A shell-and-tube heat exchanger is to heat an acidic liquid that flows in unfinned tubes of inside and outside diameters Di=10mm and D0=11mm, respectively. A hot gas flows in the shell side. To avoid corrosion of the tube material, the engineer may specify either a Ni-Cr-Mo corrosion-resistant material alloy (ρm=8900 kg/m^3, Km = 8W/m*k) or a polyvinyllidene fluoride (PVDF) plastic (ρp = 1780kg/m^3, Kp =0.17 W/m*K). The inner and outer heat transfer coefficients are hi = 1500 W/m^2*K and h0=200W/m^2*K, respectively).
(a) Determine the ratio of plastic to metal tube surface areas needed to transfer the same amount of heat.
(b) Determine the ratio of plastic to metal mass associated with the two heat exchanger designs.
(c) The cost of the metal alloy per unit mass is three times that of the plastic. Determine which tube material should be specified on the basis of cost.