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June 2003

By Mike Sherer

New hood and fan technologies are doing a better job for less.  

Specifying equipment for foodservice is hard enough. When it comes to kitchen ventilation, making the right choice is positively exhausting (pun intended).

What once was considered a black art at best, however, is slowly becoming quantifiable science. That science is leading to exhaust systems that perform better and use far less energy than their predecessors of a few years ago.

What’s the big deal? For you, it means far fewer headaches when working with mechanical engineers and kitchen consultants to spec an exhaust system that will meet code. It also means you’ll end up with a system that will take more pollutants out of your kitchen, and filters that’ll take more out of your exhaust—making everyone happy, including diners, staff, fire departments and legislators who pass ever-more-stringent clean air laws.

Even better, it means the systems you spec can save you a lot of money in the long run.

For years, kitchen ventilation and exhaust systems have been designed to meet either a mish-mash of international model engineering codes, regional and local building codes, or the National Fire Protection Agency 96 standard. Alternatively, operators also have been able to shortcut some of the confusion by purchasing UL-listed equipment since most codes accept systems tested by Underwriters Laboratories.

Eight years ago, the American Society of Heating, Refrigerating and Air-Conditioning Engineers published a chapter on commercial kitchen ventilation recommendations backed up by lab research. And nearly six years ago, ASHRAE was poised to submit for public review its own model CKV code that would set industry standards and preempt old, outdated codes.

Well, six years of fine-tuning passed. Although the industry is still waiting for ASHRAE to publish its CKV standard (possibly by press time), the content already has had a major impact on the Int’l. Mechanical Code, which has adopted as many as 30 code changes in the past three years. The changes are bringing IMC standards more in line with ASHRAE’s intent to base its standard on cubic feet per minute per linear foot of cooking equipment.

It also hasn’t stopped the industry from doing the kind of research that led to the overhaul in the first place. Utilities, industry groups, universities, ventilation manufacturers and even operators continue to pour time and money into lab testing of all types of equipment under all sorts of cooking conditions. The results are new technologies and engineering that increase the performance and efficiency of kitchen exhaust systems.

A Fan Of Fans

Most of the existing codes have focused on the amount of air you need to exhaust to carry all the smoke, grease and odors out of the kitchen. Back when many of the building and mechanical codes were first written, that entailed a lot of educated guesswork, resulting in what many have long believed are exhaust volumes way beyond what’s actually needed. Using UL-listed equipment has helped operators get around some of these generous margins of error. UL, though, tests equipment for safety, not performance.

Much of the testing that led to ASHRAE 154P foc-used on how much effluent is produced by each type of cooking equipment under different load conditions. With more precise measurements of how much air has to be moved to carry away effluent for each piece of equipment, it becomes easier to spec an exhaust system based on the equipment that’s in the kitchen.

Much of today’s advancements have come in fans and fan motors, the engines that move exhaust air out of the kitchen.

“Fan technology is a fairly high state of art,” says Bob Luddy, president of Captive-Aire Systems. Constantly looking for improvements, fan manufacturers in recent years have been adapting products used in other industrial applications. Better materials, better designs, and hybrid combinations of techniques have been paying off.

Fans typically used in kitchen exhaust systems include centrifugal upblast fans, rooftop-mounted utility sets, and less common in-line axial fans.

Upblast fans (most commonly, the aluminum mushroom-shaped fans sprouting from restaurant roofs everywhere) exhaust kitchen effluent up into the air away from the roof. More manufacturers such as Acme Engineering & Manufacturing, Loren Cook and Exhausto are introducing direct-drive versions of these fans and more powerful fans that can handle additional static pressure (resistance caused by pulling air through grease filters and long or narrow ducts).

Direct-drive fans have been around a long time, but only recently have they been used in foodservice. The most obvious benefit of direct-drive fans is that there is no drive belt to wear out or break. An even bigger benefit is that direct-drive fans can run at varying speeds. In a traditional exhaust system, a fan sits on top of a duct over the kitchen, and the fan runs all day, and sometimes all night.

“A variable-speed fan allows you to adjust the exhaust volume based on what you’re cooking,” says Steen Hagensen, president of Exhausto Inc. “That saves energy to run the fan and reduces the amount of make-up air needed, which is usually tempered. When you look at the bottom line, payback on most of these fans is two years at most.”

Utility sets are usually constructed of steel, not aluminum, so they can handle higher temperatures than aluminum, often UL-listed up to 500&Mac251;F. They’re also designed to handle higher static pressure (up to 5”) and can move a greater volume of air. Originally intended for other applications, utility sets have been adapted for foodservice, and manufacturers are making improvements.

“We use utility set fans, not upblast, because of their cleanability,” says Mark Fink, engineer, worldwide strategic operations R&D, for Burger King Corp.

Proper cleaning is important, Fink says. If it isn’t done correctly, residual grease can unbalance the fan wheel and causes vibration that can tear up the motor, or worse. “We’ve had fan fires on the roof caused by friction from an unbalanced wheel setting grease on fire.”

Loren Cook now builds a utility set with a special nonstick coating that doesn’t allow grease to accumulate on the fan wheel. Most fan manufacturers also now offer utility sets for foodservice with direct drive.

In-line fans are rarely used in kitchen exhaust systems because of the amount of grease in the effluent. The only in-line fans approved for foodservice, in fact, are axial fans, that use a propeller-like blade. Loren Cook, however, recently introduced an in-line mixed-flow fan, the first tubular in-line fan that is a combination centrifugal and axial fan.

Now, variable-speed fans wouldn’t do you any good without the ability to control them.

“There’s a bright future for intelligent controls,” says Steve Melink, president of Melink Corp. “We already have them in so many areas like security, cooking equipment and lighting. This is one of the most expensive areas and last to be controlled.”

Companies like Melink now make controls for hood manufacturers that regulate fan speed, and therefore exhaust and air flow, in a variety of ways. In most of these systems, when you open up in the morning and turn on the hood, the fan will operate at an idle speed. Temperature sensors in the hood detect when cooking equipment is turned on and bump up fan speed to increase exhaust flow. Optic sensors monitor when actual cooking is taking place by “seeing” smoke or grease vapors. They send a signal to the controller to turn fan speed up to full until all the effluent is removed from the exhaust plume.

Processors can control lights and fans for up to four hoods, which means that exhaust for different cooking lines can be controlled independently. More importantly, though, it also means tremendous savings on the cost of tempering make-up air.

“The load on your HVAC system will usually be greatest at 2 p.m. or 3 p.m., when the load on your hood is the lightest,” Melink says. “Why pump in hot make-up air when nothing needs to be exhausted from the kitchen?”

Capture And Containment
Another area in which great strides have been made is capturing and containing the plume of effluent that rises off the line as food cooks. The advent of Schlieren imaging a decade ago made it possible for researchers to visualize the effluent plume. Once they could see the plume, they also could see the effects of hood design, the rate of exhaust volume, and more.

“A lot of changes have been driven not only by engineering the design of the exhaust system, but also where you bring in replacement air,” says Rich Swierczyna, operations ma-nager at the CKV lab in Wood Dale, Ill. “That has a dramatic result on capture and containment rates.”

Make-up air is part of the bigger picture of HVAC systems and indoor air quality, an issue we’ll explore in another story this summer. It’s important to note here, though, that how replacement air is supplied to an exhaust hood has a big effect on the hood’s performance, and even on how much air the exhaust system has to move to get rid of cooking effluent.

The big surprise researchers discovered was that systems designed to aid capture and containment or introduce make-up air close to the hood often weren’t all that effective. Front-face discharge, air curtains, and even backwall make-up plenums can hurt rather than help hoods capture effluent plumes if not designed carefully.

Schlieren imaging has helped manufacturers do just that. Even manufacturers now use the technique in their own development labs. Halton, for example, has used the process to improve its CaptureJet technology, first introduced in 1991, which supplies a small amount of make-up air near the front of the hood.

Others have followed suit. Gaylord Industries uses plenum boxes at the front of the hood to force a small amount of make-up air straight down at low velocities. Captive-Aire uses a perforated supply plenum to achieve the same effect of containing the effluent plume within the hood.

Hoods also are being designed so the interior shape of the hood aids the flow of the plume. Many manufacturers also offer side panels that decrease the open area of the hood and increase the air velocity within the hood.

“The general idea is to make people aware of the disadvantage of bringing in lots of room air for a big canopy hood as opposed to a small back shelf hood,” says Swierczyna. “Also, if you lower the hood closer to the appliance, you lower the flow rate needed to capture the exhaust.”

Grease Is Not Your Friend
Obviously, removing smoke, grease and odors from your kitchen is the primary purpose of a kitchen exhaust system. Sending all that effluent up the duct and out into the air would create a pretty big mess, not to mention a hazard, if it didn’t pass through some kind of filter first.

The biggest problem, and hazard, is grease. Unless grease is pulled out of the exhaust, high temperatures in the hood and duct can cause grease vapor to ignite. Manufacturers offer a variety of grease filters, ranging from simple baffles to cyclonic cartridge filters.

“All hoods have a mechanical means of knocking down grease, but they’re more efficient at removing larger particles,” says Burger King’s Fink. “We fall into the highest risk because our broilers generate the most grease and effluent, generate the most heat, which can ignite grease fires, and our exhaust systems move the most air, which can fuel a fire.”

The solution for Burger King, which has tested almost every form of grease extraction out there, is a catalytic converter. Like those found on cars, the catalytic converter in the hood over the broiler in a BK unit is heated to about 1,000&Mac251;F. The high heat of the broiler itself provides much of the energy. Grease and effluent passing through the catalyst are incinerated, leaving nothing but ash.

So what do you do if you’re not flame-broiling a zillion burgers a day? What type of filter you need depends on the type of equipment you’re using and the food you’re cooking—in other words, how much grease you’re producing.

“Grease extraction is the next big frontier,” says Rick Bagwell, president, Halton Co. “You could almost just have air make a right turn in the exhaust duct and collect most effluent that’s 20 microns or bigger in size. Mechanical filters, though, are pretty poor at removing particles below five microns in size.”

Manufacturers have developed several approaches to address this problem. First, companies like Greenheck Inc. and Veritech Filtration have developed mechanical filters that are more efficient at removing smaller particles.

Veritech’s, for example, pulls grease-laden air through small holes in a spiral pattern that increases air velocity. Called “cyclonic” motion, the idea is to get the air moving in circles so fast that even the smallest grease particles are flung out of it through inertia, or what used to be referred to as centrifugal force. Filters like this create more resistance and require more powerful fans capable of handling as much as 5” of static pressure.

Water mist hoods are another way of pulling more grease out of the exhaust effluent. The industry has had hot water wash hoods for some time. These hoods automatically wash the hood and mechanical grease extractors. Companies like Vent Master/Enodis have added cold water mist to these types of hoods. Exhaust entering the hood passes through a wall of cold water that cools it, causing grease particles to solidify and drop into a drainage trough.

One of the drawbacks of water wash hoods is that they have to be hooked up both to water intake and drainage systems in the kitchen, a costly installation prospect. Drains and grease traps also have to be cleaned more frequently and inspected often.

An alternative introduced recently is one that uses UV-C light to extract grease. Hoods using the technology have typical mechanical grease extractors to remove most of the large grease particles. Effluent then passes over special ultraviolet lamps that cause a chemical reaction, converting grease to carbon dust, carbon dioxide and water vapor. The carbon dust that collects on the lamps is easily wiped off with a damp cloth.

A few companies, including Vent Master/Enodis and Halton, already have hoods with UV-C extractors. Others plan to introduce their own versions. Gaylord Industries expects to have one at The NAFEM Show this fall.

Another factor is the grease ductwork itself, and Metal-Fab Inc. has been hard at work on designs that are less prone to grease buildup or at least easier to clean. The new Series 4G duct system appears to score big. It’s round and has no hidden nooks and corners where grease collects and evades cleaning. Also, it’s recently been UL-classified for zero clearance to combustibles.

Putting Out The Fire
Kitchen exhaust systems also are meant to remove some of the radiant heat thrown off by cooking equipment to help keep kitchen temperature more comfortable for employees. Hoods that are designed to capture and contain the effluent plume are also going to be most effective at controlling radiant heat.

“We’re taking advantage of insulation on hoods themselves to slow the temperature transfer to the kitchen, reducing our air-conditioning load in the kitchen,” Fink says. Burger King also had hoods designed with a louvered front shutter to help decrease the amount of heat escaping into the kitchen space. The design saves each unit about one-and-a-half tons of air conditioning annually, Fink says.

Heat, as we said earlier, also can cause fires. All hoods installed over grease-producing cooking equipment are required to have NFPA-approved or UL-listed fire suppression systems. Fire suppression systems must protect the cooking surface, the interior of the hood, grease extractors and ductwork. The three most common types are dry chemical, wet chemical and water systems.

The advantage of water is its cooling ability. Water, however, isn’t good at extinguishing grease fires, since grease and water don’t mix. A new development is a wet chemical agent whose extinguishing capacity is enhanced with water. The agent, PRX from Ansul Inc., quickly extinguishes a grease fire. Water sprayed on top cools the grease, preventing it from igniting, and causes the agent to foam, replenishing the blanket extinguishing the fire.

Eventually, the payoff will be products that are easier to specify and purchase. All the research that has led to both ASHRAE’s new industry standard and innovations coming from manufacturers is applying quantifiable science to the art of kitchen exhaust systems. And that means you’ll soon be able to compare apples to apples when purchasing.

“The industry has made substantial progress in the past 10 years,” says Captive-Aire’s Luddy. “In the next five years you’ll see tremendous strides toward lower cost, higher efficiency systems.”

For help with ventilation, contact the major companies below. For an even more exhaustive list, go to our online Buyers Guide at www.fermag.com.

Acme Engineering & Mfg. Corp.

Air Tech/Enodis

Ansul Inc.

Avtec/Dover Industries

Captive-Aire Systems Inc.

Carroll Mfg.

Exhausto Inc.

Facilitec Corp.

Gaylord Industries Inc.

General Ventilation

Greenheck Fan Corp.

Halton Co.

LDI Mfg.

Loren Cook Co.

Marshall Air Systems Inc.

Melink Corp.

Metal-Fab Inc.

Muckler Industries Inc.

Penn Ventilation

Southern Equipment

Vent Master/Enodis

Veritech Filtration

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