Current situation and application of compressor rotor materials

Current situation and application of compressor rotor materials

With the development of science and technology, the rotor materials of screw compressors and centrifugal compressors have made great progress and changes nowadays. The author has done some research experiments on this, and this article briefly talks about it.　

1 Centrifugal compressor rotor material

Centrifugal compressor rotors are mostly made of stainless steel, but the centrifugal air compressors on hydrogen battery cars use aluminum alloys to make rotors, while car superchargers close to this generally use stainless steel to make rotors.

Because the centrifugal air compressor on the hydrogen battery car requires smaller volume and higher speed requirements, people think it is more appropriate to use high-strength aluminum alloy to make the rotor.

For this kind of centrifugal compressor, it is very important to reduce rotor vibration. The maximum speed of this kind of compressor is as high as 144,000 rpm or 160,000 rpm. A rotor made of aluminum alloy, due to the centrifugal force produced by the unbalanced mass, will be reduced by 2 times compared with the centrifugal force produced by a rotor made of stainless steel of the same size. That is, the centrifugal force produced by the rotor made of aluminum alloy due to the same unbalanced mass is only The centrifugal force of a stainless steel rotor of the same size is one third, and the resulting vibration will be greatly reduced. This is the rare advantage of aluminum alloy rotors over rotors of other materials.

For small centrifugal air compressors, this aluminum alloy rotor has the advantages of good elasticity, light weight, easy processing and low cost

2 Twin-screw compressor rotor material　

Rotor material of oil-injected twin screw compressor

In the early development of twin-screw compressors, foreigners mainly used free-cutting steel to make oil-injected twin-screw compressor rotors. When the Chinese bought a large number of special milling machines for screw rotors in the United Kingdom, they felt that China's ductile iron was superior to free-cutting steel rotors in both cutting performance and processing capacity, and the cost was lower. Therefore, domestic oil-injected screw rotors became more common. Made of ductile iron. As my country is rich in rare earth elements and the technology of rare earth ductile iron is mature, ductile iron rotors have become screw compressor rotors with Chinese characteristics.

　The rotor of the oil-injected twin-screw compressor works under the mixed state of the oil and gas two phases. During work, due to the combined action of pneumatic and hydraulic forces, while the oil and gas are evenly sprayed to the volume of each elementary element of the compressor, they are also sprayed on the surface of the rotor and the body to form a protective layer. The tiny particles on the surface of the rotor are washed clean. Although the surface of ductile iron is not as fine as the surface of steel rotors, it is still capable of performing the functions of the rotor.

The reliability of the screw compressor rotor is guaranteed by the accuracy of the bearings. It seems that there is almost no big difference between a steel rotor and a ductile iron rotor. For the rotors of large screw compressors, people still tend to use steel rotors and feel that the reliability of steel rotors is better.

Dry screw compressor rotor material

Because people need more reliable and cleaner compressed gas, the oil-injected screw compressor and compressed gas precision filtration still cannot completely replace the dry compressor. Later, due to the difference in reliability and maintenance period, people never gave up to partially restored various types of dry compressors.　

The rotor of the dry screw compressor is different from the rotor of the oil-injected compressor. Because of the high price, high speed, and complex compression medium of the dry screw compressor, stainless steel is generally used to make the dry compressor rotor.

In order to enhance the abrasion and corrosion resistance of the rotor, different dry compressor rotor surfaces have different anti-corrosion coatings or coatings. In order to increase its abrasion resistance and corrosion resistance. With the continuous advancement of anti-corrosion coating technology, the performance of anti-corrosion coating has been further strengthened, and different compressor manufacturers have adopted different coatings or coatings.　

There are two types of coatings: chemical coating and sintered coating, and vacuum titanium plating is a more advanced coating process.

　 Although the coating is very thin, it still needs polishing. Different manufacturers use different polishing processes, most of which use manual-assisted mechanical polishing, machining center polishing, chemical and physical polishing performance is better, and electrostatic polishing is also used, and large quantities of parts use advanced heat treatment polishing technology.

Water-lubricated twin-screw compressor rotor material　

　The rotor material of the currently introduced water-lubricated twin-screw compressors is mostly stainless steel, and no water-lubricated twin-screw compressor rotors of other materials have been found. Of course, the body is also made of stainless steel.

3 Single screw compressor rotor material

Different single screw compressors have different rotor materials.

Rotor material of oil-injected single screw compressor

Domestic oil-injected single-screw compressor rotors are generally made of free-cutting steel and ductile iron, and the star wheels are made of carbon fiber reinforced polyether ether ketone.　

　 Rotor material of water-lubricated single screw compressor

In recent years, water-lubricated single screw compressors have been widely used in pharmaceutical, food, beverage, textile and other industries. For different considerations, different manufacturers use stainless steel and copper alloys to make rotors. Due to the high price of water-lubricated single-screw compressors, the rotor of which material the manufacturer uses will achieve good economic benefits. Of course, the rotors of different materials are adapted to the star wheel of a certain formula, which is mainly reinforced polyetheretherketone. The material of the star wheel of the water-lubricated single screw compressor is different from that of the oil-injected single screw compressor.

The body and rotor material of the water-lubricated single-screw compressor are matched. The hardness of the rotor material is higher, and the casting fluidity of the body material is better.

　4 Screw compressor rotor and body temperature analysis

The correct analysis of the temperature changes of the screw compressor rotor and the body will help to select the rotor material more appropriately.

Rotor working temperature and body working temperature

　In the normal working cycle of the screw compressor rotor, the temperature of each point on the rotor surface is constantly changing between the suction temperature and the discharge temperature, and the average temperature is basically in line with the intermediate value of the suction temperature and the discharge temperature.

The body temperature of the screw compressor varies with the type of machine, but it basically conforms to the law of low suction end temperature (close to the suction temperature) and high discharge end temperature (close to the discharge temperature).

In fact, in normal operation, the average temperature of the rotor is lower than the body temperature.

Although the body is constrained by the associated pipes and flanges, the corresponding thermal expansion of the rotor is smaller than that of the body. This law is different from what people feel. This law determines that it is safe to match the screw compressor rotor with the body.

Temperature change of screw compressor rotor and body in different working stages

　The rotor temperature and body temperature of the screw compressor will change greatly when it starts, works normally and starts to stop.

The temperature of screw compressor during starting

When the screw compressor is started, the gas or gas-liquid in the unit volume begins to be compressed, and the temperature rises sharply. The sharp rise in the temperature of the gas or gas-liquid and compression is a molecular state change. The temperature rises instantaneously or instantly, without delay. As the heated rotor and the body are solid, they have different thermal resistances and the temperature rises in different ways. The rotor is fully heated, and the temperature will rise before the body. Due to the structure of the body, the thermal resistance is large, and the temperature rises slower than the rotor, but the average temperature of the rotor still does not exceed the body temperature. After this state lasts for a few tenths or several seconds, the compressor enters a normal working state.

The normal working temperature of screw compressor conforms to the law of "rotor working temperature and body working temperature"

This is a relatively stable temperature state. In this state, the screw compressor is not prone to failure unless the compressor sucks in excess solid particles or the bearings are suddenly damaged.

The temperature of the screw compressor at the beginning of the shutdown

When the screw compressor starts to stop, the high-pressure and high-temperature airflow at the discharge end will immediately flow back to the suction end. The surface temperature of the suction end of the rotor immediately rises below and close to the discharge temperature, while the body temperature does not change much. It is close to the average temperature, forming a very small gap between the rotor and the body at the suction end.

If the air inlet end of the rotor of the single-screw compressor is not repaired, the engine may be locked during shutdown.

5 High-strength aluminum alloy is suitable for making rotors

The analysis of the temperature of the screw compressor rotor and the body is conducive to a more comprehensive understanding of the screw compressor rotor material. As traditional rotor materials, free-cutting steel, ductile iron, stainless steel, copper alloy, etc. can be replaced by some other materials.

The material strength of the high-strength aluminum alloy oil and gas cylinder can be no less than that of high-quality carbon steel. The processing surface of the parts is smooth, and the threaded holes are bright and smooth, which is much better than ductile iron.

The above analysis is not difficult to see that high-strength aluminum alloy has some advantages. It is no wonder that high-end automobiles have evolved into all-aluminum engines, so it is easy to understand the possibility of using aluminum alloy rotors in screw compressors.

Because the force of the rotor is different from that of the body, the temperature of each point is different, so the aluminum alloy material of the compressor rotor should be correctly matched with the aluminum alloy material of the body. The aluminum alloy grade (composition) and heat treatment are also different; the aluminum alloy hardness of the rotor Should be higher, the overall strength is also higher.

6 Experiments and development done by the author

The author has developed aluminum alloy oil and gas cylinders on twin screw air compressors. Before 2002, a German company produced a 2m3/min screw air compressor for railway vehicle brakes. The cylinders were made of aluminum alloy. However, a domestic company produces a 2.4-3m3/min railroad vehicle brake screw air compressor that lacks aluminum alloy oil and gas cylinder materials and technology, and the oil and gas cylinders are steel welded parts. The steel welded parts are heavy and easy to rust, so they are not suitable for railway vehicles, so they can barely be used when they are irreplaceable. After the company discovered this phenomenon, the author suggested that the leadership approve the development of aluminum alloy cylinders to replace steel cylinders. After approval by the leader, the author selected a special high-strength aluminum alloy as the material for the oil and gas cylinder. From the design point of view, in order to fully reflect the advantages of aluminum alloy oil and gas cylinders, the author and other engineers used finite element methods to optimize the internal structure of the oil and gas cylinders to reduce stress concentration. After several iterations, the drawing design was completed and the sample was approved for trial production.

On the basis of sample trial production, the aluminum alloy oil and gas cylinder samples were subjected to pressure test. The author achieved that the oil and gas cylinders were intact and leak-free under the 4 times working pressure hydraulic test (pressure vessels generally require 2 times working pressure water Pressure test).

At that time, Mr. Wang Shizhang, the leader of the electric locomotive introduction expert group of the Ministry of Railways (the old man participated in the first introduction of 4 Alstom electric locomotives in my country, which were used for the round trip between Baoji and Fengzhou section of Baocheng Railway to replace two or three steam Locomotive) After seeing the pressure test, they were more pleased. After their research, they thought that it could be installed on the car. The loading test came to an end, and after confirmation, the pilot test was approved. On the basis of the pilot test, transfer to the formal package.

　On this basis, the author has developed special aluminum alloy oil and gas cylinder screw air compressors for the company for EMUs, light rails, and subways. The company has been using the products developed by the author until now.

　7 Screw compressor rotor running-in agent

　In order to make the screw compressor rotor more reliable, the new machine is a success, the author recommends the mechanical fast running-in agent from Dow Corning. When the screw compressor is assembled, a small amount of this running-in agent is applied to the friction-prone parts of the rotor and the body, which can prevent accidental strains at the beginning of the factory test, which is very reasonable and cost-effective. The anti-wear repair agent developed by the author has a unique effect in this respect.