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Air flow rate (CFM) requirements are not determined by the area of the room when the subject is cooking ventilation. Cooking ventilation works by capture and containment of the rising and expanding cooking plumes generated by the hot cooking surfaces.

Hence there are two essential performance requirements:

Capture requires that the hood overlap the expanding plumes. The plumes have various shapes and speeds, but for convenience we treat them as conical from each point on a hot cooking surface, rising upwards over a range of angles from vertical to ten degrees from vertical. This leads to the capture requirement of having the hood overlap (as much as practical and affordable) the contour of this distribution of cones, at the chosen height of the hood aperture. A three-inch hood overlap of the cooktop is often mooted as a simple version of this requirement, but in some cases is inadequate for full capture.

Containment requires that the blower flow rate be high enough to ensure that the effluent in the plumes is moved to the interior side of the baffles and expelled from the house, with none allowed to reflect off of the baffles or other hood parts and escape from the hood. The nominal air speed that I suggest is adequate for residential cooking is 90 ft/min over the hood entrance aperture. 90 ft/min is equivalent to 90 CFM per square foot of hood aperture.

Other points:

The rising plumes can have a velocity of up to 1.2 m/s (somewhat less for induction) so the containment air velocity is used to block significant downward reflections at the baffle surface.

Your two hoods should be treated independently as to requirements, except when make-up air (MUA) is addressed. I"m going to skip that subject in this message.

Most importantly: Blowers are rated at zero static pressure across them, i.e., hanging free in the air. This is not the situation in the kitchen ventilation environment. Skipping the discussion of all the reasons why, I suggest you choose a blower that is rated at about 1.5X the intended actual flow rate calculated from the 90 CFM/sq. ft. requirement. Else, a more elaborate analysis is needed, and the MUA system is involved. This makes it somewhat recursive to perform.

For a given flow rate, blower noise is reduced by using large blades moving a low speeds. (Consider the Casablanca fan, for example.) So it is true that a higher rated external blower will, at a setting below its maximum, be quieter than a smaller rated external blower running full speed. Typical Wolf/Broan or Abakka blowers mounted on the roof will be quieter than the baffle hiss if silencers are used in the duct path.

In your case you will want two blowers and individual ducts, I think, with individual silencers.

MUA comments follow in next message.

kas

Make-Up Air

Please read the many MUA discourses on this forum for background.

I do not have the Fantech MUA system; however, I intend to use parts of a scaled-up one for completing my own MUA system, described elsewhere. I haven't researched all of their design's details, but I believe it has a blower to make up for the pressure losses of the MUA path, and it has a control that senses something to turn it on in proportion to the hood air flow. If this something is kitchen differential air pressure (as in my partially finished system), then one big blower MUA system will work. If it senses flow in the hood duct, then it would seem that two MUA systems would be needed without reprogramming its computer for a more elaborate algorithm. However, I encourage you, after you choose your hood and blower per the previous message, to contact Fantech and go over your goals vs. what they have to offer.

Note: the actual flow rate of the MUA system has to match (more or less -- there is always house leakage) the actual flow rate of the hoods. So, say, two 900 CFM blowers pulling 600 CFM each need to be matched by a nominal 1800 CFM MUA blower pushing 1200 CFM. Similarly, if the hood apertures require only 400 CFM each, then two 600 CFM rated blowers would be matched by one 1200 CFM rated MUA blower pushing 800 CFM. (All of this depends on what house pressure you can use -- another topic -- because the hood blowers should in principle be selected from their fan curves to operate at a particular differential pressure due to the hood venting system and selected house pressure, while the MUA blower(s) should be selected based on its duct path pressure losses and house pressure.)

kas