How fast does hot air rise?

So for a class I am designing a solar dehydrator and I want to know how fast the hot air will rise out of the dehydrator. For calculation purposes assume the hot air is 120°F @ 30% relative humidity (23 g/m³) and rising through a tube (if necessary assume the tube is 4 in. in diameter), the outside air is 80°F @ 60% relative humidity. In addition assume the hot air has a volume of 40 cubic feet.

If anybody knows how to calculate how fast it will rise through the tube that would be awesome. Otherwise I will most likely doing an educated guess.

Thanks.