K18 June 2003 Kitchen Ventilation | A Supplement to ASHRAE Journal By Richard T. Swierczyna, Associate Member ASHRAE, & Paul A. Sobiski, Associate Member ASHRAE A large portion of kitchen ventilation planning is dedicated to properly exhausting cooking effluent. Appliance layout and energy input are evaluated, hoods are located and specified, ductwork size and routing are determined, and exhaust fans are specified to re- move the proper volume of air. Unfortunately, much less time is usu- ally dedicated to planning how the exhausted volume of air will be replaced, although an air balance schedule is commonly used to indi- cate the source of the makeup air (MUA). Overlooking MUA delivery system de- tails can have a negative impact on the performance of an otherwise well-de- signed kitchen. Cross drafts and high air velocities due to improper introduction of MUA can result in failure of the hood to capture and contain effluent from the appliances. This effluent spillage may in- clude convective heat, products of com- bustion (carbon dioxide, water and potentially carbon monoxide), and prod- ucts from the cooking process, such as grease vapor and particles, odors, water vapor, and various hydrocarbon gases. project focused on how the introduction of makeup air affects the capture and containment (C&C) performance of com- mercial food service ventilation equip- ment. The investigation included combinations of hoods, appliances, cooking conditions, MUA strategies and other factors. Three hood types were tested: wall- mounted canopy, island-mounted canopy, and proximity (backshelf). Charbroilers and griddles, representing heavy-duty and medium-duty appliances respectively, were tested during idle and representative cooking conditions. The six MUA strategies included: dis- placement ventilation (base case), ceiling diffuser, front face diffuser, air curtain dif- fuser, backwall supply, and short-circuit supply (Figure 1). Certain features of the hoods and local makeup air devices were modified to represent designs and con- figurations found in commercial kitchen installations, but not necessarily the best or worst designs or configurations. Overall commercial kitchen ventila- tion issues include indoor air quality, fire prevention, safety, employee comfort and equipment first costs, energy operating costs and maintenance costs. This article presents strategies that can minimize the impact that makeup air introduction has on hood performance. To address these MUA issues, a two- year research project was sponsored by a state government energy agency 1 and large utility. Subsequent testing for sev- eral manufacturers augmented this pub- lic research initiative. This research Reprinted by permission from ASHRAE Journal, July 2003. © 2003 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Kitchen Ventilation | A Supplement to ASHRAE Journal June 2003 K19 ‘ Overlooking MUA delivery system details can have a negative impact on the performance of an other- wise well-designed kitchen. ’ To determine which MUA strategy offered the most effec- tive operation while providing full capture and containment (C&C), the research team tested the following hypothesis: If the MUA strategy were to have no effect on exhaust hood performance (i.e., equivalent to the displacement ventilation base-case condition), then it would be pos- sible to replace 100% of the air exhausted through the makeup air configuration being investigated, while main- taining C&C. It was conclusively demonstrated that each of the MUA strat- egies and specific configurations tested compromised the ex- haust hood’s ability to completely capture and contain the thermal plume and/or effluents at higher makeup airflow rates). Temperature of the locally supplied makeup air also was shown to effect hood performance as air density impacts the dynamics of air movement around the hood. Generally, hotter MUA tem- peratures (e.g., greater than 90°F [32°C]) will affect hood perfor- mance more adversely than cooler air (e.g., less than 75°F [24°C]). C KV System Performance Testing The phrase “hood capture and containment” is defined in ASTM F1704-99 Standard Test Method for the Performance of Commercial Kitchen Ventilation Systems 2 as “the ability of the hood to capture and contain grease-laden cooking vapors, convective heat and other products of cooking processes.” Capture and containment performance testing incorporated focusing schlieren and shadowgraph visualization systems to verify capture and containment in accordance with ASTM F1704-99. These technologies are a major breakthrough for visualizing thermal and effluent plumes from cooking pro- cesses. A schlieren system presents a high-contrast image of turbulent patterns due to the different air densities within the thermal plume, similar to the effect we see over hot pavement. With appliances at idle (ready-to-cook) condition, C&C evalu- ation is a relatively simple and repetitive task. A realistic surro- gate was needed to produce consistent effluent during cooking C&C evaluations. Since cooking hamburgers provide peak ef- fluent production for approximately 10 seconds during a six- minute cooking session, cooking with hamburgers was used as a baseline condition for cooking plume simulation. For charbroilers, the natural gas flow was increased to match the previously established cooking plume. The cooking plume simulator for the gas griddle was based on spraying water onto the hot cooking surface, using a pressure regulator and timed relay valve for control, and needle valves for fine-tuning. During baseline displacement ventilation C&C tests, the exhaust flow rate was reduced until spillage of the thermal plume was observed. The exhaust flow rate was then increased in fine increments until full C&C was achieved over the test condition. The airflow rate at this condition is referred to as the threshold exhaust airflow rate for complete C&C. These values provided a baseline case to judge the various MUA strategies. Evaluating the performance degradation due to cross drafts required a repeatable and practical disturbance. For this task, a pedestal-mounted fan was located diagonally from the front corner of the hood. For most of the local MUA configurations investigated, the exhaust airflow rate was set initially to the C&C rate deter- mined in the baseline displacement MUA test. The local MUA was then increased (in a balanced room condition) until the threshold of capture and containment was exceeded (i.e., spill- age observed). This MUA rate was the airflow rate reported relative to the displacement exhaust C&C rate as the maxi- mum percentage of MUA that could be supplied without im- pacting hood performance. An exception to the general procedure for local MUA C&C testing was the ceiling four-way diffuser. Testing was performed with constant 1,000 cfm (472 L/s) airflow and modulating the exhaust system to the threshold C&C condition. In addition to the described protocols, MUA rates were incrementally increased to determine the marginal increase in exhaust airflow rate. This procedure led to an exhaust-to-MUA ratio determination and index of MUA effect. The following discussion presents research results from the viewpoint of optimizing system performance. Displacement Diffusers Displacement ventilation was the baseline for the study be- cause it provided a uniform, nearly laminar bulk airflow. This low-velocity bulk airflow has proven optimal for attaining C&C K20 June 2003 Kitchen Ventilation | A Supplement to ASHRAE Journal with the lowest exhaust rate. Therefore, supply- ing makeup air through displacement diffusers as illustrated at right is an effective strategy for in- troducing replacement air. Unfortunately, dis- placement diffusers re- quire floor or wall space that is usually at a premium in the commercial kitchen. A pos- sible solution may be remote displacement diffusers (built into a corner) to help distrib- ute the introduction of makeup air into the kitchen when transfer air is not viable. Air Curtain Supply Most hood manufacturers recommend limiting the percentage of MUA sup- plied through an air curtain to less than 20% of the hood’s exhaust flow. At such low air velocities, an air cur- tain may enhance C&C de- pending on design details. However, in the cases tested, the air curtain was the worst performing strategy at higher airflows. The negative im- pact of an air curtain is clearly illustrated above by the schlieren flow visualization recorded during a test of a wall-mounted canopy hood operating over two underfired broilers. Introducing MUA through an air curtain is a risky option. An air curtain (by itself or in com- bination with another pathway) is not recom- mended, unless velocities are kept to a minimum and the designer has access to per- formance data on the specified air curtain con- figuration. Typical air curtains are easily adjusted, which could cause cooking effluent to spill into the kitchen by inadvertently creat- ing higher than specified discharge velocities. Short-Circuit Supply (Internal MUA) Internal MUA hoods were developed as a strategy to reduce the amount of conditioned air required by an exhaust system to meet code requirements. This is accomplished by intro- ducing a portion of the untempered makeup air directly into the exhaust hood reservoir. In cold climates, condensation and cooking sur- face cooling become undesirable side effects. The laboratory testing demonstrated that when short circuit hoods are op- erated with excessive inter- nal MUA, they fail to capture and contain the cooking effluent, often spilling at the back of the hood (although front spill- age is observed in the figure at right). If, how- ever, the specified exhaust rate is higher than the threshold for C&C in an exhaust-only con- figuration, the short-circuit airflow rate can be increased accordingly, creating a condi- tion of apparent benefit on a percentage ba- sis. For the short circuit configuration tested, the average MUA rate that could be introduced without causing spillage was 15% of the threshold C&C exhaust rate. Front Face Supply Supplying air through the front face of the hood is a configuration recom- mended by many hood manufacturers. In theory, air exits the front face unit horizontally into the kitchen space. However, a front face discharge with louvers or perforated face can perform poorly, if its design does not consider discharge air velocity and direction. The figure above repre- sents a poorly designed perforated face supply, which negatively affected this hood’s capture performance in the same fashion as an air cur- tain or four-way diffuser. To improve front face performance, internal baffling and/or a double layer of perforated plates may be used to improve the uniformity of airflow. In addition, greater distance be- tween the lower capture edge of the hood and the bottom of the face discharge area may de- crease the tendency of the MUA supply to interfere with hood capture and containment. In general, face discharge velocities should not exceed 150 fpm (0.75 m/s) and should exit the front face in a horizontal direction. Perforated Perimeter Supply Perforated perimeter supply is similar to a front face supply, but the air is directed down- Figure 1: Types of MUA sup- ply integrated with the hood. Displacement diffusers Impact of air curtain Excessive in- ternal MUA Poorly designed perforated front face supply Kitchen Ventilation | A Supplement to ASHRAE Journal June 2003 K21 ward (see figure at right) toward the hood capture area. This may be advantageous un- der some conditions, since the air is directed downward into the hood capture zone. For proper hood performance, discharge velocities should not exceed 150 fpm (0.75 m/s) from any section of the diffuser and the distance to lower edge of the hood should be no less than 18 in. (0.5 m). If the air is not introduced in this manner, the system begins to act like an air curtain. An increase in the plenum discharge area lowers the velocity for a given flow of MUA and reduces the chance of it affecting C&C. If the perforated perimeter supply is extended along the sides of the hood as well as the front, the increased area will permit proportionally more MUA to be supplied. Four-Way Ceiling Diffusers Four-way diffusers located close to kitchen exhaust hoods (see figure at right) can have a detrimental effect on hood performance, particularly when the flow through the diffuser ap- proaches its design limit. Perforated plate ceiling diffusers can be used in the vicinity of the hood, and a greater number of ceiling diffusers reduce air velocities for a given supply rate. To help ensure proper hood performance, air from a diffuser within the vicinity of the hood should not be directed toward the hood. If ceiling supplied air must be directed toward a hood, the air discharge velocity at the diffuser face should be set at a design value such that the terminal velocity does not exceed 50 fpm (0.25 m/s) at the edge of the hood capture area. Backwall Supply The lab testing demonstrated that the backwall supply can be an effec- tive strategy for introducing MUA (see figure at right). For the backwall sup- ply tested with a canopy hood, the av- erage MUA rate that could be introduced without causing spillage was 46% of the threshold C&C exhaust rate. To help ensure proper performance, the discharge of the backwall supply should be at least 12 in. (0.3 m) below the cooking surfaces of the appliances to prevent the relatively high velocity introduction of MUA from interfering with gas burners and pilot lights. Backwall plenums with larger discharge areas may provide increased airflow rates as long as discharge velocities remain below maximum thresh- olds. Ideally, the quantity of air introduced through the backwall supply should be no more than 60% of the hood’s exhaust flow. Other Factors that Influence Hood Performance Hood Style. Wall-mounted canopy hoods function effectively with a lower exhaust flow rate than single-island hoods. Island canopy hoods are more sensitive to MUA supply and cross drafts than wall-mounted canopy hoods. Proximity hoods exhibit lower C&C exhaust rates, and in some cases, perform the same job at one-third of the exhaust rate required by a wall-mounted hood. Cross Drafts. Cross drafts have a detrimental effect on all hood/appliance combinations, and adversely affect island canopy hoods more than wall-mounted canopy hoods. A fan in a kitchen, especially pointing at the cooking area, severely de- grades hood performance and may make capture impossible. Cross drafts required at least a 37% increase in exhaust flow rate and in some cases C&C could not be achieved with a 235% increase in exhaust rate. Cross drafts can result from portable fans, movement in the kitchen, or an unbalanced HVAC system. Side Panels and Overhang. Side (or end) panels permit a reduced exhaust rate in most cases, as they direct the replace- ment airflow to the front of the hood. The installation of side panels improved C&C performance for static conditions an av- erage of 10% to 15% and up to 35% for dynamic (cross-draft) conditions. They are a relatively inexpensive way to achieve C&C performance and reduce the total exhaust rate. Partial side panels are able to provide virtually the same benefit as full panels. One of the greatest benefits of side panels is to mitigate the negative effect of cross drafts. An increase in overhang may increase the ability to contain large volume surges from cook- ing processes that use convection and combination ovens, steam- ers and pressure fryers, although for unlisted hoods this may mean an increase in the code-required exhaust rate. MUA Strategy and C&C Exhaust Rate What was not anticipated during the design of the study was how sensitive the C&C threshold would be to the local intro- duction of MUA. Spill conditions often were observed when as little as 10% of the exhaust rate was supplied by a given MUA strategy. Figure 2 shows a generic trend for changes in exhaust airflow rate as MUA flow rate increases for a given hood/MUA system. In this generic graph, the C&C exhaust flow rate is 3,000 cfm (1400 L/s) with no locally supplied MUA. For local MUA up to 500 cfm (236 L/s), the system did not require an increase in the exhaust rate, as represented by the horizontal part of the curve. When the MUA was increased beyond the 500 cfm (236 L/s), the exhaust rate had to increase to maintain C&C. For this particular hood/MUA system, every 1 cfm (0.47 L/s) increase in MUA required a 0.75 cfm (0.35 L/s) increase in exhaust rate. In the better performing MUA strategies, more local MUA can be introduced without increasing the exhaust rate to maintain C&C. Conclusions The primary recommendation to reduce the impact that lo- cally supplied MUA may have on hood performance is to mini- Perforated peri- meter supply Backwall supply Four-way diffusers K22 June 2003 Kitchen Ventilation | A Supplement to ASHRAE Journal mize the velocity (fpm) of the makeup air as it is introduced near the hood. This can be accomplished by minimizing the volume (cfm) of makeup air through any single distribution system, by maximizing the area of the diffusers through which the MUA is supplied, or by distributing through multiple pathways. Makeup air that is supplied through displacement ventila- tion diffusers, perforated diffusers located in the ceiling as far as possible from the hood, or as transfer air from the dining room generally works well if air velocities approaching the hood are less than 75 fpm (0.25 m/s). However, makeup air introduced close to an exhaust hood has the potential to inter- fere with the hood’s ability to capture and contain. The chances of makeup air affecting hood performance increases as the percentage of the locally supplied MUA (relative to the total exhaust) is increased. In fact, the 80% rule-of-thumb for sizing airflow through an MUA system may be a recipe for trouble. The first step to reducing the MUA requirement is to lower the design exhaust rate. This can be accomplished by prudent selec- tion and application of UL-listed hoods. 3 The use of side and/or back panels on canopy hoods to increase effectiveness, miti- gate cross drafts and reduce heat gain is highly recommended. The next step in reducing MUA flow is to take credit for outside air that must be supplied by the HVAC system to meet code requirements for ventilating the dining room. Depend- ing on the architectural layout, it may be practical to transfer most of this air to the kitchen. Although this may contradict past practice, the hood performance will be superior and the kitchen environment will benefit from the contribution of the conditioned dining room air. References 1. Brohard, G., et al. 2003. Makeup Air Effects on Kitchen Exhaust Hood Performance. California Energy Commission, Sacramento, Calif. 2. ASTM. 1999. Test Method for Performance of Commercial Kitchen Ventilation Systems. Standard F 1704-99. American Society for Testing and Materials, West Conshohocken, Pa. 3. 1999 ASHRAE Handbook—HVAC Applications. Chapter 30, Kitchen Ventilation. Richard T. Swierczyna is the lab operations manager and Paul A. Sobiski is a research engineer at Architectural Energy in Wood Dale, Ill. 6,500 5,500 4,500 3,500 2,500 1,500 500 0 Exhaust Airflow Rate (cfm) 0 1,000 2,000 3,000 4,000 5,000 Makeup Airflow Rate (cfm) MUA Introduction with No Effect on C&C C&C for Exhaust Only Condition MUA Has More of an Effect on Hood Performance MUA Has Less of an Effect on Hood Performance Figure 2: Potential impact of MUA on exhaust flow rates. Advertisement in the print edition formerly in this space. . the displacement ventilation base-case condition), then it would be pos- sible to replace 100% of the air exhausted through the makeup air configuration. diagonally from the front corner of the hood. For most of the local MUA configurations investigated, the exhaust airflow rate was set initially to the