Bell sizing

Hi all its been a while as i have been away for work. i am going to be starting this build soon. I am finalizing my plan.

I am looking for some insight on proper sizing/ volume of the bell compartment for a 7" system.

I am building a large water storage tank as my thermal mass the bottom of the tank will be the top of the bell compartment. I been working all the math to figure the right sizing of things for my system. However, I still lack the information on proper sizing of the bell area. As of now the plan for the tank is 4' in diameter and 4' tall. So there will be a 4' diamter surface area to act as a heat exchanger at the top of the bell. How tall should the bell be, to get the maximum heat exchange before exhaust? I am not limited to this size tank.

Thank you for all the great information.

Will