Design a heat exchanger

[AmAyA]

Hello

 

I have to make a design for a concentric heat exchanger . The cold water (in the shell) flow rate (1.2L/min) , inltet temp. (25 C) and outlet temp.(40 C) is given. For the hot water (in the tube) the inlte temp.(70) is given and the outlet temp. should not exceed a cretin value (55 C). I also have a correlation between Re and the over all heat transfer coefficient (U) for the hot water and another one between the flow rate and U also for the hot water which I obtained form an experiment.

 

I started by calculating the heat absorbed by the cold water Qc but I can't equal it to the heat emitted by the hot water Qh, the instructor told me to use the mean temperature efficiency to relate them but I found it to be very low 0.33 and I now that heat exchangers have high efficiency! what can I do about? how can I find a relation between them?

 

After I find Qh I will find the flow rate of the hot water and then find U, from U I will find Re and then the diameter of the tube and then the length (Qh= UA dT(LMTD)). I should do this for several temp. between 55-41 C and then chose the best diameter and length based on the head loss.

 

I did make a heat exchanger design before and I solved some design problems using effectiveness but Qh=Qc in every case, I have no idea how can I theoretically find efficiency of the HE.

 

For the mean temperature efficiency I used the equation from this document (page 6)

 

http://www.irvindelapaz.com/blog/wp-content/uploads/Heat-Exchanger-Analysis.pdf