ASHRAE Journal - July 2009 - 21

Light Duty Medium Duty Heavy Duty Extra Heavy Duty Low Temperature Medium Temperature High Temperature Wood Burner Under 450°F 450°F to 600°F More Than 600°F More Than 700°F 150 cfm/linear foot 250 cfm/linear foot 300 cfm/linear foot 500 cfm/linear foot Table 1: Exhaust flow rates based on cooking duty and temperature. convection ovens, the front overhang should be 6 in. (152 mm) past the open door at a 90 degree angle from the oven. This often translates into an 18 in. (457 mm) overhang. Wiring and gas piping can be revised to provide space in the rear of the hood for the equipment to be pushed back. Hood side panels are also an aid in improving hood side overhang and side draft disturbance problems. These side panels are available from all hood manufacturers as a retrofit item. Evaluating Current Conditions requirement for heat tempered replacement air have made the short-circuit hoods obsolete. Cooking Line Thermal Comfort To solve a problem, you must know the current conditions. Start with the cooking equipment. Obtain the kitchen equipment supplier’s recommended cooking surface temperature to determine if the equipment is considered light (450°F [232°C]), medium (600°F [316°C]), heavy (700°F [371°C]), or extra heavy duty (more than 700°F [371°C] or solid fuel). Next, measure the hood’s airflow. This can be done by measuring the airflow velocity through the filter bank and using the filter’s appropriate correction factor to arrive at the exhaust rate in cfm. Another method is to measure the discharge fan velocity and the size of the discharge outlet to arrive at a cfm value. Once the exhaust cfm value has been determined, this number should be compared to current industry values. These exhaust values are based on capture and containment of the heat and effluent rising from a hot surface. The UL 710, Standard for Safety for Exhaust Hoods for Commercial Cooking Equipment requirements rating system became the airflow minimums. The hood manufacturers use the UL 710 test as the beginning basis for their hood exhaust rates, and we should use them as a minimum checkpoint. The exhaust rate is based on cfm per linear foot of hood front face, cooking temperature and cooking duty. These rates are for wall-mounted canopy hoods. Other minimum hood values are listed in the 2007 ASHRAE Handbook—HVAC Applications, Chapter 31, Table 3, as well as applicable local mechanical codes (Table 1). Hood Front Draft Turbulence The warmest workstation in a kitchen is in front of the cooking equipment. In recent years, a heating/cooling supply terminal has been located near the hood front face to improve working conditions for cooks. Also in these same design times, replacement air outlets were also located near the cooking line. These designs were the downblow slots in the front hood lip and the front face grilles/face perforations. These designs were an attempt to supply air near the cooks and not create hood face drafts that would disturb the capture and containment. Slot ceiling diffusers also were used to direct the air near the hood face, but still direct the air away from the hood face. The best solution is to revise the location and type of supply air outlet. Close off all the existing hood supply downblow grille or front face outlets, and use a low velocity laminar flow outlet (Photo 1). Hood manufacturers have created a perforated perimeter downblow supply (laminar) unit that can be installed in front of the hood. The discharge velocity is about 100 fpm (0.5 m/s) and reaches 50 fpm (0.25 m/s) at the hood lip. The conditioned air can cool the cook and not create turbulence at the hood lip. Grease in the Exhaust Duct or on the Roof The location of supply air diffusers or grilles within the general vicinity of the hood front face often creates turbulence at the hood front lip, which results in the cooking effluent spilling out from under the hood. The solution is to redirect the supply air away from the hood face with diffuser baffles or by moving the diffusers farther away. The concept of a short-circuit hood was developed to supply untempered replacement air directly to the hood’s interior space. This air was directed toward the filter bank and often caused turbulence that resulted in the effluent spilling out from under the hood. Lower exhaust rates and the July 2009 This problem is the result of the grease not being removed at the hood filter face or secondary removal system. First, check the exhaust cfm relative to the cooking temperature and the hood duty recommendations. The exhaust rate is most often found to be below the recommended cfm/linear foot. When the cfm rate is increased to the recommendations, the filters create turbulence and surface cooling, which results in the grease being captured at the filter face. A simple solution is to check with the supplier for a source that uses less grease in their products. This solution is often used for side wall exhaust systems that show grease dripping down from the wall louver outlet. Basic hood grease filters are a baffle type with which the grease is extracted by centrifugal force through an aluminum or stainless steel baffle path. Another solution is the use of an extractor cartridge, which has a longer baffle path. A newer type of filter is known as an extended surface filter. This unit has a longer baffle path and more cooling surface. However, any change of a filter unit from the basic filter to the type with more surface area will require modification to the exhaust fan because of high static pressure requirements. Avoid adding other expensive technologies, such as water cooling/cleaning units, cartridge filters, and electroASHRAE Journal 21

ASHRAE Journal - July 2009

Table of Contents for the Digital Edition of ASHRAE Journal - July 2009