Heat fouling factors/resistances

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. 

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 performance of a heat exchanger depends upon the transfer surfaces being clean and uncorroded. The performance deteriorates with time due to accumulation of deposits on the heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. In the design of heat exchangers, this added resistance is accounted for by a fouling factor or dirt factor, R. ... 

Also, Rf 0, the resistance to heat transfer (fouling resistance or fouling factor) caused by the shell-side scale is equal to the last term in the denominator of Equa-tion4.11. 

Individual heat-transfer coefficients and the fouling resistance or fouling factor, are listed... 

These re.si.rtflnces include the inside-the-jacket film heat-transfer coefficient (HTC) and fouling factor, the inside-the-reactor HTC and fouling factor, and the reactor-wall resistance. Assuming U, M, Cp and A are con-... 

The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat exchanger to decrease. The net effect of these accumulations on heat transfer is represented by a fouling factor Rj, which i.s a measure of the thermal resistance introduced by fouling. 

A double pipe (shell-and-tube) heat exchanger is constructed of a stainless steel [k = 15.1 W/m O inner lube of inner diameter O/ = 1.5 cm and outer diameter 1.9 cm and an outer shell of inner diameter 3,2 cm. The convection heat transfer coefficient is given to be h,- = 800 W/m °C on the inner surface of the tube and h = 1200 W/m °C on the outer surface. For a fouling factor of f f, - 0.0004 m °C/W on the tube side and Ri =- 0.0001 m °C/W on the shell side, determine (a) the thermal resistance of the heat exchanger per unit iength,and (6) the overall heat transfer coefficients, Ujand U based on the inner and puter surface areas 0) the tube, respectively. 

SOLUTION The heat transfer coefficients and the fouling factors on the tube and shell sides of a heat exchanger are given. The thermal resistance and the overall heat transfer coefficients based on the inner and outer areas are to be determined. 

Consider a double-pipe heat exchanger with a lube diameter of 10 cm and negligible tube thickness. The total thermal resistance of the heat exchanger was calculated to be 0.025°CAV when it was first constructed. After some prolonged use, fouling occurs at both the inner and outer surfaces with (he fouling factors 0.000 15 CAV and 0.00015 °C/W, re-... 

Heat Transfer Coefficients 179 Overall Coefficients 180 Fouling Factors 180 Individual Film Coefficients 180 Metal Wall Resistance 182 Dimensionless Groups 182... 

In practical design calculations, we usually must consider the heat transfer resistance of the dirty deposits that accumulate on the heat transfer surface after a period of use. This problem of resistance cannot be neglected, in case the values of heat transfer coefficients h are relatively high The reciprocal of this resistance is termed the fouling factor, and this has the same dimension as h. Values of the... 

FOULING FACTORS. In actual service, heat-transfer surfaces do not remain clean. Scale, dirt, and other solid deposits form on one or both sides of the tubes, provide additional resistances to heat flow, and reduce the overall coefficient. The effect of such deposits is taken into account by adding a term l/dA to the term in parentheses in Eq. (11.27) for each scale deposit. Thus, assuming that scale is deposited on both the inside and the outside surface of the tubes, Eq. (11.27) becomes, after correction for the effects of scale. 

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. 

This subroutine is illustrated in Figure 2-57. The overall U is calculated from the sum of the heat transfer resistances shown in equation 47 (where ho is the outside film coefficient, Btu/hr. sq. ft. °F hio is the inside film coefficient, Btu/hr. sq. ft. °F and Tg is the fouling factor). The calculated surface equation 48 (where Q is the heat duty, Btu/hr. MTDc is the corrected mean temperature difference, °F) is compared with the actual surface equation 49, where ... 

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