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

Introduction to screw refrigeration compressors and precautions for use (technical sharing) Screw refrigeration compressors and piston refrigeration compressors have the same gas compression method, and both belong to volumetric compressors, which means that they all compress the gas by the change in volume. The difference is that the two compressors have different ways to achieve a change in 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 body to mesh and rotate and cooperate with the inner wall of the body and the inner wall of the suction and exhaust end seats, resulting in a change in the volume between the teeth to complete the gas Suction, compression and discharge process. 　　 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 units 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 shape and other structures, 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, at present, the screw-type refrigeration compressor has become an advanced refrigeration compressor, especially the oil-injected screw compressor has become one of the main types of refrigeration compressors, and has been widely used. 　　 Screw refrigeration compressor is mainly composed of casing, rotor, bearing, shaft seal, balance piston and energy adjustment device. 　　Chassis: It is generally a split type, which is composed of three parts of the body, the suction end seat and the exhaust end seat, which are connected by bolts. The cross-section of the inner cavity of the machine body is a horizontal figure 8 intersecting with a double circle, which is suitable for the outer cylindrical surfaces of the two meshing rotors placed inside. 　　 Rotor is a pair of intermeshing screws with special spiral tooth shape on it. Among them, the convex tooth shape is called a male screw (or male rotor), and the concave tooth shape is called a female screw (or female rotor). The gear ratio of the male screw and the female screw is generally 4:6 (the gear ratio of the compressor with a large 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 and is called the active rotor. The female screw is the driven rotor. Therefore, the male screw is usually four-headed right-handed, and the female screw is mostly six-headed left-handed. In order to serialize the screw-type refrigeration compressors, standardize and generalize the parts, China's relevant departments stipulate that the nominal diameter of the screw is 63, 80, 100, 125, 160, 200 and 315mm, and the length-diameter ratio is divided into λ=1.0 and λ=1.5 two kinds. 　　 Bearings and radial 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, and the thrust bearing is equipped with two each on the exhaust side male and female screw rods to withstand a certain internal force of the shaft. The shaft seals of screw refrigeration compressors also mostly use friction ring mechanical sealers, which are installed on the end shaft of the active rotor against the coupling. The structure and principle are the same as the shaft seals of piston refrigeration compressors. 　　Balanced piston: due to structural differences, due to the pressure difference between the suction and exhaust sides, the axial resultant force acting on the male screw is much greater than the axial resultant force acting on the female screw. Therefore, in addition to the thrust bearing, a hydraulic balancing piston is added to the male screw to reduce the load on the end face of the sliding bearing of the male screw spool and reduce the axial force that the thrust bearing bears. 　　Energy adjustment device: It is composed of spool valve, oil cylinder, oil piston, four-way electromagnetic directional valve and oil pipeline. The piston is installed at the intersection of two circles in the lower part of the cylinder wall, and the position of the slide valve can be changed to adjust the cooling capacity. 　　When the screw-type refrigeration compressor works, the volume of the element between the teeth changes periodically, so that the steam body completes the process of steam absorption, compression and exhaust during the axial movement of the rotor. 　　Safe operation Ready to work: 　　 1. Check the refrigerant, water and electrical equipment system should be normal; 2. The rotation of the test motor should not be reversed because of the screw compressor. For this purpose, the motor can be tested after removing the rubber rotating core of the coupling. The motor steering 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 to start the oil pump to make the oil cooler full of oil, and the oil level gauge indicates normal; 　　 4. Check whether all the pressure gauge valves are open and whether the thermometer socket is filled with lubricating oil; 　　 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 pressure difference of the fine oil filter does not exceed 0.1Mpa. Turn the coupling by hand, and operate the energy regulating valve at the same time, so that the unloading indication is 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. Water supply to the oil cooler, the amount of water depends on the oil temperature (the oil injection temperature is preferably 40℃-55℃); 　　 10. Close the main motor power supply 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 intermediate air supplementation are closed. Press the joint start button, the oil pump starts first, the host starts when the oil pressure reaches normal, and the suction cut-off valve is opened (when suction When the pressure of the gas system is high, it should be opened slowly to avoid excessive load). First, it should be driven to run, it should not be run for too long, stop in about 3-5 minutes and observe whether it is operating normally; 　　 2. The energy adjustment indicator runs at 0% position for 30 minutes, and observes the running status; 3. When the compressor is running normally, open the energy regulating valve, and gradually load from 0% to 100%. When the pressure ratio of the evaporation pressure and the condensing pressure increases, then open the valves of the intermediate air supply to check the working conditions of each part. Whether it is normal and reliable. 　　 (1) When the pressure difference between suction and discharge is large, and the injection pressure (that is, the difference between the oil pressure after the refined oil filtration and the discharge pressure) is small, the oil pressure should be appropriately increased by the oil pressure adjustment valve; 　　 (2) If the oil level gauge is found to be leaking, it should be repaired after stopping. The upper and lower valves of the oil level gauge should be in the fully open position when working, otherwise the valve will lose its safety protection. 　　Operation check: 　　 1. Check the pressure of suction, exhaust and oil, the temperature is within the specified range; 　　 2. Observe the opening status of all gas valves, oil valves and pipe systems, and check for leaks. A small amount of oil dripping is allowed on the compressor and oil pump shaft seal; 　　3. Observe the vibration and sound of machine operation; 　　 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 joint stop button to stop the host and the oil pump, and close the exhaust shut-off valve. After the pressure is equalized, the suction cut-off valve is closed, the oil return valve from the compressor to the oil cooler is opened, and the cooling water is closed. work process 　　1.1 Basic structure The basic structure of 　　screw refrigeration compressor is mainly composed of rotor, casing (including the middle cylinder block and suction and exhaust end seats at both ends), bearings, shaft seals, balance pistons and energy adjustment devices. The two rotors rotating in reverse according to a certain transmission ratio and meshing with each other are arranged in parallel in a cylinder with an "&" shape. 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. The left and right sides of the cylinder have suction end seats and exhaust end seats, and a pair of rotors are supported on bearings on 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 holes, and the exhaust end seat and the slide valve are provided with axial and radial exhaust holes, respectively. The suction and exhaust orifices of the compressor are carefully designed according to the needs of their working process, and the working volume can be accurately connected or partitioned with the suction and exhaust chambers according to the needs. 　　1.2 Working principle 　　Screw compressor work is completed by meshing a male rotor and a female rotor, and by means of the space surrounding the inner wall of the casing surrounding the pair of rotors. When the rotor rotates, the pair of inter-tooth volumes in the shape of "V" formed by the teeth, cogging of the rotor and the inner wall of the casing is called the elementary volume, and its volume will change periodically. 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 and then discharged. 　　1.3 Working process 　　 Schematic diagram of the working process of screw refrigeration compressor. Among them, a, b, c are views from the suction side of the rotor (generally above the rotor), showing the process of the elementary volume from the beginning of suction to the end of suction; d, e, f are from the exhaust side of the rotor (generally (Below the rotor) The view shows the process of the elementary volume 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 tooth surface contact line and the suction end is expanding, and on the exhaust side of the rotor ( In the figure, the upper part of the rotor shown in d, e, and f), the elementary volume between the contact surface of the tooth surface and the exhaust end gradually decreases. In this way, each elementary volume is moved from the suction end to the exhaust end. Next, the working process of the screw-type refrigeration compressor will be described with the volume of a certain V-shaped element shown in Fig. 6-2. 　　 1. Inhalation process The volume of the interdental element gradually expands as the rotor rotates, and communicates with the suction orifice. The gas enters the interdental element volume through the suction orifice and is called the inhalation process. When the rotor rotates by a certain angle, the volume of the element between the teeth crosses the position of the suction orifice and is disconnected from the suction orifice, and the suction process ends. It is worth noting that at this time, the basic element volumes of the female and male rotors are not connected to each other. 　　 2. Compression process In the initial stage of compression, the volume of the inter-tooth element of the active rotor and the volume of the inter-tooth element of the driven rotor move forward in isolation from each other, which is called the transfer process. The rotor continues to rotate through a certain angle, the convex teeth of the active rotor and the tooth slots of the driven rotor form a new pair of V-shaped elementary volumes. With the meshing movement of the two rotors, the elementary volume gradually decreases, achieving gas compression process. The compression process until the moment when the cell volume is connected with the discharge orifice, the exhaust process begins at this moment. 　　 3. Exhaust process As the rotor rotates, the cell volume continues to shrink, and the compressed gas with a certain pressure is sent to the exhaust chamber. This process continues until the minimum volume. 　　 With the continuous rotation of the rotor, the above-mentioned suction, compression, and exhaust processes are cyclically performed, and the volume of each elementary element works sequentially, which constitutes the working cycle of the screw refrigeration compressor. As can be seen from the above, the side where the two rotors turn to meet each other, the gas is compressed, which is called the high pressure region; on the other side, the rotors are separated from each other, and the elementary volume between the teeth sucks the gas, called the low pressure region. The high pressure area and the low pressure area are separated by the contact line between the two rotor tooth surfaces. In addition, since the gas in the volume of the suction element rotates with the rotor, it spirally moves from the suction end to the exhaust end. Therefore, the suction and discharge orifices of the screw refrigeration compressor are arranged diagonally. 　　1.4 Content ratio and additional work loss 　　 1. Internal volume ratio The volume of the inter-tooth element of the rotor is compressed as the rotation volume of the screw decreases, until the volume of the element communicates with the edge of the exhaust orifice. This process is called internal compression. The maximum volume of the elementary 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 pressure at the end of internal compression p2. The ratio of the maximum volume V1 of the variable cell 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-type refrigeration compressor is a positive displacement compressor without an air valve. The opening and closing of the suction and exhaust ports are completely determined by the geometric structure to control the suction, compression, exhaust and the required internal compression pressure. Since its structure is fixed, it has a fixed internal volume ratio, which is very different from the piston refrigeration compressor. The gas pressure at the end of the compression of the piston refrigeration compressor depends on the gas pressure in the exhaust chamber and the resistance loss of the exhaust valve. If the resistance loss of the gas valve is omitted, it can be approximated that the pressure at the end of the 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 rotor geometry, the position of the exhaust orifice, the suction pressure p1 and the type of gas, and has nothing to do with the gas pressure pd in the exhaust chamber. The final compression pressure p2 and 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 the ratio of it 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 may be unequal, which depends entirely on whether the compressor's operating conditions and design conditions are the same. 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 and the external pressure ratio should be equal or close to make the compressor obtain higher efficiency. Additional work loss When the final pressure of internal compression p2 is not equal to the gas pressure pd in the exhaust chamber, when the cell volume communicates with the exhaust orifice, the gas in the cell 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, thereby generating additional work loss.

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