Patent Publication Number: US-11040315-B2

Title: Gas-dissolved liquid producing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a gas-dissolved liquid producing apparatus for producing gas-dissolved liquid by dissolving gas in liquid. 
     Description of the Related Art 
     Cleaning of products in semiconductor device factories and manufacturing factories for electronic parts such as liquid crystals have been recently increasingly improved along with complication of producing processes and miniaturization of circuit patterns. For example, fine particles, metals, organic materials, etc. adhering to silicon wafers are removed by using special liquid (called cleaning liquid) obtained by dissolving high-purity gas, or high-purity gas and chemicals into functional water (for example, ultrapure water or the like). 
     Ozonated water in which ozone gas is dissolved in pure water is used as the functional water. Ozonated water is generally produced by an ozonated water producing apparatus, but the flow rate of ozonated water to be produced (a required flow rate of ozonated water) varies depending on a use situation at a use point. 
     A nozzle for dissolving ozone gas in pure water is used in a conventional ozonated water producing apparatus (see Japanese Patent Laid-open No. 2010-75838, for example). In the nozzle, the dissolution efficiency of ozone gas varies according to the flow rate of pure water flowing through the nozzle. In addition, the nozzle includes a region where stability of the concentration of ozonated water deteriorates depending on the concentration of ozone water (the concentration of ozone dissolved in ozonated water) and the flow rate of the ozonated water (see  FIG. 6 ). 
     However, the conventional ozonated water producing apparatus has the following problem. First, the nozzle has a flow rate (optimum flow rate) that optimizes an ozone dissolution efficiency (an efficiency at which ozone is dissolved in water). Therefore, when the flow rate of pure water supplied to the nozzle deviates from the optimum flow rate, the ozone dissolution efficiency is lowered, and this causes a problem that a larger amount of ozone gas is needed to generate ozonated water having a desired concentration, that is, the use amount of ozone gas increases. Furthermore, when the flow rate of pure water supplied to the nozzle is excessively lower than the optimum flow rate, stability of the concentration of ozonated water generated in the nozzle deteriorates. 
     The present invention has been made in view of the foregoing problem, and has an object to provide a gas-dissolved liquid producing apparatus capable of increasing a gas dissolution efficiency and also enhancing stability of the concentration of gas-dissolved liquid. 
     SUMMARY OF THE INVENTION 
     A gas-dissolved liquid producing apparatus according to the present invention includes: a gas supply unit that supplies gas serving as a raw material of gas-dissolved liquid; a liquid supply unit that supplies liquid serving as a raw material of the gas-dissolved liquid; and a gas-dissolved liquid generator that generates the gas-dissolved liquid by dissolving the gas supplied from the gas supply unit in the liquid supplied from the liquid supply unit, wherein the gas-dissolved liquid generator includes: a first gas dissolving unit having a first optimum flow rate; a second gas dissolving unit having a second optimum flow rate different from the first optimum flow rate; a flow rate detector that detects a flow rate of the liquid supplied from the liquid supply unit; and a controller that controls which one of the first gas dissolving unit and the second gas dissolving unit should be supplied with the gas supplied from the gas supply unit based on the flow rate of the liquid detected by the flow rate detector. 
     According to this configuration, in the gas dissolving liquid generator, the gas supplied from the gas supply unit is dissolved in the liquid supplied from the liquid supply unit to generate gas-dissolved liquid. The gas-dissolved liquid generator includes the two gas dissolving units (the first gas dissolving unit and the second gas dissolving unit) having different optimum flow rates, and which one of the two gas dissolving units (the first gas dissolving unit and the second gas dissolving unit) should be supplied with the gas supplied from the gas supply unit is controlled based on the flow rate of the liquid supplied from the liquid supply unit. As a result, since gas can be dissolved in an appropriate gas dissolving unit corresponding to the flow rate of the liquid, the gas dissolution efficiency can be increased, and the use amount of gas can be reduced. Furthermore, since gas can be dissolved in an appropriate gas dissolving unit corresponding to the flow rate of the liquid, the stability of the concentration of gas-dissolved liquid generated in the gas-dissolved liquid generator is enhanced. 
     Furthermore, in the gas-dissolved liquid producing apparatus of the present invention, the first gas dissolving unit and the second gas dissolving unit may be connected in series, and the controller may perform control to supply the first gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is closer to the optimum flow rate of the first gas dissolving unit than the optimum flow rate of the second gas dissolving unit, and perform control to supply the second gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is closer to the optimum flow rate of the second gas dissolving unit than the optimum flow rate of the first gas dissolving unit. 
     According to this configuration, the first gas dissolving unit and the second gas dissolving unit are connected in series, and when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the optimum flow rate of the first gas dissolving unit (the first optimum flow rate), gas is supplied to the first gas dissolving unit, and dissolution of the gas is performed in the first gas dissolving unit. On the other hand, when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the optimum flow rate of the second gas dissolving unit (the second optimum flow rate), gas is supplied to the second gas dissolving unit, and dissolution of the gas is performed in the second gas dissolving unit. As described above, dissolution of gas is performed in an appropriate gas dissolving unit corresponding to the flow rate of the liquid. 
     In the gas-dissolved liquid producing apparatus of the present invention, two arrays each including the first gas dissolving unit and the second gas dissolving unit connected in series may be provided in parallel, the first optimal flow rate may be smaller than the second optimum flow rate, and the first gas dissolving unit may be arranged on an upstream side closer to the liquid supply unit than the second gas dissolving unit. 
     According to this configuration, the first gas dissolving unit having a smaller optimum flow rate out of the two gas dissolving units (the first gas dissolving unit and the second gas dissolving unit) connected in series is arranged on the upstream side, and the second gas dissolving unit having a larger optimum flow rate is arranged on the downstream side, so that the pressure loss when the gas-dissolved water is generated in the gas-dissolved water generator can be reduced. 
     In the gas-dissolved liquid producing apparatus of the present invention, the first gas dissolving unit and the second gas dissolving unit may be connected in parallel, and the controller may perform control to supply the first gas dissolving unit with the gas supplied from the gas supply unit and the liquid supplied from the liquid supply unit when the flow rate detected by the flow rate detector is closer to the first optimum flow rate than the second optimum flow rate, and perform control to supply the second gas dissolving unit with the gas supplied from the gas supply unit and the liquid supplied from the liquid supply unit when the flow rate detected by the flow rate detector is closer to the second optimum flow rate than the first optimum flow rate. 
     According to this configuration, the first gas dissolving unit and the second gas dissolving unit are connected in parallel, and when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the optimum flow rate of the first gas dissolving unit (first optimum flow rate), gas and liquid are supplied to the first gas dissolving unit, and dissolution of the gas is performed in the first gas dissolving unit. On the other hand, when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the optimum flow rate of the second gas dissolving unit (the second optimum flow rate), gas and liquid are supplied to the second gas dissolving unit, and dissolution of the gas is performed in the second gas dissolving unit. In this way, it is possible to dissolve gas in an appropriate gas dissolving unit corresponding to the flow rate of liquid. 
     Furthermore, in the gas-dissolved liquid producing apparatus of the present invention, the gas-dissolved liquid generator may include a third gas dissolving unit that is connected to the first gas dissolving unit and the second gas dissolving unit in parallel, and has a third optimum flow rate different from both of the first optimum flow rate and the second optimum flow rate, and the controller may perform control to supply the first gas dissolving unit and the second gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is closer to a total flow rate of the first optimum flow rate and the second optimum flow rate than the third optimum flow rate, and perform control to supply the third gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is closer to the third optimum flow rate than the total flow rate of the first optimum flow rate and the second optimum flow rate. 
     According to this configuration, the three gas dissolving units (the first gas dissolving unit, the second gas dissolving unit and the third gas dissolving unit) are connected in parallel, and when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the total flow rate of the optimum flow rate of the first gas dissolving unit and the optimum flow rate of the second gas dissolving unit (the first optimum flow rate+the second optimum flow rate), gas is supplied to the first gas dissolving unit and the second gas dissolving unit, and dissolution of the gas is performed in the first gas dissolving unit and the second gas dissolving unit. On the other hand, when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the optimum flow rate of the third gas dissolving unit (the third optimum flow rate), gas is supplied to the third gas dissolving unit, and dissolution of the gas is performed in the third gas dissolving unit. In this way, the dissolution of gas can be performed in an appropriate gas dissolving unit(s) corresponding to the flow rate of the liquid. 
     In the gas-dissolved liquid producing apparatus of the present invention, the controller may perform control to supply the first gas dissolving unit and the second gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is close to an intermediate value between the total flow rate of the first optimum flow rate and the second optimum flow rate and the third optimum flow rate. 
     According to this configuration, the three gas dissolving units (the first gas dissolving unit, the second gas dissolving unit, and the third gas dissolving unit) are connected in parallel, and when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the intermediate value between the total flow rate of the optimum flow rate of the first gas dissolving unit and the optimum flow rate of the second gas dissolving unit (the first optimum flow rate+the second optimum flow rate) and the optimum flow rate of the third gas dissolving unit (the third optimum flow rate), gas is supplied to the first gas dissolving unit and the second gas dissolving unit, and dissolution of the gas is performed in the first gas dissolving unit and the second gas dissolving unit. In this way, since the gas-dissolved water can be generated in the gas dissolving units having small optimum flow rates (the first gas dissolving unit and the second gas dissolving unit), the gas dissolution efficiency when gas-dissolved water is generated can be increased. 
     In the gas-dissolved liquid producing apparatus of het present invention, the controller may perform control to supply the third gas dissolving unit with the gas supplied from the gas supply unit when the flow rate detected by the flow rate detector is close to the intermediate value between the total flow rate of the first optimum flow rate and the second optimum flow rate and the third optimum flow rate. 
     According to this configuration, the three gas dissolving units (the first gas dissolving unit, the second gas dissolving unit, and the third gas dissolving unit) are connected in parallel, and when the flow rate of liquid supplied to the gas-dissolved liquid generator is close to the intermediate value between the total flow rate of the optimum flow rate of the first gas dissolving unit and the optimum flow rate of the second gas dissolving unit (the first optimum flow rate+the second optimum flow rate) and the optimum flow rate of the third gas dissolving unit (the third optimum flow rate), gas is supplied to the third gas dissolving unit, and dissolution of the gas is performed in the third gas dissolving unit. In this way, the gas-dissolved water can be generated in the gas dissolving unit having the large optimum flow rate (the third gas dissolving unit), so that the pressure loss when the gas-dissolved water is generated can be reduced. 
     According to the present invention, the gas dissolution efficiency can be increased, and stability of the concentration of gas-dissolved liquid can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an ozonated water producing apparatus according to a first embodiment of the present invention; 
         FIG. 2  is an explanatory diagram showing an ozonated water generator in the first embodiment of the present invention; 
         FIG. 3  is an explanatory diagram showing a modification of the ozonated water generator in the first embodiment of the present invention; 
         FIG. 4  is an explanatory diagram of another modification of the ozonated water generator in the first embodiment of the present invention; 
         FIG. 5  is an explanatory diagram of an ozonated water generator in a second embodiment of the present invention; 
         FIG. 6  is a diagram showing concentration stability in the ozonated water generator in the embodiment of the present invention; 
         FIG. 7  is a view showing nozzles to be used in the second embodiment of the present invention; and 
         FIG. 8  is an explanatory diagram showing a modification of the ozonated water generator in the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A gas-dissolved liquid producing apparatus according to an embodiment of the present invention will be described hereinafter with reference to the drawings. In the present embodiment, an ozonated water producing apparatus for producing ozonated water by dissolving ozone gas in pure water will be described as an example. 
     First Embodiment 
     A configuration of an ozonated water producing apparatus according to a first embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a block diagram showing a schematic configuration of the ozonated water producing apparatus of the present embodiment. As shown in  FIG. 1 , the ozonated water producing apparatus  1  includes an ozone gas supply unit  2  for supplying ozone gas as a raw material of ozonated water, a pure water supply unit  3  for supplying pure water as a raw material of ozonated water, and an ozonated water generator  4  for generating ozonated water by dissolving ozone gas in supplied pure water. Publicly known techniques may be used for supplying ozone gas and pure water as the raw materials. 
     A flowmeter  5  and a booster pump  6  are provided between the pure water supply unit  3  and the ozonated water generator  4 . The flowmeter  5  has a function of measuring the flow rate of pure water supplied from the pure water supply unit  3  (pure water supplied to the ozonated water generator  4 ), and outputting data representing the measured flow rate (flow rate data) to the ozonated water generator  4 . The booster pump  6  has a function of adjusting the flow rate of pure water to be supplied from the pure water supply unit  3  to the ozonated water generator  4 . 
     The ozonated water generated in the ozonated water generator  4  is stored in a gas-liquid separation tank  7 . In the gas-liquid separation tank  7 , the ozonated water generated in the ozonated water generator  4  is separated into ozonated water to be supplied to a use point and surplus gas to be exhausted from an exhaust port or the like. Supply processing of supplying the ozonated water to the use point is performed by an ozonated water supply processing unit  8 . In addition, exhaust processing of exhausting the surplus gas is performed by an exhaust processing unit  9 . Publicly known techniques may be used for the supply processing of the ozonated water and the exhausting processing of the surplus gas. 
       FIG. 2  is an explanatory diagram of the ozonated water generator  4  of the present embodiment. As shown in  FIG. 2 , the ozonated water generator  4  includes two nozzles (a first nozzle  10  and a second nozzle  11 ) connected in series. The nozzle has a function of dissolving gas in liquid supplied thereto. The optimum flow rate of the first nozzle  10  is equal to, for example, 5 L, and the optimum flow rate of the second nozzle  11  is equal to, for example, 10 L. The first nozzle  10  is arranged on an upstream side of the second nozzle  11  (a side closer to the pure water supply unit  3 ). That is, the pure water supplied to the ozonated water generator  4  is supplied to the first nozzle  10 , and then supplied to the second nozzle  11 . An output valve  12  is provided on the downstream side of the second nozzle  11 . 
     As shown in  FIG. 2 , the ozonated water generator  4  is provided with two gas valves (a first gas valve  13  and a second gas valve  14 ) corresponding to the two nozzles. The ozonated water generator  4  is configured to be capable of supplying ozone gas to any one of the first nozzle  10  and the second nozzle  11  by opening or closing the first gas valve  13  and the second gas valve  14 . In the present embodiment, both the first gas valve  13  and the second gas valve  14  are inhibited to be opened at the same time. That is, both the first nozzle  10  and the second nozzle  11  are not supplied with gas at the same time. 
     Furthermore, as shown in  FIG. 2 , the ozonated water generator  4  includes a flow rate detector  15  for detecting the flow rate of pure water supplied to the ozonated water generator  4  based on the flow rate data output from the flowmeter  5 , and a controller  16  for controlling opening and closing of the two gas valves (the first gas valve  13  and the second gas valve  14 ) based on the flow rate of pure water detected by the flow rate detector  15 . By controlling the opening and closing of the first gas valve  13  and the second gas valve  14 , the controller  16  can perform control as to which one of the first nozzle  10  and the second nozzle  11  is supplied with ozone gas supplied from the ozone gas supply unit  2 . 
     For example, when the flow rate detected by the flow rate detector  15  is closer to the optimum flow rate (5 L) of the first nozzle  10  than the optimum flow rate (10 L) of the second nozzle  11  (for example, when the detected flow rate is equal to 6 L), the controller  16  performs control to supply the first nozzle  10  with the ozone gas supplied from the ozone gas supply unit  2 . On the other hand, when the flow rate detected by the flow rate detector  15  is closer to the optimum flow rate (10 L) of the second nozzle  11  than the optimum flow rate (5 L) of the first nozzle  10  (for example, when the detected flow rate is equal to 9 L), the controller  16  performs control to supply the second nozzle  11  with the ozone gas supplied from the ozone gas supply unit  2 . 
     According to the ozonated water producing apparatus  1  according to the first embodiment as described above, the ozonated water generator  4  has the two nozzles (the first nozzle  10  and the second nozzle  11 ) having different optimum flow rates, and it is controlled based on the flow rate of pure water supplied from the pure water supply unit  3  which one of the two nozzles (the first nozzle  10  and the second nozzle  11 ) should be supplied with the ozone gas supplied from the gas supply unit. As a result, ozone gas can be dissolved in an appropriate nozzle(s) corresponding to the flow rate of pure water, so that the gas dissolution efficiency can be increased and the use amount of ozone gas to obtain a predetermined ozonated water concentration can be reduced. Furthermore, since ozone gas can be dissolved in an appropriate nozzle(s) corresponding to the flow rate of pure water, the stability of the concentration of ozonated water generated in the ozonated water generator  4  is enhanced. 
     Furthermore, in the present embodiment, the first nozzle  10  and the second nozzle  11  are connected in series, and when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the first nozzle  10  (the first optimum flow rate), ozone gas is supplied to the first nozzle  10 , and dissolution of ozone gas is performed in the first nozzle  10 . On the other hand, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the second nozzle  11  (second optimum flow rate), ozone gas is supplied to the second nozzle  11 , and dissolution of ozone gas is performed in the second nozzle  11 . In this way, dissolution of ozone gas can be performed in an appropriate nozzle(s) corresponding to the flow rate of pure water. 
     Modification of First Embodiment 
       FIG. 3  shows a modification of the ozonated water generator  4  according to the first embodiment. As shown in  FIG. 3 , in this modification, two arrays each including two nozzles (a first nozzle  10  and a second nozzle  11 ) connected in series are provided in parallel. That is, the ozonated water generator  4  includes two nozzles (the first nozzle  10  and the second nozzle  11 ) in a first column and two nozzles (the first nozzle  10  and the second nozzle  11 ) in a second column. In addition, the controller  16  can perform control to supply pure water supplied from the pure water supply unit  3  to any one or both of the nozzle array on the first column and the nozzle array on the second column by switching a switching valve (not shown) provided on the upstream side of the nozzle arrays on the first column and the second column. 
     In this case, when the flow rate detected by the flow rate detector  15  is larger than the optimum flow rate of the second nozzle  11  (10 L) and also close to the total flow rate of the optimum flow rate of the first nozzle  10  and the optimum flow rate of the second nozzle  11  (15 L=5 L+10 L) (for example, when the detected flow rate is equal to 14 L), the controller  16  performs control to supply the first nozzle  10  on the first column and the second nozzle  11  on the second column with the ozone gas supplied from the ozone gas supply unit  2  and the pure water supplied from the pure water supply unit  3 . 
     Furthermore, when the flow rate detected by the flow rate detector  15  is closer to the total flow rate of the optimum flow rates of the two second nozzles  11  (20 L=10 L+10 L) than the total flow rate of the optimum flow rate of the first nozzle  10  and the optimum flow rate of the second nozzle  11  (15 L=5 L+10 L) (for example, when the detected flow rate is equal to 19 L), the controller  16  performs control to supply the second nozzle  11  on the first column and the second nozzle  11  on the second column with the ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . 
     Accordingly, it is possible to cope with a flow rate larger than the optimum flow rate of the second nozzle  11  (10 L), and dissolve ozone gas in an appropriate nozzle(s) corresponding to the flow rate of pure water. 
     Furthermore, in the present embodiment, the first nozzle  10  having a smaller optimum flow rate out of the two nozzles (the first nozzle  10  and the second nozzle  11 ) connected in series is arranged on the upstream side, and the second nozzle  11  having a larger optimum flow rate is arranged on the downstream side, so that it is possible to reduce the pressure loss when the gas-dissolved water is generated in the gas-dissolved water generator. 
     When the two arrays each including the two nozzles (the first nozzle  10  and the second nozzle  11 ) connected in series are provided in parallel, only one (for example, the first nozzle  10 ) of the two nozzles (the first nozzle  10  and the second nozzle  11 ) may be used as shown in  FIG. 4 . 
     Second Embodiment 
     Next, an ozonated water producing apparatus  1  according to a second embodiment of the present invention will be described. The ozonated water producing apparatus  1  of the second embodiment will be described hereinafter while focusing on differences from the first embodiment. Unless otherwise described hereinafter, the configuration and operation of the present embodiment are the same as the first embodiment. 
       FIG. 5  is an explanatory diagram of an ozonated water generator  4  of the present embodiment. As shown in  FIG. 5 , the ozonated water generator  4  includes three nozzles (a first nozzle  10 , a second nozzle  11 , and a third nozzle  17 ) connected in parallel. The optimum flow rate of the first nozzle  10  is equal to, for example, 5 L, the optimum flow rate of the second nozzle  11  is equal to, for example, 10 L, and the optimum flow rate of the third nozzle  17  is equal to, for example, 20 L. Furthermore, an output valve  12  is provided downstream of each of the three nozzles (the first nozzle  10 , the second nozzle  11 , and the third nozzle  17 ). 
     As shown in  FIG. 5 , the ozonated water generator  4  is provided with three gas valves (a first gas valve  13 , a second gas valve  14 , and a third gas valve  18 ) corresponding to the three nozzles. The ozonated water generator  4  is configured to be capable of supplying ozone gas to each of the first nozzle  10 , the second nozzle  13  and the third nozzle  17  independently of one another by opening or closing the first gas valve  13 , the second gas valve  14 , and the third gas valve  18 . 
     In the present embodiment, any two of the first gas valve  13 , the second gas valve  14 , and the third gas valve  18  can be opened at the same time, and all of the three valves can be opened at the same time. That is, ozone gas can be supplied to any two of the first nozzle  10 , the second nozzle  11 , and the third nozzle  17  at the same time, and ozone gas can be supplied to all of the three valves at the same time. It is needless to say that ozone gas can be supplied to any one of the first nozzle  10 , the second nozzle  11 , and the third nozzle  17  by opening the corresponding one of the first gas valve  13 , the second gas valve  14  and the third gas valve  18 . 
     By controlling the opening and closing of the three gas valves (the first gas valve  13 , the second gas valve  14 , and the third gas valve  18 ) based on the flow rate of pure water detected by the flow rate detector  15 , the controller  16  controls which nozzle(s) of the first nozzle  10 , the second nozzle  11 , and the third nozzle  17  should be supplied with ozone gas supplied from the ozone gas supply unit  2 . In addition, by switching a switching valve (not shown) provided on the upstream side of the three nozzles (the first nozzle  10 , the second nozzle  11 , and the third nozzle  12 ), the controller  16  can control which nozzle(s) of the first nozzle  10 , the second nozzle  11 , and the third nozzle  12  should be supplied with pure water supplied from the pure water supply unit  3 . 
     For example, when the flow rate detected by the flow rate detector  15  is closer to the first optimum flow rate (5 L) than the second optimum flow rate (10 L) (for example, when the detected flow rate is equal to 6 L), the controller  16  performs control to supply the first nozzle  10  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . Furthermore, when the flow rate detected by the flow rate detector  15  is closer to the second optimum flow rate (10 L) than the first optimum flow rate (5 L) (for example, when the detected flow rate is equal to 9 L), the controller  16  performs control to supply the second nozzle  11  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . Furthermore, when the flow rate detected by the flow rate detector  15  is closer to the third optimum flow rate (20 L) than the second optimum flow rate (10 L) (for example, when the detected flow rate is equal to 19 L), the controller  16  performs control to supply the third nozzle  17  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . 
     When the flow rate detected by the flow rate detector  15  is closer to the total flow rate of the first optimum flow rate and the second optimum flow rate (15 L=5 L+10 L) than the third optimum flow rate (20 L) (for example, when the detected flow rate is equal to 16 L), the controller  16  performs control to supply both the first nozzle  10  and the second nozzle  11  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . Furthermore, when the flow rate detected by the flow rate detector  15  is closer to the third optimum flow rate (20 L) than the total flow rate of the first optimum flow rate and the second optimum flow rate (15 L=5 L+10 L) (for example, when the detected flow rate is equal to 19 L), the controller  16  performs control to supply the third nozzle  17  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . 
     Furthermore, when the flow rate detected by the flow rate detector  15  is close to the intermediate value (17.5 L) between the total flow rate of the first optimum flow rate and the second optimum flow rate and the third optimum flow rate, the controller  16  performs control to supply the first nozzle  10  and the second nozzle  11  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . Alternatively, when the flow rate detected by the flow rate detector  15  is close to the intermediate value (17.5 L) between the total flow rate of the first optimum flow rate and the second optimum flow rate and the third optimum flow rate, the controller  16  may perform control to supply the third nozzle  17  with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . 
     The ozonated water producing apparatus  1  according to the second embodiment also achieves the same operation and effect as the first embodiment. That is, the ozonated water generator  4  has the three nozzles (the first nozzle  10 , the second nozzle  11 , and the third nozzle  17 ) having the different optimum flow rates, and based on the flow rate of pure water supplied from the pure water supply unit  3 , it is controlled which nozzle(s) of the three nozzles (the first nozzle  10 , the second nozzle  11 , and the third nozzle  17 ) is supplied with ozone gas supplied from the ozone gas supply unit  2  and pure water supplied from the pure water supply unit  3 . As a result, ozone gas can be dissolved in an appropriate nozzle(s) corresponding to the flow rate of pure water, so that the gas dissolution efficiency can be increased and the use amount of ozone gas for obtaining a predetermined ozone water concentration can be reduced. Furthermore, since ozone gas can be dissolved in an appropriate nozzle(s) corresponding to the flow rate of pure water, the stability of the concentration of ozonated water generated in the ozonated water generator  4  is enhanced. 
     In addition, in the present embodiment, the first nozzle  10 , the second nozzle  11 , and the third nozzle  17  are connected in parallel, and when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the first nozzle  10  (first optimum flow rate), ozone gas is supplied to the first nozzle  10 , and dissolution of the ozone gas is performed in the first nozzle  10 . Furthermore, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the second nozzle  11  (second optimum flow rate), ozone gas is supplied to the second nozzle  11 , and dissolution of the ozone gas is performed in the second nozzle  11 . Still furthermore, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the third nozzle  17  (third optimum flow rate), ozone gas is supplied to the third nozzle  17 , and dissolution of the ozone gas is performed in the third nozzle  17 . In this way, the dissolution of ozone gas is performed in an appropriate nozzle(s) corresponding to the flow rate of pure water. 
     In this case, it is possible to select an appropriate nozzle(s) corresponding to the flow rate of pure water supplied to the ozonated water generator  4  from the three nozzles connected in parallel to perform the dissolution of ozone gas, so that when the flow rate of pure water is small, a nozzle having a small optimum flow rate corresponding to the flow rate of pure water can be used. Accordingly, it is possible to avoid use of a nozzle having a large optimum flow rate, so that a region providing excellent concentration stability over the whole system becomes larger as shown in  FIG. 6 . An upper diagram of  FIG. 6  shows the concentration stability of a system in which only a nozzle having a large optimum flow rate is used, and a lower diagram of  FIG. 6  shows the concentration stability of the whole system according to the present invention. 
     Furthermore, which nozzle(s) of the three nozzles connected in parallel should be used may be determined based on a table shown in  FIG. 7 . A first line (uppermost line) of the table of  FIG. 7  shows the optimum flow rates of the nozzles, a first column (leftmost column) of the table of  FIG. 7  shows the values of ratios by which the optimum flow rates of the nozzles are multiplied. Numerical values (flow rates) as a result obtained by multiplying the optimum flow rates by the values of the ratios are shown in respective cells from the second line and second column to the fourth line and fourth column of the table of  FIG. 7 . 
     In the table of  FIG. 7 , the optimum flow rates of the nozzles are set so as to constitute a geometric progression ( 5 ,  10 ,  20 , etc.), and the values of the ratios are set so as to constitute an arithmetic progression ( 0 . 8 ,  1 . 0 ,  1 . 2 ,  1 . 4 , etc.). 
     For example, the table of  FIG. 7  shows that the nozzle having the optimum flow rate of 5 L (the first nozzle  10 ) is used when the flow rate of pure water supplied to the ozonated water generator  4  is equal to “4 L”. Likewise, the table of  FIG. 7  shows that the nozzle having the optimum flow rate of 20 L (the third nozzle  17 ) is used when the flow rate of pure water supplied to the ozonated water generator  4  is equal to “28 L”. By determining the nozzle to be used corresponding to the flow rate based on such a table, a broad flow rate range (the range from 4 L to 28 L) can be covered in a well-balanced manner. 
     In the present embodiment, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the total flow rate of the optimum flow rate of the first nozzle  10  and the optimum flow rate of the second nozzle  11  (the first optimum flow rate+the second optimum flow rate), ozone gas is supplied to the first nozzle  10  and the second nozzle  11 , and dissolution of the ozone gas is performed in the first nozzle  10  and the second nozzle  11 . On the other hand, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the optimum flow rate of the third nozzle  17  (the third optimum flow rate), ozone gas is supplied to the third nozzle  17 , and dissolution of the ozone gas is performed in the third nozzle  17 . In this way, the dissolution of ozone gas can be performed in an appropriate nozzle(s) corresponding to the flow rate of pure water. 
     Furthermore, in the present embodiment, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the intermediate value between the total flow rate of the optimum flow rate of the first nozzle  10  and the optimum flow rate of the second nozzle  11  (the first optimum flow rate+the second optimum flow rate) and the optimum flow rate of the third nozzle  17  (the third optimum flow rate), ozone gas is supplied to the first nozzle  10  and the second nozzle  11 , and dissolution of the ozone gas is performed in the first nozzle  10  and the second nozzle  11 . In this way, since gas-dissolved water can be generated with the nozzles having small optimum flow rates (the first nozzle  10  and the second nozzle  11 ), the gas dissolution efficiency when the gas-dissolved water is generated can be increased. 
     Alternatively, when the flow rate of pure water supplied to the ozonated water generator  4  is close to the intermediate value between the total flow rate of the optimum flow rate of the first nozzle  10  and the optimum flow rate of the second nozzle  11  (the first optimum flow rate+the second optimum flow rate) and the optimum flow rate of the third nozzle  17  (the third optimum flow rate), ozone gas is supplied to the third nozzle  17 , and dissolution of the ozone gas is performed in the third nozzle  17 . In this way, since gas-dissolved water can be generated with the nozzle having a large optimum flow rate (the third nozzle  17 ), the pressure loss when gas-dissolved water is generated can be reduced. 
     Modification of Second Embodiment 
       FIG. 8  shows a modification of the ozonated water generator  4  of the second embodiment. In the present modification, three nozzles (a fourth nozzle  19 , a fifth nozzle  20 , and a sixth nozzle  21 ) connected in series are provided on the rear stage of three nozzles (a first nozzle  10 , a second nozzle  11 , and a third nozzle  17 ) connected in parallel as shown in  FIG. 8 . The optimum flow rate of the first nozzle  10  is equal to, for example, 5 L, the optimum flow rate of the second nozzle  11  is equal to, for example, 10 L, and the optimum flow rate of the third nozzle  17  is equal to, for example, 20 L. Furthermore, the optimum flow rate of the fourth nozzle  19  is equal to, for example, 10 L, the optimum flow rate of the fifth nozzle  20  is equal to, for example, 15 L, and the optimum flow rate of the sixth nozzle  21  is equal to, for example, 30 L. 
     As shown in  FIG. 8 , the ozonated water generator  4  includes six gas valves (a first gas valve  13 , a second gas valve  14 , a third gas valve  18 , a fourth gas valve  22 , a fifth gas valve  23 , and a sixth gas valve  24 ) corresponding to the six nozzles. The ozonated water generator  4  is configured to be capable of supplying ozone gas to the six nozzles (the first nozzle  10  to the sixth nozzle  21 ) independently of one another by opening or closing the six gas valves (the first gas valve  13  to the sixth gas valve  24 ). 
     Therefore, it is possible to select an appropriate nozzle(s) corresponding to the flow rate of pure water supplied to the ozonated water generator  4  from the six nozzles (the first nozzle  10  to the sixth nozzle  21 ) to perform dissolution of ozone gas, so that a broader flow rate range can be covered in a well-balanced manner. 
     The embodiments of the present invention have been described by way of examples. However, the scope of the present invention is not limited to these embodiments, and the embodiment can be altered and modified according to the purpose within the scope described in the claims. 
     For example, the foregoing description has been made by exemplifying the ozonated water producing apparatus for producing ozonated water by dissolving ozone gas in pure water, but the scope of the present invention is not limited to this ozonated water producing apparatus. That is, the gas as a raw material is not limited to ozone gas, and the liquid as a raw material is not limited to pure water. For example, carbonated water may be produced by dissolving carbon dioxide in pure water or nitrogen water may be produced by dissolving nitrogen in pure water. Furthermore, hydrogen water may be produced by dissolving hydrogen in pure water. The present invention can be also applied to dissolution of gas for producing functional water. 
     As described above, the gas-dissolved liquid producing apparatus according to the present invention has an effect of increasing the gas dissolution efficiency and enhancing the stability of the concentration of gas-dissolved liquid, and is useful, for example, as an ozonated water producing apparatus for producing ozonated water by dissolving ozone gas in pure water, etc. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           1  Ozonated water producing apparatus (gas-dissolved liquid producing apparatus) 
           2  Ozone gas supply unit (gas supply unit) 
           3  Pure water supply unit (liquid supply unit) 
           4  Ozonated water generator (gas-dissolved liquid generator) 
           5  Flowmeter 
           6  Booster pump 
           7  Gas-liquid separation tank 
           8  Ozonated water supply processing unit 
           9  Exhaust processing unit 
           10  First nozzle (first gas dissolving unit) 
           11  Second nozzle (second gas dissolving unit) 
           12  Output valve 
           13  First gas valve 
           14  Second gas valve 
           15  Flow rate detector 
           16  Controller 
           17  Third nozzle (third gas dissolving unit) 
           18  Third gas valve 
           19  Fourth nozzle 
           20  Fifth nozzle 
           21  Sixth nozzle 
           22  Fourth gas valve 
           23  Fifth gas valve 
           24  Sixth gas valve 
         U Use point