How is condensate returned to the boiler along with makeup water?

The correct choice highlights the role of an open feedwater heater in the process of returning condensate to the boiler along with makeup water. An open feedwater heater operates at atmospheric pressure, allowing both condensate and makeup water to be blended together before entering the boiler system. This process is crucial because it not only utilizes the thermal energy from the condensate, thus improving the overall efficiency of the heating system, but it also ensures that any impurities that might be present are manageable before the water is reintroduced into the boiler.

The design of an open feedwater heater allows for the easy integration of condensate, as it can effectively handle varying amounts of both the returned condensate from steam systems and the necessary makeup water to replace losses. This combined approach helps maintain the appropriate water levels within the boiler while maintaining effectiveness in heat recovery.

In contrast, closed feedwater heaters operate under higher pressures and are used primarily where heat exchange occurs without allowing the condensate to mix freely with the makeup water. The direct return method and gravity feed system have more limited applications and functions when it comes to accomplishing the simultaneous return of condensate and makeup water, lacking the efficiency and benefits seen with an open feedwater heater.