Boiling and Condensation

In boiling the temperature of the liquid is the boiling point of this liquid at the pressure in the equipment. The heated surface is, of course, at a temperature above the boiling point. Bubbles of vapor are generated at the heated surface and rise through the mass of liquid. The vapor accumulates in a vapor space above the liquid level and is withdrawn. 

Boiling is a complex phenomenon. Suppose we consider a small heated horizontal tube or wire immersed in a ve.ssel containing water boiling at 373.2 K (100°C). The heat flux is q/A W/m, AT = — 373.2 K, where T is the tube or wire wall temperature and 

In the first region A of the plot in Fig. 4.8-1, at low temperature drops, the mechanism of boiling is essentially that of heat transfer to a liquid in natural convection. 

In the region B of nucleate boiling for aAT of about 5 — 25 K (9 — 45°F), the rate of bubble production increases so that the velocity of circulation of the liquid increases. The heat-transfer coefficient h increases rapidly and is proportional to AT to A7 in this region. 

The regions of commercial interest are the nucleate and film-boiling regions (P3). Nucleate boiling occurs in kettle-type and natural-circulation reboilers. 

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Several types of fluids are used as refrigerants in mechanical compression systems ammonia, halocarbon compounds, hydrocarbons, carbon dioxide, sulfur dioxide, and cryogenic fluids. A wide temperature range therefore is afforded. These fluids boil and condense isotherm ally. The optimum temperature or pressure at which each can be used can be deterrnined from the economics of the system. The optimum refrigerant can be deterrnined only... 

A liquid boils and condenses - the change between the liquid and gaseous states - at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature. In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again. 

The basic refrigeration cycle (Figure 2.1) makes use of the boiling and condensing of a working fluid at different temperatures and, therefore, at different pressures. 

C01.UF.R, J. G. Convective Boiling and Condensation (McGraw-Hill, New York, 1972). 

Kawahara A, Chung PM, Kawaji M (2002) Investigation of two-phase flow pattern, void fraction and pressure drop in a micro-channel. Int J Multiphase Plow 28 1411-1435 Kawaji M (1999) Fluid mechanics aspects of two-phase flow Flow in other geometries. In Kand-likar SG, Shoji M, Dhir VK (eds) Handbook of phase change boiling and condensation. Taylor and Francis, Washington, DC, pp 205-259... 

Collier G, Thome J (1994) Convective Boiling and Condensation, 3rd edn. Oxford University... 

Carey VP (1992) Liquid-vapour phase-change phenomena. Hemisphere, Washington, DC Collier SP (1981) Convective boiling and condensation. McGraw-Hill, New York Ha JM, Peterson GP (1998) Capillary performance of evaporation flow in micro grooves an analytical approach for very small tilt angles. ASME J Heat Transfer 120 452 57 Hetsroni G, Yarin LP, Pogrebnyak E (2004) Onset of flow instability in a heated capillary tube. Int J Multiphase Flow 30 1424-1449... 

Longo, Ed., 1963, Alkali Metals Boiling and Condensing Investigations, Quarterly Progress Reps 2 and 3, Space Propulsion Sec. General Electric Co., Sunnyvale, CA. (4)... 

Although we have looked already at boiling and condensation, until now we have always assumed that no work was done. We now see how invalid this assumption was. A heater located within the distillation apparatus, such as an isomantle, supplies heat energy q to molecules of the liquid. Heating the flask increases the internal energy U of the liquids sufficiently for it to vaporize and thence become a gas. 

Collier, J.G., Convective boiling and condensation, London, McGraw-Hill Book Company (UK) Ltd (1972). 

Elements from selenium through the middle rare earths will be present in the mixed fission product population they exhibit a wide variety of volatilities (1). The elements Y, Zr, and Nb and the rare earth oxides are high boiling and condensable at low partial pressures, whereas the noble gases, and the alkali metals Mo, Tc, Pd, Ag, Cd, Sn, Sb, Te, Ru, and perhaps Rh, are very volatile in a relative sense Sr and Ba are predicted to be of refractory or intermediate behavior. 

If the boiling and condensation cycle is repeated successively, some interesting consequences will be resulted in. Notice in the following figure that the condensed liquid becomes richer in the more volatile component A as the cycle is repeated. This process is called fractional distillation. Almost pure A may be obtained by repeating the cycle. 

Head temperature is monitored during the process. Distillation may also be used to remove a solid from a pure liquid by boiling and condensing the liquid. 

Our development in this chapter is primarily analytical in character and is concerned only with forced-convection flow systems. Subsequent chapters will present empirical relations for calculating forced-convection heat transfer and will also treat the subjects of natural convection and boiling and condensation heat transfer. 

Boiling and condensation phenomena are very complicated, as we have shown in the preceding sections. The equations presented in these sections may be used to calculate heat-transfer coefficients for various geometries and fluid-surface combinations. For many preliminary design applications only approximate values of heat flux or heat-transfer coefficient are required, and Tables 9-4 to 9-6 give summaries of such information. Of course, more accurate values should be obtained for the final design of heat-transfer equipment. 

Collier—Convective Boiling and Condensation, McGraw-Hill. 

Intermediate Reboilers and Condensers A distillation column of the type shown in Fig. 13-2a, operating with an interreboiler and an intercondenser in addition to a reboiler and a condenser, is diagramed with the solid lines in Fig. 13-38. The dashed lines correspond to simple distillation with only a bottoms reboiler and an overhead condenser. Total boiling and condensing heat loads are the same for both columns. As shown by K ihan [Am. Inst. Chem. Eng. J. Symp. Ser. 76, 192, 1 (1980)], the adclition of interreboilers and intercondensers increases thermodynamic efficiency but requires additional stages, as is clear from the positions of the operating lines in Fig. 13-38. 

Chapter 13 Heat Exchangers is moved up as Chapter 11 to succeed Boiling and Condensation" and to precede Radiation. ... 

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