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Starting up refrigeration control systems.

If we lived in a perfect world, little children could start up refrigeration control systems. Unfortunately, life is not so perfect, so a control system's startup period usually requires a combination of control skills, refrigeration skills, patience and the ability to observe the little things that are causing startup to be imperfect.

Theoretically, startup of a computerized control system is a simple computer or PLC task. In real life, startup often involves all that, plus the detective skills of Colombo to figure out why the system doesn't start up automatically. The most common source of startup problems is wiring problems, which fall into three rough categories: misplaced wires, damaged wires and finally, improperly shielded wiring. The category of misplaced wires includes improperly grounded shields.

The first task of a startup technician is to get the computer communicating with the PLC or control modules. In an RS422 communication network, this means getting the four wires from the computer connected in the right order to the four terminals on the PLC or control module. In a recent startup, two of the wires on one controller were crossed in a manner that was very difficult to see, delaying the startup for 4 hours before the startup technician said, "wait a minute ... are these wires supposed to be crossed?" The rest of the startup was easy.

Once the PLC or control modules are communicating with the central computer, it then becomes possible to make changes to the set points and other control parameters and to see where each controlled component stands, and to see whether the temperatures and pressures the controllers are seeing are reasonable.

One of the first tasks is to make sure that all the temperatures and pressures are showing readings that are consistent with reality. Sensor wires are often mislabeled or improperly connected, so you must verify that the sensors are all connected to the correct input terminals with the correct polarity. Air temperature sensor wiring can be verified by having someone with warm hands hold the temperature sensor in his hand. The person at the computer will see the temperature rise to 80-85F, which verifies the correct location of the air temperature sensor. Pressure sensors can be verified by observing the rise and fall of the actual pressure and comparing it with the pressure displayed on the computer screen. The wiring for ammonia sensors can be verified by waving an ammonia-soaked rag near the sensor. The wiring for other sensors, such as liquid line temperatures can be verified by observing their activity during operation, such as defrosts.

Digital outputs are a little more tedious to track down than analog inputs, and there usually are a lot more of them. The most straightforward way to make sure all the digital outputs are connected properly is to use the Hand/Off/Auto switches to energize each output relay to see that the correct valve, fan or pump is energized when the switch is thrown. This is a two-man job: one at the Hand/Off/Auto switch and one in the field observing the start of a motor or the opening of a valve.

Another important element of the startup process is the calibration of all analog sensors. In a computer-controlled system, this usually means simply typing the correct value of the sensor into the computer screen, letting the computer software automatically take care of the rest.
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Title Annotation:News From Hench
Author:Hench, John
Publication:Frozen Food Digest
Geographic Code:1USA
Date:Apr 1, 2004
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