Drop-wise condensation

The normal mechanism for heat transfer in commercial condensers is filmwise condensation. Drop wise condensation will give higher heat-transfer coefficients, but is unpredictable and is not yet considered a practical proposition for the design of condensers for general purposes. 

Although drop-wise condensation can produce much higher condensing film transfer coefficients, it is unpredictable, and the design is carried out on the basis of film-wise condensation. 

Fig. 4.17 Experimental results for drop-wise condensation of water at a condensation pressure of about 1 bar according to [4.24]. 1 Hampson and Ozisik 1952, graph for two different promoters [4.25] 2 Wenzel 1957 [4.26] 3 Welch and Westwater 1961 [4.27] 4 Kast 1965, chromium-plated (upper line) and plain copper surfaces [4.28] 5 Le Fevre and Rose 1965, different promoters [4.29] 6 Tanner et al. 1968, different promoters [4.30] 7 Griffith and Lee 1967, gold-plated condensation surfaces upper curve, copper, middle curve, zinc and lower curve, steel [4.31]...

Two types of condensation, drop-wise and film-wise, have been known for many years. As soon as the two types of boiling, nucleate and film, were described, certain similarities to condensation became evident. Nucleate boiling and drop-wise condensation were seen to be analogous. This is of little practical value, because no good theory of drop-wise condensation exists. However the analogy between film boiling and film condensation is fruitful, because a good theory of film condensation exists. 

Forced Circulation. The fourth is a process which was proposed by Dodge and Eshaya and extensively studied from an engineering and economic standpoint under Office of Saline Water auspices (15). A pilot plant was proposed (14), but no steps were taken to implement the proposal. The method involves heating of brine under nonboiling conditions at linear flow velocities of the order of 8 to 10 feet per second, followed by flash evaporation and compression of the flashed vapor for use as the heating medium. Drop wise condensation of the vapor was another feature of the process. A somewhat similar process has been selected for the government demonstration plant at Roswell, N. M. This plant will use vapor compression in conjunction with multiple-effect (two or three effects) evaporation, and as far as we are aware this is the first time that this combination has ever been used. 

Dodge, B. F., Eshaya, A. M., Development of a Pilot Plant for a Forced-Circulation and Drop wise Condensation Vapor-Compression Evaporator and an Experimental Program, Rept. to OSW under Contract 14-01-001-172 (September 1959). 

Promotion of Dropwise Condensation. Preliminary heat transfer experiments, using distilled water as feed, gave values of over-all heat transfer coefficient around 1000 B.t.u./sq. ft hr. ° F. and analysis indicated that the jacket condensate film provided the main resistance to heat transfer, due to filmwise condensation of the jacket steam. Following reports by Garrett (4) of successful tests using drop-wise condensation promoters, it was decided to use oleic acid for this purpose in the present experiments. The resulting improvement in jacketside coefficients enabled over-all heat transfer coefficients in excess of 5000 B.t.u./sq. ft hr ° F. to be achieved. 

There is a suggestion that peak heat fluxes in the evaporators of rotating heat pipes increase as the one-fourth power of acceleration (Costello and Adams, 1960). While the condenser performance has been less well documented, high G forces allow very thin film thicknesses and continuous irrigation of the surface, reducing the thermal resistance across it. Because of the sealed nature of heat pipes and other rotating devices, even further enhancement of condenser performance could be achieved by promoting drop-wise condensation. 

The reader who has an interest in power generation plant efficiency may like to consider how drop-wise condensation on a large power station condenser nfight improve the electricity output, and by how much. Bear in mind that only one side of the condenser is being enhanced in this way. 

A 1500 ml. flask is fitted (preferably by means of a three-necked adaptor) with a rubber-sleeved or mercury-sealed stirrer (Fig. 20, p. 39), a reflux water-condenser, and a dropping-funnel cf. Fig. 23(c), p. 45, in which only a two-necked adaptor is shown or Fig. 23(G)). The dried zinc powder (20 g.) is placed in the flask, and a solution of 28 ml. of ethyl bromoacetate and 32 ml. of benzaldehyde in 40 ml. of dry benzene containing 5 ml. of dry ether is placed in the dropping-funnel. Approximately 10 ml. of this solution is run on to the zinc powder, and the mixture allowed to remain unstirred until (usually within a few minutes) a vigorous reaction occurs. (If no reaction occurs, warm the mixture on the water-bath until the reaction starts.) The stirrer is now started, and the rest of the solution allowed to run in drop-wise over a period of about 30 minutes so that the initial reaction is steadily maintained. The flask is then heated on a water-bath for 30 minutes with continuous stirring, and is then cooled in an ice-water bath. The well-stirred product is then hydrolysed by the addition of 120 ml. of 10% sulphuric acid. The mixture is transferred to a separating-funnel, the lower aqueous layer discarded, and the upper benzene layer then... 

A. Coumarin dibromide. In a 1-1. three-necked flask, equipped with a mechanical stirrer, dropping funnel, and condenser fitted with a trap for hydrogen bromide, are placed 146 g. (1 mole) of coumarin and 200 ml. of chloroform. A solution of 160 g. (1 mole) of bromine in 85 ml. of chloroform is added drop-wise to the well-stirred solution of coumarin at room temperature over a period of 3 hours (Note 1). Excess bromine is removed by adding approximately 200 ml. of a 20% solution of sodium sulfite through the dropping funnel. The colorless chloroform layer is separated, washed with two 200-ml. portions of water, and dried over magnesium sulfate (Note 2). After the mixture has stood for only a few minutes (Note 3), the magnesium sulfate is removed by filtration and the solution of the dibromide... 

A 5-1., three-necked, round-bottomed flask is equipped with an efficient stirrer (Note 1), a reflux condenser, and a dropping funnel. Water (1850 ml.) is added, stirring is begun and continued throughout the procedure, and 783 g. (12.0 moles) of zinc dust (Note 2) and 700 ml. of acetic acid are added. The mixture is heated to reflux, the heating mantle is removed, and 1283 g. (4.00 moles) of 2,3,5-tribromothiophene (Note 3) is added drop-wise at such a rate that the mixture continues to reflux. The addition is complete in about 70 minutes. Heat is applied, and the mixture is refluxed for 3 hours. A condenser is arranged for downward distillation, and the mixture is distilled until no more or-... 

A. Triphenylarsine. In a 2-1. round-bottomed three-necked flask is placed 130 g. (5.65 gram atoms) of powdered sodium covered with 900 ml. of benzene. The flask is fitted with an Allihn condenser, a mercury-sealed mechanical Hershberg stirrer, and a 500-ml. dropping funnel in which is placed a mixture of 170 g. (0.94 mole) of arsenic trichloride and 272 g. (2.42 moles) of chlorobenzene. About 10 ml. of the arsenic trichloride-chlorobenzene mixture is dropped into the flask, and the reaction mixture is stirred and heated on a steam bath until it darkens and boils spontaneously. The steam bath is removed, and the remainder of the arsenic trichloride-chlorobenzene mixture is added drop-wise, with stirring, over a period of 1-1.5 hours at such a rate that gentle boiling is maintained (Note 1). When the addition is complete, the mixture is stirred and heated under reflux on a steam bath for 12 hours. 

The sec.-butylmalonic acid is extracted with three 200-cc. portions of ether, and the combined extracts are dried over calcium chloride overnight. The ether solution is then decanted into a 2-1. three-necked flask fitted with a mercury-sealed stirrer, reflux condenser, and dropping funnel. Five cubic centimeters of bromine is added at one time and the solution stirred until decolorized (Note 7). Then 50 cc. more bromine is added drop-wise at such a rate that the ether refluxes gently. When all the bromine has been added, 200 cc. of water is added through... 

A. 2-Methyl-2-nitropropane. To a well-stirred suspension of 650 g. (4.11 moles) of potassium permanganate in 3 1. of water, contained in a 5-1. three-necked flask fitted with a reflux condenser, a mechanical stirrer, a thermometer, and a 250-ml. dropping funnel, is added drop wise and with stirring over a 10-minute period, 100 g. (1.37 moles) of <-butylamine (Note 1). When the addition is complete, the reaction mixture is heated to 55° over a period of approximately 2 hours, and then the reaction mixture is maintained at 55° with continuous stirring for 3 hours. The dropping funnel and reflux condenser are replaced by a stopper and a still head fitted for steam distillation and the product is steam distilled from the reaction mixture (Note 2). The liquid product is separated from the denser water layer and then diluted with 250 ml. of ether and washed successively with two 50-ml. portions of aqueous 2M hydrochloric acid and with 50 ml. of water. After the ethereal solution has been dried over anhydrous magnesium sulfate, the solution is fractionally distilled at atmospheric pressure to remove the ether. The residual crude product (Note 3) amounts to 106-128 g. and is sufficiently pure for use in the next step. In a typical run, distillation of 124 g. of the crude product afforded 110 g. (78%) of the pure 2-methyl-2-nitrobutane as a colorless liquid, b.p. 127-128°, n2 d 1.3992. The material slowly solidifies on standing to a waxy solid, m.p. 25-26° (Note 4). 

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