Heat transfer coefficients in evaporators

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Though the above equations are claimed to give reasonably accurate predictions for condensation [275], they overpredict the heat transfer coefficient in evaporation by typically 50 percent or so. It has often been suggested that the turbulence would be damped near the interface and that the use of equations like Eqs. 15.265-15.267 for eddy diffusivity, which are derived from single-phase flow data, is inappropriate. An approach suggested by Levich [276] is to introduce a damping factor that reduces the eddy diffusivity to zero at the interface. Thus, a modified eddy diffusivity e is calculated from the expression... 

FIG. 11-21 Heat- transfer coefficients in LT - seawater evaporators, = ( F — 32)/l,8 to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply hy 5,6783,... 

Heat-transfer coefficients in clean coiled-tube evaporators for seawater are shown in Fig. 11-24 [Hillier, Proc. Jn.st. Mech. Eng. (London ), 1B(7), 295 (1953)]. The tubes were of copper. 

FIG. 11-25 Overall heat-transfer coefficients in agitated-film evaporators. 

Economic and process considerations usually dictate that agitated thin-film evaporators be operated in single-effect mode. Veiy high temperature differences can then be used many are heated with Dowtherm or other high-temperature media. This permits achieving reasonable capacities in spite of the relatively low heat-transfer coefficients and the small surface that can be provided in a single tube [to about 20 m" (200 ft")]. The structural need for wall thicknesses of 6 to 13 mm (V4 to V2. in) is a major reason for the relatively low heat-transfer coefficients when evaporating water-like materials. 

Estimate temperature distribution in the evaporator, taking into account boiling-point elevations. If all heating surfaces are to be equal, the temperature drop across each effect will be approximately inversely proportional to the heat-transfer coefficient in that effect. 

Coni. son,. I.M. and Mehta, R.R. Trans. Inst. Chem. Eng. 31 (1953) 208. Heat transfer coefficients in a climbing film evaporator. 

Herd, K. G., W. P. Goss, and J. W. Connell, 1983, Correlation of Forced Flow Evaporation Heat Transfer Coefficient in Refrigerant Systems, Paper B2, in Heat Exchangers for Two-Phase Applications, Vol. 27, National Heat Transfer Confi, Seattle, WA. (4)... 

A forward-feed double-effect standard vertical evaporator with equal heating areas in each effect is fed with 5 kg/s of a liquor of specific heat capacity of 4.18 kJ/kgK, and with no boiling-point rise, so that 50 per cent of the feed liquor is evaporated. The overall heat transfer coefficient in the second effect is 75 per cent of that in the first effect. Steam is fed at 395 K and the boiling-point in the second effect is 373 K. The feed is heated to its boiling point by an external heater in the first effect. 

A liquor containing 15 per cent solids is concentrated to 55 per cent solids in a doubleeffect evaporator operating at a pressure of 18 kN/m2 in the second effect. No crystals are formed. The feedrate is 2.5 kg/s at a temperature of 375 K with a specific heat capacity of 3.75 kJ/kg K. The boiling-point rise of the concentrated liquor is 6 deg K and the pressure of the steam fed to the first effect is 240 kN/m2. The overall heat transfer coefficients in... 

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. 

Example 6.3 Consider Example 6.2 for a shallow pool of methanol with its bottom surface maintained at 25 °C. Assume that natural convection occurs in the liquid with an effective convective heat transfer coefficient in the liquid taken as 10 W/m2 K. Find the surface temperature, surface vapor mass fraction and the evaporation flux for this pool. 

Brooks, C. H. and Badger, W. L. Trans. Am. Inst. Chem. Eng. 33 (1937) 392. Heat transfer coefficients in the boiling section of a long-tube, natural circulation evaporator. 

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. 

Figure 8.18. Overall heat transfer coefficients in some types of evaporations, (a) Water and sugar juice evaporators (b) Sea water evaporators. [F.C. Standiford, Chem. Eng., 157-176 (9 Dec. 1963)].

The At for freezing is the minimum in the freezer. This difference in APs is based on the fact that the heat-transfer coefficients in the evaporator will be much higher than in the... 

The forced-circulation crystallizer is a simple unit designed to provide high heat-transfer coefficients in either an evaporative or a cooling mode. Figure 12 shows a... 

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