The makeup water volume of boilers varies depending on the steam usage and the degree of condensate water recovery. The makeup water volume of condensing power station boilers is generally less than 3% of the evaporation capacity, while that of heating boilers can be as high as 100%.

 

1. Pretreatment

When the raw water is surface water, the purpose of pretreatment is to remove suspended solids, colloids, organic matter, etc. from the water. Usually, a coagulant is added to the raw water to coagulate the above impurities into large particles, which sink by their own weight and are then filtered into clear water. When groundwater or urban water is used as makeup water, it can be directly filtered. Common clarification equipment includes pulse type, hydraulic acceleration type and mechanical agitation type clarifiers; filtration equipment includes siphon filters, valveless filters and single-flow or double-flow mechanical filters, etc. To further remove organic matter from the water, an activated carbon filter can also be added.

2. Softening

Natural or artificial ion exchangers are used to convert calcium and magnesium hard salts into salts that do not form hard scale, so as to prevent calcium and magnesium hard scale from forming on the inner wall of boiler tubes. For water with high calcium and magnesium bicarbonate and high alkalinity, the hydrogen-sodium ion exchange method can also be adopted or solved in the pretreatment stage.

3. Desalination

There are many types of ion exchangers used in chemical desalination, and cation exchange resins and anion exchange resins, referred to as "cation resins" and "anion resins" for short, are commonly used. In the ion exchanger, when salt-containing water flows through the resin, the cations and anions in the salt are respectively exchanged with the cations (H+) and anions (OH-) in the resin and then removed.

 

When the alkalinity of water is high, a decarbonizer is generally required to be connected in series after the cation exchanger to remove carbon dioxide. For water with a particularly high salt content, reverse osmosis or electrodialysis processes can also be used to first desalinate the water quality, and then enter the ion exchanger for deep desalination. For drum boilers above high pressure or once-through boilers, trace silicon in the feed water must also be removed; for medium and low pressure boilers, treatment is carried out according to the content.