Multistage Systems – Analysis Of The Intercooling Process

Just as flash-gas removal was analyzed separately, the intercooling process will also be explored independently. The flow diagram of a two-stage system using only intercooling is shown in Figure 3.11a and the corresponding pressureenthalpy diagram in Figure 3.11b.

Intercooling is often used in two-stage air compression, and in this application water from a cooling tower or the tap is the cooling medium. The cooling water is assumed to be free-of-charge, so all of the reduction in power in the highstage compressor represents a net saving in power. When intercooling refrigerant, however, the cooling water that is normally available is not cold enough to desuperheat the refrigerant to the saturation temperature. The vaporization of some refrigerant, which must then be compressed by the highstage compressor, is an expenditure of power that must be deducted from the reduction in power attributable to the lower inlet temperature.

(a) Two-stage system with intercooling only, and (b) the corresponding pressure-enthalpy diagram.

In Example 3.2 the difference in power requirement attributable to intercooling appears in the high-stage compressor. When intercooling is applied, the flow rate of refrigerant handled by the high-stage compressor increases, but the
work of compression for each kg (lb) of refrigerant decreases. With the statepoints shown in Figure 3.11b, the comparison of high-stage power is as follows:

As was true of flash-gas removal only, the intercooling process with ammonia becomes more effective as the evaporating temperature drops, as Figure 3.12 shows. For the three halocarbon refrigerants shown, however, intercooling provides little or no improvement in the power requirements in contrast to the results indicated in Figure 3.4 where the halocarbons react more positively to flash-gas removal than does ammonia. Since flash-gas removal and intercooling are so frequently used in conjunction with one another, the combination of the influences will be shown later for the standard two-stage system.

Percent saving in total compressor power resulting from intercooling at the optimum intermediate temperature.

Example 3.2. The intercooling system shown in Figure 3.11a operates with ammonia at the following saturation temperatures: evaporating, -35°C (-31°F); intermediate, 0°C (32°F); and condensing, 35°C (95°F). What is the saving in power of the intercooled cycle, expressed in percent, compared to single-stage operation?

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