How to realize stepless air volume adjustment in screw compressor
1. Characteristics of screw compressor
Screw compressors are composed of a pair of parallel, intermeshing female and male screws. They are widely used in medium and large refrigeration systems or process gas compressors in refining and chemical plants. Screw compression is divided into two types: single screw and twin screw. The screw compressor usually refers to the twin screw compressor. Screw compressors have the following characteristics:
(1) The screw compressor has a simple structure and a small number of parts. There are no wearing parts such as valves, piston rings, rotors, bearings, etc., and its strength and wear resistance are relatively high.
(2) The screw compressor has the characteristics of forced gas transmission, that is, the exhaust volume is almost not affected by the exhaust pressure, no surge occurs when the exhaust volume is small, and it can still maintain the pressure in a wide range of working conditions. Higher efficiency.
(3) The screw compressor is not very sensitive to liquid hammer and can be cooled by oil injection. Therefore, under the same pressure ratio, the discharge temperature is much lower than that of the piston type, so the single-stage pressure ratio is high.
(4) The slide valve adjustment is adopted to realize stepless adjustment of energy.
2. Principle of slide valve adjustment of screw compressor
The slide valve is used for stepless control of capacity. During normal startup, this component is not loaded. The slide valve is controlled by the micro control panel through oil pressure, ultimately changing the working capacity of the compressor.
The capacity adjustment slide valve is a structural component used to adjust the volume flow in a screw compressor. Although there are many methods for adjusting the volume flow of a screw compressor, the adjustment method using a slide valve has been widely used, especially in injection molding. Oil screw refrigeration and process compressors are particularly popular. As shown in Figure 1, this adjustment method is to install an adjustment slide valve on the screw compressor body and become a part of the compressor body. It is located at the intersection of the two inner circles on the high-pressure side of the body and can move back and forth in a direction parallel to the cylinder axis.
The principle of the slide valve to adjust the volumetric flow rate of the screw compressor is based on the working process characteristics of the screw compressor. In a screw compressor, as the rotor rotates, the pressure of the compressed gas gradually increases along the axis of the rotor. In terms of spatial position, it gradually moves from the suction end of the compressor to the discharge end. After the high-pressure side of the body opens, when the two rotors begin to mesh and try to increase the gas pressure, some of the gas will bypass through the opening. Obviously, the amount of gas bypassed is related to the length of the opening. When the contact line moves to the end of the opening, the remaining gas is completely enclosed, and the internal compression process begins at this point. The work done by the screw compressor on the bypass gas from the opening is only used to discharge it. Therefore, the power consumption of the compressor is mainly the sum of the work done to compress the finally discharged gas and the mechanical friction work. Therefore, when the capacity adjustment slide valve is used to adjust the volumetric flow rate of the screw compressor, the compressor can maintain a high efficiency under the adjustment condition.
In actual compressors, it is generally not a hole in the casing, but a porous structure. The slide valve moves in a groove under the rotor and allows continuous adjustment of the size of the opening. The gas discharged from the opening will return to the suction port of the compressor. Since the compressor actually does no work on this part of the gas, its temperature does not rise, so it does not need to be cooled before it reaches the mainstream gas at the suction port. .
The slide valve can move in any direction according to the requirements of the control system. There are many ways to drive it. The most common method is to use a hydraulic cylinder, and the oil system of the screw compressor itself provides the required oil pressure. In a few machines, the slide valve is driven by a reduced motor.
Theoretically, the spool should be the same length as the rotor. Similarly, the distance required for the slide valve to move from full load to empty load needs to be the same as the rotor, and the hydraulic cylinder should also have the same length. However, practice has proven that even if the length of the slide valve is slightly shorter, good regulating characteristics can still be achieved. This is because when the bypass opening first opens near the suction end face, its area is very small, the pressure of the gas is very small at this time, and the time it takes for the rotor meshing teeth to sweep through the opening is also very short, so there will be only a small amount of Some of the gas is discharged. Therefore, the actual length of the slide valve can be reduced to about 70% of the length of the rotor working section, and the remaining part is made fixed, thus reducing the overall size of the compressor.
The characteristics of the capacity adjustment slide valve will vary with the diameter of the rotor. This is because the area of the bypass port caused by the movement of the slide valve is proportional to the square of the rotor diameter, while the volume of the gas in the compression chamber is proportional to the diameter of the rotor. Proportional to the cube of . It is worth noting that when the compressor compresses the gas, it also increases the pressure of the injected oil, and eventually discharges it together with the gas. In order for the oil to be discharged continuously, a certain exhaust volume must be reserved. Otherwise, under completely no-load conditions, oil will accumulate in the compression chamber, causing the air compressor to be unable to continue to operate. In order for oil to be continuously discharged, a volume flow rate of at least about 10% is usually required. In some cases, the volumetric flow rate of the compressor must be zero. At this time, a bypass pipe is usually arranged between the suction and exhaust. When a complete zero load is required, the bypass pipe is opened to connect the suction and exhaust. .
When using a capacity adjustment slide valve to adjust the volumetric flow of a screw compressor, the ideal situation is to keep the internal pressure ratio the same as at full load during the adjustment process. However, it is obvious that when the slide valve moves and the volumetric flow rate of the compressor becomes smaller, the effective working length of the screw becomes smaller and the time of the internal compression process also becomes smaller, so the internal pressure ratio must be reduced.
In the actual design, the slide valve is equipped with a radial exhaust hole, which moves axially with the slide valve. In this way, on the one hand, the effective length of the screw machine rotor is reduced, and on the other hand, the radial exhaust orifice is also reduced, so as to prolong the internal compression process time and increase the internal compression ratio. When the radial exhaust orifice on the slide valve and the axial exhaust orifice on the end cover are made into different internal pressure ratios, the internal pressure ratio can be maintained to be the same as that of full load during the adjustment process within a certain range. Same.
When the volume adjustment slide valve is used to simultaneously change the radial exhaust orifice size of the screw machine and the effective working section length of the rotor, the relationship between the power consumption of the screw machine and the volume flow rate is within the volume flow adjustment range of 100-50%. The power consumed decreases almost in proportion to the decrease in volumetric flow, indicating good economy of slide valve regulation. It is worth noting that in the later stage of the slide valve movement, the internal pressure ratio will continue to decrease until it is reduced to 1. This makes the power consumption and volume flow curve at this time deviate to a certain extent compared with the ideal situation. The magnitude of the deviation depends on the external pressure ratio of the screw machine. If the external pressure determined by the movement conditions is relatively small, the no-load power consumption of the screw machine may be only 20% of that at full load, while when the external pressure is relatively large, it may reach 35%. It can be seen from here that a significant advantage of using a capacity slide valve is that the starting power of the screw machine is very small.
When the regulating slide valve structure is used, the upper surface of the slide valve acts as part of the screw compressor cylinder. There is an exhaust orifice on the slide valve, and its lower part also serves as a guide for axial movement, so the requirements for machining accuracy are very high. , which will lead to increased manufacturing costs. Especially in small screw compressors, the processing cost of the slide valve will account for a large proportion. In addition, in order to ensure the reliable operation of the screw machine, the gap between the slide valve and the rotor is usually larger than the gap between the cylinder hole and the rotor. In small screw machines, this increased gap will also affect the performance of the compressor. Severe decline. In order to overcome the above shortcomings, in the design of small screw machines, several simple and low-cost regulating slide valves can also be used.
A simple spool valve design with bypass holes in the cylinder wall that correspond to the helical shape of the rotor, allowing gas to escape from these holes when they are not covered. The slide valve used is a “rotary valve” with a spiral valve body. When it rotates, it can cover or open the bypass hole connected to the compression chamber. Since the slide valve only needs to rotate at this time, the overall length of the compressor can be reduced a lot. This design scheme can effectively provide continuous capacity adjustment. However, since the size of the exhaust hole remains unchanged, the internal pressure ratio will drop when unloading begins. At the same time, due to the existence of the bypass hole on the cylinder wall, a certain amount of “clearance volume” is formed. The gas within this volume will repeatedly undergo compression and expansion processes, resulting in reduced volumetric and adiabatic efficiency of the compressor.
3. The process of adjusting the slide valve of screw compressor
By moving the slide valve left and right, the effective compression volume is increased or decreased, and the gas delivery volume is adjusted. When loading: the piston moves to the left and the slide valve moves to the left and the gas delivery volume increases; when unloading: the piston moves to the right and the slide valve moves to the right and the gas delivery volume decreases.
4. Application prospects of screw compressor slide valve adjustment
Generally, oil-free screw compressors do not use a capacity adjustment device to adjust the slide valve. This is because the compression chamber of this type of compressor is not only oil-free but also at high temperature. This makes the use of regulating slide valve devices technically difficult.
In oil-injected screw air compressors, since the compressed medium remains unchanged and the operating conditions are fixed, the capacity adjustment device of the slide valve is usually not used. A variable frequency motor is usually used to make the compressor structure as simple as possible and adapt to the needs of mass production. .
It is worth pointing out that due to the capacity adjustment device that adjusts the slide valve, the compressor can maintain high efficiency under adjusted operating conditions. In recent years, capacity adjustment devices have also been used in oil-free screw compressors and oil-injected screw air compressors. Adjusts the tendency of the slide valve.
In oil-injected screw refrigeration and process compressors, capacity adjustment slide valves are commonly used to adjust the volumetric flow rate of the screw compressor. Although this exhaust volume adjustment method is relatively complex, it can continuously and steplessly adjust the exhaust volume, and the efficiency is also high.
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