Compressor capacity control method and apparatus therefor

Abstract: A compressor capacity control method and an apparatus therefor in which an on-off type control valve (2) provided at the inlet side of a compressor body (1) is made on or off for putting the compressor body (1) in a full load operational condition or in an unload operational condition, respectively; and the pressure (P) at the load side of the compressor (18) is detected (8), and when the detected pressure (P) reaches a predetermined upper pressure limit (P max ), the on-off type control valve (2) is made off, thereby putting the compressor body (1) in an unload operational condition, while when the detected pressure (P) reaches a predetermined lower pressure limit (P min ), the on-off type control valve (2) is made on, thereby putting the compressor body 1 in a full load operational condition, whereby a magnitude (q) of load at the load side of the compressor (18) is detected (21; 27c; 8b; 32, 33), and at least one of the upper pressure limit (P max ) and the lower pressure limit (P min ) is so changed as to make the on-off switching period (Δt; Δt₁) of said on-off control valve (2) not shorter than a predetermined value

Description: BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention relates to a method of controlling a capacity of a compressor having an on-off type control valve and an apparatus therefor, more particularly, to a method of controlling a capacity of a rotary displacement type compressor such as a screw type compressor and roots type compressor, and the apparatus therefor. Herein, the compressor includes a compressor body, a piping connected with the discharge or outlet side of the compressor body at one end thereof and with a gas consumption side (load side) at the other end thereof, and a capacity control device for controlling the flow rate of the compressed gas (compressor capacity) from the compressor body to the piping at the outlet side of the compressor body. PRIOR ART A conventional capacity control apparatus of a screw type compressor having an on-off type control valve includes, for example, as shown in JP-A-58-167890, the on-off control valve arranged at the suction or inlet side of the compressor body and an air vent valve arranged at the discharge or outlet side of the compressor body. In case the compressor is operated in a "full load" condition where the on-off control valve is opened and the air vent valve is closed, the air pressure at the outlet side increases. When the air pressure at the outlet side increases beyond a predetermined set value, the on-off control valve is closed and the air vent valve is opened, thereby switching the compressor body into an "unload" operation. Then, when the air pressure in an air accumulator disposed at the outlet side decreases and reaches a value lower than a predetermined set value, the on-off control valve is opened and the air vent valve is closed, thereby switching the compressor body into a full load operation. In such a conventional capacity control apparatus, the air pressure at the outlet side of the compressor body is detected by a pressure switch, and according to the value of the detected pressure, a switching command signal is transmitted for switching the compressor body between a full load operation and an unload operation. In the above-mentioned conventional art, since each of the upper limit and the lower limit of the set air pressure values, at which the command signal for switching the compressor body between a full load operation and an unload operation is to be transmitted, is fixed, the switching of the operation of the compressor body is carried out in dependence on a constant pressure value regardless of the volume of the accumulator connected to the outlet side of the compressor body. Therefore, if a consumption speed of amount per time period (operational load) of the air from the compressor is assumed constant, the larger the volume of the piping system (including the air accumulator) connected with the outlet side of the compressor body is, the longer the interval of switching (switching period or on-off period) of the compressor body between a full load operation and an unload operation of the compressor body is. Further, in case of an extremely large (high) or extremely small (low) operational load in comparison with the compressor capacity, the switching period between a full load operation and an unload operation of the compressor becomes also longer. As a result, the time of operation of the compressor body during which the air pressure at the outlet side is higher than the pressure required by the air consuming side becomes longer, thereby causing a problem that unnecessary electric power is wasted in order to operate the compressor body. SUMMARY OF THE INVENTION An object of the present invention is to provide a compressor capacity control method and an apparatus therefor capable of reducing the operation time of the compressor body during which the compressor body produces unnecessarily high pressure, under a condition where the operational load (gas consumption speed or amount/time at the gas consumption side) is varied significantly, or the volume of the piping connected with the outlet side of the compressor body is large, and accordingly, reducing the electric power consumption. Another object of the present invention is to provide a compressor capacity control method and an apparatus therefor in which the switching period between a full load operation and an unload operation of the compressor body is maintained longer than a predetermined time length, thereby improving reliability of the capacity control apparatus while ensuring the pressure at the air consumption side to be maintained over a predetermined minimum level at all times. The above-mentioned objects of the present invention can be achieved by a capacity control method according to the present invention in which a compressor body is switched between a full load condition and an unload condition by operating an on-off control valve disposed at an inlet side of the compressor body; a pressure (P) at the discharge or load side of the compressor is detected by a pressure sensor, and when the pressure (P) reaches a predetermined upper pressure limit P max , the on-off control valve is made off, thereby putting the compressor body in the unload condition, while when the detected pressure (P) reaches a predetermined lower pressure limit P min , the on-off control valve is made on, thereby putting the compressor body in the full load condition; and magnitude (q) of the load at the load side of the compressor is detected, and according to the magnitude (q) of the load, at least one of the above-mentioned upper limit P max and lower limit P min is changed so as to make an on-off period (Δt; Δt₁) longer than a predetermined set time length value Generally, in an on-off type capacity control method, which is adopted in a capacity control method according to the present invention, when the on-off control valve is put in an "on" condition (open condition) and the compressor body is put in a full load operational condition, the pressure at the load side (outlet side of the compressor body) increases. When the pressure at the load side reaches a preset or predetermined upper pressure limit, the on-off control valve is put in an "off" condition and the compressor body is switched from the full load operation into an unload operation. The pressure at the load side then decreases at a speed corresponding to the magnitude of the load. When the pressure at the load side reaches a preset or predetermined lower pressure limit, the compressor body is switched from the unload operation to the full load operation and the pressure at the load side increases again. The above-mentioned operations are repeated. In a capacity control method according to the present invention, since the preset upper pressure limit or the preset lower pressure limit, at which value the on-off control valve is put on (opened) or off (closed), is adjusted according to the magnitude of the load, it becomes possible to maintain the switching period between the full load operation and the unload operation longer than a predetermined time length, thereby enhancing the reliability of an apparatus for carrying out the capacity control method of the present invention. The term "on-off period" means, in principle, a period (Δt) including one full load operation time and one unload operation time, these operations being alternatively repeated. The time length to be corresponded to the magnitude of the load may be also a full load period (Δt₁) or an unload period ( Δt₂ = Δt - Δt₁ ) instead of the on-off period (Δt). In other words, the full load period (Δt₁) or the unload period (Δt - Δt₁) is also a time length corresponding to the on-off period. According to one preferred embodiment of the present invention, the detection of the magnitude, more specifically relative magnitude, of the load at the load side of the compressor is carried out by detecting the compressed gas consumption rate or ratio (relative to the compressor capacity) (q) at the load side, and the detection of the compressed gas consumption rate or ratio (q) is carried out, for example, by measuring a variation speed (dP/dt) of a pressure (P) at the load side of the compressor, or by measuring the on-off period (Δt; Δt₁) of the on-off control valve. In the latter case, the preset upper pressure limit (P max ) is changed so as to make the measured on-off period (Δt; Δt₁) longer, more specifically not shorter than a predetermined value According to one preferred embodiment of the present invention, the preset upper pressure limit (P max ) is determined so as to make the measured on-off period (Δt; Δt₁) coincide with the predetermined value In this case, by making the lower limit of pressure coincide with the pressure level required at the consumption side (load side), a necessary pressure is maintained at all times. According to the present invention, the above-mentioned objects can be achieved by a compressor capacity control apparatus including an on-off control valve provided at an inlet side of a compressor body and adapted to be made "on" or "off" for putting the compressor body in a full load condition or in an unload condition, respectively; a pressure detecting means for detecting a pressure (P) at a load side of the compressor; and an on-off control means for effecting an on-off action on the on-off control valve based on a comparison of a preset or predetermined upper pressure limit (P max ) and a preset or predetermined lower pressure limit (P min ) with the detected pressure value (P) detected by said pressure detecting means, characterized in that the control means comprises a load detecting means for detecting a magnitude (q) of a load at the load side of the compressor, and a set value changing means for changing at least one of the preset upper pressure limit (P max ) and the preset lower pressure limit (P min ) so as to make an on-off period (Δt; Δt₁) of the on-off control valve not shorter than a predetermined value In a capacity control apparatus according to the present invention, similarly to in the capacity control method, since the upper pressure limit or the lower pressure limit, at which the on-off control valve is made on or off, is changed according to the magnitude of the load at the load side of the compressor body, the switching period between the full load operation and the unload operation can be changed not shorter than the predetermined time length, thereby enhancing the reliability of the capacity control apparatus. Further, in case of an extremely large (high) or extremely small (low) operational load or a great inside volume of the piping at the outlet side, the upper pressure limit is made or changed lower and the operational time at high pressure can be decreased, thereby further enhancing the energy economizing effect. According to one preferred embodiment of the present invention, the load detecting means is composed of a gas consumption rate or ratio detecting means for detecting the gas consumption rate or ratio (q) at the load side of the compressor. The gas consumption rate or ratio detecting means is composed of, for example, (1) a variation speed detecting means for detecting a pressure variation speed (dP/dt) of the compressed gas at the load side of the compressor; (2) a compressed gas flow rate detecting means at the load side of the compressor; or (3) a means for detecting the on-off period (Δt; Δt₁) of the on-off control valve. In the above (3) case, the set value changing means is so constituted that one of the preset upper limit (P max ) of pressure and the preset lower limit (P min ) of pressure is changed to make the on-off period (Δt; Δt₁) of the on-off control valve not shorter than a predetermined value In one of the preferred embodiments, for example, the set value changing means is so constituted that the upper pressure limit (P max ) is changed to make the on-off period (Δt; Δt₁) of the on-off control valve not shorter than the predetermined value In this case, since the lower pressure limit is maintained at a constant level, not only the minimum pressure can be assured, but also the upper pressure limit can be suppressed at a necessary lowest value. Further, in this case, when the operational load is extremely large (high) or when inside volume of the piping at the outlet side is large, for example, the upper pressure limit is made lower and the operational time length at high pressure can be decreased, thereby further enhancing the energy economizing effect. According to one preferred embodiment of the invention, the on-off control means includes a plurality of predetermined upper pressure limits (for example, two limits such as P max1 , P max2 ), and the set value changing means is so constituted as to switch or change the pressure upper limit (P max1 , P max2 ) according to the magnitude (q) of the load on the compressor. According to one preferred embodiment of the present invention, the on-off control means comprises a low pressure side pressure switch acting at the pressure lower limit (P min ) for switching the compressor body from the unload operation to the full load operation and a plurality of high pressure side pressure switches acting at the plurality of upper pressure limits (P max1 , P max2 ) for switching the compressor body from the full load operation to the unload operation; the capacity control apparatus further comprises a timer for measuring a time length after an action or actuation of the low pressure side pressure switch; and the set value changing means is constituted so that the high pressure side pressure switches are not actuated before at least the predetermined time length has elapsed after the compressor body is switched from the unload operation to the full load operation by combination of the plurality of high pressure side pressure switches and the timer. According to one preferred embodiment of the present invention, the compressor capacity control apparatus further comprises a pressure decreasing speed detecting means for detecting a decreasing speed (dP/dt) of the pressure (P) at the load side of the compressor in the unload condition, and the set value changing means is constituted so as to change the predetermined lower pressure limit (P min ) according to the pressure decreasing speed (dP/dt) of the pressure (P) for preventing the pressure (P) at the load side from decreasing beyond a minimum pressure (P min0 ) required at the load or consumption side when the compressor body is switched from the unload condition to the full load condition. In this configuration, it is ensured that the pressure (P) at the load side is prevented from decreasing beyond the minimum pressure (P min0 ) required at the load side when the compressor body is switched from the unload condition to the full load condition. According to one preferred embodiment of the present invention, the capacity control apparatus further comprises an air vent valve at the outlet side of the compressor body, the air vent valve being closed when said on-off valve is made on, while opened when the on-off valve is made off. The foregoing and other objects, features, as well as advantages of the invention will be made clearer from the description of preferred embodiments with reference to drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration showing an embodiment of the present invention as a whole; Fig. 2 is an operation flowchart for explaining an arithmetic processing flow of a control device; Figs. 3A and 3B are graphs showing a time-dependent variation of the pressure (P) at the outlet side and a time-dependent variation of the power (L) in an embodiment of the present invention; Fig. 3C is a flowchart showing an example for evaluating the magnitude of the load by measuring the pressure variation speed; Fig. 4 is a graph for explaining the pressure variation when the compressor body is switched from the unload operation to the full load operation; Fig. 4A is a flowchart corresponding to the control shown in Fig. 4; Fig. 5 is a block-diagram showing a concrete example of the control shown in Fig. 1; Fig. 6 is a flowchart of a processing in a case where a control having a time measuring means and a pressure switch capable of setting two kinds of the upper pressure limit is employed as the control of Fig. 1; Fig. 7 shows a diagram of an electric circuit of the control device for carrying out the embodiment shown in Fig. 6; and Figs. 7A and 7B are graphic illustrations indicating functional features of the pressure switch a arranged in the circuit of Fig. 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a compressor capacity control method and apparatus according to an embodiment of the present invention will be described below. The figure shows mainly an air-related system of a screw compressor 18 including a capacity control apparatus 17. In Fig. 1, reference numeral 1 denotes a compressor body, numeral 2 denotes a suction valve as an on-off valve disposed at an inlet side of the compressor body 1 and actuated between an open position and a closed position by means of a rod 4a of a hydraulic cylinder device 4. Reference numeral 19 denotes a suction filter disposed on the way or passage from an inlet port 14 to the suction valve 2. Numeral 13 denotes a non-return or check valve disposed at an outlet or discharge side of the compressor body 1, while numeral 12 denotes an after-cooler for cooling a compressed or pressurized gas or an exhausted from the compressor body 1. Reference numeral 11 denotes an accumulator for storing the compressed air cooled by the after-cooler 12, the compressed air in the accumulator 11 being taken out through a consumption line 16 and offered to use. Reference numeral 8 denotes a pressure sensor for detecting the pressure P in an outlet line 8a, information of the pressure P detected by the pressure sensor 8 being transmitted to a control device 9. Reference numeral 3 denotes an air vent valve actuated between an open position and a closed position by means of the rod 4a of the hydraulic cylinder device 4 similarly to the suction valve 2, and numeral 15 denotes an air vent silencer disposed between an outlet side of the air vent valve 3 and the air vent port 15. Further, reference numeral 7 denotes an oil sump, numeral 6 denotes hydraulic pump, and numeral 5 denotes a four-way electromagnetic valve, which is operated under a command or instruction or control of the control device 9 so as to select one of three operational positions thereof, thereby moving the rod 4a upwards for downwards, or stopping the same through the hydraulic cylinder device 4. In the compressor 18 having the above-mentioned arrangement, the capacity control apparatus 17 is composed of the valves 2, 3, the cylinder device 4, the pump 6, the oil sump 7, the pressure sensor 8, and the control device 9. A piping 17a at the outlet side is composed of the after-cooler 12 disposed downstream of the non-return valve 13, the air accumulator 11 and the piping 8a connecting these components. In a full load operation of the compressor body 1 of the compressor 18, the piston of the hydraulic cylinder device 4, which is connected with the suction valve 2 and the air vent valve 3 through the rod 4a puts the suction valve 2 in an open position and the air vent valve 3 in a closed position by the help of the oil supplied from the oil sump 7 through the four-way electromagnetic valve 5 by the hydraulic pump 6. In such full load operational condition, the air sucked or introduced from the inlet port 14 flows through the suction filler 19 and the suction valve (on-off control valve 2) in a fully opened condition to the compressor body 1. The high temperature and high pressure air obtained by being compressed by the compressor body 1 is fed through the non-return valve 13 and the after cooler 12 to the air accumulator 11, from which the air is sent to the air consumption line 16 and consumed. In general, in the full load condition, since the air flow rate discharged from the compressor body 1 is greater than the air consumption rate, the pressure in the outlet side piping system 17a including the air accumulator 11, namely, the pressure P at the load side or at the outlet side of the compressor 18 gradually rises. In other words, the compressor body 1 has been selected so as to have a sufficient discharge capacity. This pressure P at 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 at the load side reaches a preset or predetermined upper pressure limit P max , the control device 9 transmits a command or control signal for switching the oil path in the four-way electromagnetic valve 5 and actuating the hydraulic cylinder 4 for moving the rod 4a, thereby putting the suction valve 2 in the closed position and, at the same time, the air vent valve 3 in the open position, resulting in an unload operational condition of the compressor body 1. In the unload condition of the compressor body 1, since the air is not supplied from the compressor body 1 to the air accumulator 11, and the air in the air accumulator 11 is simply consumed, the pressure P in the outlet side piping system 17a including the air accumulator 11, namely, the pressure P at the load side of the compressor 18 gradually decreases. The control device 9 is so constructed that, when the pressure P decreases and reaches a preset or predetermined lower pressure limit P min , the four-way electromagnetic valve 5 is switched over so as to put the compressor body 1 in the full load condition again. Fig. 5 is a block diagram showing a concrete example of the control device 9 shown in Fig. 1. The control device 9 comprises a A/D (analogue-to-digital) converter 91 serving as a part of a pressure detector, ROM (read only memory) 93 and RAM (random access memory) 94 serving as a memory unit and a central processor 92 including a timer or time measuring part 92a and an arithmetic processing part 92b. The pressure value P detected by the pressure sensor 8 is converted from an analog signal to a digital signal by the A/D converter 91, and sent to the central processor or central processing unit 92. The central processing unit 92 composed, for example, of a microprocessor and including the timer 92a executes comparison and other processing utilizing the pressure signal P and the measured or counted time signal t, and then transmits an on-off command signal for controlling the capacity control valve composed by the valves 2 and 3. The ROM 93 stores preset or predetermined values such as P min , P max and Δt min which will be explained later, while the RAM 94 temporarily stores information such as operation or processing results. More particularly, in the control device 9, the detected outlet side pressure P is given through the A/D converter 91 to the processor 92, where the switching period Δt, namely the sum of a full load operational time length Δt₁, and an unload operational time length Δt₂ ( Δt = Δt₁ + Δt₂ ), is measured. The switching period Δt depends on a relative magnitude of load, i.e. a consumption rate or ratio of the compressed gas relative to a compressor capacity. Further, in the control device 9, the pressure difference ΔP between the predetermined upper pressure limit P max and the predetermined lower pressure limit P min is so changed or reset the switching period Δt fall within a predetermined range Δt min - Δt max (in the example in Fig. 2, Δt becomes equal to Δt min ). Then, at least one of the P max and P min is changed and compared with the detected pressure value P, and a switching command signal for the four-way valve 5 is delivered. The time lengths of Δt₁, and Δt₂ may be repeatedly measured for a suitable period of time, and an average value in several cycles may be determined. This average switching period Δt is compared with the predetermined value Δt min , and the pressure difference ΔP is so changed, if necessary, that the relation Δt min ≦ Δt ≦ Δt max can be satisfied. Fig. 2 shows an example of a processing flow of the operation in the control device 9. In this example, the upper pressure limit P max or the pressure difference ΔP, namely, P max - P min , is adjusted so as to make the Δt becomes to coincide with ▵t min . The value of ▵t min is determined in consideration of conditions such as the minimum operational speeds of the valves 2, 3 of the capacity control device 17. If suitable or desired, other operational factors may be taken into consideration. There should be firstly set the predetermined lower pressure limit P min , an initial set value of ▵P, and the minimum period value ▵t min (step 20), where the upper pressure limit P max = P min + ΔP . Then, in a condition that the P max and P min have been set, the actual switching period ▵t is measured (step 21). Further, in step 23, Δt is compared with Δt min . In case of Δt = Δt min , the already set ΔP is continuously used as ΔP' (step 24a). In step 23, in case of Δt < Δt min or Δt > Δt min , the pressure difference ΔP is changed to ΔP' defined as follows (step 24b or 24c respectively): Thus, the following relation is obtained: P' max = P min + ΔP' = P min + ΔP (Δt min /Δt) In case the relative air consumption rate through the air consumption line 16 fluctuates, the processing is returned to the step 21 so that the above-mentioned control is continuously effected. According to this embodiment, since the pressure difference ΔP between P min and P max is controlled so as to make the switching period Δt decrease to Δt min , it becomes possible to minimize the variation or amplitude of the pressure at the load side. Figs. 3A and 3B are graphs showing a time-dependent variation of the pressure P at the outlet side and a time-dependent variation of the power L or the energy required per unit time for driving the compressor body 1 in the above-mentioned embodiment where the lower pressure limit P min is fixed while only the upper pressure limit P max is changed based on the reset or changed data ΔP'. A case of curve a in Figs. 3A and 3B is now described. In a full load condition of the screw compressor body 1, since the air flow rate supplied from the compressor body 1 to the air accumulator 11 through the line 8a is greater than the air consumption rate Qc consumed through the line 16, the pressure P at the outlet side gradually increases. When the pressure P reaches the upper pressure limit P max , the compressor body 1 is switched into an unload operation under the control of the control device 9. In the unload operational condition, since the air is not supplied from the compressor body 1 to the air accumulator 11, the pressure P at the outlet side decreases as the air in the accumulator 11 is consumed. When the pressure P at the outlet side reaches a predetermined lower pressure limit P min , the compressor body 1 is switched from the unload operation to the full load operation under the control of the control device 9, thereby increasing the pressure P at the outlet side. Then, the same process is repeated. Defining the pressure difference between the set upper pressure limit P max and the set lower pressure limit P min as ΔP and a ratio, i.e. consumption ratio or load ratio or relative load, of the consumption rate Qc with respect to an air flow rate Qs from the compressor body 1 as q (i.e. Qc = q Qs (in this case, both being mass flow rates)), the time length Δt₁ which is required for increasing the pressure P at the outlet side from P min to P max in the full load condition and the time length Δt₂ which is required for decreasing the pressure P at the outlet side from P max to P min in the unload condition are expressed as follows: where, V : accumulator volume, i.e. a volume of piping system 17a at the outlet side, Ps : suction pressure of the compressor, Qs : suctioned air flow rate of compressor. From the equation (4) above, it will be understood that if the air consumption rate q is substantially constant, Δt becomes shorter when the ΔP decreases. Further, defining the minimum value of Δt which is determined by the mechanical restriction of the capacity control apparatus 17 as Δt min and ΔP at this time as ΔP min , the following equation is obtained: During an actual operation of the compressor 18, by measuring Δt at ΔP, the following equation is obtained: ΔP min = ΔP Δt x Δt min (6) In case the air accumulator volume V is sufficiently large in comparison with the air flow rate Qs from the compressor body 1, Δt becomes relatively long even when ΔP is relatively small. In consequence, in this case, variation or fluctuation in the pressure P at the outlet side can be suppressed in a small range by decreasing Δt to Δt min . Considering tolerances, Δt may be controlled to be slightly longther than Δt min . Further, it is also possible to set an upper limit Δt max for Δt so as to prevent an excessive increase of Δt and to maintain Δt shorter than Δt max . Further, it may be also possible to similarly control ΔP (P min ) while maintaining P max constant instead of maintaining P min constant and changing P max . In this case, however, it is preferred to control P min not to be lower than a necessary minimum pressure P min0 . The curve b in Fig. 3 shows a time-dependent variation of the pressure P at the outlet side and a time-dependent variation of the power L, when the lower pressure limit P min is fixed and the pressure difference ΔP is decreased so as to approach ΔP', smaller than the value in the before-mentioned curve a . The average powers in cases of the curves a and b are compared with each other as follows, assuming the time-dependent variation of the power L is substantially linear: Average power L ave for curve a Average power L' ave for curve b where, L min : power at minimum pressure P min , L max : power at upper pressure limit P max , L' max : power at upper pressure limit P' max , L o : power in an unload operation. Further, from equations (2), (3) and (4), the following equations are obtained: Further, if assumed the relative load or air consumption ratio q (ratio of consumption flow rate Qc to compressor body discharge flow rate Qs, both being volumetric flow rates at the same pressure or mass flow rates) is constant in the curves a and b , following equations are deduced: where Δt' = Δt₁' + Δt₂' (4b), and L ave - L' ave = 1 2 (L max - L' max ) x q. (10) Since L max > L' max , the average power is smaller in case of b than in case of a . By measuring Δt₁ and Δt₂ in a condition where the air consumption ratio q does not change rapidly, a constant K (= V/PsQs) depending on the features PsQs of the compressor body 1 and the features V of the piping system 17a can be obtained. Using this constant K, an equation 1 - kΔP/Δt₁ = q is deduced from equation (2), and an equation ΔP = (ΔP/Δt₁)(1 - KΔP/Δt₁)Δt is deduced from equation (9). Further, when the variation of the pressure P during the period Δt₁ is assumed linear, the following relation is obtained: ΔP = dP/dt₁(1 - K dP/dt₁)Δt In consequence, when the constant K is once obtained, by measuring the pressure variation speed dP/dt₁ (or dP/dt₂) in a full load operation or in an unload operation, ΔP min can be obtained as follows: ΔP min = dP/dt₁ (1 - K dP/dt₁)Δt min (11) Otherwise, in the same manner, ΔP min = dP/dt₂ (1 - K dP/dt₂)Δt min (11') According to equations (11) and (11'), in case the accumulator volume V is great, or the compressor load q (i.e. air consumption ratio through the consumption line 16) is extremely small or extremely large (q is deviated largely from 1/2), in other words, dP/dt₁ or dP/dt₂ is small, ΔP min becomes smaller. As a result, P max or P' max is set to be small, and accordingly, the compressor body 1 is not driven in a condition of an unnecessarily high pressure, thereby decreasing the required power L. As shown in flowchart 27 in Fig. 3c, if Δt₁ and Δt₂ are measured only for relatively short period, for example, Δt₁ + Δt₂, or n (Δt₁ + Δt₂), n being 2 or 3, after an operation start of the compressor (27a). The constant K dependent on the volume V of the actual piping 17a including the volume of the consumption line 16 is determined (27b). Then, changing speed dP/dt₁ or dP/dt₂ of the pressure P is measured (27c). Then, ΔP min is calculated (27d) based on equations (11) and (12) including parameters the constant K and the set value t min . Thus, the pressure difference ΔP is set at an optimal value ΔP min at which Δt becomes equal to Δt min at all times even when the air consumption ratio q continuously changes. The measurement of dP/dt₁ or dP/dt₂ is carried out by sampling the values of the pressure P detected by the pressure sensor 8 with desired time intervals δt by use of the processor 92, and obtaining δP/δt by use of the pressure difference δP between pressures at two adjacent sampling points. When the outlet side pressure decreasing speed dP/dt is high, it should be noted there is required some time length for switching the compressor body 1 from the unload operation to the full load operation, in other words, for switching the valve 2 of the capacity control apparatus 17 from the closed condition to the open condition and the air vent valve 3 thereof from the open condition to the closed condition. As shown in Fig. 4, even if the control operation of the control apparatus 17 is started at point A, there may be a fear that the pressure P at the outlet side undershoots or decreases sharply beyond the set lower pressure limit as shown by the curve e , thereby causing the pressure p at the outlet side lower than the required minimum pressure P min0 . In order to prevent a fear of this kind, in the embodiment of the present invention shown in Fig. 4, the relation between the pressure decreasing speed dP/dt and the pressure difference ΔPc exceeding the pressure lower limit P min is measured in advance and stored in the memory 93 or 94 in form of a table or an equation (step 25a in Fig. 4A). Next, dP/dt is detected in operation (step 25b), and the set pressure lower limit is changed from a value P min to a value P o for making the extreme minimum value of the pressure P at the outlet side equal to P min0 (step 25c). In this example, since the operation of the capacity control apparatus starts at point B where the value of the pressure P becomes P o , the pressure P at the outlet side is prevented from decreasing beyond P min0 as shown with the curve d in Fig. 4. In this case, P o is obtained by the following equation: P o = P min0 + ΔPc where, The affix "i" means a set of data measured and stored in advance in the step 25a. In case a plurality of sets of data are stored in the step 25a, a set of data approximate to the value of dP/dt measured in the step 25b may be selected. In certain circumstances, ΔPc may be obtained through an interpolation using two sets of stored data. Further, the relative magnitude of the load, namely air consumption ratio q can be determined based on the air flow rate Q L through the line 16 detected by the flow meter 8b, which corresponds to the magnitude of the load. Fig. 6 is a flowchart in case the control device 9 includes a time counting section and pressure switch means capable of setting two kinds (P max1 , P max2 ) of the pressure upper limit P max . Firstly, in a step 30, two upper pressure limits P max1 and P max2 (P max1 < P max2 ) and a minimum value of Δt₁ are set in advance. P max1 is set, for example, at a pressure level which the pressure P at the outlet side reaches when Δt₁ becomes substantially equal to under a usual magnitude load, while P max2 is set at a pressure level which the pressure P at the outlet side reaches when Δt₁ becomes substantially equal to in in case of a very small load, for example, in case the compressed gas is not consumed in fact. In a step 31, when the full load operation of the compressor body 1 is started, the measurement of the pressure P at the outlet side and the measurement of the time length t after starting are started. In a step 32, the measured pressure P is compared with the lower one P max1 of the set upper pressure limits. In case P < P max1 , the processing is returned to the step 31, and the steps 31 and 32 are repeated until P becomes equal to P max1 , strictly spearing, P becomes equal to or greater than P max1 . When P becomes equal to or greater than P max1 , the processing advances to a step 33, where the latest measured value t is compared with the smallest switching time length When the t is greater than the smallest switching time length Namely, when the magnitude of the load or the relative magnitude q is substantially equal to an assumed one, the processing advances to a step 35, where the compressor body 1 is switched to an unload operation. In other words, the (relative) magnitude of the load is evaluated in the steps 32 and 33. On the other hand, when the measured value t is smaller than namely, when the (relative) magnitude of the load is smaller than assumed one, the processing advances to a step 34, where the detected pressure P is compared with the upper one P max2 of the set upper pressure limits. The steps 31 to 34 are repeated until the condition P ≧ P max2 has been satisfied, as continuing the full load operation. When P becomes equal to or greater than P max2 , namely when the t becomes substantially equal to the processing advances to the step 35, where the compressor body 1 is switched to the unload operation. The set value is determined in due consideration of the mechanical restriction for timing lengths required for actuating the on-off type control valve 2 and the air vent valve 3 and of a compromise between the merit obtained when the on-off switching period is excessively short and the demerit of the significant reduction in mechanical life of the control valves 2 and 3, the electromagnetic valve 5 thereby and so on. Fig. 7 is an illustration showing an electric 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) having a hystreresis feature of making the switch "on" when the detected pressure value P is smaller than P min , and making "off" when the detected pressure value P is beyond P max1 . The pressure switch 41 is a differential pressure switch (Fig. 7B) having a hystreresis feature of making the switch "on" when the detected pressure value P is smaller than P min , and making off when the detected pressure value P is beyond P max2 . Numeral 42 denotes a relay for making on or off a switch P1X, numeral 43 a relay for making on or off a switch P2X, and numeral 44 a relay for making on or off a switch 46X. Numeral 45 denotes a timer which starts to count time when the switch 46X is made on, which makes the normally-closed switch T off when the time length reaches the predetermined set value Reference characters 5a denotes a solenoid of the electromagnetic valve 5 in Fig. 1, and reference characters 5b a surge absorber. The electric power supply line 48 is energized when the compressor body 1 starts operating. By virtue of this arrangement of the electric circuit 9a, when the compressor body 1 is started to be operated, the switches P1X and P2X are made on by means of the relays 42 and 43, respectively, because at the beginning the pressure switches 40 and 41 are in on-condition due to the relation P < P min , and the switch 46X is made on by means of the relay 46X. Therefore, when the solenoid 5a of the electric valve 5 starts to be energized and a full load operation of the compressor body starts, the timer 45 starts counting the elapsed time t. In case the relative air consumption rate or air consumption ratio q is small or the load is small, even if the pressure P reaches P max1 , t does not reaches thereby maintaining the switch T in a closed condition. Therefore, even if the relation P > P max1 is satisfied as making the pressure switch 40 off and the switch P1X off, the compressor 18 continues to operate in the full load condition, because the relay 44 is maintained in a set condition until the relation P > P max2 or is satisfied. In case of a high load, t becomes greater than before the relation P > P max1 is satisfied. Since the switch T is made off when t is greater than the switch 46X is made off, at the same time when the switch P1X is made off under the condition P > P max1 , whereby the solenoid 5a is deenergized and accordingly, the compressor body 1 is brought to an unload operation. By virtue of the above mentioned arrangement, a control device for controlling the on-off type control valve is constituted only by a plurality of pressure switches and a simple electric circuit, thereby effecting an energy economy at low cost and providing an apparatus in which the switching period between the full load operation and the unload operation can be maintained above the predetermined value. According to the present invention, since the set lower pressure limit or the set upper pressure limit is changed for controlling the on-off type control valve at the inlet side of the compressor body in dependence on the compressed gas consumption flow rate or ratio or the magnitude of the load, it becomes possible to maintain the switching period between the full load operation and the unload operation greater than the predetermined value, and to prevent a frequent switching of operations even in case of a high load, thereby enhancing the reliability of the apparatus. Further, if the minimum pressure value required by the gas consumption side is adopted as the set lower pressure limit, the pressure level required by the gas consumption side can be maintained at all times. In addition, since, by changing the set upper pressure limit according to the absolute or relative magnitude of the load so as to maintain the on-off period of the on-off type control valve within a set time range, the set upper pressure limit becomes lower in case of a extremely large or extremely small operational load or of a great inside volume of the piping system at the outlet side, an compressor operation time operated in a high pressure is decreased, thereby enhancing the energy economizing effect.

Claims: A compressor capacity control method in which an on-off type control valve (2) provided at the inlet side of a compressor body (1) is made on or off for putting the compressor body (1) in a full load operational condition or in an unload operational condition, respectively: a pressure (P) at a load side of the compressor (18) is detected (8); and when the detected pressure (P) reaches a predetermined upper pressure limit (P max ), the on-off type control valve (2) is made off, thereby putting the compressor body (1) in the unload operational condition, while when the detected pressure (P) reaches a predetermined lower pressure limit (P min ), the on-off type control valve (2) is made on, thereby putting the compressor body (1) in the full load operational condition, characterized in that a magnitude (q) of load at the load side of the compressor (18) is detected (21; 27c; 8b; 32, 33), and at least one of said upper pressure limit (P max ) and the lower pressure limit (P min ) is so changed as to make the on-off switching period (Δt; Δt₁) of said on-off type control valve (2) not shorter than a predetermined value A compressor capacity control method claimed in Claim 1, wherein the detection of said magnitude (q) of the load at the load side of the compressor (18) is carried out by measuring a consumption rate (q) of the compressed gas at the load side. A compressor capacity control method claimed in Claim 2, wherein the detection of said magnitude (q) of the load at the load side of the compressor (18) is carried out by measuring a variation speed (dp/dt) of the pressure at the load side of the compressor (18). A compressor capacity control method claimed in Claim 2, wherein the detection of said consumption rate (q) of the compressed gas is carried out by measuring (21; 32, 33) the on-off switching period (Δt; Δt₁) of said on-off type control valve (2) and said upper pressure limit (P max ) is so changed as to make the measured on-off switching period (Δt; Δt₁) not shorter than a predetermined minimum value A compressor capacity control method claimed in Claim 4, wherein said upper pressure limit (P max ) is so changed as to make the measured on-off switching period (Δt; Δt₁) coincide with the predetermined minimum value A compressor capacity control apparatus including an on-off type control valve (2) provided at the inlet side of a compressor body (1) and adapted to be made on or off for putting the compressor body (1) in a full load operational condition or in an unload operational condition, respectively; a pressure detecting means (8) for detecting a pressure (P) at a load side of the compressor (18); and an on-off control means (2-7, 9; 2-7, 9a) for effecting an on-off action on the on-off type control valve (2) based on a comparison of a predetermined upper pressure limit (P max ) and a predetermined lower pressure limit (P min ) with the detected pressure value (P) detected by said pressure detecting means (8), characterized in that said control means (21; 27c; 8b; 32, 33) comprises a load detecting means (9, 21, 9a, 32, 33) for detecting a magnitude (q) of a load at the load side of the compressor (18), and a set value changing means (9, 24a-c; 9a, 34) for changing at least one of said upper pressure limit (P max ) and said lower pressure limit (P min ) so as to make the on-off switching period (Δt; Δt₁) of said on-off control valve (2) not shorter than a minimum predetermined value A compressor capacity control apparatus according to Claim 6, wherein said load detecting means comprises a gas consumption rate detecting means for detecting a gas consumption rate (q) at the load side of the compressor (18). A compressor capacity control apparatus according to Claim 6, wherein said gas consumption rate detecting means comprises a pressure variation speed detecting means (9, 27) for detecting a pressure variation speed (dP/dt) of the compressed gas at the load side of the compressor (18). A compressor capacity control apparatus according to Claim 7, wherein said gas consumption rate detecting means comprises a compressed gas flow rate detecting means (8b) at the load side of the compressor (18). A compressor capacity control apparatus according to Claim 6, wherein said load detecting means comprises a means (9, 21; 9a, 32, 33) for deterring the on-off switching period (Δt; Δt₁) of said on-off type control valve 2, and said set value changing means (9, 24a-c; 9a, 34) is so constituted that at least one of said upper pressure limit (P max ) and the lower pressure limit (P min ) is so changed as to make the on-off switching period (Δt; Δt₁) of said on-off type control valve (2) not shorter than a predetermined minimum value A compressor capacity control apparatus according to Claim 10, wherein said set value changing means (9, 24a-c; 9a, 34) is so constituted that said upper pressure limit (P max ) is so changed as to make the measured on-off switching period (Δt; Δt₁) not shorter than the predetermined minimum value A compressor capacity control apparatus according to Claim 11, wherein said on-off control means (2-7, 9, 2-7, 9a) includes a plurality of predetermined upper pressure limits (P max1 , P max2 ), and said set value changing means (9a, 34) is so constituted as to change the upper limit of pressure (P max1 , P max2 ) according to the magnitude (q) of the load of the compressor (18). A compressor capacity control apparatus according to Claim 6, wherein said on-off control means (2-7, 9, 2-7, 9a) comprises a low pressure side pressure switch (40, 41) acting at the lower pressure limit (P min ) for switching the compressor body (1) from the unload operation to the full load operation and a plurality of high pressure side pressure switches (40, 41) acting at the plurality of upper pressure limits (P max1 , P max2 ) for switching the compressor body from the full load operation to the unload load operation; the capacity control apparatus further comprises a timer (45) for measuring a time length after an action of the low pressure side pressure switch (40, 41); and said set value changing means (9a, P1X, P2X, T, 44) is constituted so that said high pressure side pressure switches (40, 41) are not actuated until at least a predetermined time length has elapsed after the compressor body (1) is switched from the unload load operation to the full load operation by combination of said plurality of high pressure side pressure switches (40, 41) and said timer (45). A compressor capacity control apparatus according to Claim 6, wherein said compressor capacity control apparatus further comprises a pressure decreasing speed detecting means (25b) for detecting a decreasing speed (dP/dt) of the pressure (P) at the load side of the compressor (18) in the unload operational condition, and said set value changing means (9, 25c) is constituted so as to change the predetermined lower pressure limit (P min ) according to said decreasing speed (dP/dt) of said pressure (P) for preventing the pressure (P) at the load side from decreasing beyond a minimum pressure (P min0 ) required at the load side when the compressor body (1) is switched from an unload condition to a full load condition. A compressor capacity control apparatus according to Claim 6, wherein said compressor capacity control apparatus further comprises an air vent valve (3) at the outlet side of the compressor body (1), said air vent valve (3) being closed when said on-off type control valve (2) is made on, and opened when said on-off type control valve (2) is made off.

Inventors: OKITA JUNJI, TSURU SEIJI, OKITA, JUNJI, TSURU, SEIJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: F04C29
IPC codes: F04C28, F04C29, F04B49, F04C18