Hood Fight
Comments (29)
Robert Brown
6 years agoRelated Discussions
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opaone
6 years agoRobert Brown
6 years agolast modified: 6 years agokaseki
6 years agoI did not look at the actual designs of the two candidate hoods -- my bad; my comment on maximizing depth was directed at aperture. I evidently assumed too much about ModernAire's hood design.
Generally, for reasons of efficiency in various respects, residential hoods that roughly approximate commercial hood architecture will be most effective. One commercial approach to aiding capture when hood width is limited is to use "side curtains," also called "side panels" or "end skirts." These could be provided with a custom residential hood.
The curtains do not need to drop all the way to the cooktop if drafts are minimal, rather they only have to intercept the main effluent plume -- nominally a 10-degree from vertical truncated inverted cone aligned with the pan edges.
(from California Energy Commission Design Guide: Improving Commercial Kitchen
Ventilation System Performance, updated 11.20.02)If greasy cooking is expected to rarely occur, then removable skirts might provide both capture when needed and more open window view when not. Otherwise, greasy cooking may have to be limited to the central burners where the effluent cone is fully captured.
To be fair, it is difficult to fully analyze what a given hood can do. Normally, we depend on the velocity of the rising effluent plumes to bring them to the hood aperture where the flow (if adequate) will bring them to and through the baffles. Suppose that one significantly exceeded the 90 CFM/sq.ft. actual flow rate rule of thumb and evaluated, via computational fluid dynamics, the actual flow of the ventilation air rising to the hood versus that from the burners and hot cooking surfaces. The presence of the counter top will tend to align the ventilation flow inward near the burners, and this will provide some modest influence on the horizontal components of the plume elements' flow vectors as the two flows merge. This behavior will have the effect of slightly narrowing the plume below the hood edges. Using even a short hood side skirt could be helpful in such a case. The cost of depending on this is noisier ventilation.
Robert Brown
6 years agolast modified: 6 years agoopaone
6 years agoRobert Brown
6 years agokaseki
6 years agoRe: parametric hood: This link showed up near the top of a duck-duck-go search: http://ths.gardenweb.com/discussions/2329631/parametric-vent-hood
The link in that thread doesn't work, so one might have to search for the hood that was noted in the question. I suspect my comment in that thread still applies to similar experiments in how to build a good looking, but perhaps non-performing hood. Maybe the hood was designed using parametric CAD software.
Commercial hoods are optimized for efficient capture and containment, but even they depend on a correct balance of aperture and flow rate. In the schlieren images below, the second shows adequate flow rate, the first slightly inadequate flow rate. Imagine attempting to capture that rising hot flow with an edge slotted "hood." Although the images are of hot air flow off of a commercial griddle, hot greasy pans behave with similar characteristics.
kaseki
6 years agolast modified: 6 years agokaseki
6 years agolast modified: 6 years agoRobert Brown
6 years agoRobert Brown
6 years agoRobert Brown
6 years ago- PRO
Trevor Lawson (Eurostoves Inc)
6 years ago 
plllog
6 years agoRobert Brown
6 years agokaseki
6 years agoRobert Brown
6 years agohomechef59
6 years agolast modified: 6 years agoRobert Brown
6 years agolast modified: 6 years agoYep. It’s too bad I can’t put in a 42” hood. I’d have to trim the window molding. That’s not happening. I’d trim it 1.5” in from the top down 14” and the hood would sit right inside.
kaseki
6 years agoRobert Brown
6 years agolast modified: 6 years agokaseki
6 years agoRobert Brown
6 years agolast modified: 6 years agoopaone
6 years agolast modified: 6 years ago- PRO
Trevor Lawson (Eurostoves Inc)
6 years ago kaseki
6 years agolast modified: 6 years agoIt is perhaps an aspect of full disclosure to note that if I am not criticizing a hood's form, I have assumed that it has the general characteristics of a commercial hood as approximated by the usual manufacturers for residential applications. This means that I expect the aperture being analyzed to be relatively unblocked, and that unless otherwise noted, the filtration is by baffle filters.
Further, I didn't look up the ModernAire hood that the OP first listed. Having done so just now, I cannot determine from the spec sheet what the aperture plan view is, hence, I can't criticize it for any blockages I don't know about. Anyone having a photo or drawing of the aperture view of this hood, please feel free to include it in this thread.
I think the phrasing of opaone: " ... minimal containment area of the Modernaire hood ..." may be referring to the capture volume (volume before the baffles). If so, then I would argue that except in dynamic cases where a large volume of greasy effluent is released at once from some large cooking vessel, the static capture and containment rate of a hood depends much more on its flow rate relative the the effluent up velocity (up to 1.2 m/s for gas cooking) and not so much on capture volume, assuming that the hood aperture overlaps the cooking pans by enough to capture most of the expanding plumes.
In commercial hoods, one may see that the actual area of the baffles is often much less that that of the hood aperture, and that a tapering hood inner structure mates the hood entry aperture with the baffle section. (Also, the baffles are required to be steeper than 45 degrees to the horizontal.) In such cases, the effluent flow is being concentrated, and the specific flow rate (CFM/sq.ft.) has to be higher at the baffles to ensure containment. Plume reflections on the way to the baffles as well as at the top of the baffles can cause turbulence that the large capture volume helps constrain as opaone has noted.
Where the baffles mostly fill the aperture area and have nearly the same specific flow rate as the aperture, I don't believe the capture volume is as important. My hood captures and contains adequately operating at roughly the desirable 90 CFM/sq.ft. specific flow rate, when induction wok cooking at the edge of the hood, or searing more centrally.
If the ModernAire baffles are horizontal and flush with the aperture, then I will have to rely on Trevor's observations, because in such a case with no capture volume, there could be a tendency for the effluent to slide in the direction of the baffles toward the front edge without being completely contained by the baffle gaps. However, CFD by intuition is fraught with risk, so this is only a suspicion.
kas
kaseki
6 years agolast modified: 6 years ago
This photo from kitchensource.com appears to be the 24-inch version of the PS1018. I guess we need to know what the aperture depth is from the light bar to the rear blockage for the 27-inch version. For the Wolf/Independent hood pictured in my message above, the clear aperture front to back is 26 inches.
With respect to the rear blockage at the wall, this is usually not a problem with cabinets, because the reflected plume components will be entrained in the main flow except at the edges where the cabinets perform the function of reflection back into the flow stream. With only window frames, there might be spillage. This is a case where small side curtains can help without being too obtrusive.
A to-scale side drawing of the hood and the range with a wok or such on it would allow calculation of the angle being captured inside the light bar. A desirable value is 10 degrees from vertical, pan edge (wok hot zone edge) to the inside of the light bar.
ModernAire is known for making custom variants of their hoods, so one can imagine a missed plume vs. missed money trade study.
It also appears* from the kitchensource photo that the baffle configuration is, er, atypical. Perhaps what we need to know is what the configuration is above the light bar.
Edit: Other photos I've since found show more conventional baffles; perhaps this photo is deceptive.
kaseki