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 inertial force, good dynamic balance performance, stable operation, small base vibration, and small foundation.
3. The screw refrigeration compressor has a simple structure and a small number of parts. There are no wearing parts such as air valves and piston rings. Its main friction parts, such as rotors and bearings, have relatively high strength and wear resistance, and the lubrication conditions Good, so the machining amount is less, the material consumption is low, the operation cycle is long, the use is relatively 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 gas 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, the efficiency can still be kept high.
5. The slide valve is used for adjustment, which can realize stepless adjustment of energy.
6. The screw compressor is not sensitive to liquid inlet, and can be cooled by oil injection, so 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.
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 air, while the screw air compressor does not have an intake and exhaust valve group, and the intake air is only regulated by the opening and closing of a regulating valve. When the rotor rotates, the tooth groove space of the main and auxiliary rotors is the largest when it reaches the opening of the intake end wall. The air is completely exhausted, and when the exhaust is over, the tooth groove is in a vacuum state. When it turns 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 reminder When the air fills the entire tooth groove, the end surface of the intake side of the rotor turns away from the air inlet of the casing, and the air between the tooth grooves is sealed.
2. Closing and conveying process:
When the main and auxiliary rotors are inhaled, the tooth peaks of the main and auxiliary rotors are sealed with the casing, and the air is sealed in the tooth grooves and no longer flows out, that is, [the sealing process]. The two rotors continue to rotate, and the tooth crests and tooth grooves match at the suction end, and the matching surfaces gradually move toward the exhaust end.
3. Compression and fuel injection process:
During the conveying process, the meshing surface gradually moves toward 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]. While compressing, lubricating oil is also sprayed into the compression chamber due to the pressure difference to mix with the chamber air.
4. Exhaust process:
When the meshing end surface of the rotor turns to communicate with the casing exhaust, (the pressure of the compressed gas is the highest at this time), the compressed gas starts to be discharged until the meshing surface of the tooth crest and the tooth groove moves to the exhaust At this time, the 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 rotors and the air inlet of the casing reaches the maximum. Long, its 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-wall structure with exhaust channels inside, which has high strength and good noise reduction effect; the internal and external forces of the body are basically balanced, without open or semi-closed Withstand the risk of high pressure; the shell is a steel structure with high strength, beautiful appearance and light weight. Adopt vertical structure, the compressor occupies a small area, which is beneficial to the multi-head arrangement of the chiller; the lower bearing is immersed in the oil tank, and the bearing is well lubricated; the axial force of the rotor is reduced by 50% compared with the semi-closed and open type (the motor shaft on the exhaust side Balance function); no risk of horizontal motor cantilever, high reliability; avoid the influence of screw rotor, slide valve, motor rotor self-weight on the matching accuracy, improve reliability; good assembly process. Oil-free pump screw vertical design, so that there will be no 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 adopts differential pressure oil supply; the requirement for differential pressure of the system is low, and it has the function of bearing lubrication protection in case of emergency, avoiding the lack of oil lubrication of the bearing, which is conducive to the start-up of the unit in transitional seasons.
Disadvantages: exhaust cooling is adopted, and the motor is at the exhaust port, which may easily cause the motor coil to burn out; in addition, it cannot be eliminated in time when a fault occurs.
2. Semi-hermetic screw compressor
The motor is cooled by liquid spray, the working temperature of the motor is low, and the service life is long; the open compressor uses air-cooled motor, the working temperature of the motor is high, which affects the life of the motor, and the working environment of the machine room is poor; the motor is cooled by exhaust gas, the working temperature of the motor is very high, Motor life is short. Generally, the external oil separator has a large volume, but its efficiency is very high; the built-in oil separator is combined with the compressor, and its volume is small, so the effect is relatively poor. The oil separation effect of the secondary oil separation can reach 99.999%, which can ensure good lubrication of the compressor under various working conditions.
However, the plunger-type semi-hermetic screw compressor speeds up through gear transmission, the speed is high (about 12,000 rpm), the wear is large, and the reliability is poor.
3. Open screw compressor
The advantages of the open unit are:
1) The compressor is separated from the motor, so that the compressor can be used in a wider range;
2) The same compressor can be used with different refrigerants. In addition to using halogenated hydrocarbon refrigerants, ammonia can also be used as refrigerants by changing the materials of some parts;
3) Motors with different capacities can be equipped according to different refrigerants and operating conditions.
4) The open type is also divided into single-screw and twin-screw
The single-screw compressor consists of a cylindrical screw and two symmetrically arranged plane star wheels, which are installed in the casing. The screw groove, the casing (cylinder) inner wall and the star gear teeth form a closed volume. The power is transmitted to the screw shaft, and the star wheel is driven by the screw to rotate. The gas (working fluid) 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 the reciprocating piston compressor. When the teeth of the star wheel move relatively in the screw groove, the closed volume gradually decreases and the gas is compressed.
Working principle of screw compressor and comparison of fully enclosed, semi-hermetic and open types
The screw of the single-screw compressor has 6 screw grooves, and the star wheel has 11 teeth, which is equivalent to 6 cylinders. 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.
As we all know, 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 annual growth rate of screw compressor sales in the world was 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-screw compressors account for more than 80% of the entire screw compressor market due to their 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 the star wheel of the single-screw compressor belong to a pair of spherical worm pairs, and the screw shaft and the star wheel shaft must be kept vertical in space; the female and male rotors of the twin-screw compressor are equivalent to a pair of gear pairs, and the male and female rotor shafts are kept parallel. . Structurally speaking, the cooperation accuracy between the screw and the star wheel of the single-screw compressor is difficult to guarantee, so the reliability of the whole machine is lower than that of the twin-screw.
2. Drive mode
Both types of compressors can be directly connected to the motor or driven by a belt pulley. When the speed of the twin-screw compressor is high, the speed-up gear needs to be increased.
3. Cooling capacity adjustment method
The air volume adjustment methods of the two compressors are basically the same, both of which can adopt continuous adjustment of the slide valve or stepwise adjustment of 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 decreases.
4. Manufacturing cost
Single-screw compressor: Ordinary bearings can be used for screw and star wheel bearings, and the manufacturing cost is relatively low.
Twin-screw compressor: Due to the relatively large load on the two-screw rotors, it is required to use high-precision bearings, and the manufacturing cost is relatively high.
5. Reliability
Single-screw compressor: The star wheel of the single-screw compressor is a vulnerable part. In addition to the high requirements for the material of the star wheel, the star wheel needs to be replaced regularly.
Twin-screw compressor: There are no wearing parts in the twin-screw compressor, and the trouble-free running 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 axial and radial position accuracy requirements are very high, so the assembly and maintenance convenience of the single-screw compressor is lower than that of the twin-screw compressor.
The main disadvantages of the open 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 high speed, the airflow noise is large, and the noise of the compressor itself is also relatively large, which affects the environment;
(3) Complex oil system components such as separate oil separators and oil coolers need to be configured, and the unit is bulky and inconvenient to use and maintain.
Four, three screw compressor
The unique geometric structure of the three-rotor determines that it has a lower leakage rate than the double-rotor compressor; the three-rotor screw compressor can greatly reduce the load on the bearing; the reduction of the bearing load increases the exhaust area, Thereby improving efficiency; it is very important to reduce unit leakage under any load condition, especially when operating under partial load condition, the impact is even greater.
Load self-regulation: When the system changes, the sensor responds quickly, and the controller performs related calculations, so as to quickly and correctly self-regulate; self-regulation is not limited by actuators, guide vanes, solenoid valves and slide valves, and can be directly, quickly and reliably performed .
Post time: Feb-10-2023
