ANS:A - a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.

In a heat exchanger, one transfer unit means: a section of the exchanger in which change in temperature of one stream equals the average driving force in the section. Explanation:

1. Definition: Transfer units (TU) are a concept used in the analysis of heat exchangers to quantify the effectiveness of heat transfer. One transfer unit represents a certain amount of heat transfer or temperature change occurring in the exchanger.
2. Meaning: When we say "one transfer unit," we are referring to a section of the heat exchanger where the change in temperature of one fluid stream is equal to the average driving force in that section. In other words, the temperature change of the fluid stream undergoing heat exchange matches the driving force for heat transfer in that particular section.
3. Driving Force: The driving force for heat transfer in a heat exchanger is the difference in temperature between the hot and cold fluid streams. The larger the temperature difference, the greater the driving force for heat transfer.
4. Importance: Understanding transfer units is crucial for analyzing the performance of heat exchangers and optimizing their design for efficient heat transfer. It allows engineers to assess how effectively heat is transferred between the two fluid streams and identify areas for improvement.