10/28/2014 The costs and risks of damper-based controls in kitchen ventilation | Consulting-Specifying Engineer

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04/08/2014

The costs and risks of damper-based controls in kitchen ventilationEngineers must think in terms of total safety and optimal energy efficiency when designing kitchen ventilation systems of thefuture.Stephen K. Melink, PE, Melink Corp., Milford, Ohio

There is only one practical application for installingmodulating dampers in high-temperature grease ducts overkitchen hoods serving commercial cooking equipment: Ahigh-rise building in an urban setting with a kitchen havingmultiple hoods totaling over 10,000 cfm that are located on alower level where there is no other way to exhaust theeffluent other than design a single chase and duct to asingle fan located on a much higher level.

It is typically more cost-effective on a construction andoperating basis, as well as less risk-prone from a liabilitystandpoint, to minimize the length of high-temperaturegrease ducts. This is why, for example, most hotel, hospital,and other large commercial kitchens are designed as part ofa single-floor building and only connected to a multi-storybuilding. This design eliminates the first cost of installing aduct 2, 5, 10, or more floors, the waste of valuable space ina high-rise, and the risk of extending a potential fire hazardany further than necessary. And it facilitates a dedicated fanper hood design without the need for dampers—the benefitsof which will be explained later in this article.

But even in a high-rise building, purposely designing “obstructions” in a long, high-temperature grease duct that is otherwisedesigned to convey heat, smoke, and grease vapors out and away from the building is problematic for three reasons:

Liability concerns: The longer the high-temperature grease duct, the greater the probability of distributing grease into areas ofthe building beyond the kitchen. Because grease is a combustible substance, this poses a potential risk. This is why codesrequire regular cleaning of kitchen hood, ducts, and fans. High-rise buildings with long ducts and obstructions are inherentlymore exposed from a liability standpoint than are single-story buildings with short ducts and no obstructions.

Energy penalties: Long ducts with multiple 90-deg turns and dampers add resistance to airflow and require the fan to operateat a higher speed than otherwise necessary to move a specified air quantity. Given that one purpose of these dampers is topurportedly save fan energy, it is a step in the wrong direction. In fact, if the cooking load is fairly steady and/or the controls arenot highly reliable, the long ducts, additional 90-deg turns, and installed dampers will increase rather than decrease overallenergy usage.

Maintenance issues: Modulating dampers are constantly cycling and have a limited number of cycles before they willeventually fail. Even a million-cycle rating could be limited to a couple years of operation depending on the variability of cookingoperations and the effect of the heat, grease, and quarterly cleanings on their overall life. No architect or engineer wants tohave a damper fail in a high-rise building that is serving hundreds of employees, patients, and/or visitors. Who will inspect,repair, and replace these relatively inaccessible devices before such an occurrence?

Therefore, engineers and consultants should give seriouspause before designing modulating dampers into high-temperature grease ducts in high-rise buildings. Yes, thedampers might be allowed by code and approved by a listingorganization, but that does not eliminate the three problemsmentioned above—and their associated costs and risks.

Fire dampers are different than modulating dampers. Fire dampers are designed to be 100% open all the time unless there is afire, in which case they go 100% closed. They are specifically designed for applications like high-rise buildings to contain any

10/28/2014 The costs and risks of damper-based controls in kitchen ventilation | Consulting-Specifying Engineer

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fire, in which case they go 100% closed. They are specifically designed for applications like high-rise buildings to contain anysmoke and fire rather than allow them to spread to other floors. And they typically rely on simple gravity and a fusible link—which easily can be replaced on a regular schedule—to provide fail-safe performance. Some fire dampers may use anactuator, which is subject to mechanical maintenance in a way that gravity fire dampers are not, but it is far less prone to repairand maintenance issues than constantly moving modulating dampers.

Motorized back-draft dampers are also different than modulating dampers because they are either 100% opened or closed—and cycle only up to two times a day. They are specifically designed for applications in the north where extremely cold anddense outside air can “drop” through the duct and affect indoor temperatures, which in turn can increase the heating load.Fortunately, they only operate before cooking starts and after cooking ends, and do not require the use of air pressure sensors.And though this is a subsystem with moving parts, the relatively minimal cycling and simple open/close sequence makes themless prone to repair and maintenance issues. Plus, they can be installed in a more accessible and serviceable location such asa mezzanine or the outlet of the fan.

Modulating dampers are constantly moving and can cycle over a thousand times per day depending on the cooking load.Therefore, at the very least, they require a much more robust maintenance schedule than the above-mentioned dampers. Andgiven the notorious lack of preventive maintenance in the food service industry—especially for equipment above the ceiling thatyou cannot readily see or access—this again should give an engineer pause. Airflow-proving switches make sense in manyapplications, but how long will pressure sensors last ingrease ducts before becoming clogged with either grease orwater? Is anyone monitoring this?

Above and beyond the concern of installing modulatingdampers inside high-temperature grease ducts, there is theconcern of how these dampers are actually controlled tosupposedly save energy. Again, a grease duct is not a goodplace to install pressure sensors. But how else do youcontrol the dampers and the VFD? And what happens whenthe pressure sensor becomes fouled? Who will clean, repair,and/or replace these sensors on a regular basis?

It has been said that if one installs an ultraviolet system, thisshould take care of this particular problem. But will it? Theindustry needs to see case studies and testimonials showingproven energy savings before it assumes this to be correct.It would seem that even with a UV system, there will still beparticulate matter that can get inside the duct. Besides, whatprevents duct cleaners from spraying these sensors with hotwater or steam, even if on a reduced cleaning schedule?

Lastly, there is the concern that all these moving dampers will only cause hood air imbalance problems. In other words, if hood1 suddenly sees cooking activity, and then hood 5 sees cooking activity 3 seconds later while hood 2 no longer sees cookingactivity and hood 7 starts seeing a lot of heat but no cooking, the dampers and VFD would be in a constant hunting mode. Thevariables are too many and happen too fast for the dampers and single VFD to be able to respond appropriately and reliably forall hoods. Moreover, most manufacturers of damper-based controls have only one setting between open and closed, makingthe real-time hood-air balance far less precise and energy-efficient than a fan per hood design with VFDs capable ofmodulating the fans speeds infinitely in the 30%-100% range.

Of course, if optimal—not just minimal—energy savings can be validatedthrough independent research, proven case studies, and customertestimonials, then let objective data be your final guide. But performancemust be proven over time and not just upon initial start-up. Wear and tearover the months and years will reveal the truth.

Engineers should think in terms of total safety and optimal energy efficiency when designing kitchen ventilation systems of thefuture. Design-in a level of critical redundancy with at least two and possibly more exhaust fans so that no single fan can “bringdown” an entire kitchen. A fan has several moving parts—motor, fan, and belt—not to mention any modulating dampers insidethe duct, and one of these will inevitably fail at some point. Don’t let a catastrophic failure happen as a result of your designwith only one fan, or with a broken modulating damper over a char-broiler. The last thing you want is a call at midnight from anangry customer—or worse, from its attorney. Possible smoke and fire inside a large public space with lots of people is not agood combination. Multiple fans without maintenance-prone modulating dampers help minimize this risk.

Engineers should also design-in optimal energy efficiency by controlling each hood/fan independently of the others. A fan perhood design minimizes duct runs, eliminates multiple 90-deg turns, and minimizes the temptation to install modulating dampers—all three of which reduce static pressure losses and ensure superior energy savings. This also allows more infinite fan speedcontrol such that the fans can modulate between 30% and 100% rather than just between 80% and 100%. Combined, thesefactors can provide thousands of dollars of additional energy savings per year.

In the end, great design is about maximizing long-term savings and minimizing long-term costs and risks.

Stephen K. Melink is founder and president of Melink Corp. He is a NEBB certified professional, and the inventor of demandventilation controls for commercial kitchens.

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10/28/2014 The costs and risks of damper-based controls in kitchen ventilation | Consulting-Specifying Engineer

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BRUNO , PA, United States, 05/28/14 12:13 PM:

EXCELLENT INFORMING ARTICLE