Spraying, atomizing and powder coating can be achieved with slightly different processes. Sometimes pressure is controlled and sometimes flow is controlled. In this application, we are controlling pressure to an orifice, which is basically controlling flow.
This drawing shows a QB2 and an R-Series volume booster controlling pressure to a known orifice to achieve a specific flow rate for atomizing the powder. The 2-loop concept in this drawing is a unique and cost-effective way to indirectly control flow through pressure control. If you are interested in directly controlling the molecules (flow) to your process, learn more here.
The orifice in this drawing is more about limiting the flow to the process. But, since the 2nd loop transducer is sensing pressure on the downstream side of the orifice, pressure at the tank is what is ultimately being commanded.