Heat Tank Using External Exchanger

[IronMike]

What is the best way to estimate size of exchanger in a tank recirculation loop to heat a tank of fluid from one temperature to another in a given time using a steady (and limited) flow of heating fluid at constant supply temperature? It is unsteady state in that the process inlet to the exchanger increases over the time cycle until it hits target. I thought of assuming a heat exchanger size and calculating time or calculating the size directly from the duty. However, the outlet temperatures of the heating fluid and process fluid are not known. Any tips on where to look?

Thanks in advance.

 

[TBP]

You'll have to just pick the outlet temps for each fluid, and if you don't have the process flow, you'll need to select it as well. The limiting factor is the input available from the hot side of the HX. Run the calcs, and see what you get. It'll likely be a bit of trial & error as you adjust flows & temps. Remember that the LMTD of the HX will drop off as the tank heats up.

 

[IronMike]

Thanks TBP, I haven't done one in quite a while. For the liquid streams I assumed an outlet temperature for one, then calculated the outlet for other from m*Cp*dT. Also assumed exchanger size and calculated log mean dT and heat transferred, BTU/h. Changed assumed outlet until heat change in fluids equalled heat transfer of exchanger. I could go further and do some differential stuff to calculate an exact time, but I'll chicken out and use the end condition where delta T is lowest so that my exchanger is large enough to handle the heat load when the tank is at the target temp. - Your input was appreciated. - IronMike