Heat transfer coefficient, in condensation

TABLE 8.12. Overall Heat Transfer Coefficients in Condensers, Btu/(hr)(sqft)(°F)a... 

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... 

Figure 18. Heat transfer coefficients vs. mass vapor quality for upward flow R21 in heat exchanger with plain fins (20.3FPI). Wall superheat ranged from 0.9 to 1.4 K. a) evaporation for two mass fluxes, b) Comparison of heat transfer coefficients in condensation/evaporation modes at G=50 kg/m s. Mark shows the transition to the annular modes.

Heat transfer coefficient in condensation Mean condensation heat transfer coefficient for a single tube... 

Enhanced surfaces can often significandy increase the effective heat-transfer coefficient in condensation, especially if the condensing heat-transfer coefficient is the limiting factor in the overall heat-transfer-coefficient equation. Such enhancements include low fins on horizontal tubes, which increase the heat-transfer area, and fluting on vertical tubes and plane surfaces, which thins the condensate film over part of the surface by surface-tension effects. However, these improvements are limited by condensate retention between the fins and flooding of the drainage paths [7, 34],... 

If solvent recovery is maximized by minimizing the temperature approach, the overall heat-transfer coefficient in the condenser will be reduced. This is due to the fact that a large fraction of the heat transfer area is now utilized for cooling a gas rather than condensing a Hquid. Depending on the desired temperature approach the overall heat-transfer coefficients in vent condensers usually range between 85 and 170 W/m K (ca 15 and 30 Btu/h-ft. °F). 

Vertical Tubes For the following cases Reynolds number < 2100 and is calculated by using F = Wp/ KD. The Nusselt equation for the heat-transfer coefficient for condensate films may be written in the following ways (using liquid physical properties and where L is the cooled lengm and At is — t,) ... 

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. 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

No, H. C., and M. S. Kazimi, 1982, Wall Heat Transfer Coefficients for Condensation Boiling in Forced Convection of Sodium, Nuclear Sci. Eng. 57 319-324. (4)... 

A triple-effect evaporator is fed with 5 kg/s of a liquor containing 15 per cent solids. The concentration in the last effect, which operates at 13.5 kN/m2, is 60 per cent solids. If the overall heat transfer coefficients in the three effects are 2.5, 2.0, and 1.1 kW/m2K, respectively, and the steam is fed at 388 K to the first effect, determine the temperature distribution and the area of heating surface required in each effect The calandrias are identical. What is the economy and what is the heat load on the condenser ... 

A salt solution at 293 K is fed at the rate of 6.3 kg/s to a forward-feed triple-effect evaporator and is concentrated from 2 per cent to 10 per cent of solids. Saturated steam at 170 kN/m2 is introduced into the calandria of the first effect and a pressure of 34 kN/m2 is maintained in the last effect. If the heat transfer coefficients in the three effects are 1.7, 1.4 and 1.1 kW/m2K respectively and the specific heat capacity of the liquid is approximately 4 kJ/kgK, what area is required if each effect is identical Condensate may be assumed to leave at the vapour temperature at each stage, and the effects of boiling point rise may be neglected. The latent heat of vaporisation may be taken as constant throughout. 

Heat-transfer coefficients in this book have the units of Btu/[(h)(ft2)(°F)], where the ft2 term refers to the surface area of the surface condenser. The °F term refers to the condensing steam temperature, minus the average tube-side cooling-water temperature. 

A closed-loop refrigerant condenser ought to be one of the cleanest services in a process plant. Even seal- or lube-oil leaks affect the evaporator efficiency, rather than the condenser. I have measured rather high [e.g., 140 Btu/[(h)(ft2)(°F)] heat-transfer coefficients in such condensers, even after that condenser has been in service for several years since its last cleaning. 

Because of the higher heat-transfer rates, dropwise condensation would be preferred to Him condensation, but it is extremely difficult to maintain since most surfaces become wetted after exposure to a condensing vapor over an extended period of time. Various surface coatings and vapor additives have been used in attempts to maintain dropwise condensation, but these methods have not met with general success to date. Some of the pioneer work on drop condensation was conducted by Schmidt [26] and a good summary of the overall problem is presented in Ref. 27. Measurements of Ref. 35 indicate that the drop conduction is the main resistance to heat flow for atmospheric pressure and above. Nucleation site density on smooth surfaces can be of the order of 10 sites per square centimeter, and heat-transfer coefficients in the range of 170 to 290 kW/m2 °C [30,000 to 50,000 Btu/h ft2 °F] have been reported by a number of investigators. 

Using Eq. (9-28) as a starting point, develop an expression for the average heat-transfer coefficient in turbulent condensation as a function of only the fluid properties, length of the plate, and temperature difference i.e., eliminate the Reynolds number from Eq. (9-28) to obtain a relation similar to Eq. (9-10) for laminar condensation. 

Overall heat-transfer coefficient in all condensers = 150 Btu/(hXft2X°F). 

For the following conditions, calculate the heat-transfer coefficient when condensing at a rate of 54,000 lb/h (24,493.9 kg/h) on the outside of a tube bundle with a diameter of 25 in (0.635 m) with nine baffle sections each 12 in (0.305 m) long. The bundle contains 532 tubes with an outside diameter of 0.75 in (0.019 m). The tubes are on a triangular pitch and are spaced 0.9375 in (0.02381 m) center to center. Assume equal amounts condense in each baffle section. 

Because the heat-transfer coefficient for condensing steam is of order 104, the Bi —> < > limit in Table 5-2 is a good choice and dj = k. Because we know the desired temperature at the center, we can calculate 0/0 and then solve (5-21) for the time. 

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