Boiler scale thermal conductivity

The thermal conductivity of an average boiler scale is 2.2 (W/m K) and that of complex silicate scales is 0.2-0.23 (W/m K). Since the furnace peak wall flux can be over 300,000 (W/m ) it may readily be seen that a small thickness of scale can raise the metal temperature into the creep region, resulting in very expensive repairs. 

Note 1 B. Kamp [Z. tech. Thysik, 12, 30 (1931)] shows the effect of increased porosity in decreasing thermal conductivity of boiler scale. Partridge [University of Michigan, ng. Research Bull, 15,1930] has published a 170-page treatise on Formation and Properties of Boiler Scale. 

The properties which determine heat transfer through a deposit layer of given thickness are thermal conductivity, emissivity, and absorptivity. These properties vary with deposit temperature, thermal history, and chemical composition. Parametric studies and calculations for existing boilers were carried out to show the sensitivity of overall furnace performance, local temperature, and heat flux distributions to these properties in large p.f. fired furnaces. The property values used cover the range of recent experimental studies. Calculations for actual boilers were carried out with a comprehensive 3-D Monte Carlo type heat transfer model. Some predictions are compared to full-scale boiler measurements. The calculations show that the effective conduction coefficient (k/As)eff of wall deposits strongly influences furnace exit temperatures. 

A common problem in boilers is the occurrence of calcium oxide build-up on the heating elements. This is not a corrosion problem in itself, because it is caused by a chemical reaction in the water at high temperatures. However, a scale deposit present on a metal surface may cause corrosion under the deposit. This type of underdeposit corrosion can be aggravated when corrosive species such as sulfides and/or chlorides are present in the water. While scale deposits reduce the thermal conductivity of the steel, and thereby increase energy costs, corrosion of the heating element can lead to a catastrophic tubing failure, which requires costly repairs. 

Scale formation is thus a major problem of failure in steam boilers where hard water is used. Thermal conductivity of silicate scale is so low (0.0008 W/cm K) that a buildup of as little as 0.05 mm can cause boiler failture. 

Industrial chemical cleaning involves the use of reactive chemicals to remove unwanted deposits from the surfaces of various pieces of process equipment. Included are components of power-generating units such as boilers and condensers, heat exchangers in refineries and petrochemical plants, and other industrial equipment such as digesters in paper mills. The chemical removal of unwanted surface deposits is conducted for many reasons. The first reason is to eliminate scales that contribute to increased corrosion. Examples of these types of deposits include iron oxides and copper found on the watersides of many types of process equipment. The second reason is to increase heat transfer. While steel may have a thermal conductivity of... 

This book tells the reader how to make measurements and conduct tests in industrial combustion systems including full-scale burners, furnaces, heaters, boilers, flares, and thermal oxidizers. 

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