Heat exchanger tube-side fouling coefficient

Heat-transfer coefficient for cross flow over an ideal tube bank Fouling coefficient on outside of tube Heat-transfer coefficient in a plate heat exchanger Shell-side heat-transfer coefficient Heat transfer coefficient to vessel wall or coil Heat transfer factor defined by equation 12.14 Heat-transfer factor defined by equation 12.15 Friction factor... 

At one Midwest refinery, on a 180,000 BSD crude unit, two parallel, 6000 square ft, titanium twisted tube bundles have been in service for a number of years. Crude tower overhead vapors plus steam are condensing on the shell side. Crude is on the tube side. Fouling on both the shell side and the tube side appears to be quite minimal. However, the heat-transfer coefficient is bad only about 20-25 Btu/hr/ ft /°F, even when the exchanger is clean. 

This is an exchanger that was designed to operate with a low tube-side velocity. Note how there is a rapid loss in the heat-transfer coefficient, as the tubes foul and plug, as a result of low velocity. The loss in heat-transfer coefficient U stops only when the terminal-tube velocity is reached in the unplugged tubes. 

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done ... 

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. 

Acetone (s = 0.79) at 250°F is to be sent to storage at 100°F and at a rate of 60,000 Ib/hr. The heat will be received by 185,000 Ib/hr of 100 percent acetic acid (s = 1.07) coming from storage at 90°F and heated to 150°F. Pressure drops of 10.0 psi are available for both fluids. Assuming that the fouling factor on the tube side is 0.001 and that on the shell side is 0.003, calculate the heat transfer coefficients for the tube and shell sides, the overall heat transfer coefficient for the exchanger, outside area of unit, and the heat transferred. 

If fouling due to chemical reaction is anticipated on the organic liquid side of the tube, operating at the higher velocity would reduce the problem since chemical reactions are temperature sensitive. Furthermore the shear force will be increased by a factor of around 4 which is also likely to reduce the extent of the fouling. In addition, because of the increased overall heat transfer coefficient at the higher liquid velocity, a heat exchanger based on these data would require a smaller heat... 

To determine the heat transfer coefficient ki in Eq. (7-14), see, for example [0.1, 12A-126] covering the field of heat transfer problems in evaporation processes. During solution evaporation and crystallization, fouling and incrustation of the heat exchange areas has to be considered, which may lead to substantially lower evaporation rates in the evaporator. With incrustation on the product side of the evaporator tube wall, the heat transfer resistance is... 

Standard construction for shell-and-tube exchangers includes nickel tubes and carbon steel in cooling water service on the shell side. Compact heat exchangers also are widely used in caustic service. They have found a growing market in chlor-alkali plants. Their high heat-transfer coefficients and resistance to fouling reduce the surface area required. This is especially valuable when using expensive materials such as nickel, titanium, and the Hastelloys. 

This U value is called as clean U value because fouling resistances (/ j, / o) are not taken into account in equation (6.2). The film coefficients, hi and h, can be calculated based on the fluids physical properties and the geometry of the heat exchanger. For example, for U-tube exchangers with streams all liquid or aU vapor (no boiling and condensing), the correlation (Dittus and Boelter, 1930) is used to estimate the tube side Nusselt... 

The thermal performance of the designed heat exchanger can be checked by calculating the overall heat transfer coefficient. This required calculating the tube side and shell side heat transfer coefficients, the tube wall contribution to the resistance, and the appropriate fouling resistance. The overall heat transfer coefficient, based on the outside surface area of the tubes is... 

The corrected overall heat transfer coefficient is within the design range (140-260 Btu/ft h °F). The assumed value should match U-value estimated from the heat exchanger design specifications that depends on the film heat transfer coefficient of tube side and shell side, fouling factor, and metal resistance. 

The range of overall heat transfer coefficients (U) is about 10 - 200 Btu/hr fC°F. Table 8-11 lists the U-values for various types of equipment, and fouling factors of some flowing media are shown in Table 8-12. Table 8-13 illustrates a heat exhanger specification form. Factors governing the selection of process fluids in the tube and shell sides of an exchanger are illustrated in Table 8-14. 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign. ... 

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