Characteristics and trends of assembled centrifugal compressors

I. Introduction

   Assembled centrifugal compressor is also called assembling integral gear speed-increasing centrifugal compressor, and it has developed rapidly in the world in recent years. There is a great demand for this kind of compressor in our country, and it is widely used in various fields such as metallurgy, air separation plant, petrochemical, pharmaceutical, power station and so on. Since my country could not produce assembled compressors before, it basically relied on imports. According to preliminary statistics, nearly 100 compressors of different types are imported each year. After years of research and development, our company has successfully developed this assembled compressor and initially formed a series, which not only has a place in the domestic market, but also some products have been sold abroad. According to the main characteristics of the assembled compressor, this article introduces the advantages, key technologies and the development trend of localization of the compressor.

   2. The structural characteristics and advantages of the assembled compressor

  Different from the general single-shaft multi-stage centrifugal compressor, the assembled centrifugal compressor adopts the structure of multi-shaft multi-speed, one cooling per stage, all levels of axial air intake, gear box and volute integrated. Figure 1 is a general sectional view of a 3-stage SVK assembled centrifugal compressor.

   Due to the difference in structure, the assembled centrifugal compressor has the advantages that the general single-shaft multi-stage centrifugal compressor cannot have.

   1. A process closer to isothermal compression

   When analyzing the compression process of a compressor, it can usually be divided into isothermal process, adiabatic process and variable process, among which the compression work of isothermal process is the smallest. Of course, complete isothermal compression is impossible, but the isothermal efficiency can be used to measure the power consumption of the compressor, that is to say, the closer the compression process is to the isothermal process, the more power is saved and the higher the efficiency. The assembled centrifugal compressor adopts a cooling method for each stage, so that the actual compression process is as close to isothermal compression as possible. Therefore, this type of compressor is more energy-saving and more efficient than other types of compressors under the same conditions.    2. The impellers at all levels run at a better speed

  The impeller has different flow coefficients and energy head coefficients at different speeds. In the design of a multi-stage centrifugal compressor, the flow coefficient, energy head coefficient, speed, diameter and other parameters of each impeller must be matched, otherwise the unit will not work in the best state. The structure of the traditional single-shaft multi-stage centrifugal compressor forces more impellers to work at the same speed, which has a certain impact on the performance matching and working conditions of the compressor. The assembled centrifugal compressor adopts a multi-shaft and multi-stage method, with only two impellers at the same speed, which optimizes the overall performance of the unit. At the same time, due to the multi-shaft structure, each stage of the impeller can achieve axial air intake, reducing the loss of inlet airflow and improving the efficiency of the unit.

   3. Wider working condition adjustment range

  In order to make the working condition range of the unit wider, the assembled centrifugal compressor is equipped with a blade regulator at the inlet. Since the inlet regulator is very close to the impeller inlet in terms of structure, the blade regulator can not only adjust the pressure, flow and other parameters of the compressor according to the changes of the user's pipe network by means of inlet throttling, but also can change the inlet air flow and the impeller. The angle of the rotation direction (intake pre-selection adjustment) changes the power consumption of the compressor, which really plays a role in energy saving.

   4. Compact structure, small footprint, easy installation

The structure of the assembled compressor is very compact. It can be seen from the cross-sectional view that its volute is directly assembled on the gear box, and in the overall arrangement, the compressor body (including the gear box), the oil tank and the lubricating oil system, The electric motor and even the intermediate gas cooler of the smaller model are assembled on a common base. It not only covers a small area, but also is very convenient for installation and commissioning. The foundation construction is relatively simple, and the on-site construction time is short. Of course, the assembled centrifugal compressor also has its shortcomings: due to structural limitations, the current can only reach 7 to 8 stages of compression, which is not applicable when the unit pressure ratio is too large; due to structural limitations, seal size and installation The method determines that it cannot be used as a raw material compressor in petrochemical enterprises for the time being.

   Three, the key technology of assembled compressor

  Assembled centrifugal compressor has many advantages, but its design and manufacture are relatively complicated. When developing this kind of compressor, not only the structure of the unit must be researched and tested, but also many key unit technologies must be researched and developed in advance or in parallel.

   1. High-performance model-level basic level

   is the core part of the compressor and the key technology of compressor design. The level of basic performance directly affects the overall efficiency of the compressor. The model stage suitable for assembled compressors is mainly high-pressure and small flow. In order to increase the pressure ratio, the impeller speed is very high, the air velocity is close to or even exceeds the speed of sound, and the flow is small, the friction loss in the flow channel is large, the flow situation is complicated, and three-dimensional viscosity is required. The analysis technology performs simulation analysis of the flow field. This type of transonic high pressure and small flow model level is difficult to study. Our company has used the imported NREC, NUMECA and domestic aerodynamic analysis software to do many years of research and development work on this half-open high Mach number model level, and successively developed DHK-65, DHK-40 model level and NRECA1, A2, Basic model level such as A3, the overall performance has reached the international advanced level.

   2. Reliable strength analysis and vibration analysis

  Assembled centrifugal compressor is a kind of high-speed rotating power mechanical equipment. At present, the speed of this compressor produced by our company has reached nearly 50,000r/min, and the highest circumferential speed of the impeller has exceeded 450m/s. In this case, the strength check of the rotating parts and the vibration analysis of the unit are particularly important. Impeller calculations include the calculation of the strength of the impeller under the action of centrifugal force, the displacement of the blades of the impeller under high-speed rotation, the calculation of the interference between the impeller hole and the shaft, and the calculation of the loose speed. Rotor vibration analysis and calculation include the calculation of the first, second, and even third-order critical speed of each rotor, the calculation of the unbalanced response of the rotor, and the vibration analysis of the shaft system.

   3. High efficiency heat exchanger

The technical parameters of    heat exchanger not only affect the water consumption and outlet temperature of the unit, but also have a great influence on the aerodynamic performance of the unit. Our company and Xi’an Jiaotong University jointly developed a cooler design program, and developed a new cooler series of assembled centrifugal compressors in combination with imported technology. In order to further improve the heat exchange efficiency of the cooler, reduce the volume of the cooler and greatly reduce the water consumption, our company has also developed an internal fin type high-efficiency cooler. Used in this series of compressors, it can improve the pneumatic performance of the whole machine.

   4. Research on gear and bearing technology

The design of the assembled centrifugal compressor generally requires continuous operation for 2 or 3 years. The speed of each working shaft is generally between 20000 and 50000 r/min, the gear speed ratio is between 10 and 20, and the gear peripheral speed can reach 120~ 150m/s, bearing peripheral speed also exceeds 80m/s. The design and manufacture of high-speed gears and bearings with this high-speed ratio are quite difficult. Therefore, the research of gear technology is also an important subject of assembled centrifugal compressors.

   5. Design and research of mechanical structure

  Because this compressor has the characteristics of multi-axis and high-speed rotation, and at the same time requires extremely compact structure, many structural design problems need to be solved when developing this compressor. For example, the structural modeling and assembly form of each part of the rotor must be subjected to detailed calculations and tests. It is necessary to ensure the reliable use of the parts themselves, but also to consider the stress and strain of the rotating parts, the vibration characteristics of the rotor, and the shaft system. The vibration characteristics, etc., is a complex problem related to multiple disciplines.

   Fourth, the localization of assembled compressors

The SVK type assembled centrifugal compressor researched by our company has formed a series of models such as SVK2, SVK4, SVK6, SVK8, SVK12, SVK16, SVK20, and SVK25, SVK32, etc., which can transport air and nitrogen, and the flow range is from 2000 to 35000m3/h, the compressor includes 1, 2 and 3 stages, the pressure ratio can be from 1.5 to 11. According to the different use conditions and requirements of users, these compressors can also be designed as 4 stages, and the pressure ratio can reach about 20. After the SVK-type assembled centrifugal compressor series products were gradually improved and a certain production capacity was formed, the development of 4-axis high-pressure assembled centrifugal compressors began. This kind of compressor can perform up to 6 stages of compression, and the pressure ratio can reach about 60. It is expected that by 2004, this multi-shaft assembled centrifugal compressor will enter the market. Now a complete series of SVK assembled centrifugal compressors have been applied in practice.

   5. Conclusion

   Assembled centrifugal compressor has many advantages, but it is not suitable for all occasions due to technical reasons. With the continuous improvement of compressor design and manufacturing technology, its application range will become wider and wider. Although my country's assembled centrifugal compressors started late and can only reach the highest level 4 compression so far, more than 60 units have been produced in recent years, which not only occupy a place in the domestic market, but also export a small amount of products abroad. It is estimated that a year later, domestically produced multi-shaft assembled centrifugal compressors of stage 5 to 6 will be available, and within 2 to 3 years it will be able to reach the annual production capacity of 80 assembled centrifugal compressors of various forms, replacing more Of imported equipment.