ANS:B - when heat transfer area required is very low, i.e (100-200 ft²).

Double pipe heat exchangers are typically used when the heat transfer area required is relatively low, typically in the range of 100-200 ft². Here's why: b) when heat transfer area required is very low, i.e., (100-200 ft²). Double pipe heat exchangers are characterized by their simple construction, compact size, and versatility. They are well-suited for applications where space is limited, and a smaller heat transfer area is sufficient to meet the process requirements. Additionally, double pipe heat exchangers are often used in situations where precise temperature control is necessary, such as in laboratory or small-scale industrial applications. While they may not provide the same level of heat transfer efficiency as larger and more complex heat exchangers, double pipe heat exchangers offer advantages in terms of cost-effectiveness, ease of installation, and maintenance.