What device limits the opening of the pre-rotation vanes during startup?

The device that limits the opening of the pre-rotation vanes during startup is best understood through the function of a load limiting mechanism. This mechanism is essential in maintaining efficient operation of the HVAC system and preventing excessive strain on components during startup.

When a system first begins operating, there is a potential for high torque and energy demand, which can lead to mechanical stress. The load limiting mechanism serves to control how much the pre-rotation vanes can open, helping to manage the airflow and pressure within the system. By doing this, it helps to stabilize the system as it transitions towards its normal operating conditions.

In contrast, options like the control valve, temperature sensor, and pressure switch have other specific functions. A control valve typically regulates the flow of media within the system rather than managing vane positions. Temperature sensors help monitor the thermal conditions, which is not directly related to the startup process of vanes. Pressure switches are designed to react to certain pressure levels, but they don't specifically adjust the opening of the vanes during startup. Thus, the load limiting mechanism is indeed the key device that ensures a controlled and safe startup process for the pre-rotation vanes.