Film boiling condensing vapours

The equation given by Bromley (1950) can be used to estimate the heat-transfer coefficient for film boiling on tubes. Heat transfer in the film-boiling region will be controlled by conduction through the film of vapour, and Bromley s equation is similar to the Nusselt equation for condensation, where conduction is occurring through the film of condensate. 

Both the vapour evaporated from the boiling film and the concentrated product are discharged from the evaporator to a vapour-liquid separator from which the product is pumped, the vapour passing to the next effect of the evaporator, or the condenser. Compared with tubular evaporators, plate evaporators can offer important advantages in terms of headroom, floorspace, accessibility and flexibility. 

The vap. press, curve of solid iodine is indicated by PO, Fig. 16 that of liquid iodine by 00 and the effect of press, on the m.p. of iodine by ON. At the triple point 0 these curves meet. Fig. 18 shows a similar curve for water. The curve PO thus represents the sublimation curve or hoar-frost line OC. the boiling or vaporization curve, i.e. the effect of press, on the b.p. of the liquid. The same phenomenon occurs with water, iodine, etc., and the principle involved is the same as indicated in the law represented by Clapeyron-Clausius equations with respect to the lowering of the m.p. by an increase of press. Consequently, if the vap. press, of iodine be less than that of th,e triple point, the solid does not melt, but rather sublimes directly without melting at the triple point at 114-15° (89 8 mm.) and A. von Richter at 116 1° (90 mm.). According to R. W. Wood, if the condensation of iodine vapour occurs above —60°, a black granular deposit is formed, but below that temp, a deep red film is produced. 

Faraday reported that a current of steam passed into solutions of salts, acids, sugar, etc., raised the solution to the b.p. (over 100°), but the steam leaving the solution had a temperature of 100° as read by a thermometer. Gay-Lussac 5 said this had been found by Desormes, Clement, and Champy about 1810 but not published. Gay-Lussac thought the steam from a boiling solution had the temperature of the surface of the liquid, but condensed as a film of water on the thermometer bulb, which registered 100° if loss of heat from the thermometer were prevented, this condensation would not occur and the temperature of the vapour would be that of the solution. The question has often been discussed (mostly by industrial chemists), sometimes in favour of Faraday s, sometimes in favour of Gay-Lussac s, finding. Schreber s experiments favoured Faraday s Reissmann found exactly the opposite result. 

Similarly to Nusselt s film condensation theory, in the condensation of vapour mixtures, the heat flux transferred increases with the driving temperature difference — 1 0. According to Nusselt s film condensation theory, the heat transfer coefficient decreases with the driving temperature difference according to a ( oo- o) 1/4 (4-12). The heat flux increases in accordance with q co — q)3/4. Fig. 4.21a shows clearly that a minimum for the transferred heat flux exists at a certain temperature oa. This is because the temperature difference dj — r)(j between the condensate surface and the wall, which is decisive for heat transfer, also assumes a minimum this can be explained by the boiling diagram, Fig. 4.21. 

We will presume a sufficiently large temperature difference dd — dd0, such that the vapour in its initial state A condenses, and a condensate accumulates. This is indicated by point B in Fig. 4.21b. The temperature at the phase interface is then equal to the boiling point of the liquid mixture and the composition of the accumulated condensate is identical to that of the vapour. This is known as local total condensation. The wall temperature, which is assumed to be constant, is characterised by point C. The line BC corresponds to the temperature difference —dd0 that is decisive for the heat flux q. If Nusselt s film condensation theory was also valid for vapour mixtures, then q — dd0)3/4. If dd — dd0 is kept constant... 

The apparatus is completed by a receiver for the bottoms (J) and a transition piece (K) with a connection to the vacuum gauge. For deodorization work or for the processing of mixtures with a small proportion of low-boiling material the transition piece (K) is replaced by the deodorization piece (O). Through the lateral inlet the vapour of a low-boiling product can be passed counter to the falling liquid film in order to enhance the flow toward the condenser and to prevent recondensation of the distillate. A siphon prevents condensation in the bottoms receiver. 

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