The ABCs of CKV
Hoods and commercial-kitchen-ventilation (CKV) systems are changing every day—faster than their technology is being updated. Almost as soon as you get a grip on concepts such as capture and containment, replacement air, energy efficiency and tuning kitchen exhaust to balance with the rest of a building’s air system, new regulations throw fresh challenges at you. It’s too much for most non-specialists to keep up with, but you don’t have to go it alone. Suppliers’ engineering departments and savvy consultants typically step up with technical assistance.
On the regulatory side, model codes such as those from the Building Officials and Code Administrators Int’l., Int'l. Association of Plumbing and Mechanical Officials (with its Uniform Mechanical Code) and Int’l. Code Council (with its Int'l. Mechanical Code) historically have specified minimum exhaust volumes and speeds. Then UL 710, “Standard for Exhaust Hoods for Commercial Cooking Equipment,” came along certifying hoods that could perform properly at lower exhaust rates, making them "listed."
Currently, most jurisdictions have either/or requirements. For example, the ICC requires a minimum exhaust flow rate of 200-500 cfm per linear foot of an unlisted Type I (grease and smoke) wall canopy hood, depending on the type of equipment under it. An appropriate UL-listed hood may be accepted with 10%-30% lower exhaust volumes. Less exhaust means you spend less money on heating, cooling, fan energy, etc.
New Regs Change The Game
Effective July 2014, the California Energy Commission's Title 24 Building Energy Efficiency Standards will adopt American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1, "Energy Standard for Buildings Except Low-Rise Buildings," which sets maximum allowable exhaust rates, among other requirements. In many cases, Title 24 mandates demand-controlled kitchen ventilation (DCKV) systems reduce air volume during light-load periods. Most experts assume California’s standards tend to influence the rest of the country, so figure your local rules will be changing if they haven’t already. Go to bit.ly/1vrKgDy, page 201, for details on the new maximum exhaust rates and exceptions.
Setting maximum allowable exhaust rates is going to affect what you put under the hood. If you’re thinking about placing a charbroiler under an island canopy in the middle of your counter in Los Angeles, you might want to think again.
Despite changes to come, the tried-and-true best practices of good CKV engineering haven’t changed much. We asked some ventilation-savvy consultants to share their advice about various types of hoods, how they differ and what you can do to maximize their effectiveness.
TRADITIONAL WALL CANOPY
The Versatile Commercial-Kitchen Workhorse
Think of wall canopies as a core or baseline for kitchen hoods. They’ve been around forever for good reason: They have some inherent efficiency advantages in both initial and operating costs.
“Wall canopies are less expensive,” says Marcin Zmiejko, senior associate at William Caruso & Associates, Denver. “And they lend themselves to more options for airflow control.”
Although wall canopies put cooks’ backs to the rest of the kitchen, hampering awareness and sight-lines, the upsides are many.
The job of a hood is to exhaust effluent, which means controlling airflow. Up against a wall, a canopy has one side mostly taken care of: Little or no disruptive crosscurrents of air come from the wall side. Additionally, because of a phenomenon called the Coanda Effect, cooking vapor and grease-laden effluent tends to flow toward the wall and follow it straight up into the hood. Only three sides are left to manage.
Disruptive airflow can come from many sources. Busy cooks create turbulence, Zmiejko points out, and the closer they are to the hood, the more disruptive their movements can be. Doors opening can create drafts that disrupt the exhaust plume being drawn into the hood. Four-way diffusers, especially ones located in the ceiling too near the hood edge, can create turbulence.
To capture and contain the exhaust plume at the lowest reasonable exhaust volume, consultants suggest several techniques. The first method has to do with overhang: The hood lip typically should extend 6 in. or more (often more) beyond the front edge of the equipment being exhausted. The overhang offers a bigger “grab” area for rising heat, smoke and effluent plumes from cooking surfaces and opened equipment doors. Depending on the type of equipment being exhausted, as much as 12 in. or even more overhang may be advisable. If you have a deep piece of equipment, such as a combi, you’ll need a deeper overhang. Centering the most exhaust-intensive (heaviest-duty) equipment under the hood also improves capture.
Other tools are available, too. “You can hang metal panels from the sides of the hood," which Zmiejko says works well to block cross drafts. But check the path of kitchen foot traffic first. Depending on how panels are installed, staffers rushing by too closely can hit their heads, he notes.
An alternative to panels is wing walls, short-depth walls built perpendicularly from the wall on one or both sides of the cook line and hood. However, closing the cook line on three sides might introduce a new set of airflow limitations. Trade-offs of side walls and panels could include less light, blocked sight lines and slower food flow.
A footnote on airflow: Air out equals air in, right? So all of the exhausted air gets replaced by air from somewhere—and where makes a big difference. If there are gaps between the back of your equipment and the back wall, air rising through those gaps can disrupt the exhaust draw. Tests from the Commercial Kitchen Ventilation Laboratory, owned by Pacific Gas & Electric and operated by the Fisher-Nickel Inc.-run Food Service Technology Center (FSTC), San Ramon, Calif., have proved that installing a shroud or cowl between the equipment and the wall helps to stop the disrupting turbulence.
Additionally, makeup air is best introduced at very low, gentle velocities. Most CKV-system manufacturers incorporate air-supply plenums on the front of canopies or locate perforated plenums elsewhere in the kitchen or even in the dining room. The trick is to let makeup air drift in where it can get captured by the exhaust.
Production, Flow And Challenges
Even with all of the advantages of wall canopies, they’re not right for every scenario. Maybe you have big production volumes and need more space at the back-of-house, a more open flow and more access from prep areas to cook lines. Maybe your staffers need better communication and sightlines. Maybe you want to do some display cooking in the front-of-house, with chefs facing customers.
If so, island canopies fit the bill. They give you all of the above advantages, albeit with some trade-offs. First, island canopies add complexities in construction, installation and interfacing with the rest of the building in general. Island canopies need to be bigger than wall canopies over like equipment, and they often have larger overhangs, so they cost more in construction. They also weigh more.
Because they suspend from the ceiling, island canopies are more susceptible to ambient-air turbulence from all directions. Consequently, they need to run at higher exhaust rates for effective capture and containment. Most back-of-house island canopies are longer because they tend to be in larger, high-volume kitchens; because they are longer, they must exhaust more air. There are bigger numbers at stake to achieve efficiency.
Because of the larger exhaust volume, dedicated makeup air, supply air, ductwork, etc., become a more complex issue. “Sometimes different people are paying for different parts of the building,” Zmiejko explains. “Who’s providing makeup air? Who’s providing the makeup-air plenum?” In a multitenant building, coordination can be a challenge.
“When we get to project design, we face challenges with existing buildings … ventilation goals can be different and difficult,” says Ken Schwartz, FCSI, president and CEO of SSA Foodservice Design + Consulting, Pinellas Park, Fla.
“One of the first things to look at is HVAC. It can have positive or negative impact on kitchen ventilation. Who is the mechanical engineer for the HVAC? We want to collaborate with that person quickly.
“If you’re looking at a hood system in a relatively small space, suddenly ceiling space gets very busy with makeup air and supply air.
“Islands can be more problematic just by virtue of what happens with HVAC. … We are very proactive with the design and specification of the whole hood system, not just the canopy. How should we design the ductwork? How does [the environment] impact the location of the hood?”
Double Islands, Double Fun
If you need even more throughput and cooking space, you may want a double island canopy, which doubles everything. ICC requirements for the amount of cfm per linear foot are lower than for single canopies, but because there are twice as many linear feet, the total amount of air exhausted is another huge increase over a single canopy. Untempered makeup air—and the plenums to introduce it into the space—also necessitate more logistics to ensure that makeup air distributes into the space properly. Fortunately, the hood suppliers’ engineers eat, sleep and breathe these projects, and they can provide the detailed calculations you’ll need to achieve a balanced pressure and draw.
Single or double island—many of the challenges and solutions are similar. The main differences are in scale.
“Placement of equipment—function first—must make sense,” says Tarah Schroeder, FCSI, principal with Ricca Design Studios, Greenwood Village, Colo. “Put heavier-duty equipment, like charbroilers, toward the center,” she says. Steamers with vapor effluent can go on the ends. Her advice is important for any kind of hood, but even more critical for island hoods.
“Often, you’ll have to size an island canopy larger than a wall canopy,” Schroeder continues, alluding to a number of factors from the extra cfm of air required to the additional overhang depth and length. Beyond proper equipment placement, there are some pieces of equipment that really shouldn’t go under an island hood at all. “It’s important to choose the type of equipment for an island carefully. Maybe you shouldn’t have a charbroiler under it. Maybe you should consider doing more induction instead,” she says.
Those choices have a big impact: The ICC code says in cook lines where a variety of light-, medium-, heavy- and extra-heavy-duty equipment is included, the exhaust rate required for the heaviest-duty piece of equipment is the rate required for the entire hood.
Island Tips And Tricks
On the supply side, huge advances have been made in understanding supply air, including tempered transfer air (already conditioned air from other parts of the building) as well as dedicated untempered makeup air. As mentioned, balancing the air sources with exhaust volumes is key. Again, “air out equals air in” is the mantra, regardless of the type of hood. Other than that, how you balance it will depend on the nature of your building (multitenant or independent space), your climate (humidity, heat and cold have effects), etc.
On the demand side, the big leap has been demand-controlled kitchen ventilation (DCKV), in which an exhaust fan ramps up and down in response to cooking activity. When you’re not cooking, the fan idles. When you start cooking, the fan gets to work. The payoff is in lower energy consumption when in idle mode. Some systems use thermal sensors, some use optical and others use infrared (see FER, April 2013, p. 24, bit.ly/1uRKfsa).
Another variable in DCKV is sensor placement. Put it up in the collar farther from the cooking surface and fan response might be a little slower, although maybe not enough to be concerned with.
A recent DCKV evolution is the dampered hood system, an installation of multiple hoods that run on a single fan. A damper system can shut down idle hood sections while sections over active cooking surfaces continue as normal.
As for directing capture and containment, you have choices. Hanging side panels, that as previously mentioned work so well for a wall canopy hood, are not a great choice for island canopies. With two or more sides open, you’d probably still have airflow issues. Plus, panels would defeat the advantages of food flow and sight lines; many island canopies are installed front-of-house and incorporated into the decor.
Sometimes it’s hard to tell whether island hood problems are caused by too much air volume or too little. More volume, in the form of a bigger hood, bigger fan or higher fan speed, might be an option, but lab tests demonstrate that increasing cfm often is not the answer, as it can create its own self-defeating turbulence. In fact, decreasing fan size or speed sometimes is the solution. Schwartz notes that in some cases, especially for island and display applications, a bigger fan turning at a lower rpm strikes the right balance and produces less noise in the process, which is especially useful in display-cooking installations.
Another new option comes under the heading of “and now for something completely different.” One supplier, thinking outside of the box, offers a type of bottom-up air curtain, so to speak. Fixtures attached down and around the perimeter of the cooking appliances produce a curtain of upward-flowing air. It’s a pressure zone—a soft barrier that reduces the ability of drafts to disrupt the effluent plume on its way to the hood.
And, speaking of disrupters, a reminder about ceiling diffusers: Check them out. If they’re too close to the hood—especially if they’re diffusing in the direction of the hood—they will cause more problems than they solve. Four-way diffusers are notorious plume disrupters, and they show up in the ceiling near canopy edges when the CKV designer and the mechanical engineer have a “failure to communicate.”
BACK-SHELF PROXIMITY HOOD
The Right Tool For The Right Job
Another common option, widely used in QSRs, is a back-shelf proximity hood. When the application is right, there’s nothing better. You just have to know its limitations.
To oversimplify, think of a back-shelf proximity hood as a wall canopy, but mounted lower. It’s shorter in depth than a wall canopy—in fact it has a negative overhang. Dropping the hood closer to the equipment—typically 12-36 in. above the cook surface—aids in capture and containment. But the depth of the proximity hood canopy is typically only 20-36 in.; it doesn’t completely cover larger, heavier equipment.
The back-shelf design is good for installations where the ceiling is low or where you need to put some equipment that has to be exhausted but might not require a larger, more expensive hood with a high cfm exhaust rate. Light- and medium-duty equipment, such as fryers, griddles and countertop equipment, are prime candidates. Even then, not all countertop equipment is suitable for a back-shelf proximity hood just because it fits under one.
“If you’re using a back-shelf with a charbroiler, you’re in trouble,” jokes Gary Petry, project administrator, JMK Foodservice Consulting & Design, Rowlett, Texas. “Charbroilers and woks,” echoes Justin Silverthorn, FCSI, CEC, CMC, principal, Advanced Foodservice Solutions, Mountville, Pa. If you are using charbroilers and woks, he says, “You have to be very attentive to grease extraction and heat gain,” which are not likely to be handled well by a back-shelf proximity hood.
The shallow coverage on this compact hood style leaves the plume vulnerable to disruption. “Part of what disturbs the plume is the building air [movement and quality of air in the environment], and sometimes it’s as simple as culinary staff moving around in the cooking area,” he says.
Still, there’s great potential efficiency in these compact hoods for light- and medium-duty equipment. It’s all about matching the right hoods with the right applications.
No matter your equipment lineup, when it comes to engineering the right CKV system, you’re never alone. Hood makers can help you sort through all of the issues. Check out their websites for starters (go to fermag.com, click on Buyers Guide, choose Ventilation Hoods and Systems and a country and click the Search button). Many of them are loaded with good information. Do online searches for ventilation topics. ASHRAE, Fisher-Nickel Inc., the Foodservice Consultants Society Int’l. (its 2006 “Commercial Kitchen Ventilation” white paper is excellent) and a host of other reputable sources are available.
Look Ma, No Hoods!
What if you could wave a magic wand and ventilate your kitchen without a hood?
Well, most of you—with medium to heavy cooking loads—can’t, at least not yet. But if you’re only doing light- or medium-duty cooking, Halton’s Ventilated Ceiling System might open up new opportunities for you. Not widely known in the U.S., mainly because of the nature of light-duty cooking, the system has been listed domestically for seven or eight years and in Europe for decades, according to Jeff Hilbert, managing director of sales-U.S.A. Currently, a handful of U.S. and Canadian operations are using it.
Hilbert says the system isn’t meant for open-flame cooking or anything that produces heavy grease-laden vapors. But that leaves a world of lighter applications. “It’s especially well-suited for demonstration kitchens and anywhere you want or need open sight lines,” he says. Business and industry and teaching kitchens would be good fits, too; correctional facilities and cook-chill ops would be candidates as well.
How It Works
The system, custom engineered for each installation, uses a structural grid for the ceiling and four types of modular 300-series stainless cassettes: centrifugal-exhaust extraction, supply-air, lighting and “blanks” or passive cassettes. The extraction cassettes are easily removable and washable. Each one flows at 250-300 cfm, while the supply-air cassettes run at 80% of the exhaust volume. The lighting cassettes run at 110V and come in your choice of incandescent or fluorescent.
Placement of each type of cassette is based on where your cooking equipment is situated. The modularity makes updates and modifications relatively easy. You can add or rearrange cassettes as your kitchen changes.
“Maintenance varies with load,” much like any other kind of system, Hilbert says. “Since you’re not doing heavy-duty cooking, you don’t have much grease-laden vapor.” A simple clip system makes the extractors easy to remove, then you wash them in a pot sink or dishwasher.
It’s a sophisticated system and more expensive than alternative exhaust systems, Hilbert says. But it runs very quietly, is easy to access and affords a wide-open line of sight across the entire kitchen.
ACCUREX FIRE READY HOOD
range hood that can suppress stovetop fires, addressing the challenge
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over a 30- or 36-in. residential range, the hood incorporates a UL-listed, selfcontained,
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