Evaporation heat transfer coefficients

Sindlady, heating surface area needs are not direcdy proportional to the number of effects used. For some types of evaporator, heat-transfer coefficients decline with temperature difference as effects are added the surface needed in each effect increases. On the other hand, heat-transfer coefficients increase with temperature level. In a single effect, all evaporation takes place at a temperature near that of the heat sink, whereas in a double effect half the evaporation takes place at this temperature and the other half at a higher temperature, thereby improving the mean evaporating temperature. Other factors to be considered are the BPR, which is additive in a multiple-effect evaporator and therefore reduces the net AT available for heat transfer as the number of effects is increased, and the reduced demand for steam and cooling water and hence the capital costs of these auxiUaries as the number of effects is increased. 

Lewis relationship The ratio of the convective heat-transfer coefficient to the evaporative heat-transfer coefficient. 

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)... 

V. N. Schultz, D. K. Edwards, and I. Catton, Experimental Determination of Evaporative Heat Transfer Coefficients on Horizontal, Threaded Tubes, AlChE Symp. Ser. (73/164) 223-227,1977. 

Flash/transported (indirect convection) because the contact time is very short, transported driver is ideal for temperature sensitive material examples, foodstuffs coffee, maize gluten, maleic acid, oxalic acid, starch, proteins for stearates, PVC, adipic acid, aluminum oxide, CMC, dicalcium phosphate, fumaric acid, melamine. The inlet gas temperatures for flash dryers range from 175 to 750 °C and the typical exit gas temperatures range from 50 to 200 °C the exit air temperature is usually 20 °C greater than exit dry solid temperature. Gas velocity 3-30 m/s (usually 20 m/s) or 2.5 to 3 times the terminal velocity of the particles gas requirement 1-5 Nm /kg solid or 1-10 kg air/kg solid 4000-10 000 kJ/kg water evaporated. Heat transfer coefficient for gas drying h = 0.2 kW/m K and wall to gas/particles U = 0.1 kW/m K. At AT = 550 °C (the difference between inlet and exit gas temperature), the air usage/evaporation rate is 2 or 7.2 m /s/kg/s of water evap. At AT = 220 °C 4 or 14.4 m /s/kg/s of water evap. At AT about... 

TRO = Evaporator refrigerant side outlet temperature UA = A number proportional to the evaporator heat-transfer coefficient... 

The over-prediction of Tc may be a result of underestimation of evaporative heat transfer coefficients in the model, and may possibly also be affeeted by the estimations of distribution of blood flow to the core (trunk node) and shell. 

For mass transfer, the evaporative heat transfer coefficient he becomes uncertain at very low air speeds, however, an approximation can be estimated from the natural convection at a plane surface he. 

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. 

Heat Transfer from Various Metal Surfaces In an early work, Pridgeon and Badger [Jnd. Eng. Chem., 16, 474 (1924)] pubhshed test results on copper and iron tubes in a horizontal-tube evaporator that indicated an extreme effect of surface cleanliness on heat-transfer coefficients. However, the high degree of cleanhness needed for high coefficients was difficult to achieve, and the tube layout and... 

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)... 

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