In the refrigeration unit, the refrigeration oil mainly plays the role of lubrication, sealing, cooling, cleaning and noise reduction. During the use of the refrigeration oil, due to the high temperature, it will be decomposed by heat to produce carbon deposits, causing wear of the compressor parts. The oil decomposition products will react chemically with the refrigerant to reduce the refrigeration effect, and the acidic substances produced will cause strong corrosion to the compressor. Therefore, the refrigeration oil must be replaced after it has been used for a certain period of time and has deteriorated to a certain extent.
Generally speaking, the industry recommends that the oil change cycle for open units is 2000-3000h, and the oil change cycle for semi-enclosed units is about 6000h.
For chiller manufacturers, the replacement cycle of compressor refrigeration oil is related to the process control of their production process. If the cleanliness control of the evaporator and condenser of the refrigeration system and the system pipelines is better, then relatively speaking, there will be fewer pollutants entering the compressor, and the replacement cycle can be relatively longer.
It is recommended that the lubricating oil be replaced and the oil filter be cleaned after the chiller is turned on for the first time and the cumulative operation time is 2500 hours (or 3 months). Because the residues of the system assembly will accumulate in the compressor after the formal operation. After that, it should be replaced every 8,000 hours (or 2 years). In fact, due to the influence of the unit environment, operating conditions, aging of sealing gaskets, etc., the refrigeration oil will be mixed with foreign matter or moisture, and the high temperature effect will cause the refrigeration oil to be contaminated or deteriorated.
1. Refrigeration oil change index
Generally, the indicators for judging whether the refrigeration oil can continue to be used are: appearance, viscosity, acid value, moisture, pour point, flash point, compressive strength, copper sheet corrosion, mechanical impurities and insoluble matter.
Appearance
Whether the quality of the refrigeration oil changes or not should be determined through certain chemical and physical analysis. In normal use, the quality of the oil can also be intuitively judged from the appearance and color of the oil.
When the refrigeration oil contains impurities or moisture, its transparency decreases, and when the refrigeration oil deteriorates. Its color will become darker, so you can drop a drop of refrigeration oil on a clean white filter paper. If the oil stain is light and uniform, the quality of the refrigeration oil is acceptable; if the oil stain is distributed in a group of concentric circles, the refrigeration oil contains impurities; if the oil stain is distributed in brown spots, the refrigeration oil has deteriorated and cannot be used. High-quality refrigeration oil should be colorless and transparent. After a period of use, it will turn light yellow. As the use time increases, the color of the oil will gradually darken and the transparency will deteriorate. If the refrigeration oil turns orange or reddish brown, it should be replaced.
Viscosity
The resistance to fluid flow under gravity is called viscosity. In lubricating oil products, its viscosity characteristics are often expressed by kinematic viscosity values, and the corresponding test method is GB/T265. The kinematic viscosity value of refrigeration oil is the main indicator reflecting the lubrication performance of oil products, and it is also the basis for many countries in the world to formulate refrigeration oil grades.
Different types of refrigeration compressors require refrigeration oils of different viscosities. Therefore, choosing refrigeration oil with appropriate viscosity is an important factor in ensuring the normal lubrication of refrigeration compressors, reducing mechanical wear and reducing power consumption. Too high viscosity will not only fail to guarantee the low-temperature performance of the oil, which is not conducive to heat transfer, but also reduce the power of the machine; too low viscosity will not form the proper oil film at the friction part, resulting in increased equipment wear and even "cylinder pulling" and "sticking" and other faults.
Acid value
The number of milligrams of potassium hydroxide required to neutralize all free acids in 1 gram of oil is the acid value. Acid value is an important indicator of refrigeration oil, and its corresponding test method is GB/T4945 or GB/T7304. The size of the acid value in refrigeration oil reflects the refining depth and oxidation degree of the oil to a certain extent.
Moisture
In the refrigeration cycle system, even if there is only a small amount of water, ice plugging will occur at the low-temperature throttling device and cause the refrigeration oil to produce floccules prematurely. At high temperatures, moisture will also reduce the stability of the oil. On the other hand, the closed refrigeration oil is in direct contact with the motor coil, and the presence of water will damage the insulation and even cause motor breakdown accidents.
Pour point
The lowest temperature at which the oil can just flow is called the pour point of the oil. The corresponding test method is GB/T3535. During the operation of the chiller, 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 cleanly. This requires the oil to have good low-temperature fluidity (low pour point) in the refrigeration system to avoid blocking the throttling part and causing the chiller to stop working or attaching 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 the oil vapor generated during heating mixes with the air and flashes when it contacts the flame. The corresponding test method is GB/T3536. This item is an indicator of the evaporation tendency and safety of refrigeration oil, and can also indicate the quality of refrigeration oil. If the flash point of refrigeration oil is reduced, it means that the oil is mixed with light oil or some of the oil has decomposed to form low-boiling decomposition products. Generally speaking, the flash point of refrigeration oil is 15-30℃ higher than the compressor discharge temperature.
Compressive strength
Compressive strength is also called breakdown voltage or dielectric strength. It refers to the voltage applied to the refrigeration oil in a container with an electrode. When the voltage gradually increases to a certain value, the resistance of the oil suddenly drops to zero, and 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. The corresponding test method is GB/T507. The dielectric strength of a fully enclosed compressor is not less than 25KV.
Copper sheet corrosion
The copper sheet corrosion test is currently the most important corrosion determination method for industrial lubricants. It is a test of the tendency of oil products to corrode metal surfaces in contact under specified conditions. The commonly used corrosion test method for refrigeration oil is GB/T5096. When the refining depth of the oil is not enough, a small amount of sulfide and trace organic acid in the oil will corrode the metal, and these corrosive substances will also accelerate the deterioration of the lubricating oil and produce harmful substances such as sludge and carbon deposits. Therefore, the corrosion resistance of oil products can reflect the refining depth and deterioration degree of oil products to a certain extent.
Mechanical impurities and insolubles
All solid impurities floating and precipitating in lubricating oil are collectively referred to as mechanical impurities. Mechanical impurities are one of the routine monitoring items for the lubrication management of chiller compressors. Because mechanical impurities in oil products will accelerate the abnormal wear of the compressor, and will also block the oil circuit and filter, causing lubrication failure of the compressor. Mechanical impurities are one of the indicators to determine whether the refrigeration oil needs to be changed. The determination method is carried out according to GB/T511. Usually, 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.
2. Refrigeration oil replacement method and steps
Different compressors have different methods for replacing refrigeration oil. This article takes screw compressors as an example.
1. 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.
2. The lubricating oil of the compressor is released from the drain valve on the oil filter. When the lubricating oil is almost exhausted, add a certain amount of nitrogen from the high-pressure end detection joint of the compressor to accelerate the discharge of the lubricating oil with pressure, stop injecting pressure, and close the drain valve. Then remove the oil filter of the compressor, release all the lubricating oil in the oil tank, and then clean the oil filter and install it to tighten it.
3. 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 for the contact point of the vacuum pump to prevent the vacuum pump from being damaged by the refrigeration oil. When performing vacuum pressure maintenance or leak detection, observe whether the pressure rebounds.
4. Inject lubricating oil, connect it to the drain valve with a clean plastic tube, and put the other end of the plastic tube into the oil barrel (the plastic tube is placed at the bottom of the oil barrel). The oil barrel should be closed when filling oil to reduce air pollution to the oil.
3. Precautions for replacing refrigeration oil in compressors
Refrigeration oils of different brands cannot be mixed, especially mineral oil and synthetic ester oils cannot be mixed; if different brands of refrigeration oil are replaced, be careful to remove the original refrigeration oil remaining in the system. Some oils have the property of hygroscopicity, so do not expose the refrigeration oil to the air for a long time. During installation, shorten the exposure time as much as possible and do a good job of vacuuming;
If the system has a compressor motor burnout failure, pay special attention to removing the remaining acidic substances in the system when replacing the new machine, and check the acidity of the refrigeration oil after 72 hours of commissioning. It is recommended to replace the refrigeration oil and dry filter to reduce the possibility of acid corrosion. After that, check or replace the refrigeration oil again after about one month of operation; if the system has had an accident of water ingress, pay special attention to removing the water. In addition to replacing the refrigeration oil, pay special attention to detecting the acidity of the oil and replace the new oil and dry filter in time.
