[Kogara]

Hello, I'm a third year chemical engineering student and i have to design a shell-and-tube heat exchanger.
My two fluids are:
- Tube side: Acetic acid at 8.90kg/s (from 303 to 343K).
- Shell side: Water at 2.5kg/s (from 350 to 310K).

This is my problem: When i want to find the correlation factor F, i do the calculation of R and S and i found R=1 and S=0.85.
When i have a look to the chart , i can see that this value of S is after the "R=1" curve.
Following the lecture of my teacher who said: "If F>0.75 inachievable, use single tube-side pass; then F become 1", i use this F=1 and Np=1.
I found an area A=125.208m².

The problem is when i do the calculation of the tube side velocity, i found (in the best case) u=0.234m/s and the suggested range is between 1 and 2 m/s...

This is because i use Np=1 because when i use Np=6 or 8, my heat exchanger is working...

So i really don't know what can i do for finding my tube side velocity between 1 and 2 if i have to use a single tube side pass...

I hope someone is going to help me about that and if i'm not clear, i can add all the informations you want..