and thus the working fluid serves as a lubricant for all rotating components, eliminating cross-leakage effects in conventionally lubricated systems. The cycle efficiency for a Brayton system is a function primarily of the peak cycle, or turbine inlet, temperature and the system pressure ratio, as shown for a typical Brayton system^ in Figure rV-B-l-c-5. These calculations assume efficiencies of 88% and 90%, respectively, for the compressor and turbine, with a recuperator effectiveness of 0.90. The figure illustrates that for a given turbine inlet temperature, an optimum pressure ratio (ratio of turbine inlet to compressor inlet pressure) exists for maximum efficiency. Absolute pressure levels are determined by size and weight tradeoffs. Higher pressures reduce heat exchanger size, but the weight of other system components increases due to structural considerations. The type of working fluid used in the cycle affects the number of stages required for the turbine and compressor, as well as heat exchanger size. These effects are illustrated in Figure IV-B-l-c-6. Figure IV-B-l-c-6a illustrates the decrease in the required number of stages for molecular weights from 4 (helium) to 40 (argon). Mixtures of these two gases (or other gases) will yield molecular weights between these two extremes. The main point of this figure is that turbine and compressor size and weight will decrease with increasing molecular weight of the working fluid. Figure IV-B-l-c-6b shows how relative recuperator size (the recuperator is the largest heat exchanger in the system) increases with molecular weight. As stated previously, recuperation increases overall cycle efficiency. However, to accomplish this, certain penalties associated with a recuperative heat exchanger must be paid. These penalties in efficiency and size (which are analogous to weight) are shown in Figure IV-B-l-c-7. Figure IV-B-l-c-7a indicates a rate of 1% decrease in cycle efficiency for a two percent rise in recuperator pressure loss. While it is desirable to keep the pressure loss as low as possible in the recuperator for cycle efficiency reasons, this can only be accomplished by making the heat exchanger larger, as shown in Figure IV-B-l-c-7b. Thus it is seen that careful system tradeoffs are necessary to achieve optimum cycle performance. Recuperator performance (heat exchanger effectiveness) is also an important parameter, and it too has an effect on cycle efficiency. This effect is shown in Figure IV-B-l-c-8a, where it is seen that a decrease in recuperator effectiveness is accompanied by a corresponding decrease in cycle efficiency (recuperator effectiveness values for typical space power systems are in the 0.90 region). The figure also illustrates how the optimum cycle pressure ratio decreases with an increase in effectiveness. Figure IV-B-l-c-8b shows the relative variation of recuperator size with effectiveness and illustrates the extreme weight penalty which must be paid for effectiveness values approaching 1.0.
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