Sheet Metal Journal, the insight - the information

A kitchen exhaust system is more than a hood in the kitchen area to capture, contain, and remove vapor, smoke and grease from cooking operations. Baffle filters are also part of the package. The most important component is the exhaust fan and how it integrates with the replacement air.

A hood canopy is used to contain the rising thermal plume produced by hot cooking processes. The hood includes a filter bank to remove grease and to prevent flame penetration into the ductwork. Air movement transports the heat and contaminants through the filters, up the ductwork and out of the building. A fan provides the air movement.

The design airflow for a listed hood is based on the minimum amount of air required to capture and contain the thermal plume. The codes relate this to cfm per linear foot of hood based on cooking surface temperatures.

The four categories are as follows:
• Light duty such as ovens and steamers up to 400 ˚F
• Medium duty such as large kettles and ranges up to 400 ˚F
• Heavy duty such as upright broilers and woks up to 600 ˚F
• Extra heavy duty such as solid fuel equipment up to 700 ˚F

The design exhaust rate is the desired airflow in cfm/ft times the length of the hood as indicated in Table 1 reproduced from 2003 ASHRAE Handbook-Applications.

Most systems place the exhaust fan at or near the outlet of the duct system. Placing the fan at the outlet end keeps the duct under negative pressure. A roof-mounted fan is often used for ease of service and cleaning. The purposes of the fan other than to move air are also to keep contaminants off the roof surface and discharge the grease-laden air away from intakes.

The up-blast power roof ventilator (PRV) is the fan most often selected for this duty.This type of fan is compact and with the proper roof curb discharges the exhaust at least 40 inches above the roof surface to satisfy requirement in Section 4-8.2.1 of NFPA 96 designed to keep grease off the roof surface. PRV fans should bear the UL 762 rating for high temperature. Consult with your local fan supplier to make sure that you are getting a fan that can produce the volume and pressure that you require for your application. If the up-blast PRV is not appropriate other types such as utility sets and horizontal centrifugal fans may be capable of higher static pressures. See Figure 1 for typical exhaust outlets.

Replacement air for the air exhausted should come mainly from the kitchen area. The strategy used to introduce replacement (makeup) air can significantly impact hood performance and should be a key factor in the design of kitchen ventilation systems.

To improve performance and energy efficiency consider some of the following:
• Group appliances according to effluent production.
• Select UL-Listed hoods appropriate for the cooking equipment.
• Use direct fired MUA heating if heating is necessary.
• Have fan supplier select the proper fan for cfm and static pressure.
• Require building air balancing and system commissioning as part of contract.

Lack of attention to one or more of the components listed previously is likely to cause failed or substandard kitchen exhaust systems. A good reference tool for quickly estimating energy use for different kitchen designs is the Outdoor Air Calculator available at www.archenergy.com/ckv/oac.