510 Fermentation and Biochemical Engineering Handbook Often in multiple-effect evaporators the concentration of the liquid being evaporated changes drastically from effect to effect, especially in the latter effects. In such cases, this phenomenon can be used to advantage by staging one or more of the latter effects. Staging is the operation of an effect by maintaining two or more sections in which liquids at different concentra- tions are all being evaporated at the same pressure. The liquid from one stage is fed to the next stage. The heating medium is the same for all stages in a single effect, usually the vapor from the previous effect. Staging can lbstantially reduce the cost of an evaporator system. The cost is reduced because the wide steps in concentrations fromeffect to effect permit the stages to operate at intermediate concentrations, which result in both better heat transfer rates and higher temperature differences 6.0 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 I million kJ. Water has a latent heat of 480 kJ/kg at 760 mm of mercury, absolute, so the energy required to evaporate l kg of water exceeds 0. 16 cents. Therefore, the efficient utilization of energy is the most important consideration in evaluating which type of evaporation system should be selected Energy can never be used up; the first law of thermodynamics guarantees its conservation. When normally speaking of"energy use"what is really meant is the lowering of the level at which energy is available. Energy has a value that falls sharply with level. Accounting systems need to recognize this fact in order to properly allocate the use of energy level The best way to conserve energy is not to "use"it in the first place. 1221 Of course, this is the goal of every process engineer when he evaluates a process, but once the best system, from an energy point of view, has been selected, the necessary energy should be used to the best advantage. The mo efficient use of heat is by the transfer of heat through a heat exchanger with rocess-oriented heat utilization, or by the generation of steam at sufficient levels to permit it to be used in the process plant directly as heat. When heat is available only at levels too low to permit recovery in the process directly thermal engine cycles may be used for energy recovery. Heat pumps may als51 0 Fermentation and Biochemical Engineering Handbook Often in multiple-effect evaporators the concentration of the liquid being evaporated changes drastically from effect to effect, especially in the latter effects. In such cases, this phenomenon can be used to advantage by staging one or more of the latter effects. Staging is the operation of an effect by maintaining two or more sections in which liquids at different concentra￾tions are all being evaporated at the same pressure. The liquid from one stage is fed to the next stage. The heating medium is the same for all stages in a single effect, usually the vapor from the previous effect. Staging can substantially reduce the cost of an evaporator system. The cost is reduced because the wide steps in concentrations from effect to effect permit the stages to operate at intermediate concentrations, which result in both better heat transfer rates and higher temperature differences. 6.0 ENERGY CONSIDERATIONS FOR EVAPORATION SYSTEM DESIGN The single largest variable cost factor in making a separation by evaporation is the cost of energy. Ifcrude oil is the ultimate source of energy, the cost of over $126.67 per m3 ($20 per barrel) is equivalent to more than $3.33 for 1 million kJ. Water has a latent heat of 480 kJkg at 760 mm of mercury, absolute, so the energy required to evaporate 1 kg of water exceeds 0.16 cents. Therefore, the efficient utilization ofenergy is the most important consideration in evaluating which type of evaporation system should be selected. Energy can never be used up; the first law of thermodynamics guarantees its conservation. When normally speaking of “energy use” what is really meant is the lowering ofthe level at which energy is available. Energy has a value that falls sharply with level. Accounting systems need to recognize this fact in order to properly allocate the use of energy level. The best way to conserve energy is not to “use” it in the first place.[22] Of course, this is the goal of every process engineer when he evaluates a process, but once the best system, from an energy point of view, has been selected, the necessary energy should be used to the best advantage. The most efficient use of heat is by the transfer of heat through a heat exchanger with process-oriented heat utilization, or by the generation of steam at sufficient levels to permit it to be used in the process plant directly as heat. When heat is available only at levels too low to permit recovery in the process directly, thermal engine cycles may be used for energy recovery. Heat pumps may also