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View Subscription Details: Letter ( 04/30/2013 ) - HTML Format



Recently, I have visited a refrigeration plant that has an optimizer. What is it? This is device to control condensing pressure. Usually, it is small PLC that can float condensing pressure, based on fluctuation of the wet bulb temperature of ambient air. This device has 2 sensors. First sensor measures dry bulb temperature. Second sensor measures relative humidity. Only these two measurements can be seen on the screen of the PLC. Typical PLC has wet bulb approach set point. An operator can choose the value of this set point and he can control how this set point was implemented. Mentioned optimizer was designed as a black box. Nobody, except designer, knows what the set point of wet bulb approach is. A mechanic who installed this optimizer told me that he trusts the engineers who designed this device. I think that this is a wrong approach. An operating engineer should have control over every device of the refrigeration plant. He should be able to change set points and verify correct operation of the refrigeration plant.

I found two issues with this plant.

1. Relative humidity and dry bulb temperature sensors show wrong readings. How you can keep optimum condensing pressure if your sensors do not work properly? Actually this happens very often. If your plant has these sensors, make sure that they work correctly.

2. Typically, optimizer like this should be activated to control condensing pressure, when condenser capacity is oversized compared to actual refrigeration load. I found that condenser of this plant is undersized and mentioned optimizer will never be activated. Why should anybody install device witch will never be used? This can only happen when people have limited knowledge of floating condensing pressure.

Do not believe in back boxes. Believe in knowledge and experience in our industry.


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