In the last two years, the major changes to commercial kitchen exhaust system designs are the result of the 2000 International Mechanical Code (IMC), the 2001 revised National Fire Prevention Association (NFPA) 96, Underwriters Laboratories (UL) 710 Hood Listings, local pollution regulations, current duct cleaning methods, improved filtration devices, and the introduction of variable-volume control systems.

This article addresses the interrelationships of these system design changes. The areas addressed are hood exhaust requirements; filter devices; replacement air systems; variable airflow systems; duct cleaning; access, pitch and drain; stack outlets; and the effect on the external environment.

Hood Exhaust Requirements

Commercial kitchen hoods capture and contain heat, moisture, smoke, and vaporized grease. Type I hoods are installed over appliances that produce grease vapors and/or smoke, while Type II hoods are installed over heat- or moisture-producing appliances. This article will focus on the Type I hoods.

Sections 506 and 507 of the 2000 IMC address commercial kitchen hoods, grease ducts, and exhaust equipment. Hood exhaust capacity can be calculated using the formulas in Section 507.13.1, which are based on the number of exposed sides of a kitchen hood and the cooking temperature. However, a UL 710-listed hood is recommended because this method reduces the cfm requirement and the accompanying replacement air required:

IMC formula method
Hood type - Three-side open canopy
Q = 75A; A = horizontal surface area of hood, sq ft (Note: 75 = 75 cfm/sq ft)
Q = 75 x 3.5 ft x 10 ft = 2,625 cfm
Assumed: UL 710-listed unit at 250 cfm/linear ft x 10 ft = 2,500 cfm

The United States has a procedure that tests hoods for capture and containment. The test variables are based on temperature and the required cfm per linear foot of hood to capture the effluent; the test is in accordance with UL 710 standards. The listing is published both on a hood label and a listing card that describes the cfm and coating surface temperature relationship (Figure 1).

Filter Devices

The main purpose of the filter in a hood is to prevent flames from entering the exhaust duct. Also, the quantity of exhaust air also keeps a filter's surface temperature near 200 degrees F. The filter aspect of the device is to facilitate condensing of the moisture and grease vapors so it can be captured by centrifugal separation as the material passes through baffles, turns, cooling surfaces, etc.

Aluminum mesh filters are no longer allowed in Type I hoods. The simplest filter is a baffle configuration, though a greater surface area is possible with a high-velocity cartridge filter. Water mist-type hoods also cool and condense grease and moisture vapors at the filter area.

The grease filter industry is currently developing new types of baffle filters with larger surface areas that will condense vapors and moisture at the filter rather than at a cooler location in the duct.

As system designers, we should be aware that as filter types change from baffle to cartridge to an extended area, the fan static pressure requirement increases. Chapter 30 in the 1999 ASHRAE Handbook - HVAC Applications has a table that shows the typical static pressure relationship between filter types and airflow.

Replacements Air Systems

To control unwanted cooking odors in the dining area, approximately 10% of the replacement air in a commercial kitchen ventilation system should come from areas adjacent to the kitchen.

Replacement air is provided by a system that brings the air into the structure near the exhaust area. It should be filtered, heated to a minimum of 50 dgrees in cold climates, distributed to avoid drafts at the hood, and, in some cases, air conditioned or cooled. The short-circuit hood was developed to try to introduce the code formula required air directly into the hood canopy to avoid expensive conditioning. However, the use of the short circuit is fading with the use of UL 710 lower cfm listings.

Consider avoiding the creation of drafts at the hood which disturb the capture and containment of a thermal rising plume. Also, avoid a drafty four-way diffuser near the hood; hood suppliers have front-face grilles and perforated faceplates to supply air.

Today, air conditioned kitchens are becoming more and more popular to attract and keep a cooking staff. However, with the advent of kitchen air conditioning, hood suppliers have found that front-face distribution methods do not provide cook line comfort; therefore, they are providing low-velocity mass down flow diffusion units and adjustable-flow units. Rear supply plenums also can be used for untempered air in mild climates (Figure 2).

Variable Airflow Systems

As many commercial kitchen owners know, exhaust air and replacement air represent a flow of costs up the stack. These equipment and energy costs were the driving force behind the variable airflow systems study which asked: When there is no smoke, reduced heat, or reduced moisture, could the airflow system flow be reduced?

ASHRAE sponsored a research project at the University of Minnesota that resulted in NFPA 96 reducing the minimum exhaust velocity in a Type 1 grease duct from 1,500 fpm to 500 fpm. This change has opened the door for systems that reduce airflow. Reduced airflow is controlled by optical smoke and temperature sensors in the hood area and is also used for remodeling projects that have an existing Type I grease duct installed and previously designed with a higher cfm:

Old stack: 4,000 cfm at 2,000 fpm, 18-in. by 18-in.
New UL 710-listed hood: 1,350 cfm in 18-in. by 18-in.
Existing stack at 650 fpm

Efficient system design dictates that duct sizing velocity should be sized near the 2,000-fpm value. Hood collars are generally sized by the manufacturer at 1,800 fpm.

Duct Cleaning

Most duct cleaning is done by power washing the duct and hood interior. Since water is used, liquid-tight ductwork is important. Type I ductwork is black steel welded seam construction. Job site history has shown that "welded" duct is not liquid tight the first time. If the duct leaks and is wrapped and/or enclosed in a gypsum enclosure, mold will form within the wrap enclosure assembly. Obviously, mold is bad, therefore, it is important to pressure test to demonstrate liquid tightness. To ensure liquid tightness, pressure test the duct to 0.10 inches of water.

Access, Pitch, and Drain

There are several elements to consider for a kitchen's duct cleaning access, pitch, and drain. First, provide access at the roof outlets; hinged fans at the curb work well. It is important to secure additional support for aluminum fan assemblies because the tipping will stress the fan inlet/base connection. Commercial preinsulated round stacks are a good choice for multistory buildings. The preinsulated duct is similar to one-stop shopping for the liner, the insulation, the leak-tight cement, and the access.

Next, code-required access panels in horizontal and vertical ducts are required to allow for cleaning material that power washing will not remove. Consider enclosing the access door insulating wrap in a double-wall enclosure to prevent damage every time the access is used.

Third, include a low-point drain access for water used in power washing. However, pitch the horizontal duct run to the hood collar. NFPA 96 requires that horizontal duct supports must support the weight of the duct plus 800 pounds. The 800 pounds extra is for duct cleaning equipment and water.

Finally, on a tall stack project, consider providing a dry, three-quarter-inch water riser from the bottom to the roof for water access; also provide ball valves where needed for riser cleaning. And, run this water line parallel to the duct sprinkler riser. Provide valved drain access at the duct low points so that the grease/water mixture can be hosed to a suitable drain location.

Stack Outlet

Most upblast aluminum power roof ventilations discharge at about 1,000 fpm, or 11 mph. A cross wind could carry the effluent horizontal for a long distance, which often equates into recirculation of exhaust effluent into intake louvers at the same level. A solution is the high-velocity upblast stack used in laboratory design (Figure 3).

Effects on the External Environment

Many communities and neighbors are concerned about kitchen stack smoke and odors. Local pollution laws are often written so if a neighbor complains about smoke and odors, the polluter must either fix it or stop altogether. To stop pollution concerns before they start, the industry provides several control devices including electrostatic, water mist, pleated media, activated carbon, oxidizing bed, and ultraviolet. However, each of these units is costly and requires expensive, ongoing maintenance.

A simple solution to pollution is dilution. To do this, bleed outdoor air into the roof fan installation at 1:5 ratio (e.g. 5,000 cfm hood) and add 1,000 cfm of outdoor roof level air and size for 6,000 cfm. For smoke concerns, another solution is simply to reduce the fat in the cooking product.

Another point to remember is that replacement air cools the filters and causes moisture and vapor to condense at the filter face rather than in the stack or on the roof. Any time excess grease gets on the fan or on the roof, the cause is often low cfm.


The IMC is new and recognizes the variable exhaust flow requirements relative to cooking temperatures. NFPA 96 (2001) is a current standard written in code language. The UL 710 listing is allowed by the IMC and does reduce airflow requirements. Duct cleaning is mainly by power washing and liquid tightness in a design mold avoidance concern in wrapped and enclosed ducts. Pollution and odor recirculation concerns can be economically resolved by high-velocity stack outlets and dilution design.ES

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