The foregoing two sections have isolated for separate analysis the process of flash-gas removal—one of the two major processes that is available in twostage compression. The other major process is desuperheating of the discharge vapor from the low-stage compressor. The two principal advantages of interstage desuperheating, or intercooling, are the saving in compressor power and the reduction in discharge temperature from the low-stage compressor.
Both of these advantages can be demonstrated on the pressure-enthalpy diagram, as in Figure 3.8, for a compression between two given pressures. The initial temperature of the refrigerant in compression A is high compared to that of compression B. A general observation of the compression lines (lines of constant entropy) is that they become flatter as they move to regions of greater superheat. The consequence of this change of slope is that the increase in enthalpy during the compression, which indicates the power required in the compression, is greater with high inlet temperatures. In Figure 3.8, DhA is greater than DhB. Another observation from Figure 3.8 is that the discharge temperature after Compression B is lower than that after Compression A. This feature is particularly important in reciprocating compressors, but is also significant in screw compressors, despite the internal cooling inherently provided during compression by the sealing oil.
Two of the methods of achieving intercooling are shown in Figures 3.9 and 3.10. The traditional method, Figure 3.9, is to immerse the outlet of the discharge line of the low-stage compressor below the liquid level in the intercooler providing bubbling of vapor through the liquid. Some of the liquid in the vessel evaporates to provide desuperheating of the vapor. The method will usually achieve a close approach of the temperature of the discharge vapor to the liquid temperature. The second approach3 is to spray liquid into the discharge line from the lowstage compressor, as in Figure 3.10, and in so doing vaporize some of the liquid to desuperheat the discharge vapor.
The concept of bubbling superheated vapor through the liquid in the vessel, as shown in Figure 3.9, is effective as a heat-transfer process, but has several disadvantages. One is that in order to be effective, the outlet of the low-stage discharge line should be between 0.6 to 1.2 m (2 to 4 ft) below the surface of the liquid. Due to the static head of the liquid, the point of discharge will be at a slightly higher pressure than at the surface of the liquid. The compressor must, therefore, expend more energy to overcome this additional pressure. A second disadvantage of the bubbler desuperheater is that the process churns the liquid and vapor in the vessel. Because one of the functions of the vessel is to separate liquid from vapor so that only dry vapor passes on to the compressor, the turbulence makes this task more difficult.
While usually not able to achieve the degree of desuperheating possible in the bubbler, the spray method of Figure 3.10 causes less disturbance in the vessel. In some cases the supply of liquid comes directly from the condenser through an expansion valve. The superheat-control (thermostatic) expansion valve, which will be discussed further in Chapter 11, mounts a sensor on the vapor line to the high-stage compressor, and if the vapor temperature is too high, the valve opens to admit more refrigerant. A failure of this valve would permit liquid to continue to pour into the vessel. An approach that avoids this danger is possible when liquid is pumped from the vessel in the liquid recirculation concept. Centrifugal pumps handling liquid close to its saturated condition require a small flow rate even when the demand drops to zero. This bypass flow can simply serve as the desuperheating spray.