A method operates a heat exchanger unit and determines a current physical characteristic of a process fluid at or adjacent a process fluid outlet of the heat exchanger unit and predetermining a desired physical characteristic of the process fluid at which the process fluid is desired to exit the heat exchanger unit. If the current physical characteristic of the process fluid is greater than the desired physical characteristic of the process fluid, then either a current fan speed is incrementally increased or the current fan speed is set at a predetermined low set point fan speed. Alternatively, if the current physical characteristic of the process fluid is less than the desired physical characteristic of the process fluid, then either the current fan speed is incrementally decreased or the current fan speed is set at a predetermined high set point fan speed or an OFF operating mode is activated.