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Fundamentals of Venting

Why Vent?
One cubic foot of natural gas burned with just enough air for complete combustion will produce 11 cubic feet of combustion products-1 cubic foot of carbon dioxide, 2 cubic feet of water vapor, and 8 cubic feet of nitrogen. Add to that grease, smoke, heat, and water vapor from cooking and warewashing, and a food service kitchen can quickly become a potential fire and health hazard without proper ventilation.

The two basic types of venting are atmospheric (or gravity) and power venting. Atmospheric venting uses a flue and the tendency of hot gases to rise to exhaust combustion products outdoors. Atmospheric venting works well for many home appliances, but for the strenuous venting demands of most commercial food service kitchens some type of power venting, using a fan or blower to capture and exhaust the unwanted byproducts of cooking, is required. In commercial food service kitchens, this is accomplished with one or more exhaust hoods.

The basic types of exhaust hoods are Type II which is used in situations where only heat and moisture are produced and Type I for cooking processes that produce smoke and grease. Type I hoods require liquid-tight construction and a built-in fire suppression system.

Direct Vent Systems
Some appliances use a direct vent system. Direct vent appliances have a specially designed vent and air intake system. The system is designed as a part of the appliance by the manufacturer and supplemental vents are not used. No vent sizing is needed with direct vent systems.

The vent terminal clearances to building openings, such as doors and windows, vary by Btu/h input. The clearance is required to avoid flue products from being drawn into the building. Direct vent appliances 10,000 Btu/h or less must have a minimum 6-inch clearance, appliances between 10,001 to 50,000 Btu/h must have a minimum 9-inch clearance, and appliances over 50,000 Btu/h must have a minimum of 12-inches clearance. The bottom of the terminal and air intake must be at least 12 inches above grade. Local codes may impose additional requirements and should always be checked before installation of the appliance.

Factors Affecting Venting System Design and Operation
When designing a venting system, engineers take a number of factors into account before specifying a particular hood style, size, location and exhaust rate. Some of the most important considerations are:

Established guidelines - For venting purposes, food service equipment is categorized as light, medium, heavy and extra heavy-duty. As a general rule, open flame, non-thermostatically controlled appliances such as charbroilers demand more venting than closed, thermostatically controlled appliances such as ovens.

Processes - Equipment usage that creates smoke and grease during operation such as frying, griddling and charbroiling requires different venting consideration than equipment that produces only heat and moisture such as dishwashers.

Type of food to be cooked - Higher fat-content foods such as hamburgers produce more grease and smoke than low-fat foods such as chicken.

Hood style and location - Exhaust hoods come in a variety of styles and recommended installation guidelines. The style and location of the hood with respect to the equipment to be vented helps determine the required exhaust rate.

Exhaust hoods come in two basic types: Canopy hoods are designed so the inside edge overhangs or extends a horizontal distance not less than six to eight inches beyond the outer edges of the cooking surface. In contrast, non-canopy hoods do not extend completely over the cooking equipment. A non-canopy hood is designed to be as close as possible to the cooking surface, usually 18 to 24 inches above it.

Some common hood styles are:

Typical Minimum Exhaust Flow Rates

The Food Service Technology Center suggests the following steps in the design of a commercial kitchen ventilation system:

  1. Establish location and "duty" classification of appliances including menu effects. Determine preferred appliance layout for optimum exhaust ventilation.
  2. Select hood type, style and features.
  3. Size exhaust airflow rate.
  4. Select makeup air strategy, size airflow and layout diffusers.

Air for Combustion and Ventilation
Air that is removed from the kitchen through an exhaust hood must be replaced with an equal amount of outside air, a process known as makeup air. Proper venting action cannot be maintained unless there is ample air at the exhaust hood. The exhaust fan may not pull out all the undesirable products of combustion and cooking and will create a "suction" effect that makes it difficult to open doors.
Some replacement air can be provided by the HVAC system and may be adequate in some applications. However, for larger institutional food service facilities and full-service restaurants, a dedicated makeup air system may be a more practical, energy efficient and cost effective option.

An improperly designed makeup air system can negatively impact the effective operation of the exhaust hood. Design considerations include speed of the air introduced by the makeup air equipment, cross drafts from delivery and service doors and drive-through windows, and the location of the makeup air units with respect to the exhaust hoods. If not properly addressed in the design of the exhaust hood, exhaust rate, and makeup air equipment, any of these factors can result in incomplete removal of combustion and other undesirable products.

Another consideration is the temperature of the incoming makeup air. Unless pre-conditioned by the makeup air unit, air that is too cold or too hot can not only adversely affect the operation of the exhaust hood, but can also create an uncomfortable environment for kitchen workers.