What device develops a loading signal for the force draft blower speed control?

The correct choice for the device that develops a loading signal for the force draft blower speed control is the toe air flow controller. This device is specifically designed to manage the airflow into the combustion system, which directly correlates with the operation of the force draft blower. By optimizing and controlling the airflow, it ensures that the blower can adjust its speed appropriately to maintain the desired combustion conditions and efficiency.

In systems employing a toe air flow controller, it serves to regulate the amount of air introduced into the system based on feedback from the combustion process, thereby providing a signal that the blower control system uses to elevate or lower its speed as needed. This is critical in processes where maintaining the correct air-to-fuel ratio is essential for safe and efficient operation.

The other devices listed do serve important roles in pneumatic or combustion systems, but they do not directly generate the loading signal for the blower speed control in the same way the toe air flow controller does. Factors such as air pressure or flow regulation are related more to maintaining system balance and operating conditions rather than directly influencing the blower speed.