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 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.

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.