Condensation and Boiling Heat Transfer

Our preceding discussions of convection heat transfer have considered homogeneous single-phase systems. Of equal importance are the convection processes associated with a change of phase of a fluid. The two most important examples are condensation and boiling phenomena, although heat transfer with solid-gas changes has become important because of a number of applications. 

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The other studies, which included the effect of vibration on steam condensation, nucleate boiling heat transfer, and scale deposition, investigated the relation of frequency and amplitude of vibration of the heat transfer surface to increases in these heat transfer mechanisms. The study of the effect of acoustic vibrations in water on forced convection heat transfer investigated the influence of frequency and amplitude of the standing waves on increasing heat transfer rates and the flow Reynolds numbers at which increases could be obtained. 

Dissolve 12 g. of aniline hydrochloride and 6 g. of urea in 50 ml. of warm water, and then filter the solution through a fluted filter to remove any suspended impurities which may have been introduced with the aniline hydrochloride. Transfer the clear filtrate to a 200 ml. conical flask, fit the latter with a reflux water-condenser, and boil the solution gently over a gauze for about hours. Crystals of diphenylurea usually start to separate after about 30-40 minutes boiling. Occasionally however, the solution becomes supersaturated with the diphenylurea and therefore remains clear in this case, if the solution is vigorously shaken after about 40 minutes heating, a sudden separation of the crystalline diphenyl compound will usually occur. The further deposition of the crystals during the re-... 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

Kumamaru, H., Y. Koizumi, and K. Tasake, 1987, Investigation of Pre- and Post-Dryout Heat Transfer of Steam-Water Two-Phase Flow in Rod Bundles, Nuclear Eng. Design 702 71-84. (4) Kutateladze, S. S., 1952, Heat Transfer in Condensation and Boiling, USAEC Rep. AEC-tr-3770 (Translated from Mashgiz, 2d ed., pp. 76-107, State Sci. Tech. Pub. House of Literature on Machinery, Moscow-Leningrad). (2)... 

A double-effect forward-feed evaporator is required to give a product which contains 50 per cent by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are 2.8 and 1.7 kW/m2 K in the first and second effects respectively. Dry and saturated steam is available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. The concentrated solution exhibits a boiling-point rise of 3 deg K. What is the maximum permissible feed rate if the feed contains 10 per cent solids and is at 310 K The latent heat is 2330 kJ/kg and the specific heat capacity is 4.18 kJ/kg under all the above conditions. 

The economics of VRD favors separations involving components with similar boiling points (e.g., the separation of propane and propylene in an oil refinery, in a column that is typically referred to as a C3 splitter ) so that the temperatures of the top and bottom streams of the distillation column are close. This reduces the power consumption of the compressor as well as the duty (and associated heat-transfer area) of the trim condenser. Given the above, we can make the following observations and assumptions concerning the various steady-state energy flows in the process. 

Heat transfer involving a change of phase is classified as convective heat transfer even though when the solid phase is involved, the overall process involves combined and interrelated convection and conduction. Heat transfer during boiling, condensation, and solidification (freezing) all, thus, involve convective heat transfer. 

Additional methanol is injected into the gas between the two reactors. The reactors contain many tubes filled with FK-2 catalyst, where methanol and oxygen react to make formaldehyde. Reaction heat is removed by a bath of boiling heat-transfer oil. Hot oil vapor is condensed in the waste-heat boiler (5), thus generating steam at up to 40 bar pressure. Before entering the absorber (7), the reacted gas is cooled in the after cooler (6) and reheats the circulating oil from the process-gas heater (2). 

Heat transfer constraints Heat must be transferred into the liquid in the reboiler to boil off the vapor needed to provide the vapor-liquid contacting in the column. If the base temperature becomes too high and approaches the temperature of the heating source, the heat transfer rate will decrease and vapor boilup will drop. The same result occurs if the reboiler fouls and the heat transfer coefficient drops. In the condenser, heat must be transferred from the hot vapor into the coolant stream to remove the heat of condensation. If the column is operating at its maximum pressure, capacity maybe limited by condenser heat removal. 

The use of boiling heat transfer raises the maximum conversion rates that can be controlled significantly beyond that of the agitated towers filled with heat transfer tubes. The main limitation that occurs is the removal of the vapor bubbles from the polymer solution. As the viscosity of the polystyrene solution increases rapidly with conversion, this becomes most limiting when the viscosity of the polystyrene solution exceeds 1000P (dPa/s). Below this viscosity, conversion rates of 40%/h can be controlled, but above this viscosity, the polymer mass foams up into the condenser and temperature control is lost, so that the maximum conversion rate decreases rapidly at high polymer concentrations. 

Figure 186 presents the comparison of heat transfer coefficient vs. mass vapor quality for upward flow condensation and upward flow boiling modes at mass flux of 50 kg/m s. At low vapor quality the boiling heat transfer coefScient is considerably higher than that for condensation. The difference in heat transfer behavior can be explained by the absence of film mpture in case of condensation. The contact line does not exist in this case and heat transfer level is much less than in case of boiling. 

The nature of boiling heat transfer in a channel with the gap less than the capillary is also studied and presented. The condensation flow mechanisms, pressure drop and heat transfer in microchannels, role of microscale heat transfer in augmentation of nucleate boiling and flow boiling heat transfer, binary-fluid heat and mass transfers in microchannel geometries for miniaturized thermally activated absorption heat pumps, evaporation heat... 

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