Feedwater
Think back to the pot of boiling water. For the process of steam production to be continuous, as water
evaporates, it must be replaced at the same rate. In a boiler generating steam under pressure, then this
feedwater must be pumped in at a higher pressure. This is the job of the feed pump.
Another fact to consider is that whenever water is boiled the vapor is essentially pure and all of the impurities
remain behind. Eventually a pot or boiler would become encrusted with scale. Furthermore, at high saturation
temperatures, a boiler would be subject to corrosion because of oxygen present in the feedwater, and also
because the water turns slightly acidic at these temperatures. To prevent these problems the feedwater is
highly distilled (initially, in the ship's own distilling plant), deaerated (partly in the deaerating-feed heater), and
chemically treated to suppress remaining oxygen, to maintain slight alkalinity, and to inhibit scale formation.
Perhaps most important is the fact that almost all of the steam generated is recovered after use, and
condensed, to be reused as the boiler feedwater, with only enough make-up feed introduced to compensate
for losses.