A Compressor Story w/ Trevor Matthews
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Whenever we approach a failed compressor, we need to examine it carefully to get an idea of its story. For example, the copper plating could be compromised, and that could indicate that we’re dealing with acid contamination. We get acid contamination when moisture gets into the system via a poor vacuum, broken heat exchanger, or poor installation. Electrical failures can also cause a burnout, which would result in acid.
Overheating, low oil, slugging, floodback, and flooded starts are other problems that have quite a few potential causes. We need to know the compressor’s story to fix the issue completely. We need to look for other clues, including refrigerant leaks, oil viscosity or return issues, washed-out bearings, scoring, loose wires, refrigerant migration, and more that are pertinent to a compressor’s story.
Sometimes, we can’t determine a story without breaking the compressor apart. We’re clearly not going to break down a compressor every time in the field, but we can take failed compressors back and examine them later to learn from them. Before and during a teardown, our senses are our best tools for figuring out a compressor’s story. If you remove the head in the field and need to replace it, be sure to have new gaskets on hand.
The compression ratio is important to consider when looking at overheating scenarios. You can find the compression ratio by dividing the absolute discharge pressure (discharge pressure + 14.7 PSI) by the absolute suction pressure (suction pressure + 14.7 PSI). High compression ratios also point to inefficient performance, so we need to look for conditions that cause the suction pressure to be too low relative to the discharge pressure. Generally, the discharge temperature should also be below 225 degrees Fahrenheit when measured 6 inches from the compressor. A suction line that’s too warm will also result in a higher discharge temperature, so suction lines need to be insulated to prevent heat transfer in the suction line.
To prevent overheating, we need to maintain compression ratios per the manufacturer’s recommendation, use low-pressure controls within the compressor operating envelope, prevent suction pressure drop, and (sometimes) use additional cooling methods.
Flooded starts and slugging can all occur as a result of refrigerant migration. We can use discharge check valves, pump down cycles, crankcase heaters, bump start cycles, or simply keep the compressor in a warmer location.
Floodback occurs when the liquid comes back to the compressor during operation, often as the result of low airflow or metering device problems. Anything that prevents liquid refrigerant from boiling in the evaporator can cause floodback, and you will read little to no superheat Overcharging and setting the superheat properly are vital to floodback prevention.
When preparing for a compressor teardown, you will want to make sure that there is no power going to the compressor. Then, you’ll want to check the compressor type to determine how you’ll measure resistance. After checking the resistance, it would be wise to check the terminal plate and look for issues there. When you take the head off of a semi-hermetic compressor, leave two bolts before knocking it out of place. Check the gasket (if it’s still intact) and try to wipe off mechanical wear (to rule out overheating).
When dealing with a reciprocating compressor, we want to press the pistons and see how the rest of them react (to check for wrist pin wear, which the compressor in the video had). The teardown is also a great time to take off the compressor pump, check for bearing wear, and “shake hands” with the crankshaft. All of those procedures can let you know a lot more about the compressor’s story.
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