Using Charts with Caution

There are numerous trouble shooting charts and diagrams to help diagnose system problems. Probably one of the most familiar ones I’ve seen, posted numerous places is below.

The chart points out one thing clearly, you can’t diagnose a problem without multiple pieces of information about the system. I’m not sure who really gets credit for developing this chart, but the first place I came across it was in a very old Carrier training manual, and my guess is, they should get credit for it.
 Another type of performance chart is a unit specific charging chart. Using field acquired pressure and temperature of the liquid line, the chart shows you what to do to get the unit performing at design. Below is an example of that type of chart.

Both approaches seem simple enough to use, but neither will work if the system is dirty or improperly installed.
With almost any set of charging instructions you will find the following two lines;
1.      Never charge liquid into the suction port of the compressor.
2.      Add small amounts of refrigerant at a time.
Why do you suppose that is?
Liquid into the compressor will damage it. It also will cause significant swing to the system pressures and temperatures. It can also quickly overcharge the system provided the compressor can take it.
As for small amounts of charge at a time, that is because when the charge is incorrect, the system is unstable. It will take as much as 20 minutes for the change you made to be realized in the readings you are observing.
To illustrate the system performance, I recorded a dataset for an air-cooled unit cooling a water stream to 45 degrees. Below is the system performance from starting, and running to set point.

Nice stable performance, the suction pressure follows the water temperature right to the set-point. Discharge pressure slowly lowers as the system is cooled, and the pressure ratio is essentially constant.
This system, by the way, used a capillary tube as a metering device. But you will see similar characteristics, such steady motor current, in a properly charged system.
Now, let’s look at a system trying to perform with about a 20% undercharge.

As might be apparent, system stability, or lack thereof, is questionable.
The water temperature warms up some during the initial operation. The discharge pressure climbs as the system can remove all the excess heat from the unit and the process.
Pressure ratio is all over the board for almost two hours, and suction pressure climbs until the system reaches a somewhat stable operation at 4:17.
So, when you are trying to interpret the information from your gauges and temperature probes, keep in mind, the system is always trying to adapt to the process conditions and its problems all at the same time.
So, use some patients and this knowledge to proceed with care when using charts to diagnose or charge a system.
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