ANS:B - requirement of heat transfer area is low.

Double pipe heat exchangers are preferably useful when the requirement of heat transfer area is low. Here's why:

1. Compact Design: Double pipe heat exchangers have a simple and compact design, making them suitable for applications where space is limited or when a smaller heat transfer area is sufficient.
2. Ease of Installation and Maintenance: Due to their simple design, double pipe heat exchangers are relatively easy to install, operate, and maintain compared to more complex heat exchanger designs. This simplicity can lead to cost savings and reduced downtime for maintenance.
3. Versatility: Double pipe heat exchangers can handle a wide range of flow rates and temperature differentials, making them versatile for various industrial applications.

While double pipe heat exchangers can also be used for cooling high viscosity liquids or heating corrosive liquids, they are particularly advantageous when the requirement of heat transfer area is low. This is because they offer an efficient heat transfer process while minimizing the use of space and materials.