Which device is used to restrict the flow through the nozzle and flapper assembly in automatic boiler controls?

The correct choice is the flapper. In automatic boiler controls, the flapper assembly serves as a crucial component that modulates the flow of steam or water by restricting flow through the nozzle. The flapper responds to changes in pressure or flow rate, effectively opening or closing to maintain the desired operating conditions within the boiler system.

This functionality is essential for maintaining stable boiler operations, as it influences how much steam is released based on the system's demands. The precision with which the flapper can adjust the flow is vital for achieving efficient operation and safety in the boiler system. It ensures that the boiler can respond quickly to varying load demands while keeping parameters within safe limits.

The other devices mentioned have different roles. A valve actuator typically controls larger valves but does not specifically regulate flow in a nozzle and flapper assembly context. A pressure regulator is used to maintain specific pressure levels in a system rather than controlling flow direction or rate. A flow meter measures the amount of fluid passing through it but does not actively control the flow itself. Thus, the flapper is distinctly positioned as the component responsible for flow restriction in this scenario.