Hey, it’s your birthday! Go ahead and inhale those candles out! What? You don’t suck the flame off the candle? Have you ever tried?
Yes, it’s a silly anecdote and a very effective one (thanks, Dr. Livchak!). As most of us know, we blow out our candles. The physics works well and accomplishes the job quickly – so that we can get our magic wish, of course. This analogy does help to explain a regularly violated kitchen ventilation topic which comes up almost any time there is a performance issue with the capture and containment of hoods. When smoke makes it outside the hood, it is automatically assumed that the hood isn’t “sucking” enough and needs an adjustment.
A quick primer on the operation of a commercial kitchen hood, so we are all on the same page. Think about cooking burgers on a grill or charbroiler, which produces a plume of effluent and heat, rising upward. The hood canopy is an upside-down box open to this plume and capturing it inside and extracting some of it through the grease filters as it passes them. At times during the cooking, bursts of heat and effluent can be too much for the filters to handle when the smoke makes its first visit by them, plus some of this plume doesn’t even get near the filters, so the box part of the hood contains the leftovers until they can be sucked through the filters and removed.
If you’ve followed these articles thus far, you might remember the article on keeping your kitchen airflows balanced. Often, the air we are bringing into the kitchen (to replace the air that is being removed from the kitchen by the hood) can be sabotaging the performance of the hood. How? Glad you asked …
When replacement air is brought into a kitchen, it needs to be distributed through louvers, vents, diffusers, plenums, etc. It’s not good practice and would be another article, to just “dump” the air in the space through a duct with nothing on the end. The choice of where to introduce this air and how to introduce it is critical. If we have velocities that exceed 50-75 fpm at the hood lip, it’s a virtual guarantee the hood won’t work properly – even at extremely high exhaust rates. We need to step back, review how the make-up air is being introduced, and determine if this could be a cause for poor performance.
If the smoky haze of effluent and heat emanating off the cooking surface doesn’t find itself going into this “box” of a hood above it to begin with and “leans” or bends in one direction or another and finds its way outside that perimeter, then we have an issue. Increasing the airflow through the hood will not bring that smoke, grease, and heat magically into a vertical plume and rise into the hood box. It will frustrate the situation further since it’s likely due to make-up air introduction or uneven space pressurization (also make-up air-related) creating the problem.
The kitchen hood enemy number one is four-way diffusers. You know, those square 24” x 24” diffusers you see everywhere –, especially in drop ceilings. These things are great for most general HVAC work and are inexpensive, they just don’t belong in a kitchen. They were developed for office buildings! If they are in a kitchen, they need to be at least 15 feet away from any hoods and should actually be changed to a three, two- or one-way diffuser that blows away from the hood. If the air that they discharge is blown toward the hood, there is the risk that they will blow the smoky thermal plume right out from under the hood or create a low-pressure zone just outside the hood lip, which competes with the hood for the plume’s attention. Also, realize that when the exhaust is increased to try and rectify a capture and containment issue that stems from a make-up air problem like this, you need more make-up air! If you need more make-up air, the problem gets worse, and a seemingly endless cycle of increasing exhaust and make-up air flows ensues. It’s akin to chasing your tail.
To add insult to injury, the hood performance may be adequate one day and poor the next. Why would that happen? Well, air at different temperatures behaves differently. In the summer, warm air will tend to stay aloft in the space and not drop toward the floor. In the winter, cooler air will tend to do the opposite, moving toward the floor and not staying near the ceiling. At certain temperature points, the air jet leaving the diffusers may bother the hood performance more than other times, since it’s like pointing a fan at the cooking process and moving it up and down, eventually hitting the sweet spot where it really disrupts everything. Compound this with varying airflows for exhaust and make-up air due to demand controlled kitchen ventilation, which moves more air when cooking is occurring and slows the air down to save energy and noise when there isn’t any cooking, and real performance problems can happen seemingly minute by minute.
Many hood manufacturers offer their own methods of introducing make-up air, usually through low-velocity perforated panels on, or in front of, their hoods. They prefer to help head this issue off at the pass and provide a way to introduce the space-balancing make-up air without disruption to the capture and containment process. Some manufacturers have alternative specialty diffusers for use in the kitchen area, as well, which minimize the impact of the make-up air introduction.
We can’t go through every scenario in this supposed-to-be-short article, so I’ll leave with this thought. Make-up air is as important to the hood performance as is the hood’s exhaust rate. It is required to maintain balance and comfort in the space and is not as easy to understand the impact of. Partner with a good hood manufacturer and HVAC engineer to ensure your project or problem job is quickly and properly understood.
Regional Sales Manager, Halton Company
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Our recommended next article would be: What impact does a balanced building have on our lives?
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