Working principle and structure of centrifugal compressors

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I. Working Principle: The turbine (or electric motor) drives the impeller of the compressor’s main shaft to rotate; under the effect of centrifugal force, the gas is thrown into the diffuser located behind the working wheel. A thin zone is formed in the middle of the working wheel, and the gas from ahead enters the impeller through the inlet section at the center of the working wheel. As the working wheel keeps rotating, the gas is continuously thrown out, thereby maintaining a continuous flow of gas within the compressor. The gas experiences an increase in pressure due to centrifugal force, and it can leave the working wheel at high speeds. As the gas passes through the diffuser, its speed decreases gradually, with kinetic energy being converted into static pressure energy, thereby further increasing the pressure. If the pressure achieved by a single working impeller is not sufficient, the required outlet pressure can be obtained by operating multiple impellers in series. The series connection between stages is achieved through bends and return lines. This is the working principle of a centrifugal compressor. II. Basic Structure A centrifugal compressor consists of two main parts: the rotor and the stator, as shown in Figure 6-1. The rotor includes a shaft, as well as components such as an impeller fixed to the shaft, shaft sleeves, balance discs, thrust discs, and couplings. The stator consists of cylinders, various partitions positioned on the cylinder block, as well as components such as bearings. Sealing elements are also provided at the locations where gas needs to be sealed between the rotor and the stator. The functions of each component are described as follows. 1. Impeller: The impeller is the most important component in a centrifugal compressor. It is through this rapidly rotating impeller that the mechanical power from the drive unit is applied to the gas, thereby imparting energy to it. It is the only component in the compressor that performs work; it is also known as the working wheel. An impeller generally consists of a shroud, a disk, and blades, forming a closed impeller; there are also semi-open impellers without a shroud. 1. Suction chamber 2. Impeller 3. Diffuser 4. Bend 5. Recirculator 6. Volute 7, 8. Shaft end seals 9. Support bearings 10. Thrust bearings 11. Snap ring 12. Housing 13. End cover 14. Bolts 15. Thrust disk 16. Main shaft 17. Coupling 18. Wheel cover seal 19. Diaphragm seal 20. Diaphragm. Figure 6-1: Vertical sectional view of a centrifugal compressor. 2. Main shaft: The main shaft serves to support the rotating components and transmit torque. Based on its structural form. There are two types: stepped shaft and optical axis. The optical axis features a simple shape and is easy to process. 3. In multi-stage centrifugal compressors, the uneven gas forces on either side of each impeller cause a resultant force to act on the rotor in the direction of the low-pressure side; this resultant force is known as the axial force. Axial force is harmful to the proper operation of compressors; it can easily cause damage to the thrust bearings, leading to the rotor shifting toward one end. This results in the moving parts losing their correct relative position with respect to the fixed components. In severe cases, the rotor may collide with the fixed parts, resulting in an accident. A balance disk is a component that uses the pressure difference between the gases on its two sides to balance axial forces. The pressure on one side of it is the pressure in the clearance on the side of the final impeller disk, while the other side leads to the atmosphere or the intake pipe. Typically, the balance disk only balances a portion of the axial force; the remaining axial force is borne by the thrust bearings. An air seal must be installed at the outer edge of the balance disk to prevent gas leakage and maintain the pressure difference between the two sides. The balance of axial force can also be achieved through air intake on both sides of the impeller or by installing the impeller in reverse. 4. Since the balance disc only balances part of the axial force, the remaining axial force is transmitted through the thrust disc to the thrust blocks on the thrust bearings, thereby achieving force equilibrium. The contact surface between the thrust disc and the thrust blocks must be very smooth, and the gap between them should be filled with appropriate lubricating oil, so that the thrust blocks do not wear out under normal operation. When a centrifugal compressor starts up, the rotor tends to move towards the other end; to ensure that the rotor remains in its proper position, it needs to be held in place by thrust forces on both sides. This is because, at the time of startup, the gas pressure across the balance disc has not yet been established. As long as there is gas flow, the rotor will move in a direction opposite to the normal axial force. Therefore, it is necessary to have thrust forces on both sides of the rotor to prevent accidents. 5. Couplings: Due to the characteristics of centrifugal compressors, such as high-speed rotation, high power, and inevitable vibration during operation, the couplings used must be able to transmit large torques while allowing for slight radial and axial displacement. Couplings are divided into toothed couplings and diaphragm couplings; currently, diaphragm couplings are the most commonly used. These couplings do not require lubricants and are easy to manufacture. 6. Shell: Also known as the cylinder, for medium and low pressure centrifugal compressors, a horizontally split shell is generally used to facilitate assembly. The upper and lower shells are positioned using dowel pins, and they are connected together with bolts. For high-pressure centrifugal compressors, a cylindrical forged steel casing is used to withstand high pressures. The end cover of this structure is connected to the cylindrical casing using bolts. 7. When the gas from the diffuser flows out of the impeller, it still has a high flow velocity. To make full use of this portion of kinetic energy in order to increase the gas pressure, a diffuser with a gradually increasing flow area is installed behind the impeller. Diffusers generally come in various forms such as bladeless, bladed, and straight-wall diffusers. 8. In multi-stage centrifugal compressors, bends are used where the gas must change direction between stages; these bends consist of an annular space formed by the casing and partition plates. 9. The channel connected behind the bend in the returner is the returner itself; its function is to allow the airflow to enter the next stage in a uniform manner in the desired direction. It consists of partitions and guide vanes. The guide vanes are usually arc-shaped; they can be cast as one piece with the cylinder or manufactured separately and then connected together with bolts. 10. Volute: The main purpose of the volute is to gather the gas that flows out after the diffuser or the impeller and guide it out of the machine. The cross-sectional shapes of the volute include circular, plow-shaped, trapezoidal, and rectangular. 11. Sealing: To reduce the amount of air leakage through the gap between the rotor and the fixed components, sealing is often installed. Sealing is divided into internal sealing and external sealing. The function of internal seals is to prevent gas from flowing back between stages, such as the wheel cover seal at the wheel cover area, and the partition seal between the partitions and the rotor. The external seal is designed to reduce and prevent gas from inside the machine from leaking out, as well as to stop outside air from entering the machine, such as the seals at the machine’s ends. There are many types of seals in centrifugal compressors, and the commonly used ones include the following: 1) Labyrinth seal. The labyrinth seal is currently a widely used sealing device in centrifugal compressors, employed for both external and internal sealing of the compressor. The gas flow in a labyrinth seal (see Figure 6-2): as the gas flows through the gaps between the comb-shaped labyrinth seal elements, it undergoes an expansion process, with pressure dropping from P1 to P2 at the right end. This expansion occurs gradually; when the gas enters the sealing chamber through these gaps, the sudden increase in cross-sectional area causes the gas flow to form strong vortices, resulting in almost complete loss of velocity. There is a pressure difference between the gases on either side of the sealing surface. The pressure inside the sealing chamber is equal to the pressure in the gaps. According to the laws of gas expansion, as pressure decreases, velocity should increase and temperature should decrease. However, since the flow of gas through the narrow gaps is of a throttling nature, the kinetic energy obtained by the gas due to the pressure drop is completely lost in the sealing chamber and converted into useless heat energy. This heat energy then warms up the gas again, causing the temperature to rise back to its level before the pressure dropped. This process repeats itself each time the gas passes through another seal element and chamber, until the pressure reaches P2. It can be seen that a labyrinth seal utilizes the principle of throttling: each time gas passes through a tooth, its pressure decreases, and after passing through a certain number of teeth, a significant pressure drop occurs. Essentially, a labyrinth seal creates a pressure difference that acts as a resistance to the flow of gas, thereby reducing the amount of gas that can pass through. Figure 6-2: Gas flow diagram of labyrinth seals. The commonly used types of labyrinth seals include the following. The smooth type is shown in Figure 6-3; the shaft serves as the optical axis, and the seal body is equipped with comb-like teeth or embedded tooth plates, resulting in a simple structure. Figure 6-3: Smooth-type labyrinth seal; the twisted type is shown in Figure 6-4. To enhance the throttling and pressure-reduction effect of each tooth, a twisted-type labyrinth seal was developed, which provides a better sealing performance than the smooth type. Figure 6-4 shows a corrugated labyrinth seal with stepped design; as depicted in Figure 6-5, this type of seal also provides better sealing performance than the smooth-type seal. It is commonly used for sealing the impeller cover, and typically features 3 to 5 sealing teeth. 2) Oil film sealing, namely floating ring sealing. The principle of floating ring sealing relies on the film formed by high pressure between the floating ring and the shaft sleeve, which creates throttling and pressure reduction to prevent gas from flowing from the high-pressure side to the low-pressure side. Floating ring sealing enables an oil film to be formed in the gap between the ring and the shaft, while allowing the ring itself to move freely radially. Figure 6-5: Stepped labyrinth seal. Figure 6-6: Structure diagram of the spiral-groove dry gas seal. The ring on the high-pressure side is called the high-pressure ring, while the ring on the low-pressure side is called the low-pressure ring. These rings can move freely in the radial direction but cannot rotate. The pressure of the sealing oil is usually about 0.5 Kg/cm2 higher than that of the process gas; this oil enters the sealing chamber and flows toward the high-pressure side through the gap between the high-pressure ring and the shaft, where it forms an oil film that seals off the high-pressure gas. The other stream of oil flows out through the gap between the low-pressure ring and the shaft and returns to the oil tank. Usually, there are several low-pressure rings in order to achieve effective sealing. The floating ring seal is made of steel, with its end faces coated in tin bronze; the inner side of the ring is filled with babbitt alloy to prevent short-term contact between the shaft and the oil ring, as babbitt alloy serves as an anti-wear material. Floating ring seals can achieve complete leaklessness, and are widely used as shaft sealing devices for compressors. 3) Mechanical seal: Figure 6-7 shows the rotating ring sealing surface with spiral grooves. Mechanical seals are sometimes used for shaft sealing in small compressors. The differences between mechanical seals used in compressors and those used in ordinary pumps lie mainly in the high rotational speed, high linear velocity, high PV value, significant frictional heat generation, and high requirements regarding dynamic balance. Therefore, structurally, the spring and its loading mechanism are generally designed to be stationary, and the geometric shape of the rotating parts is kept as symmetrical as possible. Pins, chains, and similar elements are not used in the transmission mechanism in order to reduce the effect of centrifugal force caused by unbalanced masses. Additionally, from the perspective of frictional components and the pressure on the end faces, a double-end-face partial-balancing design is preferred, with narrow end faces; the friction coefficient of the materials forming the friction pairs should be low. Cooling and lubrication must also be enhanced to allow for rapid removal of the heat generated at the sealing surfaces. 4) Dry gas seal: With the continuous improvement and development of hydrodynamic mechanical seal technology, one important type of such seal, namely the helical groove hydrodynamic gas seal or dry gas seal, has been widely used in the petrochemical industry. Compared to oil-sealed floating ring seals, dry gas seals offer numerous advantages: they operate stably and reliably and are easy to maintain; they require fewer auxiliary systems, which reduces the workload for operators. Additionally, only a small amount of nitrogen is consumed for sealing, making them both energy-efficient and environmentally friendly. Figures 6-8 Force diagrams of dry gas seals. Figure 6-6 shows a schematic diagram of a spiral-groove dry gas seal. It consists of a moving ring 1, a stationary ring 2, springs 4, O-rings 3, 5, 8, an assembly sleeve 7, and a shaft 6. Figures 6-7 show the sealing surface obtained by precision machining of thread grooves on the moving ring surface, followed by grinding and polishing. Generally, the depth of the helical groove is around 2.5 to 10 μm. The surface parallelism of the sealing ring must be very high, being less than 1 μm, and the shape of the helical groove is approximately that of a logarithmic spiral. As shown in Figures 6-7, when the moving ring rotates, nitrogen used for sealing is drawn circumferentially into the spiral grooves; it flows from the outer diameter toward the center in a radial direction. The sealing baffle acts to prevent the gas from flowing toward the center, thereby compressing the gas and increasing its pressure. This pressure of the gas film attempts to push against the seal, thus creating the desired gas film. The typical value of this balance gap or film thickness h is 3 μm. In this way, the pressure of the sealed gas, the spring force, and the pressure of the gas film work together to ensure that the gas film has good elasticity; it possesses high stiffness, which enables stable operation and prevents the sealing surfaces from coming into contact with each other. Meanwhile, the nitrogen gas film with its high stiffness can effectively prevent leakage of the medium. The force acting on the dry gas seal is shown in Figure 6-8. Under normal operating conditions, the closing force of this seal (caused by the spring and gas forces) is equal to the opening force (caused by the gas film force). When external forces interfere and the gap decreases, the gas shear rate increases, which enhances the effectiveness of the spiral groove in allowing the seal to open; as a result, the opening force becomes greater than the closing force, and the gap returns to its original size. If external disturbances cause the gap to increase, the pressure inside the gap decreases, the opening force becomes less than the closing force, and the sealing surfaces come together again, restoring the gap to its original size. 12. Centrifugal compressors with bearings have radial bearings and thrust bearings. Radial bearings are sliding bearings; their function is to support the rotor and enable it to rotate at high speeds. Thrust bearings, on the other hand, bear the remaining axial forces on the rotor, preventing axial movement of the rotor and maintaining its axial position within the cylinder. (1) Radial bearings: Radial bearings mainly consist of a bearing housing, a bearing cover, and upper and lower half-shaft bushings. Bearing housing: It is used to hold the bearing shells; it can be cast together with the cylinder, or it can be cast separately and then supported on the machine base. The forces exerted by the rotor on the bearings are ultimately transmitted, directly or indirectly, to the machine base and the foundation through this housing. Bearing cover: It is placed on the bearing bush and maintains a certain degree of tension with the bush in order to prevent the bearing from shifting; the bearing cover is secured to the bearing housing using bolts. Bearing shells: Used to directly support the shaft journals. The circular surfaces of these bearing shells are lined with babbitt alloy; due to its good friction-reducing properties, high plasticity, and ease of casting and running-in, it is widely used in centrifugal compressors. In practice, for ease of loading and unloading, bearing shells are usually made in upper and lower halves and fastened together with bolts; currently, the thickness of babbitt used is typically between 1 and 2 mm. There are two ways in which bearing shells can be positioned within the bearing housing: one is where the bearing shell remains fixed, and the other is where it is movable – that is, there is a spherical surface on the back side of the bearing shell, which allows it to adjust its position automatically as the spindle deflects during movement, thereby ensuring even stress distribution along the entire length of the bearing shell. Lubricating oil enters the bearing through the oil holes on the side surface of the bearing. On the path of the oil as it enters the bearing, a throttle orifice plate is installed; by changing the diameter of this orifice plate, it is possible to regulate the amount of oil that enters the bearing. An annular oil groove is present in the upper part of the bearing bush, which helps to improve the circulation of the lubricating oil and cool the shaft journal. (2) Thrust bearings: Like radial bearings, thrust bearings are also divided into upper and lower halves; a positioning pin is present on the mid-surface, and they are connected using bolts. A positioning sleeve is used between the spherical housing and the spherical seat to prevent relative rotation. Due to the spherical support, adjustment can be made automatically according to the degree of shaft deflection. Thrust bearings work together with thrust discs; the thrust disc mounted on the shaft rotates along with the shaft, and the thrust transmitted by the shaft is applied to several stationary thrust blocks. A layer of babbitt is also cast on the working surface of these thrust blocks, and the thickness tolerance of these thrust blocks is less than 0.01–0.02 mm. Mitchell-type thrust bearings and Kingsbury-type bearings are widely used in centrifugal compressors. During normal operation, the axial force in a centrifugal compressor always acts in the direction of the low-pressure side, and the thrust block that bears this axial force is known as the main thrust block. When the compressor starts, the direction of the airflow force is toward the high-pressure side, and this force causes axial movement of that high-pressure side. To prevent such axial movement, another thrust block is installed; this block is located opposite to the main thrust block and is referred to as the auxiliary thrust block. A certain gap is left between the thrust disc and the thrust block to facilitate the formation of an oil film; this gap is generally within the range of 0.25 to 0.35 mm. Most importantly, the maximum value of this gap should be less than the minimum axial gap between the fixed component and the rotating component, so as to prevent the moving and stationary parts from coming into contact. The lubricating oil enters the spherical housing through the inlet at the bottom of the sphere, and then splits into two paths: one path goes to the radial bearings via the mid-surface, while the other path leads to the thrust bearings through two sets of inclined holes. Part of the oil that reaches the thrust bearings goes into the main thrust block, while the other part goes into the secondary thrust block.