JP2018115643A - Capacity control method of multistage oil free screw compressor, and multistage oil free screw compressor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease a lower limit rotational speed in speed control, without increasing a discharge temperature of a multistage oil free screw compressor.SOLUTION: A high-pressure stage compressor 11 is formed smaller than a low-pressure stage compressor 10. In an intermediate stage flow passage 20 communicating between a discharge port of the low-pressure stage compressor 10 and a suction port of the high-pressure stage compressor 11, intermediate air releasing means 21 is provided. In execution of capacity control with speed control, when a discharge temperature T1 of the low-pressure stage compressor 10 increases to become equal to or higher than a preset air leasing start temperature Ts (e.g. 230°C) lower than an emergency stop temperature Tmax (e.g. 250°C) by a predetermined temperature, air in the intermediate stage flow passage 20 is released to the atmosphere by the intermediate air releasing means 21, to suppress increase of the discharge temperature T1 of the low-pressure stage compressor 10.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a capacity control method for a multi-stage oil-free screw compressor and a multi-stage oil-free screw compressor, and more specifically, an oil-free screw compressor that does not use lubricating oil for cooling and sealing a compression working space in series. In a multistage oil-free screw compressor that further compresses the compressed gas discharged from the low-pressure stage compressor by the high-pressure stage compressor, the capacity control method is mainly performed by changing the rotational speed. And a multi-stage oil-free screw compressor that executes the capacity control method.

  Screw compressors are widely used to obtain compressed gas compressed to a predetermined pressure by compressing air, fuel gas, and other various gases.

  Of these, without using lubricating oil to cool and seal the compression working space, a non-contact state between a pair of male and female screw rotors and between the tooth tip of each screw rotor and the cylinder inner wall via a minute gap In an oil-free screw compressor that compresses the gas to be compressed by rotating the rotor at a high speed, the final target pressure may be reached depending on one compressor due to the backflow of the compressed gas through the minute gap described above. It may be difficult to increase the pressure of the compressed gas.

  For this reason, a plurality of compressors are connected in series, and the compressed gas compressed by the low-pressure stage compressor is introduced into the high-pressure stage compressor for further compression (in the case of two or more stages, the compressor is further compressed with a higher-pressure stage compressor). A compressor having a multi-stage structure is also provided which obtains a compressed gas having a final target pressure.

  In the following description, the case of a two-stage configuration in which two oil-free screw compressors composed of a low-pressure stage and a high-pressure stage are connected in series will be described as an example, but the multi-stage oil-free screw compressor of the present invention includes A multi-stage configuration of three or more stages may also be included.

  In a multi-stage oil-free screw compressor equipped with the high-pressure stage and low-pressure stage compressors described above, a motor is connected to the input shaft of each compressor via a speed increasing gear, etc., and the pressure of the compressed gas supplied to the consumer side, Therefore, a rotational speed control type capacity control is generally performed in which the rotational speed of the motor is changed by an inverter or the like according to the amount of compressed gas consumed on the consumption side.

  Various types of capacity control methods have been proposed that can reduce power consumption in a multi-stage oil-free screw compressor that performs such rotational speed control type capacity control.

  As an example of such a capacity control method, in Patent Document 1 described later, an air discharge valve is provided in the discharge flow path of the high-pressure compressor, and the air supply valve is closed and supplied to the consumer side during load operation. The rotational speeds of the low-pressure stage compressor and the high-pressure stage compressor are controlled in accordance with the pressure change of the compressed gas, and the low-pressure stage compressor and the high-pressure stage compressor are both maintained at the minimum rotational speed during no-load operation. It has been proposed to reduce the power consumption during no-load operation by opening the air valve (Patent Document 1 [0032] column).

  In addition, in Patent Document 2 described later, an air release valve (high-pressure side) provided in the discharge flow path of the high-pressure compressor for the purpose of further reducing power consumption compared to the invention described in Patent Document 1 described above. An air release valve (low pressure side air release valve) is also provided in the intermediate flow path that connects the discharge port of the low pressure stage compressor and the intake port of the high pressure stage compressor. Both the stage and low-pressure stage compressors are maintained at the minimum rotational speed, and not only the high-pressure side vent valve is opened, but the low-pressure side vent valve is also opened to reduce the intake amount of the high-pressure stage compressor. It has been proposed to further reduce the power consumption during no-load operation by reducing the power consumed for compression by the high-pressure stage compressor.

Japanese Patent Laid-Open No. 10-82391 JP 2002-138777 A

  In the above-described multi-stage oil-free screw compressor, there is a problem that the discharge temperature rises when performing a weight-reducing operation in which the rotation speed is reduced to reduce the discharge gas amount (see the above-mentioned Patent Document 1 [0008] column).

  Since this temperature rises as the rotational speed decreases, if the rotational speed of the multi-stage oil-free screw compressor is significantly reduced, the compressor of either the low-pressure stage, the high-pressure stage, or both of them is caused by thermal expansion accompanying the rise in discharge temperature. (For example, between the screw rotor or between the tip of the screw rotor and the inner wall of the cylinder) may cause a serious failure such as seizure of the compressor.

  For this reason, in a multi-stage oil-free screw compressor, the relationship between the discharge temperature at which there is a risk of failure and the rotational speed at which this discharge temperature occurs is experimentally determined in advance, and a necessary margin for this rotational speed is obtained. The added rotation speed is set as the lower limit value (setting lower limit rotation speed) of the rotation speed range during speed control, and capacity control is performed within the rotation speed range with this setting lower limit rotation speed and the rated rotation speed of the motor as the upper limit. (Rotational speed control) is performed, and capacity control (rotational speed control) that lowers the rotational speed below the set lower limit rotational speed cannot be performed.

  Such a set lower limit rotational speed can be higher than 50% of the rated rotational speed of 100%, so the motor is higher than necessary even during low-load operation or no-load operation. It is operated at a rotational speed, and extra power is consumed.

  Therefore, if the lower limit of the rotation speed range (setting lower limit rotation speed) during speed control (capacity control) can be set to a lower rotation speed while suppressing an increase in discharge temperature, the consumption of the multistage oil-free screw compressor will be reduced. Electric power can be reduced.

  In order to make it possible to reduce the rotational speed during speed control (capacity control) while suppressing such an increase in discharge temperature, the above-mentioned Patent Document 1 discloses a low-pressure stage compressor, a high-pressure stage compressor, Can be driven independently at variable speeds, and based on the discharge temperatures of the low-pressure compressor and the high-pressure compressor at the time of weight reduction operation, the rotation speeds of the motors of the high-pressure and low-pressure stages are independent according to the discharge temperature. Thus, it is possible to make the rotation speed of each compressor slower (Patent Document 1 [0015] column).

  However, in order to perform the capacity control described in Patent Document 1, it is necessary to control the rotational speeds of the low-pressure stage compressor and the high-pressure stage compressor separately. It cannot be applied to a general multi-stage oil-free screw compressor that operates the compressor and the low-pressure stage compressor at a constant fixed speed increasing ratio.

  In addition, since the rotation speed control of the low-pressure stage compressor and the high-pressure stage compressor is independently performed, not only the control itself becomes complicated, but also a motor and an inverter for driving the compressor need to be provided for each compressor. At the same time, the amount of processing performed by the control device (electronic control device) is greatly increased, so that it is necessary to adopt a control device with high processing capacity, and the configuration of the device itself is complicated and expensive.

  Therefore, the inventor of the present invention examined whether or not the set lower limit rotational speed could be reduced without increasing the discharge temperature with a simpler configuration.

  Here, in the oil-free screw compressor, no lubricating oil is used for cooling and sealing the compression working space, and minute gaps are formed between the screw rotors and between the screw rotor teeth and the cylinder inner wall. As described above, it is possible to compress the screw rotor by rotating it at a high speed. As a result, when the flow rate is low (low rotational speed), the amount of internal leakage through the gap increases, and once compressed air is compressed again. Because of compression, the amount of heat generated by compression increases and the discharge temperature rises.

  The multi-stage oil-free screw compressor used in the study adopts a high-pressure compressor that is smaller than the low-pressure compressor, but the working accuracy of the compressor is reduced by the size of the compressor. However, there is no significant difference in the size of the gap between the low-pressure compressor and the high-pressure compressor, and as a result, the smaller high-pressure compressor However, a relatively large gap is formed, and the increase in the amount of internal leakage that occurs when the rotational speed is reduced occurs more significantly in the high-pressure stage compressor than in the low-pressure stage compressor.

  Therefore, the discharge temperature rise directly caused by the increase in internal leakage is expected to be more remarkable in the high-pressure compressor.

  However, when the discharge temperature of the low-pressure stage compressor and the discharge temperature of the high-pressure stage compressor are measured in the multi-stage oil-free screw compressor used in the study, the temperature rise when the rotational speed is reduced is the discharge pressure of the low-pressure stage compressor. There was a marked increase in temperature, and the discharge temperature of the high-pressure stage compressor did not show as much as the discharge temperature of the low-pressure stage compressor.

  From the above results, when the rotational speed decreases, the reason why the discharge temperature of the low-pressure compressor is significantly higher than that of the high-pressure compressor is that the internal leakage of the high-pressure compressor is due to the low-pressure compressor. Presumably, this is due to the fact that the balance between the volumetric efficiency of the high-pressure stage compressor (the ratio of the actual air volume to the theoretical air volume) and the volumetric efficiency of the low-pressure stage compressor changes as the rotational speed decreases. did. That is, in a multi-stage oil-free screw compressor, the suction side pressure of the low pressure stage compressor (atmospheric pressure: gauge pressure 0 MPa if air) and the discharge side pressure of the high pressure stage compressor (pressure of the compressed gas supplied to the consumption side) The target pressure set as: 0.70 MPa as an example is substantially constant, and the two compressors cooperate to compress air at atmospheric pressure (gauge pressure 0 MPa) to the target pressure (0.70 MPa).

  Therefore, if the volumetric efficiency of the high-pressure compressor is significantly reduced and the balance of the volumetric efficiency of the two compressors changes, the intermediate-stage flow connecting the discharge port of the low-pressure compressor and the intake port of the high-pressure compressor is changed. It is considered that the pressure in the passage rose compared to before the rotational speed was lowered, and as a result, the compression ratio of the low-pressure compressor was increased and the discharge temperature was significantly increased.

  On the other hand, an increase in the pressure in the intermediate stage flow path results in an increase in the intake pressure for the high-pressure stage compressor, and therefore the compression ratio of the high-pressure stage compressor decreases. It is considered that the discharge temperature of the high-pressure compressor did not rise significantly, even though the amount of internal leakage increased and the amount of heat generated to compress the compressed air once again increased.

  According to the above prediction, in a multi-stage oil-free screw compressor that employs a smaller compressor than the low-pressure stage compressor as the high-pressure stage compressor, the rotational speeds of the low-pressure stage compressor and the high-pressure stage compressor are each compressed. If the discharge-side pressure of the low-pressure compressor can be reduced during weight reduction operation without the need for complicated control such as independent control according to the discharge temperature of the compressor, the discharge temperature of the low-pressure compressor will increase. It is possible to reduce both the set lower limit rotational speeds of the high-pressure stage and low-pressure stage compressors to a lower rotational speed while suppressing the above.

  Under the above prediction, the present invention is a multi-stage oil-free operation in which the low-pressure stage and the high-pressure stage are operated at a constant speed increase ratio by a completely different method from the conventional capacity control (Patent Document 1). It aims at providing the capacity | capacitance control method which can set a setting minimum rotational speed to a lower rotational speed also in a screw compressor.

  Hereinafter, means for solving the problem will be described together with reference numerals used in the embodiment for carrying out the invention. This code is used to clarify the correspondence between the description of the claims and the description of the mode for carrying out the invention. Needless to say, it is used in a limited manner to interpret the technical scope of the present invention. It is not something that can be done.

In order to achieve the above object, the capacity control method for a multi-stage oil-free screw compressor according to the present invention comprises:
In a multistage oil-free screw compressor 1 in which a plurality of compressors 10 and 11 are connected in series to form a multistage structure, and compressed gas compressed by the low pressure stage compressor 10 is introduced into the high pressure stage compressor 11 and further compressed. ,
The high-pressure stage compressor 11 is made smaller than the low-pressure stage compressor 10, and
Allowing the gas in the intermediate stage flow path 20 communicating with the discharge port of the low-pressure stage compressor 10 and the intake port of the high-pressure stage compressor 11 to be released into the atmosphere;
During capacity control performed by changing the rotational speed of each of the compressors 10 and 11 corresponding to the pressure change of the compressed gas supplied to the consumption side with the intake port 10a of the low-pressure stage compressor 10 open. When the discharge temperature T1 of the low-pressure compressor 10 rises to a preset release start temperature Ts (for example, 230 ° C.) or when the rotation speed of the low-pressure compressor 10 is set to a preset release start rotation When the speed falls below a speed Ns (for example, a rotational speed of 55% of the rated rotational speed), the capacity control is continued while releasing the gas in the intermediate-stage flow path 20 to the atmosphere. Item 1).

In the capacity control method configured as described above,
An emergency stop temperature Tmax (as an example, 250 ° C.) that is a discharge temperature T1 of the low-pressure stage compressor 10 that causes the multistage oil-free screw compressor 1 to perform an emergency stop, and a discharge temperature T1 that is lower than the emergency stop temperature Tmax by a predetermined value. A certain release start temperature Ts (230 ° C. as an example) is set,
When the discharge temperature T1 of the low-pressure compressor 10 rises to the preset release temperature Ts or higher, the gas in the intermediate-stage flow path 20 is released into the atmosphere (Claim 2). .

Alternatively, in the capacity control method having the above configuration,
A correspondence relationship between the intake air temperature Ta, the rotational speed, and the discharge temperature T1 of the low-pressure compressor 10 at the time of the capacity control in which the release of the gas in the intermediate flow passage 20 is stopped is obtained in advance, and multistage oil is obtained. An emergency stop temperature Tmax (as an example, 250 ° C.) that is the discharge temperature T1 of the low-pressure compressor 10 that makes the free screw compressor 1 emergency stop is set.
The intake air temperature Ta is measured, and the discharge temperature T1 of the low-pressure compressor 10 is lower than the emergency stop temperature Tmax by the measured intake air temperature Ta based on the correspondence relationship. The rotation speed of the low-pressure stage compressor 10 at which the temperature Ts is 230 ° C. is calculated and set as the discharge start rotation speed Ns, and the rotation speed of the low-pressure stage compressor 10 is equal to or less than the discharge start rotation speed Ns. When lowered, the gas in the intermediate stage flow path 20 can be released into the atmosphere (claim 3).

  The stop of the release of the gas in the intermediate flow path 20 to the atmosphere may be performed in any of the following cases.

A predetermined low temperature (210 ° C. as an example) with respect to the discharge temperature of the low-pressure stage compressor 10 (in the embodiment, the air release start temperature Ts: 230 ° C.) at the time when the release of the gas in the intermediate stage flow path 20 is started. ) Is set as the air discharge stop temperature Te,
When the discharge temperature T1 of the low-pressure stage compressor 10 falls below the discharge stop temperature Te, the atmospheric discharge of the gas in the intermediate stage flow path 20 is stopped (Claim 4).

Alternatively, the rotation speed of the low-pressure compressor 10 at the time when the gas in the intermediate-stage flow path 20 starts to be released into the atmosphere (in the embodiment, the discharge start rotation speed Ns: a rotation speed of 55% of the rated rotation speed). A predetermined high rotational speed (for example, a rotational speed of 60% of the rated rotational speed) is set as the deaeration stop rotational speed Ne,
When the rotational speed of the low-pressure stage compressor 10 rises above the degassing stop rotational speed Ne, the atmospheric discharge of the gas in the intermediate stage flow path 20 is stopped (Claim 5).

Alternatively, a correspondence relationship between the intake air temperature Ta, the rotational speed, and the discharge temperature T1 of the low-pressure compressor 10 at the time of the capacity control in which the release of the gas in the intermediate-stage channel 20 is stopped is obtained in advance.
The low-pressure stage compression at the time when the intake air temperature Ta is measured and the release of gas in the intermediate-stage flow path 20 is started at the measured intake air temperature Ta (30 ° C. in the embodiment) based on the correspondence. A rotation speed that is a predetermined low temperature (210 ° C. as an example) with respect to the discharge temperature of the machine (air release start temperature Ts: 230 ° C. in the embodiment) is calculated and set as the air discharge stop rotation speed Ne;
When the rotational speed of the low-pressure stage compressor 10 rises above the deaeration stop rotational speed Ne, the atmospheric discharge of the gas in the intermediate stage flow path 20 is stopped (Claim 6).

  In any of the above control methods, the capacity control can be performed with the ratio between the rotational speed of the low-pressure stage compressor 10 and the rotational speed of the high-pressure stage compressor 11 fixed at a constant level. ).

  The discharge temperature T1 of the low-pressure stage compressor 10 and the discharge temperature T2 of the high-pressure stage compressor 11 immediately after starting the release of the gas in the intermediate stage flow path 20 to the atmosphere are substantially the same temperature (both in the embodiment). It is preferable to adjust the release amount of the gas in the intermediate stage flow path 20 so as to be 215 ° C. (Claim 8).

  In addition, the discharge pressure T1 of the low-pressure compressor 10 causes the multi-stage oil-free screw compressor to be stopped at an emergency during the capacity control in which the gas in the intermediate-stage channel 20 is released into the atmosphere. The rotational speed of the low-pressure compressor 10 that is a predetermined low temperature (230 ° C. as an example) with respect to the emergency stop temperature Tmax (250 ° C. as an example) is measured in advance, and the measured rotational speed is measured in advance. Is set to the lower limit value (set lower limit rotational speed Nmin) of the rotational speed range of the low-pressure compressor 10 during the capacity control.

In addition,
During the capacity control in which the gas in the intermediate stage flow path 20 is released into the atmosphere, the discharge temperature T1 of the low-pressure stage compressor 10 makes an emergency stop temperature Tmax that causes the multistage oil-free screw compressor 1 to stop emergency (an example) 250 ° C. as a predetermined low discharge temperature (230 ° C. as an example), a correspondence relationship between the intake air temperature Ta of the low-pressure compressor 10 and the rotational speed is obtained in advance.
The intake air temperature Ta is measured, and the rotation speed corresponding to the predetermined low discharge temperature (230 ° C. as an example) is calculated from the measured intake air temperature Ta (30 ° C. in the embodiment) based on the correspondence relationship. The rotation speed thus set is set to the lower limit value of the rotation speed range of the low-pressure compressor 10 during the capacity control (set lower limit rotation speed Nmin: as an example, 45% of the rated rotation speed).

Moreover, the multistage oil-free screw compressor 1 of the present invention for executing the capacity control method is as follows.
A low-pressure stage compressor 10 and a high-pressure stage compressor 11 for introducing and compressing the compressed gas compressed by the low-pressure stage compressor 10, and with the intake port 10 a of the low-pressure stage compressor 10 opened, A capacity control device (a pressure detection means 44, an inverter 31, an intake valve 13, a control device) that performs capacity control for changing the rotational speed of each of the compressors 10 and 11 in accordance with a change in pressure of compressed gas supplied to the consumption side. 30 etc.) in a multi-stage oil-free screw compressor 1
The high-pressure stage compressor 11 is made smaller than the low-pressure stage compressor 10, and
An intermediate air discharge means 21 for releasing the gas in the intermediate stage flow path 20 communicating with the discharge port 10b of the low pressure stage compressor 10 and the intake port 11a of the high pressure stage compressor 11 to the atmosphere;
When the discharge temperature T1 of the low-pressure stage compressor 10 rises above the preset discharge start temperature Ts (230 ° C. in the embodiment) or when the rotational speed of the low-pressure stage compressor 10 is A control device 30 is provided for causing the intermediate air release means 21 to release the air when the air discharge speed decreases below a preset air release start rotational speed Ns (claim 11).

In the multistage oil-free screw compressor having the above configuration,
The control device 30 is
An emergency stop temperature Tmax (as an example, 250 ° C.) that is a discharge temperature T1 of the low-pressure stage compressor 10 that causes the multistage oil-free screw compressor 1 to perform an emergency stop, and a discharge temperature T1 that is lower than the emergency stop temperature Tmax by a predetermined value. A storage area (not shown) for storing a certain release start temperature Ts (230 ° C. as an example),
When the discharge temperature T1 of the low-pressure compressor 10 rises to the preset release temperature Ts or higher, the intermediate discharge means 21 may be configured to release the atmosphere (claim 12).

Or, in the multi-stage oil-free screw compressor configured as described above,
The control device 30 is
The correspondence relationship of changes in the intake air temperature Ta, the rotational speed, and the discharge temperature T1 of the low-pressure compressor 10 during the capacity control when the atmospheric discharge by the intermediate air release means 21 is stopped is stored, and the multi-stage oil-free screw compression is stored. A storage area (not shown) that stores an emergency stop temperature Tmax that is a discharge temperature of the low-pressure compressor 10 that causes the machine 1 to stop emergency;
Based on the correspondence relationship, a rotation speed at which the intake air temperature Ta becomes a predetermined low temperature (for example, 230 ° C.) with respect to the emergency stop temperature Tmax at the intake air temperature Ta is calculated to start the discharge. Set as rotation speed Ns,
When the rotational speed of the low-pressure stage compressor 10 is reduced to the air release start rotational speed Ns or less, the intermediate air release means 21 may be configured to perform the atmospheric discharge (claim 13).

The control device 30 has a predetermined low temperature (210 ° C. as an example) with respect to the discharge temperature of the low-pressure stage compressor (for example, the air start temperature Ts: 230 ° C. as an example) at the time of starting the atmospheric discharge by the intermediate air release means 21. Is set as the venting stop temperature Te,
The intermediate air release means 21 may be configured to stop atmospheric discharge when the discharge temperature T1 of the low-pressure compressor 10 falls below the discharge stop temperature Te (claim 14).

Alternatively, the control device 30 sets the rotation speed of the low-pressure compressor 10 at the time of starting the atmospheric discharge by the intermediate discharge means 21 (for example, the discharge start rotation speed Ns: a rotation speed of 55% of the rated rotation speed). On the other hand, a predetermined high rotational speed (for example, a rotational speed of 60% of the rated rotational speed) is set as the deaeration stop rotational speed Ne,
You may comprise so that the said atmospheric | air discharge by the said intermediate | middle air vent means 21 may be stopped when the rotational speed of the said low pressure stage compressor 10 rises more than the said air exhaust stop rotational speed Ne (Claim 15).

Alternatively, the control device 30 is
A storage area (not shown) is provided for storing a correspondence relationship between the intake air temperature Ta, the rotational speed, and the discharge temperature T1 of the low-pressure compressor 10 during the capacity control when the air release by the intermediate air release means 21 is stopped. ,
Based on the correspondence, from the measured intake temperature Ta to the intake temperature Ta, the discharge temperature of the low-pressure compressor at the start of the atmospheric discharge by the intermediate exhaust means 21 (exhaust start temperature Ts: 230 ° C. as an example) ) To calculate a rotation speed at a predetermined low temperature (210 ° C. as an example) and set it as the deaeration stop rotation speed Ne,
When the rotational speed of the low-pressure stage compressor 10 increases to the exhaust gas stop rotational speed Ne or higher, the atmospheric discharge by the intermediate air discharge means 21 may be stopped (Claim 16).

  In any of the multi-stage oil-free screw compressors 1, the motor M common to the low-pressure stage compressor 10 and the high-pressure stage compressor 11, and the rotation of the motor M at a constant speed increase ratio is the low-pressure stage compressor 10. And a power transmission means (speed increasing gear) 32 for transmitting to the high-pressure compressor 11 (claim 17).

  The intermediate temperature so that the discharge temperature T1 of the low-pressure stage compressor 10 and the discharge temperature T2 of the high-pressure stage compressor 11 immediately after the start of the air release by the intermediate air discharge means 21 become substantially the same temperature (for example, 215 ° C.). It is preferable to set the air release amount by the air release means 21 (claim 18).

The control device 30 includes:
During the capacity control when the intermediate air release means 21 is releasing to the atmosphere, the discharge temperature T1 of the low-pressure stage compressor 10 is the discharge temperature of the low-pressure stage compressor 10 before the multi-stage oil-free screw compressor 1 is emergency stopped. Stores the rotational speed of the low-pressure stage compressor that is a predetermined low temperature (230 ° C. in the embodiment) with respect to a certain emergency stop temperature Tmax (250 ° C. in the embodiment);
The rotational speed may be set to a lower limit value (set lower limit rotational speed Nmin) of the rotational speed range of the low-pressure compressor 10 during the capacity control (claim 19).

The control device 30 includes:
The low pressure that becomes a predetermined low discharge temperature (230 ° C. in the embodiment) with respect to the emergency stop temperature Tmax (250 ° C. as an example) that is the discharge temperature T1 of the low-pressure compressor 10 that makes the multi-stage oil-free screw compressor 1 emergency stop. A storage area (not shown) storing the correspondence between the intake air temperature Ta and the rotational speed of the stage compressor 10;
Based on the correspondence relationship, a rotation speed corresponding to the predetermined low temperature (230 ° C. as an example) is calculated from the measured intake air temperature Ta (30 ° C. in the embodiment),
The rotational speed is configured to be set to a lower limit value of a rotational speed range of the low-pressure compressor 10 at the time of the capacity control (set lower limit rotational speed Nmin; rotational speed of 45% of the rated rotational speed in the embodiment). (Claim 20).

  With the configuration of the present invention described above, in the capacity control method of the present invention, the lower limit of the rotation speed range that can be used during capacity control while suppressing the increase in the discharge temperatures T1 and T2 and preventing the occurrence of burn-in or the like ( The lower limit setting speed Nmin) can be set lower than that of the conventional control method, and as a result, power consumption can be reduced as compared with the case of performing the conventional capacity control method. It was.

Explanatory drawing of the multistage oil-free screw compressor of this invention. The flowchart explaining the basic operation | movement (starting, speed control, stop) of the multistage oil-free screw compressor of this invention. The flow figure explaining the emergency stop operation | movement of the multistage oil-free screw compressor of this invention, and the air discharge | release operation of an intermediate | middle stage flow path (example which controls an intermediate | middle air discharge means based on discharge pressure). The flowchart explaining the emergency stop operation | movement of the multistage oil-free screw compressor of this invention, and the air discharge operation | movement of an intermediate | middle stage flow path (example which controls an intermediate | middle air discharge means based on a rotational speed). The graph explaining the difference in power consumption when setting lower limit rotation speed Nmin is 55% of the rated value and 45% of the rated value.

  Embodiments of the present invention will be described below with reference to the accompanying drawings, but the configuration of the present invention is not limited to the embodiments shown below.

[Device configuration of multi-stage oil-free screw compressor]
In FIG. 1, the structural example of the multistage oil-free screw compressor 1 of this invention is shown.

  The multi-stage oil-free screw compressor 1 shown in FIG. 1 is a two-stage oil-free screw compressor 1 in which two compressors 10 and 11 are connected in series. The multistage oil-free screw compressor 1 to which the control method is applied includes one in which three or more compressors are connected in series.

  The multi-stage oil-free screw compressor 1 shown in FIG. 1 uses compressed gas as air, and the intake port 10a of the low-pressure compressor 10 has an intake valve 13 and an intake so that the low-pressure compressor 10 can suck air. The air is released through the filter 14.

  The discharge port 10b of the low-pressure stage compressor 10 communicates with the intake port 11a of the high-pressure stage compressor 11 via an intermediate-stage flow path 20 provided with an intercooler 22, and thereby discharges from the low-pressure stage compressor 10. The compressed air is cooled by the intercooler 22 and then introduced into the high-pressure compressor 11 to be further compressed.

  In FIG. 1, for convenience, the low-pressure stage compressor 10 and the high-pressure stage compressor 11 are shown to have the same size, but the high-pressure stage compressor 11 is smaller than the low-pressure stage compressor 10 described above. doing.

  The discharge port 11b of the high-pressure compressor 11 is communicated with a discharge flow path 40 having a check valve 41 and an aftercooler 42, and is communicated with an air working machine (not shown) via the discharge flow path 40. The compressed air can be supplied to the consumer side.

  These input shafts are connected to the output shaft of the motor M in order to rotationally drive the low-pressure compressor 10 and the high-pressure compressor 11 respectively.

  The low-pressure stage compressor 10 and the high-pressure stage compressor 11 may be provided with motors M for driving them independently, but in the embodiment shown in FIG. In order to be able to drive any of the compressors 10, 11, both the input shafts of the low-pressure stage and high-pressure stage compressors 10, 11 are connected to the single motor M via the power transmission means 32 composed of a speed increasing gear. Connected to the output shaft.

  Therefore, in this embodiment, the rotational speed of the low-pressure compressor 10 and the rotational speed of the high-pressure compressor 11 are maintained at a constant ratio determined by the speed increasing ratio of the speed increasing gear 32, and the speed of the motor M is maintained. The rotational speed is configured to increase / decrease corresponding to the increase / decrease.

  The rotational speed Nd of the motor M is controlled by the inverter 31 and is supplied to the consumption side according to the discharge pressure Pd of the high-pressure compressor 11 detected by the pressure detection means (pressure sensor) 44 provided in the discharge flow path 40. Based on the pressure of the compressed air, the control device 30 controls the inverter 31 to change the rotation speed of the motor M, so that the pressure in the discharge flow path 40 is set to a predetermined target pressure Pt ( In this embodiment, the capacity control by the speed control is performed so as to approach 0.70 MPa).

  The multistage oil-free screw compressor 1 of the present invention further includes an intermediate air release means 21 for releasing compressed air in the intermediate stage flow path 20 and a high-pressure stage discharge for discharging compressed air in the discharge flow path 40. An air means 43 is provided.

  In the illustrated embodiment, the intermediate air release means 21 has one end communicated with the intermediate stage flow path 20 on the primary side of the intercooler 22 and the other end opened to the atmosphere via the silencer 21c. The discharge valve 21a is an electromagnetic valve that opens and closes the discharge passage 21b. By opening and closing the release valve 21a according to a control signal received from the control device 30, the compression in the intermediate-stage passage 20 is performed. It is comprised so that the discharge of air can be started and stopped.

  This intermediate air release means 21 is, for example, when the air release valve 21a is opened by selecting the diameter of the air release passage 21b described above, by selecting the size of the air release valve 21a to be used, or by providing a separate throttle. The flow passage area is reduced so that a part of the compressed air flowing through the intermediate flow passage 20 is discharged at an appropriate flow rate.

  Preferably, from the state in which the intermediate discharge means 21 closes the intermediate stage flow path 20, the discharge temperature T1 of the low-pressure compressor 10 becomes the discharge start temperature Ts (230 ° C. as an example) described later, and the intermediate discharge is performed. When the discharge by means 21 is started, the intermediate discharge means 21 so that the discharge temperature T1 of the low-pressure compressor 10 immediately after the start of discharge and the discharge temperature T2 of the high-pressure compressor 11 are substantially the same. Adjust the amount of air released by.

  On the other hand, the high-pressure stage air release means 43 is connected to the discharge flow path 40 on the primary side of the aftercooler 42 and the check valve 41, and the air discharge flow path 43b opened to the atmosphere via the silencer 43c on the other end. The air discharge valve 43a is composed of an electromagnetic valve that opens and closes the air discharge flow path 43b, and the air discharge valve 43a is opened and closed by a control signal received from the control device 30 to compress the discharge flow path 40. The air can be started and stopped.

  In this embodiment, the release valve 43a for opening and closing the release passage 43b of the high-pressure stage release means 43 is configured by an electromagnetic valve that operates in response to a control signal from the control device 30. Thus, a mechanical on-off valve is provided as the air release valve 43 a, and the air release valve 43 a opens when the intake valve 13 is closed in conjunction with the opening and closing operation of the intake valve 13 provided at the air inlet 10 a of the low-pressure compressor 10. The suction valve 13 may be closed when the suction valve 13 is opened.

  In FIG. 1, reference numerals “23” and “45” are temperature detection means (temperature sensors), and the temperature detection means 23 detects the discharge temperature T1 of the low-pressure compressor 10 and outputs it to the control device 30. The temperature detecting means 45 detects the discharge pressure T2 of the high-pressure compressor 11 and outputs it to the control device 30.

  In accordance with a predetermined program stored in advance in the storage area, the control device 30 generates an inverter based on the discharge temperatures T1, T2 detected by the temperature detecting means 23, 45 and the discharge pressure Pd detected by the pressure detecting means 44. 31, the intermediate venting means 21, the high-pressure stage venting means 43 and the suction valve 13 are controlled.

  The multistage oil-free screw compressor 1 of the present invention is further provided with temperature detection means 15 that is a temperature sensor for detecting the temperature of the air sucked by the low-pressure stage compressor 10 (intake air temperature Ta). You may comprise so that the detection signal of the means 15 may be input into the control apparatus 30. FIG.

[Description of operation of multi-stage oil-free screw compressor]
The operation of the multistage oil-free screw compressor having the configuration described with reference to FIG. 1 will be described with reference to the flowcharts shown in FIGS.

<Basic operation (start / capacity control / stop)> (Fig. 2)
The basic operation from start to capacity control and stop of the multistage oil-free screw compressor 1 of the present invention will be described with reference to FIG.

  The main power supply is turned on to start the multi-stage oil-free screw compressor (S1). In this state, the operator operates the switches on the operation panel to perform a predetermined starting operation (S2).

  When the multistage oil-free screw compressor 1 can change the pressure setting (setting of the target pressure Pt) of the compressed air supplied to the consumption side, the operator sets the target pressure Pt in this starting operation.

  With the start operation by the operator, the control device 30 sets the set lower limit rotational speed Nmin based on the intake air temperature Ta detected by the temperature detecting means 15 in accordance with the correspondence stored in advance in the storage area, and the target pressure Pt input by the operator The unload start pressure Pu, which is a predetermined high pressure, is set to cause the multistage oil-free screw compressor 1 to perform a predetermined start operation (S3).

  For example, the control device 30 starts this operation by starting the motor M with the intake valve 13 closed and the air release valve 43a of the high-pressure stage air release means 43 open, and for a predetermined time, the set lower limit rotational speed Nmin. This is done by driving at

  After the start-up operation (S3) is completed, the control device 30 opens the suction valve 13 and stops the air release by the high-pressure stage air release means 43 (S4), whereby the multistage oil-free screw compressor 1 is put into a normal operation. Transition.

  Due to the shift to the normal operation, the control device 30 moves the pressure in the discharge flow path 40 (discharge pressure Pd) based on the detection signal from the pressure detection means 44 provided on the discharge side of the high-pressure compressor 11, that is, on the consumption side. The pressure of the supplied compressor air is monitored, and the rotational speed of the motor M is controlled so that the detected discharge pressure Pd approaches the aforementioned target pressure Pt (for example, 0.70 MPa) set by the operator. The suction valve 13 and the high-pressure stage air release means 43 are controlled.

  Specifically, when the discharge pressure Pd is less than the above-described unload start pressure Pu (0.75 MPa as an example) (NO in S5), the control device 30 is detected according to the correspondence stored in advance. The target rotational speed Nc with which the deviation between the discharged pressure Pd and the target pressure Pt is 0 (zero) is calculated (S9), and the output frequency is set to the inverter 31 so that the rotational speed of the motor M becomes the target rotational speed Nc. By outputting a control signal to be changed and controlling the rotational speed of the motor M (S10), compressed air corresponding to the amount of compressed air consumed on the consumption side is generated and supplied.

  On the other hand, when the discharge pressure Pd becomes equal to or higher than the unload start pressure because the consumption of compressed air on the consumption side stops or the like (YES in S5), the control device 30 closes the intake valve 13 and intakes the low-pressure compressor 10. While stopping, the air release valve 43a of the high pressure stage air release means 43 is opened, the compressed air in the discharge flow path 40 is released to the atmosphere, the discharge pressure Pd is lowered, and the power of the compressor is reduced. Is shifted to the set lower limit rotational speed Nmin (S6), and the no-load operation is continued while the discharge pressure Pd is equal to or higher than the unload start pressure Pu (NO in S7).

  During the no-load operation, when the discharge pressure Pd drops below the unload start pressure Pu again due to the restart of consumption of compressed air on the consumption side (YES in S7), the control device 30 opens the intake valve 13 and Control for changing the rotation speed Nd of the motor M to the target rotation speed Nc calculated based on the discharge pressure Pd (S9, S10) by closing the discharge valve 43a of the high-pressure stage discharge means 43 and stopping the discharge (S8). )I do.

  The above-described capacity control (S5 to S10) by speed control is repeated until the operator operates the operation panel to instruct stop processing (NO in S11), and when the stop processing command is input by the operator (S11). YES), the control device 30 closes the suction valve 13 and opens the air release valve 43a of the high pressure stage air release means 43 to start air release (S12). After normal operation, the controller 30 operates in this state for a predetermined time. The motor is stopped (S13), and the multistage oil-free screw compressor is stopped.

<Control of emergency stop / intermediate air release means (discharge temperature reference type)> (See Fig. 3)
When the main power supply is turned on and the multi-stage oil-free screw compressor is started (S1 in FIG. 2), the control device 30 performs a low-pressure stage compressor based on detection signals from the temperature detection means 23 and 45 as shown in FIG. Monitoring of the discharge temperature T1 of 10 and the discharge temperature T2 of the high-pressure compressor 11 is started (S16, S14 in FIG. 3).

  If the discharge temperature T2 of the high-pressure compressor 11 is lower than a predetermined emergency stop temperature Tmax (250 ° C. as an example), the discharge temperature T2 is continuously monitored (NO in S15), and becomes higher than the emergency stop temperature Tmax. Then (YES in S15), the control device 30 closes the intake valve 13 and opens the air release valve 43a of the high pressure stage air release means 43 (S12 in FIG. 2), stops the motor M (S13), and multistage. The oil-free screw compressor 1 is emergency stopped.

  In the case of the above-described normal stop process, the control device 30 closes the suction valve 13 and opens the discharge valve 43a of the high-pressure stage discharge means 43 (S12), and continues the operation for a certain time with the load reduced. After that, the motor M is stopped (S13). However, in the case of an emergency stop, the suction valve 13 is blocked and the high pressure is not operated for a certain period of time while the load is reduced. You may comprise so that the stop process of the motor M may be performed simultaneously with the air discharge start by the stage air discharge means 43. FIG.

  The control device 30 also monitors the discharge temperature T1 of the low-pressure compressor 10 based on the detection signal from the temperature detector 23 (S16), and the discharge temperature T1 of the low-pressure compressor 10 is stored in advance. If the emergency stop temperature Tmax (as an example, 250 ° C.) or higher is reached (YES in S17), the intake valve 13 is closed, and the high-pressure stage air release means 43 starts air discharge (S12 in FIG. 2). It stops (S13 of FIG. 2), and the multistage oil-free screw compressor 1 is made an emergency stop.

  In the case of an emergency stop, the motor stop process may be performed while the suction valve 13 is opened, and the high-pressure stage air release valve 43a is opened after the motor M stop process is performed. Also good.

  When the discharge temperature T1 of the low-pressure stage compressor 10 is lower than the emergency stop temperature Tmax (250 ° C. as an example) (NO in S17 of FIG. 3), When the temperature is lower than the discharge start temperature Ts set as a low temperature (230 ° C. as an example) (NO in S18), the control device 30 continues to monitor the discharge temperature of the low-pressure compressor 10 (return to S16).

  When the discharge temperature T1 of the low-pressure compressor 10 is equal to or higher than the discharge start temperature Ts (230 ° C. as an example) (YES in S18), the control device 30 starts to discharge the intermediate stage flow path 20 by the intermediate discharge means 21. (S19), the pressure in the intermediate stage flow path 20 is reduced.

  Thus, the discharge temperature T1 of the low-pressure stage compressor 10 is lowered by lowering the pressure of the intermediate stage flow path 20.

  During the discharge by the intermediate discharge means 21, the control device 30 compares the discharge temperature T1 of the low-pressure compressor 10 with a predetermined discharge stop temperature Te stored in advance, and the discharge temperature T1 is the discharge stop temperature. When the temperature drops below Te (210 ° C. as an example) (YES in S20), the air release by the intermediate air release means 21 is stopped (S21), and the control device 30 continues to monitor the discharge temperature T1 of the low-pressure compressor 10. (Return to S16)

  If the discharge temperature T1 exceeds the discharge stop temperature Te (NO in S20) and the discharge temperature T1 is equal to or higher than the emergency stop temperature Tmax (YES in S22), the control device 30 performs an emergency stop. (S12, S13 in FIG. 2). If the temperature is lower than the emergency stop temperature Tmax (NO in S22), the intermediate venting means 21 continues to vent (return to S19).

  This discharge stop temperature Te is set lower than the measured value (215 ° C. in this embodiment) of the discharge temperature T1 of the low-pressure compressor 10 immediately after the start of discharge by the intermediate discharge means 21, for example, experimentally in advance. The discharge temperature may be measured and stored in the control device 30, or the discharge temperature T 1 of the low-pressure compressor 10 immediately after the middle air release means 21 is released is released by the temperature detection means 23. Based on the measured value, the control device 30 may set a temperature lower than the measured value as the discharge stop temperature Te.

  In the multistage oil-free screw compressor 1 configured as described above, during normal operation, the control device 30 controls the rotational speed of the motor M so that the discharge pressure Pd becomes the target pressure Pt (0.70 MPa as an example). Thus (S9, S10 in FIG. 2), when the consumption of compressed air on the consumption side decreases and the discharge pressure Pd becomes higher than the target pressure Pt, speed control (capacity control) is performed to reduce the rotational speed of the motor M. .

  Therefore, as the consumption of compressed air on the consumption side decreases and the rotational speed of the motor M decreases from the rated value (100%), the low-pressure stage compressor 10 and the high-pressure stage compressor 11 are caused by the change in volumetric efficiency described above. In the meantime, the temperature in the intermediate stage flow path 20 (discharge temperature T1 of the low-pressure stage compressor 10) gradually increases.

  As an example, in the multi-stage oil-free screw compressor 1 of the present embodiment, the intake temperature Ta (outside air temperature) of the low-pressure stage compressor 10 is 30 ° C., the rotational speed of the motor M is the rated value (100%), and the discharge pressure Pd is During full load operation substantially equal to the target pressure Pt (0.70 MPa), the pressure in the intermediate stage flow path 20 is 0.20 MPa, the discharge temperature T1 of the low pressure compressor 10 is 190 ° C., and the discharge of the high pressure compressor 11 The temperature T2 was 190 ° C.

  On the other hand, when the rotational speed Nd of the motor M is reduced to 55% of the rated value in a state where the discharge pressure Pd and the target pressure Pt (0.70 MPa) substantially coincide with each other, the pressure in the intermediate-stage flow path 20 is 0. .23 MPa, the discharge temperature T1 of the low-pressure compressor 10 increased to 230 ° C., and the discharge temperature T2 of the high-pressure compressor 11 increased to 210 ° C.

  When the consumption of compressed air on the consumption side is greatly reduced, the rotational speed of the motor M is controlled based on a preset lower limit rotational speed Nmin (in this embodiment, the outside temperature Ta detected by the temperature detecting means 15). The set lower limit rotational speed Nmin calculated by the device 30 is reduced to 45% of the rated speed as an example), but the discharge of the low-pressure compressor 10 is performed in the process in which the rotational speed of the motor M is reduced to the set lower limit rotational speed Nmin. When the temperature reaches 230 ° C. or higher (YES in S18 of FIG. 3), the control device 30 starts to release air by the intermediate air release means 21 (S19), and reduces the pressure in the intermediate stage passage 20.

  Thereby, the compression ratio of the low-pressure stage compressor 10 is lowered, and the discharge temperature T1 of the low-pressure stage compressor 10 is lowered to a temperature lower than the discharge start temperature Ts (230 ° C.).

  As an example, in the multi-stage oil-free screw compressor 1 of the present embodiment, the pressure in the intermediate stage flow path 20 when the discharge start temperature (230 ° C.) is reached is 0.23 MPa. As a result, the pressure in the intermediate-stage flow path 20 immediately after the release was reduced to 0.20 MPa, and the discharge temperature T1 was reduced to 215 ° C.

  However, the amount of compressed air discharged from the intermediate stage flow path 20 by the intermediate discharge means 21 is adjusted by the throttle, and if the diameter of the throttle is too small, the amount of compressed air released to the atmosphere is small and the intermediate stage is reduced. The pressure drop in the passage is reduced, and the discharge temperature T1 of the low-pressure compressor 10 cannot be lowered sufficiently.

  On the other hand, when the diameter of the throttle is too large, the amount of compressed air released to the atmosphere increases, and the pressure in the intermediate stage flow path 20 is quickly reduced to reduce the discharge temperature T1 of the low pressure stage compressor 10. However, if the pressure in the intermediate-stage flow path 20 is lowered too much, the compression ratio of the high-pressure stage compressor 11 increases and the discharge temperature T2 of the high-pressure stage compressor 11 rises, resulting in an emergency stop temperature. Will approach.

  Accordingly, the amount of air discharged by the intermediate air discharge means 21 is preferably determined so that the discharge temperature T1 of the low-pressure stage compressor 10 and the discharge temperature T2 of the high-pressure stage compressor 11 are balanced, and more preferably, the intermediate discharge amount. The discharge amount is set so that the discharge temperature of the low-pressure compressor 10 immediately after the start of the discharge by the discharge means 21 and the discharge temperature T2 of the high-pressure compressor 11 substantially coincide with each other. The discharge temperature of the low-pressure compressor 10 and the discharge temperature of the high-pressure compressor 11 immediately after the start of venting were both set to 215 ° C.

  Further, the amount of compressed air released from the intermediate stage passage 20 to the atmosphere is set to be smaller than the amount of compressed air released by the high pressure stage air release means 43 during no-load operation.

  Thus, in this embodiment, the discharge temperature T1 of the low-pressure stage compressor 10 and the discharge temperature T2 of the high-pressure stage compressor 11 immediately after the start of the discharge by the intermediate discharge means 21 are both 215 ° C. Since the temperature is sufficiently lower than the stop temperature Tmax (as an example, 250 ° C.), the speed control (capacity control) is further continued in the state where the air is discharged by the intermediate air discharging means 21, and the low pressure stage and the high pressure stage compressor 10, 11 can be reduced, and as a result, the set lower limit rotational speed Nmin, which is the lower limit value of the rotational speed used for speed control (capacity control), can be reduced.

  As an example, the lower limit rotational speed Nmin is set such that the discharge temperature of the low-pressure compressor 10 in the state where the intake valve 13 provided at the intake port 10a of the low-pressure compressor 10 is opened and the intermediate air discharge means 21 is discharged. The rotational speed of the motor M at which T1 becomes 230 ° C. is experimentally determined, and a value equal to or higher than the determined rotational speed, preferably the determined rotational speed is set as a set lower limit rotational speed Nmin. % Rotation speed was set as the set lower limit rotation speed Nmin.

  Thus, when the compressed air in the intermediate stage flow path 20 is not discharged, the lower limit rotational speed is set to 55% of the rated rotational speed in order to set the discharge temperature T1 of the low pressure stage compressor 10 to 230 ° C. or lower. However, in the configuration of the present invention, the discharge temperature of the low-pressure compressor is reduced even when the lower limit rotational speed Nmin of the motor is reduced to 45% of the rated rotational speed. T1 could be suppressed to 230 ° C. or lower.

  FIG. 5 is a graph showing the difference between the air amount ratio and the power consumption ratio when the set lower limit rotational speed Nmin is 55% of the rated value and 45% of the rated value. In the figure, the solid line indicates the case where the set lower limit rotational speed Nmin is 55% of the rated value, and the alternate long and short dash line is 45%.

  When the setting lower limit rotation speed Nmin is 55% of the rated value, when the air amount ratio is 55 to 100%, the intake valve 13 is opened and the rotation speed control is performed, and when the air amount ratio is 0 to 55%. The intake valve 13 is controlled to open and close with the rotation speed constant at 55% of the rated value. When the air amount ratio is 0%, the intake valve 13 is closed at the rotation speed of 55% of the rated value, and the high-pressure stage air discharge means 43 is open.

  On the other hand, when the setting lower limit rotation speed Nmin is 45% of the rated value, the intake valve 13 is opened and the rotation speed control is performed while the air amount ratio is 45 to 100%, and the air amount ratio is 0 to 45%. Is controlled to open and close the suction valve 13 at a constant rotational speed of 45% of the rated value. When the air amount ratio is 0%, the suction valve 13 is closed at the rotational speed of 45% of the rated value and the high-pressure stage is discharged. The means 43 is open.

  By reducing the setting lower limit rotation speed Nmin by 10% from 55% to 45% of the rated value, it is possible to reduce the power consumption as shown by the hatched lines in FIG. 5, and as a result, multistage oil-free screw compression The machine can be operated with less power consumption.

  When air is discharged by the intermediate air release means 21 in this way, if the consumption of compressed air on the consumption side increases and the discharge pressure Pd becomes lower than the target pressure Pt, the control device 30 causes the discharge pressure to be reduced. The rotational speed of the motor M is increased so that Pd becomes the target pressure Pt (0.70 MPa).

  By increasing the rotational speed of the motor M, the internal leakage amount of the compressor is reduced and recompression of the leaked compressed air is suppressed, and the internal leakage amount of the high-pressure compressor 11 is reduced, thereby reducing the intermediate stage flow path. The pressure in 20 decreases, and the discharge temperature T1 of the low-pressure compressor 10 also decreases. When the discharge temperature T1 of the low-pressure compressor 10 becomes equal to or lower than the discharge stop temperature Te (210 ° C. as an example) (S20 YES in FIG. 3), the control device 30 stops the discharge by the intermediate discharge means 21 (S21). .

  In this embodiment, the discharge temperature T1 of the low-pressure stage compressor 10 and the discharge temperature T2 of the high-pressure stage compressor 11 at this time are both less than 230 ° C.

  During no-load operation, since the pressure in the discharge flow path 40 is reduced due to the release of air by the high pressure stage air release means 43, the pressure in the intermediate stage flow path 20 is also low, The discharge temperature T1 of the low-pressure compressor 10 is equal to or lower than the discharge stop temperature Te (210 ° C.), and the intermediate discharge means 21 does not discharge.

  However, even in this case, for example, when the discharge temperature T1 of the low-pressure stage compressor 10 rises without the pressure in the discharge flow path 40 being lowered due to a failure of the high-pressure stage air discharge means 43, etc. When the discharge temperature T1 of the low-pressure stage compressor 10 rises to the discharge start temperature (230 ° C.) or higher for the reason, the control device 30 starts the discharge by the intermediate discharge means 21, so the low-pressure compressor 10 The discharge temperature T1 decreases.

<Emergency stop / control of intermediate venting means (rotational speed reference type)> (See Fig. 4)
As described above, in the configuration described with reference to FIG. 3, the above-described intermediate air release means 21 is controlled based on the discharge temperature T <b> 1 of the low-pressure compressor 10 detected by the temperature detection means 23.

  In contrast, in this embodiment, the discharge temperatures T1 and T2 of the low-pressure compressor 10 and the high-pressure compressor 11 detected by the temperature detectors 23 and 45 are used only for detecting the emergency stop temperature Tmax, and are used as intermediate discharges. The opening / closing control of the air means 21 is performed based on the rotational speed Nd of the motor M.

The operation in this case will be described with reference to FIG.
When the main power supply is turned on and the multi-stage oil-free screw compressor is started (S1 in FIG. 2), the control device 30 performs high-pressure stage compression based on detection signals from the temperature detection means 23 and 45 as shown in FIG. Monitoring of the discharge temperature T2 of the compressor 11 and the discharge temperature T1 of the low-pressure compressor 10 is started (S25 and S23), and the discharge temperature T2 of the high-pressure compressor 11 or the discharge temperature T1 of the low-pressure compressor 10 is emergency stopped. When the temperature becomes equal to or higher than Tmax (S24, S26), the intake valve 13 is closed, and the high-pressure stage venting means 43 starts to vent (S12 in FIG. 2), and the motor M is stopped (S13 in FIG. 2). The point where the oil-free screw compressor 1 is brought to an emergency stop is the same as the control described with reference to FIG.

  In the configuration of the present embodiment, the correspondence area between the intake air temperature Ta of the low-pressure compressor 10 and the rotational speed of the motor M that serves as a reference for opening / closing the intermediate air discharge means 21 is stored in the storage area of the control device 30. Based on the intake air temperature Ta detected by the temperature detection means 15, the control means 30 determines the release start rotation speed Ns at which the intermediate release means 21 starts releasing and the release by the intermediate release means 21. The air discharge stop rotational speed Ne to be stopped is calculated and set (S27 in FIG. 4).

  The air release start rotational speed Ns and the air release stop rotational speed Ne are obtained by changing the rotational speed of the motor M and the discharge of the low-pressure compressor 10 at each intake air temperature Ta in a state where the intermediate air release means 21 is closed in advance by experiments or the like. By measuring the change of the temperature T1 and storing the arithmetic expression based on the measurement result in the control device 30, the control device 30 is based on the intake air temperature Ta and the above-described emergency stop temperature Tmax (250 ° C. as an example). In contrast, a rotation speed corresponding to a predetermined low release start temperature Ts (230 ° C. as an example) is set as the release start rotation speed Ns, and a predetermined lower release than the release start temperature Ts (230 ° C.). The rotational speed corresponding to the stop temperature (210 ° C. as an example) is set so as to be set as the above-described venting stop rotational speed Ne.

  As an example, in the apparatus of this embodiment, when the intake air temperature Ta is 30 ° C. and the target pressure is set to 0.70 MPa, the discharge start rotation speed is 55% relative to the rated rotation speed 100%, and the discharge stop rotation. The speed was 60%.

  The control device 30 monitors the rotation speed Nd of the motor M based on the above-described release start rotation speed Ns and release discharge stop rotation speed Ne (S28), and the rotation speed Nd of the motor M becomes equal to or less than the discharge start rotation speed Ns. Then (YES in S29), the air release by the intermediate air release means 21 is started (S30), and then the intermediate air release means in a state where the rotational speed Nd of the motor M is lower than the air release stop rotational speed Ne (NO in S31). 21 is continued (returns to S30), and when the rotation is over the discharge stop rotation speed Ne (YES in S31), the discharge by the intermediate discharge means 21 is stopped (S32), and the motor M described above is stopped. Monitoring of the rotational speed Nd is continued (return to S28).

  In the above configuration, during normal operation, the rotational speed of the motor M is controlled so that the discharge pressure Pd becomes the target pressure Pt (0.70 MPa), the consumption amount of compressed air on the consumption side is reduced, and the discharge pressure Pd becomes the target pressure. When the pressure becomes higher than the pressure Pt, the rotational speed Nd of the motor M is decreased.

  When the consumption of compressed air on the consumption side is significantly reduced, the rotational speed of the motor M is controlled based on a preset lower limit rotational speed Nmin (in this embodiment, the outside temperature Ta detected by the temperature detecting means 15). The setting lower limit rotation speed Nmin calculated by the device 30 is reduced to 45% of the rated speed as an example), but the rotation speed Nd is released in the process in which the rotation speed of the motor M is reduced to the setting lower limit rotation speed Nmin. When the rotational speed falls below the starting rotational speed Ns (for example, a rotational speed of 55% of the rated rotational speed), the intermediate venting means 21 starts venting, thereby lowering the pressure in the intermediate stage flow path 20, resulting in a low pressure. The discharge temperature T1 of the stage compressor 10 can be lowered.

  In the multistage oil-free screw compressor 1 used in this embodiment, the outside temperature Ta detected by the temperature detection means 15 is 30 ° C., the rotational speed of the motor M is the rated value (100%), the discharge pressure Pd and the target pressure Pt ( 0.70 MPa), the pressure in the intermediate flow path 20 is 0.20 MPa, the discharge temperature T1 of the low-pressure compressor 10 is 190 ° C., and the discharge temperature T2 of the high-pressure compressor 11 is When the discharge pressure Pd is the target pressure Pt (0.70 MPa) and the rotation speed of the motor decreases to the discharge start rotation Ns (rotation speed 55% of the rated rotation speed), the pressure in the intermediate passage is The discharge temperature T1 of the low-pressure compressor 10 is increased to 230 ° C., and the discharge temperature T2 of the high-pressure compressor 11 is increased to 210 ° C.

  However, if the air release by the intermediate air discharge means 21 is started at the above-described discharge start rotation speed Ns (rotation speed 55% of the rated rotation speed), the discharge temperature T1, the high pressure stage of the low pressure compressor 10 immediately after the start of discharge. Any of the compression discharge temperatures T2 can be lowered to 215 ° C., and this temperature is sufficiently lower than the emergency stop temperature Tmax (250 ° C.). Even when the temperature is further reduced, the discharge temperatures T1 and T2 can be suppressed to a predetermined low temperature (for example, 230 ° C.) or lower than the emergency stop temperature Tmax (250 ° C.).

  Accordingly, the discharge temperature T1 of the low-pressure compressor 10 is lower than the emergency stop temperature Tmax (250 ° C.) by a predetermined temperature (for example, 230, with the set lower limit rotational speed Nmin being discharged by the intermediate discharge means 21. The rotation speed of the motor at (° C.) is obtained by experiment, and a value equal to or higher than the rotation speed obtained in this experiment, preferably the rotation speed obtained in the experiment is set as the set lower limit rotation speed Nmin.

  In the multi-stage oil-free screw compressor used in this example, the rotational speed obtained in the above experiment when the intake air temperature of the low-pressure stage compressor 10 is 30 ° C. and the target pressure is 0.70 MPa is 45% of the rated rotational speed. This rotational speed was defined as the set lower limit rotational speed Nmin.

  In the multistage oil-free screw compressor 1 of the present embodiment, the rotational speed at which the discharge temperature T1 of the low-pressure stage compressor 10 is kept at 230 ° C. or lower when the intermediate air release means 21 is not vented is the rated speed. The rotational speed is 55%, and this rotational speed is a set lower limit rotational speed Nmin that can be used during speed control (capacity control) of a conventional multistage oil-free screw compressor.

  In this embodiment, the set lower limit rotational speed can be reduced by 10% to 45% of the rated rotational speed for this rotational speed (55% of the rated rotational speed), which is used for speed control (capacity control). The range of the rotational speed that can be achieved can be expanded to a lower speed side, and the power consumption during the operation of the multistage oil-free screw compressor can be reduced (see FIG. 5).

  In this embodiment, since the operation of the intermediate air release means 21 is controlled based on the rotation speed of the motor M, the control device 30 needs to be able to grasp the rotation speed of the motor M. However, the rotational speed Nd of the motor M may be detected by providing a rotational speed sensor on the motor shaft, or the target rotational speed Nc calculated when the control device 30 controls the inverter, You may comprise so that the rotational speed Nd of the motor M may be grasped | ascertained based on the frequency signal which 31 outputs with respect to the motor M.

  As described above, when the amount of compressed air consumed on the consumption side increases while the intermediate air release means 21 is releasing air, the discharge pressure Pd becomes lower than the target pressure Pt (for example, 0.70 MPa). The control device 30 increases the rotation speed of the motor M and controls the rotation speed of the motor M so that the discharge pressure Pd approaches the target pressure Pt (0.70 MPa).

  The increase in the rotational speed of the motor M reduces the internal leakage of both the low-pressure and high-pressure compressors 10 and 11 and suppresses recompression of the leaked compressed air, and the internal leakage of the high-pressure compressor 11. Is reduced, the volumetric efficiency of the high-pressure compressor 11 is improved, the pressure in the intermediate-stage flow path 20 is lowered, and the discharge temperature T1 of the low-pressure compressor 10 is lowered.

  Then, when the rotational speed Nd of the motor M rises above the degassing stop rotational speed Ne (rotational speed 60% of the rated rotational speed), the control device 30 stops the air venting by the intermediate air vent means 21. At this time, the discharge temperature T1 of the low-pressure compressor 10 and the discharge temperature T2 of the high-pressure compressor 11 are both less than 230 ° C.

  In the present embodiment, the operation was performed in a state where the air discharge by the intermediate air discharge means 21 was stopped with the air discharge stop rotational speed Ne set to the intake temperature of 30 ° C. of the low pressure compressor 10 and the target pressure set to 0.70 MPa. The rotation speed of the motor M at which the low-pressure stage discharge temperature is 210 ° C. is set. However, instead of this configuration, the control device 30 changes, for example, the discharge stop rotation speed Ne to the discharge start rotation speed Ns (55%). A predetermined value, for example, 5% may be set.

  In the configuration of the present embodiment described above, since the start and stop of the air release by the intermediate air release means 21 are controlled according to the rotation speed of the motor M, the rotation speed of the motor M becomes the set lower limit rotation speed Nmin. Even during no-load operation, the intermediate air release means 21 performs air release.

  However, during no-load operation, the pressure in the discharge flow path 40 is reduced by the discharge of air from the high pressure stage discharge means 43. As a result, the pressure in the intermediate flow path 20 is low and the low pressure stage Since the discharge temperature T1 of the compressor 10 is also equal to or lower than the discharge stop temperature Te (210 ° C. as an example), it is not necessary to release the air by the intermediate discharge means 21.

  On the other hand, if there is a negative pressure in the intermediate stage flow path 20 during no-load operation, outside air may flow into the intermediate stage flow path 20 via the intermediate air release means 21, and therefore, intermediate discharge based on the rotational speed may occur. In the configuration of this embodiment for controlling the air means 21, the control device 30 may close the air release valve 21a of the intermediate air release means 21 during no-load operation.

<Other (variation)>
In the embodiment described with reference to FIG. 3, a configuration is adopted in which the discharge by the intermediate discharge means 21 is started and stopped based on the discharge temperature T <b> 1 of the low-pressure compressor 10, while referring to FIG. 4. In the embodiment described and explained above, a configuration is adopted in which the air release by the intermediate air release means 21 is started and stopped according to the rotational speed of the motor M, but the combination of the both is used. The operation may be controlled.

  That is, the start of release by the intermediate release means 21 is performed based on the discharge temperature T1 of the low-pressure compressor 10, and the stop of release by the intermediate release means 21 is based on the rotational speed Nd of the motor M. Or, on the contrary, the start of the release by the intermediate discharge means 21 is performed based on the rotational speed Nd of the motor M, and the stop of the discharge by the intermediate discharge means 21 is a low-pressure compressor. It may be performed based on the discharge temperature T1 of ten.

1 Multi-stage oil-free screw compressor 10 Low-pressure stage compressor 10a Inlet (for low-pressure stage compressor)
10b Discharge port (for low-pressure compressor)
11 High-pressure stage compressor 11a Inlet (for high-pressure stage compressor)
11b Discharge port (for high-pressure compressor)
13 Suction valve 14 Suction filter 15 Temperature detection means (intake air temperature Ta)
20 Intermediate stage flow path 21 Intermediate air release means 21a Air release valve 21b Air release flow path 21c Silencer 22 Intercooler 23 Temperature detection means (of discharge temperature T1 of the low pressure stage compressor)
30 Control device 31 Inverter 32 Power transmission means (speed-up gear)
40 Discharge flow path 41 Check valve 42 After cooler 43 High pressure stage air release means 43a Air discharge valve 43b Air discharge flow path 43c Silencer 44 Pressure detection means (of discharge pressure Pd)
45 Temperature detection means (for the discharge temperature T2 of the high-pressure compressor)
M Motor T1 Discharge temperature (low pressure stage compressor)
T2 discharge temperature (for high-pressure compressor)
Tmax Emergency stop temperature (ex.250 ℃)
Ts Air release start temperature (ex.230 ℃)
Te air stop temperature (ex.210 ℃)
Ta Intake air temperature Nd Rotational speed (measured value)
Nc Target rotational speed (calculated value)
Ns Air release start rotation speed (ex. 55% of rated rotation speed)
Ne Rotation stop rotation speed (ex. 60% of rated rotation speed)
Nmin setting lower limit rotation speed (ex. 45% of rated rotation speed)
Pd Discharge pressure Pt Target pressure (ex.0.70MPa)
Pu unloading start pressure (ex.0.75MPa)

Claims (20)

In a multi-stage oil-free screw compressor in which a plurality of compressors are connected in series to form a multi-stage, and the compressed gas compressed by the low-pressure stage compressor is introduced into the high-pressure stage compressor for further compression.
The high-pressure stage compressor is made smaller than the low-pressure stage compressor,
Enabling the gas in the intermediate-stage flow path communicating with the discharge port of the low-pressure stage compressor and the intake port of the high-pressure stage compressor to be released into the atmosphere;
When the capacity control is performed by changing the rotational speed of each compressor corresponding to the pressure change of the compressed gas supplied to the consumer side with the intake port of the low-pressure stage compressor open, the low-pressure stage compression When the discharge temperature of the compressor rises above a preset discharge start temperature, or when the rotation speed of the low-pressure compressor drops below a preset discharge start rotation speed, A capacity control method for a multi-stage oil-free screw compressor, characterized in that the capacity control is continued while discharging the gas to the atmosphere. An emergency stop temperature that is a discharge temperature of the low-pressure stage compressor that performs an emergency stop of the multi-stage oil-free screw compressor, and the discharge start temperature that is a predetermined lower discharge temperature than the emergency stop temperature are set,
2. The multistage oil-free screw compression according to claim 1, wherein when the discharge temperature of the low-pressure stage compressor rises above the preset release temperature, the gas in the intermediate stage passage is released into the atmosphere. Capacity control method. The correspondence relationship between the intake air temperature, the rotational speed, and the discharge temperature of the low-pressure compressor at the time of the capacity control in which the release of the gas in the intermediate passage is stopped is obtained in advance, and the multi-stage oil-free screw compressor Set the emergency stop temperature, which is the discharge temperature of the low-pressure compressor,
The intake air temperature is measured, and the rotation speed of the low-pressure stage compressor at which the discharge temperature of the low-pressure stage compressor becomes a predetermined lower discharge temperature than the emergency stop temperature at the measured intake air temperature based on the correspondence relationship is determined. Calculated and set as the discharge start rotation speed, and when the rotation speed of the low-pressure stage compressor falls below the discharge start rotation speed, the gas in the intermediate stage flow path is released into the atmosphere. The capacity control method for a multi-stage oil-free screw compressor according to claim 1. A predetermined low temperature is set as the discharge stop temperature with respect to the discharge temperature of the low-pressure stage compressor at the time when the gas in the intermediate stage passage is started to be released into the atmosphere,
The multi-stage according to any one of claims 1 to 3, wherein when the discharge temperature of the low-pressure stage compressor falls below the discharge stop temperature, the atmospheric discharge of the gas in the intermediate stage flow path is stopped. Capacity control method for oil-free screw compressors. A predetermined high rotational speed is set as the degassing stop rotational speed with respect to the rotational speed of the low-pressure stage compressor at the start of the atmospheric discharge of the gas in the intermediate stage flow path;
The atmospheric discharge of the gas in the intermediate stage flow path is stopped when the rotational speed of the low-pressure stage compressor rises above the degassing stop rotational speed. Capacity control method for multi-stage oil-free screw compressors. The correspondence relationship between the intake air temperature, the rotational speed, and the discharge temperature of the low-pressure compressor at the time of the capacity control in which the release of the gas in the intermediate flow path is stopped is obtained in advance,
The intake air temperature is measured, and a predetermined lower temperature than the discharge temperature of the low-pressure compressor at the time when the atmospheric discharge of the gas in the intermediate stage flow path is started at the measured intake air temperature based on the correspondence relationship Calculate the rotation speed to be set as the degassing stop rotation speed,
The atmospheric discharge of the gas in the intermediate stage flow path is stopped when the rotational speed of the low-pressure stage compressor rises above the degassing stop rotational speed. Capacity control method for multi-stage oil-free screw compressors.   The multistage oil-free according to any one of claims 1 to 6, wherein the capacity control is performed with a fixed ratio of a rotation speed of the low-pressure stage compressor and a rotation speed of the high-pressure stage compressor fixed. Screw compressor capacity control method.   The discharge temperature of the low-pressure compressor and the discharge temperature of the high-pressure compressor immediately after starting the release of the gas in the intermediate channel are substantially the same temperature. The capacity control method for a multi-stage oil-free screw compressor according to any one of claims 1 to 7, wherein the amount of gas released is adjusted.   During the capacity control for discharging the gas in the intermediate stage flow path to the atmosphere, the discharge temperature of the low pressure stage compressor becomes a predetermined lower temperature than the emergency stop temperature at which the multistage oil-free screw compressor is emergency stopped. 2. The rotational speed of the low-pressure stage compressor is measured in advance, and the measured rotational speed is set to a lower limit value of a rotational speed range of the low-pressure stage compressor during the capacity control. The capacity | capacitance control method of the multistage oil-free screw compressor of any one of -8. The intake of the low-pressure compressor, which has a predetermined low discharge temperature with respect to the emergency stop temperature at which the multi-stage oil-free screw compressor is emergency-stopped during the capacity control in which the gas in the intermediate-stage channel is released into the atmosphere. Find the correspondence between temperature and rotational speed in advance,
The intake air temperature is measured, a rotation speed corresponding to the predetermined low discharge temperature is calculated from the measured intake air temperature based on the correspondence relationship, and the calculated rotation speed is used as the low pressure stage compressor at the time of the capacity control. The capacity control method for a multi-stage oil-free screw compressor according to any one of claims 1 to 9, wherein the lower limit value of the rotation speed range is set. A low-pressure stage compressor, and a high-pressure stage compressor that introduces compressed gas compressed by the low-pressure stage compressor and further compresses the compressed gas. In a multi-stage oil-free screw compressor equipped with a capacity control device that performs capacity control for changing the rotational speed corresponding to the pressure change of the compressed gas supplied to the consumer side,
The high-pressure stage compressor is made smaller than the low-pressure stage compressor,
Providing an intermediate air discharge means for releasing the gas in the intermediate stage flow path communicating with the discharge port of the low pressure stage compressor and the intake port of the high pressure stage compressor;
In the capacity control device, when the discharge temperature of the low-pressure stage compressor rises above a preset discharge start temperature, or the rotation speed of the low-pressure stage compressor falls below a preset discharge start rotation speed. A multi-stage oil-free screw compressor is provided, wherein a control device is provided for causing the intermediate air release means to release air to the atmosphere. The control device is
A storage area that stores an emergency stop temperature that is a discharge temperature of the low-pressure stage compressor that makes an emergency stop of the multi-stage oil-free screw compressor, and the discharge start temperature that is a predetermined lower discharge temperature than the emergency stop temperature With
12. The multi-stage oil-free screw compressor according to claim 11, wherein when the discharge temperature of the low-pressure stage compressor rises above the preset discharge start temperature, the intermediate discharge means is released into the atmosphere. . The control device is
Stores the correspondence relationship between the intake air temperature, the rotational speed, and the discharge temperature of the low-pressure compressor at the time of the capacity control in which the release of the gas in the intermediate passage is stopped, and the multi-stage oil-free screw compressor is A storage area for storing an emergency stop temperature, which is a discharge temperature of the low-pressure stage compressor to be stopped,
Based on the correspondence relationship, a rotation speed at which the intake air temperature becomes a predetermined lower temperature than the emergency stop temperature is calculated and set as the discharge start rotation speed.
12. The multi-stage oil-free screw compressor according to claim 11, wherein when the rotational speed of the low-pressure stage compressor drops below the air release start rotational speed, the intermediate air release means performs the atmospheric discharge. The control device sets a predetermined low temperature as the discharge stop temperature with respect to the discharge temperature of the low-pressure compressor at the start of the atmospheric discharge by the intermediate discharge means,
The multistage oil-free screw according to any one of claims 11 to 13, wherein when the discharge temperature of the low-pressure stage compressor drops below the discharge stop temperature, the intermediate discharge unit stops the atmospheric discharge. Compressor. The control device sets a predetermined high rotation speed as a rotation speed of the low-pressure stage compressor at the time of starting the atmospheric discharge by the intermediate discharge means as a discharge stop rotation speed;
The multistage oil according to any one of claims 11 to 13, wherein when the rotation speed of the low-pressure stage compressor is increased to be equal to or higher than the discharge stop rotation speed, the atmospheric discharge by the intermediate discharge means is stopped. Free screw compressor. The control device is
A storage area for storing a correspondence relationship between the intake air temperature, the rotational speed, and the discharge temperature of the low-pressure compressor at the time of the capacity control in which the atmospheric discharge by the intermediate discharge means is stopped;
Based on the correspondence relationship, the rotational speed at which the intake air temperature becomes a predetermined low temperature with respect to the discharge temperature of the low-pressure compressor at the start of atmospheric release by the intermediate air discharge means is calculated from the measured intake air temperature. And set it as the venting stop rotation
The multi-stage according to any one of claims 11 to 13, wherein when the rotational speed of the low-pressure stage compressor is increased to be greater than or equal to the exhaust gas stop rotational speed, the atmospheric discharge by the intermediate exhaust means is stopped. Oil-free screw compressor.   A motor common to the low-pressure stage compressor and the high-pressure stage compressor, and power transmission means for transmitting the rotation of the motor to the low-pressure stage compressor and the high-pressure stage compressor at a constant speed increase ratio. The multistage oil-free screw compressor according to any one of claims 11 to 16.   The amount of air discharged by the intermediate air discharge means is set so that the discharge temperature of the low pressure stage compressor immediately after the start of air discharge by the intermediate air discharge means and the discharge temperature of the high pressure stage compressor are substantially the same temperature. The multistage oil-free screw compressor according to any one of claims 11 to 17. The control device is
The discharge temperature of the low-pressure stage compressor is set to an emergency stop temperature that is the discharge temperature of the low-pressure stage compressor that stops the multi-stage oil-free screw compressor at the time of the capacity control in which the air is released by the intermediate air release means. And storing the rotational speed of the low-pressure compressor at a predetermined low temperature,
The multistage oil-free screw compressor according to any one of claims 11 to 18, wherein the rotation speed is set to a lower limit value of a rotation speed range at the time of the capacity control. The control device is
A predetermined low discharge temperature with respect to an emergency stop temperature, which is a discharge temperature of a low-pressure stage compressor that performs an emergency stop of the multi-stage oil-free screw compressor, during the capacity control in which the gas in the intermediate stage passage is released into the atmosphere. A storage area storing the correspondence between the intake air temperature and the rotational speed of the low-pressure stage compressor,
Based on the correspondence, the rotational speed corresponding to the predetermined low temperature is calculated from the measured intake air temperature,
The multistage oil-free screw compressor according to any one of claims 11 to 18, wherein the rotation speed is set to a lower limit value of a rotation speed range at the time of the capacity control.

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