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View Subscription Details: Letter ( 02/26/2015 ) - HTML Format

Energy Savings and Operators

 

Operators are major players in the energy saving process. However, their knowledge should be upgraded to achieve maximum energy savings.

I have heard from several people that to save energy in industrial refrigeration they want to buy a sophisticated PLC that works without input from the operators. Unfortunately, such PLC doesn’t exist. Every PLC has certain set points and to maximize energy efficiency of the refrigeration plant the best set points should be chosen, implemented and monitored. Operators should be actively involved in this process as only they can monitor plant operation 24 hours per day and 7 days per week.

For instance, several years ago I visited a refrigeration plant which had sophisticated PLC. This PLC had a feature to control the web bulb approach. This feature is very useful to float condensing pressure and thus save energy. However, to get this energy savings the right set point should be chosen and the wet bulb temperature sensor should work properly. A set point for this plant was chosen properly but the wet bulb sensor showed a wrong reading. Nobody knew how long this sensor didn’t work properly. Due to the faulty sensor this refrigeration plant overspent a lot of energy. Another refrigeration plant had the same feature but the wet bulb sensor had a wrong location. Half of the day it was showing the correct reading and the other half of the day it was showing the wrong reading. Unfortunately, in both of these cases nobody explained to the operators how to verify correct operation of this feature.

Lowering condensing pressure is a major energy savings measure. This measure can give us up to 50% of total energy savings. Operators are often very concerned with lowering condensing pressure below typical 110 – 120 psig (7.5 – 8 bars). There are several barriers to operate a plant at condensing pressure below the typical nonetheless every barrier has a solution. An expert should educate operators about these barriers and should tell them how these barriers can be overcome. First, every plant should be tested. Condensing pressure should be gradually reduced by 5 psig (0.3 bars) per week. Small steps will not harm the plant. However, certain points (liquid make up, hot gas defrosting …) of plant operation should be monitored by the operators. Assume that at 100 psig (7 bars) of condensing pressure bottom of the evaporators did not defrost properly. Defrosting process (hot gas supply, BPR setting…) should be adjusted to provide complete defrost.

I believe that operators should actively participate in the energy saving process. However, to achieve maximum savings they should be educated and guided about efficient operation and the tuning up process. 

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