The costs and risks of damper-based controls in kitchen ventilation
There is only one practical application for installing modulating dampers in high-temperature grease ducts over kitchen hoods serving commercial cooking equipment: A high-rise building in an urban setting with a kitchen having multiple hoods totaling over 10,000 cfm that are located on a lower level where there is no other way to exhaust the effluent other than design a single chase and duct to a single fan located on a much higher level.
It is typically more cost-effective on a construction and operating basis, as well as less risk-prone from a liability standpoint, to minimize the length of high-temperature grease ducts. This is why, for example, most hotel, hospital, and other large commercial kitchens are designed as part of a single-floor building and only connected to a multi-story building. This design eliminates the first cost of installing a duct 2, 5, 10, or more floors, the waste of valuable space in a high-rise, and the risk of extending a potential fire hazard any further than necessary. And it facilitates a dedicated fan per hood design without the need for dampers—the benefits of 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 otherwise designed 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 of the building beyond the kitchen. Because grease is a combustible substance, this poses a potential risk. This is why codes require regular cleaning of kitchen hood, ducts, and fans. High-rise buildings with long ducts and obstructions are inherently more 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 operate at a higher speed than otherwise necessary to move a specified air quantity. Given that one purpose of these dampers is to purportedly save fan energy, it is a step in the wrong direction. In fact, if the cooking load is fairly steady and/or the controls are not highly reliable, the long ducts, additional 90-deg turns, and installed dampers will increase rather than decrease overall energy usage.
Maintenance issues: Modulating dampers are constantly cycling and have a limited number of cycles before they will eventually fail. Even a million-cycle rating could be limited to a couple years of operation depending on the variability of cooking operations and the effect of the heat, grease, and quarterly cleanings on their overall life. No architect or engineer wants to have 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 serious pause before designing modulating dampers into high-temperature grease ducts in high-rise buildings. Yes, the dampers might be allowed by code and approved by a listing organization, but that does not eliminate the three problems mentioned 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 a fire, in which case they go 100% closed. They are specifically designed for applications like high-rise buildings to contain any smoke 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 an actuator, which is subject to mechanical maintenance in a way that gravity fire dampers are not, but it is far less prone to repair and 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 and dense 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 them less prone to repair and maintenance issues. Plus, they can be installed in a more accessible and serviceable location such as a 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. And given the notorious lack of preventive maintenance in the food service industry—especially for equipment above the ceiling that you cannot readily see or access—this again should give an engineer pause. Airflow-proving switches make sense in many applications, but how long will pressure sensors last in grease ducts before becoming clogged with either grease or water? Is anyone monitoring this?
Above and beyond the concern of installing modulating dampers inside high-temperature grease ducts, there is the concern of how these dampers are actually controlled to supposedly save energy. Again, a grease duct is not a good place to install pressure sensors. But how else do you control the dampers and the VFD? And what happens when the 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, this should take care of this particular problem. But will it? The industry needs to see case studies and testimonials showing proven energy savings before it assumes this to be correct. It would seem that even with a UV system, there will still be particulate matter that can get inside the duct. Besides, what prevents duct cleaners from spraying these sensors with hot water 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 hood 1 suddenly sees cooking activity, and then hood 5 sees cooking activity 3 seconds later while hood 2 no longer sees cooking activity and hood 7 starts seeing a lot of heat but no cooking, the dampers and VFD would be in a constant hunting mode. The variables are too many and happen too fast for the dampers and single VFD to be able to respond appropriately and reliably for all hoods. Moreover, most manufacturers of damper-based controls have only one setting between open and closed, making the real-time hood-air balance far less precise and energy-efficient than a fan per hood design with VFDs capable of modulating the fans speeds infinitely in the 30%-100% range.
Of course, if optimal—not just minimal—energy savings can be validated through independent research, proven case studies, and customer testimonials, then let objective data be your final guide. But performance must be proven over time and not just upon initial start-up. Wear and tear over 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 the future. Design-in a level of critical redundancy with at least two and possibly more exhaust fans so that no single fan can “bring down” an entire kitchen. A fan has several moving parts—motor, fan, and belt—not to mention any modulating dampers inside the duct, and one of these will inevitably fail at some point. Don’t let a catastrophic failure happen as a result of your design with only one fan, or with a broken modulating damper over a char-broiler. The last thing you want is a call at midnight from an angry customer—or worse, from its attorney. Possible smoke and fire inside a large public space with lots of people is not a good 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 per hood 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 speed control such that the fans can modulate between 30% and 100% rather than just between 80% and 100%. Combined, these factors 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 demand ventilation controls for commercial kitchens.