Heat surface area

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. 

A calorifier has a heating surface area of 2m and is required to heat a flow of water from 65°C to 80°C. The U value has been found to be 1250 W/m °C, and steam can be supplied at a gauge pressure of 2 bar in the steam chest. What rate of water flow can the calorifier handle (see Figure 22.3). From Equation (22.2) ... 

From the above it can be seen that 10 sqft of boiler heating surface was commonly considered to provide 34.5 lb of saturated steam, from and at 212 °F. Whereas, because of the much higher heat transfer rates today, modem boilers often generate anywhere from 100 to 500 Ib/hr of steam from the same heated surface area (3.5-0.7 sqft per boiler hp). 

Drier, drum, atmospheric heat surface area, 1-10 m 0.40... 

Drier, shelf, vacuum heat surface area, 1-100 0.53... 

Evaporator, agitated falling-film, s.s. heat surface area, 3-6 m 0.55... 

A single-effect evaporator with a heating surface area of 10 m2 is used to concentrate a NaOH solution flowing at 0.38 kg/s from 10 per cent to 33.3 per cent. The feed enters at 338 K and its specific heat capacity is 3.2 kJ/kg K. The pressure in the vapour space is 13.5 kN/m2 and 0.3 kg/s of steam is used from a supply at 375 K. Calculate ... 

For pyrolyses which proceed best in a heated tube, which may with advantage be packed with glass beads or with porcelain chips to increase the heated surface area, and for reactions which occur on the surface of a heated catalyst, the basic apparatus shown in Fig. 2.65(c) is often suitable. A pressure-equalising... 

Preparation method Final heat Surface area... 

Usually, the heating surface areas of all effects are equal for construction economy, therefore,... 

Assuming that all effects have the same heating surface area, Eqs. (13) and (14) give... 

The heating surface area of the apparatus is 0.016 in , the condensing surface area of the condenser is 0.2 m . The maximum working temperature lies around 220 °C so that at 25 torr the maximum rate of evaporation is 480 g/h. 

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