Evaporation 511 be used to"pump"energy from a lower to a higher level, enabling" waste heat to be recovered through process utilization Thermal efficiencies of heat exchangers are high, 90-95%. Thermal efficiencies of thermal 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 alterna tives, 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 th 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 che 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 Maximum outlet temperatures for cooling water usually are dictated by the chemistry of the cooling water. Most cooling water contains chlorides and carbonates; consequently temperatures at the heat transfer surfaces must not exceed certain values in order to minimize formation of deposits or scale which reduces heat transfer and leads to excessive corrosion. In addition locity restrictions must be imposed and observed to prevent corrosion and fouling as a result of sedimentation and poor venting. Stagnant conditions on the water side must always be avoided. In some plants, water consumption is dictated by thermal pollution restrictions Unnecessary restraints should not be imposed on the pressure drops permitted across the water side of condensers. all too often, specified design values for pressure drop aretoo low and much higher values are realized wher the unit has been installed and is operating. Not only does this result in more expensive equipment, but frequently the water flow rate is not monitored and cooling water consumption is excessive, increasing operating costs. Because cooling water consumption is governed by factors other than energy conser- vation and because cooling water velocities must be maintained above certain values, tempered water systems can be effectively used at locations whereEvaporation 51 I be used to “pump” energy from a lower to a higher level, enabling “waste” heat to be recovered through process utilization. Thermal efficiencies of heat exchangers are high, 90-95%. Thermal efficiencies of thermal 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 alterna￾tives, 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 ofthe 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. Maximum outlet temperatures for cooling water usually are dictated by the chemistry ofthe cooling water. Most cooling water contains chlorides and carbonates; consequently temperatures at the heat transfer surfaces must not exceed certain values in order to minimize formation of deposits or scale, which reduces heat transfer and leads to excessive corrosion. In addition, velocity restrictions must be imposed and observed to prevent corrosion and fouling as a result of sedimentation and poor venting. Stagnant conditions on the water side must always be avoided. In some plants, water consumption is dictated by thermal pollution restrictions. Unnecessary restraints should not be imposed on the pressure drops permitted across the water side of condensers. All too often, specified design values for pressure drop are too low and much higher values are realized when the unit has been installed and is operating. Not only does this result in more expensive equipment, but frequently the water flow rate is not monitored and cooling water consumption is excessive, increasing operating costs. Because cooling water consumption is governed by factors other than energy conser￾vation and because cooling water velocities must be maintained above certain values, tempered water systems can be effectively used at locations where