In heat exchangers, approach is defined as which of the following?

• A. The temperature difference between the entering fluids
• B. The temperature difference between the leaving and entering fluids
• C. The leaving fluids temperature difference
• D. The rate of heat transfer between fluids

Answer: (C)

Approach in a heat exchanger is the temperature difference between the leaving temperatures of the two streams. This shows how close the two streams come to leaving at the same temperature, given the chosen flow rates and heat transfer. For example, if the hot stream leaves at 60°C and the cold stream leaves at 40°C, the approach is 20°C. A smaller approach means the exchanger brings the streams nearer in temperature, indicating more effective heat transfer under the operating conditions. The other descriptions don’t reflect the standard definition: differences at inlet, differences between leaving and entering temperatures, or the rate of heat transfer are not how approach is defined.