SHORT REPORT: Ventilation Tips & Tricks

In one store, your exhaust hood works great. In another, a hood that seems to be virtually identical is having problems. What’s the problem, and maybe more to the point, what’s the solution?

“Nine times out of 10, it is NOT more exhaust,” says Don Fisher, of Fisher-Nickel Inc., operator of Pacific Gas & Electric Co.’s Food Service Technology Center in San Ramon, Calif. In fact, Fisher notes, many hoods are pumping way more air than should be necessary. They’re using energy to overcome problems that very often are easy to eliminate or reduce. And don’t forget, you’re not just using energy in the exhaust fan. You’re using energy to move and temper the makeup air as well. You’re spending a lot of money.

Fisher, whose team has been instrumental in developing every one of the 30-plus ASTM Standardized Test Methods for Foodservice Equipment, is known throughout the industry for his experience in efficiency testing all kinds of equipment. But his roots are in ventilation.

And he took his “Ventilation Tips & Tricks” presentation to noncommercial foodservice operators attending FER magazine’s Multiunit Foodservice Equipment Symposium last June at the Barton Creek Resort & Spa in Austin, Texas. He covered an extraordinary range of symptoms and solutions during the hour-long session. Included here are just a few of the points Fisher addressed:

What Type Hood Do You Need?

Each style of hood has its own distinctive characteristics and tradeoffs. And each type can be tweaked for big improvements. Not that you can design your whole operation around the hood, but being aware can help you make decisions.

To illustrate, Fisher talked about running two 3′ charbroilers under an 8′ hood of varying types. Under standard lab conditions, a standard single-island canopy hood requires 5,100 cfm of exhaust flow for capture and containment. In comparison, a standard wall canopy with a common, but insufficient, 6″ overhang requires 4,100. An engineered wall canopy with a bigger 18″ overhang and the addition of some side panels requires just 2,400. And an engineered proximity hood, also known as a backshelf type, requires just 1,250 cfm to capture and contain the same two charbroilers. These are lab conditions, good for comparison. Look at that spread, and look at the spread in operating dollars.

Short List Of Tips & Tricks

One of the key mistakes often made in specifying, Fisher said, is sizing the hood too small. Metal costs money, and it’s tempting to go for smaller hoods. But the money you save can quickly disappear in increased exhaust costs.

1. Turbulence is trouble. One of the overriding thoughts here is that anything that disrupts the smooth movement of effluent upward into the hood is a problem. Opening and closing doors create drafts, and doors near the hood can create bigger drafts near the hood. For the same reason, you don’t want diffusers near the hood, and you definitely don’t want them discharging air toward the hood, so no four-way diffusers nearby. If you have your choice in sourcing your makeup air, get it from a safe distance, at a low velocity. As one of Fisher’s slides so simply put it, “Hoods do not like high velocity.” You don’t want to “blow” makeup volume at the hood.  

2. Bigger hoods, deeper and taller, will do a better job with lower cfm. Don’t shortchange yourself on the overhangs. What happens when you open the door to the oven or the combi? Does the heat or steam escape outside the overhang? And then what? Your kitchen is unpleasant. You pay for HVAC to counteract the heat and humidity, not to mention the effluent.

3. Push back your equipment. If your equipment is near a wall, minimize the gap. That space between the equipment and the wall is an alleyway for drafts and disruption. You don’t want to give the plume a way to meander off track. Pushing back the equipment has another advantage, too—in effect it gives you some more overhang.

4. In fact, consider sealing that rear gap with something simple like a sheetmetal panel that prevents air from moving upward through that gap.

5. Lower the hood where possible. Get it closer to where the effluent is emanating from. Proximity hoods, aptly named, give you advantages here.

6. Full or partial side panels, end panels and end walls help “steer” the effluent upward a couple different ways. They prevent drafts interfering with the plume, and in effect they extend the hood downward, increasing its proximity.

7. Be careful about your equipment placement. Place heavy duty equipment such as broilers in the middle of the line below the hood. You want the biggest amount of effluent where it has the straightest shot toward the exhaust and it’s least likely to stray beyond capture. Equipment that produces lesser effluent, like ovens, can go toward the ends.

This handful of tips doesn’t begin to cover the Fisher presentation, but it’s a start. Use this checklist, and you’ll be on your way toward improvements.