Evaporation, heat calculations, examples

An example of an Al(lll) surface is shown in Fig. 8.4. From the measured work function, cj)=3.5 eV, we find k = 0.96A", The atomic distance is a = 2.88 A, which also equals the parameter m in the Morse formula. An estimation of the parameter Uq in the Morse formula, or the binding energy per pair of A1 atoms, can be made as follows The evaporation heat of aluminum is 293 kJ/mol, which is 3.0 eV per atom. Aluminum is an fee crystal, where each atom has 12 nearest neighbors. Therefore, the binding energy per pair of A1 atoms is about 0.5 eV. Substitute these numbers into Eq. (8.16) and Eq. (8.17), we find the forces at the T site and the H sites which reproduce the result of first-principle calculations by Ciraci et al. (1990a). 

For the heat calculations the reader is referred to. section 4.12, where examples have been given. The amount of heat necessary to bring the charge to the boiling point is found by formula (156), that for evaporation of the distillate and reflux by formula (158). The required condenser surface area may be calculated by means of formula (198). 

Calculating the heat transfer and water evaporation rates are illustrated by the following example. A cooling tower eools 900 gpm of water from 95 to 85 F. The problem is to determine what the heat rejeetion is, and also what is the evaporation rate. The heat rejeetion is ealeulated as follows ... 

With such data, an estimate can be made of a possible evaporator configuration for a required duty, that is, the diameter, length, and number of tubes can be specified. Then heat transfer correlations can be applied for this geometry and the surface recalculated. Comparison of the estimated and calculated surfaces will establish if another geometry must be estimated and checked. This procedure is described in Example 8.12. 

This is calculated at 180 °C for the charge of 2kmol and for a conversion of zero, which is conservative. It would be reasonable to interrupt the runaway at its very beginning, for example, at 190 °C. If we consider that the reaction rate doubles for a temperature increase of 10 K, the heat release rate would be 56kW at 190 °C. The latent heat of evaporation can be estimated from the given Clausius-Clapeyron expression ... 

With the fixed boil-off pressure and the measurement of the filling level, that is, of the hydrogen mass, all state variables of the hydrogen (M2, M3, h") are known from the T-s-diagram or a data base. From two measurements of the filling level at two times, the evaporating mass can be calculated and thence the heat input Q. For our example of the main tank, evaporating mass and heat input for two cases are shown in Table 1.5. Evaporation losses of the tank here are between 1 and 1.2% per day. 

Latent heats of evaporation may be calculated by the Clapeyron-Clausius equation ( 7.VIII L) and there is good agreement wi the experimental values. For liquefied gases, for example, the values of 4 are ... 

Example 24 Heat-Transfer Calculations A single rotating drum of 1.250-m diameter and 3 m wide is internally heated by saturated steam at 0.27 MPa. As the drum rotates, a film of slurry 0.1 mm thick is picked up and dried. The dry product is removed by a knife, as shown in Fig. 12-80a. About three-quarters of the drum s surface is available for evaporating moisture. Estimate die maYimiim drying rate when the outside air temperature Tq is 15°C and the surface temperature 50 C, and compare the effectiveness of the unit with a dryer without end effects and in which all the surface could be used for drying. Data ... 

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