Metal vent run: why convex curved steel at top of soffit? (long)
I wish to ask for some help brainstorming why there appears to be a convex curve rather than straight vertical galvanized metal at the top front of my vent run within our kitchen soffit. The soffit runs below ceiling and parallel to the counter top for 11"7" on this cooktop side (there's soffits all around this 1970's kitchen and pantry).
I have a decades old 42" under the cabinet Thermador vent hood with mesh filters, then two parallel aligned 3 1/4" x 10" vertical apertures for exhaust and a long run of metal ending in an infinite speed squirrel cage exterior fan by Trade Winds (1000 cfm motor). At least that's the way I've figured it.
What I thought I saw (with much peering over time) is a 50 inch "high run" of thin galvanized metal running up vertically behind the 36" cabinet back and continuing into a 14" wood/plaster soffit, as well as a 30 inch "low side" (on the back wall) forming a 90 degree hole with turn for the rest of the metal run. I tried to take a picture but it's too dark. I took off the wood trim between the 36" cabinets and the soffits and can see all the way above the vent hood cabinet that there is metal back on that back wall. The 14" soffits around the kitchen and in the pantry appear to be made of 3/4 " or more wood fronts covered with smooth plaster. I see no pipes in the soffit, just one electrical and this vent metal, in all of the kitchen and pantry trim I removed and peered into.
But, when I look up the 3 1/4" x 10" vertical apertures with a special LED bright light, I see a curved back metal, or curvature, at the very top of the soffit. Not a big curve at the back of the cabinet side within the soffit, but it's just not straight vertical on that "long" end run of metal. Might I have a round duct in the top back of the vent cabinets? The vent hood cabinets are shallow, not the usual 12". I just don't see how it's possible that this curve may represent a duct running parallel along the soffit, rather than vertically through the soffit, as 30" of original kitchen brick wall still marks the end of the kitchen and there is no duct running through that brick. So why is there that curve?
I'm not grasping something here in spite of the hours of peering and climbing around peeking with LED flashlights and wide screwdrivers between the cabinets and soffit. I probably need to get a video camera designed for this, better yet an HVAC specialist to come in and figure this out for me.
Any suggestions on what that curve in the vent metal is from? I welcome all input as it's got me perplexed and I hoping to nail down a range selection after I figure out the specifics of this kitchen's ventilation. Thank you very much.