The number of heat exchangers in series

[cayusbonus]

Hi, I need to design a system of heat exchangers for a steady-state process. I have inlet temperatures of the fluids, the amount of heat that must be exchanged, the max flow ratio and other data. The objective is to reach the exchange area and the length of the shell. My problem is that I have not idea about how many heat exchangers I must put in series, Better said, I don't know how the number of heat exchangers in series afect to the areas and the lenghts, when all the heat exchangers must be the same. The heat exhcanger must have one shell pass and two tubes pass.

Thanks.

 

[Black-Mind]

Since you have the inlet tempreture and all the fluid properties you can calculate the effictiveness of the heat exchanger where E=q/qmax
and then calculate the heat capacity ratio Cr
And the overall heat transfer coefficient U
Then by using E-NTU method u will have NTU=UA/Cmin
where NTU can be calculated from tables or the figures you can find those at "Fundamentals of Heat and Mass Transfer-Incropera" at chapter 11 section 4

 

[cayusbonus]

Since you have the inlet tempreture and all the fluid properties you can calculate the effictiveness of the heat exchanger where E=q/qmax
and then calculate the heat capacity ratio Cr
And the overall heat transfer coefficient U
Then by using E-NTU method u will have NTU=UA/Cmin
where NTU can be calculated from tables or the figures you can find those at "Fundamentals of Heat and Mass Transfer-Incropera" at chapter 11 section 4

Ok, I had find a solution for this.

Your mean will run if you've only one heat exchager. The problem here is that we've not the number of heat exchanger of shell&tube class in series needed, and this number must be fixed. So, one method can be readed in the Heat Exchanger Design Handbook p.272 Chapter 5. This text give you an empirical condition that determines if more than a self heat exchanger is needed, and one graphical technique for estimate this number.

But nowadays we have a powerful computers and good tools that allow us to improve this results. For example, with EES (Engineer Equation Solver) you can stablish this number reasoning that X heat exchangers in series are like one heat exchanger with X shell passes. Then, you can use the shell&tube routines increasing the 'N' number (shell passes), obtaining the F factor (or epsilon, depending on the method). You can do this with tables and graphical representations of F, epsilon etc. but is a really heavy work.

In conclussion, with the graphical technique you achieve the number of heat exchangers for a F=0,8, but increasing X, you can rise F, but with a negative exp. behavioring (the F changes are lesser when X rises).