Before you settle for that as a solution, you may wish to investigate changing the circular section to an ellipse at the problem area.
For example a 600mm dia duct (23”) could very likely be squeezed to say a 450mm (18”) high x 800 (32”) wide thus picking up an extra 150mm (6”) in head clearance
Looking for a bit more ? -  300mm (12”) high x 1200mm (48”) wide picks up an extra 300mm (12”) in head clearance.  
The downside is a little bit more expense and maybe a slight loss in velocity at the ellipse, which would be negligible in most systems. But the upside could be a safe head clearance.

I once had to install extraction ducting from bag dump stations in a very low roof space and needed to design the whole system this way.  Not ideal, but the only practical way.  There was some thought that solids might drop out and a series of trapdoors were designed into the bottom of the ducting at strategic locations, for inspection and cleaning.  
The velocity was quite high, but after 2 years of operation there was never any drop out.

regards - John Chadwick - contact detail - This e-mail address is being protected from spam bots, you need JavaScript enabled to view it