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Fuel waste due to boiler scale may be 2% for water-tube boilers and upto 5% in fire-tube boilers. Pretreatment of feedwater, adequate blowdown and boiler cleaning minimize scaling.

Scale creates a problem because its thermal conductivity is much less than the corresponding the corresponding value of bare steel. Even thin layers of scale serve as an effective insulator and retard heat transfer. The result is overheating of boiler tube metal, tube failures and loss of energy efficiency.

Scale deposits occur when calcium, magnesium, and silica, commonly found in most water supplies, react to from a continuous layer of material on the waterside of the boiler heat exchange tubes.

Independent of the size of the boiler, prevention of scale formation can produce substantial energy savings.

Scale Thickness, mm

Fuel Loss, % of Total Use

Scale Type

Normal

High Iron

Iron plus silica

1.5

1

1.6

3.5

3

2

3.1

7

4.5

3

4.7

-

6

3.9

6.2

-

Scale formation can be best dealt with in two major ways,

  • Reducing the incoming feedwater TDS
  • Maintaining the TDS levels in the boiler

Feedwater tds is maintained by chemical pretreatment of water and blowdown maintains the boiler tds levels.

Condensate being pure, its recovery to feedwater tank reduces the requirement for addition of chemicals and maintains / reduces the feedwater TDS level and also the boiler blowdown frequency, thus saving energy.