Hood Systems Basics
The science of commercial kitchen ventilation includes both exhausting air as well as providing replacement air within the cooking area. Whether a restaurant is a small free-standing site or a large institutional kitchen, managing and balancing airflow is a complex issue. It is a challenge to properly ventilate commercial kitchens, as they require moving large volumes of air through ductwork and equipment placement in very restricted spaces.
Overall design, construction, installation coordination, and maintenance are required to get optimum performance and an energy-efficient air balance from the system.
A Typical System
A typical kitchen ventilation system includes an exhaust hood or canopy, ductwork, fan system, and a means of providing adequate make-up air. The entire system must constitute a fire-safe assembly within the building.
Exhaust hoods and canopies capture heat and contaminates in the air by means of filters, extraction baffles (cartridges), and water mist systems. There are many style variations of hoods with canopy styles—a large box with and open bottom—being the most common. Styles selection is based on the type of oven and the expected contaminates to be removed. While there are several styles of hoods, all fall within two major categories:
Type I hoods carry a listing label and are manufactured and installed according to the manufacturer's and listing agencies' requirements. They are designed to handle grease and include a number of integrated components within the hood.
Type II hoods are used in the collection of steam, vapor, heat, and odors—but not grease. The two sub-classifications of Type II hoods are condensate and heat/fume.
Exhaust ductwork provides the means to transfer contaminated air, cooking heat, and grease vapors from the hood to the fan.
The duct is often run inside a shaft enclosure and that enclosure is typically constructed of gypsum board, plaster, concrete, or ceramic tiles and must be an approved continuous fire-rated enclosure.
Exhaust fans move the heat and contaminated air out of the building. All exhaust fan components must be accessible or have removable access panels for cleaning and inspection and must be designed to contain and drain any excess grease.
There are three major types of exhaust fans:
Up-blast fans are typically aluminum centrifugals that are designed for roof mounting directly on top of the exhaust stack.
Utility fans are normally roof-mounted with the inlet and outlet 90 degrees from each other and are typically used where high-static pressure losses exist.
Inline fans are typically located in the interior duct and are used where exterior fan mounting is impractical.
In order for the exhaust system to work properly, make-up air is required to replace air equal to the amount removed. Make-up air can be provided via an independent system or in combination with the building's HVAC system.
Kitchen Ventilation Concepts
To better understand why a kitchen ventilation system needs to be designed and constructed in a very specific manner, the principles behind air movement must be understood. Buildings are required to adhere to indoor air quality regulations and, depending upon the jurisdiction, sometimes exhaust air quality regulations. The food service industry must meet higher air quality regulations than standard building exhausts due to the type of contaminated air produced by cooking food.
Relevant Codes and Standards
Local codes are generally adopted from either the International Mechanical Code (IMC) or the National Fire Protection Association Standard # 96 Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations (NFPA #96) or a combination of the two with particular local issues of concern included. In many major cities, local codes are written and published by local building officials; however, such codes are frequently based on one of the national codes previously mentioned.