Round has a variety of advantages, but an equal amount of disadvantages, even more disadvantageous in a traditional sized van.
Plus side: Strength. You can fashion a stronger tank, using the same thickness material or a lighter tank of equal strength, as compared to a flat sided tank.
Also, on a round tank, you can fashion the airflow anfd water flow to smoothly enter the tank at a tangent, so as to create the "
vortex" like movement of these flows. Moving the air and water in this manner produces the lowest possible restriction.
Negative side: The biggest negative of a round tank is space. The tank takes up more usable space for a given capacity. On a big box truck, space is not so much an issue, so on
Vortex machine, you can get all the pluses, without too much negative impact of the space detriment.