Multistage Systems – Flash Gas Removal

Suitable equipment to partially expand the refrigerant and then remove the flash gas is shown schematically in Figure 3.2. Liquid refrigerant from the condenser or high-pressure receiver passes through a level-control valve. The liquid, being more dense than the vapor, separates and flows on to the expansion valve of the evaporator. A flash-gas compressor draws off the vapor from the separating vessel or flash tank and compresses it to the condensing pressure where it joins the vapor from the main compressor.

A two-stage compression system with flash-gas removal.

Because the level-control valve in Figure 3.2 is appearing for the first time in this book, it is important to realize that it performs the function of an expansion valve. In controlling the level of liquid in the vessel, the valve opens wider if the liquid level begins to drop. Similarly, if the liquid level begins to rise, the valve closes more. The pressure in the flash tank is controlled by the pumping capacity of the flash gas compressor in relation to the flow rate of liquid passing on to the evaporator. For a given flow rate of liquid leaving the
vessel, a low pumping capacity of the flash-gas compressor results in a high intermediate pressure. This is true, because at steady-state operation the flow rate of vapor that forms due to flashing must equal the rate of removal by the compressor. If this pumping rate is low, the process adjusts by generating only a low flow rate of vapor, which is associated with a small drop in pressure through the valve. The pressureenthalpy diagram displaying the statepoints of the refrigerant in the equipment of Figure 3.2 is shown in Figure 3.3.

Pressure-enthalpy diagram of flash-gas removal process

The magnitude of saving from flash-gas removal depends on the thermodynamic properties of the refrigerant and the magnitude of the temperature lift from the evaporator to the condenser. Figure 3.4 shows the percent saving in total compressor power for several refrigerants and for a range of evaporating temperatures. Flash gas is removed at an optimum intermediate temperature, and this temperature will be explained further in Section 3.10. The percent saving increases as the evaporating temperature drops, which supports the contention that flash-gas removal and, in general, two-stage compression are most effective at low evaporating temperatures.

The halocarbons exhibit higher percent savings than does ammonia, but it should not be assumed that two-stage systems provide greater improvement in efficiency when using halocarbons. The other process associated with two-stage compression (intercooling) must yet be incorporated.

Percent saving in total compressor power resulting from flash-gas removal at the optimum intermediate temperature.

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