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View Subscription Details: Letter ( 12/30/2013 ) - HTML Format

Good and bad investments in energy efficiency (1)


Today many companies are focused on energy efficiency of their refrigeration plants. They invest in sophisticated PLCs, VFDs and etc. However, very often limited energy savings are achieved. Why?

Unfortunately, many managers think that significant investments in energy saving equipment will lead to significant energy savings. This is incorrect, for instance, look at investments in PLCs. Ideal refrigeration plant PLC should have 2 parts.

The first part should determine optimum (the best) set points and operating strategies, while the second part of PLC should implement these set points and operating strategies. There are several PLCs on the market which can implement certain set points and operating strategies. Unfortunately, PLC capable of determining optimum set points and operating strategies does not exist. This means that a human should determine the mentioned set points and operating strategies. The huge question is Who should do that?

Very often this task was given to the operating engineers and this is fundamentally wrong. Yes, operating engineers can do initial optimization, however, final optimization of the refrigeration plant operation is the most complicated part of the energy saving process. This final optimization should be done by the experts. I have seen many refrigeration plants where operating engineers attempted to optimize operation of their refrigeration plants. Unfortunately, these plants have achieved only 5 - 30% of potential energy savings. Wrong set points and operating strategies are the major reasons for poor investments in PLCs.

Modern refrigeration plant PLC should have the following features:

1.     Floating condensing pressure based on wet bulb temperature of ambient air. Minimum allowable condensing pressure should be determined. Typically, it is below 100 psig (7 bars).

2.     Floating suction pressure based on production requirements.

3.     Flexible frequency of hot gas defrosting based on rate of frost formation.

4.     Optimized loading and unloading of the compressors.

5.     Optimized condensers sequence.

6.     Minimized energy use of the evaporator fans.


If your PLC has these features, it is a good one. However, PLC is just a tool to save energy. Only craftsmen can use this tool properly. To improve energy efficiency of your facility, find the right person who will help you to tune up your refrigeration plant PLC.


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