Why a flue pipe at all?

[SReynolds]

There is a popular video on youtube with a gent demonstrating smithing inside what appears to be a very modern shop. It is well ventilated per the noise of the fans. The forge sets in the center of the shop and there is no chimney/flue pipe anywhere in sight.

anything wrong with that?

I have a portable forge set in the center of my shop (with 40' ceiling and a vented coupla. There is no problem working in there. Unless you dump a load of coal on the hot coke fire. What little coal that converts into coke won't bother anyone inside the shop.

[Charles R. Stevens]

Most if us dont have 40' celings

[HWooldridge]

I knew a guy who had a tall shop and a big HVLS wall fan (the type used in greenhouses) was mounted in one end gable. He had several forges throughout the shop and simply turned on the fan when he lit a fire.  The smoke would then spiral up and out through the opening; one of the cleanest shops I'd ever seen with regard to soot not accumulating on every horizontal surface.  Of course, this was in Central Texas where winters are mild - might not want to do this in the cold white north...

[Charlotte]

Actually, the soot and dust of coal is an appearance problem as well as a lung problem so flues are well advised but not necessarily required.  However, remember that not everything you see is what hurts you. Coal and coke produce huge amounts of carbon monoxide and more harmful invisible and potentially injurious compounds. Industrial hygiene professional would have a field day shutting us down if they brought in their little sniffers. That more of us don't get sick is a testimony to the resilience of the human body and the respect with which we treat our work area.