Boiling liquids, heat transfer

Boiling liquids, heat-transfer coefficients for, 597 Bonds, interest rate on, 248-249 Book value definition of 277 in replacement analysis, 332 Box-Wilson design, 766, 769 Break-even chart, 155-156... 

Rohsenow, W. H., A Method of Correlating Heat Transfer Data for Surface Boiling of Liquids, Heat Transfer Div. ASME, Atlantic City, NJ. Meeting Nov. 25, (1951) Paper No. 51-A-llO. 

Film Boiling and Heat Transfer in Liquid-Deficient Regions 274... 

FILM BOILING AND HEAT TRANSFER IN LIQUID-DEFICIENT REGIONS... 

Melting of reactor tubes during runaway reaction is only to be feared in multitubular reactors if the respective tube is not surrounded by liquid heat transfer medium. Thus, appropriate design must ensure that running dry of reactor tubes cannot occur. In the case of corrosive reaction gases, provision for the detection of leaks causes by corrosion must be made, particularly when pressurized or boiling water is used as coolant. 

The occuring of dry-out may explain why, when combined with heat flux dependent boiling, the heat transfer coefficient decreases with the mass velocity. The greater the mass velocity, the more probable dry-out should be, because the liquid film is increasingly dragged from the wall due to shear stress. Thus dry-out should cause a decrease in the heat transfer coefficient with the mass velocity. 

The onset of nucleate boiling (line XX in Fig. 15.88) occurs above the x = 0 line at low heat flux (i.e., there is a net bulk superheat of the liquid) but at qualities less than 0 for high heat fluxes, corresponding to the region of subcooled nucleate boiling. For heat transfer to a single-phase liquid, the wall superheat (ATsat)w can be calculated from... 

P. J. Marto and V. J. Lepere, Pbol Boiling Heat Transfer From Enhanced Surfaces to Dielectric Liquids, / Heat Transfer (my. 292-299,1982. 

Liquid level. When liquid level falls below the top of the bundle, the unflooded tubes heat vapor rather than perform nucleate boiling. Since vapor heating has a much lower heat transfer coefficient than nucleate boiling, reboiler heat transfer suffers. Further, the tube wall temperature in the unflooded portion approaches the heating fluid temperature, which may overheat tube metal. For these reasons, it is important to maintain the desirable liquid level in the reboiler. 

Dowtherm n The trade name (Dow Ghemical Go.) of a liquid heat-transfer medium consisting of biphenyl and phenyl ether in eutectic ratio. High-boiling and stable, it is less used today than formerly because of its suspected carcinogenicity. Bp, 258° C. Used for heat transfer. 

The variety of regimes during the forced convection boiling in tubes or ducts requires different correlations in order to determine the heat transfer coefficient related to the respective boiling mechanisms. The well-established correlations have been developed for nucleate boiling controlled heat transfer - when evaporation occurs at the inner tube surface - and convective boiling heat transfer - when evaporation occurs at the liquid film interface. 

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. 

The lower Emit of applicability of the nucleate-boiling equations is from 0.1 to 0.2 of the maximum limit and depends upon the magnitude of natural-convection heat transfer for the liquid. The best method of determining the lower limit is to plot two curves one of h versus At for natural convection, the other ofh versus At for nucleate boiling. The intersection of these two cui ves may be considered the lower limit of apphcability of the equations. 

Pressure drop due to hydrostatic head can be calculated from hquid holdup B.]. For nonfoaming dilute aqueous solutions, R] can be estimated from f i = 1/[1 + 2.5(V/E)(pi/pJ ]. Liquid holdup, which represents the ratio of liqmd-only velocity to actual hquid velocity, also appears to be the principal determinant of the convective coefficient in the boiling zone (Dengler, Sc.D. thesis, MIT, 1952). In other words, the convective coefficient is that calciilated from Eq. (5-50) by using the liquid-only velocity divided by in the Reynolds number. Nucleate boiling augments conveclive heat transfer, primarily when AT s are high and the convective coefficient is low [Chen, Ind Eng. Chem. Process Des. Dev., 5, 322 (1966)]. 

Highest heat-transfer coefficients are obtained in FC evaporators when the liquid is aUowed to boil in the tubes, as in the type shown in Fig. 11-122 7. The heating element projects into the vapor head, and the hquid level is maintained near and usuaUy slightly below the top tube sheet. This type of FC evaporator is not well suited to salting solutions because boiling in the tubes increases the chances of salt deposit on the waUs and the sudden flashing at the tube exits promotes excessive nucleation and production of fine ciystals. Consequently, this type of evaporator is seldom used except when there are headroom hmitations or when the hquid forms neither salt nor scale. 

A low-boiling-point liquid, in boiling off, has a good heat transfer coefficient to help cool the wall and buy time. Calculate the time required to heat up the liquid and vaporize the inventory. If the time is less than 15 minutes... 

Plate and Frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids slurries. High heat transfer. Not well suited for boiling or condensing limit 350-500°F by gaskets. Used for Liquid-Liquid only not gas-gas. 0.8-1.5... 

Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers... 

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