food grade product line by Water Services Ltd

Deposit formation.

The dissolved solids in boiler water tend to concentrate as steam is generated. The dissolved solids can come out of solution and form scale. The dissolved solids also can form a sludge in the boiler which can form a deposit.

a. Some dissolved solids become less soluble as the water temperature increases.

This includes the salts of calcium and magnesium [(CaCO3 , CaSO4 , MgCO3 , Mg(OH)2 ]. As a result of this property, these materials will tend to form scale in the hotter areas of the boiler.

b. Scale can occur locally due to evaporation as illustrated in figure below. When a steam bubble forms on a heated surface, a thin film of water between the bubble and the tube wall becomes more concentrated with the dissolved materials. This thin layer also can be as much as 30 o F hotter than the average boiler water temperature. These local conditions can cause the precipitation of the solids and the local formation of scale.

c. Sludge deposits can form when the precipitated materials in the boiler water become attached to surfaces.

LOCALIZED SCALE FORMATION PROCESS.