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View Subscription Details: Letter ( 06/19/2007 ) - HTML Format

Better control


A common misconception in industrial refrigeration is that better control (new PLC, additional feature for existing PLC) of a refrigeration plant will save energy.

Energy savings can be achieved only by implementing better set points and better operating strategies. Better control without improvement of set points and operating strategies will not save energy.

 Example. A cold storage refrigeration plant has 1 condenser and 3 compressors of equal capacities. A chief engineer has gotten a proposal from a contractor to install wet bulb approach feature for refrigeration plant PLC. Is this a good idea?

The wet bulb approach needs to balance capacities of the compressors and the condensers. This refrigeration plant operates 2 or 3 compressors during the periods of warm weather (summer operation) and 1 or 2 compressors during the periods of cool weather (winter operation). My estimation has shown that capacities of the compressors and the condenser are balanced when 2 or 3 compressors are running. The refrigeration plant is imbalanced only when 1 compressor is running.

 Summer operation. When 2 or 3 compressors are run, the capacities of the compressors and the condenser are balanced. Therefore, there is no need for the wet bulb approach.

 Winter operation. 1 or 2 compressors are run during this period. The wet bulb approach could be beneficial when only 1 compressor is running.

However, during the periods of cool weather, optimum head pressure based on wet bulb approach will be very low (less than 100 psig). Unfortunately, we can not run the refrigeration plant at this pressure and this plant will operate at minimum allowable head pressure (120 psig). Therefore, energy will not be saved by using the wet bulb approach during the winter operation.

 Conclusion. The wet bulb approach feature will not save energy for this refrigeration plant.

Every refrigeration plant requires a custom approach to optimize their performance. The same energy saving feature (wet bulb approach) could be beneficial to one refrigeration plant (April 2007 newsletter) but useless to another.   


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