This is post number ten of a series.
Retail Booster Systems
Two-stage compression is used for transcritical low temperature applications because the discharge temperature of R744 is high and will potentially result in lubricant breakdown. Figure below shows a simple two-stage booster or externally-compounded system:
The refrigerant from the low-temperature loads is drawn into the low-stage compressors. The discharge from these compressors goes into the suction of the high-stage compressors.
The refrigerant from the medium-temperature (MT) loads is drawn into the suction of the high-stage compressors. The refrigerant from the receiver pressure-regulating valve is also drawn into the suction of the high-stage compressors. The flash gas from the receiver pressure-regulating valve and the suction gas from the medium temperature loads provide some interstage cooling. This is usually enough to maintain the discharge temperature of the high-stage compressors below the level at which the lubricant will deteriorate. Additional interstage cooling can also be provided if required.
In the next article of this series we’ll take a closer look at retail cascade systems.
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