Evaporation overall heat transfer coefficients

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

Some workers have attempted to base the design of humidifiers on the overall heat transfer coefficient between the liquid and gas phases. This treatment is not satisfactory since the quantities of heat transferred through the liquid and through the gas are not the same, as some of the heat is utilised in effecting evaporation at the interface. In fact, at the bottom of a tall tower, the transfer of heat in both the liquid and the gas phases may be towards the interface, as already indicated. A further objection to the use of overall coefficients is that the Lewis relation may be applied only to the heat and mass transfer coefficients in the gas phase. 

A single-effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50 per cent of solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. If the overall heat transfer coefficient is 3 kW/m2 K, calculate the heating surface required and the amount of steam used if the feed to the evaporator is at 294 K and the condensate leaves the heating space at 352.7 K. The specific heat capacity of a 10 per cent solution is 3.76 kJ/kgK, the specific heat capacity of a 50 per cent solution is 3.14 kJ/kgK. 

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 single-effect evaporator is used to concentrate 0.075 kg/s of a 10 per cent caustic soda liquor to 30 per cent. The unit employs forced circulation in which the liquor is pumped through the vertical tubes of the calandria which are 32 mmo.d. by 28 mmi.d. and 1.2 m long. Steam is supplied at 394 K, dry and saturated, and the boiling-point rise of the 30 per cent solution is 15 degK. If the overall heat transfer coefficient is 1.75 kW/m2 K, how many tubes should be used, and what material of construction would be specified for the evaporator The latent heat of vaporisation under these conditions is 2270 kJ/kg. 

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 ... 

Overall heat transfer coefficients for any form of evaporator depend on the value of the film coefficients on the heating side and for the liquor, together with allowances for scale deposits and the tube wall. For condensing steam, which is a common heating medium, film coefficients are approximately 6 kW/m2 K. There is no entirely satisfactory... 

A single-effect evaporator operates at 13 kN/m2. What will be the heating surface necessary to concentrate 1.25 kg/s of 10 per cent caustic soda to 41 per cent, assuming a value of the overall heat transfer coefficient U of 1.25 kW/m2 K, using steam at 390 K The heating surface is 1.2 m below the liquid level. 

A triple-effect evaporator concentrates a liquid with no appreciable elevation of boiling point. If the temperature of the steam to the first effect is 395 K, and vacuum is applied to the third effect so that the boiling point is 325 K, what are the approximate boiling points in the three effects The overall heat transfer coefficients may be taken as 3.1, 2.3, 1.3 kW/m2 K in the three effects, respectively. 

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)].

Overall heat transfer coefficient U = 310Wm 2K 1 Heat exchange area (assumed to be constant) A = 5.5 m2 Mean cooling water temperature Tc = 20 °C Latent heat of evaporation (Isopropanol) AHv = 700kjkg ... 

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

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 density 1100 kg/m3 passes through the inside of 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 flowrate is 1.75 x 10 4 m3/s and to 370 K when the flowrate is 3.25 x 10 4 m3/s. What is the heat transfer area and the value of the overall heat transfer coefficient when the flow rate is 1.75 x 10 4 m3/s ... 

The same airflow as in Prob. 10-69 is to be cooled in a finned-tube exchanger with evaporating Freon in the tubes. It may be assumed that the Freon temperature remains constant at 35°F and that the overall heat-transfer coefficient is 125 W/m2 °C. Calculate the exchanger area required in this case. Also calculate the reduction in heat transfer which would result from cutting the air flow rate by one-third. 

When true scale formation occurs, the overall heat-transfer coefficient may be related to the time the evaporator has been in operation by the straight-line equation ... 

FIG. 11-2S Overall heat-transfer coefficients in agitated-film evaporators. °C = (°F - 32)/1.8 to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply by 5.6783 to convert centipoises to pascal-seconds, multiply by lO h... 

The data from the experiments showed that the overall heat transfer coefficient (for the whole absorber) varied from a minimum of 133 W/m -K at a concentrated solution flow rate of 0.0106 kg/s to a maximum of 403 W/m -K at 0.0361 kg/s. A relatively constant, high value of the coolant-side heat transfer coefficient, 2600 W/m -K, was seen for all the cases tested, in which the flow remains laminar. The absorber was found to be solution-side limited. Heat transfer rates as high as 16.2 kW, representative of residential heat pumps, were achieved in this small envelope however, the performance was somewhat lower than that predicted by the preliminary model of Garimella [34], The 21% smaller surface area of the prototype [37] compared to the modeled design [34] accounted for some of the discrepancy. Also, the single-pressure absorber test facility used in the experiments supplied vapor to the absorber at conditions representative of desorber outlet conditions (as 86.7% NH3) rather than evaporator... 

If scale forms inside and/or outside the tube walls, additional resistance terms (Fig. 4) should be added to Eq. (2). Some typical values of overall heat-transfer coefficients are given in Table 3 for various evaporator designs. 

Sludge is concentrated in a triple-effect evaporator to recover vitamin The first effect operates at 8 psig, whereas the last effect operates at a temperature of 110°F. If the overall heat transfer coefficients are 400, 320, and 240 for the first, second, and third effects, respectively, estimate the liquor boiling temperature in each stage. Neglect any boiling point elevation. 

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