What issue arises when air has excessive velocity in a heat exchanger system?

When air has excessive velocity in a heat exchanger system, it can indeed hinder the proper transfer of heat. This occurs because high air velocity can lead to turbulent flow, which, while it may seem beneficial for heat transfer, can actually disrupt the thermal boundary layer that forms around the heat exchanger surfaces. When the boundary layer is disrupted, it reduces the time that the air is in contact with the heat transfer surface, ultimately diminishing the efficiency of heat exchange. Moreover, excessive velocity can lead to noise and vibration issues, and can also create hot spots or uneven temperature distributions, further compromising the desired heat transfer performance.

In contrast, enhanced heat transfer associated with turbulent flow typically occurs within optimal velocity ranges; beyond these ranges, the benefits can diminish. Other options, like causing water to vaporize or increasing pressure, do not directly relate to the core issue of heat transfer efficiency associated with excessive air velocity.