Tube Wall Resistance

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

Tables 10-16, 10-17, 10-18, and 10-18A give general estimating overall coefficients, and Table 10-19 gives the range of a few common film coefficients. Table 10-20 illustrates the effect of tube-wall resistance for some special construction materials. Table 10-20A lists estimating coefficients for glass-lined vessels. Also see Reference 215. See Table 10-24 for suggested water rates inside tubes.

Figure 10-45 can be used to solve the overall coefficient, U, equation for the clean coefficient, composed of the tube-side and shell-side film coefficients only. Correction for tube-side and shell-side scaling and tube-wall resistance can... 

Calculate the overall coefficient using Equation 10-37. Neglect the tube-wall resistance, unless special situations indicate that it should be included. 

Usually the tube wall resistance can be neglected, but if you doubt its effect, add to the resistances. 

For overall U cooling, assume water side fouling = 0.002 propylene side fouling = 0.001 neglect tube wall resistance... 

The tube density coefficient, c[), is given in Table 10-27. The tube wall resistance cannot be ignored for reboilers. Based on the outside tube diameter, ... 

Calculate the required area, based on the film coefficient of steps 4 and 5 together with fouling and tube wall resistances A = Q/U At. 

Plot this point as on Figure 10-123. Because the correlation is based on 14 BWG Admiralty tubes, no correction -was made for tube -wall resistance. 

Assume a temperature drop across the boiling film, Atj,. Neglect tube wall resistance. 

Table 5.17 gives estimates of the film transfer coefficients of an existing shell-and-tube heat exchanger, assuming the tube-wall resistance to be negligible. 

If the recommended factor of 0.7 on the refractory area is used, the effective area of the tubes is [22.0-1- (0.7)(3.18)]/22.0= 1.10 mVm of actual area. The exact evaluation of the outside tube temperature from the known oil temperature would involve a knowledge of the oil-film coefficient, tube-wall resistance, and rate of heat flow into the tube, the evaluation usually involving trial and error. However, for the present purpose the temperature drop through the tube wall and oil film will be assumed to be 41.7°C ( 5°F), making the tube surface temperatures 357°C (675°F) and 468°C (875°F) the average is 412°C (775°F). The radiating gas temperature is... 

The scale or fouling resistances represent a necessary safety factor that increases the surface of the heat exchanger. This enables the full process duty requirements to be attained between cleaning periods. When an exchanger is first placed in operation there is no dirt or scale on the tubes consequently, the overall resistance consists of the two film and the tube wall resistances. During operation, dirt or scale accumulates on the surface of the tubes and the overall heat transfer rate decreases as the dirt buildup increases. The rate of this scale or dirt depends on the cleanliness or fouling tendencies of the process fluids. 

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 selection of materials for these applications is often a compromise between the requirements of the process flow and the type of water. Associated with such heat exchangers are pumps, pipes, and valves to distribute the water and return it to source. The various metals commonly used in heat exchangers have quite different thermal conductivities (Table 8.9). However, the thermal conductivity of the metal wall is only one component of the resistance to heat transfer in a heat exchanger tube. In a condenser (i.e., where steam is condensing on cold tubes), for example, the resistance to heat transfer through a tube wall is made up of five main components as illustrated in Fig. 8.15 [8]. The tube wall resistance is comparatively small so that changes in thermal conductivity from the use of different metals in not necessarily very significant. 

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