ANS:D - density

While all the parameters listed can influence the design and operation of a shell and tube heat exchanger, pressure is typically not the most important factor in determining the routing of the fluid within the exchanger. The primary factors that usually govern the routing of fluids in a shell and tube heat exchanger are:

1. Corrosiveness & fouling characteristics: These characteristics determine the material selection for the tubes, as well as the choice of tube side or shell side for the more corrosive or fouling-prone fluid to minimize maintenance and operational issues.
2. Viscosity: Viscosity affects the flow behavior of the fluid, which can influence the choice of routing to optimize heat transfer efficiency and minimize pressure drop.
3. Density: Density also affects fluid flow behavior and pressure drop, influencing the routing of fluids to ensure proper flow distribution and heat transfer across the exchanger.

While pressure is important for ensuring proper flow rates and preventing leakage, it is often not the primary factor in deciding the routing of fluids within the exchanger. Instead, pressure considerations tend to focus on maintaining appropriate pressure differentials for effective heat transfer and avoiding structural issues related to pressure containment.