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 Re0.8 Pr0.4
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?