The treatment and conditioning of boiler feed water must satisfy three main objectives:

Continuous heat exchange

Corrosion protection

Production of high quality steam

Water softening is the reduction or removal of impurities from water outside the boiler. In general, this is important when the amount of one or more of the feed water impurities is too high to be tolerated by the boiler system in question. Water softening is one of the first treatment which can be used to tailor make feed-water for a particular system. 

The water softening equipment purify make-up water or feed water. Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all of calcium & magnesium  ions are removed and replaced with sodium ions.