ANS:C - to obtain higher heat transfer co-efficient and shorter tube.

Multipass heat exchangers are primarily used to obtain higher heat transfer coefficients and shorter tube lengths. Here's why:

1. Higher Heat Transfer Coefficients: Multipass heat exchangers allow the fluid to make multiple passes over the heat transfer surface, which increases the contact time between the fluid and the surface. This extended contact time enhances heat transfer rates and, consequently, increases the heat transfer coefficient. Higher heat transfer coefficients result in more efficient heat exchange, making multipass heat exchangers desirable for applications requiring high rates of heat transfer.
2. Shorter Tube Lengths: By allowing the fluid to make multiple passes through the heat exchanger, multipass configurations can achieve the desired heat transfer without requiring excessively long tubes. Shorter tube lengths reduce the overall size and footprint of the heat exchanger, making it more compact and easier to integrate into a system.

While simplicity of fabrication, low heat load, and reduction of pressure drop may also be factors influencing the choice of heat exchanger design in certain situations, the primary advantage of multipass heat exchangers is their ability to enhance heat transfer coefficients and achieve shorter tube lengths, leading to more efficient heat exchange.