Foodservice Equipment Reports

SPECIAL REPORT: Fine-Tuning Your Air

Air’s free, and there’s plenty of it, right? Well, maybe not in foodservice kitchens. Trying to get the air supply balanced in a commercial kitchen often feels like being in the middle of a Goldilocks adventure. There’s too much air, there’s too little; it’s too hot, it’s too cold….

Until recent years, trying to resolve those issues was more art than science. Which historically left a lot of you trying to sort things out in the field by trial and error—much like sitting in a few chairs, tasting several bowls of porridge, and sleeping in a couple beds to find out which suits you best.

Fortunately, in recent years groups like the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the Commercial Kitchen Ventilation Laboratory, which is operated by Pacific Gas & Electric Co.’s Food Service Technology Center in San Ramon, Calif., have worked hard to inject some science into kitchen ventilation.

And manufacturers have used that science to develop new products that do a better job of achieving the goals of ventilation systems. If you attended The NAFEM Show in February, you may have seen a lot of new products from CKV component manufacturers aimed at bringing down those costs. Most of what’s new is evolutionary, not revolutionary, but every stride manufacturers make toward greater efficiency means lower energy bills, higher productivity and faster payback.

Which is a good thing because there’s a lot of money at stake. Restaurants use four to five times as much energy per square foot as any other commercial building. You’re running equipment that spins your utility meters like roulette wheels in Vegas. Depending on where you’re located, HVAC in restaurants accounts for anywhere from a quarter to almost half your energy load. And guess what? Kitchen ventilation is responsible for as much as 75% of your total HVAC bill.

Comfort And Safety

The goals of CKV are pretty simple. The first is safety. Kitchen ventilation is designed to exhaust heat, grease and smoke from the cooking area, which reduces the possibility of fire. And just in case, most hoods include fire suppression systems—and most codes require them.

Comfort is the other primary objective of good ventilation. Cooler, less humid kitchens are more comfortable to work in, making kitchen staff more energetic, alert and productive.

Front of house benefits from good CKV too. Hoods help clear the air, which prevents grease, smoke and odors from spreading through the kitchen and into the dining room.

Until about 10 years ago, the general thinking was if you moved a large enough volume of air through the kitchen and up the hood, you’d suck out all the heat, smoke and grease and keep the kitchen more comfortable to boot. But that meant hood fans worked overtime to maintain a high cubic ft./min. volume, and that volume created greater demand for makeup air and tempering, putting more strain on the HVAC and/or makeup air units on the roof.

With energy costs climbing, and not about to come down anytime soon, lab researchers and manufacturers started looking into ways to lower cfm rates while still maintaining kitchen safety and comfort. Efforts to make CKV more efficient in the past decade have come in several forms. These include monitoring and controlling air volume, designing hoods that do a better job of capturing and containing effluent at lower exhaust volumes, building more effective grease filtration systems, and integrating front- and back-of-house HVAC.

Air On Demand

The big money required to operate CKV systems goes toward moving air. Hood fans use electricity, and the more air they move, all other things being equal, the bigger your electric bills. What you’ll hear from a lot of folks in the business these days, especially manufacturers, is all about low cfm. The lower, the better, they say. But moving the right amount of air at any given time—depending on what and how much product you’re cooking or even what equipment is on and idling—is more important than simply lowering your CKV system’s cfm rate.

Twenty years ago, variable-speed fans, especially in kitchen hoods, were a rarity. Ten years ago there was perhaps one manufacturer making controls for variable-speed fans in hoods. Now the logical extension of variable-speed fans, demand ventilation, is all the rage, and sophistication has risen significantly.

Demand ventilation systems do exactly what their name suggests—they provide variable ventilation based on the demands of the kitchen. Manufacturers use thermal, infrared or optical sensors to detect heat and smoke. The sensors in turn regulate fan speeds, increasing or decreasing the cfm exhaust rate based on how much effluent your equipment is generating.

Most demand ventilation systems use at least two types of sensors in combination, but each system is different. Temperature sensors, usually mounted in the duct behind the grease filters, measure heat in the exhaust plume and vary fan speed accordingly.

Optical sensors detect smoke and grease in the effluent and increase fan speed when food is actually cooking. These sensors need to be cleaned regularly to function properly. One maker uses a tiny air curtain to keep grease from getting on the sensor lens.

Infrared sensors detect heat at the source—a griddle top or fryer vat, for example. They send signals to the fan controller based on what’s happening with the cooking equipment and adjust the fan speed lower, for example, when frozen product is lowered into a fryer vat. As the product cooks, fan speed increases.

Fan Of New Fans

Another fan-related development, the shift from belt- to direct-drive fans, has improved energy efficiency too, by about 10%. And now that most fan motors are electronically commutated motors, they’re capable of infinite variability; controllers can send signals directly to the motor to vary speed.

The systems can save as much as 50% in energy costs to run hood exhaust fans. Even better, the lower the air volume going up your hood, the less makeup air you have to introduce into the kitchen. Since that makeup air is generally tempered, and you’re tempering less, you end up saving a ton more in HVAC costs.

About five years ago, many municipalities began updating their mechanical codes to come in line with the then-updated ICC Code, which had added a requirement mandating an interlock to automatically activate fans any time cooking equipment was on. Today, that means in many places your hood exhaust fan turns on any time cooks fire up the line. With demand ventilation, sophisticated computer controls decide what cfm rate is required to safely remove effluent, which makes inspectors happy. And since fan speeds increase only when the system senses something cooking, you save money.

Aside from the operating savings, two other big benefits of demand ventilation are increased kitchen comfort and reduced kitchen noise. Since the volume of air moving through the kitchen is based on the heat and effluent from the cooking, the hotter the line the more air being exhausted up the hood and replaced with tempered air from somewhere else, keeping staff cooler. One system even sends a signal not only to the exhaust hood fan, but to the supply fan (whether a makeup air unit, or MAU, or main HVAC) to help balance the air throughout the restaurant.

Noise abatement is another advantage of these sophisticated demand systems. A noisy kitchen actually raises stress levels, and productivity can suffer. With demand ventilation, fans run at noisy high speeds only when necessary. When the line is idling and cooks are prepping and doing other tasks, kitchens stay quieter.

In the past few years, more manufacturers have jumped into the market for demand ventilation controls. Today, both controls and sensors are more sophisticated than they were even three years ago when we last wrote about kitchen ventilation. And communications modules also give you far more options in the ways in which you can monitor the system and its energy use.

Capture And Containment

Another approach manufacturers have taken to reducing cfm requirements is through more effective and efficient hood design. The advent of Schlieren photography, which allows visual imaging not just of effluent but of heat flow, has been invaluable in this effort. Now, hood designers can literally see what’s working, and how, and what isn’t.

With measuring technology in place, standardized testing became possible. And with the help of ASTM’s F1704-05 Standard Test Method for Capture and Containment Performance of Commercial Kitchen Exhaust Ventilation Systems, engineers have been able to refine hood designs to improve their effectiveness.

The result has been rapid improvements in sizing and shaping. Schlieren photography now shows the traditional rectangular boxes hung over cooking lines create a lot of turbulence. That turbulence impedes flow, and the exhaust fan is left to try to overpower the turbulence with a higher exhaust rate. Which increases the turbulence in a vicious circle.

Newer designs can address all that. Rounded lips, careful placement in relationship to walls and air sources, and even adding something as simple as side panels to a hood can greatly improve capture and containment and bring exhaust rates down.

Several years ago, one manufacturer introduced a hood with an arched interior surface that directs the plume into the filters and exhaust stream. More recently, another manufacturer introduced a hood with passive capture and containment design features using principles such as the Bernoulli Effect to draw rising heat toward the back wall and direct it upward to the grease extractors.

The most common active capture and containment measure is some sort of air curtain in front of the hood. Air (often makeup air) is typically directed down in front of the lip of the hood through a perforated plenum. The downdraft helps contain the effluent plume within the hood. One manufacturer has added the same principle to side panels on some of its hoods.

Getting The Grease Out

And then there’s the matter of grease. CKV systems are designed to remove grease from your kitchen exhaust before sending it up the stack. One reason for this, obviously, is to reduce the potential for grease fires in your ductwork. Another reason is that more municipalities are enforcing strict limitations on what you can exhaust into the outside air. California, especially, has enacted pretty tight air quality regulations in several areas of the state, a trend that will grow.

Cooking effluent is made up of grease and smoke particles of all sizes, and when it comes to grease extraction, you have to sweat the small stuff. The Bay Area Air Quality Management District in San Francisco, for example, classifies particulate emissions in three categories: coarse (dust and so forth), PM 10 (particles 10 microns or smaller), and PM 2.5 (particles 2.5 microns and smaller). Commercial cooking, according to BAAQMD, accounts for about 7% of the PM 10 in San Francisco.

Most mechanical filters—baffles, etc.—remove particles 10 microns or larger in size, and do a pretty decent job on grease particles 5 microns or larger. Particles in the 2.5- to 5-micron range and grease vapor are tougher to extract. The lower cfm of new demand ventilation hoods makes the task that much more difficult.

Manufacturers have been up to the task, however. A big help has been an ASTM test method (F2519-05 Standard Test Method for Grease Particle Capture Efficiency of Commercial Kitchen Filters and Extractors) that manufacturers can use to determine how efficient their filters truly are and for which size particles. The test method, developed by the FSTC and the University of Minnesota, also lets you compare apples to apples when it comes to evaluating the effectiveness of filters.

Most ventilation systems these days use a couple of different types of filters in combination to remove as much “stuff” from cooking exhaust as possible. The front line of defense is a simple baffle filter, a mechanical filter that stops the big particles, but not much else. That may be all you need if you have a light-duty cook line (ovens, steamers, ranges and food warmers). But it won’t cut it if you generate heavy-duty grease.

Into The Vortex

Cyclonic filters have become pretty standard as a preferred mechanical filter. These are designed in a way that spins and accelerates the exhaust. Centrifugal force separates grease particles from the airstream and deposits them on side walls or baffles. Newer versions of this type of filter improve on the principle to compensate for lower fan speeds.

Another type of mechanical filter uses a different filter medium to capture smaller exhaust particulate. Some makers use a ceramic bead bed in these multi-stage filters to extract grease. The filter combinations extract 95% of grease particles above 4 microns in size in one case, and 80% of particles 1 micron or larger in another. The designs also don't cause as large a drop in static pressure as some other filters.

The disadvantage of these filters is that they sometimes require cleaning more frequently (e.g., once a shift vs. once a day), and are more difficult to clean than most other mechanical filters.

The use of ultraviolet light in multi-stage filtration has grown in acceptance. UV-C bulbs generate ozone, which breaks down and transforms grease into small dust molecules that don’t stick to ductwork.

Most UV filtration systems situate the UV bulbs behind the mechanical filters. The problem is that the bulbs are in the airstream, so any grease or vapor that’s “trapped” by the ozone ends up collecting on the bulbs as well as the surrounding ductwork. When the bulbs become coated with dust, they are ineffective.

At least one manufacturer has designed its UV-light system with easy access to the bulbs through a door in the front of the hood. That encourages frequent cleaning to keep the bulbs operating efficiently.

A new product released at The NAFEM Show generates ozone with a UV-C bulb in a unit that can be mounted almost anywhere. That keeps the bulb out of the airstream so it doesn’t need cleaning. The manufacturer says the unit reduced grease and odor by 50% to 70% in field tests. When combined with a good mechanical filter like other UV-light systems, the unit can help prolong the life of fan motors and reduce the frequency of duct cleaning.

Easy On The Makeup

Nature, it’s said, abhors a vacuum. CKV systems are no exception. Air that goes out, or up the stack, must be replaced from somewhere else. For years, figuring out how to supply makeup air has confounded designers and engineers.

The big questions? How to introduce makeup air—floor diffusers, four-way ceiling diffusers, perforated supply plenums, etc.—and where to introduce it—near the hood, far away, etc.—have a big impact on hood performance as well as employee comfort. Tempered air may have a big impact on your HVAC bills.

A lot of people in the industry have long argued that since the hood is where air is exhausted, it makes most sense that the hood—or nearby—is the best place to introduce makeup air. Their logic is that the makeup air doesn’t have to be tempered, since it’s going right into the hood and up the stack anyway, and if properly introduced—through a perforated plenum, for example—it can actually help capture and contain cooking effluent inside the hood.

Often, however, makeup air introduced near the hood is hard to balance, can create turbulence in the effluent plume instead of directing it into the hood, and may be uncomfortable for line cooks if it’s not tempered.

Recently, manufacturers have offered a couple of different solutions to these problems. One, for example, offers dual perforated supply plenums. One, mounted directly in front of the hood, supplies dedicated makeup air at low velocity, which the maker says aids capture and containment. Another narrower plenum mounts in front of the first and introduces HVAC air to the cooking line to keep cooks comfortable.

Another solution embraced by manufacturers is a static pressure sensor mounted in the duct behind the filters. The sensor controls makeup air supply independently from the exhaust fan speed, which helps ensure pressure equalization, balancing what goes out with what comes in. You still have to worry about how and where you introduce that makeup air to prevent turbulence in the effluent plume.

A more radical approach one manufacturer introduced a few years back has been adopted by at least one other manufacturer, and may be more widely used in the future. The idea is to eliminate makeup air entirely, or at least the MAU, and replace the exhausted air with fully conditioned air from your HVAC rooftop unit, or RTU.

The idea is that MAUs are designed to replace air going up the hood, so that even if it’s tempered—either heated or cooled—it’s still outside air. Since kitchens tend to get humid as well as warm, MAUs generally don’t make kitchen environments more comfortable.

It’s also more difficult to remove moisture from air once it’s in a space that’s conditioned than it is to condition outside air before it enters the space. Since the kitchen needs to replace air exhausted by the hood, why not replace that air with not just tempered, but conditioned air?

What these manufacturers have done is create direct outdoor air systems that replace kitchen air exhausted up the hood with 100% conditioned air at low velocity from the direct outside air unit. Using a DOA can reduce total airflow in some cases more than 45% and create a more comfortable environment.

While initial cost of these systems is more than traditional CKV packages, installation costs are the same, and quick payback make them cost-competitive.

Ultimately, while CKV is only a portion of total building HVAC, it is an integral part. To design the most effective and energy-efficient system possible, you need to look at the big picture and integrate your approach.

With the growing sophistication in equipment and controls, energy-efficient CKV systems are helping more operators save money and even qualify for LEED certification, enhancing their green reputations. And that’s not just a lot of hot air.

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