The hoods at the link appear to be 19 or 22 inches deep (front to back). Strive for as much front-to-back depth as available in your price range because it will improve capture of the rising and expanding cooking plume effluent. I'm not sure what "commercial style cooking" means, but unless the baffle pitch angle is greater than 45 degrees, the assembly is non-compliant with NFPA 90 for commercial installations. Lower angles typical of residential designs are fine for residential purposes. For a higher altitude view of the subject matter, you may wish to review my comments at: http://ths.gardenweb.com/discussions/3554712/advice-re-kitchen-hood-imperial-fantech?n=2 kas

Ah. My apologies for abusive engineering descriptions. Sometimes I tend to unconsciously assume that all the previous stuff here has been read and digested, so all I have to do is present a reminder. Capture: The plume expands as it rises. The actual expansion angle is greater than 10 degrees outward, but ten degrees encompasses most of the higher velocity components for relevant cases. This means that the cone is wider the higher one measures above the pan base. At 36 inches hood height, for example, with a tangent of 10 degrees equal to 0.176, the plume has expanded outward by 36 inches times 0.176 or 6.4 inches. Similarly, for 30 inches, the plume has expanded by 30 times 0.176 = 5.3 inches. (Exactness is used here to clarify the calculation, not assert a defined boundary exists at the plume.) Suppose a pan base's maximum extent from the wall was 19 inches (which would be true of my induction cooktop if it were against a wall). Then one should aim for a hood aperture that was 19 + 6.375 = ~ 25 inches from the wall if 36 inches up. A 27-inch hood with lights in front of the aperture is close enough. Hence, your drawings have the angle going the wrong way. Imagine a really large traffic cone (without it weighted base). Slice it where it is the same diameter as the pan base and invert it over the pan. This inverted, truncated traffic cone, or rather a set of them perched on all the burners or hobs, can be imagined as defining the required boundary of the hood aperture. The desired hood aperture for good capture is independent of whether the structure of the hood is all metal or is metal inside of wood. Depending on your configuration, it may be best to space a smaller hood outwards and fill the gap at the wall side with some sheet metal. Side cabinets can help perform the function of side curtains in commercial configurations. (Note re pillog's comment. There is a small zone of high flow rate at the edges of the hood, and this has the effect of a slight increase in effective aperture.) Containment: Your analysis is correct. If the aperture is 6 sq. ft., then the desirable actual flow rate at full power is 90 x 6 = 540 cubic ft./min. Now where did I pull the magic number 90 from. Briefly, there are studies that have been published of plume velocities as a function of surface temperature, with induction and with gas. The gas cooking velocity (because the gas combustion products are entrained in the plume) causes more than a meter per second of plume rising velocity. The velocity with induction is typically less than a m/s. At the pan, the hood has NO effect on the velocity, and this essentially holds until the plume is close to the hood aperture. Another way to put it is that the hood captures the plume due to the plume's upward momentum (mass times velocity), which will not change until accelerated or decelerated by some force. When the plume reaches the baffles, it can intercept a gap and pass into the baffle space, or it can hit a metal part and potentially reflect. To keep the reflection in check, the air flow velocity of the hood has to be enough to change the reflection momentum to near zero and let the plume part be pulled into the baffles. When the air velocity is 90 ft/min average across the baffle space, it is about twice that at the baffle gaps, and this velocity is equal to that of the plume velocity. Also, I have observed this level working for the plume over my induction wok, and Greenheck suggests 85 ft/min for similar cooking (Figure 4, page 9 of the previous reference). So think of 90 ft/min as a suggestion that I don't think will prove to be too low. Too low of a hood velocity (CFM/sq. ft.), hence too low of a hood flow rate (below). Enough velocity (below) Blower specification: Yes, the factor of 1.5 is used to make up for the fact that blowers are specified by themselves (usually) hanging in free air, and what you have is a restrictive system that has to deal with baffle restriction, duct, and make-up air pressure losses. The relationship is given by the so-called "fan curve." For example, the Broan blower fan curve immediately below is just about done at well under an inch of water column pressure. Your requirements, depending particularly on baffles and MUA, may require a more "powerful" (higher nominal CFM) blower, as in the second figure. If the only loss were baffles, then the first figure would be barely good enough, because the baffles might have a pressure loss of about one or two tenths of an inch, w.c., at 90 CFM/sq. ft., depending on their aerodynamics. Throw in ducting and pressure losses in the MUA system (a whole giant topic of its own), and the tenths of inches build up leading variously to a need for one of the higher nominal flow rate models.Ekes out 540 CFM at a very low pressure drop (above) A blower with the 1.5 factor achieved (above). Keep in mind 1.5 is only a guess. A blower that would make up for many installation crimes (above), but which drives an unnecessarily more expensive MUA system to keep house pressure up. kas

Well, let's see if I'm up to four questions all at once. Rule no. 1, do not believe VaH sales persons trying to explain blower operation unless you have some other means of determining their veracity, and most importantly, their understanding of the fluid dynamics of air. Rule no. 2. Blowers that suck air also blow air and cannot know what is the cause of the pressure differential across the blades, at least at the tiny pressure differences that apply to hood blowers. Pulling air could be a problem when the intake is at feet of water column reduced pressure, not inches. This does not mean that the blowers will be unaffected by operating into a duct bend, or from a hood transition, etc., but in general, even a VaH blower is pulling air from one side and pushing with the other side, and its blower, like all others, is constructed for one direction of air flow and to achieve a particular flow rate performance at a tolerable sound pressure level. I don't keep up with VaH, but I thought that their 1200 CFM (claimed) hood had two 600 CFM blowers, not four 300 CFM blowers. Perhaps they are using two motors, each with two cages. Fan blade tip noise is caused by the turbulence of the air spilling from the blade tip. It results from operating at a high speed exacerbated by spilling from a crude airfoil shape. Casablanca ceiling fans move a lot of air but the turbulence is at most modest due to the low speed of each blade tip. Duct noise is from irregular air flow in ducts. It generally won't be laminar at the flow rates of interest (1000 - 2000 ft/min). Pressure loss of bends is the result of degrading the flow due to increased side wall interaction as the moving air tries to pile into the blockage ahead before turning. Smoother longer turns reduce the effect. VaH Magic Lung blowers use squirrel cage designs that are good, I guess, for throwing grease at the hood walls, but have a weakness -- they don't like added pressure loss relative to some other popular fan configurations, such as axial in-line blowers or centrifugal carousel types such as Wolf/Broan/Abbaka roof blowers, so actual flow due to inadequate MUA or constrictive ducting will have a greater effect on actual flow rate than these other designs. It is certainly possible to build an external chase for ducting and even for the silencer (about 4 inches larger in diameter). Number of bends is worth minimizing, but it is always possible to specify a blower to provide the needed flow rate at a given pressure loss if the bend number cannot be minimized. Roof blowers usually also work mounted on walls. One secondary effect is that on roofs, sunlight UV generally degrades and eliminates any grease that condenses on the blades and is expelled onto the shingles, whereas on siding that is in shade the grease may become noticeable. Commercial blowers operated in up-blast or side-blast configurations likely will not blow grease onto house surfaces, but condensation within the unit may lead to some release around the unit. Downward ducting has a couple of deficiencies. The process of turning a rising plume to a downward direction requires additional energy (equivalent to pressure loss) and likely causes the grease particulates that escape the baffles to condense by impingement early rather than being expelled. To keep the typically long horizontal underfloor duct path from dripping, a continuous down tilt will be desirable, and in a typical basement this may lead to requiring a hole in concrete rather than in a wood wall. Some municipalities may not approve such paths. The "heat rising" view is probably less relevant because the duct velocity will be at least 10X the velocity of the intake to the hood. Last, an observation. My hood blower is rated for 1500 CFM and is roof mounted. Actual flow into my hood is about 1000 CFM with presently passive MUA. This is 100 ft/min air flow across the hood entry aperture. I have a 10-inch duct with LD-10 silencer. Flow is 1800 ft/min in the duct. There are three bends, all less than 90 degrees to get the vertical hood direction to a shallower direction in my attic that encompasses the silencer, then two more partial turns to match the roof blower's intake angle. Note that configurations of this type not only allow room for the silencer, but also allow the hood exhaust to be placed more optimally on the roof, if needed, than exactly above the hood. I hear mainly baffle hiss at the cooktop, with a slight underlying rumble, likely due to imperfect fan carousel balance interacting with imperfectly stiff roof planking. Edit: I feel comfortable asserting the baffle pressure loss, and MUA pressure loss, dominates all the pressure losses in my duct path. tl;dr: An external blower downstream from a silencer will always be quieter than a blower in the hood for a given flow rate so long as the external blower is not constrained to being significantly smaller than the in-hood blower. Other noise will depend on whether the baffle size and shape is reasonable for the air flow rate and the duct size is such that the air flow in the duct is in the range of 1000 to 2000 ft/min. Let me know what I forgot to answer.

You probably looking at 900-1200 CFM they usually run anywhere between 2-11k. depending on the style. Make sure you have a makeup air provided for that size of a vent. Most brands I used were Wolf or Vent A Hood Designer Series hoods... Take a look at model PRH18454SS by Vent a Hood Designer series it's reasonably priced and it's a more basic model for that size and you can find one @ around 2k. Good luck