Introduction to screw refrigeration compressors and precautions for use (technology sharing)

Introduction to screw refrigeration compressors and precautions for use (technology sharing) 　Screw refrigeration compressors and piston refrigeration compressors are the same in terms of gas compression, and both belong to positive displacement compressors, that is to say, they both compress gas by volume changes. The difference is that the two compressors have different ways of realizing the change of working volume. Screw refrigeration compressors are divided into single screw compressors and twin screw compressors. Among them, the twin-screw compressor uses two screws with helical tooth grooves placed in the machine body to mesh and rotate and cooperate with the inner wall of the machine body and the inner wall of the suction and exhaust end seats to cause the change of the volume between the teeth, thereby completing the gas The process of suction, compression and discharge. 　　 Screw compressors can be divided into two types: oil-free and oil-injected. Oil-free screw compressors were mainly used to compress air when they came out in the 1930s, and were only used in refrigeration devices in the 1950s. In the 1960s, a screw-type refrigeration compressor with oil injection in the cylinder appeared, and its performance was improved. In recent years, with the continuous improvement of tooth profile and other structures, the performance has been greatly improved. In addition, the screw compressor has no clearance volume, high efficiency, and no wearing parts such as suction and exhaust valve devices. Therefore, the current screw refrigeration compressor has become an advanced refrigeration compressor, especially the oil-injected screw compressor has been one of the main types of refrigeration compressors, and has been widely used. 　　 Screw type refrigeration compressor is mainly composed of casing, rotor, bearing, shaft seal, balance piston and energy adjustment device. 　　Chassis: generally split type, composed of three parts: the body, the suction end seat and the exhaust end seat, which are connected by bolts. The cross section of the internal cavity of the machine body is a horizontal figure 8 with double circles intersecting, which is suitable for the outer cylindrical surfaces of the two meshing rotors placed in it. The rotor is a pair of intermeshing screws with a special spiral tooth profile. Among them, the convex tooth is called the male screw (or male rotor), and the concave tooth is called the female screw (or female rotor). The gear ratio of male screw to female screw is generally 4:6 (the gear ratio of the compressor with high flow rate can be 3:4, when the compression ratio is as high as 20, the gear ratio can be 6:8). In most cases, the male screw is directly connected to the motor, which is called the active rotor, and the female screw is the driven rotor. Therefore, the male screw is mostly right-handed with four heads and the female screw is mostly left-handed with six heads. In order to serialize screw refrigeration compressors, standardize and generalize parts, relevant departments of our country stipulate that the nominal diameter of the screw is 63, 80, 100, 125, 160, 200, and 315mm, and the ratio of length to diameter is divided into λ=1.0 and λ=1.5 two kinds. Bearings and spoke seals: The female and male screws of the screw refrigeration compressor are supported by sliding bearings (main bearings) and centripetal thrust ball bearings. The main bearing is correctly installed and fixed in the suction and exhaust end seats with a pin. Two thrust bearings are installed on the male and female screws on the discharge side to withstand a certain shaft force. The shaft seals of screw refrigeration compressors also mostly use friction ring mechanical seals, which are installed on the coupling-end shaft of the active rotor. The structure and principle are the same as those of piston refrigeration compressors. 　　Balanced Piston: Due to the difference in structure, the resultant axial force acting on the male screw is much larger than the axial resultant force acting on the female screw caused by the pressure difference between the suction and exhaust sides. Therefore, in addition to a thrust bearing, an oil pressure balance piston is added to the male screw to reduce the load of the male screw rod on the end face of the sliding bearing and reduce the axial force borne by the thrust bearing. 　　 Energy regulating device: It is composed of slide valve, oil cylinder, oil piston, four-way electromagnetic reversing valve and oil pipeline. The piston is installed at the intersection of the two circles at the bottom of the cylinder wall, and the position of the slide valve can be changed to adjust the cooling capacity. 　　 When the screw refrigeration compressor is working, the volume of the elementary element between the teeth changes periodically, so that the process of suction, compression and exhaust is completed when the steam moves along the axial direction of the rotor. 　　 safe operation Ready to work: 　　 1. Check that the refrigerant, water and electrical equipment system should be normal; 2. Test the rotation of the motor. Since the screw compressor should not be reversed, you can try to rotate the motor after removing the rubber rotating core of the coupling. The rotation of the motor should be counterclockwise when viewed from the side of the compressor. direction; 　　3. Check the oil level of the oil separator. The correct oil level is when the oil pump is turned on to fill the oil cooler with oil, and the oil level gauge indicates normal; 　　4. Check whether all pressure gauge valves are open and whether the thermometer socket is filled with lubricant; 　　 5. Check or open all valves on the oil circuit, they should be fully open; 6. Start the oil pump. Check the direction of the oil pump. The oil pressure is not less than 0.05-0.3Mpa gauge pressure (can be adjusted by regulating valve). The differential pressure of the oil filter should not exceed 0.1Mpa. Turn the coupling by hand and operate the energy regulating valve at the same time to make the unloading indication from 0%-100%, then from 100%-0%, and then stop the oil pump; 　　7. Pull the compressor coupling without jamming; 　　 8. Open the exhaust shut-off valve on the compressor and close the oil return shut-off valve from the compressor to the oil cooler; 　　9. Supply water to the oil cooler, the amount of water depends on the oil temperature (the injection temperature is preferably 40℃-55℃); 　　 10. Close the main motor power supply and release the control power supply. The power indicator is normal. start up: 1. At the position where the energy adjustment indicator is at 0%, all valves related to the intermediate air supplement are closed, press the joint start button, the oil pump starts first, when the oil pressure reaches normal, the main engine starts, and at the same time opens the suction stop valve (when suction When the air system pressure is high, it should be opened slowly so as not to make the load excessive). First, drive and run, it is not suitable to run for too long, about 3-5 minutes, stop and observe whether the operation is normal 　　2, the energy adjustment indicator runs at 0% for 30 minutes, and observe the operating conditions; 3. When the compressor is running normally, open the energy regulating valve and gradually load from 0% to 100%. When the pressure ratio between the evaporation pressure and the condensing pressure increases, open the valves of the intermediate air supplement to check the working conditions of each part. Is it normal and reliable. 　　(1) When the suction and discharge pressure difference is large, and the fuel injection pressure (that is, the difference between the oil pressure after the fine filter oil and the discharge pressure) is small, the oil pressure should be appropriately increased by the oil pressure regulating valve; 　　(2) If the oil level gauge is found to be leaking, it should be checked and repaired after parking. The upper and lower valves of the oil level gauge should be in the fully open position when working, otherwise the valve will lose safety protection. 　　 Operation check: 　　 1. Check the pressure of suction, exhaust and oil, and the temperature is within the specified range; 2. Observe the opening status of all air valves, oil valves and pipe systems, and check for leaks. A small amount of oil dripping on the compressor and oil pump shaft seals is allowed; 　　3. Observe the vibration and sound of the machine; 　　4, check the cooling water system; 　　 5. Observe the current and voltage of the motor. 　　Parking: 　　 First reduce the energy adjustment indication from 100% to 0%, then press the combined stop button to stop the main engine and the oil pump, and close the exhaust shut-off valve. After equalizing the pressure, close the suction stop valve, open the oil return valve from the compressor to the oil cooler, and close the cooling water. work process 　　1.1 Basic structure The basic structure of the screw refrigeration compressor is mainly composed of the rotor, the casing (including the cylinder block in the middle and the suction and exhaust end seats at both ends, etc.), bearings, shaft seals, balance pistons and energy adjustment devices. Two rotors that rotate in opposite directions and mesh with each other according to a certain transmission ratio are arranged in parallel in a cylinder in the shape of "&". The rotor has a special spiral tooth shape, the convex tooth shape is called the male rotor, and the concave tooth shape is called the female rotor. Generally, the male rotor is the active rotor and the female rotor is the driven rotor. There are suction end seats and exhaust end seats on the left and right of the cylinder, and a pair of rotors are supported on the bearings of the left and right end seats. There is a small gap between the rotor and between the rotor and the cylinder and the end seat. The suction end seat and the upper part of the cylinder are provided with axial and radial suction orifices, and the exhaust end seat and the sliding valve are respectively provided with axial and radial exhaust orifices. The suction and discharge orifices of the compressor are carefully designed according to the needs of its working process, and the working volume can be accurately connected or separated from the suction and discharge chambers according to the needs. 　　1.2 Working principle 　　The work of the screw compressor is accomplished by means of a male rotor and a female rotor that are in meshing motion, and with the help of the space of the inner wall of the casing surrounding the pair of rotors. When the rotor rotates, the volume between a pair of teeth in the shape of a "V" formed by the rotor teeth, tooth slots and the inner wall of the casing is called the elementary volume. The axial direction of the rotor moves from the suction port side to the exhaust port side, and the refrigerant gas is sucked in and compressed to a certain pressure before being discharged. 1.3 Working process 　　 Schematic diagram of the working process of a screw refrigeration compressor. Among them, a, b, and c are the views from the suction side of the rotor (usually above the rotor), indicating the process of the primitive volume from the start of the suction to the end of the suction; d, e, f are from the exhaust side of the rotor (generally above the rotor) The view below the rotor shows the process from the beginning of compression to the end of exhaust. On the suction side of the two rotors (the upper part of the rotor shown in a, b, and c in the figure), the volume of each element between the contact line of the tooth surface and the suction end is expanding, and on the exhaust side of the rotor ( The upper part of the rotor shown in d, e, and f), the volume of the element between the tooth surface contact line and the exhaust end gradually decreases. In this way, the volume of each elementary element is moved from the suction end to the exhaust end. The following describes the working process of the screw refrigeration compressor with the volume of a certain V-shaped element shown in Figure 6-2. 　　1. Inhalation process The volume of the element between the teeth gradually expands as the rotor rotates and is connected to the suction orifice. The gas enters the volume of the element between the teeth through the suction orifice, which is called an inhalation process. When the rotor rotates at a certain angle, the volume of the element between the teeth crosses the position of the suction port and is disconnected from the suction port, and the suction process ends. It is worth noting that at this time, the volume of the elementary elements between the teeth of the female and male rotors are not connected to each other. 　　2. Compression process At the beginning of compression, the volume of the element between the teeth of the driving rotor and the volume of the element between the teeth of the driven rotor advance forward in isolation from each other, which is called the transfer process. The rotor continues to rotate through a certain angle, and the protruding teeth of the driving rotor and the cogging grooves of the driven rotor form a pair of new V-shaped elementary volumes. With the meshing movement of the two rotors, the elementary volume gradually decreases to achieve gas compression. process. The compression process is until the moment when the volume of the elementary element communicates with the discharge orifice, at which point the discharge process begins. 　　3. Exhaust process As the volume of the elementary element keeps shrinking when the rotor rotates, the compressed gas with a certain pressure is sent to the exhaust chamber, and this process continues until the volume is minimum. 　　With the continuous rotation of the rotor, the above-mentioned suction, compression, and discharge processes are carried out cyclically, and the volume of each elementary element works one after another, forming the working cycle of the screw refrigeration compressor. It can be seen from the above that the side where the two rotors turn to meet, the gas is compressed, is called the high pressure zone; on the other side, the rotors are separated from each other, and the primitive volume between the teeth sucks in the gas, which is called the low pressure zone. The high pressure zone and the low pressure zone are separated by the contact line between the two rotor tooth surfaces. In addition, since the gas in the suction element volume rotates with the rotor, it spirals from the suction end to the exhaust end. Therefore, the suction and discharge ports of the screw refrigeration compressor are arranged diagonally. 1.4 Internal volume ratio and additional power loss 　　1. Inner volume ratio The volume of the element between the teeth of the rotor is compressed with the reduction of the rotating volume of the screw until the volume of the element communicates with the edge of the exhaust port. This process is called the internal compression process. The maximum volume of the primitive volume at the end of suction is V1, the corresponding gas pressure is the suction pressure p1, the volume at the end of internal compression is V2, and the corresponding gas pressure is the final pressure of internal compression p2. The ratio of the maximum volume V1 of the variable element volume at the end of suction to the volume V2 at the end of internal compression is called the internal volume ratio ev of the screw refrigeration compressor. That is, the screw refrigeration compressor is a positive displacement compressor with no air valve. The opening and closing of the suction and discharge ports is completely determined by the geometric structure to control the suction, compression, discharge and required internal compression pressure. Because its structure has been determined, it has a fixed internal volume ratio, which is very different from piston refrigeration compressors. 　　 The gas pressure at the end of compression of the piston refrigeration compressor depends on the gas pressure in the discharge chamber and the resistance loss of the discharge valve. If the resistance loss of the gas valve is omitted, it can be approximated that the pressure at the end of compression of the piston refrigeration compressor is equal to the gas pressure in the discharge chamber. The final compression pressure p2 in the screw refrigeration compressor is related to the geometry of the rotor, the position of the exhaust port, the suction pressure p1 and the type of gas, but has nothing to do with the gas pressure pd in the discharge chamber. The final internal compression pressure p2 is related to the suction pressure The ratio of p1 is called the internal pressure ratio ei. That is, the variable index of the compression process in the formula. The gas pressure (back pressure) pd in the exhaust chamber is called the external pressure, and its ratio to the suction pressure p1 is called the external pressure ratio e. The external pressure ratio and internal pressure ratio of screw refrigeration compressors can be equal or unequal, depending on whether the operating conditions of the compressor are the same as the design conditions. The internal pressure ratio depends on the position of the orifice, while the external pressure ratio depends on the operating conditions. Generally, the internal pressure ratio should be equal to or close to the external pressure ratio so that the compressor can obtain higher efficiency. Additional work loss. When the end pressure of internal compression p2 and the gas pressure pd in the exhaust chamber are not equal, when the element volume is connected to the exhaust orifice, the gas in the element volume will undergo constant volume compression or constant volume expansion to make the gas The pressure and the exhaust chamber pressure pd tend to balance, resulting in additional work loss.