ANS:C - double pipe type with fin on air side.

The most suitable option for heating air with steam in a heat exchanger depends on various factors such as efficiency, cost, space constraints, and specific application requirements. However, typically, for heating air with steam, the most common and effective choice is the shell and tube type heat exchanger. Therefore, the correct option is: shell and tube type. Explanation:

1. Plate type heat exchanger: While plate heat exchangers are efficient and compact, they are more commonly used for liquid-to-liquid heat transfer applications. They may not be as suitable for heating air with steam due to potential issues with pressure drop and limited heat transfer area for gases.
2. Double pipe type with fin on steam side: Double pipe heat exchangers are versatile and can be used for various applications, but they are not typically preferred for heating air with steam, especially when high heat transfer rates are required. Fins on the steam side might improve heat transfer efficiency, but they may not be optimal for this specific application.
3. Double pipe type with fin on air side: Similar to the previous option, this configuration might enhance heat transfer efficiency, but it may not be the most suitable for heating air with steam, especially if high heat transfer rates are needed.
4. Shell and tube type: Shell and tube heat exchangers are well-suited for high-pressure and high-temperature applications such as heating air with steam. They provide a large heat transfer area and can handle significant temperature differentials between the fluids. This type of heat exchanger offers good efficiency and reliability for heating air with steam in various industrial processes.