DE69107010T2 - Method and device for controlling the volume of a compressor. - Google Patents

Description

BACKGROUND OF THE INVENTION TECHNICAL AREA

The present invention relates to a method for controlling a capacity of a compressor with a switching control valve and a device therefor, in particular to a method for controlling a capacity of a rotary motion compressor such as a screw compressor and a Roots compressor and a device therefor. The compressor has the following components: a compressor part, a pipe connection which is connected to the outlet or output side of the compressor on the one side and to a gas consumption side (load side) on the other side, and a capacity control device for controlling the flow rate of the compressed gas (compressor capacity) from the compressor into the line on the outlet side of the compressor.

STATE OF THE ART

In a conventional capacity control device of a screw compressor with a switching control valve, for example, as shown in JP-A-58167890, the switching control valve is arranged on the suction or inlet side of the compressor and a discharge valve is arranged on the discharge or outlet side of the compressor. When the compressor is operated at full load with the switching control valve opened and the air discharge valve closed, the air pressure on the discharge side increases. When the air pressure on the discharge side increases above a predetermined set value, the switching control valve is closed and the air discharge valve is opened, switching the compressor to an idle operation. Then, when the air pressure in an air accumulator arranged on the outlet side decreases and reaches a value that is below a predetermined set point, the switching control valve is opened and the air outlet valve is closed, causing the compressor to operate at full load.

In such a conventional capacity control device, the air pressure on the discharge side of the compressor is detected by a pressure switch, and a signal corresponding to the value of the detected pressure is transmitted so that the compressor can be switched between full load and idle operation.

In the above-mentioned prior art, since both the upper and lower limits of the air pressure setpoints at which the control signal for switching the compressor between full load and idle operation is to be sent out are fixed, the switching of the operating mode of the compressor is carried out in dependence on a constant pressure value, regardless of the volume of the air pressure vessel connected to the outlet side of the compressor. Therefore, if a consumption rate of quantity per period (operating load) of air by the compressor is assumed to be constant, the interval of switching (switching period or on-off period) of the compressor between full load and idle operation of the compressor is longer, the larger the volume of the piping system (including the air pressure vessel) connected to the outlet side of the compressor. In addition, even when the operating load is extremely large (high) or extremely small (low) compared to the compressor capacity, the switching period between full load and idle operation of the compressor becomes longer. Therefore, the operating time of the compressor during which the air pressure on the discharge side is higher than the pressure required from the air consumption side becomes longer, causing a problem that unnecessary electric power is consumed to operate the compressor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compressor capacity control method and apparatus capable of shortening the operating time of the compressor during which the compressor generates unnecessarily high pressure under a condition where the operating load (gas consumption rate or amount/time on the gas consumption side) is changed considerably or the volume of the piping connected to the outlet side of the compressor is large, thereby reducing the electric power consumption.

Another object of the present invention is to provide a compressor capacity control method and apparatus in which the switching time between full load and idle operation of the compressor is maintained for a time longer than the predetermined time, thereby improving the reliability of the capacity control apparatus while guaranteeing that the pressure on the air consumption side is always kept above a predetermined minimum value.

The above objects of the present invention can be achieved by a capacity control method according to the invention, in which a compressor is switched between full load and idle operation by operating a switching valve arranged on the inlet side of the compressor; a pressure (P) on the outlet or load side of the compressor is detected by a pressure sensor, and when the pressure (P) reaches a predetermined upper pressure limit Pmax, the switching control valve is closed, whereby the compressor is put into idle operation, but when the detected pressure (P) reaches a predetermined lower pressure limit Pmin, the switching control valve is opened, whereby the compressor is put into full load operation; a magnitude (q) of the load on the load side of the compressor is detected, and depending on the magnitude (q) of the load, either the above-mentioned upper limit Pmax or the lower limit Pmin is changed so as to extend an on-off period (Δt; Δt₁) beyond a predetermined time period set value (Δtmin; Δt₁min).

In general, the pressure on the load side (discharge side of the compressor) increases in a switching power control method used in a power control method according to the invention when the switching control valve is opened and the compressor is put into full load operation. When the pressure on the load side reaches a preselected or predetermined upper pressure limit, the switching control valve is closed and the compressor is switched from full load to idle operation. The pressure on the load side then decreases at a rate corresponding to the size of the load. When the pressure on the load side reaches a preselected or predetermined lower pressure limit, the compressor is switched from idle operation to full load operation and the pressure on the load side increases again. The above processes repeat.

Since, in a power control method according to the invention, the preselected upper pressure limit or the preselected lower pressure limit at which the switching control valve is opened or closed is set depending on the size of the load, it becomes possible to extend the switching time between full load and idle operation beyond a predetermined period of time, thereby increasing the reliability of the device for carrying out the power control method according to the invention.

The term "on-off period" basically means a period (Δt) that includes a full-load operating time and an idling operating time, with these operating modes being repeated alternately. The period of time that is to be adjusted to the size of the load can also be a full-load period (Δt₁) or an idle period (Δt₂ = Δt - Δt₁) instead of the on-off period (Δt). The full-load period (Δt₁) or the idle period (Δt - Δt₁) is therefore also a period corresponding to the on-off period.

According to a preferred embodiment of the present invention, the detection of the magnitude, more precisely the relative magnitude, of the load on the load side of the compressor is carried out by determining the consumption rate or the consumption ratio (in relation to the compressor capacity) (q) of the compressed gas on the load side and the consumption rate or the consumption ratio (q) of the compressed gas is determined, for example, by determining the variation rate (dP/dt) of a pressure (P) on the load side of the compressor or the on-off period (Δt; Δt�1) of the switching control valve is measured.

In the latter case, the preselected pressure upper limit (Pmax) is changed so that the measured on-off period (Δt;Δt₁) becomes longer, more precisely, not shorter than a predetermined value (Δtmin; Δt₁min). According to a preferred embodiment of the present invention, the preselected pressure upper limit (Pmax) is determined so that the measured on-off period (Δt;Δt₁) is equal to the predetermined value (Δtmin; Δt₁min). In this case, by setting the pressure lower limit equal to the pressure required on the consumption side (load side), a required pressure is maintained during the whole time.

According to the present invention, the above-mentioned objects can be achieved by a compressor power control device which contains the following elements: a switching valve which is arranged on an inlet side of a compressor and is suitable for opening or closing in order to put the compressor into full load or idle operation, a pressure detection device for detecting a pressure (P) on a load side of the compressor, and a switching control device which opens or closes a switching valve based on a comparison of a preselected or predetermined pressure upper limit (Pmax) and a preselected or predetermined pressure lower limit (Pmin) with the pressure value (P) detected by the pressure detection device, which is characterized in that the control device contains the following elements: a load detection device for detecting a size (q) of a load on the load side of the compressor and a setpoint changing device for changing at least the preselected pressure upper limit (Pmax) or the preselected pressure lower limit (Pmin) so as to not let an on-off period (Δt; Δt₁) of the switching valve become shorter than a predetermined value (Δtmin; Δt₁min). Since in a power control device according to the invention, similar to In the capacity control method, the upper pressure limit or the lower pressure limit at which the switching valve is opened or closed is changed according to the size of the load on the load side of the compressor, the switching time between full load and idle operation can be changed to a value not shorter than the predetermined time period, thereby improving the reliability of the capacity control device. In addition, when the operating load is extremely large (high) or extremely small (low) or the internal volume of the discharge side piping is large, the upper pressure limit is reduced and the high pressure operation time can be shortened, thereby enhancing the energy saving effect.

According to a preferred embodiment of the present invention, the load detecting means consists of means for detecting the gas consumption rate or gas consumption ratio (q) on the load side of the compressor. The means for detecting the gas consumption rate or gas consumption ratio consists of, for example: (1) variation rate detecting means for detecting a pressure change rate (dP/dt) of the compressed gas on the load side of the compressor; (2) compressed gas flow rate detecting means on the load side of the compressor; or (3) means for measuring the on-off period (Δt; Δt₁) of the switching control valve.

In the above case (3), the set value changing means is arranged to change either the preset upper limit pressure (Pmax) or the preset lower limit pressure (Pmin) so that the on-off period (Δt; Δt₁) of the switching control valve is determined to be not shorter than a predetermined value (Δtmin; Δt₁min). In a preferred embodiment, for example, the set value changing means is arranged to change the upper limit pressure (Pmax) so that the on-off period (Δt; Δt₁) of the switching control valve is determined to be not shorter than a predetermined value (Δtmin; Δt₁min). is determined. In this case, since the lower limit pressure is maintained at a constant value, not only can a minimum pressure be guaranteed, but also the upper limit pressure can be suppressed at a required minimum value. In addition, in this case, when the operating load is extremely large (high) or when the internal volume of the discharge side piping is large, the upper limit pressure is reduced and the high pressure operating time can be shortened, thereby further enhancing the energy saving effect. According to a preferred embodiment of the invention, the switching control means includes a plurality of predetermined pressure limit values (e.g., two limit values such as Pmax1, Pmax2), and the set value changing means is arranged such that the upper limit pressure (Pmax1, Pmax2) is switched or changed depending on the magnitude (q) of the load acting on the compressor. According to a preferred embodiment of the present invention, the switching control device includes a low-pressure side pressure switch which switches the compressor from idle to full-load operation at the lower pressure limit (Pmin) and a plurality of high-pressure side pressure switches which switch the compressor from full-load operation to idle operation at the upper pressure limits (Pmax1, Pmax2); the capacity control device further includes a timing device for measuring a time period after actuation of the low-pressure side pressure switch; and the setpoint changing device is designed such that the high-pressure side pressure switches are not actuated until at least the predetermined time period (Δt₁min) has elapsed after the compressor has been switched from idle operation to full-load operation by the plurality of high-pressure side pressure switches cooperating with the timing device.

According to a preferred embodiment of the present invention, the compressor capacity control device further includes a pressure reduction speed detecting device for detecting the reduction speed (dP/dt) of the pressure (P) on the load side of the compressor in idle operation, and the setpoint changing device is designed to change the predetermined lower limit pressure (Pmin) according to the pressure decreasing rate (dP/dt) of the pressure (P) so as to prevent the pressure (P) on the load side from falling below a minimum pressure (Pmin0) required on the load (consumption) side when the compressor is switched from idle to full load operation. This configuration guarantees that the pressure (P) on the load side will not fall below the minimum pressure (Pmin0) required on the load (consumption) side when the compressor is switched from idle to full load operation.

According to a preferred embodiment of the present invention, the capacity control device further includes an air outlet valve on the outlet side of the compressor, the air outlet valve being closed when the switching valve is opened and being opened when the switching valve is closed.

The foregoing and other objects, features and advantages of the invention will become more apparent from the description of the preferred embodiments with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of an embodiment of the present invention as a whole;

Fig. 2 is a block diagram for explaining the arithmetic processing flow of a control device;

Figs. 3A and 3B are graphs of a time-dependent change in the pressure (P) on the outlet side and a time-dependent change in the power (L) in an embodiment of the invention;

Fig. 3C is a flow chart showing an example of sizing the size of the load by measuring the pressure change rate;

Fig. 4 is a graph for explaining the pressure change when the compressor is switched from idle operation to full load operation;

Fig. 4A is a flow chart corresponding to the control device in Fig. 4;

Fig. 5 is a block diagram of a concrete example of a control device shown in Fig. 1;

Fig. 6 is a flow chart of a processing in which a control device having a time measuring device and a pressure switch capable of setting two different upper pressure limits is used as the control device of Fig. 1;

Fig. 7 is an electric circuit diagram of the control device for carrying out the embodiment shown in Fig. 6; and

Fig. 7A and 7B are graphs showing functional characteristics of the pressure switch as arranged in Fig. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to Fig. 1, a method and a device for compressor power control according to an embodiment of the invention is described below. The figure mainly shows an air-operated system of a screw compressor 18 with a power control device 17.

In Fig. 1, 1 denotes a compressor, 2 is an intake valve as a switching valve which is arranged on the inlet side of the compressor 1 and is switched back and forth between the open and closed positions by means of a rod 4a of a hydraulic cylinder device 4. 19 denotes an intake filter arranged on the way or between an inlet port 14 and the intake valve 2. 13 is a blow valve which is arranged on the outlet or discharge side of the compressor 1, while 12 is an aftercooler for cooling the compressed gas or the discharge from the compressor 1. 11 denotes a pressure vessel for storing the compressed air cooled by the aftercooler 12, the compressed air in the pressure vessel 11 being led out via a consumption line 16 and thus made available for use. 8 denotes a pressure detection device for detecting the pressure P in an outlet line 8a, wherein the information about the pressure P collected by the pressure detection device is forwarded to a control device 9.

3 denotes an air outlet valve which is switched between an open and a closed position via a rod 4a of the hydraulic cylinder device 4, similar to the intake valve 2, and 15 denotes an air outlet muffler which is arranged between an outlet side of the air outlet valve 3 and the air outlet port 15.

In addition, 7 denotes an oil pan, 6 a hydraulic pump, and 5 a four-position electromagnetic valve (four-way valve) which is operated under the control of the control device 9 to assume one of the three possible positions, whereby the rod 4a is moved up or down or stopped by the hydraulic cylinder device 4.

In the compressor 18 having the above arrangement, the capacity control device 17 consists of the valves 2, 3, the cylinder device 4, the pump 6, the oil pan 7, the pressure detection device 8 and the control device 9. A piping system 17a on the outlet side consists of an aftercooler 12 arranged behind the impact valve 13, the air accumulator 11 and the piping system 8a for connecting these elements.

In full load operation of the compressor 1 of the compressor 18, the piston of the hydraulic cylinder device 4, which is connected to the intake valve 2 and the outlet valve 3 via the rod 4a, brings the intake valve 2 into an open position and the outlet valve into a closed position, by means of oil from the oil pan 7 via the electromagnetic four-way valve 5 through the hydraulic pump 6. In such full load operation, the air sucked in or introduced at the inlet opening 14 flows through the intake filter 19 and the fully open intake valve (switching control valve 2) to the compressor 1. The very hot and highly compressed air obtained by the compression by the compressor 1 is passed through the impact valve 13 and the Aftercooler 12 is fed to the pressure vessel 11, from where the air is passed on to the air consumption line 16 for consumption.

Generally, since the flow rate of air discharged from the compressor 1 is greater than the air consumption rate during full load operation, the pressure of the discharge-side piping system 17a including the pressure vessel 11, namely the pressure P on the load side or the discharge side of the compressor 18, gradually increases. In other words, the compressor is selected to have a sufficient discharge capacity. The pressure P on the load side is detected by the pressure sensor 8, and the detected pressure value P is sent to the control device 9. When the detected pressure value P on the load side reaches a predetermined or preselected pressure upper limit Pmax, the control device 9 sends a command or control signal for switching the oil path in the electromagnetic four-way valve 5 and for operating the hydraulic cylinder 4 to move the rod 4a, thereby bringing the suction valve 2 into the closed position and simultaneously bringing the air outlet valve 3 into the open position, thereby switching the compressor 1 into idle operation.

Since, during idle operation of the compressor 1, the air is not supplied from the compressor 1 to the air pressure tank 11 and the air in the air pressure tank 11 is simply consumed, the pressure P in the discharge side piping system 17a including the air pressure tank 11, namely the pressure P on the load side of the compressor 18, gradually decreases. The control device 9 is designed so that the electromagnetic four-way valve 5 is switched when the pressure P decreases and reaches a preselected or predetermined pressure lower limit Pmin, thus returning the compressor to full load operation.

Fig. 5 is a block diagram showing a concrete example of the control device of Fig. 1. The control device 9 contains an A/D (analog-digital) converter 91 which is part of a pressure detection device a ROM (read only memory) 93 and a RAM (random access memory) 94 serving as storage means; and a central processing means 92 including a time adjuster or time measuring part 92a and an arithmetic processor 92b. The pressure value P detected by the pressure sensor 8 is converted from an analog to a digital signal by the A/D converter 91 and sent to the central processing means or central processing unit (CPU) 92. The central processing unit 92, which consists of, for example, a microprocessor with a time adjuster 92a, performs comparisons and other processing steps using the pressure signal P and the measured or counted time signal t, and then sends a switching command signal for controlling the capacity control valve consisting of the valves 2 and 3. The ROM 93 stores predetermined or preselected values such as Pmin, Pmax and Δtmin, which will be explained later, while the RAM 94 temporarily stores information such as operation or processing results.

More specifically, in the control device 9, the detected discharge side pressure P is passed through the A/D converter 91 to the processor 92, where the switching period Δt, namely the sum of a full load operating period Δt1 and an idle operating period Δt2 (Δt = Δt1 + Δt2), is measured. The switching period Δt depends on a relative magnitude of the load, ie, the consumption rate or a consumption ratio of the compressed gas in relation to the compressor capacity. Furthermore, in the control device 9, the pressure difference ΔP between the predetermined upper limit pressure Pmax and the predetermined lower limit pressure Pmin is changed or reset so that the switching period Δt falls within a predetermined range Δtmin - Δtmax (in the example of Fig. 2, Δt becomes Δtmin). Then, at least Pmax or Pmin is changed and compared with the detected pressure value P, and a switching control signal is supplied to the four-way valve 5. The periods Δt₁ and Δt₂ may be repeatedly measured for an appropriate period of time and an average value of several cycles are determined. This average switching period Δt is compared with the predetermined value Δtmin and the pressure difference ΔP is changed, if necessary, so that the condition Δtmin ≤ Δt ≤ Δtmax can be met.

In Fig. 2, an example of the processing flow of the operation in the control device 9 is shown. In this example, the upper pressure limit Pmax or the pressure difference ΔP, namely Pmax - Pmin, is set so that Δt matches Δtmin. The value of Δtmin is determined taking into account conditions such as the minimum operating speeds of the valves 2, 3 of the capacity control device 17. If appropriate or desired, other operating factors may be taken into account. First, the predetermined lower pressure limit Pmin, an initial set value of ΔP and the minimum time period value Δtmin (step 20) should be set, where the upper pressure limit Pmax = Pmin + ΔP. Then, under the condition that Pmax and Pmin have been set, the actual switching time Δt is measured (step 21). Next, in step 23, Δt is compared with Δtmin. In the case of Δt = Δtmin, the already set ΔP continues to be used as ΔP' (step 24a). In step 23, in the case of Δt < Δtmin or Δt > Δtmin, the pressure difference ΔP is changed to ΔP' as defined below (step 24b or 24c):

ΔP'= ΔP Δtmin/Δt (1)

This gives the following ratio:

P'max = Pmin + ΔP' = Pmin + ΔP (Δtmin/Δt)

When the relative air consumption speed through the air consumption line 16 fluctuates, the processing returns to step 21 so that the above control is continuously performed.

In this embodiment, since the pressure difference Δp between Pmin and Pmax is controlled so that the switching period Δt is reduced to Δtmin, it becomes possible to minimize the variation or the amplitude of the pressure on the load side.

Fig. 3A and 3B are graphs of the time-dependent variation of the pressure P on the discharge side and the time-dependent variation of the power L or the energy required per unit time for driving the compressor 1 in the above-mentioned embodiment, wherein the lower limit pressure Pmin is fixed while only the upper limit pressure Pmax is changed based on the adjusted or changed data ΔP'.

The case of a curve a in Fig. 3A and 3B will now be described. Since, during full load operation of the screw compressor 1, the flow rate of the air supplied from the compressor 1 to the pressure tank 11 through the pipe 8a is greater than the consumption rate Qc of the air consumed through the pipe 16, the pressure P gradually increases on the discharge side. When the pressure P reaches the upper limit pressure Pmax, the compressor 1 is switched to idle operation under the control of the control device 9. Since no air is supplied from the compressor 1 to the air accumulator 11 during idle operation, the pressure on the discharge side decreases while the air in the pressure tank is consumed. When the pressure P on the discharge side reaches a predetermined lower pressure limit Pmin, the compressor 1 is switched from the idle operation to the full load operation under the control of the control device 9, thereby increasing the pressure P on the discharge side. Then, the same operation is repeated. While the pressure difference between the upper pressure set point Pmax and the lower pressure set point Pmin is defined as ΔP and a ratio, i.e., the consumption ratio (or load ratio or the relative load) of the consumption rate Qc depending on an air flow rate Qs from the compressor 1 is defined as g (i.e., Qc = q Qs (in this case, both are mass flow rates)), the time period Δt₁ required for increasing the pressure P on the discharge side from Pmin to Pmax in the full load operation and the time period Δt₂ required for decreasing the pressure P on the discharge side from Pmax to Pmin in the idle operation are expressed as follows:

Δt₁ = VΔP/(1-q)PsQs (2)

Δt₂ = vΔP/qPsQs' (3)

where

V: pressure vessel volume, i.e. volume of the pipe system 17a on the outlet side,

Ps: compressor suction pressure,

Qs: Air intake rate of the compressor.

Δt = Δt�1 + Δt₂ (4a)

=vΔP/q(1-q)PsQs (4)

From the above equation (4), it can be seen that when the air consumption rate q is substantially constant, Δt becomes shorter as ΔP decreases. In addition, by defining the minimum value of Δt determined by the mechanical limitations of the power control device 17 as Δtmin and ΔP at that time as ΔPmin, the following equation is obtained:

Δtmin = V/q(1-q)PsQs x ΔPmin (5)

During actual operation of the compressor 18 by measuring Δt at ΔP, the following equation is obtained:

ΔPmin = ΔP/Δt x Δtmin (6)

When the air pressure tank volume V is large enough compared to the air flow rate Qs from the compressor 1, Δt becomes relatively long even if ΔP is relatively small. Consequently, in this case, a variation or fluctuation of the pressure P on the discharge side can be restricted to a small range by reducing Δt to Δtmin.

Taking tolerances into account, Δt can be controlled to be slightly longer than Δtmin. In addition, it is also possible to set the upper limit Δtmax of Δt so that an excessive increase in Δt is avoided and Δt remains shorter than Δtmax. Furthermore, it may also be possible to control ΔP (Pmin) in a similar way while keeping Pmax constant, instead of keeping Pmin constant and increasing Pmax. In this case, however, it is preferred to control Pmin so that it is not lower than the required minimum pressure Pmin0.

Curve b in Fig. 3 shows a time-dependent variation of the pressure P on the output side and a time-dependent variation of the power L when the pressure lower limit Pmin is fixed and the pressure difference ΔP is reduced to a value close to ΔP' which is smaller than the value in the above-mentioned curve a. The average power values in curves a and b are compared as follows, assuming that the time-dependent variation of the power L is substantially linear:

Average power Lave for curve a

Lave = [Lmin + Lmax]/2 x Δt₁/Δt + Lo x Δt₂/Δt (7)

Average power L'ave for curve b

L'ave = [Lmin + L'max]/2 x Δt'₁/Δt' + Lo x Δt'₂/Δt' (7)

where

Lmin: Power at minimum pressure Pmin,

Lmax: power at pressure limit Pmax,

L'max: power at pressure limit Pmax,

Lo: Power at idle.

In addition, from equations (2), (3) and (4) the following equations are obtained:

Δt₁/Δt = q, Δt₂/Δt = (1 - q) (9)

If it is also assumed that the relative load or air consumption ratio q (ratio of consumption flow rate Qc to compressor discharge flow rate Qs, both of which are volumetric flow rates at the same pressure or mass flow rates) in curves a and b is constant, the following equations can be derived:

Δt₁'/Δt' = q, Δt₂'/Δt' = 1 - q (9a)

and

where Δt' = Δt�1' + Δt₂' (4b),

Lave - L'ave = ½ (Lmax - L'max) x q (10)

Since Lmax >L'max, the average power in case of b is smaller than in case of a.

By measuring Δt1 and Δt2 in a state where the air consumption ratio q does not change rapidly, a constant K (=V/PsQs) which depends on the characteristics PsQs of the compressor 1 and the characteristics V of the piping system can be obtained:

With this constant K one can

an equation 1 - kΔP/Δt₁ = q from equation (2) and

derive an equation Δp = (ΔP/Δt₁)(1 - KΔP/Δt₁)Δt from equation (9).

If we also assume that the variation of the pressure P during the time Δt1 is linear, we obtain the following relationship:

ΔP = dP/dt₁ (1 - K dP/dt₁)Δt

Consequently, once the constant has been determined by measuring the pressure variation rate dP/dt₁ (or dP/dt₂) in full load or idle operation, ΔPmin can be determined as follows:

ΔPmin = dP/dt₁ (1 - K dP/dt₁)Δtmin (11)

Accordingly

ΔPmin = dP/dt₂ (1 - K dP/dt₂)Δtmin (11')

According to equations (11) and (11'), ΔPmin becomes smaller when the pressure vessel volume V is large or the compressor load q (i.e., the air consumption ratio via the consumption line 16) is extremely large or extremely small (q deviates considerably from 1/2), i.e., dP/dt₁ or dP/dt₂ is small. Consequently, Pmax or P'max is set small, and therefore the compressor is not operated at an unnecessarily high pressure, thereby reducing the required power L.

If, as shown in the flow chart 27 of Fig. 3c, Δt1 and Δt2 are measured only over a relatively short period of time, e.g. Δt1 + Δt2 or n(Δt1 + Δt2), where n can be either 2 or 3, after a start of operation of the compressor (27a), the constant K, which depends on the volume V of the effective line 17a including the volume of the consumption line 16, is determined (27b). Then the rate of change dP/dt1 or dP/dt2 of the pressure P is measured (27c). Then, ΔPmin is calculated (27d) based on equations (11) and (12) including the parameters of the constant K and the set value tmin. Thus, the pressure difference ΔP is set to an optimum value ΔPmin at which Δt always becomes equal to Δtmin even if the consumption ratio q changes continuously. The measurement of dP/dt1 or dP/dt2 is made by taking the values of the pressure P detected by the pressure detecting device 8 at desired time intervals δt using the processor 92 and obtaining δP/δt by means of the pressure difference δP between the pressures at two consecutive sampling points.

When the discharge side pressure reduction rate dP/dt is high, a certain period of time is required for switching the compressor from the idle operation to the full load operation, that is, for switching the valve 2 of the capacity control device 17 from the closed state to the open state and the air discharge valve 3 from the open state to the closed state. As shown in Fig. 4, even if the control operation of the control device 17 starts at point A, there is a risk that the discharge side pressure P drops sharply below the lower limit pressure as shown by curve e, thereby making the discharge side pressure P lower than the required minimum pressure Pmin0. In order to eliminate this danger, in the embodiment of the invention shown in Fig. 4, the relationship between the pressure reduction rate dP/dt and the pressure difference ΔPc falling below the lower pressure limit Pmin is measured in advance and stored in the memory 93 or 94 in the form of a table or equation (step 25a in Fig. 4A). Then, dP/dt is calculated during of operation is detected (step 25b), and the lower target limit is changed from a value Pmin to a value Po, thereby setting the extreme minimum value of the pressure P on the outlet side equal to Pmin0 (step 25c).

In this example, since the operation of the capacity control device starts at point B where the value of the pressure P becomes equal to Po, the pressure P on the outlet side is prevented from falling below Pmin0, as shown by curve d in Fig. 4. In this case, Po is obtained by the following equation:

Po = Pmin0 + ΔPc

where

ΔPc = ΔPci x [dP/dt]/(dP/dt)i (12)

The suffix "i" means a data group measured and stored in advance in step 25a. When multiple data groups are stored in step 25a, a data group close to the value of dP/dt measured in step 25b can be selected. Under certain circumstances, ΔPc can be determined by interpolating two stored data groups.

Then, the relative size of the load, namely the air consumption ratio q can be determined based on the air flow velocity QL through the line 16 measured by the flow meter 8b, which corresponds to the size of the load.

Fig. 6 is a flow chart for the case where the control device 9 contains a time counting part and pressure switching device capable of setting two types (Pmax1, Pmax2) of the pressure upper limit Pmax.

First, in a step 30, two pressure upper limits Pmax1 and Pmax2 (Pmax1 < Pmax2) and a minimum value Δt₁min of Δt₁ are set in advance. Pmax1 is set to a pressure value that the pressure P on the outlet side reaches, for example, when Δt₁ becomes substantially equal to Δt₁min at a normal large load, while Pmax2 is set to a pressure value which the pressure P on the discharge side reaches when Δt₁ becomes substantially equal to Δt₁min at a very small load, for example when the compressed gas is not actually used up. When the full load operation of the compressor starts, the measurement of the pressure P on the discharge side and the measurement of the time period t after the start are started in a step 31. In a step 32, the measured pressure P is compared with the lower Pmax1 of the upper target limits. If P < Pmax1, processing goes back to step 31, and steps 31 and 32 are repeated until P becomes equal to Pmax1, more precisely, P becomes equal to or greater than Pmaxi. If P becomes equal to or greater than Pmax1, processing proceeds to step 33 where the last measured value t is compared with the smallest switching time Δt₁min. If the time t is greater than the smallest switching time Δt₁min, namely, if the magnitude of the load or the relative magnitude q is substantially equal to an assumed magnitude, processing proceeds to step 35 where the compressor 1 is switched to idle operation. In other words, the (relative) magnitude of the load is evaluated in steps 32 and 33. On the other hand, if the measured value t is smaller than Δt₁min, that is, if the (relative) magnitude of the load is smaller than an assumed magnitude, the processing proceeds to a step 34 where the detected pressure P is compared with the upper Pmax2 of the pressure upper limits. The steps 31 to 34, that is, a continuous full load operation, are repeated until the condition P ≥ Pmax2 is satisfied. If P becomes equal to or larger than Pmax2, namely, if t becomes substantially equal to Δt₁min, the processing proceeds to a step 35 where the compressor 1 is set to idle operation. The set point Δt₁min is determined taking into account the mechanical limitation of the times required to operate the switching control valve 2 and the air exhaust valve 3 and a compromise between the advantage of an extremely short switching period and the disadvantage of a considerable shortening of the service life of the control valves 2 and 3, the electromagnetic valve 5, etc.

Fig. 7 shows an electrical circuit 9a which can be used instead of the control device 9 in Fig. 1 for the embodiment shown in Fig. 6. The pressure switch 40 is a differential pressure switch (Fig. 7A) with the hysteresis property that the switch is turned on when the detected pressure value P is less than Pmin and it is turned off when the detected pressure value P is greater than Pmax1. The pressure switch 41 is a differential pressure switch (Fig. 7B) with the hysteresis property that the switch is turned on when the detected pressure value P is less than Pmin and it is turned off when the detected pressure value P is greater than Pmax2. 42 is a relay for turning on or off a switch P1X, 43 is a relay for turning on or off a switch P2X, and 44 is a relay for turning on or off a switch 46X. 45 is a timer which starts measuring time when the switch 46X is turned on and turns off the rest switch T when the time reaches the predetermined set value Δt1min. 5a is an electromagnet of the electromagnetic valve 5 in Fig. 1, and 5b is an overvoltage protector. The electric power supply line 48 is energized when the compressor 1 is started.

When the compressor 1 starts operating, this arrangement of the electrical circuit 9a switches the switches PIX and P2X via the relays 42 and 43, respectively, since at the beginning the pressure switches 40 and 41 are in the on state, and the switch 46X is turned on via the relay 46X. Therefore, when the electromagnet 5a of the electric valve 5 is energized and the full load operation of the compressor starts, the timing device starts measuring the elapsed time t. When the relative air consumption speed or air consumption ratio q is small or the load is light, even if the pressure P reaches the value Pmax1, t does not reach Δt₁min, thereby keeping the switch T in the closed state. Therefore, when the condition P > Pmax1 is satisfied by turning off the pressure switch 40 and the switch P1X, the compressor 18 continues to operate at full load because the relay 44 is kept in a set state until the condition P > Pmax2 or t ≈ Δt₁min is satisfied. When the load is large, t becomes larger than Δt₂min before the condition P > Pmax1 is satisfied. Since the switch T is turned off when t is larger than Δt₁min, the switch 46X is turned off, at the same time, when the switch P1X is turned off under the condition P > Pmax1, thereby turning off the solenoid 5a, the compressor 1 is switched to idle operation.

The above arrangement provides a control device for controlling a switching control valve only by a number of pressure switches and a simple electrical circuit, thereby saving energy in a cost-effective manner, and a device in which the switching period between the full load and idle operation can be kept above a predetermined value.

In the present invention, since the lower or upper pressure set limit value for controlling the switching control valve on the inlet side of the compressor is changed depending on the consumption flow rate of the compressed gas or the size of the load, it becomes possible to adjust the switching period between full load and idle operation greater than a predetermined value and thus prevent frequent switching between operating modes even under high load, thereby increasing the reliability of the device.

Furthermore, if the minimum pressure required on the gas consumption side is taken as the lower pressure target limit, the minimum pressure required on the gas consumption side can be maintained all the time.

In addition, by changing the upper pressure set limit value depending on the absolute or relative size of the load to restrict the on-off period of the switching control valve to a predetermined range, the upper pressure limit becomes lower when the operating load is extremely large or extremely small or when the internal volume of the piping system on the discharge side is large, the high-pressure operating time of a compressor is shortened, thereby enhancing the energy-saving effect.

Claims (15)

1. Method for controlling the performance of a compressor arrangement, in which a switching valve (2) on the inlet side of a compressor (1) is opened or closed in order to put the compressor (1) into full load or idle operation: the pressure (P) on the load side of the compressor arrangement (18) is detected (8); and when the detected pressure (P) reaches a predetermined upper pressure limit (Pmax), the switching valve (2) is closed, whereby the compressor (1) is put into idle operation, while when the detected pressure (P) has reached a predetermined lower pressure limit (Pmin), the switching valve is opened, whereby the compressor (1) is put into full load operation, characterized in that a load value (q) on the load side of the compressor arrangement (18) is detected (21; 27c; 8b; 32, 33) and at least one of the two limit values, pressure upper limit (Pmax) and pressure lower limit (Pmin), is changed so that the open/closed switching period (Δt, Δt₁) of the switching valve (2) does not fall below a predetermined value (Δtmin; Δt₁min). 2. Method according to claim 1, characterized in that the detection of the magnitude (q) of the load on the load side of the compressor arrangement (18) is accomplished by measuring the consumption rate (q) of the compressed gas on the load side. 3. Method according to claim 2, characterized in that the detection of the magnitude (q) of the load on the load side of the compressor arrangement (18) is accomplished by measuring the rate of change (dp/dt) of the pressure on the load side of the compressor arrangement (18). 4. Method according to claim 2, characterized in that the detection of the consumption rate (g) of the compressed gas is accomplished by measuring (21; 32, 33) the open/close switching period (Δt; Δt₁) of the switching valve (2) and the pressure upper limit (Pmax) is changed so that the measured open/close switching period (Δt; Δt₁) does not fall below a predetermined minimum value (Δtmin, Δt₁min). 5. Method according to claim 4, characterized in that the pressure upper limit (Pmax) is changed so that the measured open/close switching period (Δt; Δt₁) corresponds to the predetermined minimum value (Δtmin; Δt₁min). 6. Device for regulating the performance of a compressor arrangement with a switching valve (2) on the inlet side of a compressor (1) which is suitable for being opened or closed in order to put the compressor (1) into full load or idle operation; with a pressure detection device (8) for detecting the pressure (P) on the load side of the compressor arrangement (18) and with an open/close control device (2-7, 9; 2-7, 9a) for opening and closing the switching valve (2), which is based on the comparison of a predetermined upper pressure limit (Pmax) with a predetermined lower pressure limit (Pmin), the detected pressure value (P) being detected by the pressure detection device (8), characterized in that the control device (21; 27c; 8b; 32, 33) has the following devices: a load detection device (9, 21, 9a, 32, 33) for detecting the size (q) of the load on the load side of the compressor arrangement (18) and a setting value changing device (9, 24a-c; 9a, 34) for changing at least one of the two limit values pressure upper limit (Pmax) and pressure lower limit (Pmin) so that the open/closed switching period (Δt, Δt₁) of the switching valve (2) does not fall below a predetermined value (Δtmin; Δt₁min). 7. Device according to claim 6, characterized in that the load detection device has a gas consumption detection device for detecting the gas consumption rate (q) on the load side of the compressor arrangement (18). 8. Device according to claim 6, characterized in that the gas consumption detection device has a pressure change rate detection device (9, 27) for detecting the pressure change rate (dP/dt) of the compressed gas on the load side of the compressor arrangement (18). 9. Device according to claim 7, characterized in that the gas consumption detection device on the load side of the compressor arrangement (18) has a compressed gas throughput detection device (8b). 10. Device according to claim 6, characterized in that the load detection device has a device (9, 21; 9a, 32, 33) for detecting the open/close switching period (Δt; Δt₁) of the switching valve (2) and that the value changing device (9, 24a-c; 9a, 34) is designed such that at least one of the two limit values, the upper pressure limit (Pmax) and the lower pressure limit (Pmin), is changed so that the open/closed switching period (Δt, Δt₁) of the switching valve (2) does not fall below a predetermined minimum value (Δtmin; Δt₁min). 11. Device according to claim 10, characterized in that the value changing device (9, 24a-c; 9a, 34) is designed so that the open/close switching period (Δt, Δt₁) of the switching valve (2) does not fall below a predetermined minimum value (Δtmin; Δt₁min). 12. Device according to claim 11, characterized in that the open/close control device (2-7, 9, 2-7, 9a) has several predetermined pressure upper limits (Pmax1, Pmax2) and the set value changing device (9a, 34) is designed such that it changes the pressure upper limit (Pmax1, Pmax2) according to the size (q) of the load of the compressor arrangement (18). 13. Device according to claim 6, characterized in that the open/close control device (2-7, 9, 2-7, 9a) has a pressure switch (40, 41) on the low pressure side, which switches the compressor (1) from idle to full load at the lower pressure limit (Pmin), and several pressure switches (40, 41) on the high pressure side, which switch the compressor (1) from full load to idle at the upper pressure limits (Pmax1, Pmax2); that the power control device further has a time adjustment device (45) which measures a period of time after an operation of the pressure switch (40, 41) on the low pressure side; and that the set value changing device (9a, P1X, P2X, T, 44) is designed such that the pressure switches (40, 41) are only actuated when at least a predetermined period of time (Δt₁min) has elapsed after the compressor (1) has been switched from idle to full load by the interaction of the pressure switches (40, 41) on the high pressure side and the timing device (45). 14. Device according to claim 6, characterized in that the compressor capacity control device further comprises a pressure drop rate detecting device (25b) for detecting the drop rate (dP/dt) of the pressure (P) on the load side of the compressor arrangement (18) during idling and the set value changing unit (9, 25c) is designed to change the predetermined pressure lower limit (Pmin) according to the drop rate (dP/dt) of the pressure (P) so that the pressure (P) on the load side does not fall below a minimum pressure (Pmin0) required on the load side when the compressor (1) is switched from idling to full load. 15. Device according to claim 6, characterized in that the compressor capacity control device on the outlet side of the compressor (1) further comprises an air outlet valve (3) which is closed when the switching valve (2) is opened and which is opened when the switching valve (2) is closed.