Stratified flow, condensation

Lineham,J. H., M. Petrick, and M. M. El-Wakil, The Condensation of A Saturated Vapor on a Subcooled Film Drying Stratified Flow, AlChE Sym. on Heat Transfer, V. 66, No. 102. 

Two flow models are used to estimate the mean condensation coefficient in horizontal tubes stratified flow, Figure 12.45a, and annular flow, Figure 12.45. The stratified flow model represents the limiting condition at low condensate and vapour rates, and the annular model the condition at high vapour and low condensate rates. For the stratified flow model, the condensate film coefficient can be estimated from the Nusselt equation, applying a suitable correction for the reduction in the coefficient caused by... 

Figure 12.45. Flow patterns in condensation, (a) Stratified flow (b) Annular flow...

For condenser design, the mean coefficient should be evaluated using the correlations for both annular and stratified flow and the higher value selected. 

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... 

The following equations can be used to predict heat-transfer coefficients for condensation inside horizontal tubes For stratified flow,... 

Heat-transfer coefficient in condensation Mean condensation heat-transfer coefficient for a single tube Heat-transfer coefficient for condensation on a horizontal tube bundle Mean condensation heat-transfer coefficient for a tube in a row of tubes Heat-transfer coefficient for condensation on a vertical tube Condensation coefficient from Boko-Kruzhilin correlation Condensation heat transfer coefficient for stratified flow in tubes Local condensing film coefficient, partial condenser Convective boiling-heat transfer coefficient... 

Condensation heat transfer coefficient for stratified flow in tubes Local condensing film coefficient, partial condenser... 

In horizontal tubes, at low flow velocities, the liquid film that develops is not annular, but stratified. The condensate collects in the lower part of the tube, whilst the upper walls are wetted by the liquid. Stratified flow appears if the dimensionless vapour velocity is... 

Stratified. During condensation within horizontal tubes, when the vapor velocity is very low (i.e., jf is less than 0.5), the flow will be dominated by gravitational forces, and stratifica-... 

Convective condensation in horizontal and vertical tubes is most important with two flow patterns annular film flow and stratified flow. 

In stratified flow, the stratified layer at the lower part of the tube free-flow area is influenced primarily by shear effects, while a thin film covers the upper portions of the inner tube wall and stratifies under the influence of gravity. The heat transfer conditions in two regions are quite different, but it is a standard practice to correlate heat transfer based on the entire perimeter. In Table 17.25, a correlation based on the modified Nusselt theory is given for stratified flow, developed by Chato [87] and modified by Jaster and Kosky, as reported by Carey [76]. Consult Carey [76] and Butterworth [81] for a detailed analysis of related phenomena. The most recent condensation correlations are given by Dobson and Chato [89]. 

As far as horizontal pipes are concerned, the amount of water is not an important factor. In fact, as water is generally heavier than oil, gas, and condensed products, iu the case of stratified flow, it may separate outo the lowest surfaces, geueraUy at the 6 o clock position. In this case, the expected corrosion will occur only on the water wetted surfaces. 

In the case of stratified flow, corrosion is also hkely to occur at the top of the line, due to the condensation of water droplets from the wet gas. The effect of inhibition is poor in this case, and experience shows that corrosion rates at the top of the line can he assumed to he 10% of the predicted rate in fully immersed conditions, with a maximum of about 0.3 mm/yr, irrespective of the COj content. To increase the inhibition efficiency in stratified flow, periodic pig launches should be made to allow an inhibitor film to form at the top of the line. 

When flow velocities are lower, the flow is partly annular, with a higher concentration of liquids in the bottom half of the pipe, i.e. the film of liquids is thicker. Lower flow velocities allow some free liquid to collect in the bottom of the line, and slug flow predominates. As the flow rate declines, stratified flow predominates and the stream has separate gas and hquid phases. The top portion may not be regularly contacted at aU by liquid, except that condensed from the gas. A vapour-phase inhibitor or some means of introducing a periodic batch that contacts the top of the fine should be considered. In stratified flow, partitioning of the inhibitor between oil and water layers is important Slug flow increases requirements for the ability to withstand shear stress. 

FIGURE 1423 Idealized condensate profile during The coefficient Q depends on the fraction of the tube cir-stratifled flow, in-tube condensation within a horizon- cumference that is stratified. Jaster and Kosky [143] have... 

Inside Horizontal Tubes Condensation inside horizontal tubes can be predicted assuming two mechanisms laminar film condensation and vapor shear dominated condensation in which the two-phase flow is in the annular region. For laminar film condensation the further assumption is made that the rate of condensation on the stratified layer of liquid running along the bottom of the tube is negligible. Consequently, this layer of liquid must not exceed values assumed without being approximately accounted for. 

To avoid thermal stressing in the event that the second valve fails to close, a skirt is integrated into the scram tank. The thermally stratified water behind the skirt remains at its given elevational level after the water level inside the skirt drops due to the outflow of cold water. After a certain time the driving steam comes in contact with the cold parts of the skirt and condenses (see Fig. 6). Thus, the pressure inside the scram tank is reduced by condensation of steam. When the pressures in the RPV and in the scram tank are the same, flow within the scram system ceases despite the fact that both valves are still open. 

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