Economizer Cycles

The primary method of improving vapor compression system cycle performance is increasing the refrigeration effect per unit mass of refrigerant by cooling the temperature of the liquid refrigerant prior to expansion to the primary evaporator. The colder the liquid refrigerant's temperature prior to flashing, the lower the mass flow and power required. Three methods for accomplishing this are: subcooling with a thermal economizer, use of an open economizer, or use of a closed economizer.

A thermal economizer, or sub-cooler, is a circuit in which some of the tubes in the first water pass of the condenser are below the refrigerant liquid level in the condenser. The refrigerant liquid level is maintained to cover the subcooler tubes in order to increase the cooling of liquid refrigerant. By reducing the required total mass flow of refrigerant through the system, brake power is reduced and cycle efficiency increased. In low-temperature applications, subcoolers are not used due to the risk of freezing water in the subcooler tubes in the event of a pump failure.

Open and closed economizers are commonly used for improving cycle efficiency in process refrigeration systems. An open, or flash, economizer is a tank that is vented to the inlet of one of the compression interstages. Liquid refrigerant is cooled prior to entering the cooler as some refrigerant is allowed to flash, cooling the remaining liquid to the saturation temperature corresponding to the inlet pressure of the compressor interstage. The closed economizer takes the liquid from the condenser and splits the flow into two streams. Most of the flow is directed through the tubes of a shell-and-tube heat exchanger, while the remainder goes to the shell side through a control value and is flashed to cool the refrigerant in the tubes. The gas generated is vented to the inlet of one of the compressor stages.

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