The system pressure, P, must be specified and varied to determine its effect on system performance. The mass flow rate of ammonia-water from the condenser, m6, defines the size scale of the system. For example, this mass flow of ammonia-water solution could be increased to increase the evaporator load and likewise adjusted to obtain the load desired. Finally the temperature of the generator, T8, needs to be high enough to generate enough ammonia to run the cycle, but not so high that it would stress the materials used to hold the fluids.
Similarly to the effect of Te on system performance is the effect of Tc on system performance (Figure 3-2). With a fixed Te of 275 K, a fixed P of 4.01 bar, and a fixed Tg of 400 K, the COP again tends to infinity as the temperature lift goes to zero, except now Tc is approaching Te. However, the effect of Tc on COP is less pronounced than the effect of Te on COP.
Figure 3-3 shows the results of fixing Te, Tc, and Tg and varying the system pressure. As system pressure goes up, the COP decreases in a nearly linearly fashion. When Figures 3-1, 3-2, and 3-3 are accompanied by Figure 3-4, an interesting limitation of the system is unveiled. In Figure 3-4, the bubble point and dew point temperatures are plotted versus concentration for the ammonia-butane system in VLE. The optimum system pressure it the lowest pressure for which the ammonia-butane system will still exist. First, looking at the set of curves at 4 bar, the lowest possible temperature where ammonia-butane system can exist is the saturation temperature of pure ammonia at 4 bar or 271.3 K. This limits the temperature in the evaporator to be above 271.3 K since below this, the butane in the evaporator will not boil.
Figure 3-5 shows this effect in a plot of butane vapor concentration in the evaporator at state point 6 vs. the evaporator temperature. As Te decreases at a fixed system pressure, the concentration of butane in the vapor also decreases and reaches zero at an evaporator temperature of 271.3 K. Lowering the system pressure allows for a lower Te shown by the other curves in Figure 3-5, however, this has the undesirable effect of lowering the maximum possible condenser temperature.
The highest possible temperature in the condenser is limited by the saturation temperature of pure butane at 4 bar, or 315 K (see Figure 3-3). Above 315 K, the nearly pure butane leaving the condenser at state point 1 would contain vapor which is undesirable for use in the evaporator. The maximum Tc and minimum Te are shown as a P-T plot for the pure substances ammonia and butane in Figure 3-6. It is seen here that the temperature lift is completely specified by the system pressure and the chosen working fluids.
Finally, the generator temperature was varied while holding Te, Tc, and P fixed, and the results are shown in Figure 3-7. Interestingly, changing the generator temperature has no effect on the COP. Increasing the generator temperature (increasing the heat transferred to the generator) increases the amount of vapor ammonia generated for use in the evaporator. This, in turn, increases the amount of butane evaporated in the evaporator which increases the evaporator load. However, the ratio of Qe to Qg (the COP) remains constant.
