adoration 517 Steam jet thermo-compressors can be used with either single or multiple-effect evaporators. As a rule-of-thumb, the addition of a thermo- compressor will provide an improved steam economy equivalent to an additional effect, but at a considerably lower cost. Thermo-compressors have low efficiencies which further diminish when the jet is not operated at its design point. Thermo-compressors in a typical operation can entrain one pound of vapor per pound of motive steam. They areavailable in a wide range of materials of construction, and can have a wide range of design and operating conditions. They should be considered only when high pressure motive steam is available, and when the evaporator can be operated with low pressure steam. Motive steam pressures above 60 psig usually are required to justify using thermo-compressors. Steam condensate from thermo- compressors often is contaminated with trace amounts of product and may have to be treated before being returned to the steam generator It is relatively easy to design an evaporator using thermo-compression for a given set of operating conditions. However, once the thermo-compres sor has been designed and fabricated, its performance characteristics are basically fixed. The design of a thermo-compression evaporation system should include an analysis of the consequences of changing operating points The characteristics of a thermo-compressor make it difficult to predict performances at conditions different from the design point, so accurate prediction of evaporator performance at other than design conditions be comes impossible. Because of the unpredictable performance of thermo-compressors control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermo-compressors in parallel. Thi permits capacity control without loss in energy economy. Thermo-compres sion evaporators are used for single or double-effect systems where low operating temperatures and improved economy are desired. It costs less to add a thermo-compressor instead of an additional effect, and both have about the same effect on energy economy. The temperature differences across the thermo-compressor should be below 15C. This evaporator system is not as flexible as multiple-effect systems because of the unpredictable variation of performance characteristics for the thermo-compressor under changing operating conditionsEvaporation 51 7 Steam jet thermo-compressors can be used with either single or multiple-effect evaporators. As a rule-of-thumb, the addition of a thermo￾compressor will provide an improved steam economy equivalent to an additional effect, but at a considerably lower cost. Thermo-compressors have low efficiencies which further diminish when the jet is not operated at its design point. Thermo-compressors in a typical operation can entrain one pound of vapor per pound of motive steam. They are available in a wide range of materials of construction, and can have a wide range of design and operating conditions. They should be considered only when high pressure motive steam is available, and when the evaporator can be operated with low pressure steam. Motive steam pressures above 60 psig usually are required to justig using thermo-compressors. Steam condensate from thermo￾compressors often is contaminated with trace amounts of product and may have to be treated before being returned to the steam generator. It is relatively easy to design an evaporator using thermo-compression for a given set of operating conditions. However, once the thermo-compres￾sor has been designed and fabricated, its performance characteristics are basically fixed. The design of a thermo-compression evaporation system should include an analysis ofthe consequences of changing operating points. The characteristics of a thermo-compressor make it difficult to predict performances at conditions different from the design point, so accurate prediction of evaporator performance at other than design conditions be￾comes impossible. Because of the unpredictable performance of thermo-compressors, control of evaporators using them is more difficult than for a conventional system where it is necessary to set only steam and feed rates to maintain a constant evaporation rate. One way to provide flexibility with better operating stability is to use two or more thermo-compressors in parallel. This permits capacity control without loss in energy economy. Thermo-compres￾sion evaporators are used for single or double-effect systems where low operating temperatures and improved economy are desired. It costs less to add athermo-compressor instead of an additional effect, and both have about the same effect on energy economy. The temperature differences across the thermocompressor should be below 15OC. This evaporator system is not as flexible as multiple-effect systems because of the unpredictable variation of performance characteristics for the thermo-compressor under changing operating conditions