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1. Overview of the development of centrifugal compressors Centrifugal compressors are a type of turbine compressor. They have the characteristics of large gas processing capacity, small volume, simple structure, smooth operation, easy maintenance, and no gas pollution. With the achievements in gas dynamics research, the efficiency of centrifugal compressors has been continuously improved, and due to the successful development of key technologies such as high-pressure sealing, processing of small-flow narrow impellers, and multi-oil wedge bearings, a series of problems in the development of centrifugal compressors towards high pressure and wide flow ranges have been solved. The application range of centrifugal compressors has been greatly expanded, so that it can replace reciprocating piston compressors in many occasions. 2. Working principle and basic structure of centrifugal compressor 1. Working principle Generally speaking, the main goal of increasing gas pressure is to increase the number of gas molecules per unit volume, that is, to shorten the distance between gas molecules. In order to achieve this goal, in addition to the volumetric compression method that uses extrusion elements to squeeze the gas, there is also a gas dynamics method, which uses the machine's power element (high-speed rotating impeller) to do work on the gas, so that the pressure of the gas in the centrifugal force field is increased, and the kinetic energy is also greatly increased. Later, when flowing in the expansion channel, this part of the function is converted into static pressure energy, which further increases the gas pressure. This is the working principle of the centrifugal compressor or the principle of supercharging. 2. Basic structure The functions of each component (or flow component) in the compressor flow path are described below. suction chamber: The first-stage inlet of each section of the compressor is equipped with a suction chamber, whose function is to evenly introduce the gas from the inlet pipe to the inlet of the impeller to reduce the flow loss when the gas enters. impeller: The impeller is the most important component in a centrifugal compressor. The driving machine uses the high-speed rotating impeller blades to perform work on the gas to obtain energy from the gas. It is the only actuating component in the compressor, so it is also called the working wheel. The impeller is generally a closed impeller composed of a wheel cover, a disc and blades, or a semi-open impeller without a wheel cover. diffuser: When the gas flows out of the impeller, it has a very high speed. In order to convert this part of the speed into pressure energy as much as possible, a flow space with a gradually expanding flow section is set up around the outer edge of the impeller. This is a diffuser. The diffuser is an annular channel composed of front and rear baffles. Among them, those without blades are called vaneless diffusers, and those with blades are called vaned diffusers. curve: In order to guide the gas flowing out of the diffuser to the next stage for re-compression, an annular channel, called a bend, is set up around the diffuser to change the gas from the centrifugal direction to the centripetal direction. The curve is an annular space composed of a partition and the inner wall of the cylinder. refluxer: In order to make the air flow evenly enter the impeller inlet of the next stage in a certain direction (usually axial direction), a recirculator is set at the outlet of the curve, so that the gas can flow evenly and centripetally by relying on the guiding effect of the blades in the recirculator, and then flow into the impeller of the next stage. The recirculator is composed of two baffles and blades installed between the baffles. volute: The function of the volute is to collect the airflow from the diffuser (or impeller) in an orderly manner and lead it out of the compressor. In some cases, because the airflow speed in the volute has decreased, the volute can also play a certain role in expanding pressure. The intermediate stages of the compressor are generally composed of impellers, diffusers, bends and returners. ； The first stage also has a suction chamber, the last stage has a volute, but there is no return flow device, and the last stage may not have a diffuser. In addition to the above-mentioned flow elements that directly compress the gas, the centrifugal compressor also has the following important components:: 1. Seals: In order to reduce the leakage of gas in the machine from high pressure to low pressure, wheel cover seals are installed at the outer diameter of the inlet rings of impellers at all levels, and there are interstage seals between stages. This seal is usually a non-contact labyrinth seal (or comb seal). In addition, there is also a seal where the rotating shaft extends outside the machine, which is called a shaft end seal, or shaft seal for short. The type of shaft seal is also commonly used for low-pressure seals: side palace seals, and for high-pressure seals, floating ring oil film seals or mechanical seals are mostly used. 2. Balance plate: In order to reduce or balance the unbalanced axial force caused by the gas pressure in the centrifugal compressor rotor, a rotating disk is usually installed on the shaft near the last stage impeller, which is a balance plate. There is also a labyrinth seal between the outer edge of the balance plate and the cylinder housing. Make the inside of the balance plate communicate with high-pressure gas, and the other side with low pressure (or compressor air inlet). The residual axial force on the rotor that is not completely balanced by the balance plate is borne by the thrust bearing. 3. Bearings: Centrifugal compressors are high-speed rotating machines, except for small compressors that use flow bearings. Most of them use special types of dynamic pressure sliding bearings, including support bearings (or radial bearings) and thrust bearings. The support bearing bears the gravity and other radial forces of the compressor rotor, while the thrust bearing mainly bears the unbalanced axial force on the rotor and ensures the axial positioning of the rotor to avoid collision between the rotor and the stator when the machine is running. The centrifugal compressor rotor is a high-speed, light-load rotor. In order to ensure that it can form an ideal oil film in the bearing and prevent the abnormal phenomenon of bearing oil film oscillation, multiple tilting pad bearings or special sliding bearings such as ellipses are generally used. Multi-piece tilting pad bearings use several thousand tiles to slightly swing near their fulcrum to form multiple oil wedges, so that the journal of the high-speed rotating shaft can receive sufficient oil lubrication in a timely manner and ensure stable operation. 4. Coupling: Since centrifugal compressors have the characteristics of high-speed rotation, large functions, and inevitable vibration during operation, the coupling used must be able to transmit large torque and allow a small amount of radial and axial displacement. Therefore, tooth-type couplings are commonly used, relying on tooth-type meshing to transmit torque. This type of coupling requires lubricant. In recent years, a tympanic membrane coupling has been created abroad, which uses a diaphragm to transmit torque, and the diaphragm can also be slightly deformed. This coupling requires no lubricant, is easy to manufacture, and is very popular. In addition to the useful motors for medium and small compressors, the drivers of centrifugal compressors are generally directly driven by steam turbines or gas turbines. This can not only meet the requirements of high power and high speed, but also directly use the by-product of the factory - high-pressure steam or high-temperature gas as power. In addition, the use of this type of driver can also enable the compressor to adjust the speed of the compressor to adjust the compressor flow or pressure. This adjustment method is more economical. Centrifugal compressors also have cooling water systems and demanding lubricating oil systems. Sometimes there is a speed increase box to increase the speed of the entire compressor or a certain rotor. The automatic control system of centrifugal compressors has higher requirements than piston compressors. In addition to conventional operating parameter measurement and display, there are also safety facilities such as surge control system, shaft displacement and vibration indication and alarm, and automatic parking. 3. Performance and adjustment of centrifugal compressors The main parameters that reflect the performance of centrifugal compressors are volume air intake Qj, pressure ratio ε (or exhaust pressure ㄗ, pressure difference △ㄗ, and energy head h), power N and efficiency η. As the air intake changes, other performance parameters will also change accordingly, so the performance change relationship of the compressor is often expressed in the form of curves, such as the pressure ratio curve ε-Qj (or ㄗ-Qj, △ㄗ-Qj and h-Qj curves), the power curve N-Qj and the efficiency curve eta-Qj, etc. There are also general performance curves expressed using dimensionless parameters, such as: ψ—curve (i.e. energy head coefficient-flow coefficient), etc. Figure 4-49 is the performance curve of a centrifugal compressor. It is a graphical representation of the operating conditions of the compressor. It is the basis for selecting the model specifications, operation and performance adjustment of the compressor. Therefore, it is very important to understand the characteristics of these performance curves and understand the various factors that affect the performance of the compressor, so as to flexibly grasp and use it. Qj in the performance curve represents the volumetric flow rate of the compressor in the air intake state, eta is often represented by the variable efficiency etapol¬¬¬, and N generally refers to the shaft power. (1) The performance curve of a centrifugal compressor stage generally has the following characteristics: 1. As the flow rate decreases, the pressure ratio that the compressor can provide will increase. At minimum flow, the pressure ratio reaches its maximum. Conversely, if the back pressure of the compressor decreases, its flow rate will automatically increase. The relationship between the flow rate and the pressure ratio of the centrifugal compressor is one-to-one, and the relationship between the flow rate and other parameters is also a corresponding relationship, which is reflected in each performance curve. 2. The centrifugal compressor has two limit flows: maximum flow and minimum flow. ； Of course, the discharge pressure also has maximum and minimum values. 3. The efficiency curve has a maximum efficiency point, and the efficiency drops rapidly when leaving this point. ； 4. Power N is roughly proportional to Ghrh, so the power curve generally slopes upward as Qj increases. However, when the ε-Qj curve slopes downward quickly, the power curve may first slope upward and then gradually slope downward. (2) Maximum flow condition and surge condition 1. Maximum flow condition As mentioned above, the condition when the flow reaches the maximum is the maximum flow condition. There are two possibilities for causing this situation:: First, the air flow at a throat in the middle stage reaches a critical state. At this time, the volumetric flow rate of the gas is already at its maximum value. No matter how much the back pressure of the compressor decreases, the flow rate cannot increase further. This situation is especially called a "blocking" condition. Another situation is that the flow channel has not reached a critical state, that is, a "blocking" condition has not yet occurred. However, under a large flow rate, the compressor has a large flow loss inside the machine, and the exhaust pressure it can provide is very small, almost close to zero energy head (ε≈1). It is only enough to overcome the flow resistance of the exhaust pipe to maintain such a large flow rate. This is also the maximum flow condition of the compressor. 2. Surge operating condition The operating condition of the centrifugal compressor at the minimum flow rate is called surge operating condition. The cause of surge is first examined from the flow within the stage. The root cause of surge is that the flow rate of the compressor is too small, less than the minimum flow rate of the compressor (or because the back pressure of the compressor is higher than its maximum discharge pressure), resulting in severe gas rotation separation in the machine. The external cause is that the pressure of the pipe network is higher than the discharge pressure that the compressor can provide, causing gas backflow and large air flow pulsation. The frequency and amplitude of pulsation are related to the capacity of the pipe network. The larger the capacity of the pipe network, the lower the frequency of pulsation and the greater the amplitude of pulsation. On the contrary, if the capacity of the pipe network is small, the frequency of pulsation will be high but the amplitude will be small. Surge is extremely harmful, but it cannot be eliminated from the design of the machine so far. It can only be avoided during operation. Anti-surge is to target the causes of surge. When surge is about to occur, immediately try to increase the flow rate of the compressor. There are two specific methods to prevent surge.: ①Partial air flow venting method: When the air intake volume of the compressor is reduced to close to the surge condition, the flow sensor 1 sends a signal to the servo motor 2, causing it to act to control the actuator, that is, to open the anti-surge vent valve 3. As a result, part of the air flow is vented, the compressor back pressure is immediately reduced, the flow rate is automatically increased, and the working condition is far away from the surge condition. Using this method will waste part of the compression work and lose part of the gas. ②The principle of the partial air flow reflux method is the same as the above-mentioned venting method. The only difference is that the gas passing through the anti-surge valve flows back to the machine air inlet pipe for recovery. This method is suitable for handling gases that are toxic, flammable, explosive or have high economic value and are not suitable for venting. This method also wastes part of the compression work. In addition, there are other methods to prevent surge, such as changing the rotation speed of the compressor. Although the above anti-surge measures can avoid the occurrence of surge and protect the machine, the compressor should not be operated with the anti-surge valve open for a long time, which will cause a lot of waste. The production operating system should be checked to find out the external causes that affect compressor surge and solve them. This is the fundamental method to prevent surge. The minimum flow condition and maximum flow condition of the centrifugal compressor have been discussed above. It can be seen that the stable working condition area is between these two extreme conditions. Measuring the performance of the compressor stage requires not only higher pressure and higher efficiency, but also a wider stable operating zone. 4. Vibration and vibration isolation of high-speed rotors. Centrifuges are high-speed rotating machines. Vibrations are inevitable during operation, and sometimes severe vibrations occur. Therefore, vibration is also one of the important problems of centrifuges. The purpose of studying the vibration characteristics of the centrifuge is to reduce the vibration generated by the centrifuge during operation to ensure its normal operation. The cause of centrifuge vibration mainly comes from the imbalance of the rotating part. If the unbalanced mass is large, the vibration will be serious, otherwise the vibration amount will be small. In order to avoid and reduce vibration, the working speed of the centrifuge (that is, the frequency of unbalanced forces and torques) should be kept away from the critical speed of its system during design. ； This is a measure on the one hand, and on the other hand it is to ensure the quality of manufacturing and assembly. If the manufacturing and assembly do not meet the specified technical conditions, such as the balance of the rotor, processing accuracy, matching requirements and uniformity of material quality, it will also cause and intensify the vibration of the centrifuge. In addition, attention should also be paid to ensuring the balance of the machine during use and operation. Uneven distribution, partial leakage, collapse, large foreign matter mixed in, and movement of connecting parts will also cause vibration. Therefore, the vibration problem of a centrifuge must be analyzed on a case-by-case basis. For example, if a centrifuge originally operated with very little vibration and its rotating part was overhauled and disassembled, and the vibration intensified, then you should consider whether the balance of the rotor is affected. If necessary, you need to conduct a new balance test of the rotor. The vibration is not large when idling but becomes larger after feeding. In many cases, new machines are often used well, but the vibration becomes larger and larger after being used for a long time. This requires analysis and research from the wear and corrosion of the rotating parts, material conditions, and whether the connecting parts (including anchor bolts) are loose. For centrifuges with finalized products, the rotation speed must not be changed at will without careful calculation. ； It is also not allowed to arbitrarily repair welding, remove or add parts and masses on the high-speed rotating rotor. From the perspective of manufacturing and assembly, the key issue to avoid vibration is to strive to balance the rotating part to minimize the unbalanced forces and moments that cause vibration. The centrifuge rotor (including drum and shaft, etc.) must be balanced tested and calibrated after the parts are processed and assembled. The balance test includes static balance and dynamic balance. Static balance Static balancing devices include guide rail type, balance type, roller type, etc. Generally, the guide rail type is commonly used. The cross-sections of guide rails are round, rectangular, rhombus and trapezoid. Among them, circular cross-section has the highest accuracy. But it is generally only used to balance light parts. The method to check the static balance of the rotor is: Place the rotor as a whole on two horizontal hard rails and observe whether it can achieve "balance at any time", that is, it can be balanced at any position. When the center of mass shifts, the rotor can only stay when its center of mass is at the lowest position. At this time, you can add a mass at a certain radius opposite the center of mass and above the rotor to achieve "casual balance", or balance it by subtracting one mass in the direction of the center of mass. Whether a part only needs static balancing or dynamic balancing is mainly related to its working speed n and aspect ratio L/D. Generally, it can be selected according to Figure 10-1. In the figure, below line a is the static balance area, and above line b is the dynamic balance area. The area between the two lines is mainly used for more important parts, but does not have strict vibration requirements. In actual production, the static balance of parts is generally sufficient to achieve "incidental balance". In dynamic balancing, for long axial dimensions, there is often not only a centrifugal inertia force G, but also a centrifugal inertia moment. The centrifugal inertia force can be balanced during static balancing, but a centrifugal inertia couple will be generated during rotation, M=ce. This imbalance of the rotor is called dynamic unbalance. After balancing, the rotor is marked on the corresponding part connecting the drum and the shaft. Generally, it is not allowed to be disassembled at will. If it must be disassembled, it should be installed according to the original markings to avoid affecting the balance. 5. Requirements for friction pair materials of commonly used mechanical seal materials: 1. High mechanical strength, pressure resistance, high stiffness and small deformation. 2. Good self-lubrication, dry wear resistance and high load resistance. 3. The material pairing performance is good, the friction state of the sealing end face is improved, there is no excessive wear and corrosion of the pairing material, and the self-lubricating property is good. 4. Good wear resistance and extended service life. 5. Good thermal conductivity, large thermal conductivity, and good heat dissipation effect. 6. Good heat resistance, improving the high temperature resistance of dynamic and static rings. 7. Good thermal shock resistance and improved thermal crack resistance. 8. Strong corrosion resistance, corrosion resistance, erosion resistance, extended service life. 9. Small thermal expansion coefficient, heat deformation resistance and good dimensional stability. 10. Good processing performance, easy to process, cut and form. 11. Low density and good air tightness.