Fouling coefficient

U = Overall heat transfer (fouled) coefficient. Equation 10-37... 

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

Since no serious problems exist with corrosion, the materials of constmction can be cheaper and the thicknesses reduced. Fouling coefficients do not need to be incorporated into the design calculations, as the surfaces remain clean indefinitely. This in itself provides a secondary benefit in that the pitching of fins may be reduced without the risk of reducing flow. 

Typical values for the fouling coefficients and factors for common process and service fluids are given in Table 12.2. These values are for shell and tube exchangers with plain (not finned) tubes. More extensive data on fouling factors are given in the TEMA standards (1999), and by Ludwig (2001). 

Take the fouling coefficients from Table 12.2 methanol (light organic) 5000 Wm brackish water (sea water), take as highest value, 3000 Wm 2oC-1... 

Fouling coefficient Shell diameter/baffle spacing... 

For heating media other than steam and process side fouling coefficients different from 6000 W/m2oC, the design heat flux taken from Figure 12.59 may be adjusted as follows ... 

Take steam condensing coefficient as 8000 W/m2oC, fouling coefficient 5000 W/m2oC butane fouling coefficient, essentially clean, 10,000 W/m2oC. 

The heat transfer through the resistance created by the outside (shell-side) fouling is quantified by a fouling coefficient given by ... 

Table 15.2 gives typical values of fouling coefficients1-4. Fouling is often quoted as a fouling factor. This is simply the reciprocal of the fouling coefficient. 

As pointed out previously, fouling is dependent on time, fluid velocity, temperature and other factors. Table 15.3 illustrates this with typical fouling coefficients for crude oil as a function of fluid velocity and temperature. 

It is also interesting to consider the heat transfer coefficient of the tube wall relative to the film and fouling coefficients. Table 15.4 tabulates the coefficients for a variety of materials for some standard tube sizes. It should be noted that the sizes in Table 15.4 are not universally standard. 

Table 15.3 Fouling coefficient for crude oil as a function of temperature and fluid velocity (W-m ...

The exchanger type to be used is 1 shell-pass, 2 tube-passes. Because crude oil will have a greater tendency to foul than kerosene, crude oil will be allocated to the tube-side. Assume the fouling coefficient for kerosene on the shell-side is 5000 W m 2 K 1 and for the crude oil on the tube-side is 2000 W m 2 K 1. 

The effect of the fouling on the shell-side flow is to increase the cross-flow and increase the overall heat transfer coefficient for a fixed pressure drop (assuming the same fouling coefficients in both cases). 

Given the uncertainties associated with the calculations, especially those on the shell-side, a sensible design basis for the heat transfer area specification would be the shell-side flow characterized by the clean condition. Of course, the fouling coefficients for the shell-side and tube-side should be included to account for the surface fouling resistance. 

If the fouling coefficients are assumed to be unchanged, the new overall heat transfer coefficient is given by ... 

For the simulations, the heating was assumed to be supplied using saturated steam on the shell-side with a combined condensation and fouling coefficient of 5700 W m-2 K-1. The fouling coefficient on the tube-side was assumed to be 5700 W m 2 K 1. 

The correlation should be used with caution outside the range 0.6 < Tr < 0.8 and should not be used below a pressure of 0.3 bar. When dealing with a clean, nondegrading material, the process fouling coefficient should be increased to around 11,000 W m 2 K 1, but should be reduced to 1400 to 1900 W m 2 K 1 for material that has a tendency to polymerize17. If a shell-side coefficient of process fouling coefficient different from 5700 W m 2 K 1 is required, the corrected overall heat transfer coefficient can be calculated from17 ... 

This does not need to be corrected using Equation 15.102 as the steam condensing film coefficient and process fouling coefficient agree with the assumptions on which the correlation is based. 

For the three heat exchangers from Exercise 1, make a first estimate of the order of magnitude of the overall heat coefficients from tabulated values of film transfer coefficients and fouling coefficients. Neglect the resistance from the tube walls. 

Assume the fouling coefficients to be 10,000 W-m-2-K 1 and 5000 W rn 2 K 1 for isopropanol and cooling water respectively. For an assumed cooling water velocity of 1 m-s 1, estimate the heat transfer area for ... 

The two film coefficients are 100 each, the two fouling coefficients are 2000 each, the tube outside diameter is 0.1 ft, wall thickness is 0.01 ft, and thermal conductivity of the metal is 30 ... 

TABLE 8.6. Typical Ranges of Individual Film and Fouling Coefficients [A Btu/(hr)(sqft)(°F)]... 

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