Final answer:
The heat transfer rate of a heat exchanger is directly proportional to the air velocity, but the exact increase in heat transfer rate cannot be determined without more information.
Step-by-step explanation:
The heat transfer rate of a heat exchanger is directly proportional to the air velocity. When the air velocity is increased to 400 ft/min, the heat transfer rate will also increase. However, the exact increase in heat transfer rate can only be determined if we have the specific relationship between air velocity and heat transfer rate for the given heat exchanger. Without that information, we cannot determine which of the given options (a. 400 btu/hr-sq ft-f, b. 455 btu/hr-sq ft-f, c. 500 btu/hr-sq ft-f, d. 350 btu/hr-sq ft-f) is correct.