Successful capture and containment is the purpose of a kitchen ventilation system; of which one component is the hood. Capture and containment are also processes. Capture is the process of getting the rising and expanding cooking plume into the hood; containment is the process of getting the plume past the baffles [or other filter] and ejecting it to the outdoors [usually].

Capture is mainly effected by having the hood overlap the plume at the height of the bottom of the hood. Some capture volume below the baffles may be helpful for averaging transitory fume collection (opening a pan cover), but the overlap is the most important part. Capture is very modestly aided by the blower flow rate (CFM) due to the rapid fall-off in blower air velocity away from the hood aperture.

However, containment is all about getting the plume past the baffles instead of having it bounce off of the baffles and other sheet metal parts. Reflection of the plume would occur due to conservation of momentum if there were no air flow to aid containment.

Once the plume and blower-induced air flow is above the baffles, (which should have collected some of the larger-sized end of the grease particle spectrum), the plume is extremely unlikely to escape back down. Volume above the baffles is mainly needed to straighten air flow for smooth transition into the ducting, to provide room for an internal blower, if used, and to equalize the flow rate across the baffle extent. Very narrow (top-to-bottom depth) hoods may not have uniform flow unless the filter is fairly restrictive (mesh filter, say). The volume above the baffles is not intended to be a reservoir for fumes.

So, goals should be: (a) your best trade-off between performance (includes noise), aesthetics, and affordability, (b) hood entrance aperture overlapping the rising and expanding plume, (c) achieving an actual 90 CFM per square foot of hood entrance aperture (blower has to be rated higher than this), (d) providing a suitably sized duct to balance low restriction with a goal of achieving 1000 to 2000 ft/min duct air velocity, and (e) providing a path or paths for sufficient make-up air. There are additional details that may be gleaned from various past hood topics here.

kas

Okay, I'm going to try to say what I've just learned in English, and hope Kas will come back and correct me if I get it wrong.

1. You need your hood to be wider than the cooking area to get the whole plume of cooking fumes/steam/whatever, which angles out, up from the pots to hood height.

2. If you have some extension of the hood below the baffles (which many hoods do), it can help wrangle the plume, especially when it gets a heavy shot of stuff into it, while it's being sucked up.

3. The plume will bounce off the baffles without the fan to suck it up past them.

4. The plume doesn't come back down once it's sucked up into the hood because it's being pulled toward the duct rather than pushed back through the baffles.

5. The fanned and baffled air is turbulent and tumbly. The shape of the area above the baffles helps straighten it out so it can go out the hole (duct).

6. The space above the baffles also gives you room for the fan(s) (like on mine!).

7. If you have a very skinny (top to bottom) canopy, you need mesh filters, which are more restrictive to air flow, to even out the air as it comes through. I assume this is so that there won't be traffic jams? (I had always wondered why so many high end hoods had mesh even though baffles seemed to be preferred in general. Now I know!)

8. The only part a skinny hood doesn't have that a big one does is the straightening the air geometry (see #5 and #7). There is no need to collect air inside the canopy.

9. There's a whole lot of technical stuff involving duct aperture, duct length, number of bends, temperature inside duct, source of make-up air, conditioning of make-up air, relative effects of local vs. remote blower, in-line supplementary blowers, etc., that are beyond my ken. I could learn it, but I don't want to. I've picked up a lot of crumbs of what Kas has shared over the years, but to me it boils down to get an HVAC guy (or woman) who has the technical knowledge and skill to do the job right.

5. I was particularly thinking about a flat top hood mated to a perpendicular duct where the flow has to take a sharp turn causing turbulence and pressure loss at the transition. Most hoods will not be all that great in that area, but pyramidal hoods are expected to be somewhat better than narrow flat ones.

7. [The air molecule traffic jams are straightened by a legion of Maxwell's demons acting as cops. :] Essentially, the flow through any part of the filter will be dependent on the pressure loss from below the filter through it to the duct opening (think external blower for a moment). It is desirable that this be consistent across the filter area to equalize flow. This is most easily done by assuring that the greatest pressure loss is at the filter. A very narrow hood will be more restrictive, internally, from the farthest reaches of the filter area and might cause loss of flow rate at the hood edges unless the filter is commensurately more restrictive.

kas

Thanks, Kas!