Calculate the surface area available inside heat exchanger

Question

You are working in a company that has developed a process to produce DNA vaccines from E.coli. After the fermentation step a lysis step is used to release the DNA from the cells resulting in a sticky mixture. You find that an aggressive cleaning solution heated to 80°C is required to clean the sticky mixture off the inside of the lysis tank. A counter-current shell-and-tube heat exchanger containing 100 tubes is used to heat the cleaning solution from 10°C to 80°C at a mass flow rate of 8.6 kg s^-1. The tubes have external and internal diameters of 11 and 10mm, respectively, and a thermal conductivity of 17 W m^-1 K^-1. Water enters the shell at 90°C at a mass flow rate of 22 kg s^-1. The shell side heat transfer coefficient is 2500 W m^-2 K^-1.  

         You are given the following data: 

         The following correlation applies for turbulent flow in pipes:

         Nu = 0.023 Re^0.8 Pr^0.4

 

         Cleaning Solution:

         Thermal conductivity = 0.5 W m^-1 K^-1

         Viscosity = 1 x 10^-3 Pa s

         Specific heat capacity = 3.68 kJ kg^-1 K^-1

            Density = 1,100 kg m^-3

 

         Water

         Specific heat capacity = 4.18 kJ kg^-1 K^-1

 

a)      Calculate the surface area available inside the heat exchanger. 

b)      You notice that with time the aggressive cleaning solution causes corrosion in the heat exchanger resulting in a build up of layers of rust deposits. If you were to buy a new heat exchanger, suggest ways you could re-specify its design so as to limit the damage by corrosion?