Films heat transfer

FIG. 11 -22 Kunz and Yerazunis Correlation for falling-film heat transfer. 

The fin efficiency is found from mathematically derived relations, in which the film heat-transfer coefficient is assumed to be constant over the entire fin and temperature gradients across the thickness of the fin have been neglected (see Kraus, Extended Suiface.s, Spartan Books, Baltimore, 1963). The efficiency cui ves for some common fin configurations are given in Figs. Il-3(k7 and 11-30 ,... 

The inside film heat transfer eoeffieient (h ) ean be ealeulated from the following Nusselt number eorrelation ... 

In the ease of steam eondensation, a film heat transfer eoeffieient hj is used. In the ease of liquid eireulation, veloeities will he very low heeause of the large eross-seetional area. 

Empirieal dimensionless group eorrelations have been used in the seale-up proeess. In partieular, the eorrelation for the inside film heat transfer eoeffieient for agitated, jaeketed vessels has been employed for the seale-up to a larger vessel. Reaetion ealorimeters are often used to give some indieation of heat transfer eoeffieients eompared to water in the same unit. Conelation for plant heat transfer is of the general form... 

Figure 10-50A. Tube-side film heat transfer coefficient for water. (Used by permission Kern, D. Q., Process Heat Transfer, 1= Ed., 1950. McGraw-Hill, Inc. All rights reserved. Original adapted from Eagle and Ferguson, Proc. Royal Society A 127, 450, 1930.)...

Figure 10-50C. Tube-side (inside tubes) liquid film heat transfer coefficient for Dowtherm . A fluid inside pipes/tubes, turbulent flow only. Note h= average film coefficient, Btu/hr-ft -°F d = inside tube diameter, in. G = mass velocity, Ib/sec/ft v = fluid velocity, ft/sec k = thermal conductivity, Btu/hr (ft )(°F/ft) n, = viscosity, lb/(hr)(ft) Cp = specific heat, Btu/(lb)(°F). (Used by permission Engineering Manual for Dowtherm Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Figure 10-50D. Tube-side (inside pipes or tubes) liquid film heat transfer coefficient for Dowtherm A and E at various temperatures. (Used by permission Engineering Manual for Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Sinek and Young present a design procedure for predicting liquid-side falling film heat transfer coefficients within 20% and overall coefficients within 10%. 

The net effective surface true film heat transfer rate is obtained by correcting the coefficient for the bare surface thus, fouling is excluded ... 

The thesis of Steward indicates that the overall liquid film and mass transfer coefficients were functions of the gas flow rate and the column pressure and are independent of the liquid flow rate and inlet air temperature. The gas film heat transfer coefficient was found to be a function only of the air flow rate. 

From these correlations it is possible to calculate the film heat transfer coefficient and the pressure loss for laminar flow. This coefficient, combined with that of the metal and the calculated coefficient for the service fluid together with the fouling resistance, is then used to produce the overall coefficient. As with turbulent flow, an... 

Stroebe, G., Baker, E., and Badger, W., Boiling film heat transfer coefficients in a long tube vertical eavporator, Trans. Am. Inst. Chem. Engrs. 35,17 (1939). 

In most cases where convective heat transfer is taking place from a surface to a fluid, the circulating currents die out in the immediate vicinity of the surface and a film of fluid, free of turbulence, covers the surface. In this film, heat transfer is by thermal conduction and, as the thermal conductivity of most fluids is low, the main resistance to transfer lies there, Thus an increase in the velocity of the fluid over the surface gives rise to improved heat transfer mainly because the thickness of the film is reduced. As a guide, the film coefficient increases as (fluid velocity)", where 0.6 < n < 0.8, depending upon the geometry. 

Figure 9.26. Distribution of the film heat transfer coefficient round a cylinder with flow normal to the axis for...

A jacketed reaction vessel containing 0.25 nv1 of liquid of specific gravity 0.9 and specific heat 3.3 kJ/kg K is heated by means of steam fed to a jacket on the walls. The contents of the tank are agitated by a stirrer rotating at 3 Hz. The heat transfer area is 2.5 nr ami the steam temperature is 380 K. The outside film heat transfer coefficient is 1.7 kW/m2 K and the 10 mm thick wall of the tank has a thermal conductivity of 6.0 W/m K... 

Assume that the film heat transfer coefficient for the liquid in the tubes is proportional to the 0.8 power of the. velocity, the transfer coefficient for the condensing steam remains constant at 3.4 kW/m- K and that the resistance of the tube wall and scale can be neglected. 

An air stream at approximately atmospheric temperature and pressure and containing a low concentration of carbon disulphide vapour is flowing at 38 m/s through a series of 50 mm diameter tubes. The inside of the tubes is covered with a thin film of liquid and both heat and mass transfer are taking place between the gas stream and the liquid film. The film heat transfer coefficient is found to be 100 W/mzK. Using a pipe friction chan and assuming the tubes to behave as smooth surfaces, calculate ... 

Obtain the Taylor-Prandtl modification of the Reynolds Analogy between momentum transfer and mass transfer (equimolecular counterdiffusion) for the turbulent flow of a fluid over a surface. Write down the corresponding analogy for heat transfer. State clearly the assumptions which are made. For turbulent flow over a surface, the film heat transfer coefficient for the fluid is found to be 4 kW/m2 K. What would the corresponding value of the mass transfer coefficient be. given the following physical properties ... 

Inside film heat transfer coefficient 496 INSINGER, T. H. 486, 492, 564 Institution of Chemic al Engineers 516 Instruments, capillary tube viscometer 196... 

Here, Um is the film heat transfer coefficient between the reactor and the reactor wall, Uj is the film heat transfer coefficient between the reactor wall and the jacket. Am is the area for heat transfer between the reactor and the wall and Aj is the area for heat transfer between the wall and the jacket. 

Fick s Law 62, 223, 226, 637 Filling and emptying tanks 512 Film heat transfer coefficient 140, 627 Filter bed 579... 

Below a Reynolds number of about 2000 the flow in pipes will be laminar. Providing the natural convection effects are small, which will normally be so in forced convection, the following equation can be used to estimate the film heat-transfer coefficient ... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

Calculate the film heat transfer coefficients for each stream see method given below. 

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