Boiler Blowdown Systems
Even after water treatment, a small amount of impurities will still be present in the boiler feed water. At this level of concentration the impurities would be harmless. However, most of the impurities are left behind in the boiling water as steam is produced. To replace the steam more feed water is added. Which also contains impurities. Over a period of time these impurities increase in the water in the boiler, and become concentrated to a level which can cause problems such as fouling or corrosion.
Chemicals may be added directly to boilers to prevent scale from forming on the metal surfaces in the boiler. These chemicals work by turning the scale-forming substances into a silt or slurry which will not stick to metal. These slurries, however, get in the way of heat transfer and could eventually fill the boiler with solids as they will become concentrated in the same way as dissolved impurities.
High concentrations of impurities and silts can be prevented by regularly removing a small amount of water from the boiler. This is known as boiler blowdown.