Cooking up ventilation
This month's panel discusses ventilation best practices in commercial kitchens, including codes and standards, and different energy saving and techniques.
This month's panel discusses ventilation best practices in commercial kitchens, including codes and standards, and different energy saving and techniques.
CSE: What actions or product applications help prevent grease fires in kitchen ventilation systems?
Peter D'Antonio : Most grease fires are a result of poor maintenance of the exhaust system. Grease fires are avoidable through proper operation and maintenance. A good O&M plan will ensure the system is properly air balanced, makes provisions for kitchen staff to perform routine maintenance, and contracts qualified outside services to inspect and clean the grease exhaust system periodically based on the amount of effluent produced and filtration in place.
Steve Melink : I think canopy-style hoods should be less susceptible to grease fires since they hang higher above the cooking equipment. The greater the physical separation between the heat source, boilers, the fuel source, and the grease-laden filters, the less likely an ignition will occur. Using the same logic, cooking equipment that is only 36-in. high is better than similar equipment that is 72-in. high. In addition, there should be horizontal duct runs, so that grease is less likely to collect on the bottom wall and become a potential fuel source in between cleanings.
Brian Rivet : Capturing and removing the grease particulate at the hood will prevent hood plenums and ducts from building up substantial quantities of grease between scheduled duct cleanings. When capturing the grease near the source, the risk of grease fires in the ventilation system lessens. Select grease filters that have been tested to the ASTM F2519-2005 standard to understand the filter efficiencies and best match them to the cooking operation. Standard baffle filters will not provide efficient removal of grease particles from the airstream in most applications, especially on high-grease-producing cooking operations. Opt for other mechanical filters such as centrifugal filters and dual stage filters.
Tony McGuire : I do not know of any particular venting applications that will avoid grease fires because they are generally the result of inadequate maintenance or mistakes made on cooking lines themselves. This is why new requirements for cleaning and maintenance were included in the 2008 edition of NFPA 96, “Ventilation Control and Fire Protection of Commercial Cooking Operations.”
CSE: When are VFDs on kitchen fans appropriate?
McGuire : VFDs can be excellent methods of reducing operating costs when the kitchen design provides the right opportunities for modifying flow rates; however, many kitchens do not have the ability to do so. This becomes a matter of kitchen operations. A key question is, “What are the skills of the staff that will be turning equipment on and off?” Then, to determine whether VFDs are appropriate, you must look at the entire restaurant's HVAC design as to how air will be moving during the course of a day's operation. While the large chains have studied and developed many engineered approaches to ventilation and exhausts, the smaller, independent businesses continue to have little to no knowledge on the matter.
D'Antonio : VFDs are appropriate where the kitchen exhaust must run continuously, but the cooking operation has a high diversity of operating schedules and appliance type. If appliances are idle a majority of the day, there is good potential for a VFD to reduce energy use. A primary consideration is that the reduced velocity is adequate for capture and containment at the hood.
Rivet : VFDs have been commonly used on fans, which are part of a variable volume kitchen system. These systems use temperature sensors and optic sensors to detect cooking activity under the hood, then send a signal to a processor that automatically adjusts the exhaust and supply rates. Variable volume systems' typical applications include large cooking operations, such as batch cooking or facilities that operate 12 to 24 hours per day.
Melink : VFDs should be specified as much as possible so that direct-drive rather than belt-drive fans can also be specified. Since the belt is the weak link in the overall kitchen ventilation system, the more we design these out of kitchens, the better Demand ventilation is slowly but surely becoming an industry standard, and there is no reason to use both belt-drive fans and VFDs.
CSE: How should engineers handle roof venting issues—stacks above rooflines, neighbors' complaints, wind, etc.?
D'Antonio : First, follow the code for venting requirements and always augment that with site-specific needs related to aesthetics, potential odor, and noise control. Mixed-use applications are typically problematic about odor control spreading to nearby residential units. Fan selection is also very important in this regard, with utility set fans providing the best throw distances. Select the lowest possible fan rpm to reduce noise and consider vibration control. Odor can be controlled to a greater level with various secondary particulate removal devices, which are located downstream of the primary filtration in the hood; however, these devices may be costly to install and maintain.
Melink : Lower-cost aluminum up-blast fans usually work satisfactorily in light- and medium-duty applications. Rugged steel centrifugal fans usually work better in heavy-duty applications where there are high heat and grease loads. Properly designed parapet walls can help reduce the aesthetic, noise, and wind issues that can bother neighbors and adversely affect fan performance.
McGuire : First, prevailing winds may be prevailing, but that does not mean they always blow in the same direction. Distances that may seem more than adequate for dilution of cooking odors can disappear as the chef begins to season the menu items. One of my tougher design situations was one in which we put a hood exhaust fan on the roof of a 10-story building. Its discharge was approximately 75 ft from the closest intake. Unfortunately, the ground-floor restaurant that the system served came up with a menu that used garlic to an extreme. Office tenants on the top half of the building began smelling garlic around 10 a.m. while the kitchen was preparing for a busy lunch schedule. The ultimate solution, which we used, contained a level of carbon filtration of exhaust air. Yes, we needed to filter the exhaust for odor control. It required an outlay for installation and a continuing maintenance cost, but it was the appropriate solution to the situation.
CSE: How should engineers provide adequate make-up air for exhaust fans? For pre-conditioning air with heat exchangers? To avoid depressurization?
D'Antonio : Make-up for commercial kitchen exhaust should be provided in a manner to maintain the kitchen area at a negative pressure with respect to the dining area. This will ensure proper odor control. Make-up air can be introduced in a variety of locations for this purpose. One example is to provide 85% to 90% of the exhaust requirement at or near the kitchen hood with the remaining 10% to 15% make-up introduced at the dining area HVAC unit. Since the kitchen exhaust is grease-laden air, it typically is not conducive to heat exchange. To effectively employ heat exchange on make-up air, a particulate removal system should be installed to treat exhaust stack effluent. To avoid depressurization of the kitchen and to satisfy codes, the make-up air must be interlocked to operate with the exhaust fans.
Melink : Ideally, the make-up air should be provided in a distributed rather than concentrated manner and as far from the hood as possible to minimize drafts that can interfere with proper hood capture. Most make-up air units have heating capability, but the trend is toward both heating and cooling/dehumidifying capability.
Rivet : There are various ways to provide make-up air to the space so it is important to evaluate the source and the quality of the outside air being supplied before choosing how to introduce it. If using tempered air, the system should introduce the air using perforated ceiling diffusers evenly distributed throughout the room. Diffusing tempered air contributes positively to the comfort in the space. When supplying non-tempered or marginally tempered air, use supply plenums around the perimeter of the hood that introduce the air downward at low velocities, in order to keep the air near the hood, which reduces the negative effects the air could have on the comfort in the space. Also, a back/rear plenum that introduces air behind and below the appliances can be used, reducing the diffusion of the make-up air into the space.
CSE: What are the advantages of direct-drive motors and belt-driven motors? Are certain applications ideal for each?
Rivet : Direct-drive motors are most applicable when maintenance is a concern and when air volume and static pressure requirements are low enough to meet direct-drive performance limitations. Benefits include:
Elimination of bearings and belts that wear over time, mitigating maintenance on the fans
Elimination of energy losses due to belts; thus, direct-drive fans are in general slightly more efficient
Reduced risk of downtime due to broken belts. Belt-driven motors are ideal for most applications because they have very broad performance ranges and provide greater flexibility in the field for balancing.
Performance flexibility; different belt and pulley combinations can be used to adjust for slight variations in static pressure and air volume
High air volume and high static pressure capabilities
Lower cost; typically, they are less expensive than direct-drive motors.
Melink : From reduced maintenance and fire safety concerns to improved air balance and energy efficiency, direct-drive fans are the way of the future. Disadvantages include slightly higher cost, a motor that is more exposed to the airstream on up-blast fans, and a motor that is typically more difficult to replace if that ever becomes necessary. But the advantages of direct-drive far outweigh the disadvantages.
McGuire : Personally, I like using direct-drive motors for most grease hood exhaust designs. The prime advantages come from knowing that the fan itself is running when electricity is on. In addition, proper selection will limit failure during times of increased temperatures in the airstream. The belt-driven fans must have cowls over the belts to keep them out of the airstream. The significant problem is that of belt maintenance. The higher temperatures can lead to shortened life for belts.
CSE: What codes/standards do engineers need to be aware of when working in these facilities?
D'Antonio : The following codes, standards, and test protocols govern commercial kitchen ventilation. The 2006 International Mechanical Code addresses commercial kitchen hoods, exhaust ducts, and exhaust equipment in Sections 506 and 507. “Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations,” in NFPA 96, details the minimum fire safety requirements related to the design, installation, operation, inspection, and maintenance of all public and private cooking operations, excluding single-family residential usage. These requirements include, but are not limited to, all manner of cooking equipment, exhaust hoods, grease-removal devices, exhaust ductwork, exhaust fans, dampers, fire extinguishing equipment, and all other auxiliary or ancillary components or systems that are involved in the capture, containment, and control of grease-laden cooking effluent. The 2007 ASHRAE Handbook: HVAC Applications addresses kitchen ventilation. UL 710 test protocol was established to determine minimum exhaust rates for 400, 600, and 700 F surface temperature appliances. The airflow portion of the test is a visual one made by a UL inspector, who observes the removal of visible cooking vapors. If, in the judgment of the inspector, the hood captures and contains this visible effluent, then the tested exhaust airflow rate passes the test. What UL 710 does not test for is the amount of heat escaping the hood at a given airflow. Listed hoods can have a fire damper to protect ductwork and maintain temperatures below 375 F or can function without a fire damper. UL 710 does not cover evaluation of the exhaust hoods with respect to their grease extraction efficiency. UL 1046-test protocol was established for grease filters used in exhaust systems with Type I restaurant-type cooking equipment. Grease filters are investigated to insure they remove grease from the effluent—drain-off of the collected grease—in such a manner that it does not fall back onto the cooking surface, and limit the projection of flames into the exhaust ductwork when fire breaks out on the upstream face of the filter, and after exposure to grease-laden air. UL Subject 762, “Power Roof Ventilators for Restaurant Exhaust Appliances” covers roof- or wall-mounted ventilators for restaurant exhaust appliances.
Melink : The International Mechanical Code, NFPA 96, and the listed hood performance ratings provided by the manufacturer are the main ones. However, state and local codes may be different and dictate other approaches to designing and installing the kitchen ventilation system.
Rivet : I agree. The most recognized code in the kitchen ventilation industry is the International Mechanical Code (IMC 2006). In some areas, the International Building Code 2006 is realizing greater recognition. The predominant standard in commercial kitchen is NFPA 96, 2008 edition.
McGuire : Local codes always dictate the minimal permissible levels of design, but that may not be the correct way to engineer the system. The engineer must remember that a code only dictates the worst design permitted by law. NFPA 96 (2008) gives the latest thinking for our topic and should be part of the reference materials used for design. The International Mechanical Code has been regularly updating and improving the requirements for commercial hood exhaust systems.
CSE: How does proper kitchen venting play into codes? What should engineers be aware of?
McGuire : Codes are written to protect the public, while also protecting property in certain areas. Since codes become part of a law in whatever jurisdiction adopts them, they need to be considered legally binding. Building officials commonly take a standard code, such as the International family of codes, and then modify it to meet specifics for the jurisdiction. They, in turn, pass it on for adoption by the legislators. In 1998, NFPA issued its document, NFPA 96. It was significantly revised in 2001, in 2004, and again in the latest 2008 edition. Likewise, the International Mechanical Code was issued in 1998, 2000, 2003, and 2006. Each of these issues contains major changes in Section 506 “Commercial Hood Ventilation System Ducts and Exhaust Equipment.” Each edition incorporated important items. However, legislators were not sitting around waiting to pass new laws to make these changes binding by law, and few would consider adopting something they had not seen. Such an approach would require accepting changes without a vote. What has happened is a vast variation in legal requirements across the United States. All the changes made in these codes, however good they are, do not guarantee that they will be accepted by local authorities since the local building ordinances may not be revised for years. This has resulted in the need for a thorough level of communication with authorities to assure that the design is acceptable.
Melink : The main objective of the kitchen ventilation system is to properly capture and remove the heat and smoke generated from the cooking process to ensure a comfortable and safe working environment. As long as your design accomplishes this main objective, you are complying with most, if not all, the code requirements. While this may seem simple on the surface, the reality is there are numerous factors to consider, and experience is an important credential. Fortunately, there is more literature available on the subject today than 10 to 20 years ago, and this can help one climb the learning curve more quickly.
Peter D'Antonio, PE
PCD Engineering Services Inc.
Tony McGuire, PE, FASHRAE
McGuire Engineers Inc.
Steve Melink, PE
Sales & Marketing Mgr.
Kitchen Ventilation Systems
Greenheck Fan Corp.