Kitchen hood ventilation 1

[mazou]

Hello, I need some info on ventilation considerations for my kitchen range hood. I have a 1000 CFM blower that needs to vent outside. I am mounting the hood on an exterior wall to have the shortest duct lenght. The manufaturer of the hood recommends 10" round duct. But the hood itself has a 4.5" x 22" rectangular exit.Actually, there are 2 openings of 4.5"x8" included in the 4.5"x22" and the manufacturer recommends a transition to bring this to 10" round. My question is : Can I simply extend the rectangular duct coming out of the hood to the outside? If so, can I extend the 2 actual opening (4.5"x8") separately to the outside, each with its own wall cap? The reason I ask is that I am space limited either above or behind the hood. I have 12" free space above the hood and I doubt it is possible to transition to 10" round and exit with 90degree elbow. The outside wall is approx 5" thick (very old house built with real 2"X4"!!!) so I am looking to vent directly behind the hood but again, a 10" trasition will extend too far on the outside. What do you suggest?
Thank you very much

 

[tombmech]

Turning vanes in rectangular elbows are always useful, but at 4-1/2" x 8", it hardly makes any difference. Add to that the fact that the hood blower is sized to handle a reasonable pressure drop, and you will have plenty of fan to keep the flow up. This is because the hood mfr will have accounted for some run length of the 10" duct in a variety of applications. You may still be better off - depending on how close the holes are - by cutting a single, rectangular hole in the wall and letting both outlets dump through it.

 

[mazou]

Thank's for the input. I will probably run a single rectangular duct directly back of the hood to the outside and have a custom made wall cap made up. I wasn't sure if there could have been any wind speed issues or turbulence from one big vent...But i guess that with only about 5" to go, there won't be enough to help or indur the windflow. Thankss again...