JPH04159491A - Method and device for controlling volume of screw compressor - Google Patents

Abstract

PURPOSE:To improve an effect of saving energy and reliability of a device by changing each upper or lower limit pressure preset value of an on-off control valve in accordance with a compressed gas use amount in a compressor load side, and controlling an on-off period to the preset value or more. CONSTITUTION:Air is sucked from a suction port 14, to pass through a suction filter 19 and a suction valve (on-off type control valve) 2 in a fully-closed condition, and advanced into a compressor main unit 1. Air of high temperature and high pressure, compressed by the compressor main unit 1, is fed into an air storage tank 11 after passing through a check valve 13 and an aftercooler 12. Further, a hydraulic piston 4 opens/closes the suction valve 2 and a blow-off valve 3 by oil supplied from an oil storage tank 7 by a four-way electromagnetic valve 5 and an oil hydraulic pump 6. Here, a delivery side pressure is detected by a pressure sensor 8, and further, a control unit 9, when the delivery side pressure reaches, for instance, an upper limit(pressure preset)value, switches an oil passage of the four-way electromagnetic valve 5 to actuate the hydraulic piston 4. Simultaneously with closing the suction valve 2, the blow-off valve 3 is opened to place the compressor 1 in a no-load operational condition.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method and apparatus for controlling the capacity of a screw compressor equipped with an on-off type control valve on the suction side.

[Conventional technology]

A conventional capacity control device for a screw compressor equipped with an on-off type control valve has an on-off type control valve and a discharge blow-off valve, as described in, for example, Japanese Patent Publication No. 1-33676. The pressure on the discharge side is increased, and when it exceeds a certain set value, the on/off control valve is closed and the air discharge valve is opened to switch to no-load operation, and then when the pressure inside the storage tank on the discharge side decreases and becomes below a certain set value. The on-off control valve is opened and the blowoff valve is closed to switch to full-load operation.

In such conventional devices, the pressure on the discharge side of the compressor is detected by a pressure switch, and a command signal for switching between full load and no load operation is issued in accordance with this detected value.

[Problem to be solved by the invention]

In the above conventional technology, the pressure setting value that issues the command signal for switching between full load and no load operation is fixed, so full load operation and no load operation are performed at a constant pressure value regardless of the capacity of the storage tank on the discharge side of the compressor. was switching. Therefore, if the air consumption (operating load) on the discharge side is constant, the larger the capacity of the discharge piping system (including the air storage tank), the greater the switching interval time (switching period or (on/off cycle) was getting longer. Further, even when the operating load is small, the switching cycle between full load and no load operation becomes longer.

For this reason, there is a problem in that the compressor operates for a long time at a discharge pressure higher than the required pressure on the air consumption side, and unnecessary power is consumed.

Furthermore, as the amount of air used on the discharge side increases, the switching cycle between full load and no-load operation becomes shorter, and the frequent repetition of full-load and no-load operation causes the problem of reduced reliability of the equipment. .

The purpose of the present invention is to provide a screw compressor that can improve energy saving effects by reducing operating time due to unnecessary high pressure when the operating load (gas consumption amount on the consumption side) is small or when the capacity of the discharge side piping system is large. The object of the present invention is to obtain a capacity control method and device.

Another object of the present invention is a capacity control method for a screw compressor that improves the reliability of the device by increasing the switching cycle between full load and no-load operation of the device to a certain value or more, and that can always ensure the required pressure value on the consumption side. and equipment.

[Means to solve the problem]

The first feature of the present invention to achieve the above object is to control an on-off control valve provided on the suction side of the screw compressor by the pressure on the discharge side of the compressor to perform full-load operation and no-load operation. In a screw compressor, the amount of compressed gas used on the load side of the compressor is detected, and depending on this amount, an upper limit pressure setting value for turning off the on-off control valve or a lower limit pressure setting value for turning on the on-off control valve is set. The present invention provides a capacity control method for a screw compressor, in which at least one of the on-off control valves is changed to control the on-off period of the on-off control valve to a predetermined setting value or more. Note that the amount of compressed gas used on the compressor load side can also be determined from the pressure change rate on the compressor load side.

A second feature of the present invention is that in a screw compressor that performs full load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is detected. When the pressure value reaches the upper limit pressure set value, the on-off type control valve is turned off and the compressor is operated under no load, and when the detected pressure value reaches the lower limit pressure set value, the on-off type control valve is turned off. The present invention provides a capacity control method for a screw compressor, in which the compressor is operated at full load by turning on the on-off control valve, and the upper limit pressure set value is corrected so that the length of the on-off cycle of the on-off type control valve falls within a set range.

A third feature of the present invention is that in a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is detected. pressure detection means for turning on and off the on-off type control valve, and an upper limit pressure set value and a lower limit pressure set value for turning on and off the on-off type control valve, and compares these pressure set values with the pressure value from the pressure detection means to perform the on-off type control valve. A screw compression device comprising a control means for turning on and off a valve, and a set value correction means for correcting at least one of the upper limit pressure set value or the lower limit pressure set value so that the on-off cycle of the on-off type control valve is equal to or higher than a set value. It is in the machine's capacity control device.

A fourth feature of the present invention is that in a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is A pressure detector is provided to detect the on-off type control valve, and a lower limit pressure setting value and an upper limit pressure setting value are provided for turning on and off the on-off type control valve, and these pressure settings are compared with the pressure value from the pressure detector to detect the a control device for on/off controlling an on-off type control valve; a means for detecting the magnitude of the load on the compressor load side; and the upper limit pressure set value or the lower limit pressure set value depending on the magnitude of the load from the load detecting means. The present invention provides a capacity control device for a screw compressor, comprising set value correcting means for correcting at least one of the above and setting the on/off period of the on/off type control valve to be equal to or greater than a predetermined set value.

A fifth feature of the present invention is that in a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is detected. In order to control the on-off type control valve, it has a pressure detector, a lower limit pressure set value and an upper limit pressure set value for turning on and off the on-off type control valve, and compares these pressure set values with the pressure value from the pressure detector to turn on and off the on-off type control valve. Capacity of a screw compressor comprising: a control device for controlling on/off of a control valve; and a set value correction means for correcting the upper limit pressure set value so that the length of the on/off cycle of the on/off type control valve falls within a set range. Located in the control device.

The set value correction means detects the amount of compressed gas used (load size) on the compressor load side, and corrects the upper limit pressure set value accordingly.

The sixth feature of the present invention is that the screw compressor is equipped with an on-off type control valve provided on the suction side and an air blowing valve provided on the discharge side thereof, so that full load operation and no load operation are performed depending on the pressure on the compressor discharge side. In a screw compressor that operates by switching, at least one of the upper limit pressure set value for switching from full load operation to no load operation or the lower limit pressure set value for switching from no load operation to full load operation is A capacity control device for a screw compressor is configured to correct the switching period according to the magnitude of the compressor load so that it is equal to or greater than a set value.

In addition, if the lower limit pressure set value is kept constant and only the upper limit pressure set value is corrected according to the compressor load so that the full load/no-load switching cycle is within the set range, the minimum pressure can be ensured. Moreover, since the upper limit pressure can be set to the minimum necessary pressure, the energy saving effect is further improved.

Further, instead of setting the upper limit pressure setting value steplessly, a plurality of upper limit pressure setting values may be prepared in advance, and the upper limit pressure setting value may be switched depending on the compressor load.

Furthermore, the pressure drop rate during no-load operation is detected, and the lower limit pressure set value is set according to the pressure drop rate to prevent the discharge side pressure from dropping below the required minimum pressure when switching from no-load operation to full-load operation. By correcting it, you can always ensure the required minimum pressure.

A seventh feature of the present invention is that in a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is detected. It has a pressure detector, and a lower limit pressure set value and an upper limit pressure set value for turning on and off the on-off type control valve, and compares these pressure set values with the pressure value from the pressure detector to perform the on-off type control. A control device for a screw compressor includes a control device that controls the on-off type control valve so that the on-off cycle does not become shorter than a certain period of time.

The eighth feature of the present invention is that the screw compressor is equipped with an on-off type control valve provided on the suction side and an air blowing valve provided on the discharge side thereof, so that full load operation and no load operation are performed depending on the pressure on the compressor discharge side. In a screw compressor that operates by switching between High-pressure side pressure switches having different set pressures, and a timer for measuring the time after the pressure switch with a lower set pressure among the plurality of high-pressure side pressure switches is activated, and the plurality of high-pressure side pressure switches and Capacity control of a screw compressor configured to use a timer to stop switching between full load and no load operation for a certain period of time after switching between full load and no load operation until the pressure switch with the higher set pressure is activated. It's in the device.

[Effect]

With the above configuration, when the on-off type control valve is turned on and the compressor is operated at full load, the discharge side pressure increases and when it reaches the upper limit pressure set value, the on-off control valve is turned off and the compressor is operated at full load. The machine switches from full load operation to no load operation. The pressure on the discharge side decreases according to the amount of gas used on the load side, and when the lower limit pressure setting is reached, the compressor switches from no-load operation to full-load operation, and the pressure on the discharge side increases again.

Repeat the same below.

In the present invention, the magnitude of the load, that is, the amount of compressed gas used on the consumption side, is determined by detecting the rate of pressure increase during full load operation or the rate of pressure decrease during no load operation, and is determined according to the magnitude of the load. Since the lower limit pressure set value or upper limit pressure set value for turning on and off the on-off type control valve is corrected, the switching cycle between full load and no load operation can be controlled to a predetermined constant value or higher. The reliability of the device can be improved, and by setting the required pressure value on the consumption side as a lower limit value, the required pressure can be ensured at all times.

By detecting the pressure drop rate during no-load operation and setting this lower limit pressure setting value in consideration of this, the discharge side pressure when switching from no-load operation to full-load operation is below the required minimum pressure. It is possible to reliably prevent this from dropping.

In addition, by correcting the upper limit pressure set value according to the size of the compressor load so that the length of the on-off cycle of the on-off type control valve is within the set range, it is possible to When the capacity of the pump is large, the upper limit pressure setting value becomes lower, so the operating time at unnecessary high pressure can be reduced, and the energy saving effect can be improved.

〔Example〕

An embodiment of the capacity control device for a screw compressor according to the present invention will be explained below with reference to FIG. 1. FIG. 0 shows the flow of the air system of the screw compressor and the capacity control device.

When the screw compressor is operating at full load, air sucked in from the suction port 14 passes through the suction filter 19, passes through the suction valve (on-off control valve) 2 in a fully closed state, and enters the compressor body 1. The high-temperature, high-pressure air compressed by the compressor body passes through check valve 1.
3. The air is sent to the air storage tank 11 through the aftercooler 12. The air stored in the air storage tank 11 is sent to the air consumption line 16 and consumed. In addition, in this state, the air discharge valve 3
is closed. In addition, the hydraulic piston 4 that drives the suction valve 2 and the air discharge valve 3 opens the suction valve 2 with oil supplied from the oil storage tank 7 by the four-way solenoid valve 5 and the hydraulic pump 6.
The blow-off valve 3 is pushed to the closed position.

Generally, during full load operation, the compressor discharge amount is greater than the '/S cost air amount, and the discharge side pressure increases.

This discharge side pressure is detected by a pressure sensor 8 and sent to a control device 9. In the control device 9, when the detected discharge side pressure P reaches a preset upper limit (pressure setting) value Pmax,
A command is issued to the four-way solenoid valve 5 to switch the oil passage of the four-way solenoid valve 5, operate the hydraulic piston 4, and close the intake valve 2.
At the same time, the blowoff valve 3 is opened to put the compressor 1 into a no-load operating state. Air leaking into the compressor 1 from the suction valve 2 exits the compressor 1, passes through the blow-off valve 3, passes through the blow-off silencer 1°, and is blown into the atmosphere from the blow-off port 15.

In the no-load operating state, air is not supplied from the compressor main body 1 to the air storage tank 11, and the air in the air storage tank 11 is consumed. P (referred to as pressure) decreases, and when the pressure decreases to a preset lower limit pressure P win, the control device 9 switches the four-way solenoid valve 5 to return to the full load operating state.

The control device 9 includes a pressure detection section, a storage section, and a time measurement section.

It has a built-in calculation section and detects the discharge side pressure.

Measure the switching interval time (switching period Δt1 + Δtz) Δt between the full load operating time Δt and the no-load operating time Δt2, and calculate Δt.
The switching period Δt of P is within the setting range (Δtmln~Δt
wax), set the upper limit pressure setting value P +aa
Calculate the pressure difference ΔP between X and the lower limit pressure setting value P□, and
At least one of maXI P mln is corrected and compared with the detected pressure value P to issue a switching signal for the four-way solenoid valve 5. The timing of Δt and Δt2 is repeated for a certain period of time, the average value of several cycles is determined, and the average switching period Δt and the preset Δt□ are determined. Δt
If the condition -+n<Δt Δj +max is not satisfied, ΔP is corrected so as to satisfy this condition.

An example of the processing flow of the arithmetic unit of the control device 9 is shown in FIG.

First of all, lower limit pressure setting value P□. .

ΔP initial value setting (p, i, +ΔP = P-ax...
(upper limit pressure setting value) and the minimum value Δt□ of the switching cycle are set (step 20). Next, the actual switching period Δt is measured (step 21). Furthermore, step 2
3, compare Δt with Δtllln, Δt=Δjmtn
If so, the ΔP set as shown in 24a is used as it is as the corrected ΔP', and the set value in step 20 is corrected as the correction data A. Further, in step 23, Δt Δt+w. Or Δt〉Δt□,
In this case, it is necessary to correct ΔP so that Δt becomes a large value or a small value, but in this example, 24b
.

As shown in 24c, the pressure difference ΔP' after correction is given as correction data A, and the set value ΔP in step 20 is corrected.

According to this embodiment, P
Since the pressure is controlled to be the required minimum value, the fluctuation width of the discharge side pressure can be minimized.

FIG. 3 shows the lower limit pressure setting value P w in the above embodiment.
FIG. 4 is a diagram showing a change in discharge side pressure and a change in power in an example in which in is fixed and only the upper limit pressure set value P wax is changed based on correction data A. FIG.

To explain case a in the figure, when the screw compressor is operating at full load and the amount of air supplied from the compressor is greater than the amount of air consumed, when the discharge side pressure reaches the upper limit pressure set value P□8. , the screw compressor switches from full load operation to no load operation. There is no supply of air from the screw compressor, which is in a no-load operating state, and as the air in the storage tank is consumed, the discharge side pressure decreases. When the discharge side pressure decreases to the pressure lower limit pressure set value P m l n, the compressor switches from no-load operation to full-load operation, and the discharge side pressure increases. The same operation is repeated thereafter. If the differential pressure between the upper limit P+may and the lower limit P0° of the set pressure value is ΔP, and the ratio of the consumed air volume to the compressor air volume Q is q, the discharge side pressure during full load operation increases from P win to P max. The time Δ required for this and the time Δt2 required for the discharge side pressure to decrease from P m&X to P m1n during no-load operation are expressed by the following equation.

Δt=Δt1+Δt2 q(1q)PsQs According to the above equation, if the air consumption rate q is constant, Δt becomes shorter by decreasing ΔP. On the other hand, Δt generally needs to be greater than a certain value due to mechanical limitations of the capacity control device. The minimum value of 8t is Δjmin, and ΔP at that time is Δ
When P m I n, it becomes ■. Now, if we measure Δt at a certain ΔP in the actual operating state, the pressure difference ΔP m t becomes Δj man
n can be determined by Δt.

If the storage tank volume is sufficient compared to the compressor air volume, Δ
Since t has a margin, by shortening Δt to Δt, .

The box in FIG. 2 shows the change in pressure on the discharge side and the power when the lowest pressure P mln is fixed with respect to a and ΔP is reduced to ΔP'. Comparing the average power of a and b, it is as follows;

Average power of a Lave 2 Δt Δt (7) Average power of b L'ave (8) On the other hand, as is clear from equations (2), (3), and (4), Δ
t Δ t and the air consumption rate q is constant at a and b, then Lave L'ave" (Lmax L'
wax) X q...(10) Since L 11aX>L'+maX, b is
The average power is smaller than that.

Also, when the air consumption rate q is stable, Δt is applied.

If Δt2 is measured, once the compressor K is determined from equations (4) and (9), the rate of pressure change during full load operation or no load operation d P/d t, (or d P/d t,)
By measuring ΔP 1llIn , it is possible to obtain ΔP 1llIn using the following equation.

According to ...(11) or ...(11'), when the storage tank volume is large or the compressor load is small, P wax is set small as shown by P'maX, so there is no problem. The compressor is not driven at the required high pressure, making it possible to reduce power.

Also, as shown in Figure 3, Δt□ and Δt2 are measured only in the initial state at the start of operation, and thereafter the pressure change rate dP
, /d tl and calculate ΔP and I□, it is possible to always set the optimal setting value ΔP even when the air consumption rate changes every moment.

Figure 4 shows the pressure drop rate dP,/dto of the discharge side pressure.
If is large, time is required to switch from no load to full load, so the discharge side pressure P may drop far beyond the lower limit pressure setting value Pan1□ as shown in curve 1iAc. The discharge side pressure P is the lower limit pressure setting value P0
This is an example in which the temperature is prevented from falling below . In this example, the relationship between the pressure drop rate dP,/dto and the overpressure ΔPc that decreases beyond the lower limit pressure set value P111n is measured in advance, and dPo/dto during operation is measured.
dt, and the discharge side pressure becomes P□. Don't go below.

Set the lower limit pressure setting value to Po. GaraP. Since the pressure is controlled so as to be raised to 1, the pressure does not fall below the minimum pressure P1lln.

It will be explained in more detail. In the case of curve C, the discharge side pressure is P
At point A when l1ln is reached, a signal is sent to the capacity adjustment device and the no-load operation is switched to full-load operation.

Due to the operating time of the capacity adjustment device (on/off type control valve, blow-off valve, etc.), the discharge side pressure exceeds Pl, 1 and becomes Δ.
The pressure valve of Pc only decreases. On the other hand, first measure the pressure drop rate dPa/dt and ΔPC0, and set the lower limit pressure setting value P□ according to dP/dt. From P
If the pressure is increased to 0, the capacity adjustment device starts operating at point B, so the discharge side pressure will not drop below Pmln. po in this case is determined by the following formula.

P clt.

FIG. 5 is a block diagram showing a specific example of the control device 9 shown in FIG. 1. The pressure signal detected by the pressure sensor 8 is converted from an analog signal to a digital signal by an A/D converter 91, and sent to the central processing module w92. The central processing unit (92 has a clock circuit, which performs comparison and arithmetic processing based on the pressure signal and the clock signal, and outputs an on/off signal for the capacity control valve. Also, the circuit has a Δjmtn+
It has a read-only memory (ROM) 93 that stores setting values such as Pm1n+P and &×, and a random access memory (RAM) 94 that temporarily stores calculation results and the like.

Figure 6 is the control system shown in Figure 1! 9, it has a timer section and has two types of upper limit pressure setting values p wax (P wraxxrP m
aX2) is a diagram showing the flow when using a pressure switch that can be set.

First, in step 30, P waxlp P maX2
1Δt1□. Set in advance. In step 31, full load operation of the compressor is started, and at the same time measurement of the discharge side pressure P is started, and Δt is measured at the same time. In step 32,
The measured P is compared with the smaller upper limit pressure setting value Pmax□, and the process is repeated until the condition of P≧Pox□ is satisfied.

When P≧Pyoa2□.

Proceeding to step 33, the above Δt and the minimum switching time Δ
tlllin, and if Δt is smaller than the minimum switching time Δt1□□, it means that the load is small, so the process proceeds to step 35 and switches to no-load operation. If Δt is larger than Δt1m1n, the load is large, so the process proceeds to step 34, and full-load operation is continued until the detected pressure value P becomes larger than the larger upper limit pressure set value PmaXz, so that P≧Pmaxz. The process then proceeds to step 35 and is switched to no-load operation. Set value Δt engineering □. is determined in consideration of mechanical constraints such as the time required to operate the on-off type control valve 2 and the blow-off valve 3, and the mechanical life of the control valve and solenoid valve due to on/off switching.

FIG. 7 is an electrical circuit diagram of a control device for realizing the embodiment shown in FIG. 6 above. 40 means that the detected pressure value P is P
41 is a pressure switch that turns ON when P is smaller than P mln and turns OFF when P is larger than Pmax□; 42 is a relay that turns switch PIX ON-OFF; 4
3 is a relay that turns the switch P2X ON-OFF, 44 is a relay that turns ON-OFF the switch 46X, and 45 is a timer that starts measuring time when the switch 46X is turned ON, and a certain set time ti□. When this is reached, the switch T is turned on. With such an electric circuit configuration, when the compressor is operated, switches PIX and PIX are turned on by relays 42 and 43, and switch 46X is turned on via relay 46X. Therefore, at the same time as full load operation starts, the tie -45 is increased by the elapsed time Δt
Measure yo. Because the load is large, Δt will exceed t before P reaches Pmax□.

When it reaches, switch T is closed, so P〉wax□
Even if the conditions are met and switch PIX is turned off, P
> Continue full load operation until P-ax□. When the load is small, the condition P>P-a-□ is satisfied immediately after the start of full-load operation. Therefore, since the switch T is not turned on during t□ and Δt and 1n, P > P
When the switch PIX is turned off under the conditions of , , □, the switch 46X is turned off at the same time, resulting in sudden load operation.

With this configuration, it is possible to configure a control device that controls an on-off type control valve using only multiple pressure switches and a simple electric circuit, and it is possible to improve the energy saving effect at low cost, and also to reduce the total load to no load. It is possible to realize a device in which the load switching cycle is greater than or equal to a certain value.

〔Effect of the invention〕

According to the present invention, the lower limit pressure set value or the upper limit pressure set value is corrected in order to turn on and off the on-off type control valve provided on the compressor suction side according to the amount of compressed gas used on the consumption side, that is, the size of the load. Because of this, the switching cycle between full load and no load operation of the compressor can be kept above a certain value, preventing frequent operation switching even when the load is large, and improving the reliability of the equipment. .

Further, by setting the minimum required pressure on the consumption side as the lower limit value, the required pressure can be ensured at all times.

Furthermore, by correcting the upper limit pressure set value according to the size of the compressor load so that the length of the on-off cycle of the on-off type control valve is within the set range, it is possible to When the capacity is large, the upper limit pressure setting value becomes lower, so the compressor operation time at unnecessary high pressure can be reduced, and the energy saving effect can be improved.

[Brief explanation of drawings]

Fig. 1 is an overall configuration diagram showing an embodiment of the present invention, Fig. 2 is a flowchart showing an example of the calculation processing flow of the control device shown in Fig. 1, and Fig. 3 is a discharge side pressure in an embodiment of the present invention. Figure 4 is a diagram showing the change in discharge side pressure when switching from no-load operation to full-load operation, and Figure 5 is a diagram showing the change in the control device shown in Figure 1. A block diagram showing a specific example, FIG. 6 is a flowchart of the process when a pressure switch having a timer unit and capable of setting two types of upper limit pressure set values is used as the control device in FIG. 1, and FIG.
FIG. 3 is an electrical circuit diagram of a control device for realizing the embodiment shown in the figure. DESCRIPTION OF SYMBOLS 1...Compressor body, 2...Suction valve on/off type control valve, 3...Blowout valve, 4...Piston, 5...Four-way solenoid valve, 6...Oil pump, 8... -Pressure sensor (pressure detector), 9...control device, 11...air storage tank. The last 3 days were also a cow act 5 Nikko 6 (3)

Claims (1)

[Claims] 1. A screw compressor in which an on-off control valve provided on the suction side of the screw compressor is controlled by pressure on the discharge side of the compressor to perform full load operation and no load operation, Detecting the amount of compressed gas used on the compressor load side, and changing at least one of an upper limit pressure setting value for turning off the on-off control valve and a lower limit pressure setting value for turning on the on-off control valve according to this usage amount. . A method for controlling the capacity of a screw compressor, characterized in that the on-off cycle of the on-off control valve is controlled to be equal to or higher than a predetermined set value. 2. A capacity control method for a screw compressor according to claim 1, characterized in that the amount of compressed gas used on the compressor load side is determined based on the pressure change rate on the compressor load side. 3. In a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, the pressure on the load side of the compressor is detected and the pressure value is determined. When the upper limit pressure set value is reached, the on-off type control valve is turned off and the compressor is operated with no load,
When the detected pressure value reaches a lower limit pressure set value, the on-off control valve is turned on to operate the compressor at full load, and the length of the on-off cycle of the on-off control valve is within a set range. A capacity control method for a screw compressor, comprising correcting the upper limit pressure set value. 4. A capacity control method for a screw compressor according to claim 3, characterized in that the upper limit pressure setting value is corrected so that the length of the on-off cycle of the on-off control valve is constant. 5. In a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off control valve provided on the suction side of the screw compressor, pressure detection means for detecting the pressure on the load side of the compressor; A control means having an upper limit pressure setting value and a lower limit pressure setting value for turning on and off the on-off type control valve, and comparing these pressure setting values with a pressure value from the pressure detection means to turn on and off the on-off type control valve; and a set value correction means for correcting at least one of the upper limit pressure set value and the lower limit pressure set value so that the on/off cycle of the on-off type control valve is equal to or higher than the set value. Capacity control device. 6. In a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, a pressure detector for detecting the pressure on the load side of the compressor; Control that has a lower limit pressure set value and an upper limit pressure set value for turning on and off the on-off type control valve, and controls the on-off type control valve on and off by comparing these pressure set values and the pressure value from the pressure detector. a device, a means for detecting the magnitude of the load on the compressor load side, and an on-off type that corrects at least one of the upper limit pressure set value or the lower limit pressure set value according to the magnitude of the load from the load detection means; 1. A capacity control device for a screw compressor, comprising set value correction means for setting the on/off cycle of a control valve to a predetermined set value or more. 7. In a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, a pressure detector for detecting the pressure on the load side of the compressor; It has a lower limit pressure setting value and an upper limit pressure setting value to turn on and off the on-off type control valve. A control device that controls the on-off type control valve on and off by comparing these pressure set values and the pressure value from the pressure detector, and a control device that controls the on-off type control valve so that the length of the on-off cycle of the on-off type control valve falls within a set range. A capacity control device for a screw compressor, comprising: set value correction means for correcting an upper limit pressure set value. 8. The capacity of a screw compressor according to claim 7, wherein the set value correction means corrects the upper limit pressure set value according to the amount of compressed gas used (load size) on the compressor load side. Control device. 9. A capacity control device for a screw compressor according to claim 8, wherein the amount of compressed gas used on the compressor load side is determined based on the pressure change rate on the compressor load side. 10.Equipped with an on-off type control valve provided on the suction side of the screw compressor and a blowoff valve provided on the discharge side thereof, and operates by switching between full load operation and no-load operation depending on the pressure on the compressor discharge side. In a screw compressor, at least one of the upper limit pressure setting value for switching from full load operation to no-load operation or the lower limit pressure setting value for switching from no-load operation to full-load operation is set to a setting value that changes the switching cycle between full-load operation and no-load operation. A capacity control device for a screw compressor, characterized in that it is configured to perform correction according to the magnitude of compressor load as described above. 11. The screw according to claim 10, wherein the lower limit pressure setting value is constant, and only the upper limit pressure setting value is corrected according to the compressor load so that the full load/no-load switching cycle is within the set range. Compressor capacity control device. 12. In claim 10 or 11, the compressor load is
A capacity control device for a screw compressor, characterized in that the rate of change in pressure on the discharge side of the compressor is detected and determined during at least one of full-load operation and no-load operation. 13. The capacity control device for a screw compressor according to claim 11, wherein a plurality of upper limit pressure set values are prepared in advance, and the upper limit pressure set values are switched according to the compressor load. 14. In claim 11, the pressure drop rate during no-load operation is detected, and the pressure drop rate is determined according to the pressure drop rate so that the discharge side pressure does not fall below the required minimum pressure when switching from no-load operation to full-load operation. A capacity control device for a screw compressor, characterized by correcting a lower limit pressure set value. 15. In a screw compressor that performs full-load operation and no-load operation by turning on and off an on-off type control valve provided on the suction side of the screw compressor, a pressure detector that detects pressure on the load side of the compressor, and the on-off control valve provided on the suction side of the screw compressor. It has a lower limit pressure set value and an upper limit pressure set value for turning the on-off type control valve on and off, and controls the on-off type control valve on and off by comparing these pressure set values and the pressure value from the pressure detector, A control device for a screw compressor, comprising a control device that controls an on-off type control valve so that an on-off cycle does not become less than a certain time. 16.Equipped with an on-off type control valve provided on the suction side of the screw compressor and a blowoff valve provided on the discharge side thereof, and operates by switching between full-load operation and no-load operation depending on the pressure on the compressor discharge side. In screw compressors, there is a low-pressure side pressure switch that operates at the lower limit pressure to switch from no-load operation to full-load operation, and multiple high-pressure switches with different set pressures that operate at the upper limit pressure to switch from full-load operation to no-load operation. A side pressure switch and a timer for measuring the time after the pressure switch with the lower set pressure among the plurality of pressure side pressure switches is activated, and the plurality of high pressure side pressure switches and the timer are used to control the full load. Capacity control of a screw compressor, characterized in that the switching between full load and no load operation is stopped for a certain period of time after switching between operation and no load operation until the pressure switch with the higher set pressure is activated. Device.