Commercial kitchen ventilation design: what you need to know

Engineered Systems, Feb, 2003 by John Clark

At least three major industry-related groups have made changes in codes or expectations for exhaust systems in the last two years. Stir in the need to stay on top of improved equipment options, local codes, and a variety of usual design issues, and designing today's commercial kitchen is no piece of cake. Read on to master this recipe of old and new concerns.

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.

REPLACEMENT 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[degrees] 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).