THE EFECT OF MAKEUP AIR ON KITCHEN HOODS

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... at higher makeup airflow rates). Temperature of the locally supplied makeup air also was ... of makeup air affecting hood performance increases as the ...

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Nội dung Text: THE EFECT OF MAKEUP AIR ON KITCHEN HOODS

Reprinted by permission from ASHRAE Journal, July 2003. © 2003 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 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 project focused on how the introduction of makeup air affects the capture and containment (C&C) performance of com- mercial food service ventilation equip- size and routing are determined, and exhaust fans are specified to re- ment. The investigation included move the proper volume of air. Unfortunately, much less time is usu- combinations of hoods, appliances, cooking conditions, MUA strategies and ally dedicated to planning how the exhausted volume of air will be other factors. replaced, although an air balance schedule is commonly used to indi- Three hood types were tested: wall- mounted canopy, island-mounted cate the source of the makeup air (MUA). canopy, and proximity (backshelf). Overlooking MUA delivery system de- Overall commercial kitchen ventila- Charbroilers and griddles, representing tails can have a negative impact on the tion issues include indoor air quality, fire heavy-duty and medium-duty appliances performance of an otherwise well-de- prevention, safety, employee comfort and respectively, were tested during idle and signed kitchen. Cross drafts and high air equipment first costs, energy operating representative cooking conditions. velocities due to improper introduction costs and maintenance costs. This article The six MUA strategies included: dis- of MUA can result in failure of the hood presents strategies that can minimize the placement ventilation (base case), ceiling to capture and contain effluent from the impact that makeup air introduction has diffuser, front face diffuser, air curtain dif- appliances. This effluent spillage may in- on hood performance. fuser, backwall supply, and short-circuit clude convective heat, products of com- To address these MUA issues, a two- supply (Figure 1). Certain features of the bustion (carbon dioxide, water and year research project was sponsored by a hoods and local makeup air devices were potentially carbon monoxide), and prod- state government energy agency1 and modified to represent designs and con- ucts from the cooking process, such as large utility. Subsequent testing for sev- figurations found in commercial kitchen grease vapor and particles, odors, water eral manufacturers augmented this pub- installations, but not necessarily the best vapor, and various hydrocarbon gases. lic research initiative. This research or worst designs or configurations. K18 June 2003 K i t c h e n Ve n t i l a t i o n | A S u p p l e m e n t t o A S H R A E J o u r n a l
To determine which MUA strategy offered the most effec- For charbroilers, the natural gas flow was increased to match tive operation while providing full capture and containment the previously established cooking plume. The cooking plume (C&C), the research team tested the following hypothesis: simulator for the gas griddle was based on spraying water onto If the MUA strategy were to have no effect on exhaust the hot cooking surface, using a pressure regulator and timed hood performance (i.e., equivalent to the displacement relay valve for control, and needle valves for fine-tuning. ventilation base-case condition), then it would be pos- During baseline displacement ventilation C&C tests, the sible to replace 100% of the air exhausted through the exhaust flow rate was reduced until spillage of the thermal makeup air configuration being investigated, while main- plume was observed. The exhaust flow rate was then increased taining C&C. in fine increments until full C&C was achieved over the test It was conclusively demonstrated that each of the MUA strat- condition. The airflow rate at this condition is referred to as the egies and specific configurations tested compromised the ex- threshold exhaust airflow rate for complete C&C. These values haust hood’s ability to completely capture and contain the provided a baseline case to judge the various MUA strategies. thermal plume and/or effluents at higher makeup airflow rates). Evaluating the performance degradation due to cross drafts Temperature of the locally supplied makeup air also was shown required a repeatable and practical disturbance. For this task, a to effect hood performance as air density impacts the dynamics pedestal-mounted fan was located diagonally from the front of air movement around the hood. Generally, hotter MUA tem- corner of the hood. peratures (e.g., greater than 90°F [32°C]) will affect hood perfor- For most of the local MUA configurations investigated, the mance more adversely than cooler air (e.g., less than 75°F [24°C]). exhaust airflow rate was set initially to the C&C rate deter- ‘Overlooking MUA delivery system details can have a negative impact on the performance of an other- wise well-designed kitchen.’ C KV System Performance Testing mined in the baseline displacement MUA test. The local MUA The phrase “hood capture and containment” is defined in was then increased (in a balanced room condition) until the ASTM F1704-99 Standard Test Method for the Performance threshold of capture and containment was exceeded (i.e., spill- of Commercial Kitchen Ventilation Systems2 as “the ability of age observed). This MUA rate was the airflow rate reported the hood to capture and contain grease-laden cooking vapors, relative to the displacement exhaust C&C rate as the maxi- convective heat and other products of cooking processes.” mum percentage of MUA that could be supplied without im- Capture and containment performance testing incorporated pacting hood performance. focusing schlieren and shadowgraph visualization systems to An exception to the general procedure for local MUA C&C verify capture and containment in accordance with ASTM testing was the ceiling four-way diffuser. Testing was performed F1704-99. These technologies are a major breakthrough for with constant 1,000 cfm (472 L/s) airflow and modulating the visualizing thermal and effluent plumes from cooking pro- exhaust system to the threshold C&C condition. In addition to cesses. A schlieren system presents a high-contrast image of the described protocols, MUA rates were incrementally increased turbulent patterns due to the different air densities within the to determine the marginal increase in exhaust airflow rate. This thermal plume, similar to the effect we see over hot pavement. procedure led to an exhaust-to-MUA ratio determination and With appliances at idle (ready-to-cook) condition, C&C evalu- index of MUA effect. The following discussion presents research ation is a relatively simple and repetitive task. A realistic surro- results from the viewpoint of optimizing system performance. gate was needed to produce consistent effluent during cooking C&C evaluations. Since cooking hamburgers provide peak ef- Displacement Diffusers fluent production for approximately 10 seconds during a six- Displacement ventilation was the baseline for the study be- minute cooking session, cooking with hamburgers was used as cause it provided a uniform, nearly laminar bulk airflow. This a baseline condition for cooking plume simulation. low-velocity bulk airflow has proven optimal for attaining C&C K i t c h e n Ve n t i l a t i o n | A S u p p l e m e n t t o A S H R A E J o u r n a l June 2003 K19
with the lowest exhaust The laboratory testing rate. Therefore, supply- demonstrated that when ing makeup air through short circuit hoods are op- displacement diffusers as erated with excessive inter- illustrated at right is an nal MUA, they fail to effective strategy for in- capture and contain the troducing replacement Displacement cooking effluent, often Excessive in- air. Unfortunately, dis- diffusers spilling at the back of the ternal MUA placement diffusers re- hood (although front spill- quire floor or wall space that is usually at a age is observed in the figure at right). If, how- premium in the commercial kitchen. A pos- ever, the specified exhaust rate is higher than sible solution may be remote displacement the threshold for C&C in an exhaust-only con- diffusers (built into a corner) to help distrib- figuration, the short-circuit airflow rate can ute the introduction of makeup air into the be increased accordingly, creating a condi- kitchen when transfer air is not viable. tion of apparent benefit on a percentage ba- sis. For the short circuit configuration tested, Air Curtain Supply the average MUA rate that could be introduced Most hood manufacturers without causing spillage was 15% of the recommend limiting the threshold C&C exhaust rate. percentage of MUA sup- plied through an air curtain Front Face Supply to less than 20% of the Supplying air through hood’s exhaust flow. At such Impact of air the front face of the hood low air velocities, an air cur- curtain is a configuration recom- tain may enhance C&C de- mended by many hood pending on design details. However, in the cases manufacturers. In theory, tested, the air curtain was the worst performing air exits the front face unit Poorly designed strategy at higher airflows. The negative im- horizontally into the perforated front pact of an air curtain is clearly illustrated above kitchen space. However, a face supply by the schlieren flow visualization recorded front face discharge with during a test of a wall-mounted canopy hood louvers or perforated face can perform poorly, operating over two underfired broilers. if its design does not consider discharge air Introducing MUA through an air curtain is a velocity and direction. The figure above repre- risky option. An air curtain (by itself or in com- sents a poorly designed perforated face supply, bination with another pathway) is not recom- which negatively affected this hood’s capture mended, unless velocities are kept to a performance in the same fashion as an air cur- minimum and the designer has access to per- tain or four-way diffuser. formance data on the specified air curtain con- To improve front face performance, internal figuration. Typical air curtains are easily baffling and/or a double layer of perforated adjusted, which could cause cooking effluent plates may be used to improve the uniformity to spill into the kitchen by inadvertently creat- of airflow. In addition, greater distance be- ing higher than specified discharge velocities. tween the lower capture edge of the hood and the bottom of the face discharge area may de- Short-Circuit Supply (Internal MUA) crease the tendency of the MUA supply to Internal MUA hoods were developed as a interfere with hood capture and containment. strategy to reduce the amount of conditioned In general, face discharge velocities should air required by an exhaust system to meet code not exceed 150 fpm (0.75 m/s) and should requirements. This is accomplished by intro- exit the front face in a horizontal direction. ducing a portion of the untempered makeup air directly into the exhaust hood reservoir. In Perforated Perimeter Supply Figure 1: Types of MUA sup- cold climates, condensation and cooking sur- Perforated perimeter supply is similar to a ply integrated with the hood. face cooling become undesirable side effects. front face supply, but the air is directed down- K20 June 2003 K i t c h e n Ve n t i l a t i o n | A S u p p l e m e n t t o A S H R A E J o u r n a l
ward (see figure at right) toward the hood Other Factors that Influence Hood Performance capture area. This may be advantageous un- Hood Style. Wall-mounted canopy hoods function effectively der some conditions, since the air is directed with a lower exhaust flow rate than single-island hoods. Island downward into the hood capture zone. canopy hoods are more sensitive to MUA supply and cross drafts For proper hood performance, discharge than wall-mounted canopy hoods. Proximity hoods exhibit lower velocities should not exceed 150 fpm (0.75 C&C exhaust rates, and in some cases, perform the same job at m/s) from any section of the diffuser and Perforated peri- one-third of the exhaust rate required by a wall-mounted hood. the distance to lower edge of the hood meter supply Cross Drafts. Cross drafts have a detrimental effect on all should be no less than 18 in. (0.5 m). If the hood/appliance combinations, and adversely affect island air is not introduced in this manner, the system begins to act like canopy hoods more than wall-mounted canopy hoods. A fan in an air curtain. An increase in the plenum discharge area lowers a kitchen, especially pointing at the cooking area, severely de- the velocity for a given flow of MUA and reduces the chance of grades hood performance and may make capture impossible. it affecting C&C. If the perforated perimeter supply is extended Cross drafts required at least a 37% increase in exhaust flow rate along the sides of the hood as well as the front, the increased and in some cases C&C could not be achieved with a 235% area will permit proportionally more MUA to be supplied. increase in exhaust rate. Cross drafts can result from portable fans, movement in the kitchen, or an unbalanced HVAC system. Four-Way Ceiling Diffusers Side Panels and Overhang. Side (or end) panels permit a Four-way diffusers located close to reduced exhaust rate in most cases, as they direct the replace- kitchen exhaust hoods (see figure at ment airflow to the front of the hood. The installation of side right) can have a detrimental effect on panels improved C&C performance for static conditions an av- hood performance, particularly when erage of 10% to 15% and up to 35% for dynamic (cross-draft) the flow through the diffuser ap- conditions. They are a relatively inexpensive way to achieve proaches its design limit. Four-way diffusers C&C performance and reduce the total exhaust rate. Partial side Perforated plate ceiling diffusers can panels are able to provide virtually the same benefit as full be used in the vicinity of the hood, and a greater number of panels. One of the greatest benefits of side panels is to mitigate ceiling diffusers reduce air velocities for a given supply rate. the negative effect of cross drafts. An increase in overhang may To help ensure proper hood performance, air from a diffuser increase the ability to contain large volume surges from cook- within the vicinity of the hood should not be directed toward ing processes that use convection and combination ovens, steam- the hood. If ceiling supplied air must be directed toward a ers and pressure fryers, although for unlisted hoods this may hood, the air discharge velocity at the diffuser face should be mean an increase in the code-required exhaust rate. 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. MUA Strategy and C&C Exhaust Rate What was not anticipated during the design of the study was Backwall Supply how sensitive the C&C threshold would be to the local intro- The lab testing demonstrated that duction of MUA. Spill conditions often were observed when as the backwall supply can be an effec- little as 10% of the exhaust rate was supplied by a given MUA tive strategy for introducing MUA (see strategy. Figure 2 shows a generic trend for changes in exhaust figure at right). For the backwall sup- airflow rate as MUA flow rate increases for a given hood/MUA ply tested with a canopy hood, the av- system. In this generic graph, the C&C exhaust flow rate is 3,000 erage MUA rate that could be Backwall supply cfm (1400 L/s) with no locally supplied MUA. For local MUA introduced without causing spillage up to 500 cfm (236 L/s), the system did not require an increase was 46% of the threshold C&C exhaust rate. in the exhaust rate, as represented by the horizontal part of the To help ensure proper performance, the discharge of the curve. When the MUA was increased beyond the 500 cfm (236 backwall supply should be at least 12 in. (0.3 m) below the L/s), the exhaust rate had to increase to maintain C&C. For this cooking surfaces of the appliances to prevent the relatively particular hood/MUA system, every 1 cfm (0.47 L/s) increase in high velocity introduction of MUA from interfering with MUA required a 0.75 cfm (0.35 L/s) increase in exhaust rate. In gas burners and pilot lights. Backwall plenums with larger the better performing MUA strategies, more local MUA can be discharge areas may provide increased airflow rates as introduced without increasing the exhaust rate to maintain C&C. long as discharge velocities remain below maximum thresh- olds. Ideally, the quantity of air introduced through the Conclusions backwall supply should be no more than 60% of the hood’s The primary recommendation to reduce the impact that lo- exhaust flow. cally supplied MUA may have on hood performance is to mini- K i t c h e n Ve n t i l a t i o n | A S u p p l e m e n t t o A S H R A E J o u r n a l June 2003 K21
6,500 MUA Has More of an Effect on Hood Performance Exhaust Airflow Rate (cfm) 5,500 MUA Introduction 4,500 with No Effect on C&C MUA Has Less of an 3,500 Effect on Hood Performance 2,500 C&C for Exhaust Only Condition 1,500 500 0 0 1,000 2,000 3,000 4,000 5,000 Makeup Airflow Rate (cfm) Figure 2: Potential impact of MUA on exhaust flow rates. 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 Advertisement in the print edition formerly in this space. 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. K22 June 2003 K i t c h e n Ve n t i l a t i o n | A S u p p l e m e n t t o A S H R A E J o u r n a l