| 摘要 |
An arithmetic mean value (PT1 + PT2)/2 of a detected value of a first pressure sensor (5) on a positive pressure side and a detected value of a second pressure sensor (6) on a negative pressure side is found so that when the arithmetic mean value is greater than a set mean value, a first switching valve (7) is opened only for an optical set time and when the arithmetic mean value is less than the set mean value, a second switching valve (8) is opened only for another optional set time. A detected differential pressure (PT1 - PT2) between a detected value of the first pressure sensor (5) and a detected value of the second switching valve (8) is found so that when the detected differential pressure is greater than a set differential pressure, a pump (4) is controlled to reduce its output and when the detected differential pressure is less than the set differential pressure, the pump (4) is controlled to increase its output. It is possible to enhance energy efficiency while keeping positive and negative pressures generated at inlet and outlet ends of a single pump in a desired range.
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