ANS:D - same in either 'a', b' or 'c'

1. Parallel Flow: In a parallel flow heat exchanger, both fluids enter the exchanger at the same end and flow parallel to each other to the other end. While parallel flow configurations are simple and easy to construct, they often result in a smaller temperature difference between the two fluids along the length of the exchanger. This smaller temperature difference reduces the rate of heat transfer, requiring a larger surface area to achieve the desired heat transfer duty. Therefore, parallel flow configurations typically require more surface area compared to other configurations for the same heat transfer duty.
2. Mixed Flow: Mixed flow heat exchangers combine aspects of both parallel and counterflow configurations, with the fluids entering the exchanger at different ends and flowing partially parallel and partially counter to each other. Mixed flow configurations aim to achieve a balance between the simplicity of parallel flow and the efficiency of counterflow. However, because they do not maximize the temperature difference between the fluids like counterflow configurations do, they generally require more surface area compared to counterflow configurations for the same heat transfer duty.
3. Counterflow: In a counterflow heat exchanger, the two fluids flow in opposite directions to each other. This configuration maximizes the temperature difference between the fluids along the length of the exchanger, resulting in a more efficient heat transfer process. Because of this increased efficiency, counterflow configurations require the least amount of surface area to achieve a given heat transfer duty compared to parallel and mixed flow configurations.
4. Same in Either 'a', 'b', or 'c': This option suggests that the surface area required is equal in all flow configurations. However, this is not the case. As explained above, the counterflow configuration typically requires the least amount of surface area for the same heat transfer duty due to its ability to maximize the temperature difference between the fluids. Therefore, this statement is incorrect.

In summary, while all flow configurations have their advantages and disadvantages, in heat exchangers where one of the fluids is condensing or evaporating, the surface area required is typically the least in the counterflow configuration due to its ability to maximize the temperature difference between the fluids along the length of the exchanger, resulting in more efficient heat transfer.