JP2011179502A - Compressor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce power consumption in no-load and low load periods, in a rotation speed variable oil-free screw compressor having a low pressure stage compressor and a high pressure stage compressor. <P>SOLUTION: This oil-free screw compressor 100 has a low pressure stage compressor body 1 and a high pressure stage compressor body. Motive power of a motor 4 driven by an inverter 8 is transmitted to these compressor bodies via gears 5, 6 and 7. A low pressure stage gas blowoff two-way valve 21 is arranged in a pipe 20 branching off from the middle of an air pipe 9 for connecting the low pressure stage compressor body and the high pressure stage compressor body, and a high pressure stage gas blowoff two-way valve 16 is arranged in a pipe 15 branching off from a delivery air pipe 11 arranged on the delivery side of the high pressure stage compressor body. In no-load operation, a control device 18 commands the inverter so that a rotating speed of the motor becomes a preset lower limit rotating speed, and instructs to open the low pressure stage gas blowoff two-way valve. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a rotational speed variable oil-free screw compressor and an operation control method thereof, and more particularly, to a rotational speed variable oil-free screw compressor having a low-pressure stage compressor body and a high-pressure stage compressor body and an operation control method thereof. .

  In a conventional rotary speed variable screw compressor, for example, as described in Patent Document 1, a low-pressure stage screw compressor and a high-pressure stage screw compressor are connected in series, and a cooler is connected between the two compressors. It was. A motor is connected to each of the low-pressure stage screw compression section and the high-pressure stage screw compression section, and this motor is driven at a variable speed by an inverter. In the variable speed screw compressor constructed as described above, when the flow rate is low, both the low pressure stage screw compression part and the high pressure stage screw compression part become low rotation, and the amount of internal leakage cannot be ignored. The low pressure stage screw compression section and the high pressure stage screw compression section were operated at the minimum rotation to control the air discharge.

JP-A-10-82391

  In an oil-free screw compressor having a two-stage compressor of a low-pressure stage and a high-pressure stage, the power consumption at no load is smaller than that at full load as compared with a single-stage oil-free screw compressor. Therefore, even if the method described in the above publication is applied during no-load operation, there is a problem that the power consumption does not decrease much as compared with the conventional method in which the suction throttle valve is throttled.

  The present invention has been made in view of the above-mentioned problems of the conventional technology, and an object of the present invention is to provide a variable-speed oil-free screw compressor having a low-pressure stage compressor and a high-pressure stage compressor. It is to reduce the consumption power of time.

  A first feature of the present invention that achieves the above object is that a variable speed rotary screw compressor has a variable speed low pressure stage compressor body and a high pressure stage compressor body. The object of the present invention is to provide an air release means for releasing compressed air from the middle of the pipe connecting the main stage compressor body to the atmosphere. In this feature, it is preferable that an intercooler and an air discharge means are provided in the middle of the pipe connecting the low pressure stage compressor body and the high pressure stage compressor body, and an after cooler is provided on the discharge side of the high pressure stage compressor body. In addition, a check valve is provided on the upstream side of the aftercooler, and another air release valve for discharging compressed air discharged from the high pressure stage compressor body is provided between the high pressure stage compressor body and the check valve. It is also possible to provide air from a vent valve and another vent valve during no-load operation or low-load operation.

  In addition, a pressure detector for detecting the pressure of high-pressure air discharged from the high-pressure stage compressor body and a discharge pressure signal detected by the pressure detector are input to the discharge side of the high-pressure stage compressor body, You may make it provide the control apparatus which outputs the control signal which controls this. A low-pressure stage compressor body, an electric motor that rotationally drives the high-pressure stage compressor body, and an inverter that drives the electric motor, and the control device controls the inverter based on a discharge pressure signal detected by the pressure detector. Also good.

  More preferably, a suction throttle valve is provided on the suction side of the low-pressure stage compressor body, and another discharge means is provided on the discharge side of the high-pressure stage compressor body, and the other discharge means is interlocked with the suction throttle valve. Is.

  In order to achieve the above object, the second feature of the present invention is that the low speed compressor main body and the high speed compressor main body are changed in rotation speed so as to perform the operation according to the consumption air amount on the use side. The method of controlling the oil-free screw compressor of the low pressure stage compressor body in the range of air consumption based on the pressure detected by the pressure detector provided on the discharge side from the maximum air amount to a preset air amount In the no-load operation where the air consumption is almost zero, the low-pressure stage compressor body and the high-pressure stage compressor body are determined in advance for each compressor body. The compressed air is discharged from the discharge means interposed in the pipe connecting the low pressure stage compressor body and the high pressure stage compressor body while operating at the set lower limit rotational speed. Load operation and no load It is intended to repeat the rolling.

  Preferably, in the load operation, the low-pressure stage compressor body and the high-pressure stage compressor body are changed in proportion to the amount of air consumed. Preferably, in the no-load operation, compressed air discharged from the high-pressure stage compressor body is discharged.

  In order to achieve the above object, the third feature of the present invention is that the low speed compressor main body and the high speed stage compressor main body are changed in rotational speed so as to operate according to the amount of air consumed on the use side. In the oil-free screw compressor operation control method, when the amount of air consumed based on the pressure detected by the pressure detector provided on the discharge side exceeds a preset air amount, between the upper limit rotational speed and the lower limit rotational speed. When the load operation is performed and the amount of air consumed is less than the set air amount and the pressure of the compressed air discharged from the low pressure stage compressor body is equal to or higher than the atmospheric pressure, the low pressure stage compressor body and the high pressure stage compressor body are An operation including a no-load operation in which compressed air is discharged from an air discharging means interposed in a pipe to be connected is performed.

  Then, the suction throttle valve provided on the suction side of the low-pressure stage compressor body is linked to the discharge means for compressed air compressed by the high-pressure stage compressor body. The throttle valve may be controlled to be throttled.

  As described above in detail, according to the present invention, in an oil-free screw compressor including a two-stage compressor body having a variable rotation speed, the low-pressure stage compressor body, the high-pressure stage compressor body, Since the compressed air can be discharged to the atmosphere from between, the power consumption of the oil-free screw compressor at no load can be greatly reduced. Furthermore, power consumption can be reduced even during low-load operation in which no-load operation and load operation at the set lower limit rotational speed are repeated.

It is a block diagram of one Example of the inverter drive type oil free screw compressor which concerns on this invention. It is a figure explaining the operating method of the oil free screw compressor shown in FIG. It is a figure explaining the power consumption characteristic of the oil free screw compressor shown in FIG. It is a block diagram of the other Example of the inverter drive type oil free screw compressor which concerns on this invention. It is a figure explaining the pressure characteristic at the time of a no-load driving | operation of the oil free screw compressor shown in FIG.

  Several embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram of an oil-free screw compressor, and FIGS. 2 and 3 are diagrams for explaining an operation method of the oil-free screw compressor shown in FIG.

  The oil-free screw compressor 100 includes a low-pressure stage compressor body 1 and a high-pressure stage compressor body 2. The low-pressure stage compressor 1 holds a pair of male and female rotors in a casing having a cooling jacket formed on the outer periphery. The pair of rotors rotate synchronously when the timing gears attached to the shaft end portions of the rotors mesh with each other. A pinion gear 6 is attached to the end of the rotary shaft 1A opposite to the timing gear attachment end of one rotor. Similarly, the high-pressure stage compressor 2 holds a pair of male and female rotors in a casing having a cooling jacket formed on the outer periphery. The pair of rotors rotate synchronously when the timing gears attached to the shaft end portions of the rotors mesh with each other. A pinion gear 7 is attached to the end of the rotary shaft 2A opposite to the timing gear attachment end of one rotor.

  The two pinion gears 6 and 7 mesh with a bull gear 5 attached to a bull shaft coupled to the rotating shaft 4A of the motor 4. The motor 4 is a variable speed motor driven by an inverter 8. The pinion gears 6 and 7 and the bull gear 5 are accommodated in the gear casing 3. The lower part of the gear casing is a reservoir for lubricating oil for lubricating the bearings of the compressor bodies 1 and 2, the bull gear 5, and the pinions 6 and 7.

  A filter 14 for filtering the ambient air and supplying it to the low-pressure stage compressor body 1 is attached to the suction flow path of the low-pressure stage compressor body 1, and a suction port 14A is formed on the downstream side of the filter. Has been. An intercooler 10 is provided between the discharge side of the low-pressure stage compressor body 1 and the suction side of the high-pressure stage compressor body 2, and the intercooler 10 is connected to the low-pressure stage compressor body 1 by an air pipe 9. Thus, the high pressure stage compressor body 2 is connected by air piping 9A. An aftercooler 13 is connected to the downstream side of the high-pressure stage compressor body 2 by an air pipe 11 via a check valve 12.

  A low-pressure stage discharge pipe 20 is branched from the middle of the air pipe 9 that connects the low-pressure stage compressor body 1 and the intercooler. The low-pressure stage exhaust pipe 20 is provided with a low-pressure stage exhaust two-way valve 20. Similarly, a high-pressure stage discharge pipe 15 is branched from the upstream side of the check valve 12 in the middle of the air pipe 11 connecting the high-pressure stage compressor body 2 and the aftercooler 13. The high-pressure stage vent pipe 15 is provided with a high-pressure stage vent two-way valve 16. In order to supply the compressed air cooled by the aftercooler 13 to the use side, a discharge air pipe 23 is provided downstream of the aftercooler 13.

  A pressure detector 17 for measuring the pressure of the compressed air discharged from the oil-free screw compressor 100 is attached in the middle of the discharge air pipe 23. The pressure detected by the pressure detector 17 is input to the control device 18.

  The operation of this embodiment configured as described above will be described below. When the motor 4 is operated, the rotational force of the motor 4 is transmitted to the low pressure stage compressor body 1 and the high pressure stage compressor body 2 through the bull gear 5 and the pinion gears 6 and 7. As a result, the pair of rotors provided in the low-pressure stage compressor main body 1 and the high-pressure stage compressor main body 2 rotate synchronously to compress the air that is the working gas. The ambient air for compression sucked from the suction port 14A is compressed by the low-pressure stage compressor body 1, and the temperature rises as the pressure rises. This high-temperature compressed gas is guided to the intercooler 10 through the air pipe 9 and cooled by the intercooler 10. The compressed air cooled by the intercooler 10 is guided to the high-pressure compressor main body 2 through the air pipe 9A, and is further pressurized to a predetermined discharge pressure and rises in temperature. The compressed air whose temperature has risen is guided to the aftercooler 13 through the air pipe 11, cooled by the aftercooler 13, and then supplied from the discharge air pipe 23 to the use side.

  When the amount of consumed air on the use side decreases, the discharge pressure detected by the pressure detector 17 increases. The detected discharge pressure is input to the control device 18. When the discharge pressure increases, the control device 18 outputs a command signal for decreasing the rotation speed of the motor 4 to the inverter 8. When the rotational speed of the motor 4 is reduced, the rotational speed of the rotor provided in the low-pressure stage compressor body 1 and the high-pressure stage compressor body 2 is reduced, and the amount of air discharged from the oil-free screw compressor 100 is reduced.

  That is, when the amount of air consumed decreases and the amount of air discharged from the oil-free screw compressor may be about 100% to about 50% of the specified amount of discharge air, the control device 18 keeps the discharge pressure constant. 4 is controlled in proportion to the discharge air amount ratio as shown in FIG. 2 (operation range D). On the other hand, when the discharge air amount may be about 50% or less of the specified discharge air amount, the control device 18 commands the discharge air decompression operation. Specifically, if the discharge pressure detected by the pressure detector 17 exceeds the set upper limit pressure preset in the control device 18, the control device 18 instructs the inverter to maintain the set lower limit rotation speed. At the same time, in order to open the high-pressure stage air release two-way valve 16, an opening command is given to the high-pressure stage air release two-way valve 16. By opening the high-pressure stage discharge two-way valve 16, the compressed air compressed by the high-pressure stage compressor body 2 is released to the atmosphere without being guided to the aftercooler 13.

  By the way, in this embodiment, the air pipe 20 is branched from the middle of the air pipe 9 on the discharge side of the low-pressure stage compressor body 1. A low pressure stage two-way valve 21 is interposed in the pipe 20. The reason is as follows. The range in which the amount of air discharged from the oil-free screw compressor is from 100% to about 50% of the specified discharge air amount is a region of load operation. In this load operation region, since it is desired to supply the entire amount of compressed air to the use side, the control device 18 instructs the low pressure stage discharge two-way valve 21 to close the low pressure stage discharge two way valve 21. As a result, the entire amount of compressed air compressed by the low-pressure stage compressor body 1 is supplied to the high-pressure stage compressor body 2.

  During no-load operation when compressed air consumption on the use side is reduced and it is no longer necessary to supply compressed air to the use side, the rotational speeds of the low-pressure stage compressor body 1 and the high-pressure stage compressor body 2 are set to the set lower limit values. In addition, the control device 18 commands the inverter 8. At the same time, an open command is output to the low pressure stage discharge two-way valve 21 to release a part of the compressed air compressed by the low pressure stage compressor body 1 to the atmosphere.

  When the supply amount of compressed air to the user side is small, that is, during low load operation where the discharge air amount is about 50% or less of the specified discharge air amount, the low pressure stage compressor body 1 and the high pressure stage compressor body 2 The control device 18 commands the inverter 8 so that both rotation speeds become the set lower limit values. Then, the control device 18 controls the low-pressure stage exhaust two-way valve 21 and the high-pressure stage exhaust two-way valve 16 so as to repeat the above-described no-load operation and load operation. In any of the above operations, the amount of compressed air consumed is determined based on the pressure detected by the pressure detector 17 provided in the discharge air pipe 23.

  FIG. 3 shows changes in the power consumption of the oil-free screw compressor when the control device 18 controls the inverter 8 and the low-pressure stage discharge two-way valve 21 and the high-pressure stage discharge two-way valve 16 in this way. In FIG. 3, the horizontal axis is the value obtained by dividing the discharge air amount of the oil-free screw compressor by the specified discharge air amount, and the vertical axis is the power consumption when the discharge air amount becomes 100%. This is the power consumption of the oil-free screw compressor. The F line in the figure is shown for comparison, and the power consumption when the conventional capacity control method using a suction throttle valve that opens and closes according to the load on the suction side of the low-pressure stage compressor body 1 is used. Is a change. In this control method, both the low-pressure stage compressor body and the high-pressure stage compressor body are operated at a constant rotational speed, and compressed air is discharged during no-load operation. The point f is the power consumption during no-load operation when this conventional capacity control method is applied.

  The G line in FIG. 3 is also shown for comparison with the present invention, and is a variable speed type that does not have the low-pressure stage two-way valve 16 but has only the high-pressure stage two-way valve 16. This is a change in power consumption of an oil-free screw compressor. This conventional oil-free screw compressor includes a low-pressure stage compressor body and a high-pressure stage compressor body, and each compressor body is operated by an inverter-driven motor. Point g is the power consumption during no-load operation when this conventional rotational speed control method is applied.

  The H line in FIG. 3 is the power consumption characteristic of the oil-free screw compressor when the control method according to the present invention described above is applied. When the discharge air amount ratio is 100% to 50%, the power consumption changes in proportion to the discharge air amount ratio. When the discharge air amount ratio is 50% or less, the change becomes milder than that at the time of a large flow rate (100% to 50%), but less power is consumed than the conventional F-line and G-line. Moreover, the point h indicating the power consumption during no-load operation is clearly below the points f and g.

  Incidentally, the power consumption of the oil-free screw compressor is the sum of the power for compressing air and the mechanical loss generated in the bearings. During no-load operation, the rotational speed of the compressor body is controlled to about half the rotational speed during full-load operation, so the percentage of mechanical loss is small and most of the power required for air compression. In this embodiment, part of the compressed air compressed by the low-pressure stage compressor body during no-load operation is released to the atmosphere, so the amount of compressed air supplied to the high-pressure stage compressor body is only the amount of discharged air. Decrease. The power consumed by air compression is approximately proportional to the amount of air that the compressor body inhales. Therefore, if 50% of the compressed air compressed by the low-pressure compressor body is released, the power consumed by air compression in the high-pressure compressor body Is almost halved. Therefore, when the power consumed by air compression in the full load operation is approximately equal between the low pressure stage compressor body and the high pressure stage compressor body, if 50% of the compressed air compressed by the low pressure stage compressor body is discharged, the low pressure stage The power consumed by air compression in the stage compressor body and the high pressure stage compressor body can be reduced by 25%.

  Since the unload efficiency is generally higher in the two-stage compressor, the power consumption when the two-stage compressor is unloaded is relatively smaller than that in the single-stage compressor. Therefore, the difference between the power consumption during no load (point f) in the conventional capacity control method and the power consumption during no load (point g) in the rotational speed control method is very small. On the other hand, according to this embodiment, when the amount of discharged air is small as shown in FIG. 3, the compressed air compressed by the low-pressure stage compressor is discharged to the atmosphere to reduce the compression work of the high-pressure stage compressor. Therefore, power consumption is reduced. When the capacity of a two-stage oil-free screw compressor having a low-pressure stage compressor body having a constant rotational speed and a high-pressure stage compressor body is controlled using a suction throttle valve, the pressure of the compressed air discharged from the low-pressure stage compressor body Therefore, it is difficult to release the compressed air compressed by the low-pressure compressor main body to the atmosphere.

  Another embodiment of the present invention will be described with reference to FIGS. FIG. 4 is an overall configuration diagram of an inverter-driven oil-free screw compressor according to the present invention, and FIG. 5 shows changes in discharge pressure when the oil-free screw compressor shown in FIG. 4 is operated at different rotational speeds. FIG. This embodiment is different from the embodiment shown in FIG. 1 in that a suction throttle valve 31 is provided at the suction port of the low-pressure stage compressor body 1 and that the suction throttle valve 31 is used instead of the high-pressure stage discharge two-way valve 16. The air release valve 32 interlocked with the opening / closing of the air release valve 32 is provided, and the air release silencer 33 is provided on the secondary side of the air release valve 32.

  In this embodiment configured as described above, the control device 18 at the time of a load operation for supplying compressed air to the use side uses the oil-free required amount of air on the use side obtained based on the discharge pressure detected by the pressure detector 17. The inverter 8 is commanded to rotate the motor 4 so that the compressor can supply it. At the same time, an instruction is given to open the suction throttle valve 31.

  At the time of no-load operation in which compressed air is not supplied to the use side, the control device 18 instructs the inverter 8 to close the suction throttle valve 31, and instructs the inverter 8 so that the rotational speed of the motor 4 becomes the set lower limit rotational speed. Furthermore, the control device 18 also commands opening the air release valve 32. During this no-load operation, the rotational speed of the low-pressure stage compressor body 1 is the set lower limit rotational speed, so if the amount of air sucked into the low-pressure stage compressor body 1 decreases, the low-pressure stage compression that is the secondary side of the suction throttle valve 31 The suction pressure of the machine body 1 decreases. However, in order to share this suction throttle valve 31, a large type, for example, a 22 kW two-stage compressor, is used for 100 kW. Therefore, even if the suction throttle valve 31 is throttled, the suction side pressure is extremely low. Will not drop. As a result, the discharge pressure of the low-pressure stage compressor body 1 that is a value obtained by multiplying the suction pressure by the pressure ratio can be made positive. Therefore, if the low-pressure stage discharge two-way valve 21 is opened, the compressed air compressed by the low-pressure stage compressor body 1 can be discharged to the atmosphere. Thereby, the amount of compressed air supplied to the high-pressure stage compressor body 2 can be reduced. When the suction throttle valve 31 is adapted to the rated power, the suction throttle valve is controlled so that the discharge pressure of the low-pressure compressor does not become negative.

  At the time of low load in which only a small amount of compressed air is supplied to the use side, the control device 18 commands the inverter 8 so that the rotation speeds of the low-pressure stage compressor body 1 and the high-pressure stage compressor body 2 become lower limit values. Then, the control device 18 controls the suction throttle valve 31 and the exhaust two-way valves 21 and 32 so as to repeat the above-described no-load operation and load operation.

  In FIG. 5, the pressure of each part of the oil-free screw compressor in a present Example is shown. FIG. 5 shows a no-load operation. The horizontal axis is the ratio to the rated rotational speed. It can be seen that the pressure of the compressed air discharged from the low-pressure stage compressor body 1 exceeds the atmospheric pressure when the rotational speed of the low-pressure stage compressor body 1 is approximately 60% or less of the rated value. Therefore, it can be seen that in the no-load operation set to 50% of the rated speed, the compressed air compressed by the low-pressure stage compressor body 1 can be discharged to the atmosphere.

  DESCRIPTION OF SYMBOLS 1 ... Low pressure stage compressor main body, 2 ... High pressure stage compressor main body, 4 ... Motor, 8 ... Inverter, 9 ... Air piping, 11 ... Air piping, 12 ... Check valve, 16 ... High pressure stage air-release two-way valve, DESCRIPTION OF SYMBOLS 17 ... Pressure detector, 18 ... Control apparatus, 21 ... Low-pressure stage release two-way valve, 31 ... Suction throttle valve, 32 ... Release valve

Claims (11)

  An air release having a low-pressure compressor main body and a high-pressure compressor main body having a variable rotation speed, and releasing compressed air to the atmosphere from the middle of a pipe connecting the low-pressure compressor main body and the high-pressure compressor main body. An oil-free screw compressor with variable rotation speed, characterized by comprising means.   An intercooler and an air discharge means are provided in the middle of a pipe connecting the low-pressure stage compressor body and the high-pressure stage compressor body, and an aftercooler is provided on the discharge side of the high-pressure stage compressor body, respectively. Item 2. A variable-speed oil-free screw compressor according to Item 1.   A pressure detector that detects the pressure of high-pressure air discharged from the high-pressure stage compressor body, and a discharge pressure signal detected by the pressure detector are input to the discharge side of the high-pressure stage compressor body, and the discharge means The variable-speed oil-free screw compressor according to claim 1 or 2, further comprising a control device that outputs a control signal for controlling the motor.   The low pressure stage compressor body and an electric motor that rotationally drives the high pressure stage compressor body and an inverter that drives the motor, and the control device controls the inverter based on a discharge pressure signal detected by the pressure detector. The variable-speed oil-free screw compressor according to claim 3.   A suction throttle valve is provided on the suction side of the low-pressure stage compressor body, and another air release means is provided on the discharge side of the high-pressure stage compressor body, and the other air release means is interlocked with the suction throttle valve. 5. The variable rotation speed oil-free screw compressor according to claim 1.   This is a control method for a variable-speed oil-free screw compressor that operates according to the amount of air consumed on the user side by changing the rotation speed of the low-pressure stage compressor body and the high-pressure stage compressor body. When the amount of air consumed based on the pressure detected by the provided pressure detector is within the range of the preset air amount from the maximum air amount, load operation is performed to change the rotational speed of the low-pressure stage compressor body and the high-pressure stage compressor body. In the no-load operation where the amount of air consumption is almost zero, the low-pressure stage compressor body and the high-pressure stage compressor body are operated at a preset lower limit rotational speed for each compressor body, and the low-pressure stage compressor body Compressed air is discharged from the discharge means interposed in the pipe connecting the high-pressure compressor main body, and the load operation and the no-load operation are repeated when the amount of consumed air is less than the set air amount. Rotational speed Method of controlling the oil-free screw compressor of deformable.   7. The control method for an oil-free screw compressor with variable rotation speed according to claim 6, wherein in the load operation, the low-pressure stage compressor body and the high-pressure stage compressor body are changed substantially in proportion to the amount of air consumed. .   The method for controlling an oil-free screw compressor of variable rotation speed according to claim 6 or 7, wherein in no-load operation, compressed air discharged from the high-pressure stage compressor body is discharged.   An operation control method for an oil-free screw compressor of a variable rotation speed type that performs operation according to the amount of air consumed on the use side by changing the rotation speed of the low pressure stage compressor body and the high pressure stage compressor body, the discharge side When the amount of air consumed based on the pressure detected by the pressure detector provided on the vehicle exceeds a preset air amount, a load operation is performed between the upper limit rotational speed and a preset lower limit rotational speed. When the pressure of the compressed air discharged from the low-pressure stage compressor body is equal to or higher than the atmospheric pressure when it is below the set air amount, it is interposed in a pipe connecting the low-pressure stage compressor body and the high-pressure stage compressor body. An operation control method for an oil-free screw compressor of a variable rotational speed type, wherein an operation including a no-load operation in which compressed air is discharged from an air release means is performed.   The suction throttle valve provided on the suction side of the low-pressure stage compressor main body and the discharge means for compressed air compressed by the high-pressure stage compressor main body are linked so that the suction throttle is less than the set air quantity. The operation control method for an oil-free screw compressor of a variable rotation speed type according to claim 9, wherein control is performed so that the valve is throttled.   A check valve is provided on the upstream side of the aftercooler, and another air release valve for releasing compressed air discharged from the high pressure stage compressor body is provided between the high pressure stage compressor body and the check valve. 2. The variable speed oil-free screw compressor according to claim 1, wherein the variable speed oil-free screw compressor is provided in the exhaust valve and vents from the vent valve and another vent valve during no-load operation or low-load operation.

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