ENERGY CONSIDERATIONS FOR EVAPORATION SYSTEM DESIGN

The single largest variable cost factor in making a separation by evaporation is the cost of energy. If crude oil is the ultimate source of energy, the cost of over 126.67 per m ( 20 per barrel) is equivalent to more than 3.33 for 1 million kJ. Water has a latent heat of 480 kJ/kg at 760 mm of mercury, absolute, so the energy required to evaporate 1 kg of water exceeds 

16 cents. Therefore, the efficient utilization of energy is the most important consideration in evaluating which type of evaporation system should be selected. 

Thermal efficiencies of heat exchangers are high, 90-95%. Thermal efficiencies of diermal engine cycles are low, 10-20%. Heat pumps permit external energy input to be reduced by a factor of 4 to 5 however, the energy required in a heat pump is in the work form, the most expensive energy form. 

Utility consumption, of course, is one of the major factors which determine operating cost and, hence, the cost of producing the product for which a plant has been designed. In order to select the proper equipment for a specific application it is important to be able to evaluate different alternatives, which may result in a reduction of utility usage or enable the use of a less costly utility. For example, the choice of an air-cooled condenser versus a water-cooled condenser can be made only after evaluating both equipment costs and the costs of cooling water and horsepower. 

When heating with steam, a selection of the proper steam pressure level must be made when designing the evaporator. No definite rules for the selection can be established because of changing plant steam balances and availability. However, it is generally more economical to select the lowest available steam pressure level which offers a saturation temperature above the process temperature required. Some evaporator types require relatively low temperature differences. Some products may require low temperature in order to reduce fouling or product degradation. 

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