Introduction and selection of refrigeration oil

First, the role of refrigeration oil (: / Yang)

Refrigeration oil is commonly referred to as lubricating oil in refrigeration engineering and plays an important role in the operation of refrigeration compressors. There are mainly the following aspects:

1. Lubrication Reduces friction and wear of moving parts of the machine and prolongs service life.

2. Reducing the temperature The refrigerating machine oil continuously circulates in the refrigerating compressor, which can take away a lot of heat generated during the working process of the refrigeration compressor, keep the machine at a lower temperature, thereby improving the efficiency and reliability of the refrigeration compressor.

3. Sealing function The refrigerating machine oil seals between the shaft seal and the cylinder and the piston to prevent refrigerant leakage.

4. Providing power for the unloading mechanism In the refrigeration compressor with the unloading device, the hydraulic pressure of the refrigerating machine oil is used as the power of the unloading mechanism.

Second, the performance index and selection of lubricants (a) performance indicators of lubricants

1. Viscosity Viscosity is a major performance indicator of lubricating oil. Different refrigerants have different requirements on viscosity, such as R12.

If the lubricant can dissolve with each other, the viscosity of the lubricant will decrease, so a lubricant with a higher viscosity should be used. The viscosity of the lubricating oil in the compressor is too large and too small to be good. Excessive viscosity causes the compressor friction power and friction heat generation to increase, the starting torque increases, and the machine efficiency decreases. If the viscosity is too small, the wear of the bearing or the like is accelerated because the required oil film cannot be established. Therefore the viscosity must be moderate.

The viscosity of the lubricating oil varies greatly with temperature (for example, when the temperature is raised from 50 ° C to 100 ° C, the viscosity of the mineral oil is reduced to 1/3 - 1 / 6 of the original value). Therefore, a lubricant with a small temperature influence on viscosity should be used.

2. Cloud point The cloud point of the lubricating oil is the temperature at which the wax starts to precipitate in the lubricating oil when the temperature is lowered to a certain value (that is, the lubricating oil becomes cloudy). The lubricating oil used in the refrigeration compressor should have a cloud point lower than the evaporation temperature of the refrigerant. Especially in the fluorine system, a part of the lubricating oil is dissolved in the refrigerant and flows to the refrigeration system with the refrigerant. If paraffin is precipitated in the oil, it will accumulate in the throttle valve to cause blockage, or accumulate in the evaporator. Hot surface, weakening heat transfer.

3. Freezing point The lubricating oil is cooled to the temperature at which it stops flowing under the test conditions, called the freezing point. For refrigeration

For the lubricant of the compressor, the freezing point should be as low as possible. Generally the freezing point should be below -40 °C. When the lubricating oil and the refrigerant dissolve each other, the freezing point will decrease.

4. Flash point Lubricating oil (in the open oil sump) is heated to the point where the steam is in contact with the flame.

The temperature is called the flash point. The lubricating oil used in the refrigeration compressor should have a flash point 25-35 ° C higher than the exhaust steam temperature to avoid burning and coking of the lubricating oil. Generally, the lubricating oil for ammonia, R12 and R22 should have a flash point of 160-170 ° C or higher.

5. Learning stability and oxidation resistance Lubricating oil should have good chemical stability and oxidation resistance. Otherwise, under the catalysis of high temperature or metal, it will react with refrigerant and other harmful substances such as coke and acid.

6. Water content and mechanical impurities The lubricating oil should not contain water, because the water will not only lower the evaporation pressure, but also increase the evaporation temperature, and it will aggravate the chemical changes of the oil and the role of corrosive metals. Moisture also causes "plating of steel" in the Freon compressor, causing copper parts to react with Freon to decompose copper and accumulate on the steel surface of bearings, valves and other parts. As a result, the thickness of these watches is increased, the gap of the bearing is broken, and the machine is malfunctioning. This phenomenon occurs in both closed and semi-hermetic compressors.

Generally, the new oil does not contain moisture and mechanical impurities, because the lubricating oil used in the refrigerator has undergone strict dehydration treatment in the production process. However, dehydrated lubricating oil has strong hygroscopicity, so it should be avoided to contact with air during storage and refueling.

The substance remaining after being dissolved and diluted with gasoline or benzene and filtered with filter paper is called a mechanical impurity of the lubricating oil. The mechanical impurities in the lubricating oil will accelerate the wear of the parts and reduce the insulation performance of the oil and block the lubricating oil passage, so the impurities are also as small as possible, generally not exceeding 0.01%.

7. Breakdown voltage Breakdown voltage is an indicator of the insulation performance of lubricating oil. Pure lubricating oil has good insulation performance, but when it contains impurities such as moisture, fiber, dust, etc., the insulation performance will be lowered.

Semi-closed and fully enclosed compressors generally require a breakdown voltage of 25kV or more. Because the lubricant is in direct contact with the motor windings.

(II) Specifications and selection of domestic refrigeration oils The current specifications of refrigeration oils in China are produced in accordance with the standards of ZBE34003-86 promulgated by Petrochemical Corporation.

The product of this standard is divided into five viscosity grades of N15, N22, N32, N46 and N68 according to the central value of kinematic viscosity at 40 °C, which can be used for the refrigerator with ammonia as the refrigerant. The main performance indicators are shown in Table 2-1. However, the previously issued refrigerating machine oil specifications are divided into four grades of 13, 18, 25 and 30 according to the kinematic viscosity value at 50 °C. When selecting, please refer to the new and old viscosity grade comparison table of refrigeration oil.

In practice, the general R12 compressor is N32 (No. 18), the R22 compressor is N46 (No. 25), and the ammonia compressor is N22 (No. 13) or N46 (No. 25).

Table 2-1 Specifications and main performance indicators of domestic refrigeration oil

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