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
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
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
Establish location and "duty"
classification of appliances including menu effects. Determine preferred appliance
layout for optimum exhaust ventilation.
Select hood type, style and features.
Size exhaust airflow rate.
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
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.