JP2007085360A - Method for operating screw compressor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce generation of a power in the low load region of a screw compressor and to prevent generation of drain in an oil separator due to repetition of frequent starting and stop. <P>SOLUTION: In an operation method of a screw compressor capable of speed change operation driven by a motor using an inverter, rotation speed control operation coping with change of load by changing rotation speed of a compressor and no load operation establishing rotation speed of the compressor to a lower limit rotation sped used in a rotation speed control operation and reducing delivery pressure by discharging compressed air delivered from the screw compressor to atmospheric air are provided. Under a load condition lighter than predetermined load, loaded operation and no load operation establishing rotation speed control operation are repeated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an operating method of a screw compressor and a screw compressor that adjust the capacity of a compressor by changing the rotational speed of a drive motor using an inverter.

In the conventional screw compressor, as described in Japanese Patent Laid-Open No. 7-35079, the compressor has a suction throttle valve at its suction port, and this suction throttle valve is set according to the rotation speed signal of the inverter. It was opened and closed with a solenoid valve that opens and closes.
JP 7-35079 A

In the above prior art, the suction throttle valve is closed and the capacity is controlled at the time of low load, so the following points are not fully considered.
The suction throttle valve is fully closed with the compressor speed reduced at low load:
1. Insufficient power reduction.
2. As the discharge pressure of the compressor remains at the specified pressure, the suction pressure decreases, so the amount of oil supply increases compared to the full load. An increase in the amount of oil supply during low-speed rotation causes an increase in driving power and required drive torque, and an inverter drive in which the drive torque decreases in the low-speed rotation range may cause an inverter trip or the like.
3. Since the amount of work of the compressor is reduced in a state where the discharge pressure of the compressor is higher than the specified pressure, the capacity of the upper oil cooler is relatively increased and the oil supply temperature to the compressor is lowered. Along with this, the discharge temperature of the compressor decreases, and the possibility of generation of drain in the oil separator increases.

  By the way, in order to improve power at low load, (a) a state in which the suction throttle valve is fully closed to release the pressure in the oil separator, and (b) a state in which the suction throttle valve is fully opened. A method of repeating the two states (a) and (b) has also been proposed. However, in this method, a pressure margin for shifting to the states (a) and (b) is necessary, and therefore, there is a risk of lowering the set pressure. And even when the control pressure is secured, the operation frequency of the suction throttle valve and the electromagnetic valve increases, and there is a risk that the durability of these valves will be reduced.

  An object of the present invention is to reduce the power of the compressor while maintaining and improving the reliability of the compressor, and to reduce the generation of drain in the oil separator.

  In order to achieve the above object, the first feature of the present invention is that, in a driving method of a screw compressor capable of a speed change operation driven by an electric motor using an inverter, a change in load by changing the number of revolutions of the compressor. Rotation speed control operation corresponding to, and the rotation speed of the compressor is set to the lower limit rotation speed used in this rotation speed control operation, and the compressed air discharged from the screw compressor is discharged to the atmosphere to reduce the discharge pressure No load operation is performed, and when the load is lower than a predetermined load, the load operation set to the lower limit rotation speed of the rotation speed control operation and the no load operation are repeated.

  And in this feature, when the no-load operation has continued for at least a predetermined time, or when the time ratio between the no-load operation and the load operation in the low load operation exceeds a predetermined ratio, the operation has become one of Sometimes, the screw compressor is stopped; the load is reduced, the compressor speed becomes the lower limit speed, the suction throttle valve is closed, and the discharge pressure of the compressor is reduced to start the speed control operation. When switching to an operation mode in which no-load operation and load operation are repeated, the control upper limit pressure P1 is set to P1> P0 with respect to the set pressure P0 of the rotational speed control operation; the compressor pressure decreases and the compressor When the engine is restarted, it is desirable to continue the operation of the compressor for a predetermined time (t1) even if the load decreases, and to stop the compressor after the predetermined time (t1) has elapsed.

  In order to achieve the above object, the second feature of the present invention includes a pair of male and female rotors rotatably supported by bearings, and includes an electric motor that drives the rotor and an inverter that controls the electric motor. In the screw compressor, the rotational speed control means for controlling the capacity of the screw compressor by changing the rotational speed of the electric motor by an inverter, and the screw compressor is driven at the minimum rotational speed set in the rotational speed control means. And a capacity control means for performing a no-load operation in which the compressed air discharged from the screw compressor is discharged to the atmosphere to reduce the discharge pressure.

  In this feature, the upper limit value of the discharge pressure when the compressor is switched to no-load operation is P1, and the lower limit of the discharge pressure when the compressor is switched to the load operation that opens the suction throttle valve at the minimum rotational speed. When the value is P2 and the set pressure for the rotational speed control operation is P0, the capacity control means sets P1 and P2 so that P1> P0, P2> = P0; When the compressor is automatically stopped, the pressure is increased to P3 where P3> P0 with respect to the set pressure P0 and then stopped; when the pressure at which the compressor is restarted is P4, Control is performed so that P4> = P0; a discharge pressure detecting means for detecting the discharge pressure of the compressor is provided on the discharge side of the compressor, the set pressure (P0) in the rotational speed control, and the upper limit pressure (P1) of the capacity control ) And lower limit pressure (P2) , A storage means for storing a pressure (P3) for automatic stop and a restart pressure (P4) after the automatic stop, and a rotation speed control means and a capacity control means based on the detected pressure detected by the discharge pressure detection means. Switching means for switching the operation and input means for changing the values of P0, P1, P2, P3, and P4 stored in the storage means are provided; set pressure (P0) in the rotational speed control, and capacity control Storage means for storing upper limit pressure (P1), lower limit pressure (P2), pressure to automatically stop (P3), restart pressure after automatic stop (P4), and each setting based on set pressure (P0) Pressure setting means for calculating and setting the pressure (P1, P2, P3, P4); a discharge pressure detecting means for detecting the discharge pressure is provided, and a rotation speed control means based on the discharge pressure detected by the discharge pressure detecting means While controlling the rotation speed, this discharge Based on the discharge pressure of the force detected by the detecting means, it is desirable to operate the capacity control means in the low load than a predetermined load.

  In the present invention configured as described above, for example, the electric motor is controlled using an inverter in the range of 100% to 30% of the rated discharge air amount of the compressor, and the capacity is controlled by changing the rotation speed of the compressor. When the amount of air used drops below 30%, load operation that fixes the compressor speed to 30% load (lower limit speed) and no-load operation that reduces the compressor discharge pressure Since repeated control is performed, power consumption is significantly reduced.

  For example, the compressor is automatically stopped when no-load operation continues for 10 minutes. Alternatively, for example, the compressor is set to automatically stop in a load region of 10% or less.

  Also, when the load decreases, for example, when the amount of air used drops below 30%, the compressor speed is maintained at the set value (lower limit number of revolutions), and when the amount of air used decreases further, the lower limit Continue operation at the frequency. When the set pressure in the rotation speed control region is P0, if the discharge pressure of the compressor reaches the pressure P1 where P1> P0, the discharge pressure of the compressor is reduced so that no-load operation is started. ing. The above method is also performed when the compressor has an automatic stop function.

  Further, the pressure P2 (lower limit pressure) for returning from the no-load operation to the load operation is set to a pressure equal to or higher than the set pressure P0. When the amount of air used is 100% to 30% of the rated discharge air amount of the compressor, the number of revolutions is controlled, and operation is performed so that the pressure is constant near the set pressure P0. On the other hand, when the air usage is 30% or less, the no-load operation and the load operation at the lower limit rotational speed are repeated according to the air usage between the pressures P1 and P2 where P1> P2> = P0.

  When the amount of air used is further reduced and the conditions for automatic stop of the compressor are established, a period of load operation at the lower limit rotational speed is provided, and the pressure rises to P3 where P3> P0 with respect to the set pressure P0 described above. Then stop the compressor. When the air consumption starts and the pressure drops to P4 where P4> = P0, the compressor is restarted.

  Note that pressures such as P1, P2, P3, and P4 may be detected using a pressure sensor, and each set pressure may be stored in a storage device. Each set pressure P1-P4 may be calculated and set automatically by the calculation device when the pressure P0 is set. Moreover, you may provide the input means to change a setting value manually. Thereby, each set pressure can be set appropriately and the set value can be easily changed.

According to the present invention, when the screw compressor is operated at a low load and the rotation speed of the compressor is reduced, the following effects are obtained.
(1) When the load is lower than a predetermined load, a load operation set to the lower limit rotation speed of the rotation speed control operation and a no-load operation for reducing the discharge pressure of the compressor are repeated. The reduction effect of is increased.
(2) Since the discharge air from the compressor is discharged during no-load operation and the discharge pressure is reduced, it is possible to prevent the amount of oil supply from increasing more than necessary at the time of low-speed rotation and preventing an increase in the required driving torque. Inverter-driven compressors can reduce the risk of inverter trips.
(3) During no-load operation, the compressed air discharged from the screw compressor is discharged into the atmosphere to reduce the discharge pressure and reduce the discharge pressure of the compressor. Can be prevented.
(4) Since it is possible to keep the pressure constant by controlling the rotational speed, the minimum required pressure can be set as the target set pressure (P0) in the rotational speed control region. Power can be reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings for some examples.
In FIG. 1, the schematic diagram of the screw compressor apparatus based on the Example of this invention is shown. The air sucked from the suction filter 1 passes through the suction throttle valve 2, is compressed between the rotors 3 of the screw compressor, and is discharged from the discharge port 4. Lubricating oil is injected into the rotor 3 of the screw compressor 12 for cooling the compression heat generated by the compression and for lubrication and sealing. The compressed air discharged together with the lubricating oil from the discharge port 4 flows into the oil separator tank 5 and is separated from the lubricating oil by the oil separator element 6, and sequentially passes from the discharge pipe 7 through the check valve 8 and the pressure regulating valve 9. Then, it flows into the aftercooler 10, is cooled in the aftercooler 10, and then is discharged to an external device (not shown).

  On the other hand, the lubricating oil is separated from the compressed air in the oil separator tank 5 and guided from the bottom of the oil separator tank 5 to the oil cooler 11. Lubricating oil cooled by the oil cooler 11 and uncooled lubricating oil not passing through the oil cooler are mixed in the temperature control valve 13 to lubricate the screw compressor 12. The oil cooler 11 and the aftercooler 10 are cooled by the cooling air of the cooling fan 14.

  The rotor 3 shaft of the screw compressor and the motor 16 shaft are connected by a belt 15 for rotation. The electric motor 16 can be operated at a variable speed by an inverter 17. A pressure sensor 18 is provided on the downstream side of the check valve 8 to detect the pressure discharged from the screw compressor 12. The output signal of the pressure sensor 18 is input to the input / output unit 19. The control device unit 20a has storage means and a PID function. Then, the stored set pressure is compared with the pressure detected by the pressure sensor 18, and a frequency at which the detected pressure becomes the target pressure P 0 is given to the inverter 17 to change the rotation speed of the electric motor 16. The various pressure set values stored in the storage means and the control unit 20a are automatically set to appropriate values only by setting the target pressure P0. In addition, the setting value can be changed using the setting input unit and the display unit 20b connected to the control unit 20a. Further, the setting input unit and the display unit 20b are provided with a display unit (LED, liquid crystal element, etc.), and displays a set value of pressure and an operating frequency.

  The valve plate 2a of the suction throttle valve 2 provided on the upstream side of the screw compressor 12 operates in the closing direction when the piston 2b receives pressure from the electromagnetic valve 21 side. That is, when the electromagnetic valve 21 is opened, the high pressure in the oil separator 5 is guided to the suction throttle valve 2, and pressure is applied to the piston 2b. Further, a part of the air in the oil separator 5 is discharged to the suction side of the suction throttle valve 2 through the discharge pipe 22 at the same time when the electromagnetic valve 21 is opened. At this time, the flow rate is adjusted by the orifice 23. Instead of the pipe 22, it may be configured to directly vent to the atmosphere. The electromagnetic valve 21 compares the set pressure set automatically or manually in the storage means and the control unit 20a with the pressure signal from the input / output unit 19 input to the storage unit and the control unit 20a. Opened and closed based on the result.

  The operation of the screw compressor device configured as described above will be described below. If the oil-cooled screw compressor is driven by an electric motor using an inverter, it is possible to reduce the rotational speed of the compressor 12 with a decrease in the amount of air used. A reduction effect is obtained. This has been well known in the past, but if the rotational speed control is performed in the entire region of the discharge air amount, the following disadvantages are caused.

That is, it is necessary to use a volume control using a suction throttle valve or the like in a low rotation speed or small air volume region,
(1) When the number of revolutions is low, the number of revolutions of the cooling fan 16a provided integrally with the motor 16 also decreases at the same time, the motor cannot be cooled, and the motor coil temperature exceeds a predetermined temperature range.
(2) Since oil supply to the compressor 12 is performed using the differential pressure inside the oil separator 5 and the screw compressor 12, the amount of air discharged from the screw compressor is greatly reduced due to a low rotational speed. However, the amount of oil supply does not decrease, liquid compression of oil occurs inside the screw compressor, and it becomes overloaded,
This causes a malfunction.

  In order to avoid these problems, various methods such as providing a fan driven by a dedicated motor to cool the electric motor, or providing a valve for adjusting the amount of oil supply in the low rotation speed range can be considered. The structure becomes complicated and is not realistic.

  Therefore, in the one described in Japanese Patent Application Laid-Open No. 7-35079, in the small air amount region, the capacity control by the rotational speed control is not performed, and the suction throttle valve 2 is closed at the same time when the lower limit rotational speed is reached. It was in the driving state. However, this method can be expected to save energy compared with the conventional method, but it is still insufficient. Accordingly, in the present invention, the apparatus is configured as shown in FIG. 1 to control the capacity of the screw compressor. The detailed flow is shown in FIG.

  In the operating range of the air amount of about 30% to 100% with respect to the specified discharge air amount, the drive frequency of the electric motor 16 is changed by the inverter and the rotation speed is controlled. On the other hand, when the discharge air amount falls within an operating range of 30% or less of the specified discharge air amount, the compressor discharge pressure detected by the pressure sensor 18 reaches the set pressure P1 stored in the storage means and the control output unit. In this case, the rotational speed of the screw compressor is maintained at the set lower limit rotational speed in the rotational speed control. Then, the electromagnetic valve 21 is opened and the suction throttle valve 2 is closed. Further, the discharge pressure of the compressor 3 is reduced to switch to capacity control for no-load operation. Thereby, the pressure in the discharge port 4 of the compressor 12 falls, and it becomes possible to reduce power consumption significantly with respect to a prior art.

  The ratio of the power consumption to the discharge air amount ratio in this case is shown in FIG. In FIG. 2, line A is the power consumption characteristic according to the conventional method, and line B is the power consumption characteristic according to one embodiment of the present invention. When the discharge air amount ratio is in the vicinity of 0%, the power consumption is reduced to about half compared to the conventional case.

  Since the discharge pressure is reduced, the amount of oil supplied to the compressor rotor 3 can also be reduced, and there is no possibility of causing an abnormal increase in torque generated when the lubricating oil is liquid-compressed. In addition, when the screw compressor is operated at a low load, the oil supply temperature is lowered and drainage is likely to be generated in the oil separator 5, but the pressure in the oil separator 5 is also decreased during no-load operation, so that drainage is possible. The nature becomes less.

  Furthermore, as a result of maintaining the compressor speed at this set lower limit speed and operating the suction throttle valve 2 in the closed state, the time of no-load operation during which the discharge pressure of the compressor 3 decreases, and the set lower limit speed The rotation speed of the compressor is held, and the load operation time during which the suction throttle valve 2 is opened is determined by the storage means and the timer means incorporated in the control output unit. When the former ratio is, for example, a load of 10% or less, or when the former operation time continuously exceeds 3 minutes, the compressor is stopped. Further, the pressure is continuously monitored by the pressure sensor 18 even during stoppage, and the compressor is restarted when the pressure drops to the set pressure P4 stored in the storage means and the control unit. If the operation of the compressor is controlled in this way, the power consumption characteristics are as shown by line C in FIG. 2, and the power can be reduced in the operation region where the amount of air consumption is small.

  In the above embodiment, when the amount of air consumption decreases and the compressor speed reaches the set lower limit speed, the target set pressure P0 in the speed control area and no-load operation (hereinafter, the speed is kept constant). The operation state of reducing the discharge pressure of the compressor 3 at the same time as closing the suction throttle valve 2 is referred to as no-load operation.) The upper limit pressure P1 for starting the same is the same, and the air consumption is the switching point of the control method. If they coincide with each other, an unstable ON-OFF command is generated in the solenoid valve 21, which may cause hunting of the suction throttle valve 2.

  In a general constant-speed motor-driven screw compressor, when the capacity control for closing only the suction throttle valve is not performed, the compressor specification pressure P0 * and the set upper limit pressure P1 * for entering no-load operation Are set to the same pressure. This is because the motor is designed so as to have an allowable maximum output in the full load operation state at the specified pressure. In other words, if the set upper limit pressure P1 * for starting no-load operation is higher than the specified pressure P0 *, the motor will be overloaded, while the set upper limit pressure P1 * for starting no-load operation will be lower than the specified pressure P0 *. Then, it is because the malfunction that it enters into a no-load driving | operation before it reaches | attains a specification pressure arises.

  On the other hand, in the present invention, since no-load operation is started at the lower limit rotational speed of the rotational speed control region by the inverter, there is no restriction on the compressor driven by the constant speed motor, and the target set pressure P0 in the rotational speed control region is Even if the start pressure P1 of the no-load operation is set higher than (that is, the specification pressure in the case of a compressor driven by a constant speed motor), no problems such as overload of the motor occur. Therefore, in the present invention, P1 is set so that P1> P0. For example, when P0 is 0.69 Mpa, P1 is set to 0.79 Mpa. By setting in this way, it is possible to give a time delay between the rotation speed control region and the constant speed control at the lower limit rotation speed, and there is no possibility that the above-described hunting occurs.

  Depending on the set pressure, control conditions suitable for the compressor may be exceeded, which may cause inconvenience. Therefore, a method of automatically calculating an appropriate value when the target set pressure P0 is input and determining P1 to P4 is used. An example of a method for determining P1 to P4 is shown below. Assume that the pressure P0 is P0 = 0.69 MPa. When the load becomes low in this state, the maximum pressure is increased by 0.098 MPa, and the calculation of P1 = P0 + 0.098 = 0.79 MPa is performed. Here, since the discharge pressure of the safety valve is 0.93 MPa, the control upper limit pressure satisfies the condition that the discharge pressure is equal to or less than this discharge pressure. Next, the condition of the pressure P2 that can be stopped is P2> = P1. In other words, load operation is performed until P1 = P2, and when P1 = P2, the operation is switched to the unload operation. Therefore, the time from when switching to this unload operation until the pressure drops to P0 is calculated, and this time is predetermined. If it is longer than the time, it is stopped, and if it is within the predetermined time, the control device is operated so that the load operation is performed. The pressure P3 for returning to the load operation is P3 = <P0. The pressure P4 for restarting after the operation is stopped is set to P4 = P0−0.098 MPa = 0.59 MPa.

  When the pressures P0 to P4 are set in this way, when P0 = 0.83 MPa or more, P1 = (P0 + 0.098)> 0.93 MPa, which exceeds the discharge pressure of the safety valve. Therefore, an empirical formula suitable for each specification represented by P1 = P0 + (0.07 / P0) MPa is stored in the storage means of the compressor control device. In other words, each set pressure can be automatically determined only by inputting P0 by storing an arithmetic expression corresponding to the specification in accordance with the pressure setting of each compressor. In this embodiment, the relationship between each set pressure is given by an arithmetic expression, but discrete values may be interpolated and used. Further, although this relational expression is stored in the storage means, an expression stored in an external storage means such as a floppy (registered trademark) disk may be used.

  By the way, conventionally, when a constant speed motor is used, the generation of a pressure difference due to the opening / closing operation of the suction throttle valve or the electromagnetic valve has been unavoidable. For example, even when the minimum required pressure was 0.59 Mpa, the load operation and the no-load operation were repeated between 0.69 Mpa and 0.59 Mpa. On the other hand, in the rotational speed control method using an inverter, it is possible to change the rotational speed while keeping the pressure constant by PID control. The minimum required pressure 0.59 Mpa is set as the target set pressure P0 in the rotational speed control region. Therefore, it is not necessary to increase the pressure to a high pressure unnecessarily, and a power saving effect is obtained. However, during no-load operation in a low load region where the rotational speed is not controlled, for example, when the pressure is controlled between 0.59 Mpa and 0.49 Mpa and the pressure is reduced to 0.59 Mpa or less, this effect is also halved. That is, if the pressure drops below 0.59 Mpa, P0 cannot be lowered after all if there is a problem.

  Therefore, the pressure sensor 18 detects that the compressed air is consumed and the discharge pressure is lowered after entering the no-load operation. Further, the pressure (lower limit pressure) P2 when returning to the load operation is set to be equal to or higher than the target set pressure P0 in the rotation speed control region. As a result, even when the amount of air used is further reduced from the rotational speed control region, it is always possible to operate at a pressure higher than the target set pressure in the rotational speed control region. Even if the control pressure is increased, the increase in driving power is very small because it is in the low load region.

  Next, a modification of the present invention will be shown. This modification is about control when load decreases. When the load decreases, automatic stop and automatic restart are performed by the timer function described above. At this time, once the automatic stop condition of the compressor is satisfied, the load operation is forcibly performed once, and the discharge pressure of the compressor is once increased to P3 where P3> P0 with respect to the target set pressure P0 in the rotational speed control region. And then stop the compressor. Further, the pressure P4 for restarting the compressor is set to P4> = P0 with respect to the target set pressure P0 in the rotation speed control region. As a result, the compressor can be stably controlled even in a low load region where the rotational speed is kept constant. That is, according to this modification, since the target set pressure in the rotation speed control region can be maintained in all load regions, the power saving effect of the rotation speed control by the inverter described above that the compressor can be operated at the minimum required pressure. Can demonstrate to the maximum.

In the above embodiment, the set value of the control pressure is stored in the storage means 20a and can be displayed when necessary using the display and input means 20b. These set values are automatically calculated and determined when the control pressure P0 is input, but can be easily performed using the display and input means 20b. For example, in the example in which the maximum pressure is limited by the pressure, the above five control pressures P0, P1, P2, P3, P4 are
P1 = P3 = P0 + (0.07 / P0) Mpa
P2 = P4 = P0
And is stored in advance in the storage means, and usually only P0 is changed. In this case, the operating pressure can be easily changed.

  Needless to say, each set pressure can be determined manually. For example, P1 = P2 = P3 = P0 + X, P4 = P0−Y, and the values of X and Y are input using an input switch or the like provided on the control panel surface while watching the values displayed on the display device every moment. To do. Here, 0.001> X or Y> 0.098.

  In addition, a timer for starting counting after the automatic restart is provided in the storage means and the control unit 20a, and the compressor is not stopped for a predetermined time t1 even when the load is reduced, and the compressor is still maintained after this time t1 has elapsed. The compressor is automatically stopped when the load condition satisfies the automatic stop condition. As a result, it is possible to prevent the compressor from stopping before the oil temperature sufficiently rises and draining from occurring in the oil separator 5 due to frequent operation and stop of the compressor.

  FIG. 3 shows an example of changes in air consumption and pressure when the above-described screw compressor control method is used, and FIG. 4 shows an example of a flow of operation control of the screw compressor.

  In the present invention, in the screw compressor combining the rotational speed control using the inverter and the capacity control and the operation method thereof, there is no operation region of the conventional method in which only the suction throttle valve is closed during the capacity control operation and the capacity control is performed. Thereby, reduction of power and generation of drain can be reduced.

  As described above, according to the present embodiment, by inputting the control set pressure, pressure setting is facilitated in capacity control at low load, and there is no mechanical inconvenience due to pressure conditions. Moreover, since the function which can change and confirm an input value is provided, the driving | running state of a compressor can always be grasped | ascertained and the reliability in compressor operation can be improved.

It is a mimetic diagram of a screw compressor device concerning one example of the present invention. It is a graph which shows the characteristic of consumption power. It is a graph which shows an example of the change of air usage-amount and a pressure. It is a flowchart of an example of operation control.

Explanation of symbols

2 ... Suction throttle valve, 5 ... Oil separator, 6 ... Oil separator element, 8 ... Check valve, 9 ... Pressure regulating valve, 16 ... Electric motor, 17 ... Inverter, 18 ... Pressure sensor, 19 ... Input / output unit, 20a... Storage unit and control unit, 20b... Display and setting input unit, 21.

Claims (3)

In the operating method of the screw compressor capable of variable speed driving driven by an electric motor using an inverter,
A rotation speed control operation corresponding to a load change by changing the rotation speed of the compressor, and setting the rotation speed of the compressor as a lower limit rotation speed used in the rotation speed control operation, and discharging from the screw compressor With no-load operation to discharge the compressed air to the atmosphere and reduce the discharge pressure,
When the load is lower than a predetermined load, a screw compressor operating method, wherein the load operation set to the lower limit rotation speed of the rotation speed control operation and the no-load operation are repeated.   After the load is reduced and the rotational speed of the compressor is set to the lower limit rotational speed of the rotational speed control operation, the compressed air discharged from the compressor is discharged to the atmosphere to reduce the discharge pressure; 2. The upper limit value P <b> 1 of the discharge pressure of the compressor is set to P <b> 1> P <b> 0 with respect to the set pressure P <b> 0 of the rotation speed control operation when the operation mode is switched to repeat the load operation. Operation method of screw compressor.   When the compressor pressure is reduced and the compressor is restarted, the compressor operation is continued for a predetermined time (t1) even if the load is reduced, and the compressor is stopped after the predetermined time (t1) has elapsed. The operating method of the screw compressor of Claim 2 characterized by the above-mentioned.

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