Fuel wastage due to scaling of the boiler may be between 2% to 5% in water or fire tube boilers. Pretreatment of feedwater, adequate blowdown and regular boiler cleaning helps in minimising scaling.
Scale is a problem because its thermal conductivity is much less than the corresponding value of bare steel. Even a thin layer of scale acts as an effective insulator and retards heat transfer. This causes the boiler tube metal to overheat leading to tube failure and also reduced energy efficiency.
Scale deposits occur when calcium, magnesium, and silica, commonly found in most water supplies, react to form a continuous layer on the waterside of the boiler heat exchange surface.
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
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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 total dissolved solids (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 frequency of boiler blowdown. This results in energy saving.