First focus on the process that occurs in the expansion valve, where liquid enters and a mixture of liquid and vapor leaves. The familiar vertical line on the pressure-enthalpy diagram, such as process 3-4 in the standard vapor compression cycle of Figure 2.32, indicates a progressive increase in the fraction of vapor. This vapor is unable to absorb heat from the evaporator, but requires compressor power to return it to the condenser pressure. To seek more insight into this process, explore the expansion of ammonia saturated liquid at 30°C (86°F) and a pressure of 1163.8 kPa (168.8 psia) to a pressure of 289.9 kPa (42.1 psia) corresponding to a saturation temperature of -10°C (14°F). Figure 3.1a shows the process taking place in one step. Figure 3.1b shows the equivalent process but with the expansion occurring as follows: (a) a drop in pressure to 572.1 kPa (83.0 psia), (b) a separation of the liquid and vapor, (c) separate expansion of liquid and vapor to the final pressure of 289.9 kPa (42.1 psia), and finally (d) combining the mixture of liquid and vapor at point 6 with the vapor at 5 to achieve at point 7 the same conditions that occurred after the single-stage expansion. Table 3.1 shows conditions at the statepoints of Figure 3.1b with the flow rates that would occur for a total flow rate of 1 kg/s.
The purpose of comparing the two techniques for conducting the expansion is to focus on the flash gas that progressively develops as the expansion proceeds. In particular, when the vapor at point 3 is expanded to the low pressure this vapor is useless for refrigeration. Its enthalpy is too high to achieve any refrigeration in the evaporator, and its temperature of -2.15°C (28.1°F) is actually higher than the -10° C (14°F) that will be the evaporating temperature. In order to drop the temperature of the vapor at point 5 to -10° C (14°F), a small amount of liquid at 6 must vaporize to match the state at point 7 to that which occurs in the single-stage expansion. Not only is the vapor at point 3 useless for refrigeration, the continued expansion to the low pressure only entails the expenditure of power to recompress it. The conclusion is that the vapor at point 3 should be removed and compressed back to the condensing pressure without expanding it further to the low pressure. This objective defines the function of flash-gas removal.