Heat fouling factor

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. 

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). 

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

Figure 10-144. Approximate relationship of the overall coefficient fouled, and the fouling factor of inside tubes for predicting the economical use of finned tubes in shell and tube units. (Used by permission Williams, R. B., and Katz, D. L. Performance of Finned Tubes and Shell and Tube Heat Exchangers, 1951. University of Michigan. Note For reference only, 1950 costs.)...

Ganapathy, V, Nomograph Relates Clean and Dirty Heat Transfer Coefficients, Fouling Factor, Heating/Piping/Air Conditioning, Jan. (1979) p. 127. 

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

Table 12.9. Fouling factors (coefficients), typical values for plate heat exchangers...

F specific energy (= force constant), kj/kg FF fouling factor (heat transfer), J/(m2 s °C)... 

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 ... 

A fouling factor of 2000 kcal h m °C is assumed. The overall heat transfer resistance 1/1/ based on the outer tube surface is... 

The correlations of friction in lines that will be presented are for new and clean pipes. Usually a factor of safety of 20-40% is advisable because pitting or deposits may develop over the years. There are no recommended fouling factors for friction as there are for heat transfer, but instances are known of pressure drops to double in water lines over a period of 10 years or so. 

Heat transfer coefficients are empirical data and derived correlations. They are in the form of overall coefficients U for frequently occurring operations, or as individual film coefficients and fouling factors. 

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