In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reach saturation point, they are thrown out of the water in the form of precipitates on the inner walls of the boiler. If the precipitation takes place in the form of loose and slimy precipitate, it is called sludge. On the other hand, if the precipitated matter forms a hard, adhering crust/coating on the inner walls of the boiler, it is called scale.
Sludge is a soft, loose, and slimy precipitate formed within the comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. Sludges are formed by substances that have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc.
What is sludge in the boiler?
Boiler sludge is a deposit that forms when suspended materials present in the boiler water settle on, or adhere to, hot boiler tubes or other surfaces. Sludge may be formed from a combination of whatever suspended materials are in the water, including loose corrosion products, insoluble mineral precipitates, and oil.
What are scales in the boiler?
Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. in a boiler causes impurities to concentrate. This interferes with heat transfers and may cause hot spots.
Define scale and sludge. Give the causes, disadvantages of scales in boiler.
Difference Between Sludge and Scale in Boiler
Sludge
1 Due to heating, the salt-containing water will get concentrated and changes into loose and slimy precipitates called sludge.
2 Sludge is not harmful to the boiler.
3 Sludge can be removed from the bottom of the boiler from time to time.
4 Sludge doesn’t cause clogging and corrosion.
5 Sludge increases the discarding cost of waste.
Scale
1 When these precipitates become hard and adherent called scales.
2 Scales are harmful to the boiler.
3 Scale can be removed by either external or internal treatment or hammering.
4 Scale causes clogging and corrosion.
5 Scale increases the maintenance and operation cost.
Boiler sludge is a deposit that forms when suspended materials present in the boiler water settle on or adhere to, hot boiler tubes or other surfaces. Sludge may be formed from a combination of whatever suspended materials are in the water, including loose corrosion products, insoluble mineral precipitates, and oil.
Decrease in Efficiency of Boiler: The excessive scale formation may cause clogging of boiler tubes which decreases the efficiency of the boiler.
Wastage of Fuel: Scale is a poor conductor of heat. This results in a reduced rate of heat transfer, and thus the evaporative capacity of the boiler will be reduced.