Condensation outside vertical surfaces

Figure 10-71. Condensing steam film coefficients for vertical surfaces or horizontal tubes. G 7n,/ restricted to < 1,090. For theoretical h , for horizontal tubes, use and multiply results by 0.8. G = condensate mass flow per unit tube outside circumference, vertical tubes, lb/(hr) (ft). (Used by permission Devore, A. Petroleum Refiner, V. 38, No. 6, 1959. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

Condensation Outside Horizontal Tubes. Figure 8.14(d) shows a condenser with two tube passes and a shell side provided with vertically cut baffles that promote side to side flow of vapor. The tubes may be controlled partially flooded to ensure desired subcooling of the condensate or for control of upstream pressure by regulating the rate of condensation. Low-fin tubes often are advantageous, except when the surface tension of the condensates... 

These equations are strictly valid only for a plane vertical surface. However, they can be used for inside or outside vertical tubes with small error because the condensate film is thin compared with the diameter of a typical tube. Because of rippling and other nonidealities, the predicted coefficients are about 10-20% below experimental values. 

Film thidcness S is typically two or three orders of magnitude smaller than the tube diameter it can therefore be found, for flow either inside or outside a tube, from the equation for a flat plate, Eq. (5.77). Since there is a temperature gradient in the film, the properties of the liquid are evaluated at the average film temperature 7., given by Eq. (13.11). For condensation on a vertical surface, for which cos) = 1, Eq. (5.77) becomes... 

A guide to the overall performance of a wide variety of typical vertical evaporator tubes with condensing outside and vaporization inside is given in Fig. 11.25. This survey by Alexander and Hoffman [164] was specifically directed at vertical-tube evaporators for desalination systems. It is seen that the best surfaces yield increases in overall coefficient up to 200 percent. 

Let us first consider condensation of a vapor on a vertical surface. In this case the condensate in the form of a liquid laminar film will flow down the surface (see Figure 7-4). As can be seen, maximum velocity occurs on the outside of the film and minimum velocity (zero) at the solid surface. An analytical treatment for the mean heat transfer coefficient for the entire surface yields... 

Evaporators, Horizontal-Tube Type - The basic horizontal-tube evaporator is illustrated in Figure 12. The body of this evaporator is the liquor compartment and is in the form of a vertical cylinder. It is closed, top and bottom, with dished heads, although the bottom may be conical. The lower body ring is provided on opposite sides with steam compartments, closed on the outside by cover plates and on the inside by tube sheets. Between these tube sheets are fastened a number of horizontal tubes. The two steam chests with their connecting mbes form the steam compartment, and the tube wall heating surface. Steam is introduced into one steam chest and as it flows through the tubes it washes non-condensed gases and condensate ahead of it, so that these are withdrawn from the opposite steam chest. 

It should be noted that the Eqs. (11.19) and (11.21) are also valid for condensation on the outside surface of vertical tubes [44], provided the radius of the tube is large compared to the film thickness as indicated in Fig. 11.9. Since condensate films are typically very thin in practical situations, this condition is usually met. 

Stagnant saturated steam at 100°C is to be condensed on a 15-cm long vertical tube with an outside diameter of 25 mm. What tube surface temperature is needed to maintain a condensation rate of 0.6 kg per hour ... 

Saturated steam at 55°C is to be condensed at a rale of 10 kg/h on the outside of a 3-cm-outer-diameler vertical tube whose surface is maintained at 45 C by the cooling water. Determine the required lube length. 

The inner reference cylinder can be moved vertically and maintained concentric with the outer section. With the inner cylinder raised, a metal him can be evaporated to form the outer cylinder of the capacitor. The reference surface is prepared by coating the outside of the glass cylinder with a mixture of stannous oxide+1% antimony pentoxide. For most evaporated films, the condenser has a capacity of 30 pf. It is essential to screen effectively from electromagnetic and electrostatic interference. This is done by earthing the filament, an inner stannous oxide coating and an outer coating of liquid platinum. 

DEHUMIDIFYING CONDENSERS. A condenser for mixtures of vapors and noncondensable gases is shown in Fig. 15.9. It is set vertically, not horizontally like most condensers for vapor containing no noncondensable gas also, vapor is condensed inside the tubes, not outside, and the coolant flows through the shell. This provides a positive sweep of the vapor gas mixture through the tubes and avoids the formation of any stagnant pockets of inert gas that might blanket the heat-transfer surface. The modified lower head acts to separate the condensate from the uncondensed vapor and gas. 

Medwell and Nicol [98, 99] were among the first to study the effects of surface roughness on condensate films. They condensed steam on the outside of one smooth and three artificially roughened pipes with pyramid-shaped roughness. All were oriented vertically, and the con-... 

A major study of condensing on the outside of vertical enhanced tubes has been carried out at Oak Ridge National Laboratory in connection with geothermal Rankine cycle condensers. About 12 tubes were tested with ammonia, isobutane, and various fluorocarbons. The report by Domingo [163] on R-ll concluded that the best surface was the axially fluted tube, followed, in order, by the deep spirally fluted tube, spiral tubes, and roped tubes. The composite (vapor and tube wall) heat transfer coefficient was as much as 5.5 times the smooth-tube value. This high performance was further improved to a factor of 7.2 by using skirts to periodically drain off the condensate. 

Velkoff and Miller [336] investigated the effect of uniform and nonuniform electric fields on laminar film condensation of Freon-113 on a vertical plate. With screen grid electrodes providing a uniform electric field over the entire plate surface, a 150 percent increase in the heat transfer coefficient was obtained with a power expenditure of a fraction of one watt. Choi and Reynolds [337] and Choi [338] recently reported data for condensation of Freon-113 on the outside wall of an annulus in the presence of a radial electric field. With the maximum applied voltage of 30 kV, the average heat transfer coefficients for a 25.4-mm outside diameter by 12.7-mm inside diameter annulus were increased by 100 percent. 

Fig. 7.3-1 Condensers operated with cooling water, (a) Condensation on the outside surfaces of horizontal tubes (b, c) condensation on the inside surfaces of vertical tubes in cocurrent (b) or countercurrent (c) flow of vapor and condensate...

There is another type of arrangement for the measurement of surface potentials. An insulated plate connected with an electrometer is held above the film the plate and the substrate surface form a condenser whose capacity changes if the plate is moved in a direction vertical to the solution surface. If the plate be brought by means of an outside potential to the same potential as the substrate, no current flows when the plate is moved. This gives a means of measuring the potential at the surface. The horizontal plate may be made to vibrate by mechanical means and the resulting current flow used to locate equipotentials (50, 51). 

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