Look at a brand called Kobe. They have hoods that are 6 & 9 inches tall, that have 800 cfm and can vent vertically using 6 or 7 inch round. they are quiet and you can purchase them with baffle type filters or oil cups. Your roof cap should be heavy duty with a good back draft damper.

If you're replacing the duct you should definitely make it larger. There are only a few 600 CFM hoods that work with 6" duct - most work with 8" duct. 1200 CFM hoods usually require 10" duct. The thing to do is pick out the hood you want and then select the duct size to match the hoods requirements.

weissman or others - Do you know if there are any vent wall caps / dampers which will provide the equivalent of a 10 inch round duct volume but in a rectangular configuration?

Not sure what size rectangle I would need.

My dilemma is if I get a unit that vents vertically (I can get one that I want at an excellent price) - I don't have cabintes overhead, just a 6 inch valance and about 3 inches of crown moulding. So to keep the duct hidden - I want to do a 90 degree turn with a wide but not to tall rectangular duct. I can get the duct work custom made (my brother is in the sheetmetal business) but he doesn't work with dampers / end caps.

If my recollection of geometry is right - a 10 inch round duct has an area of 78.5 - so I would need something rectangular to give me the equivalent. I can find 3 1/4 x 10 rectangular caps - but that is only equivalent to a 7 inch round duct. I am toying with the idea of using 2 - but ideally would find a wider vent cap

The transition from round to rectangular may be important, depending on how closely you match areas.

Companies that make roof exhaust fans make housings for them, and in some cases they also make inline fans that connect to the same housings. These housings should have dampers, or at least bird screens if the dampers are nearer the hood.

90-degree turns will cause significant pressure loss, so require careful construction to minimize it. Better is straight up.

While commercial ducts require welding of sections to minimize fire escape, residential ducts do not. They are generally slip fit. It is up to the baffles/screens/"magic lungs" to block fire propagation and keep the worst of the grease out of the upper ducting. I am not aware of any requirements for double or triple wall ducting for kitchens. If you need one your wok is too hot. :)

Ducts should be sized to establish a flow rate of 1000 to 2000 feet per minute at maximum flow. So define the fan you are going to use, estimate losses in the system (duct loss calculators can be found on-line; the hood losses may be a guess without manufacturer information), reduce the fan's rated flow rate (cubic feet per minute) at zero inches of water static pressure to a lower number (flow vs. pressure curves for several fan sizes are available at Broan's website to use as examples, see also Greenheck's website for a long application note), divide the cfm by 1500 ft/min to get square feet, convert to square inches by mulitplying by 144 to obtain duct area; diameter follows by solving Area = pi*diameter-squared/4 for the diameter. [sorry for the rediculously long sentence] If the calculated flow with losses from the calculation above is too low for your cooking style/stove design/insert dozen other factors here), increase fan size and recalculate.

kas