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View Subscription Details: Letter ( 04/25/2005 ) - HTML Format

PLC (Programmable logical controller)

In our days majority of industrial refrigeration plants are using PLCs (programmable logical controllers) to control their operation. Very often, it is not an easy task for a manager or a chief engineer to choose the right PLC. Hopefully, my newsletters will help you to make the right choice.


According to my classification, there are 2 types of PLCs:

  • “black box”
  • “of the shelf”

“Black box”, usually, is designed specifically for industrial refrigeration and it has

several energy saving features. Manufacturer can only reprogram this type of PLC and

you have to deal only with the manufacturer regarding upgrades, spare parts, additional

features and etc.


“Of the shelf” is PLC, usually, common for many industries. You need a team (energy efficiency consultant and a programmer) to program this type of PLC. In this case you have the opportunity to choose consultant, programmer, parts supplier and etc.

Sometimes this type of PLC is preferred, because it should interact with other PLCs at your facility.


Regardless of type every PLC should have several major energy savings features:

  • condensing pressure control
  • defrost control
  • suction pressure control
  • load shifting control
  • compressors sequence control

The most important feature is condensing (head) pressure control. The right strategy and right set points for this control, very often, can give you over 50 % of total energy savings.


There are 2 types of condensing pressure control:

  • floating condensing pressure
  • fixed condensing pressure

Fixed condensing pressure is a certain set point of your choice, for example 140 psig.

PLC will keep head pressure from 138 psig to 142 psig, if dead band is set to 4 psig. But

you can get much more energy savings by floating condensing pressure instead of

keeping this pressure at a certain set point.


To maximize energy savings in industrial refrigeration we have to look at the refrigeration plant as a whole system. This is a system approach. It is a very difficult task to optimize efficiency of the whole plant. To simplify this task we will divide our plant into high side and low side.

  • High side includes compressors and condensers.
  • Low side includes compressors and evaporators.
  • Floating condensing pressure will help us to minimize high side energy consumption.

What is the floating condensing pressure?

Industrial refrigeration plants, usually, have evaporative condensers. Wet bulb temperature of ambient air has a dominant influence on the capacity of these condensers.

This temperature is going up and down during the day. To conserve energy of high side, condensing temperature (pressure) should follow wet bulb temperature at certain temperature difference. This temperature difference between condensing temperature and wet bulb temperature is wet bulb approach.


To run refrigeration plant efficiently, we have to keep optimum wet bulb approach. This approach will help us to balance condensers’ capacity and compressors’ capacity at optimum level to minimize total power consumption of high side.


In next newsletter I will give you an example of floating condensing pressure.

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