JPH07107396B2 - Oil-free screw compressor anti-rust method and anti-rust device and oil-free screw compressor equipped with anti-rust device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oil-free screw compressor rust preventive method and a rust preventive device, which are suitable for preventing rust generated on a rotor or the like when the oil-free screw compressor is out of operation. And an oil-free screw compressor provided with the rust preventive device.

[0002]

2. Description of the Related Art Conventional oil-free screw compressors are
For example, as described in Japanese Patent Application Laid-Open No. 56-14895, water contained in a gas sucked into a compressor is first condensed and condensed by cooling the gas discharged from the compressor. Then, the gas is separated by an oil / gas separator, introduced into a water-containing oil / moisture separator, and the oil-in-water liquid separated by the oil / water separator is circulated through a filter to further produce oil. A method has been proposed in which oil-in-water is separated and the oil-in-water is again passed through a filter to remove water contained in the injection oil in the oil circulation system of the oil injection type screw compressor.

However, the above-mentioned prior art does not take into consideration that the rotor is corroded by the moisture contained in the gas when the compressor is stopped, and there is a problem that the rotor is corroded.

Therefore, in order to solve the above problems, conventionally, for example, a method of applying a coating on the surface of the rotor and an inner surface of the casing and a method of using a rotor formed of a corrosion resistant material such as stainless steel have been implemented. There is.

[0005]

The above-mentioned conventional techniques have the following problems, respectively.

(1) The method of applying the rust preventive paint is such that the rust preventive paint cannot withstand the high temperature of the compressor and the environmental conditions at the time of high speed rotation, and the rust preventive effect is lost due to abrasion and peeling. I will end up.

(2) The method of using a corrosion resistant material such as stainless steel has many problems in slidability and economy, and cannot be put to practical use.

An object of the present invention is to enable rust prevention of a loader or the like when the compressor is stopped without applying a rust preventive paint to the rotor or forming the rotor with a corrosion resistant material such as stainless steel. An object of the present invention is to provide a method for preventing rust of an oil-free screw compressor, a rust preventive device, and an oil free screw compressor provided with the rust preventive device.

[0009]

In order to achieve the above object, the rust preventive method for an oil-free screw compressor according to the first aspect of the present invention is such that when the compressor is operated, the suction side of the one-stage compressor is kept for a predetermined time. Close the installed intake valve, open the blow-off valve provided on the discharge side of the final stage compressor, drive all compressors, supply cooling water to the intercooler installed between the compressors, and perform unloading operation. The gas and water in the path from the suction valve to the blowoff valve are removed.

Further, the rust preventive method for the oil-free screw compressor according to the second aspect of the invention is such that when the compressor is stopped, the suction valve installed on the suction side of the first-stage compressor is closed for a predetermined time, and the final-stage compressor is closed. Open the blow-off valve installed on the discharge side, drive all the compressors, supply cooling water to the intercooler installed between the compressors to perform unloading operation, and from the intake valve to the blow-off valve. When the dew point temperature between the compressors reaches a preset value before removing the gas and water in the path up to the predetermined time, the cooling water to all the compressors and the intercooler To stop the supply of.

Further, in the rust preventive method for the oil-free screw compressor according to the third aspect of the present invention, when the compressor is stopped, the suction valve provided on the suction side of the first-stage compressor is closed for a predetermined time and the final-stage compressor is closed. Open the blowoff valve provided on the discharge side, perform all the unloading operation by driving all the compressors, discharge the gas and moisture in the path from the suction valve to the blowoff valve from the blowoff valve, When the dew point temperature between the compressors reaches a preset value before the predetermined time is reached, the driving of all the compressors is stopped.

The rust preventive device for an oil-free screw compressor according to the fourth aspect of the present invention comprises a suction valve on the suction side of the one-stage compressor,
An intercooler between the compressors, an exhaust valve on the discharge side of the final stage compressor, a valve drive means for opening and closing the intake valve and the exhaust valve, and when shutting down the compressor, Control means for closing the intake valve and opening the blow-off valve via the valve driving means, driving all the compressors, supplying cooling water to the intercooler to perform unloading operation, and the control means. And a timer for driving for a predetermined time.

The rust preventive device for an oil-free screw compressor according to the fifth aspect of the present invention comprises a suction valve on the suction side of the one-stage compressor,
An intercooler between the compressors, an exhaust valve on the discharge side of the final stage compressor, a valve drive means for opening and closing the intake valve and the exhaust valve, and when shutting down the compressor, Control means for closing the intake valve and opening the blow-off valve via the valve driving means, driving all compressors, supplying cooling water to the intercooler to perform unloading operation, and the control means. When a dew point temperature between the timer for driving for a predetermined time and the compressor is detected and the detected temperature reaches a preset value before reaching the predetermined time set by the timer, the control means is operated. And a dew point temperature detecting means for stopping the unloading operation.

The rust preventive device for the oil-free screw compressor according to the sixth aspect of the present invention comprises a suction valve on the suction side of the one-stage compressor,
A discharge valve of the final stage compressor is provided with a blow-off valve and a valve drive means for opening and closing the suction valve and the blow-off valve, and when the compressor is stopped, the above-mentioned valve drive means is used. The intake valve is closed, the blow-off valve is opened, and at the same time, the control means for driving all the compressors to perform the unloading operation, the timer for driving the control means for a predetermined time, and the dew point temperature between the compressors are detected. However, when the detected temperature reaches a preset value before reaching the predetermined time set by the timer, it includes dew point temperature detecting means for stopping the unloading operation via the control means. is there.

The oil-free screw compressor provided with the rust preventive device of the seventh invention has a suction valve on the suction side of the first-stage compressor, an intercooler between the compressors, and a discharge of the final-stage compressor. After cooler, blow valve, pressure switch on the side,
Valve driving means for opening and closing the intake valve and the blow-off valve, and opening the suction valve through the valve-driving means, closing the blow-off valve, driving all the compressors, the intercooler and the When cooling water is supplied to the aftercooler to perform compression operation, when the pressure on the discharge side of the aftercooler reaches a preset value and the pressure switch detects it, and when the compressor is stopped, The intake valve is closed via a valve drive means, the blowoff valve is opened, all the compressors are driven, and control means for supplying cooling water to the intercooler to perform unloading operation is provided, And, when stopping the operation of the compressor, a timer for unloading operation for a predetermined time through the control means,
The dew point temperature between the compressors is detected, and when the detected temperature reaches a preset value before reaching the predetermined time set by the timer, the unloading operation is stopped via the control means. The dew point temperature detecting means and the switching means for switching and connecting any one of the circuit connecting the dew point temperature detecting means, the timer and the OR circuit, or the circuit having the pressure switch to the control means.

An oil-free screw compressor provided with the rust preventive device of the eighth aspect of the invention is a suction valve on the suction valve side of the compressor, an aftercooler, a blower valve, a pressure switch on the discharge side, and the suction valve. Valve driving means for opening and closing the valve and the blow-off valve, and opening the suction valve through the valve driving means, closing the blow-off valve, driving the compressor, and supplying cooling water to the aftercooler. When the discharge side pressure of the aftercooler reaches a preset value and the pressure switch detects it, and when the compressor is stopped, the suction valve is operated via the valve driving means. And opening the blow-off valve, driving the compressor, stopping the supply of cooling water to the aftercooler to perform an unload operation, and suspending the operation of the compressor. When the control A timer for operating predetermined time unloading through the stage to detect the dew point temperature between the compressor,
When the detected temperature reaches a preset value before reaching the predetermined time set by the timer, the dew point temperature detecting means for stopping the unloading operation via the control means, and the dew point temperature detecting means. , The above timer and O
A switching means for switching and connecting either the circuit connecting the R circuit or the circuit having the pressure switch to the control means.

[0017]

According to the first and fourth aspects of the invention, when the compressor is stopped for a long period of time, the control means closes the intake valve via the valve drive means for a predetermined time set by the timer, and the blowoff valve is turned on. As it opens, it drives all the compressors and supplies cooling water to the intercooler to unload all the above compressors, so that the gas and moisture in the path from the intake valve to the blowoff valve are released. Discharge from valve. Then, after a lapse of a predetermined time, the unloading operation is stopped via the control means. Therefore, it is possible to prevent rust when the compressor is stopped.

In the second and fifth aspects of the invention, when the compressor is stopped, the control means closes the intake valve via the valve drive means for a predetermined time set by the timer, and the blowoff valve is turned on. While opening, all the compressors are driven, cooling water is supplied to the intercooler, and all the compressors are unloaded.Therefore, gas and moisture in the path from the intake valve to the blowoff valve are Discharge from the blowoff valve. Then, when a predetermined time elapses or when the detected value of the dew point temperature between the compressors by the dew point temperature detection means reaches a preset value before the predetermined time, the unload operation is stopped via the control means. To do. Therefore, the unloading operation time by the compressor can be shortened.

Further, in the third and sixth aspects of the invention, when the operation of the compressor is stopped, the control means closes the intake valve via the valve drive means for a predetermined time set by the timer, and the blowoff valve is turned on. Since all the compressors are opened and the compressors are unloaded while being opened, the suction temperature of the compressors after the second stage rises, which promotes the removal of water. Thereafter, when the detected value of the dew point temperature between the compressors by the dew point temperature detection means reaches a preset value before the predetermined time by the timer, the unload operation is stopped via the control means. Therefore, the unloading operation time by the compressor can be further shortened.

In the seventh aspect of the invention, the control means and the pressure switch are connected by the switching means during the normal compression operation. In this state, the control means opens the intake valve through the valve drive means, closes the blowoff valve, drives all compressors, and supplies cooling water to all intercoolers and aftercoolers. The compressor performs a compression operation, and the compressed gas is discharged after being cooled by the intercooler and the aftercooler. Then, when the pressure of the gas on the discharge side of the aftercooler reaches a preset pressure, the pressure switch outputs an electric signal to the control means. Therefore, the control means closes the suction valve and opens the blow-off valve via the valve driving means, so that all the compressors are switched to the unloading operation.

Then, when the pressure of the gas on the discharge side of the aftercooler drops below a preset pressure, the electrical signal from the pressure switch is cut off.
The control means again causes the compressor to perform a compression operation.

Next, when the operation of the compressor is stopped, the control means is switched and connected to the OR circuit by the switching means, and the control means is operated through the valve drive means for the predetermined time set by the timer, and the normal compression is performed. Similar to the unloading operation during operation, the intake valve is closed, the blowoff valve is opened, all compressors are driven, and cooling water is supplied to the intercooler.
Since all the compressors are operated in the unloading mode, gas and water in the path from the suction valve to the blowoff valve are discharged from the blowoff valve. Then, when a predetermined time elapses or before the predetermined time, the detected value of the dew point temperature between the compressors by the dew point temperature detection means reaches a preset value, the unloading operation is performed via the control means. Stop. Therefore, the switching means can perform normal compression operation and rust prevention when the compressor operation is stopped.

The eighth invention is a case where the compressor in the seventh invention is a single stage.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the first and fourth inventions will be described below with reference to FIG. As shown in FIG.
At the suction side of the stage compressor 1, a suction valve 4 for adjusting the amount of intake air
Is installed, and the discharge side is connected to the suction side of the two-stage compressor 2 via the intercooler 3. On the discharge side of the two-stage compressor 2, a blowoff valve 5 for adjusting the amount of discharge air is installed. The intake valve 4 and the blow-off valve 5 are coated with an anticorrosion coating on their inner surfaces, respectively, and are connected to a pair of valve driving means 6. The valve driving means 6 is a control means 10 described later.
Hydraulic supply source 7 driven by the command
And a piston 9. The hydraulic cylinder 8 simultaneously closes the suction valve 4 and opens the blow valve 5 via the piston 9 by the hydraulic pressure from the hydraulic supply source 7, and the command from the control means 10 is stopped. At this time, the supply of the hydraulic pressure from the hydraulic pressure supply source 7 is stopped, and the suction valve 4 and the blowoff valve 5 maintain the above state. The control means 10 is connected to the timer 11 and gives a command to the hydraulic pressure supply source 7 for a predetermined time set by the timer 11 and drives the compressors 1 and 2 and the intercooler 3.

Next, a method of preventing corrosion when the compressors 1 and 2 are out of operation will be described.

When the compressors 1 and 2 are stopped, the control means 10 gives a command to the valve drive means 6 for a predetermined time set by the timer 11 to close the intake valve 4 and blow off the blowoff valve 5. And the compressors 1 and 2 are driven to supply cooling water to the intercooler 3. Therefore, the compressors 1 and 2 perform unloading operation, and discharge gas and water in the path from the suction valve 4 to the blowoff valve 5 from the blowoff valve 5.

Then, after a lapse of a predetermined time, the control means 10 stops the command to the valve driving means 6 and also stops the driving of the compressors 1 and 2 to cool the cooling water to the intercooler 3. Stop the supply. Therefore, even if the compressors 1 and 2 are out of operation, the rotor and the like can be rust-proofed.

Next, FIG. 2 showing one embodiment of the second invention and the fifth invention will be described.

The embodiment shown in FIG. 2 is different from the embodiment shown in FIG. 1 in that the dew point temperature detecting means 12 is provided between the first-stage compressor 1 and the second-stage compressor 2, and the dew-point temperature is different. The difference is that the OR circuit 13 is installed between the detection means 12 and the timer 11 and the control means 10, and other elements are the same, so the same reference numerals are given and the differences will be described.

The dew point temperature detecting means 12 detects the dew point temperature between the compressors 1 and 2, and when the detected value reaches a preset value, an electric signal is sent to the OR circuit 1.
Output to 3. The OR circuit 13 outputs the electric signal from the timer 11 for a predetermined time while the dew point temperature detecting means 12 is in operation.
When an electrical signal is output from the control means 10, the driving of the control means 10 is stopped.

Therefore, in this embodiment, since the compressors 1 and 2 are supplied for operation, the time required for unloading operation can be shortened.

Next, one embodiment of the third invention and the sixth invention will be described with reference to FIG.

The third invention and the sixth invention are the same as the second invention.
Compared with the invention of 5 and the fifth invention, the operation of the compressors 1 and 2 is stopped, so that the driving of the intercooler 3 is stopped when the compressors 1 and 2 are unloaded, and Since they are the same, the differences will be described.

In this embodiment, the driving of the intercooler 3 is stopped when the compressors 1 and 2 are in the unloading operation. Therefore, the temperature sucked into the two-stage compressor 2 rises and the removal of water is promoted.

Therefore, since the compressors 1 and 2 are stopped, the unloading time can be further shortened.

Next, FIG. 3 showing an embodiment of the seventh invention.
Will be described. The same parts as those in FIGS. 1 and 2 are designated by the same reference numerals.

In FIG. 3, 14 is a valve driving means,
It is composed of a hydraulic pressure supply source 7, a solenoid valve 15, a hydraulic pressure switching valve 16, a hydraulic cylinder 8, and a piston 9. The solenoid valve 15 gives an excitation signal and a demagnetization signal to the hydraulic pressure switching valve 16 in response to a signal from a driving means 18 installed in a control means 17 described later. The hydraulic pressure switching valve 16 switches the hydraulic pressure from the hydraulic pressure supply source 7 by the excitation signal from the electromagnetic valve 15 and supplies the hydraulic pressure to the hydraulic cylinder 8, and the hydraulic pressure thereof causes the suction valve 4 to pass through the piston 9. Open, the blow-off valve 5 is closed, and the suction valve 4 is closed and the blow-off valve 5 is opened by a demagnetization signal from the solenoid valve 15.
The control means 17 is composed of a drive means 18 and a switching means 19. The switching means 19 is the driving means 1
8 is switched and connected to either the OR circuit 13 or the pressure switch 20. The pressure switch 20 is installed on the discharge side of the two-stage compressor 2, and the gas pressure on the discharge side of the aftercooler 21 installed on the discharge side of the two-stage compressor 2 is
When the preset value is reached, the drive means 1
An electrical signal is output to 8. The drive means 18 outputs a command to the solenoid valve 15, and the compressor 1,
2 Above intercooler 3 and above aftercooler 21
Supply or stop the cooling water. Reference numeral 22 denotes a check valve, which is closed by the drive means 18 when the compressors 1 and 2 are unloading-operated to prevent backflow from the aftercooler 21.

Next, a normal compression operation method and a rust preventive method when the operation is stopped will be described.

First, in the case of performing a normal compression operation, when the driving means 18 is switched and connected to the pressure switch 20 side by the switching means 19, the driving means 18 is connected to the valve driving means 1 described above.
4 to open the intake valve 4 and close the blowoff valve 5. Further, the driving means 18 drives the compressors 1 and 2 and supplies the cooling water to the intercooler 3 and the aftercooler 21, so that the compressors 1 and 2 perform a normal compression operation and are compressed. The gas is discharged through the aftercooler 21. Then, when the pressure of the gas on the discharge side of the aftercooler 21 reaches the value set by the pressure switch 20, the pressure switch 20 outputs an electric signal to the driving means 18. Therefore, the drive means 18 gives a command to the valve drive means 14 to close the intake valve 4 and open the blowoff valve 5 to perform an unload operation. Next, when the pressure of the gas on the discharge side of the aftercooler 21 drops, the drive means 18 causes the normal compression operation.

Next, when the compressors 1 and 2 are stopped, the switching means 19 causes the drive means 18 to operate in the OR circuit 1.
When the connection is switched to the 3 side, the drive means 18 gives a command to the valve drive means 14 to close the intake valve 4 and open the blowoff valve 5 for a predetermined time set by the timer 11. Further, the drive means 18 drives the compressors 1 and 2 and supplies the cooling water to the intercooler 3, so that the compressors 1 and 2 perform the unload operation, and the suction valve 4 and the blowoff valve 5 are operated.
The gas and moisture in the path up to are discharged from the blowoff valve 5. Then, when the dew point detection value between the compressors 1 and 2 by the dew point temperature detection means 12 reaches a preset value before or after the elapse of a predetermined time, the dew point temperature detection means 12 Since an electric signal is output by the OR circuit 13, the OR circuit 13 is demagnetized, the supply of the pressure oil from the hydraulic pressure supply source 7 of the valve drive means 14 via the drive means 18 is stopped, and the compressor 1, 2 and the driving of the intercooler 3 are stopped.

Therefore, by switching the switching means, it is possible to easily perform rust prevention during normal compression operation and when the compressor operation is stopped.

Next, FIG. 4 showing an embodiment of the eighth invention will be described.

The embodiment shown in FIG. 4 is different from the embodiment shown in FIG. 3 in that the intercooler 3 and the two-stage compressor 2 are not installed because the compressor has one-stage compression. , And others are the same, and therefore, the same reference numerals are used and the description thereof is omitted.

In the above embodiment, the case of two-stage compression and one-stage compression is shown, but the present invention is not limited to this and can be applied to a compressor having three or more stages.

[0045]

Since the present invention is constructed as described above, it has the following effects.

(1) According to the first and fourth aspects of the invention, when the compressor is stopped, all the compressors and intercoolers are unloaded for a predetermined time preset by the timer and sucked. Gas and water in the path from the valve to the blowoff valve are discharged from the blowoff valve 5. Therefore, the rotor and the like can be rust-prevented when the compressor is stopped.

(2) Due to the above (1), it is possible to prevent a serious accident such as inability to restart due to the rusted lock of the rotor,
The reliability can be improved.

(3) Due to the above (1), disassembling and repairing the compressor by the lock with rust requires a great number of man-hours and expenses, and can prevent a large damage to the user.

(4) Due to the above (1), the performance deterioration due to the corrosion of the rotor can be prevented, the energy can be saved, and the economical effect is great.

(5) According to the second and fourth aspects of the invention, all the compressors and intercoolers are unloading-operated to remove gas and water in the path from the intake valve to the blowoff valve. When the dew point temperature between the compressors reaches a preset value before the predetermined time set by the timer is reached, the driving of all the compressors and intercoolers is stopped. Therefore, the unloading operation of all the above compressors and intercoolers can be shortened.

(6) According to the third and sixth aspects of the invention, when all the compressors are unloaded, the driving of all the intercoolers is stopped. The suction temperature of the machine rises, which can accelerate the removal of water. Therefore, the dew point temperature between the compressors reaches the preset value at an early stage, and the unloading operation time can be further shortened.

(7) According to the seventh invention, by switching the switching means, the normal compression operation and the unload operation are continuously performed, and the unload operation when the compressor operation is stopped is performed. Therefore, with normal compression operation,
It is easy to switch the operation of the compressor when the compressor is stopped. In addition, since water is removed by the unload operation during normal compression operation when the compressor is stopped, there is no need to install a special mechanism for removing water, so the configuration is simple and the operability is simple. Can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a rust preventive device when a compressor is stopped according to a first embodiment of the present invention.

FIG. 2 is an explanatory view showing a rust preventive device when the compressor is stopped according to a second embodiment of the present invention.

FIG. 3 is an explanatory view showing a case where a rust preventive device when a compressor is stopped is installed in a normal compression operating device which is a third embodiment of the present invention.

FIG. 4 is an explanatory view showing a case where a rust preventive device when a compressor is stopped is installed in a normal one-stage compression operating device which is a fourth embodiment of the present invention.

[Explanation of symbols]

 1, 2 Compressor 3 Intercooler 4 Suction valve 5 Blowoff valve 6 Valve driving means 10, 17 Control means 11 Timer 12 Dew point temperature detecting means 18 Driving means 19 Switching means 20 Pressure switch 21 Aftercooler

Claims (8)

[Claims] 1. A predetermined time period 1 when the compressor is stopped.
Close the suction valve installed on the suction side of the stage compressor, open the blow-off valve installed on the discharge side of the final stage compressor, drive all compressors, and cool the intercooler installed between the compressors with cooling water. Is supplied to perform an unloading operation to remove gas and moisture in the path from the intake valve to the blowoff valve, and a rust preventive method for an oil-free screw compressor. 2. A predetermined time period 1 when the compressor is stopped.
Close the suction valve installed on the suction side of the stage compressor, open the blow-off valve installed on the discharge side of the final stage compressor, drive all compressors, and cool the intercooler installed between the compressors with cooling water. To perform unloading operation to remove gas and moisture in the path from the suction valve to the blowoff valve, and before the predetermined time is reached, the dew point temperature between the compressors reaches a preset value. When it reaches, the driving of all the compressors is stopped, and the supply of the cooling water to the intercooler is stopped, and the rust preventive method for the oil-free screw compressor. 3. A predetermined time period 1 when the compressor is stopped.
The suction valve installed on the suction side of the stage compressor is closed, the blowoff valve installed on the discharge side of the final stage compressor is opened, and all compressors are driven to perform the unloading operation, and the discharge valve discharges the above The gas and water in the path to the valve are discharged from the blowoff valve, and when the dew point temperature between the compressors reaches a preset value before the predetermined time is reached, the drive of all the compressors is stopped. A method for preventing rust of an oil-free screw compressor, which is characterized by: 4. A suction valve on the suction side of the first-stage compressor, an intercooler between the compressors, a blow-off valve on the discharge side of the final-stage compressor, and the intake valve and the blow-off valve are opened and closed. A valve drive means, and when stopping the operation of the compressor, the suction valve is closed via the valve drive means for a certain period of time, the blow-off valve is opened, and all the compressors are driven.
A rust preventive device for an oil-free screw compressor, comprising: a control unit that supplies cooling water to the intercooler to perform an unloading operation; and a timer that drives the control unit for a predetermined time. 5. A suction valve on the suction side of the first-stage compressor, an intercooler between the compressors, a blow-off valve on the discharge side of the final-stage compressor, and the suction valve and the blow-off valve are opened and closed. Valve drive means and when the compressor is stopped, the suction valve is closed via the valve drive means, the blow-off valve is opened, and all the compressors are driven to operate the intercooler. Control means for supplying cooling water to perform unloading operation, a timer for driving the control means for a predetermined time, and a dew point temperature between the compressors are detected, and the detected temperature reaches a predetermined time set by the timer. A rust preventive device for an oil-free screw compressor, comprising dew point temperature detecting means for stopping the unloading operation via the control means when the value reaches a value previously preset. 6. A suction valve on the suction side of the one-stage compressor, a blowoff valve on the discharge side of the final stage compressor, and valve drive means for opening and closing the suction valve and the blowoff valve, and Control means for closing the intake valve and opening the blow-off valve via the valve driving means when the compressor is stopped, and driving all the compressors for unloading operation, and the control means. When a dew point temperature between the timer for driving for a predetermined time and the compressor is detected and the detected temperature reaches a preset value before reaching the predetermined time set by the timer, the control means is operated. A rust preventive device for an oil-free screw compressor, which is provided with dew point temperature detection means for stopping the unloading operation. 7. A suction valve on the suction side of the first-stage compressor, an intercooler between the compressors, an aftercooler, a blow valve, a pressure switch on the discharge side of the final-stage compressor, the suction valve and the above-mentioned. Valve driving means for opening and closing the blow-off valve, opening the suction valve through the valve driving means, closing the blow-off valve, driving all the compressors, and cooling water to the intercooler and the aftercooler. Is supplied to perform a compression operation, when the pressure on the discharge side of the aftercooler reaches a preset value and the pressure switch detects it, and when the compressor is stopped, the valve drive means is used. The intake valve is closed, the blowoff valve is opened, all the compressors are driven, and a control means for supplying cooling water to the intercooler to perform an unloading operation, and suspending the operation of the compressor Do At this time, a dew point temperature between the timer for unloading for a predetermined time and the compressor is detected through the control means, and the detected temperature is a preset value before reaching the predetermined time set by the timer. The dew point temperature detecting means for stopping the unloading operation via the control means, and any one of the circuit connecting the dew point temperature detecting means, the timer and the OR circuit, or the circuit having the pressure switch. An oil-free screw compressor provided with a rust preventive device, which is provided with a switching means for switching and connecting to a control means. 8. A suction valve on the suction valve side of the compressor, an aftercooler, a blowoff valve, a pressure switch on the discharge side, valve drive means for opening and closing the suction valve and the blowoff valve, and the valve drive means. Open the suction valve, close the blower valve, drive the compressor, supply cooling water to the aftercooler to perform compression operation, and set the discharge side pressure of the aftercooler in advance. Value is reached by the pressure switch and when the compressor is shut down,
Control means for closing the intake valve via the valve driving means, opening the blowoff valve, driving the compressor, stopping the supply of cooling water to the aftercooler and performing unloading operation. And a timer for unloading the compressor for a predetermined time when the compressor is stopped, and a dew point temperature on the discharge side of the compressor is detected, and the detected temperature is set by the timer. Or a circuit connecting the dew point temperature detecting means for stopping the unloading operation via the control means and the dew point temperature detecting means, the timer and the OR circuit when the preset value is reached before reaching the predetermined time. An oil-free screw compressor provided with a rust preventive device, comprising: switching means for switching and connecting one of the circuits having the pressure switch to the control means.