Stove vent pipe question

A thread in another newsgroup put a bee in my bonnet to ask this. I have a microwave installed over my stove. I wasn't there when the decision was made to install it as a recirculating vent. It's useless.
Don't get me wrong, I understand why they did it. The oven is on an inside wall and how to get the vent pipe to go to the outside, which leads to my question. Bear with me.
Do you think there is any reason why the vent pipe can't be a rectangular shape rather than round? If it was flat it woud go along the top of the cabinets and wouldn't be visible. I think.
But who knows, maybe that shape would cause a grease buildup or something I wouldn't think of. Is there a good reason why vent pipes usuallly are round?
Thanks. nancy
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Nancy, Vent pipes are usually round because they are easy to work with, can be turned in any direction, etc. If rectangle works for you, that would be OK. Just be sure the square inches of the duct opening is the same size. Chances are, the top of the MW is square anyway and a transition piece to round is needed. When you get to the top of the cabinet with a rectangle, if you turn front or back, that is simple, but if you go left or right, that is more difficult to find the right piece. A sheetmetal place that does ducts can make whatever you need though.
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Thanks so much, you understood exactly what I was getting at. It would have to go right immediately above the microwave, but after that it's a straight shot to the outside wall. Maybe 8, 10 feet.
I get what you're saying, the vent would need to have the same capacity as a round one would.
nancy
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wrote:

Nancy, Your stove fan vent pipe can be any shape you want. But they are often traditionally round to make cleaning more effective. That may sound strange since most home owners never clean their vent ducting.
caveat: In today's litigious world you may want to check your local building codes, just to be on the safe side. And yes getting to the outside world with ducting can be a real hassle. People often add soffits just to hide the vent ducting. But I've also seen people put the round spiral ducts out in the open and paint them as a decoration. :)
Joshua
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Good idea, I didn't think of that.

I have seen that, too, I can say that would go over like a lead balloon. Heh. I have a cathedral ceiling over these cabinets, so a soffit is not going to happen. I'm going to put something up there to approximate this vent idea of mine to see if I can hide one up there. The crown molding (moulding?) along the top is a pretty good size, so I have a chance.
Thanks a million for the answers.
nancy
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