Adapting between flat and round ducting

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Does adapting between round and flat ducting on main intake or extract create any issues?

The way I was planning intake would have to go from round ducting from MVHR unit to flat ducting to get through the narrow gap in roof space and back to round ducting again to external vent.

The overall area would remain roughly same - round duct is 160mm, flat is 300x80mm  overall area remains at 0.02m2. 

But is changing the shape create additional problems around turbulence or others which would make this type of setup not suitable?