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View Subscription Details: Letter ( 11/20/2009 ) - HTML Format

We float condensing pressure


Today many companies claim that to save energy they float condensing pressure of the refrigeration plants. Essentially, during summer operation the condensing pressure will float itself based on fluctuation of the wet bulb temperature and refrigeration loads. To save energy, this pressure should be floated at optimum level to minimize high side (compressors + condensers) power use. The compressor and condenser capacities should be matched. When the refrigeration load and compressor capacity is reduced, condenser capacity should be reduced as well.

When should the condensing pressure be floated?

Floating should be done during periods of warm weather. However, when weather is hot, majority of the refrigeration plants will have maximum refrigeration load and all condensers will operate at full capacity (no floating). The refrigeration plants with significant and frequent fluctuation of the refrigeration loads will benefit the most from floating condensing pressure. During the periods of cool weather, the refrigeration plants will be operated at minimum condensing pressure (no floating). I think that periods of warm (not hot) weather is the best time to float condensing pressure. However, evaporative condensers should not be undersized, otherwise they will run at full capacity and the refrigeration plant will never reach an optimum condensing pressure.

How great are the savings of floating condensing pressure?

Floating condensing pressure does not save compressor energy, but it saves condenser energy. The condenser energy is not significant and typically in the range of 8-10% of total refrigeration plant energy use. My research has shown that properly floated condensing pressure will save 15-20% of condenser energy use. This means that these energy savings will be in the rage of 1-2% of total refrigeration plant energy use.


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