Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Laminar flow, condensation

Heat Transfer Coefficients for Laminar-Flow, Condensation, Boiling, and Compact Heat Exchangers... [Pg.436]

N tubes in a vertical row where the total condensate flows smoothly from one tube to the one beneath it, without splashing, and still in laminar flow on the tube, the mean condensing coefficient h i for the entire row of N tubes is related to the condensing coefficient for the top tube hi by... [Pg.566]

For a gas in laminar flow over a condensed phase sample of length L, the mass transport across the boundary layer, in terms of the flux of molecules from the sample to die gas phase, is therefore... [Pg.104]

The preceding equation automatically allows for the effect of the number of vertical rows of horizontal tubes as proposed by Kem and cited later in this discussion. The flow should be streamlined (laminar) flow, with a Reynolds Number of 1,800- 2,100 for the condensation, see Figures 10-67Aand 10-67B. [Pg.119]

The plate heat exchanger, for example, can be used in laminar flow duties, for the evaporation of fluids with relatively high viscosities, for cooling various gases, and for condensing applications where pressure-drop parameters are not excessively restrictive. [Pg.397]

Two cases are considered. The first, the laminar flow of a thin film down an inclined surface, is important in the heat transfer from a condensing vapour where the main resistance to transfer lies in the condensate film, as discussed in Chapter 9 (Section 9.6.1). The second is the flow in open channels which are frequently used for transporting liquids down a slope on an industrial site. [Pg.94]

The basic equations for filmwise condensation were derived by Nusselt (1916), and his equations form the basis for practical condenser design. The basic Nusselt equations are derived in Volume 1, Chapter 9. In the Nusselt model of condensation laminar flow is assumed in the film, and heat transfer is assumed to take place entirely by conduction through the film. In practical condensers the Nusselt model will strictly only apply at low liquid and vapour rates, and where the flowing condensate film is undisturbed. Turbulence can be induced in the liquid film at high liquid rates, and by shear at high vapour rates. This will generally increase the rate of heat transfer over that predicted using the Nusselt model. The effect of vapour shear and film turbulence are discussed in Volume 1, Chapter 9, see also Butterworth (1978) and Taborek (1974). [Pg.710]

The Nusselt Equations apply to laminar flow of the condensing film. For horizontal condensation the equations... [Pg.338]

Transition from laminar to turbulent flow within the condensed film can occur when the vapor is condensed on a tall surface or on a tall vertical bank of horizontal tubes [45] to [47]. It has been found that the film Reynolds number, based on the mean velocity in the film, um, and the hydraulic diameter, D, can be used to characterize the conditions under which transition from laminar flow occurs. The mean velocity in the film is given by definition as ... [Pg.570]

Eqs. (11.55) and (11.57) are valid for laminar flow. If flat plate flow is used as a rough guide, it will be seen that these equations can be expected to yield accurate results for about Re < 60. This Reynolds number is twice the value for a flat plate because condensation occurs on both sides of the tube. [Pg.577]

The rate of condensation on a vertical surface is controlled by the force of gravity acting on the condensed liquid film. A consideration of Eq. (11.20) shows for example that for a vertical plate the mean heat transfer rate from the plate with laminar flow in the film is proportional to gw. Attempts have therefore been made to increase condensation rates by using centrifugal forces instead of the gravitational force to drain the condensed liquid film from the cold surface [55], The simplest example of this would be condensation on the upper surface of a cooled circular plate rotating in a horizontal plane. This situation is shown in Fig. 11.23. A Nusselt-type analysis of this situation will be considered in the present section. [Pg.597]

Vapor can condense on a cooled surface in two ways. Attention has mainly been given in this chapter to one of these modes of condensation, i.e.. to him condensation. The classical Nusselt-type analysis for film condensation with laminar film flow has been presented hnd the extension of this analysis to account for effects such as subcooling in the film and vapor shear stress at the outer edge of the film has been discussed. The conditions under which the flow in the film becomes turbulent have also been discussed. More advanced analysis of laminar film condensation based on the use of the boundary layer-type equations have been reviewed. [Pg.600]

In a steam condenser there are 81 tubes arranged in a square array, i.e.. there are 9 columns of tubes with 9 tubes in each column. Saturated steam at 40°C condenses on the tubes. Each tube has an outside diameter of 1 cm and has a wall temperature of 35°C. Assuming laminar flow, calculate the condensation rate per meter length of the tubes. [Pg.602]

Consider laminar film condensation on a vertical plate when the vapor is flow ing parallel to the surface in a downward direction at velocity, V. Assume that a turbulent boundary layer is formed in the vapor along the outer surface of the laminar liquid film. Determine a criterion that will indicate when the effect of the shear stress at the outer edge of the condensed liquid film on the heat transfer rate is less than 59c. Assume that pv [Pg.602]

In trying to calculate the Reynolds number we find that it is dependent on the mass flow of condensate. But this is dependent on the heat-transfer coefficient, which is dependent on the Reynolds number. To solve the problem we assume either laminar or turbulent flow, calculate the heat-transfer coefficient, and then check the Reynolds number to see if our assumption was correct. Let us assume laminar film condensation. At atmospheric pressure we have... [Pg.499]

Select the calculation method to be used. Condensation inside horizontal tubes can be predicted assuming two mechanisms. The first assumes stratified flow, with laminar film condensation. The... [Pg.296]

Select the calculation method to be used. Condensation on the outside of banks of horizontal tubes can be predicted assuming two mechanisms. The first assumes laminar condensate flow the second assumes that vapor shear dominates the heat transfer. The following equations can be used to predict heat-transfer coefficients for condensation on banks of horizontal tubes For laminar-film condensation,... [Pg.301]

Calculate hc, the condensing coefficient if laminar-film condensation is assumed. Because an equal amount condenses in each increment, hc will be the same in each increment. The average condensate flow rate in each increment will be Wj = WJ9 = 54,000/9 = 6000 lb/h. Now,... [Pg.302]

Horizontal In-Shell Condensers The mean condensing coefficient for the outside of a bank of horizontal tubes is calculated from Eq. (5-93) for a single tube, corrected for the number of tubes in a vertical row. For undisturbed laminar flow over all the tubes, Eq. (5-97) is, for realistic condenser sizes, overly conservative because of rippling, splashing, and turbulent flow (Process Heat Transfer, McGraw-Hill, New York, 1950). Kern proposed an exponent of -Ve on the basis of experience, while Freon-11 data of Short and Brown General Discussion on Heat Transfer, Institute of Mechanical Engineers, London, 1951) indicate independence of the number of tube rows. It seems reasonable to use no correction for inviscid liquids and Kern s correction for viscous condensates. For a cylindrical tube bundle, where N varies, it is customary to take N equal to two-thirds of the maximum or centerline value. [Pg.864]

Many numerical models make additional assumptions, valid if only some specific questions are being asked. For example, if one is not interested in the start-up phase or in changing the operation of a fuel cell, one may apply the steady state condition that time-independent solutions are requested. In certain problems, one may disregard temperature variations, and in the free gas ducts, laminar flow may be imposed. The diffusion in porous media is often approximated by an assumption of isotropy for the gas diffusion or membrane layer, and the coupling to chemical reactions is often simplified or omitted. Water evaporation and condensation, on the other hand, are often a key determinant for the behaviour of a fuel cell and thus have to be modelled at some level. [Pg.152]


See other pages where Laminar flow, condensation is mentioned: [Pg.604]    [Pg.604]    [Pg.1041]    [Pg.1080]    [Pg.308]    [Pg.500]    [Pg.339]    [Pg.477]    [Pg.161]    [Pg.53]    [Pg.64]    [Pg.199]    [Pg.142]    [Pg.308]    [Pg.500]    [Pg.563]    [Pg.578]    [Pg.583]    [Pg.607]    [Pg.903]    [Pg.330]   
See also in sourсe #XX -- [ Pg.558 , Pg.559 , Pg.560 , Pg.561 , Pg.562 , Pg.563 , Pg.564 , Pg.565 , Pg.566 , Pg.567 , Pg.568 , Pg.569 ]




SEARCH



Laminar flow, condensation vertical plate

Nusselt model of condensation laminar flow

© 2024 chempedia.info