Structure and principle of centrifugal compressor

Working principle and structure of centrifugal compressor 1. Working Principle Centrifugal refrigeration compressor has various structural forms, such as single stage, double stage and multi stage. The single-stage compressor is mainly composed of suction chamber, impeller, diffuser and volute, as shown in Figure 6- 1. For multi-stage compressors, there are also components such as elbows and return pipes. A working impeller and its supporting fixed components (such as suction chamber, diffuser, elbow, reflux device or volute, etc.). ) constitutes the first stage of the compressor. The main shaft of multistage centrifugal refrigeration compressor is equipped with multiple impellers, which work in series to obtain higher pressure ratio. The intermediate stage of multistage centrifugal refrigeration compressor is shown in Figure 6-2. In order to save compression power consumption and not make the exhaust temperature too high, the multistage centrifugal refrigeration compressor can be divided into several stages, each including one to several stages. The waste gas from the low-pressure part needs intermediate cooling before being sent to the high-pressure part. 1-inlet adjustable guide vane 2-suction chamber1-impeller 2-diffuser 3-impeller 4-volute 5-diffuser 6-spindle 3-elbow 4-single-stage centrifugal type of reflux device is shown in Figure 6- 1. The working principle of the refrigeration compressor is that when the compressor impeller 3 rotates, the refrigeration gas enters the impeller wheel from the suction chamber 2 through the inlet adjustable guide vanes 1, and the gas rotates with the impeller under the push of the impeller blades. Due to centrifugal force, gas flows radially along the impeller channel and leaves the impeller. At the same time, a low pressure is formed at the inlet of the impeller, and the gas is continuously sucked by the suction pipe. In this process, the impeller does work on the gas, so that its kinetic energy and pressure energy increase, and the pressure and flow rate of the gas increase. Then, the gas enters the diffuser 5 and the volute 4 with gradually enlarged cross sections at high speed, and the flow velocity gradually decreases, and most of the kinetic energy of the gas is converted into pressure energy, and the pressure is further increased, and then it is led out of the compressor. For multi-stage centrifugal refrigeration compressor, in order to make the pressure of refrigerant gas continue to rise, elbow and reflux device are used to introduce the gas into the next stage impeller for compression, as shown in Figure 6-2. Due to the different working principles of the compressor, the centrifugal refrigeration compressor has the following characteristics compared with the reciprocating piston refrigeration compressor: ① Under the same refrigeration capacity, its overall volume is small, its weight is light and its floor space is small. Under the same refrigeration conditions and refrigeration capacity, the piston refrigeration compressor is 5~8 times heavier than the centrifugal refrigeration compressor (including gear speed increaser) and occupies about 2 times the area. ② No reciprocating parts, good dynamic balance characteristics, small vibration and simple foundation requirements. At present, for small and medium-sized assembly units, the compressor can be directly installed in the single-cylinder evaporator. 0? On the condenser, there is no need to design another foundation, which is convenient to install. ③ Less wearing parts, long continuous operation cycle, low maintenance cost and long service life. (4) Lubricating oil and refrigerant are basically not in contact, which improves the heat transfer performance of evaporator and condenser. ⑤ It is easy to realize multi-stage compression throttling and multi-evaporation temperature operation of the same refrigerator. ⑥ Stepless adjustment can be carried out economically. Imported guide vanes are used to automatically adjust energy, which has good adjustment range and energy saving effect. ⑦ For large refrigerators, if the industrial steam turbine with high economy is directly driven, the variable speed adjustment can be realized and the energy saving effect is better. Especially for industrial enterprises with waste heat steam, it can also realize energy recovery. ⑧ When the rotating speed is high, it is generally necessary to set a speed increaser for the motor-driven equipment. Moreover, the requirement of shaft end sealing is high, which increases the manufacturing difficulty and structural complexity. Pet-name ruby when the condensation pressure is too high, or the refrigeration load is too low, the compressor unit will surge and cannot work normally. Attending the refrigeration capacity is small and the efficiency is low. At present, centrifugal refrigeration units can be roughly divided into two categories: one is water chillers, whose evaporation temperature is above -5℃, which are mostly used in large central air conditioners or industrial processes for preparing cold water above 5℃ or salt water slightly below 0℃; The other is a low-temperature unit, the evaporation temperature is -5~-40℃, which is mostly used in chemical processes with large refrigeration capacity. In addition, centrifugal refrigeration unit can also be used in beer industry, artificial dry ice field, frozen soil, low temperature laboratory and heat pump system with both cold and warm water supply. Centrifugal refrigeration compressors are usually used in occasions with large refrigeration capacity, with closed centrifugal refrigeration compressors within 350~7000kW and open centrifugal refrigeration compressors within 7000~35000kW. 2. Structure and function of main parts Due to the difference of evaporation temperature and refrigerant in use, the number of cylinders, stages and stages of centrifugal refrigeration compressor are quite different, and the overall structure is also different, but its basic components will not change. The structure and function of its main components are briefly described as follows. (1) suction chamber The function of suction chamber is to guide the gas from evaporator or interstage cooler evenly to the inlet of impeller. In order to reduce the disturbance and separation loss of airflow, the cross section of the suction chamber along the airflow direction is generally tapered to slightly accelerate the airflow. The structure of the suction chamber is relatively simple, including axial air intake and radial air intake, as shown in Figure 6-3. For the single-stage cantilever compressor, the compressor is placed in the combined air conditioning unit above the evaporator and condenser, and the elbow suction chamber with radial air intake is commonly used (Figure 6-3b). However, because the suction inlet of the impeller is axial, the suction chamber with radial air inlet needs to be provided with a diversion elbow. In order to make the air flow into the impeller evenly after turning, sometimes a deflector is added at the corner of the suction chamber. The suction chamber shown in Figure C is usually used for multi-stage compressors with double support bearings, and the first stage impeller has a through shaft. A) axial air intake suction chamber b) radial air intake elbow suction chamber c) radial air intake semi-volute suction chamber (2) Inlet guide vane Inlet guide vane is installed on the casing in front of the inlet of the first stage impeller of the compressor to adjust the refrigeration capacity. When the guide vane rotates, it changes the direction and size of airflow entering the impeller. When turning the guide vane, lever or wire rope adjusting mechanism can be used. The lever type is shown in Figure 6-4. The imported guide vane is actually a chrysanthemum valve composed of Jojo 1- pinion 2- gear ring 3- rotating blade 4- servo motor 5- bellows 6- connecting rod 7- lever 8- handwheel 1- guide vane 2- driven gear 3- wire rope 4- transition gear 5- driving gear shaft rotating blade 3. Figure 6-5 of each blade shows the wire rope transmission, in which a driving gear 5 drives six driven gears 2 to rotate through the wire rope 3, thus driving seven guide vanes 1 to open. In order to make the wire rope move on a fixed track and prevent it from slipping out of the driving gear and the driven gear, seven transition wheels 4 are installed. The driving gear is driven by the guide vane adjustment actuator to rotate the driving gear according to the adjustment signal of the refrigeration unit. The imported guide vane is made of cast copper or aluminum, and the vane has two profiles: wing profile and symmetrical wing profile, which are selected by hand polishing. The rotating shaft of the inlet guide vane is equipped with a copper bushing, and a small amount of lubricant should be injected between the rotating shaft and the bushing and each connecting part to ensure the flexible rotation of the mechanism. (3) Impeller Impeller, also known as working wheel, is the only part in the compressor that does work on gas. After the impeller rotates with the spindle at high speed, the blades of the impeller do work on the gas. Due to the centrifugal force of rotation and the expansion flow in the impeller, the pressure and velocity of gas after passing through the impeller are improved. Impellers can be divided into three types according to their structural types: closed, semi-open and open. Usually closed and semi-open, as shown in Figure 6-6. The closed impeller is composed of wheel cover, blades and disks, and most refrigeration compressors for air conditioning are closed. Semi-open impeller has no wheel cover, and one side is open, only blades and disks, which is used in occasions with high single-stage pressure. With the wheel cover, the internal air leakage loss can be reduced and the efficiency can be improved. However, when the impeller rotates, the stress of the wheel cover is large, so the circumferential speed of the impeller cannot be too large, which limits the improvement of the single-stage pressure ratio. Because the semi-open impeller has no wheel cover, it is suitable for bearing centrifugal inertia force, which is beneficial to the strength of the impeller and makes the peripheral speed of the impeller higher. At present, the circumferential speed of steel semi-open impeller can reach 450~540m/s, and the single-stage pressure ratio can reach 6.5. A) Closed b) Blade of semi-open centrifugal refrigeration compressor impeller can be divided into single circular arc, double circular arc, straight blade and three-dimensional blade according to shape. The single-stage impeller of air-conditioning compressor mostly adopts three-dimensional blades with curved surface and twisted shape, which is complicated to process and requires high precision. When freon refrigerant is used, cast aluminum impeller is usually used, which can reduce the processing requirements. (4) When the diffuser gas flows out of the impeller, the flow velocity is very high, which can generally reach 200~300m/s, accounting for a large proportion of the work done by the impeller on the gas. In order to fully convert this kinetic energy into pressure energy, and at the same time make the gas have a lower reasonable flow rate when entering the next stage, a diffuser is arranged behind the impeller, as shown in Figure 6-2. The diffuser usually consists of two parallel walls perpendicular to the impeller shaft. If there are no vanes between two parallel walls, it is called a vaneless diffuser. If vanes are provided, they are called vane diffusers. The cross section of the annular channel in the diffuser gradually becomes larger, and the speed of gas flow gradually decreases and the pressure gradually increases. The vaneless diffuser is simple in structure and convenient to manufacture, and there is no impact loss because there is no blade blockage in the flow passage. In air-conditioning centrifugal refrigeration compressor, vaneless diffuser is generally used in order to adapt to its wider working range. Vane diffusers are often used in multistage compressors in cryogenic equipment. (5) Elbow and reflux In a multistage centrifugal refrigeration compressor, elbow and reflux are used to guide the gas flowing out of the diffuser to the next stage impeller. The function of the elbow is to guide the airflow from the diffuser outlet to the inlet of the reflux device, so that the airflow changes from centrifugal direction to centripetal direction. Reflux is to guide the airflow evenly to the inlet of the next stage impeller. Therefore, there are vanes in the backflow channel, which make the gas flow along the bending direction of the vanes and enter the next stage impeller along the axial direction. In the refrigeration cycle with multi-stage throttling and intermediate air supply, there is intermediate air supply between stages, so the reflux device of centrifugal refrigeration compressor also has interstage air supply structure. Figure 6-7 shows three types of aeration, of which type B and type C are not conducive to the uniformity of air flow at the inlet of the next stage impeller, but can reduce the axial distance. (6) The function of volute is to collect the gas flowing out from diffuser or impeller (without diffuser) and discharge it to condenser or intercooler. Figure 6-8 shows a volute commonly used in centrifugal refrigeration compressor, and its flow cross-section gradually increases along the rotation direction of the impeller (that is, the rotation direction of the incoming air flow) to adapt to the uneven flow along the circumference, and also plays a role in slowing down and expanding the air flow. The volute is generally installed behind the diffuser of each last stage, and some last stages directly install the volute behind the impeller without diffuser, as shown in Figure 6-9. Where a is a diffuser installed in front of the volute; A) there is a diffuser in front of the volute; B) there is an impeller in front of the volute; C) There is an asymmetric inner volute; B) The volute is directly installed behind the impeller. In this kind of volute, the air velocity is relatively high, so a diffuser is generally installed behind the volute; Because the volute is directly behind the impeller, it has a great influence on the work of the impeller, which increases the unevenness of the airflow at the outlet of the impeller; C stands for asymmetric inner volute, which is often used in single-stage air conditioning units. This volute is placed on one side of the impeller, and the outer diameter of the volute is unchanged. By reducing the inner diameter, the flow section is increased. The cross section of volute is usually circular and trapezoidal. There is an oil drain hole at the bottom of the volute of Freon water chiller, and a high-pressure gas conduit connected with the oiler is arranged in the horizontal position. The high-pressure gas which is inflated and sealed everywhere is led out from the volute. (7) Sealing For closed units, it is unnecessary to adopt shaft seal components to prevent refrigerant from leaking outwards. However, in the compressor, in order to prevent inter-stage gas leakage or oil-gas mutual leakage, various types of gas seals and oil seal components must be adopted, and shaft seal devices are also needed for open compressors. The following types of seals are usually used in centrifugal refrigeration compressors. 1) labyrinth seal, also known as comb seal, is mainly used for interstage sealing, such as the inner seal of wheel cover and shaft sleeve and the seal at the balance plate. Labyrinth seal consists of many units separated by comb teeth. It uses the labyrinth of comb teeth to make the gas leak to the low pressure side, and throttles, expands and depressurizes many times (because the gas pressure in each cell is lost after passing through a gap), thus reducing the leakage. Labyrinth seal has many structures, and the common one is shown in Figure 6- 10. Sawtooth seal is better than smooth seal, and is often used to seal shaft sleeve and balance disk, but it is complicated to manufacture and requires strict axial positioning. Step seal is mainly used for wheel cover seal. A) Inlaid serrated seal b) Integral smooth seal c) Step seal1-shaft seal shell 2-spring 3, 7-O ring 4-stationary ring seat 5-stationary ring 6-moving ring 2) Mechanical seal is mainly used to seal the shaft end of the open compressor where the rotating shaft passes through the shell. Mechanical seal has many structural forms, including single-ended type consisting of a static ring and a moving ring, double-ended type consisting of two static rings and a moving ring, or double-ended type consisting of two static rings and two moving rings. Fig. 6- 1 1 shows a double-ended mechanical seal consisting of a moving ring 6 and two stationary rings 5. The sealing surface is the contact surface between the stationary ring and the moving ring, and the spring 2 presses the stationary ring on the moving ring through the stationary ring seat 4. O-rings 3 and 7 prevent gas from leaking from the gap. When the compressor works, lubricating oil with a pressure about 0.05~0. 1MPa higher than the gas pressure is introduced into the shaft seal cavity, which pushes away the static rings pressed on both sides of the moving ring to form a sealing oil film, which not only reduces the friction loss, but also plays a role in cooling and strengthening the sealing effect. When stopping, the oil pressure drops, but the constant pressure box still maintains a certain oil pressure in the shaft seal cavity, and the spring presses the static ring against the moving ring to form a good stop seal. The advantages of mechanical seal are good sealing performance, close to absolute seal and compact structure. But the disadvantages are easy to wear, short service life, low linear speed of friction pair and limited specific pressure of sealing surface. A) One-piece oil seal b) Inflatable oil seal 3) Oil seal Figure 6- 12a shows a simple one-piece oil seal. Single piece oil is encapsulated on both sides of the bearing, which is usually made of aluminum and copper. The diameter clearance is 0.2~0.4mm, which is greater than the radial clearance of the bearing. Fig. 6- 12b shows the gas-filled seal. In the centrifugal refrigeration compressor for air conditioning, inflatable seal is mainly used. An annular cavity is formed in the middle of the labyrinth gear row cut by the integral casting aluminum alloy lathe. High-pressure gas slightly higher than the oil pressure is introduced into the annular cavity from the volute of the compressor, and the high-pressure gas flows out from both ends of the sealing teeth in the cavity, with one end sealing the oil and the other end entering the compressor. Generally speaking, the diameter gap of teeth is 0.2 ~ 0.6 mm. In addition to the above main components, centrifugal refrigeration compressor has other components. For example, the balance disk reduces the axial thrust; Thrust bearing for bearing residual axial thrust of rotor and radial bearing for supporting rotor. In order to make the compressor run continuously, safely and efficiently, it is necessary to set up some auxiliary equipment and systems, such as speed increaser, lubrication system, cooling system, automatic control and monitoring and safety protection system. -and: /viewthread.php? tid= 136088