Boiling Natural circulation

Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface. 

Circulation of liquid past the heating surface is induced by boiling (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit liquid flow from the top tubesheet to the bottom tubesheet. The downcomer flow area is generally approximately equal to the tubular flow area. Downcomers should be sized to minimize liquid holdup above the tubesheet In order to improve heat transfer, fluid dynamics and minimize foaming. For these reasons, several smaller downcomers scattered about the tube nest are often the better design. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

Table 10-26 lists some values of maximum flux and critical AT for pool boiling. The values are very useful when quick data must be estimated or when guides and limits must be established. They are also applicable to natural circulation boiling in tubes. 

Figure 10-103. Kern correlation for natural circulation boiling and sensible film coefficients—outside and inside tubes. (Used by permission Kern, D.Q. Process Heat Transfer, Ed., 1950. McGraw-Hill Book Company. All rights reserved.)...

Boiling Nucleate Natural Circulation (Thermosiphon) Inside Vertical Tubes or Outside Horizontal Tubes... 

Natural circulation reboilers are effective and convenient units for process systems operating under pressure. They are usable in vacuum applications but must be applied with care, because the effect of pressure head (liquid leg) on the boiling point of the fluid must be considered. The temperature difference between the heating medium and boiling point of the fluid may be so small as to be impractical, regardless of the tube length in a vertical unit. 

Lottes (1961) found that the predictions based on Romie s analysis shown above agreed with ANL data within 4% at 600 psia (4.1 MPa) and within 6% at 1,200 psia (8.2 MPa), for a natural-circulation boiling system. In addition, Lottes also found that Hoopes s data for flow of steam-water mixtures through orifices appeared to verify Romie s analysis for AJA2) — 0. 

Flow boiling is distinguished f rom pool boiling by the presence of fluid flow caused by natural circulation in a loop or forced by an external pump. In both systems, when operating at steady state, the flow appears to be forced no distinction will be made between them, since only the flow pattern and the heat transfer are of interest in this section. 

Pressure drop oscillations (Maulbetsch and Griffith, 1965) is the name given the instability mode in which Ledinegg-type stability and a compressible volume in the boiling system interact to produce a fairly low-frequency (0.1 Hz) oscillation. Although this instability is normally not a problem in modern BWRs, care frequently must be exercised to avoid its occurrence in natural-circulation loops or in downflow channels. 

Blumenkrantz, A., and J. Taborek, 1971, Application of Stability Analysis for Design of Natural Circulation Boiling Systems and Comparison with Experimental Data, AlChE Paper 13, Natl. Heat Transfer Conf, Tulsa, OK. (6)... 

Guemeri, S. A, and R. D. Tatty, 1956, A Study of Heat Transfer to Organic Liquids in Single Tube Natural Circulation Vertical Tube Boilers, AIChE Chem. Eng. Prog. Symp. Ser. 52(18) 69—77. (4) Gungor, K. E., and R. H. S. Winterton, 1986, A General Correlation for Flow Boiling in Tubes and Annuli, Int. J. Heat Mass Transfer 29(3) 351—358. (4)... 

Brooks, C. H. and Badger, W. L. Trans. Am. Inst. Chem. Eng. 33 (1937) 392. Heat transfer coefficients in the boiling section of a long-tube, natural circulation evaporator. 

Description The unique reactor design uses a simple vertical U-shaped leg connected to a horizontal gas-liquid separation vessel. Reactant gases are fed to the bottom of the U where they dissolve and combine under sufficient static pressure to prevent boiling in the reaction zone. Above this zone, the heat of reaction produces vapor bubbles that flow upwards into the horizontal vessel. A natural circulation of EDC is induced by the density difference in the two legs of the U. ... 

Natural circulation of the boiling medium is obtained by maintaining sufficient liquid head to provide for circulation. 

Natural circulation in the standard short tube evaporator depends upon boiling. Should boiling stop, any solids suspended in the liquid phase will settle out. The earliest type of evaporator that could be called a forced-circulation device is the propeller calandria illustrated in Fig. %(e). Basically a standard evaporator with a propeller added in the downcomer, the propeller calandria often achieves higher heat transfer rates. The propeller is usually placed as low as possible to avoid cavitation and is placed in an extension of the downcomer. The propeller can be driven from above or below. Improvements in propeller design have permitted longer tubes to be incorporated in the evaporator. 

When a liquid is boiled under natural circulation inside a vertical tube, relatively cool liquid enters the bottom of the tube and is heated as it flows upward at a low velocity. The liquid temperature rises to the boiling point under the pressure prevailing at that particular level in the tube. Vaporization begins, and the upward velocity of the two-phase liquid-vapor mixture increases enormously. The resulting pressure drop causes the boiling point to fall as the mixture proceeds up the tube and vaporization continues. Liquid and vapor emerge from the top of the tubes at very high velocity. 

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