The working principle of screw compressor and comparison of fully enclosed, semi-closed and open type

The core problemThe working principle of screw compressor and comparison of fully enclosed, semi-closed and open type

1. Compared with reciprocating piston refrigeration compressors, screw refrigeration compressors have a series of advantages such as high speed, light weight, small volume, small footprint and low exhaust pulsation.

　　2. The screw refrigeration compressor has no reciprocating mass inertia force, good dynamic balance performance, stable operation, small vibration of the base, and a small foundation.

The working principle of screw compressor and comparison of fully enclosed, semi-closed and open type

3. The screw refrigeration compressor has a simple structure, a small number of parts, no vulnerable parts like gas valves, piston rings, etc. Its main friction parts such as rotors, bearings, etc., have relatively high strength and wear resistance, and lubrication conditions Good, so the machining volume is small, the material consumption is low, the operation cycle is long, the use is more reliable, the maintenance is simple, and it is beneficial to realize the automation of operation.

4. Compared with the speed compressor, the screw compressor has the characteristics of forced air delivery, that is, the displacement is almost not affected by the discharge pressure, and there is no surge phenomenon when the displacement is small. Within the range of conditions, high efficiency can still be maintained.

　　5. The slide valve is used to adjust, which can realize stepless adjustment of energy.

　　6. ​​Screw compressors are not sensitive to liquid intake and can be cooled by oil injection. Therefore, under the same pressure ratio, the exhaust temperature is much lower than that of the piston type, so the single-stage pressure ratio is higher.

　　7. There is no clearance volume, so the volumetric efficiency is high.

　　The working principle and structure of screw compressor:

　　 1. Inhalation process:

The suction port on the intake side of the screw type must be designed so that the compression chamber can fully inhale, while the screw type air compressor has no intake and exhaust valve groups. The intake only depends on the opening and closing of a regulating valve. When the rotor rotates, the tooth groove space of the main and auxiliary rotors has the largest space when it turns to the opening of the inlet end wall. At this time, the tooth groove space of the rotor communicates with the free air of the inlet, because the tooth groove space is The air is completely exhausted. When the exhaust is completed, the tooth groove is in a vacuum state. When it is turned to the air inlet, the outside air is sucked in and flows into the tooth groove of the main and auxiliary rotors along the axial direction. Screw air compressor maintenance reminds that when the air fills the entire tooth groove, the air inlet side end surface of the rotor turns away from the air inlet of the casing, and the air between the tooth grooves is closed.

　　 2. The process of sealing and conveying:

　　 At the end of the suction of the main and auxiliary rotors, the peaks of the main and auxiliary rotors are sealed with the casing. At this time, the air is closed in the grooves and no longer flows out, that is, [close process]. The two rotors continue to rotate, and their tooth peaks and grooves coincide at the suction end, and the anastomotic surface gradually moves to the exhaust end.

　　3. Compression and injection process:

　　 During the conveying process, the meshing surface gradually moves to the exhaust end, that is, the tooth groove between the meshing surface and the exhaust port gradually decreases, the gas in the tooth groove is gradually compressed, and the pressure increases, which is the [compression process]. At the same time of compression, the lubricating oil is injected into the compression chamber to mix with the chamber air due to the pressure difference.

　　 4. Exhaust process:

When the meshing end surface of the rotor of the screw air compressor is turned to communicate with the exhaust of the casing, (the pressure of the compressed gas is the highest at this time) the compressed gas begins to be discharged until the meshing surface of the tooth peak and the tooth groove moves to the exhaust At this time, the tooth groove space between the meshing surface of the two rotors and the exhaust port of the casing is zero, that is, the exhaust process is completed. At the same time, the length of the tooth groove between the meshing surface of the rotor and the intake port of the casing reaches the maximum Long, the inhalation process is going on again.

　　 1. Fully enclosed screw compressor:

The body adopts a high-quality, low-porosity cast iron structure with small thermal deformation; the body adopts a double-walled structure with exhaust channels inside, which has high strength and good noise reduction effect; the forces inside and outside the body are basically balanced, without opening, semi-closed Withstand the risk of high pressure; the shell is made of steel, with high strength, beautiful appearance and light weight. Using vertical structure, the compressor occupies a small area, which is conducive to the arrangement of multiple heads of the chiller; the lower bearing is immersed in the oil tank, and the bearing is lubricated; the axial force of the rotor is reduced by 50% compared with the semi-closed and open type (motor shaft on the exhaust side) There is no risk of the horizontal motor cantilever, and the reliability is high; it avoids the influence of the screw rotor, the slide valve, and the motor rotor on the matching accuracy, and improves the reliability; the assembly process is good. The oil-free pump screw vertical design prevents oil shortage when the compressor is running or shutting down. The lower bearing is immersed in the oil tank as a whole, and the upper bearing uses differential pressure to supply oil; the system has low pressure differential requirements. In emergency situations, the bearing lubrication protection function can prevent the bearing from lacking lubrication, which is beneficial to the opening of the unit during the transition season.

　　 Disadvantages: the use of exhaust cooling, the motor is at the exhaust port, it is easy to cause the motor coil to burn out; in addition, failures cannot be eliminated in time.

　　 Two, semi-hermetic screw compressor

The motor is cooled by spraying liquid, and the motor has a low operating temperature and a long life; the open compressor uses air to cool the motor, and the motor has a higher operating temperature, which affects the life of the motor, and the working environment of the machine room is poor; if the exhaust cooling motor is used, the motor has a high operating temperature. The motor life is short. Generally, the external oil separator has a large volume, but the efficiency is high; the built-in oil separator is combined with the compressor, and the volume is small, so the effect is relatively poor. The secondary oil separation effect can reach 99.999%, which can ensure the compressor has good lubrication under various working conditions.

　　 But the plunger type semi-hermetic screw compressor is increased by gear transmission, the speed is high (about 12000 rpm), the wear is large, and the reliability is poor.

　　 Three, open type screw compressor

The advantages of

　　 open type unit are:

　　1) The compressor is separated from the motor, which makes the compressor applicable to a wider range;

　　2) The same compressor can be applied to different refrigerants. In addition to halogenated hydrocarbon refrigerants, ammonia can also be used as a refrigerant by changing the materials of some parts;

　　3) Motors of different capacities can be equipped according to different refrigerants and operating conditions.

　　4) Open type is also divided into single screw and twin screw

　　Single-screw compressor consists of a cylindrical screw and two symmetrically arranged plane planetary gears which form a meshing pair, which is installed in the casing. The screw groove, the inner wall of the casing (cylinder) and the star gear constitute a closed volume. The power is transmitted to the screw shaft, and the screw drives the star wheel to rotate. The gas (working medium) enters the screw groove from the suction chamber, and is discharged through the exhaust port and the exhaust chamber after being compressed. The role of the star wheel is equivalent to the piston of a reciprocating piston compressor. When the star wheel teeth move relative to each other in the screw groove, the enclosed volume is gradually reduced and the gas is compressed.

　　The working principle of screw compressor and comparison of fully enclosed, semi-closed and open type

　　The screw of the single screw compressor has 6 screw grooves, the star wheel has 11 teeth, which is equivalent to 6 cylinders, and the two star wheels mesh with the screw grooves at the same time. Therefore, each rotation of the screw is equivalent to 12 cylinders working.

　　 It is well known that screw compressors (including twin screw and single screw) account for the largest proportion of rotary compressors. From the perspective of the international market, during the 20 years from 1963 to 1983, the world's screw compressor sales grew at an annual rate of 30%. At present, twin screw compressors account for 80% of medium capacity compressors in Japan, Europe and the United States. As single-screw compressors and twin-screw compressors within the same working range, in comparison, twin-screws account for more than 80% of the entire screw compressor market due to its good processing technology and high reliability. Screw compressors account for less than 20%. The following is a brief comparison of the two compressors.

　　1. Structure

The screw and star of a single-screw compressor belong to a pair of spherical worm pairs, and the screw shaft and the star wheel shaft must be spaced perpendicularly; the female and male rotors of a twin-screw compressor are equivalent to a pair of gear pairs, and the female and male rotor shafts are kept parallel. . From the structural point of view, it is difficult to ensure the matching accuracy between the screw and the star wheel of the single-screw compressor, so the operating reliability of the whole machine is lower than that of the twin-screw.

　　2. Drive method

　　 Both compressors can be directly connected to the motor or driven by a pulley. When the speed of the twin-screw compressor is high, the speed increasing gear is required.

　　3. Cooling capacity adjustment method

　　The air volume adjustment methods of the two compressors are basically the same, and both can be adjusted continuously by the slide valve or stepwise adjustment by the plunger. When the slide valve is used for adjustment, the twin-screw compressor needs one slide valve, while the single-screw compressor needs two slide valves at the same time, so the structure becomes complicated and the reliability is also reduced.

　　 4. Manufacturing cost

　　Single screw compressor: ordinary bearings can be used for screw and star wheel bearings, and the manufacturing cost is lower.

　　 Twin-screw compressor: Due to the relatively large load of the two-screw rotors, bearings with higher precision are required, and the manufacturing cost is higher.

　　 5. Reliability

　　Single screw compressor: The star wheel of the single screw compressor is a vulnerable part. In addition to the higher requirements for the material of the star wheel, the star wheel needs to be replaced regularly.

　　 Twin screw compressor: There are no vulnerable parts in the twin screw compressor, and the trouble-free operation time can reach 40,000 to 80,000 hours.

　　6. ​​Assembly and maintenance

　　 Since the screw shaft and the star wheel shaft of the single-screw compressor must be kept vertical in space, the accuracy of the axial and radial positions of the single-screw compressor is very high, so the convenience of assembly and maintenance is lower than that of the twin-screw compressor.

　　The main disadvantages of the open-type unit are:

　　 (1) The shaft seal is easy to leak, which is also the object of frequent maintenance by users;

　　 (2) The equipped motor rotates at a high speed, and the airflow noise is large, and the compressor itself is also noisy, which affects the environment;

　　 (3) It is necessary to configure a separate oil separator, oil cooler and other complicated oil system components. The unit is bulky and inconvenient to use and maintain.

　　Four and three screw compressors

The unique geometric structure of the three-rotor determines that it has a lower leakage rate than the compressor with a dual-rotor design; the three-rotor screw compressor can greatly reduce the load on the bearing; the reduction of the bearing load makes the exhaust area larger, So as to improve efficiency; it is very important to reduce unit leakage under any load condition, especially when it is operating under partial load, the impact is even greater.

Load self-regulation: When the system changes, the sensor responds quickly, and the controller performs relevant calculations to quickly and correctly self-regulate; self-regulation is not restricted by actuators, guide vanes, solenoid valves and slide valves, and can be directly executed quickly and reliably .