The service life of lubricating oil mainly depends on the service environment of the compressor. The moisture and dust in the air are the key factors. The filtering accuracy of the oil filter is generally not less than 15 microns, that is, the dust less than 15 microns will circulate with the lubricating oil. When the dust concentration below 15 microns is too high, the oil must be changed. The moisture in compressed air leads to the emulsification of lubricating oil and loss of lubricating effect. Metal corrosion caused by moisture, and a large number of rust less than 15 microns are also accompanied by lubricating oil circulation. Oil change, like dialysis for patients with kidney disease, is an important means to maintain the service life of the compressor. The use of lubricating oil with short replacement cycle is more conducive to the service life of the machine.

1. Necessity of compressor oil change

Compressor oil is affected by heating, oxidation, high-speed shearing, dust pollution and harmful gases during use. Oxygen reacts with hydrocarbons in the lubricating oil to slowly oxidize the oil, darken the color and increase the viscosity. Finally, it may be so serious that the oxide cannot be dissolved in the oil, but deposited somewhere in the system as a brown mucus layer, which is very easy to block the control oil passage in the components, aggravate the friction of ball bearings and valve cores, and affect the normal operation of the system. Oxidation also produces corrosion and acidification. The oxidation process starts slowly. The accumulated sediment and acid value increase and the viscosity increases. When it reaches a certain degree, the oil should be changed. Otherwise, the oxidation will continue, the oxidation speed will gradually accelerate, and the viscosity will rise, resulting in the increase of working oil temperature. With the continuous strengthening of oxidation, the viscosity of compressor oil will increase significantly. When the oxidation rate changes from linear to power curve, the viscosity increases faster, and people may not want to see the results.

2.Compressor oil temperature

Under normal temperature and pressure, the oxidation process of lubricating oil is very slow, so the lubricating oil is not easy to oxidize and deteriorate during storage and transportation. However, when the temperature of lubricating oil or oxygen pressure increases, the oxidation rate of lubricating oil increases rapidly.
 The temperature of circulating lubricating oil of piston compressor and centrifugal compressor has little relationship with the temperature of compressed gas, while that of oil injected screw compressor is different. The temperature of compressor oil changes between the exhaust temperature of compressor and the temperature of lubricating oil after cooling. Most of the time, the temperature of the compressor oil is equal to and close to the exhaust temperature of the main engine. The service life of oil injected screw compressor oil is much more strongly affected by thermal oxidation.
Under normal temperature and pressure, the oxidation process of lubricating oil is very slow, so the lubricating oil is not easy to oxidize and deteriorate during storage and transportation. However, when the temperature of lubricating oil or oxygen pressure increases, the oxidation rate of lubricating oil increases rapidly.
The temperature of circulating lubricating oil of piston compressor and centrifugal compressor has little relationship with the temperature of compressed gas, while that of oil injected screw compressor is different. The temperature of compressor oil changes between the exhaust temperature of compressor and the temperature of lubricating oil after cooling. Most of the time, the temperature of the compressor oil is equal to and close to the exhaust temperature of the main engine. The service life of oil injected screw compressor oil is much more strongly affected by thermal oxidation.

3. Real life of compressor oil

The service life of screw air compressor oil should be the minimum service life under normal working conditions. It is difficult to accurately determine the service life of lubricating oil, and the provisions of various manufacturers are also different. The general provisions are: class I mineral oil is 2000 hours, class II mineral oil can be 4000 (later reduced to 2000) hours, and class III mineral oil can be 6000 (later reduced to 4000) hours, Pao can reach 8000 (6000) hours and polyol ester can reach 12000 (8000) hours.
 The service life of compressor oil is specified by the lubricating oil manufacturer. If the oxidation stability measured by rotating oxygen bomb method is less than 50min, and the oxidation speed begins to accelerate when it is less than 50min (the oil change standard is generally 60min), the service life of compressor oil is considered to be over. The service life of different types of compressor oils is quite different, and their oxidation stability depends on the properties and formula of base oil.
 The service life of compressor oil with good class III oil and some Pao and ester oil can reach 5000 ~ 6000 hours (the exhaust pressure of air compressor is 0.7 ~ 0.8 MPa), the time when its acid value reaches 2.0mgkoh/g ≥ 10000 hours, and the rotating oxygen bomb value can reach 1500 ~ 2000min. Please note that there seems to be some contradiction between the data of "service life up to 5000 ~ 6000 hours" and "≥ 10000 hours", which will be further discussed later.

3.1 type of compressor oil

Now there are more famous compressor oils. Only the base oils of compressor oils include: mineral oil, secondary hydrogenation mineral oil, hydroisomerization class III base oil, natural gas synthesis class III base oil, Pao, polyether, ester oil (including diester and polyol ester), silicone oil, etc.
Compressor oils on sale include ordinary compressor oil (based on mineral oil), synthetic oil and semi synthetic oil. Among them, synthetic oil is relatively chaotic. Synthetic oil is the base oil obtained by chemical synthesis. Shell calls the hydroisomerized class III base oil as xhvi synthetic oil with ultra-high viscosity index. This base oil is obtained by deep refining of mineral oil, and more wonderful is GTL class III base oil synthesized from natural gas.
As for semi synthetic compressor oil, there is some confusion. The emergence of semi synthetic oil is aimed at the fact that simple mineral oil is difficult to meet the working conditions of the compressor. By adding some synthetic oil with special structure to improve its functions in some aspects, it is a technical solution that not only ensures the performance but also reduces the increase of cost as much as possible. The performance of semi synthetic oil is not a simple average of the performance of synthetic oil and mineral oil. Selecting a reasonable semi synthetic oil blending scheme can sometimes get unexpected performance.
The question is, what is the content of mineral oil base oil in semi synthetic oil? What is the type and content of synthetic oil? As class III base oil belongs to unconventional base oil, part of class III base oil is formed by hydroisomerization of mineral oil, and part is GTL class III base oil synthesized from natural gas. In recent years, people have distinguished class III base oil from mineral oil and other synthetic oil, which is called class III oil. This name is appropriate, reasonable and clear in connotation.

3.2 the real life of compressor oil is closely related to the compressor

Due to the results publicized by the lubricating oil manufacturer, users do not know much about the impact of the compressor itself on the real life of compressor oil, especially the oil injection screw air compressor oil. The real situation is that even if the same compressor oil is used on compressors of different types, different parameters, different compression media and different working conditions, the real life is very different. See the following analysis:
 1) If the exhaust temperature of oil injected screw air compressor is reduced by 8 ~ 10 ℃, the service life of compressor oil will be doubled;
 2) The exhaust pressure of single-stage oil injection screw compressor ranges from 0.3 to 1.3 MPa. Due to different exhaust temperature and oxygen partial pressure, the oxidation speed of air compressor oil is very different. The real life of lubricating oil of low-pressure air compressor with exhaust pressure of 0.3 MPa is 3 to 5 times longer than that of air compressor with exhaust pressure of 1.3 MPa;
 3) The lubricating oil of nitrogen compressor is not affected by oxygen, and its real life is much longer than that of the same type of air compressor;
 4) The oleic acid value of similar screw compressors compressing CO2 gas is easy to exceed the standard and the real life is short;
 5) Due to the low oxygen partial pressure and low exhaust temperature at all levels, the two-stage compressed atmospheric oil injection screw air compressor not only has high energy efficiency, but also the real life of lubricating oil is about 2 times longer than that of single-stage air compressor of the same type;
 6) The compressor oil life of single-stage screw air compressor with high compression ratio is short;
 7) Due to the low exhaust temperature during the frequency conversion period, the shear effect of the compressor on the lubricating oil is lighter than that of the ordinary volume regulating air compressor, and the real life of the lubricating oil is longer. The real life of the lubricating oil is different due to the different frequency conversion time and frequency conversion depth;
 8) In order to improve energy efficiency, some air compressor manufacturers and users have increased the fan of air-cooled oil injection screw air compressor and reduced the exhaust temperature of screw compressor, which not only reduces the power consumption, but also prolongs the service life of compressor oil and achieves the effect of killing two birds with one stone;
 9) The real life of centrifugal compressor oil and piston compressor oil is much longer than that of oil injection screw air compressor.

Next:  Great Knowledge of Elang Screw Air Compressor Oil Change (Part Two)