Fouling factor

Fouling generally reduces heat transfer by imposing an additional resistance. It s effect is accounted for by use of 

There may be a fouling factor for both the inside and the outside of the pipe. Adding these resistances to the others  

By means of the electrical analogy, we are able to guess the form of the correction without having to go through the analysis. 

In all the problems we have worked so far, we have been given the values of the 1-phase heat transfer coefficients (the A/ and Now we are going to turn our attention to how these can be estimated if they are not otherwise known. The empirical correlations appropriate for a given situation usually depend on three questions  

What is the geometry (e.g flow inside pipe, flow across bank of tubes, flow around sphere) 

Heat transfer may be degraded in time by corrosion, deposits of reaction products, organic growths, etc. These effects are accounted for quantitatively by fouling resistances, l/hf. They are listed separately in Tables 8.4 and 8.6, but the listed values of coefficients include these resistances. For instance, with a clean surface the first listed value of V in Table 8.4 would correspond to a clean value of U = 1/(1/12 — 0.04) = 23.1. How long a clean value could be 

After a period of operation the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems, or the surfaces may become corroded as a result of the interaction between the fluids and the material used for construction of the heat exchanger. In either event, this coating represents an additional resistance to the heat flow, and thus results in decreased performance. The overall effect is usually represented by a fouling 

Tabl 10-2 Table of Normal Fouling Factors, According to Ref. 2. 

Fouling factors must be obtained experimentally by determining the values of U for both clean and dirty conditions in the heat exchanger. The fouling factor is thus defined as 

An abbreviated list of recommended values of the fouling factor for various fluids is given in Table 10-2, and a very complete treatment of the subject is available in Ref.[9], 

Suppose the water in Example 10-2 is seawater above I25°F and a fouling factor of 0.0002 nr °C/W is experienced. What is the percent reduction in the convection heat-transfer coefficient  

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The rate of heat-transfer q through the jacket or cod heat-transfer areaM is estimated from log mean temperature difference AT by = UAAT The overall heat-transfer coefficient U depends on thermal conductivity of metal, fouling factors, and heat-transfer coefficients on service and process sides. The process side heat-transfer coefficient depends on the mixing system design (17) and can be calculated from the correlations for turbines in Figure 35a. 

Fouling factor, water side 0.0002 heating or cooling streams are shown at top of columns as C, D, F, G, etc. to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply hy 5.6783 to convert hours per square foot-degree Fahrenheit-British thermal units to square meters per second-kelvin-joules, multiply hy 0.1761. 

Psig In Psig Out F in F out Velocrty, FPS Fouling Factor F, Max Delta-P... 

Cooley. Specify twin coolers with non-interrupting transfer valve. Spevily materials of construction, utilities, and fouling factor,... 

Experience and judgment as to fouling severity are required to estimate fouling factors (FF , FFj) to determine the overall heat transfer... 

Ri and Ro are fouling factors. Fouling factors are normally included to allow for the added resistance to heat flow resulting from dirt, scale, or corrosion on the tube walls. The sum of these fouling factors is normally taken to be 0.003 hr-ft--°F/Btu, although this value can vary widely with the specific service. 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

The percentage effect of the fouling factor on the effective overall heat transfer coefficient is considerably more on units with the normally high value of a clean unfouled coefficient than for one of low value. For example, a unit with a clean overall coefficient of 400 when corrected for 0.003 total fouling ends up with an effective coefficient of 180, but a unit with a clean coefficient of 60, when corrected for a 0.003 fouling allowance, shows an effective coefficient of 50.5 (see Figure 10-39). 

Fouling factors as suggested by TEMA are shown in Table 10-12. These values are predominantly for petroleum... 

The fouling factors are applied as part of the overall heat transfer coefficient to both the inside and outside of the... 

Figure 10-40A. Fouling factors as a function of temperature and velocity. (Used by permission W. L. Nelson, No. 94 in series, Oil and Gas Journal. PennWell Publishing Company.)...

Figure 10-43A. Predict fouling by nomograph, Part 1. Calculation of R value for fouling factor use in conjunction with Figures 10-43B and 10-43C. (Used by permission Zanker, A., Hydrocarbon Processing. March 1978, p. 146. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

B = constant, equal to fouling factor at infinity flow rate F or ff = fouling factor... 

As the value of B or the fouling factor increases with time, the engineer can determine when the condition will... 

Note the relative effects of the tube wall resistance when compared to the fouling factors in this case. 

Figure 10-39 presents the effect of total fouling on the overall coefficient. For example, if a clean nonfouled coefficient is corrected to the fouled condition by one overall fouling factor, the effect of changing the expected amount of fouling to another value can be readily determined. 

Fouling factors typical to process fluids and materials of construction are included. Multiply by 4.882 for conversion to kcal/(hr) (m )(°C). 

Use same fouling factors as for first trial. Overall coefficient. 

Calculate the overall fouling coefficient, adding the appropriate fouled factors to clean, U. ... 

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