Pumps – Condition responsive control of pump drive motor – By control of electric or magnetic drive motor
Reexamination Certificate
2001-03-29
2003-05-13
Tyler, Cheryl J. (Department: 3746)
Pumps
Condition responsive control of pump drive motor
By control of electric or magnetic drive motor
C417S243000, C417S252000, C417S253000, C417S310000
Reexamination Certificate
active
06561766
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a variable rotational speed oil free screw compressor and a control method therefor and, more particularly, to a variable rotational speed oil free screw compressor having a low-pressure stage compressor body and a high-pressure stage compressor body and a control method therefor.
In a conventional variable rotational speed oil free screw compressor, for example, as disclosed in Japanese Patent Laid-Open No. 82391/1998, a low-pressure stage screw compressor body and a high-pressure stage screw compressor body are connected to each other in series, and a cooler is provided between the two compressing sections. A motor is connected to each of the low-pressure stage screw compressor body and the high-pressure stage screw compressor body, and the motor is driven at a variable speed by an inverter. In the variable rotational speed oil free screw compressor constructed above, with a small flow rate, both of the low-pressure stage screw compressor body and the high-pressure stage screw compressor body rotate at low speed, so that an amount of internal leakage cannot be ignored. Therefore, a blow-off valve is connected to an outlet pipe, by which blow-off control is carried out while the low-pressure stage screw compressor body and the high-pressure stage screw compressor body are operated at the lowest rotational speed.
With an oil free screw compressor having two stages of low-pressure and high-pressure stages, power consumption with no load is smaller than that with full load as compared with a single-stage oil free screw compressor. Therefore, even if the method disclosed in the above-described Publication is applied at the time of no-load operation, there is a disadvantage that power consumption is not decreased so much as compared with a conventional method, in which a suction throttle valve is throttled.
BRIEF SUMMARY OF THE INVENTION
The present invention has been contrived in view of the above problems of the prior art, and has its object to provide a variable rotational speed oil free screw compressor having a low-pressure stage compressor body and a high-pressure stage compressor body, in which power consumption is reduced both at no load and at low load.
A first feature of the present invention for attaining the above object is a variable rotational speed oil free screw compressor, comprising a low-pressure stage compressor body and a high-pressure stage compressor body, which are variable in rotational speed, and blow-off means for blowing off compressed air to the atmosphere midway a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body. In this feature, it is preferable that an inter-cooler and blow-off means is provided midway the pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body, and an after-cooler is provided on a discharge side of the high-pressure stage compressor body. Also, another blow-off valve may be provided between the high-pressure stage compressor body and a check valve to blow off compressed air discharged from the high-pressure stage compressor body, and wherein compressed air is blown off through the blow-off valve and another blow-off valve at the time of no-load or low-load operation.
Also, a pressure detector may be provided on the discharge side of the high-pressure stage compressor body for detecting pressure of high-pressure air discharged from the high-pressure stage compressor body, and a controller may be provided to receive a signal of discharge pressure detected by the pressure detector and output a control signal for controlling the blow-off means. The variable rotational speed oil free screw compressor may further comprise an electric motor for rotatingly driving the low-pressure stage compressor body and the high-pressure stage compressor body, and an inverter for driving the electric motor, and wherein the controller controls the inverter based on a signal of discharge pressure detected by the pressure detector.
Preferably, a suction throttle valve is provided on a suction side of the low-pressure stage compressor body, and another blow-off means is provided on a discharge side of the high-pressure stage compressor body, another blow-off means interconnecting with the suction throttle valve.
A second feature of the present invention for attaining the above object is a method of controlling a variable rotational speed oil free screw compressor adapted to operate in accordance with a volume of consumed air on a usage side while changing rotational speeds of a low-pressure stage compressor body and a high-pressure stage compressor body, the method comprising the steps of: performing a load operation to change rotational speeds of the low-pressure stage compressor body and the high-pressure stage compressor body in a region, in which a volume of consumed air based on pressure detected by a pressure detector provided on a discharge side ranges from a maximum air volume to a preset volume of air; operating the low-pressure stage compressor body and the high-pressure stage compressor body at set lower limit rotational speeds preset every compressor body in a no-load operation, in which a volume of consumed air is substantially zero, and blowing off compressed air from blow-off means provided in a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body; and repeating the load operation and the no-load operation when a volume of consumed air is equal to or smaller than a set air volume.
Preferably, during the load operation, the rotational speeds of the low-pressure stage compressor body and the high-pressure stage compressor body are changed substantially in proportion to a volume of consumed air. Also preferably, during no-load operation, compressed air discharged from the high-pressure stage compressor body is blown off.
A third feature of the present invention for attaining the above object is a method of controlling a variable rotational speed oil free screw compressor adapted to operate in accordance with a volume of consumed air on a usage side while changing rotational speeds of a low-pressure stage compressor body and a high-pressure stage compressor body, the method comprising the steps of: performing operations including a no-load operation to blow off compressed air from blow-off means provided in a pipe connecting between the high-pressure stage compressor body and the low-pressure stage compressor body when a volume of consumed air based on pressure detected by a pressure detector provided on a discharge side is not exceeding a preset volume of air and pressure of compressed air discharged from the low-pressure stage compressor body is at least the atmospheric pressure.
A suction throttle valve provided on a suction side of the low-pressure stage compressor body may be made to interlock with blow-off means for compressed air compressed by the high-pressure stage compressor body to make control to throttle the suction throttle valve when a volume of consumed air is not exceeding a set air volume.
REFERENCES:
patent: 1422376 (1922-07-01), Richards
patent: 1612639 (1926-12-01), Metzgar
patent: 3216648 (1965-11-01), Ford
patent: 3367562 (1968-02-01), Persson et al.
patent: 4815950 (1989-03-01), Aoki et al.
patent: 5195874 (1993-03-01), Odagiri
Nishimura Hitoshi
Ohta Hiroshi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Tyler Cheryl J.
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