Tank Heater - Flow Area Calculation

[Peasea]

This is for the design of a heater stabbbed inside a storage tank for crude oil service. The heat transfer fluid is ammonia which is circulated in 2" or 4" straight tubes. Ammonia circulates inside the linear tubes by thermosyphon effect. The tank ID is about 24 ft and is 26 ft tall. While designing this system, I am getting a very flow velocity of 0.00010 m/s for the shell side flow for the specified flow rate of 500 BOPD which is again due to the low area that we are getting - about 9.43 m2 (102 ft2). The equation that we are currently using to calculate the shell flow area is - Shell IDX Baffle spacing X clearance X tube pitch. For shell ID we are taking the tank ID. Is this the right approach since it is difficult to fathom the fluid going past the entire c/s of the tank. There are no baffles in this assembly. So does it mean that the baffle spacing = tank ID = 24 ft.

Any help in this regard is highly appreciated.

Regards

PC