What is the oil-free air compressor used?

Oil-free compression classification

oil-free air compressor is divided according to whether the compressed air contains oil, which corresponds to oil air compressor or micro-oil air compressor.

Oil-free air compressor according to different forms of compression or host structure is different, several common oil-free air compressor:

1. Dry oil-free screw air compressor

2. Water-lubricated oil-free screw air compressor

3. Oil-free piston air compressor

4. Oil-free scroll air compressor

5. Oil-free rotary tooth air compressor

6. Centrifugal air compressor

among them, the oil-free piston has been phased out, this product has become less and less, no longer discussed.

Oil-free scroll and oil-free rotary teeth and centrifugal air compressors are discussed separately in special chapters.

In this section, I will only talk about dry oil-free screw and water-lubricated oil-free screw air compressor

dry oil-free screw introduction

the dry oil-free screw is mainly a twin-screw compressor. There is no lubrication in the compression cavity. There is only lubricating oil in the gearbox, which can be said to be dry.

There is a gap between the rotors, do not contact each other, and have a synchronous gear structure. The rotors rely on synchronous gears to transmit torque and positioning.

Both the male and female rotors at the intake and exhaust ends have a shaft seal structure to separate the medium gas from the lubricating oil.

The surface of the rotor has a special coating. Because there is no contact, the first compression pressure is not very high. To increase the pressure, use two-stage compression.

Compression effect is best isothermal compression, the actual can not do isothermal compression, so in the first stage after the compression of the intercooler and trap (cooling, drainage), the second stage after the compression also has the rear cooler.

The pressure of the first stage of compression is about root number 2, and then this pressure enters the second stage of compression, and the pressure discharged by the second stage of compression meets the system requirements, so the second stage compression ratio is higher, the working environment is more severe, and the service life is shorter than that of the first stage.

Due to the high speed and high temperature in the head, the head shell is a one-time lost foam technology for cooling. The nose housing is completely separate from the rotor. The shell is generally cooled by oil.

Water-lubricated oil-free introduction

water lubricated oil-free air compressor is generally a single screw air compressor, which is characterized by the use of water for lubrication of the medium is water, the use of water-based as a seal, completely oil-free, pure oil-free compressed air output.

Because the water is sprayed into the compression chamber, the temperature is only about 40-50 degrees, the constant temperature compression, the volume efficiency is higher, and the efficiency is 15% higher than that of the dry oil-free screw air compressor. The price is cheap.

Why are water-lubricated oil-free machines generally single-screw rather than twin-screw?

The reason is that when the single screw is running, the screw and the star wheel have a speed difference, one-way compression, air is easy to compress, and the speed is low.

The twin-screw is co-extrusion, which requires high speed, and the lubrication of water is not enough compared to the oil density, and the compressed air is easy to flow backwards.

Need to pay attention to two points

1. The lubrication part is water, preferably pure water.

Air contains absolutely no oil, but it contains water.

Application of oil-free screw

in the field of high gas quality requirements, such as textile, metallurgy, food, chemical, pharmaceutical, petroleum and air separation and other places that need pure oil-free compressed air, oil-free screw compressor can provide high-quality compressed gas to meet various needs, so the application prospect is broad.

In the food manufacturing and packaging industry, when the oil-injected screw compressor is used to prepare compressed gas, the performance of the lubricating oil in the compressor has been greatly reduced and acidic after many times of high-temperature oxidation and condensation emulsification in the production process. Not only can it not lubricate the subsequent equipment, but it will destroy the normal lubrication. The use of oil-free screw compressor can avoid the negative impact of invalid lubricating oil on the equipment.

In pharmaceutical and biological engineering, the pollution of bacteria and bacteriophage in compressed gas can not be ignored. The pure compressed gas provided by oil-free screw compressor can avoid the breeding of bacteria and bacteriophage in the gas.

In the electroplating industry, there are some problems in the production process, such as the surface of the coating, burning, pinhole, crack and so on. People use stirring to solve such problems, and the stirring needs the support of compressed air.

In the automotive spray industry, impure gases often result in inferior coatings. If the compressed air contains oil, there are some scattered or concentrated small bumps on the surface of the coating. These blisters are generally produced in a coating under the topcoat, and the blisters are caused by water vapor or dirt under the coating. In addition, oily compressed air may also lead to the appearance of dot-like distribution of small potholes on the surface of the wet coating, forming crater-like silica pits, and sometimes visible substrates at the bottom of the pits, commonly known as fish eyes. Nowadays, the automobile spraying industry has begun to apply oil-free screw compressors to prepare pure gas for spraying, so that the spraying level of domestic automobiles has been improved.

In the textile industry, air-jet looms require dry and oil-free compressed air. In the production process, the fine nozzle will blow compressed air onto the wire harness to form a vortex, thereby giving the yarn shape, elasticity and elasticity. The pure compressed air provided by the oil-free screw compressor can ensure the quality of the finished cloth.

Dry oil-free screw surface coating

rotor materials and profile rotor materials In dry screw compressors, because there is no lubrication of lubricating oil and the removal of tiny particles, the gas will corrode the rotor, the inner wall of the casing and the air passage. Therefore, on the basis of traditional oil-injected screw rotor materials, anti-corrosion layers, such as polytetrafluoroethylene, molybdenum disulfide, Teflon, etc., are usually sprayed to ensure that the rotor is not deformed and corroded.

Water-lubricated oil-free screw rotor technology

in the water jet screw compressor, due to the injection of water into the compression cavity, although the cooling, sealing and maintaining the purity of the gas are obtained, the efficiency of the compressor is improved, and the exhaust temperature is also well controlled. However, water will inevitably cause corrosion to the rotor and other components, so it is necessary to carefully select the rotor material.

The choice of rotor material is generally as follows:

1.NSB steel rotor.

2. Polymer polymer ceramic rotor, this material has good wear resistance, and has good reliability and applicability.

3. Engineering plastic rotor, the use of engineering plastic rotor, opened up a new economic way of screw compressor rotor material and processing means, and provides good conditions for the development of water jet screw compressor. Precision injection molding of engineering plastic rotor, stainless, low cost, while reducing the noise and vibration of the screw compressor. It creates good conditions for the use of water spray seal cooling technology in the compression chamber. Screw rotor profile for screw compressor, profile design is very important, need to consider the rotor profile meshing requirements, continuous contact line, processing performance and other factors.

Technical difficulties of oil-free screw

technical difficulties of dry screw compressor

1. Rotor deformation.

The performance of a dry screw compressor depends to a large extent on the sealing gap when the rotor is meshed.

There are many leakage channels between the rotor and between the rotor and the casing, and the leakage of fluid through the gap directly affects the volumetric efficiency and the total efficiency.

In the actual operation process, the rotor undergoes thermal and force elastic deformation under the action of thermal and force boundary conditions, which makes the actual operating gap and the design gap difference. If the gap is too large, the leakage will increase, the volume efficiency will decrease, and if the gap is too small, the accident of burn and death will easily occur.

2. Coating failure

the inferior rotor coating will fall off after a period of operation. The rotor is exposed to the air and bears the influence of impurities and temperature changes in the air. Ultimately, it leads to a reduction in operating performance and even damage to the machine.

Technical difficulties of water jet screw compressor

1. The manufacture of ceramic rotor due to the wear resistance of ceramic materials, not suitable for machine processing, so the use of precision casting, and casting process is one of the difficulties. In general, defects such as pores and cracks of ceramic products need to be detected by X-ray instrument with microscopic focus, and the consistency of density of ceramic blanks needs to be monitored by ultrasonic instruments.

2. Clean up due to the presence of water, it will inevitably cause a certain corrosion effect on the screw compressor host and cooling water system, and will form a certain scale in the pipeline. After the scale is formed in the cooling system, it will reduce the cross-sectional area of the water flow channel, increase the resistance of the water circulation, and hinder the normal heat exchange. With the continuous thickening of the scale layer, it will cause serious poor cooling of the equipment and bring a series of adverse effects, such as increasing the power consumption of the compressor, reducing the intake volume, reducing the exhaust volume, etc. Therefore, the scale treatment method is also very important. Scale treatment methods generally have chemical reagent method, physical method, artificial method, etc.

3. Water-lubricated bearings need to change the material properties and lubrication structure of traditional metal bearing systems, and integrate bearings with dynamic sealing devices and other components, using water as a lubricating medium.

At present, there are relatively few manufacturers that can provide water-lubricated bearings, and SKF in Sweden is actively engaged in research and development in this area.