What is defined as surface and continuous blow down in relation to boiler feed water?

The choice stating that feed water continually skimmed from the surface is defined as surface and continuous blow down is correct because blow down refers to the process of removing a portion of the water from a boiler to control the concentration of impurities and to maintain water quality.

Surface blow down specifically targets the contaminants that accumulate on the surface of the boiler water. As impurities float to the top, skimming this water helps in maintaining optimum performance while preventing scaling and foaming. This action is continuous as it is usually done to maintain consistent water quality and pressure, ensuring that the feed water entering the system is of an appropriate purity level.

In contrast, other options refer to different processes or actions associated with boiler operation. For instance, water drained from the bottom of the boiler typically relates to bottom blow down, which focuses on removing heavier sediment that settles at the bottom. Water added to maintain the level describes the make-up water necessary to replace water lost through evaporation or blow down, rather than explicitly relating to the blow down process itself. Scheduled maintenance might involve deeper inspections or repairs that could require manual removal of water, but this is separate from the ongoing management of water chemistry through blow down.