Heat transfer Falling film

A minimum flowrate is required to induce a film for falling film equivalent. This minimum rate can be determined as below  

Once the film has been formed, a lower terminal flow rate can be achieved without destroying the film. This rate can be determined as below  

If the minimum rate is not achieved a film cannot be formed. If the terminal rate is less than that predicted by Equation 7.42, the film will break and form rivulets. Part of the tube surface will not be wetted and the result will be reduced heat transfer and increased fouling potential. 

Falling film units must be plumb. Tilted units will achieve lower heat transfer rates and offer higher fouling potential. Units with long tubes must be rigidly supported to counter the effects of wind and transmitted vibrations. 

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FIG. 11 -22 Kunz and Yerazunis Correlation for falling-film heat transfer. 

Sinek and Young present a design procedure for predicting liquid-side falling film heat transfer coefficients within 20% and overall coefficients within 10%. 

For vertical condensers, condensate can be readily subcooled if required. The subcooling occurs as falling-film heat transfer, so the procedure discussed for falling-film heat exchangers can be used to calculate heat-transfer coefficients. 

The relationships developed for these two cases are different and will be discussed below. Equipment using falling film heat transfer can be classified into vertical and horizontal systems. The vertical systems can include falling films on the inside or outside of tubes, or alternatively (in plate-type evaporators) on vertical flat plates. Generally, the liquid films are sufficiently thin to be treated as equivalent to the flat plate case for all of these configurations. Another important case is that of falling films on tube banks, as illustrated in Fig. 15.141 the... 

Falling film Heat-transfer area, A, ft 150-4,000 ft Cp = 10,800 4 Stainless steel tubes, carbon steel shell... 

Figure 7-8 Falling film heat transfer. No interfacial shear = 0).

For the falling-film mass transfer process as shown in Fig. 8.3, when the Ma number exceeds the critical value Macr, the linear stability analysis is not valid, and the nonlinear disturbance should be considered. Xiao [12] solved the following nonlinear disturbance equation for the process with heat and mass transfer as follows ... 

Film Heat Transfer Coefficient Value Most of the experimental data for liquid metals in forced convection have been obtained for round tubes. Since a large fraction of heat transfer to liquid metals in forced convection is by molecular and electronic conduction, the velocity and temperature distribution of the fluid in the channel is expected to have a noticeable effect. Until data are obtained for the reference channel, however, the data for round tubes is used with the equivalent diameter of the channel replacing the diameter of the tube. Most of the round tube data fall below the L.yon-Martinelli theoretical prediction, and therefore 85% of the Lyon-Martinelli Nusselt Number is used as the best average value in the range of Peclet Number of interest (500-1000). The factor shown in Table X represents the expected accuracy of experimental data. 

Points 2 and 4 are the main ones governing the choice of reac tor type. The high gas/liquid ratio restricts the choice to types d, e, i, and k of Fig. 23-25, but because of the high rate of heat transfer that is needed the choice falls to the falling film or tubular types. 

Heat transfer equipment has a great variation in heat transfer area per unit of material volume. Table 4.4 compares the surface compactness of a variety of heat exchanger types. Falling film evaporators and wiped film heat exchangers also reduce the inventory of material on the tube side. Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required. 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

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