Steam fouling factor

In practice, we must consider the heat transfer resistance of the dirt or scale which has been deposited on the metal surface, except when values of U are small, as in the case of gas heater or cooler. Usually, we use the so-called fouling factor h(, which is the reciprocal of the dirt resistance and hence has the same dimension as the film coefficient h. The dirt resistance sometimes becomes controlling, when U without dirt is very large - as in the case of liquid boiler heated by saturated steam. Thus, in case the dirt resistance is not negligible, the overall resistance for heat transfer l/Ll is given by the following equation ... 

Two fouling factors are obtained from Ref. El (p.845, Table 12) and converted into metric equivalents. A coefficient representative of air streams is used for the reaction-gas stream (F, = 0.002). The second coefficient is a typical value for treated boiler-feed water or steam (F, = 0.001). 

As shown in Chap. 11 [Eq. (11.35)], the overall resistance to heat transfer between the steam and the boiling liquid is the sum of five individual resistances the steam-film resistance the two scale resistances, inside and outside the tubes the tube-wall resistance and the resistance from the boiling liquid. The overall coefficient is the reciprocal of the overall resistance. In most evaporators the fouling factor of the condensing steam and the resistance of the tube wall are very small, and they are usually neglected in evaporator calculations. In an agitated-film evaporator the tube wall is fairly thick, so that its resistance may be a significant part of the total. 

The reduction of heat transfer coefficient can be treated as an empirical "fouling factor." Data published indicates that the apparent fouling factor can be correlated for condensing steam in the presence of noncondensables as below ... 

The capacity of these systems should normally be ample to support design power levels. The steam generator and condenser designs include fouling factors to accommodate reasonable degrees of scaling. 

The heat transfer coefficients of vacuum condensers depend on many different factors, particularly on the inert gas content and the flow rate in the condensation area. Relatively high heat transfer coefficients exist, for example, in turbine condensers, where sometimes enormous quantities of turbine exhaust steam are condensed under vacuum. The heat transfer coefficients depend on the condenser design and the inert gas content. Values from 2500 to 5000 W (m K) are common. In this case, pure water vapour has to be condensed, so that on the vapour side no fouling is to be taken into consideration. On the water side, the fouling depends on the cooling water quality, and therefore appropriate fouling factors have to be considered. 

Assume that the bottoms is on the shell side and steam is inside the tubes. Because the bottoms is almost pure, assume that it vaporizes at 138°F, whereas the steam condenses at 220°F. Therefore, ATlm = Ar = 220 - 138 = 82. From Table 13.5, under vaporizers, with propane on the shell side and steam condensing on the tube side, U = 200-300 Btu/ F-fF-hr. Note that this includes a fouling resistance of 0.0015 (hr-fF-°F)/Btu. The correction factor. Ft, is 1, regardless of the number of passes or flow directions, because at least one fluid is at a constant temperature in the exchanger. From Eq. (13.7), using 200 Btu/°F-ft -hr for U,... 

Efficient and trouble-free operation of boilers is very important when industrial plants generate their own power or when steam is required for manufacturing operations. It is, therefore, very important for engineers to understand the factors which lead to the fouling and corrosion of various components of boilers, and also to imderstand the methods which lead to the smooth and risk-free operation of boilers. 

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