Condensation heal transfer

In direct contact heal exchange, there is no wall to separate hot and cold streams, and high rales of heal transfer are achieved. Applications include reactor off-gas quenching, vacuum condensers, desuperheating, and humidification. Water-cooling lowers are a particular example of a direct contact heal e.xchanger. In direct contact cooler-condensers, the condensed liquid is frequently used as the coolant. 

Rhodes, F.H. and Younger, K.R. Ind. Eng. Chem. 27 (1935) 957. Rate of heal transfer between condensing organic vapours and a inetal tube. 

A shell-and-tube heat exchanger is used as an ammonia condenser with ammonia vapor entering the shell at 50°C as a saturated vapor. Water enters the single-pass tube arrangement at 20°C and the total heat transfer required is 200 kW. The overall heat-transfer coefficient is estimated from Table I0-I as 1000 W/m2 °C. Determine the area to achieve a heat exchanger effectiveness of 60 percent with an exit water temperature of 40°C. What percent reduction in heal transfer would result if the water flow is reduced in half while keeping the heat exchanger area and V the same ... 

The overhead stream from the column is a vapor that passes though a condenser. The condensed liquid is divided into two equal streams one is taken off as the overhead product (distillate) and the other (the reflux) is returned to the column. The bottom stream from the column is a liquid that is partially vaporized in a reboiler. The liquid stream emerging from the reboiler is taken off as the bottoms product, and the vapor is returned to the column as boilup. Negligible heat is lost from the column, so that the only places in the system where external heal transfer takes place are the condenser and the reboiler. 

Cross-flow over tube banks is commonly encountered in practice in heal transfer equipment such as the condensers and evaporators of power plants, refrigerators, and air conditioners. In such equipment, one fluid moves through the tubes while the other moves over the tubes in a perpendicular direction. 

Consider a vertical plate of height L and width b maintained at a constant temperature r, that is exposed to vapor at the saturation temperature The downward direction is taken as the positive x-direction with the origin placed at the lop of the plate whete condensation initiates, as shown in Fig. 10-24. The surface temperature is below the saluratioii temperature (7 j < r <) and thus the vapor condenses on the surface. The liquid film flows downward under the influence of gravity. The film thickness S and thus the mass flow rate of the Condensate increases with x as a result of continued condensation on the existing film. Then heal transfer from the vapor to the plate must occur through the film, which offers resistance to heat transfer. Obviously the thicker the film, ihe larger its thermal resistance and thus the lower the rate of heal transfer. 

FIGURE 10-26 Nondimensionalized heal transfer coefficients for the wave-free laminar, wavy laminar, and turbulent flow of condensate on vettical plates. 

Reconsider Prob. 10-57. Using EES (or other) software, investigate the effect of the condenser pressure on the rale of heal transfer and the rate of condensation of the steam. Let the condenser pressure vary from 3 kPa to 15 kl a. Plot Ihc rale of heat transfer and the rate of coiiden sation of the steam as a function of the condenser pressuie, and discuss Ihe result.s. 

Saturaied aruraonia vapor at 30°C is passed over 20 vertical flat plates, each of which is 10 cm high and 15 cm wide. The average surface temperature of the plates is 10°C. Estimate the average heal transfer coefficient and the rate of ammonia condensation. 

Steam at 40°C condenses on the outside of a 3-cm diameter thin horizontal copper rube by cooling water that enters the tube at 25°C at an average velocity of 2 m/s and leaves at 35°C. Determine the rale of condensation of steam, the average overall heal transfer coefficient between the steam and the cooling water, and the mbe length. 

At a distance x down a vertical, isothermal flat plate on which a saturated vapor is condensing in a continuous film, the thickness of the liquid condensate layer is 8. The heal transfer coefficient at this location on the plate is given by (a) kJS (b) Shf (c) Sh ... 

In a binary geothermal power plant, the working fluid isobulane is to be condensed by air in a condenser at 75°C = 255.7 kJ/kg) at a rate of 2.7 kg/s. Air enters the condenser at 21 C and leaves at 28 C (see Fig. PI 1-65). The heal transfer surface area based on the isobuiane side is 24 m. Determine the mass flow rate of air and the overall heat transfer coefficieni. 

Steam is to be condensed on the shell side of a 2-shell-passes and S-tube-passes condenser, with 20 lubes in each pass. Oooling water enters the tubes a rale of 2 kg/s. If the heal transfer area is 14 m and the overall heal transfer... 

Analysis The schematic of the wall as well as the different elements used in its construction are shown in Figure 14-25. Condensation is most likely to occur at the coldest part of insulation, which Is the part adjacent to the exterior sheathing. Noting that the total thermal resistance of the wall is 3.05 m °C/W, the rate of heal transfer through a unit area A = 1 m of the wall is... 

Furthermore, if ihe molecules condeaae, the total enthalpy given up would include the latent heat. Sherwood ei al. descriha how the above equations along with appropriate mass transfer expressions aed phase equilibrium relationships are employed in the analysis of condensers and other processes involving simultaneous heal transfer and mass transfer. 

A shell and tube heat exchanger is used for preheating the feed to an evaporator. The liquid of specific heat 4.0 kJ/kg K and specific gravity 1.1 passes through the inside of the tubes and is heated by steam, condensing at 395 K on the outside. The exchanger heats liquid at 295 K to an outlet temperature of 375 K when the flow rate is 175 cinVs and to 370 K when file flowrate is 325 cm /s. What is the heal transfer area and the value of the overall heat transfer coefficient when the flow is 175 cm /s ... 

In considering the heat that is transferred, the method first put forward by NussELT(%i and later modified by subsequent workers is followed. If the vapour condenses on a vertical surface, the condensate film flows downwards under the influence of gravity, although it is retarded by the viscosity of the liquid. The flow will normally be streamline and the heal flows through the film by conduction. In Nusselt s work it is assumed that the temperature of the film at the cool surface is equal to that of the surface, and at the other side was at the temperature of the vapour. In practice, there must be some small difference in temperature between the vapour and the film, although this may generally be neglected except where non-condensable gas is present in the vapour. 

Hannemann. R. and Mikic, B., An Experimental Investigation into the Effect of Surface Thermal Conductivity on the Rate of Heat Transfer in Dropwise Condensation." Int. J. Heal Mass Transfer, Vol. 19. p. 1309, 1976. 

SOLUTION Water is healed by steam in a circular tube. The tube length required to heat the water to a specified temperature is to be determined. Assumptions 1 Steady operating conditions exist. 2 Fluid properties are constant. 3 The convection heat transfer coefficient is constant. 4 The conduction resistance of copper tube is negligible so that (he inner surface temperature of the tube is equal to the condensation temperature of steam. 

We can have a similar argument for vapor that enters the condenser as superheated vapor at a temperature 7,. instead of as saturated vapor. In this case the vapor must be cooled first to 3 , before it can condense, and this heat must be transferred to the wall as well. The amount of heat released as a unit mass of superheated vapor at a temperature 7,. is cooled to 75, is simply Cf,r[Ty - 7jjt), where Cp,. is the specific heat of the vapor at the average temperature of (Ty + 7sj,)/2. The modified latent heal of vaporization in this case becomes... 

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