly flammable. In fact, it qualifies as a hydrocarbon due to its inherent chemistry. Regardless of what state it is in, vapor, liquid or solid, it ignites easily and burns very rapidly, necessitating special provisions to accomplish a fire-resistance rating based on an internal grease fire as well as an external fire. Special provisions also include the necessity for proof that any adjacent firestop must be compatible with the grease duct system.

In North America, grease ducts must be in compliance with National Fire Protection Association (NFPA) 96 (NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations) as well as the local building codes and fire codes. Cleaning takes place typically every 3 months, 6 months, or annually, depending on the nature of the appliances below the hood. For instance, woks require quarterly grease duct cleaning, whereas normal stoves may necessitate the grease duct to be cleaned only every 6 months. Compliance must be proven through certificates issued by the cleaning and maintenance contractors. Purpose-designed fire suppression systems inside the hoods must also be routinely maintained. Proper cleaning must be enabled through the use of approved, fire-resistant access panels. Grease ducts should be kept as short as possible to minimize grease build-up.

A proprietary duct system that has its own inherent fire-resistance rating can be used, or a metallic duct, either field fabricated or UL certified factory-built designs. Field fabricated is typically made from 16 gauge carbon steel, all welded, per local codes, which is then externally treated with fireproofing. Factory-built designs are UL tested to the UL 1978 test standard and are made from lighter gauge stainless steel and offered in single wall and multiple double wall insulated designs. Typical materials used for fireproofing field fabricated designs are:

  • ceramic fibers
  • rockwool
  • calcium silicate
  • vermiculite boards pressed and bonded with sodium silicate
  • intumescents
  • endothermic materials, sometimes on their own, sometimes in combination with ceramic fiber

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