Can a screw compressor be adjusted for displacement without using an inverter motor?
First, the characteristics of the screw compressor
Screw compressor consists of a pair of parallel, intermeshing yin and yang screw composition, widely used in medium and large refrigeration systems or refining plant process gas compressor. Screw compression is divided into single screw and twin-screw two kinds, usually said screw compressor refers to the twin-screw compressor. Screw compressor has the following characteristics:
(1) screw compressor structure is simple, the number of parts is small, there is no such as valves, piston rings and other wear parts, rotors, bearings, etc., strength, wear resistance are relatively high.
(2) screw compressor has the characteristics of forced gas transmission, that is, the exhaust volume is almost independent of the exhaust pressure, in the small exhaust volume does not occur when the phenomenon of wheezing, in a wide range of operating conditions, can still maintain a high degree of efficiency.
(3) screw compressor is not very sensitive to liquid shock, can be used to spray oil cooling, so in the same pressure ratio, the discharge temperature is much lower than the piston type, so the single-stage pressure ratio is high.
(4) the use of slide valve adjustment, can realize the energy stepless adjustment.
Second, the principle of screw compressor slide valve adjustment
Slide valve is used to stepless control of capacity, in the normal start-up, the part is not loaded, the slide valve is controlled by the micro-control board through the oil pressure, and ultimately change the working capacity of the compressor.
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The capacity adjusting slide valve is a structural element used to adjust the volume flow rate in a screw compressor. Although there are various methods of regulating the volume flow rate of a screw compressor, the adjustment method using a slide valve has gained wide application, especially in oil-injected screw refrigeration and process compressors. As shown in Figure 1, this adjustment method is in the body of the screw compressor, installed a regulating slide valve, become part of the compressor body. It is located in the body of the high-pressure side of the intersection of the two inner circle, and can move back and forth in the direction parallel to the cylinder axis.
Slide valve regulating screw compressor volume flow principle, is based on the characteristics of the working process of the screw compressor. In the screw compressor, with the rotation of the rotor, the pressure of the compressed gas along the direction of the axis of the rotor gradually increases, in the spatial position, from the suction end of the compressor gradually moved to the exhaust end. After an opening is made in the high pressure side of the body, some of the gas is bypassed through the opening as the two rotors begin to engage and attempt to increase the gas pressure. 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 gas bypassed from the opening is used only to discharge it, so that the power dissipated by the compressor is mainly the sum of the work done to compress the gas finally discharged and the work of mechanical friction. Therefore, when the capacity adjustment slide valve to adjust the screw compressor volumetric flow, the compressor can be adjusted to maintain a higher efficiency under operating conditions.
In the actual compressor, generally not a hole in the casing, but a multi-hole structure. The slide valve moves in a groove under the rotor, allowing continuous adjustment of the size of the opening. The gas discharged through the opening is returned to the compressor suction port, and since the compressor does not actually do work on this portion of the gas, its temperature does not rise, and it does not need to be cooled before it reaches the main stream of gas at the suction port.
The slide valve can be moved in either direction as required by the control system and can be driven in a number of ways, the most common being by means of a hydraulic cylinder, with the screw compressor’s own oil circuit system providing the required oil pressure. In a few machines, the slide valve is driven by a motor after reduction.
Theoretically, the length of the slide valve should be the same as the rotor. Similarly, the distance the slide valve has to travel from full load to no load should be the same as that of the rotor, and the hydraulic cylinders should be of the same length. In practice, however, it has been shown that even with a slightly shorter length of slide valve, good regulation characteristics can still be achieved. This is because when the bypass opening near the suction side is first opened, its area is very small, the pressure of the gas is very small, and the time taken by the rotor to engage the teeth to sweep through the opening is also very short, so only a small portion of the gas is discharged. Therefore, the actual length of the slide valve can be reduced to about 70% of the length of the working section of the rotor, and the remaining part is made fixed, so that the overall size of the compressor can be reduced.
The characteristics of the capacity-adjusting slide valve vary with rotor diameter because the area of the bypass port caused by the movement of the slide valve is proportional to the square of the rotor diameter, whereas the volume of gas in the compression chamber is proportional to the third power of the rotor diameter. It is important to note that the compressor compresses the gas while simultaneously increasing the pressure of the injected oil and causing it to eventually discharge with the gas. In order for the oil to be continuously discharged, a certain amount of discharge volume must be retained. Otherwise, under completely empty conditions, oil will accumulate more and more in the compression chamber, resulting in unsustainable operation of the compressor. In order for the oil to be continuously discharged, a minimum volume flow of approximately 10% is usually required. In some cases, it is required that the compressor volume flow must be zero, in this case, usually in the suction and discharge between the arrangement of the bypass pipe, when it is necessary to completely 0 load, let the bypass pipe open, so that the suction and discharge connected.
With the capacity of the slide valve to adjust the volumetric flow of screw compressors, the ideal situation is in the process of regulating to maintain the internal pressure ratio with the same as the full load. But it is clear to see, when the slide valve to move the compressor volume flow becomes small, the effective working length of the screw becomes smaller, the internal compression process time also becomes smaller, so the internal pressure ratio must be reduced.
In the actual design, the slide valve are open on the radial exhaust orifice, which moves axially with the slide valve. In this way, on the one hand, the effective length of the screw machine rotor in the reduction, on the other hand, the radial exhaust orifice is also in the reduction, in order to extend the time of the internal compression process, increase the internal compression ratio. When the radial exhaust orifice on the slide valve and the axial exhaust orifice on the end cap to make a different internal pressure ratio, it can be adjusted in a certain range of the process, to maintain the internal pressure ratio and the same as the full load.
The use of volume adjustment slide valve at the same time to change the screw machine radial exhaust orifice size and rotor length of the effective working section, screw machine power consumption and volumetric flow rate of the relationship between the volume flow rate in the 100-50% volume flow rate within the scope of the adjustment, the power consumed by the screw machine can be almost with the reduction of volumetric flow rate decreased in direct proportion, indicating that the slide valve adjustment has a good economic performance. It is worth noting that in the latter 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 volumetric flow rate curve at this time compared with the ideal situation, resulting in a certain deviation. The magnitude of the deviation depends on the magnitude of the external pressure ratio of the screw machine. If the external pressure, which is determined by the motion conditions, is small, the no-load power consumption of the screw machine may be only 20% of the full-load power consumption, whereas it may reach 35% when the external pressure is large. It can be seen here that one of the significant advantages of using capacity slide valves is that the starting power of the screw machine is very small.
The use of adjustable slide valve structure, the upper surface of the slide valve acts as a part of the screw compressor cylinder, the slide valve has a discharge orifice, the lower part of which also has to play a guiding role in the axial movement, so the requirements for machining accuracy is very high, which will lead to an increase in manufacturing costs. Especially in the small screw compressor, slide valve processing costs will occupy 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, usually larger than the gap between the cylinder bore and the rotor, in the small screw machine, this increased gap will also make the performance of the compressor is seriously reduced. In order to overcome the above shortcomings, in the design of small screw machine, can also be used in several shapes of simple and low-cost regulating slide valve.
A simple slide valve design, the cylinder wall with the rotor spiral shape corresponding to the bypass holes, when these holes are not covered, the gas can be discharged from these holes. The slide valve used is a “rotary valve”, the valve body is spiral, when it rotates, it can cover or open the bypass holes connected to the compression chamber. Since the slide valve only needs to rotate at this point, the overall length of the compressor can be greatly reduced. This design solution is effective in providing continuous capacity regulation. However, since the size of the discharge orifice remains unchanged, the internal pressure ratio decreases as soon as the compressor is unloaded. At the same time, due to the presence of bypass holes in the cylinder wall, a certain amount of “clearance volume” is formed. This volume of gas will repeatedly undergo compression and expansion process, resulting in the compressor volume efficiency and adiabatic efficiency reduction.
Third, the process of screw compressor slide valve adjustment
Through the left and right movement of the slide valve, increase or decrease the effective compression volume, adjust the size of the gas delivery. When loading: the piston moves left to drive the slide valve left, the volume of gas delivery increased; load reduction: the piston moves right to drive the slide valve right, the volume of gas delivery decreased.
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Fourth, the application of screw compressor slide valve adjustment prospects
General oil-free screw compressor does not use the adjustable slide valve capacity adjustment device, this is because this type of compressor compression chamber is not only oil-free, but also in the high temperature. This makes the adoption of adjustable slide valve device has many technical difficulties.
In the oil-injected screw air compressor, due to the compression medium is unchanged and the operating conditions are fixed, usually do not use the adjustable slide valve capacity adjustment device, usually using variable frequency motor to make the compressor structure as simple as possible, to adapt to the needs of mass production.
It is worth pointing out that, due to adjust the capacity of the slide valve adjustment device, the compressor can be adjusted to maintain high efficiency, in recent years, in the oil-free screw compressor and oil-injected screw air compressor, there is a trend to use the capacity of the adjustable slide valve.
In the oil-injected screw refrigeration and process compressors, the general use of capacity regulating slide valve to regulate the volumetric flow of the screw compressor. This type of displacement adjustment is more complicated, but it allows continuous stepless adjustment of the displacement, and it is also more efficient.