What prevents windmilling of an idle forced draft blower while the other blower is online?

The most effective mechanism to prevent windmilling of an idle forced draft blower while another is operational is the use of automatic shutters. When one blower is running, the automatic shutters close off the inlet of the idle blower, preventing airflow from entering and subsequently stopping it from windmilling. Windmilling can occur when air continues to flow through an idle fan, causing it to spin without being driven by a motor. By automatically closing the shutters, the system efficiently isolates the non-operational blower, thus mitigating unnecessary wear and preventing inefficient operation.

Other methods, such as manual shut-off valves, could also be used to close off airflow, but they require manual operation and aren't automatic, making them less efficient for this purpose. Pressure sensors and flow regulators play supportive roles in monitoring and controlling airflow and pressure within the system, but they do not specifically prevent windmilling in the same direct manner as automatic shutters do.