Condensation inside and outside vertical tubes

For condensation inside and outside vertical tubes the Nusselt model gives  

Equation 12.51 will apply up to a Reynolds number of 30 above this value waves on the condensate film become important. The Reynolds number for the condensate film is given by  

The presence of waves will increase the heat-transfer coefficient, so the use of equation 12.51 above a Reynolds number of 30 will give conservative (safe) estimates. The effect of waves on condensate film on heat transfer is discussed by Kutateladze (1963). 

Boyko and Kruzhilin (1967) developed a correlation for shear-controlled condensation in tubes which is simple to use. Their correlation gives the mean coefficient between two points at which the vapour quality is known. The vapour quality x is the mass fraction of 

In a condenser the inlet stream will normally be saturated vapour and the vapour will be totally condensed. 

Fv = vertical tube loading, condensate rate per unit tube perimeter, kg/m s for a tube bundle 

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It is normal practice to assume that integral condensation occurs. The conditions for integral condensation will be approached if condensation is carried out in one pass, so that the liquid and vapour follow the same path as in a vertical condenser with condensation inside or outside the tubes. In a horizontal shell-side condenser the condensate will tend to separate from the vapour. The mean temperature difference will be lower for differential condensation, and arrangements where liquid separation is likely to occur should generally be avoided for the condensation of mixed vapours. 

These equations are strictly valid only for a plane vertical surface. However, they can be used for inside or outside vertical tubes with small error because the condensate film is thin compared with the diameter of a typical tube. Because of rippling and other nonidealities, the predicted coefficients are about 10-20% below experimental values. 

Heat transport with phase change such as in boiling or condensation is an efficient method to transfer heat because latent heat per unit mass is very large compared to the sensible heat. For single component fluid, the interface temperature difference involved for heat transfer in evaporation and condensation is relatively small. However, when more than one component is present in a system the temperature difference can be higher. An example is condensation of vapors in the presence of noncondensable gases. The two-phase heat transfer relevant to reactors includes pool boiling, evaporation in a vertical channels, and condensation inside or outside the tubes. 

Design equations are given in Volume 6, Chapter 12, for condensation both inside and outside horizontal and vertical tubes, and the importance of avoiding flooding in vertical tubes is stressed. 

Fig. 7.3-1 Condensers operated with cooling water, (a) Condensation on the outside surfaces of horizontal tubes (b, c) condensation on the inside surfaces of vertical tubes in cocurrent (b) or countercurrent (c) flow of vapor and condensate...

Evaporators, Horizontal-Tube Type - The basic horizontal-tube evaporator is illustrated in Figure 12. The body of this evaporator is the liquor compartment and is in the form of a vertical cylinder. It is closed, top and bottom, with dished heads, although the bottom may be conical. The lower body ring is provided on opposite sides with steam compartments, closed on the outside by cover plates and on the inside by tube sheets. Between these tube sheets are fastened a number of horizontal tubes. The two steam chests with their connecting mbes form the steam compartment, and the tube wall heating surface. Steam is introduced into one steam chest and as it flows through the tubes it washes non-condensed gases and condensate ahead of it, so that these are withdrawn from the opposite steam chest. 

Estimate the heat-transfer coefficient for steam condensing on the outside, and on the inside, of a 25 mm o.d., 21 mm i.d. vertical tube 3.66 m long. The steam condensate rate... 

The evaporating pans of M, Derosse differ from those of Rotii in the shape of the condensing vessel, which in the former s consists of a tube bent like a cracker, the folds of which lie in vertical plane. Instead of effecting the condensation by water, Derossb employs sirup which, being allowed to flow upon the tipper fold of the tubes, trickles down from one fold to another. In condensing the steam in the inside of the tube, the sirup which flows down the outside of the tubes becomes itself heated, and parts with a portion of its water while the heat of the tube and its vertical position determine an ascending current of air, which removes the vapor of water as it is formed. 

Condensation may be performed inside or outside tubes, in horizontal or vertical positions. In addition to the statements made in the previous section about the merits of tube side or shell side When freezing can occur, shell side is preferable because it is less likely to clog. When condensing mixtures whose lighter components are soluble in the condensate, tube side should be adopted since drainage is less complete and allows condensation (and dissolution) to occur at higher temperatures. Venting of noncondensables is more positive from tube side. 

Figure 8.14. Some arrangements of shell-and-tube condensers, (a) Condensate inside tubes, vertical upflow. (b) Inside tubes, vertical downflow, (c) Outside tubes, vertical downflow, (d) Condensate outside horizontal tubes. (HEDH, 1983, 3.4.3).

A vertical, short-tube evaporator is shown in Fig. 2. A bundle of short tubes (A), 4-8 ft long and 2-A in. in diameter is placed in a vertical shell (B) in which the evaporating liquor is introduced. Steam condenses outside the tubes causing boiling of the liquor. The liquor spouts upward inside the tubes and returns through the downtake. Concentrated liquor is removed from the bottom of the evaporator (C) and liquid vapor is removed at (D). The cross-sectional area of the downtake is 25% of the total cross-sectional area of the tubes. 

Laminar film condensation on vertical or inclined plates, and on the inside or outside of a vertical tube. 

DEHUMIDIFYING CONDENSERS. A condenser for mixtures of vapors and noncondensable gases is shown in Fig. 15.9. It is set vertically, not horizontally like most condensers for vapor containing no noncondensable gas also, vapor is condensed inside the tubes, not outside, and the coolant flows through the shell. This provides a positive sweep of the vapor gas mixture through the tubes and avoids the formation of any stagnant pockets of inert gas that might blanket the heat-transfer surface. The modified lower head acts to separate the condensate from the uncondensed vapor and gas. 

A guide to the overall performance of a wide variety of typical vertical evaporator tubes with condensing outside and vaporization inside is given in Fig. 11.25. This survey by Alexander and Hoffman [164] was specifically directed at vertical-tube evaporators for desalination systems. It is seen that the best surfaces yield increases in overall coefficient up to 200 percent. 

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