May 2006
Hood Efficiency Matters
By Jennifer Hicks

Got a capture and containment problem? Crank up the exhaust. For years that’s been the typical approach to dealing with ventilation hoods that don’t deal effectively with effluent.

Trouble is, in nine out of 10 cases, adding more exhaust is exactly the wrong way to make a hood work better, says Don Fisher, Fisher-Nickel Inc./PG&E Food Service Technology Center. And more troubling, simply upping the exhaust alone leaves the real source of problems unattended.

Fortunately, says Fisher, the path to significantly increasing hood efficiency is paved with some easy, low- or no-cost solutions for existing hoods, and easy-to-implement design tips for new hoods. And he’s got the research to back up his claims: a recently released study from the American Society of Heating, Refrigerating and Air-Conditioning Engineers called “Effect of Appliance Diversity and Position on Commercial Kitchen Hood Performance.” Its pages are full of startlingly simple fixes that can drive your exhaust flow rates down.

Reviewing the ASHRAE-funded research at the Multiunit Foodservice Equipment Symposium in early February, Fisher laid out several key areas examined by researchers, including overhang, rear gap, side panels and hood mounting height.

“Small changes can make a big difference,” Fisher emphasized, “and your specs for hood placement and design <I>can<I> result in higher operating value.”

Minding The Gap

The first topics—overhang and rear gap—focus on existing hoods and can be addressed for little or no cost. As Fisher reviewed slides showing poor equipment placement under hoods, he said listeners should take a look at how far forward all appliances are sitting under their stores’ hoods. When appliances sit too far forward, the resulting limited overhang can’t capture effluent effectively.

Meanwhile, with a wide rear gap—that space between your equipment and the wall—you’re asking your hood to work harder because it’s pulling in air from behind the appliance as well as the effluent coming off of it.

Using Schlieren imaging in his presentation slides, Fisher showed the group the performance difference between 6” of front overhang over three fryers compared to 18” of overhang over the same fryers. With just 6” of overhang, significant effluent escapes the hood at 240 cfm/ft., but with 18”, the group could clearly see full capture and containment.

“Accomplishing this increase in overhang [with an existing hood] is easier said than done,” Fisher told the MUFES audience of multiunit chain spec/buyers and equipment suppliers. Wherever possible, pushing back equipment will help, he said. “A couple inches of appliance push-back can have an effect, and 18” makes a huge difference.”

Equipment placement can be complicated when designers line up ovens, ranges and fryers at the front and leave an ineffective overhang for some equipment. Thus, at the very least, Fisher recommended not lining up the fryer with the combi oven, which tends to migrate toward the front of the hood.

Meanwhile, take a look at your rear gaps, Fisher said. With equipment pushed back that gap lessens, but another approach is to seal the gap entirely with a strip of steel. When the ASHRAE study looked at this option, the results were remarkable.

As shown in the chart on page 49, when using mixed appliances under 6” of overhang an unsealed rear gap required a flow rate of 510 cfm/ft., while sealing that gap dropped the rate to just 340 cfm/ft. Similarly, under 18” of overhang the sealed rear gap also helped drive down the exhaust rate by 30%.

Panels Working For You

On to side panels. “We’ve all asked, ‘What’s the benefit of going from a 4’ partial side panel to a full side panel or end wall?’ Now we have the data,” Fisher said. The ASHRAE project, he noted, looked at the effects on capture and containment when adding five styles of side panels: full, 4’ x 4’ tapered, 3’ x 3’ tapered, 2’ x 2’ tapered and 1’ x 1’ tapered.

Using multiple configurations of appliances under 10’ wall-canopy hoods, researchers looked at the relationship between side panels and overhang and the benefits of both. In one sequence, fryers were placed beneath hoods with 6”, 12” and 18” of front overhang. As expected, the hoods with no side panels recorded the highest exhaust flow rates no matter what the overhang; the differences between 6”, 12” and 18” of overhang amounted to flow rates of 330, 280 and 240 cfm/ft., respectively. (See chart above.)

By adding tapered 1’ x 1’ side panels, exhaust flow rates came down to 310 cfm/ft. for 6” of overhang, 260 cfm/ft. for 12” of overhang, and 200 cfm/ft. for 18” of overhang. Researchers then placed 2’ x 2’ side panels and watched flow rates drop to 280, 230 and 160 cfm/ft. for the three overhangs.

Interestingly, this is where the significant rate reductions ended, Fisher said. Moving up to tapered 3’ x 3’ and 4’ x 4’ panels did not produce significant rate reductions beyond the 2’ x 2’ results in the 18’ overhang category, at least for the lineup of fryers tested. However, testing other appliances such as underfired broilers showed there was a benefit to using the larger 3’ x 3’ or 4’ x 4’ panels over the smaller 2’ x 2’ panel. When in doubt, specify the largest panel you can.

The conclusion: Something as simple and inexpensive as adding partial side panels to your existing hood can result in up to a 30% reduction in exhaust flow.

“Each time you think side panels aren’t as important as they are, you find out you’re wrong,” Fisher said.

A Little Lower, Please

Next up: hood mounting height. It seems logical enough, but a hood that’s mounted too high just won’t handle effluent the way it’s supposed to. Still, Fisher and others have seen enough hoods mounted too high to know it might be a problem.

So the ASHRAE study looked at capture and containment in a wall-mounted canopy hood placed five different ways over three broilers. The hood measured 10’ long by 4’ deep by 2’ high. To begin tests were done with the hood mounted with 7’ 6”of space from floor to hood and 4’ 6” of distance from appliance to hood. From there, researchers brought down the overall hood height to simultaneously reduce the distance from appliance to hood.

In particular, Fisher zeroed in on the effect of going from 7’ 6” to 6’ 6” in floor-to-hood height when one end broiler of three was operating. After removing that foot of space in hood height—which also meant a 1’ reduction in distance from appliance to hood—the study found a reduction in exhaust flow rate of 600 cfm/ft.

If you do specify a 7’6” mounting height, be sure to place heavy-duty appliances such as broilers in the middle of the line.

At The Drawing Board

Hood height itself is one area where a new spec can improve efficiency. Fisher said the project looked at a wall-mounted canopy with heights of 2’ and 3’. With only the center broiler of three running, a 2’-high hood required an exhaust flow rate of 310 cfm/ft., compared to just 200 cfm/ft. under a 3’-high hood. That’s a 35% reduction in flow rate accomplished by adding 1’ of height to the spec. (See chart above.)

Another tip that emerged from the ASHRAE project: update your specs to add 1’ of depth height to your hood. Adding just 1’ can reduce exhaust flow rate by about 20% depending on overhang and rear clearance, according to the research.

There’s also the issue of four-way diffusers placed near the hood, Fisher said. The intrusive negative impact of high-velocity air blowing on your hood cuts significantly into its efficiency.

And finally, island canopies require the most exhaust and offer little ability to control air flow. For efficiency’s sake, avoid spec’ing single-canopy island hoods, he concluded.

No matter which approach you choose, it’s clear that tightening your specs can help you in both new hood design and troubleshooting. For more on Research Project 1202, contact ASHRAE at 800/527-4723 or Don Fisher at dfisher@fishnick.com.