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.