[I feel like I need to parse that one paragraph blob in a word processing app just to decode its content. Only having read it thrice, I'll limit my comments for now to hood construction and general requirements.]

In order to obtain relatively equal flow through the filter area, whether baffles or mesh, there has to either be some height above the baffles (probably commensurate with their airfoil length), or similarly above the mesh (narrow dimension), or the mesh has to be so restrictive that it chokes the flow equally at all points (not good). So of all the noted configurations that weren't already rejected by the OP, I would vote for narrowing her consideration to whichever ones provide enough height for the selected filter type and dimensions.

Baffles have advantages over mesh filters, but both can filter if the meshes are kept clean.

Less tortuous duct paths are preferred over more tortuous paths.

Full power actual flow rate (CFM) should be roughly 90 cfm/sq. foot of hood aperture, if possible. A rated capability of the blower of 1.5x that above should accommodate pressure losses in ducting, hood, filters, transitions, and MUA restrictions.

Duct area should allow the air velocity in the duct at full power (90 cfm/sq. ft times the aperture area in square feet divided by the duct area in square feet) to be between 1000 and 2000 feet per minute. Aim toward the lower value if all the ducting remains warm in winter, or the higher value if passing through a cold attic.

kas

Thank you kas, I was hoping you would respond. Sorry you had to reread 3x to try to decode. I had to read your next to last paragraph 3x to understand the necessary calculations. Haven't done that yet, but will make my calculations. Further Imperial recommendations for my situation was to go up the required 18" vertical inside the cabinet, then angle 45 degrees out the top of the cabinet and to the wall behind. They then recommend at least 5 ft. straight duct run from the first 45 degree angle before angling up to the roof.
Kas, I do have a question for you. With ducting angling above the cabinets, could I get by with just painting the duct that is exposed on the kitchen side, or would you enclose with fireproof sheetrock? Other questions-- I notice some hood manufacturers recommend a T-shaped roof cap. Do you or anyone else have any thoughts or recommendations on roof cap style?

This post was edited by Texasgal47 on Wed, May 14, 14 at 15:15

Although I am a belt-suspenders-duct tape kind-o-guy, normally residential hood ducts do not pick up enough grease to support a fire, particularly if firestopping baffles or mesh filters are in place and the filters also do their job as grease collectors. Otherwise, firecode sheetrock would be required all the way to the roof.

I think what you do on the outside of the ducts is an aesthetic issue.

My roof cap is a centrifugal blower with its own housing and damper, with its air output aimed down the roofline. It is made by Broan and sold by Wolf. Although it could be buried by snow, it seems here in southern NH that it doesn't, and in any case, such snow would have to be fast falling wet snow rapidly turning cold, because anything movable can be blasted off the roof in front of the fan position, or melted by warm impinging air.

However, others may not be so lucky, and an external fan of the upblast type (what one typically sees on a restaurant roof, but smaller) may be a better choice. My secondary kitchen fan is of this type, obtained from NuTone many years ago.

If the blower is in-line or at the hood, and not so powerful, then a T-shaped cap can certainly be designed to keep out rain and provide some height above the snow. I wouldn't want to make recommendations without having some experience with them. I think some sort of damper should be present at the roof cap, and at the hood (just above is ok). Duct mounted dampers can be obtained from Fantech.

kas

Thank you kas for your thorough reply--greatly appreciate all your help.