How to Obtain the Transfer Function for a Heating System with Dual Tanks?

[hamshie.k]

i need a help to obtain transfer function for a tank, where a heater is placed. the liquid temperature should be maintained at 45C . no inlet or out let. how to obtain the transfer function?? for analysisng about the controller?

 

[hamshie.k]

Actually the system is like this. a tank is placed inside another tank. consider outer tank as A and inner as B. the heating coil is placed inside tank B, for the purpose of heat distribution in the liquid inside tank A. tank A has inlets and outlet.
but tank B water level is stationary no inflow or out flow. assume there won't be a mass loss as vapour.
the required temp in outer tank A is 40 C. so i have build the relationship and obtain transfer function.

please guide me . pleasezzzzz