The Case of Overheating
Sure, the compressor failed. But in The Case of Overheating, the real culprit was a system failure. Unfortunately, that doesn't let the compressor completely off the hook. System problems like overheating can lead to compressor and other damage. What follows is a line-up of the causes and effects of overheating on one of the compressors mentioned in the article from The News, the K-body.
| Section 1: The Effects of Overheating On Valve Plates. |
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This valve plate is discolored from high discharge temperature. Discharge temperatures must be controlled. The discoloration is from damaged, burnt oil. Think what is happening to the lubricating abilities of the oil in the cylinders and wrist pin areas. |
| Most of us have seen shiny new valve plates. What is pictured here is not the normal color of a valve plate. Even after years of operation the valve plate should appear new. Discoloration is not normal wear and tear. Discoloration is a sign of system problems. The discoloration is a symptom, not the cause of a problem. |
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Discharge temperatures are easy to measure. Use a reliable temperature indicator, and read the discharge line temperature 6 inches from the compressor connection. Discharge line temperature should not exceed 225° F. You can refer to the Application Engineering Bulletin 17-1260 in our Online Product Information section for additional details. |
| Section 2: Piston Damage |
| Piston and cylinder scoring is the result of the lubricant not being able to do its job. This type of wear could be the result of liquid refrigerant returning to the compressor and washing the lubricant away. This could also be the direct result of high discharge temperature thinning the lubricant so much that it cannot properly lubricate the surface. The big difference is the presence of heat. If liquid refrigerant did the damage, no indication of high temperatures would be present. Look at the valve plate, suction and discharge reeds, piston tops and cylinder walls for discoloration. Then you can make a determination of what caused the wear. |
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Cast iron pistons, as compared to aluminum, have a special coating on them. This coating is flat in color and appearance. If the piston appears shiny, like the top edge here, this is excessive wear. In order to see this the piston must be removed from the compressor. |
| Here it appears as if the entire length of the piston has been worn. You may look at the cylinder for indications of wear. Typically the cylinder will be worn if the piston is worn, but not always. For cylinder wear look for vertical marks or for the cross hatch to be worn away. The cross hatch is an “X “ ing pattern as the result of honing operation of the cylinders. |
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| Section 3: Other Effects Of Overheating |
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This is the upper end of the connecting rod and is called the wrist pin hole. A wrist pin is inserted through the piston and rod connecting the two together. Here we see an elongated pin hole. This could be the result of high temperatures breaking down the lubrication. If lubrication is lost the components will wear. Upon removal of the head and valve plate push one piston to the bottom of the stroke and the opposite piston will come to top dead center (TDC). With this piston at TDC the top of the piston should be flush with the machined compressor deck. Here this piston would have stayed below the deck because of the great amount of wear that we see. This would result in a loss of capacity and a noisy compressor. |
| The upper portion of the connecting rod with the wrist pin hole depends upon splash or misting lubrication to keep the wrist pin and pin hole from scoring and wear. Here is an example of a pin hole that has become oval due to wear. The compressor in this case would become noisy and lose capacity since the piston can no longer be pushed up near the valve plate, leaving a lot of compressed gas in the cylinder. |
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| Section 4: For More Information
More detailed compressor technical information and technical bulletins can be found in our ONLINE PRODUCT INFORMATION, which you can access through the Detailed Product Information link at the top of this page. The following bulletins found here offer more detailed information about refrigerant control and compressor operation: 4-1301, 4-1311, 4-1312, 17-1238, 17-1243, 22-1182. The Copeland Compressor Service and Operating Seminar (COSS) and Copeland's A/C Training courses can also help walk you through identifying and repairing floodback problems.
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