Precautions for Replacing Refrigeration Compressor Refrigerant Oil
2024-09-13
As the "blood" of a refrigeration unit, refrigeration oil mainly plays roles such as lubrication, sealing, cooling, cleaning, and noise reduction. During use, due to the action of high temperature, refrigeration oil will decompose due to heat and generate carbon deposits, leading to wear of refrigeration compressor parts. Oil decomposition products will chemically react with the refrigerant, resulting in reduced refrigeration effect. At the same time, the acidic substances generated will strongly corrode the compressor. Therefore, after the refrigeration oil deteriorates to a certain extent after being used for a certain period of time, it must be replaced.
Generally speaking, the oil change period for open-type units recommended in the industry is 2000 - 3000 hours, and the oil change period for semi-hermetic units is about 6000 hours. The replacement period of compressor refrigeration oil is related to the process control of its production process. If the cleanliness control of the evaporator and condenser of the refrigeration system and the system pipelines is relatively good, then relatively speaking, there are fewer pollutants entering the compressor, and the replacement period can be relatively lengthened.
Generally speaking, the oil change period for open-type units recommended in the industry is 2000 - 3000 hours, and the oil change period for semi-hermetic units is about 6000 hours. The replacement period of compressor refrigeration oil is related to the process control of its production process. If the cleanliness control of the evaporator and condenser of the refrigeration system and the system pipelines is relatively good, then relatively speaking, there are fewer pollutants entering the compressor, and the replacement period can be relatively lengthened.
Main technical indicators of refrigeration oil
Generally, the indicators for judging whether refrigeration oil can continue to be used are: appearance, viscosity, acid value, moisture content, pour point, flash point, dielectric strength, copper strip corrosion, mechanical impurities and insolubles.
Generally, the indicators for judging whether refrigeration oil can continue to be used are: appearance, viscosity, acid value, moisture content, pour point, flash point, dielectric strength, copper strip corrosion, mechanical impurities and insolubles.
Appearance
Whether the quality of refrigeration oil has changed should be determined through certain chemical and physical analyses. During normal use, the quality of the oil can also be intuitively judged from the appearance and color of the oil. High-quality refrigeration oil should be colorless and transparent. After being used for a period of time, it will turn light yellow.
Whether the quality of refrigeration oil has changed should be determined through certain chemical and physical analyses. During normal use, the quality of the oil can also be intuitively judged from the appearance and color of the oil. High-quality refrigeration oil should be colorless and transparent. After being used for a period of time, it will turn light yellow.
Viscosity
The kinematic viscosity value of refrigeration oil is the main indicator reflecting the lubrication performance of the oil, and it is also the basis for many countries in the world to formulate refrigeration oil grades. If the viscosity is too large, not only can the low-temperature performance of the oil not be guaranteed and heat transfer is not conducive, but also the power of the machine will be reduced; if the viscosity is too small, the required oil film cannot be formed at the friction parts, resulting in increased equipment wear and even failures such as "cylinder scoring" and "adhesion".
The kinematic viscosity value of refrigeration oil is the main indicator reflecting the lubrication performance of the oil, and it is also the basis for many countries in the world to formulate refrigeration oil grades. If the viscosity is too large, not only can the low-temperature performance of the oil not be guaranteed and heat transfer is not conducive, but also the power of the machine will be reduced; if the viscosity is too small, the required oil film cannot be formed at the friction parts, resulting in increased equipment wear and even failures such as "cylinder scoring" and "adhesion".
Acid value
Acid value is an important indicator of refrigeration oil. The corresponding test method is GB/T 4945 or GB/T 7304. The acid value of refrigeration oil reflects the refining depth and oxidation degree of the oil to a certain extent.
Acid value is an important indicator of refrigeration oil. The corresponding test method is GB/T 4945 or GB/T 7304. The acid value of refrigeration oil reflects the refining depth and oxidation degree of the oil to a certain extent.
Moisture
In the refrigeration circulation system, even if only a small amount of moisture exists, ice blockage will occur at the low-temperature throttling device and the refrigeration oil will prematurely produce flocculates. At high temperatures, moisture will also reduce the stability of the oil. On the other hand, closed refrigeration oil is in direct contact with the motor coil. The presence of moisture will destroy the insulation and even cause a motor breakdown accident.
In the refrigeration circulation system, even if only a small amount of moisture exists, ice blockage will occur at the low-temperature throttling device and the refrigeration oil will prematurely produce flocculates. At high temperatures, moisture will also reduce the stability of the oil. On the other hand, closed refrigeration oil is in direct contact with the motor coil. The presence of moisture will destroy the insulation and even cause a motor breakdown accident.
Pour point
The lowest temperature at which the oil can just flow is called the pour point of the oil. When the refrigerant is compressed, some oil mist often enters the pipeline. Even after passing through the oil-gas separator, the two cannot be separated completely. This requires that the oil has good low-temperature fluidity (low pour point) in the refrigeration system to avoid clogging the throttling part and causing the water chiller to stop working or adhering to the evaporator heat exchange tube to reduce the heat transfer efficiency.
The lowest temperature at which the oil can just flow is called the pour point of the oil. When the refrigerant is compressed, some oil mist often enters the pipeline. Even after passing through the oil-gas separator, the two cannot be separated completely. This requires that the oil has good low-temperature fluidity (low pour point) in the refrigeration system to avoid clogging the throttling part and causing the water chiller to stop working or adhering to the evaporator heat exchange tube to reduce the heat transfer efficiency.
Flash point
The flash point of refrigeration oil refers to the lowest temperature at which flashover occurs when the oil vapor generated during heating is mixed with air and comes into contact with a flame. If the flash point of refrigeration oil is reduced, it indicates that light oil is mixed in the oil or some of the oil has decomposed to form low-boiling-point decomposition products. Generally speaking, the flash point of refrigeration oil is 15 - 30 °C higher than the outlet temperature of the compressor.
The flash point of refrigeration oil refers to the lowest temperature at which flashover occurs when the oil vapor generated during heating is mixed with air and comes into contact with a flame. If the flash point of refrigeration oil is reduced, it indicates that light oil is mixed in the oil or some of the oil has decomposed to form low-boiling-point decomposition products. Generally speaking, the flash point of refrigeration oil is 15 - 30 °C higher than the outlet temperature of the compressor.
Dielectric strength
Dielectric strength, also known as breakdown voltage or dielectric strength, refers to placing refrigeration oil in a container equipped with an electrode and applying voltage. When the voltage gradually increases to a certain value, the resistance of the oil suddenly drops to zero. A strong current passes through the oil in the form of sparks or arcs. The critical voltage value at this time is called dielectric strength. The main factors affecting the breakdown voltage of refrigeration oil are moisture, impurities, temperature, etc.
Dielectric strength, also known as breakdown voltage or dielectric strength, refers to placing refrigeration oil in a container equipped with an electrode and applying voltage. When the voltage gradually increases to a certain value, the resistance of the oil suddenly drops to zero. A strong current passes through the oil in the form of sparks or arcs. The critical voltage value at this time is called dielectric strength. The main factors affecting the breakdown voltage of refrigeration oil are moisture, impurities, temperature, etc.
Copper strip corrosion
The copper strip corrosion test is currently the most important corrosion determination method for industrial lubricating oils. Under specified conditions, it is a test for the corrosion tendency of the oil on the metal surface in contact. The corrosion resistance of the oil can reflect the refining depth and deterioration degree of the oil to a certain extent.
The copper strip corrosion test is currently the most important corrosion determination method for industrial lubricating oils. Under specified conditions, it is a test for the corrosion tendency of the oil on the metal surface in contact. The corrosion resistance of the oil can reflect the refining depth and deterioration degree of the oil to a certain extent.
Mechanical impurities and insolubles
Mechanical impurities are one of the routine monitoring items for compressor lubrication management of water chillers. Because mechanical impurities in the oil will accelerate abnormal wear of the compressor. At the same time, it will also block the oil circuit and filter, resulting in lubrication failure of the compressor. Mechanical impurities are one of the indicators for judging whether refrigeration oil needs to be changed. Generally, mechanical impurities below 0.005% are considered to be free of mechanical impurities. If it exceeds 0.005%, it contains mechanical impurities and the refrigeration oil is judged to be unqualified.
Mechanical impurities are one of the routine monitoring items for compressor lubrication management of water chillers. Because mechanical impurities in the oil will accelerate abnormal wear of the compressor. At the same time, it will also block the oil circuit and filter, resulting in lubrication failure of the compressor. Mechanical impurities are one of the indicators for judging whether refrigeration oil needs to be changed. Generally, mechanical impurities below 0.005% are considered to be free of mechanical impurities. If it exceeds 0.005%, it contains mechanical impurities and the refrigeration oil is judged to be unqualified.
Steps for changing refrigeration oil
Different compressors have different methods for changing refrigeration oil. This article takes a screw compressor as an example.
Different compressors have different methods for changing refrigeration oil. This article takes a screw compressor as an example.
- Recover the system refrigerant to the condenser side, discharge the refrigerant in the compressor, and discharge the refrigeration oil from the oil drain angle valve of the compressor.
- For the lubricating oil of the compressor, drain it from the oil drain valve on the oil filter. When the lubricating oil is almost drained, add a certain amount of nitrogen from the high-pressure side detection joint of the compressor to accelerate the discharge of the lubricating oil with pressure. Stop injecting pressure and close the oil drain valve. Then disassemble the oil filter of the compressor, drain all the lubricating oil in the oil sump, and then clean the oil filter and install it tightly.
- Add lubricating oil according to the standard amount of the compressor, and evacuate the system at the same time. It is recommended to select the high (low) pressure side angle valve or other relatively high points as the connection point of the vacuum pump to prevent the vacuum pump from being damaged by pumping out refrigeration oil. When performing vacuum holding pressure or leak detection, observe whether the pressure rebounds.
- Inject lubricating oil and connect a clean plastic tube to the oil drain valve. The other end of the plastic tube is placed in the oil barrel (the plastic tube is placed at the bottom of the oil barrel). When injecting oil, the oil barrel should be closed to reduce the pollution of air to the oil.
Precautions for changing refrigeration oil in compressors:
- Different grades of refrigeration oil cannot be mixed, especially mineral oil and synthetic ester oil cannot be mixed;
- If different grades of refrigeration oil are replaced, pay attention to removing the remaining original refrigeration oil in the system;
- Some oils have the characteristic of moisture absorption, so do not expose the refrigeration oil to the air for a long time. Shorten the exposure time as much as possible during installation and perform vacuum pumping operations well;
- If the compressor motor in the system has burned out, pay special attention to removing the remaining acidic substances in the system when replacing with a new machine. After seventy-two hours of commissioning and operation, check the acidity of the refrigeration oil. It is recommended to replace the refrigeration oil and the drying filter to reduce the possibility of acid corrosion. After that, detect or replace the refrigeration oil again after about one month of operation;
- If there has been a water inlet accident in the system, pay special attention to removing the moisture completely. In addition to replacing the refrigeration oil, pay special attention to detecting the acidity of the oil product and replace the new oil and drying filter in time.
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