Condensation vapor velocity, effect

Armbruster and Mitrovic [62] observed that liquid falls from tube to tube in three patterns discrete droplets, jets or columns, and sheets, depending on the flow rate (i.e., film Reynolds number) and fluid properties. In addition, depending on the tube arrangement and spacing, the condensate may cause ripples, waves, and turbulence to occur in the film splashing may occur, as well as nonuniform rivulet runoff of condensate because of tube inclination or local vapor velocity effects. As a result, it is impossible to arrive at an analytical expression to describe these complex bundle phenomena. In general, the effect of inundation may be accounted for using... 

The large effective heat capacity of the liquid-solid slurry absorbent enables relatively small slurry flows to absorb the carbon dioxide heat of condensation with only modest absorber temperature rise. This contrasts with other acid gas removal processes in which solvent flows to the carbon dioxide absorber are considerably larger than flows determined by vapor-liquid equilibrium constraints. Large flows are required to provide sensible heat capacity for the large absorber heat effects. Small slurry absorbent flows permit smaller tower diameters because allowable vapor velocities generally increase with reduced liquid loading (8). 

Carpenter, E.F. and Colbum, A.P.. The Effect of Vapor Velocity on Condensation Inside Tubes," in Proceedings. General Discussion on Heat Transfer. Inst. Mech. Eng.-ASME, New York, pp. 20-26. 1951. 

Calculate the heat-transfer coefficient using both mechanisms, and select the higher value calculated as the effective heat-transfer coefficient hL. The vapor-shear effects vary for each typical baffle section. The condenser should be calculated in increments, with the average vapor velocity for each increment used to calculate vapor-shear heat-transfer coefficients. 

If vapor velocities are high enough, the vapor shear on the condensate interface initiates turbulence in the condensate film and causes it to flow off the surface more rapidly. These effects increase the... 

During shell-side condensation in tube bundles, neighboring tubes disturb the vapor flow field and create condensate that flows from one tube to another under the action of gravity and/or vapor shear stress forces. The effects of local vapor velocity and condensate inundation must, therefore, be properly accounted for when calculating the average heat transfer in the bundle. Marto and Nunn [53], Marto [54], and Fujii [55] provide details of these phenomena. 

The effect of vapor velocity on finned tube condensation is less than that on a smooth tube [98-100], Cavallini et al. [101] proposed the following relationship for the average heat transfer coefficient on a finned tube during forced convection conditions ... 

Tube Bundles. The average heat transfer coefficient in a bundle of finned tubes is influenced by both vapor shear and condensate inundation, although the effects are not as large as for smooth tubes [88,102-107]. At low vapor velocities, Webb and Murawski [107] express the local coefficient for the Mh row in terms of the local film Reynolds number ... 

Condensation in vertical tubes depends on the vapor flow direction and its magnitude. During downflow of vapor, if the vapor velocity is very low, then the condensate flow is controlled by gravity, and the Nusselt results for a vertical flat plate are applicable (unless the tube inside diameter is very small and tube wall curvature effects become important [36]). 

Annular. When the vapor velocity is high enough (j > 1.5), gravitational effects can be neglected, and the condensate collects as a thin annular film around the inside of the tube walls, with no stratification. A significant portion of most condensers operate in this flow regime. There are numerous predictive models described in the literature for annular flow. Laminar flow models predict heat transfer coefficients that are too low, and turbulent models must be used. The most commonly used models are listed in Table 14.1. All models have a form for the local Nusselt number... 

W. M. Rohsenow, J. H. Webber, and A. T. Ling, Effect of Vapor Velocity on Laminar and Turbulent Film Condensation, Trans. ASME, 78, pp. 1637-1643,1956. 

D. W. Nobbs and Y. R. Mayhew, Effect of Downward Vapor Velocity and Inundation on Condensation Rates on Horizontal Tube Banks, Steam Turbine Condensers, National Engineering Laboratory Report No. 619, East Kilbride, Glasgow, pp. 39-52,1976. 

A. G. Michael, P. J. Marto, A. S. Wanniarachchi, and J. W. Rose, Effect of Vapor Velocity During Condensation on Horizontal Smooth and Finned Tubes, Heat Transfer for Phase Change, ASME HTD-Vol. 114, pp. 1-10,1989. 

When vapor is moving at a large approaching velocity, the shear stress between the vapor and the condensate surface must be taken into account (i.e., shear forces are large compared to gravity force). A good review of the work devoted to this problem is found in Rose [85], who provided a detailed discussion of film condensation under forced convection. In Table 17.24, a correlation derived by Fuji et al. [86] and suggested by Butterworth [81] is included for the vapor shear effect. The same equation can be applied for a tube bundle. In such a situation, the approach velocity u should be calculated at the maximum free-flow area cross section within the bundle. 

In the absence of vapor shear effects, the heat transfer coefficient around the lower tubes in a bundle should decrease. However, in general, it is difficult to predict the actual value in a tube bundle depending on the influence of vapor and condensate velocities, turbulence effects, vapor flow direction, tube bundle layout, pressure, heat transfer surface conditions, and so on. 

The recommended procedure is to calculate the heat transfer coefficient using both mechanisms and select the higher value as the effective heat transfer coefficient (h). For baffled condensers, the vapor shear effects vary for each typical baffle section. The condenser should be calculated in increments with the average vapor velocity (Vy) for each increment used to calculate vapor shear heat transfer coefficients. When the heat transfer coefficients for laminar flow and for vapor shear are nearly equal, the effective heat transfer coefficient (h) is increased above the higher of the two values. The table below permits the increase to be approximated ... 

Rohsenow, W.M., Webber, J.H., and Ling, A.H. (1956) Effect of vapor velocity on laminar and turbulent film condensation. Journal of Heat Transfer-Transactions of the ASME, 78, 1637-1643. 

Only two possibilities exist for explaining the existence of cloud formation in the atmosphere. If there were no particles to act as cloud condensation nuclei (CCN), water would condense into clouds at relative humidities (RH) of around 300%. That is, air can remain supersaturated below 300% with water vapor for long periods of fime. If this were to occur, condensation would occur on surface objects and the hydrologic cycle would be very different from what is observed. Thus, a second possibility must be the case particles are present in the air and act as CCN at much lower RH. These particles must be small enough to have small settling velocity, stay in the air for long periods of time and be lofted to the top of the troposphere by ordinary updrafts of cm/s velocity. Two further possibilities exist - the particles can either be water soluble or insoluble. In order to understand why it is likely that CCN are soluble, we examine the consequences of the effect of curvature on the saturation water pressure of water. 

Non-local mass exchange The effective mass flow is non-local (Case A) when atoms at a step edge can directly exchange with a vapor reservoir (through evaporation-condensation) or with an overall terrace reservoir that forms by fast direct adatom hops between different terraces. In such cases, we assume that step velocity is proportional to the chemical potential difference between the step and the reservoir ... 

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]

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