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 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 on a 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 decrease the length of high-temperature grease ducts.
This is why, such as, 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 need 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 need the fan to work at a higher speed than otherwise necessary to move a specified air measure.
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, added 90-deg turns, and installed dampers will increase 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 of 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?
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