Condensing vapor film-type condensation

Condensation Mechanisms Condensation occurs when a saturated vapor comes in contact with a surface whose temperature is below the saturation temperature. Normally a film of condensate is formed on the surface, and the thickness of this film, per unit of breadth, increases with increase in extent of the surface. This is called film-type condensation. 

Film-type condensation is more common and more dependable. Dropwise condensation normally needs to be promoted by introducing an impurity into the vapor stream. Substantially higher (6 to 18 times) coefficients are obtained for dropwise condensation of steam, but design methods are not available. Therefore, the development of equations for condensation will be for the film type only. 

FIG. 5-9 Chart for determining film coefficient for film-type condensation of pure vapor, based on Eqs. 5-88 and 5-93. For vertical tubes multiply by 1.2. If 4F/ J exceeds 2100, use Fig. 5-10. is in U.S. customary units to convert feet to... 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Film-type condensation is considered to be the usual condition for most pure vapors, although drop-type condensation gives transfer coefficients many times larger when it does occur. For practical purposes, film-type is considered in design. 

To give a few examples, //-values for film-type condensation of water vapor (when a film of condensed water would cover the entire cooling surface) will range from 4000 to 15 000 kcal h" m - °C b and those for boiling water would be in the range of 1500 - 30 000 kcal h m - °C E... 

Film-type condensation of vapors Outside horizontal tubes NvDont At/ J wn/ ), . , . 3100 McAdamsf Pure saturated vapors p- < 2000 bnj Physical properties are for condensate. McAdams t Pure saturated vapors iw/rDoP/ < 2000 Physical properties are for condensate. 

Film-type condensation of vapors Outside horizontal tubes... 

DROPWISE AND FILM-TYPE CONDENSATION. A vapor may condense on a cold surface in one of two way.s, which are well described by the terms dropwise and film type. In film condensation, which is more common than dropwise condensation, the liquid condensate forms a film, or continuous layer, of liquid that flows over the surface of the tube under the action of gravity. It is the layer of liquid interposed between the vapor and the wall of the tube that provides the resistance to heat flow and therefore fixes the magnitude of the heat-transfer coefficient. 

COEFFICIENTS FOR FILM-TYPE CONDENSATION. The basic equations for the rate of heat transfer in film-type condensation were first derived by Nusselt. " The Nusselt equations are based on the assumption that the vapor and liquid at the outside boundary of the liquid layer are in thermodynamic equilibrium, so that the only resistance to the flow of heat is that offered by the layer of condensate flowing downward in laminar flow under the action of gravity. It is also assumed that the velocity of the liquid at the wall is zero, that the velocity of the liquid at the outside of the film is not influenced by the velocity of the vapor, and that the temperatures of the wall and the vapor are constant. Superheat in the vapor is neglected, the condensate is assumed to leave the tube at the condensing temperature, and the physical properties of the liquid are taken at the mean film temperature. 

PI CAL USE OF NUSSELT EQUATIONS. In the absence of high vapor velocities, experimental data check Eqs. (13.13) and (13.14) well, and these equations can be used as they stand for calculating heat-transfer coefficients for film-type condensation on a single horizontal tube. Also, Eq, (13.14) can be used for film-type condensation on a vertical stack of horizontal tubes, where the condensate falls cumulatively from tube to tube and the total condensate from the entire stack finally drops from the bottom tube. The average coefficient for the stack of tubes is less than that for one tube it is given by the equation ... 

Let us now turn our attention to a more detailed discussion of transitions that occur between different types of films, and in particular to the way in which these transitions are influenced by the temperature, the structure of surfactant molecules and the composition of the medium. The direct transition from gaseous and vapor films to liquid and solid condensed ones is a two-dimensional first-order phase transition that is quite similar to a three-dimensional vapor condensation. A decrease in the area per molecule in the adsorption layer in the region of gaseous films causes a gradual increase in pressure up to the level that corresponds to the condensation pressure of a saturated two-dimensional vapor, 7rc, at an area per molecule equal to sc (see Fig. 11-21). The subsequent compression of film is not accompanied by an increase in the two-dimensional pressure the two-dimensional vapor transforms into the two-dimensional condensed state, which can be either liquid expanded, liquid condensed, or solid, depending on the nature of the... 

For SO3 condensers, the falling film type condenser (SO3 vapors on the shell side and a trickling stream of water on the inside of tubes) is generally found to be safer as compared to a shell and tube type unit wherein water is circulated under pressure. 

Normally, when a vapor condenses on a surface such as a vertical or horizontal tube or other surfaces, a film of condensate is formed on the surface and flows over the surface by the action of gravity. It is this film of liquid between the surface and the vapor that forms the main resistance to heat transfer. This is called film type condensation. 

Film-condensation coefficients for vertical surfaces. Film-type condensation on a vertical wall or tube can be analyzed analytically by assuming laminar flow of the condensate film down the wall. The film thickness is zero at the top of the wall or tube and increases in thickness as it flows downward because of condensation. Nusselt (HI, Wl) assumed that the heat transfer from the condensing vapor at 7, K, through this liquid film, and to the wall at 7 K was by conduction. Equating this heat transfer by conduction to that from condensation of the vapor, a final expression can be obtained for the average heat-transfer coefficient over the whole surface. 

FIG. 11-122 Evaporator types, a) Forced circulation, (h) Siibmerged-tiihe forced circulation, (c) Oslo-type crystallizer, (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (ij) Horizontal-tube evaporators. G = condensate F = feed G = vent P = product S = steam V = vapor ENT T = separated entrainment outlet. 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

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