Mean Metal Temperature

[Zanna83]

My question is about Mean metal temperature calculation in shell and tube heat exchangers. This parameter is foundamental when calculating the thickness of a fixed tubesheet in a BEM heat hexchanger... the question is... if inlet and outlet temperature fore each side of the exchanger are given... how do i calculate shell MMT and tube MMT ?

I know that you can find something on TEMA specification but the calculation reported there is complex and does not cover all the cases. I need a shorter (approximated) and more general way to calculate MMT.

Thanks in advance for your help

Bye

 

[davefitz]

first you need to decide if you are designing the shell or the tubesheet- 2 different issues.

second, I am sure that if it is a TEMA HX, then you are best served by using the TEMA formula, if provided .

If such a formula is not provided by TEMA, the it seems to be a simple case of calculating the overall heat transfer across the shell ( or tube sheet). You need to know the inside film conv HT coef, the inside fouling factor, the shell or tube sheet conductivity and thickness, the outside fouling factor, the outside conv HT coef., and the max fluid temps inside and outside the surface in question. The value of q/a=(T2-T1)/U*, where U* = 1/(R1+R2+...+Rn), where the Ri's are the resistances to HT caused by each heat transfer coef.

Once the worst q/a is known, then it is a simple matter to calculate the temperatures at each interface and MMMT.