Pumps – Condition responsive control of drive transmission or pump... – Plural pumps with individual or relative control
Reexamination Certificate
2000-03-16
2001-09-11
Walberg, Teresa (Department: 3742)
Pumps
Condition responsive control of drive transmission or pump...
Plural pumps with individual or relative control
C417S002000, C417S007000
Reexamination Certificate
active
06287083
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a compressed air production facility for compressing gas such as air, and in particular, to a compressed air production facility in which a variable speed compressor adapted to change its rotational speed so as to adjust its capacity, and a constant speed compressor are operated in parallel.
For example, a compressed air production facility which is capacitively controlled, is disclosed in Japanese Laid-Open Patent No. H9-250485. The compressor disclosed in this document is provided in its compressed air outlet port with a pressure sensor which detects a discharge pressure. With this arrangement, PID control is carried out for the rotational speed of the compressor with the use of the thus detected discharge pressure in order to capacitively control the compressor.
Conventionally, in such an arrangement that a plurality of compressors which are connected with one another in parallel, are operated, load pressures (start pressures) and unload pressures (rest pressures) of the compressors are set to be different from one another bit by bit. In this case, a change in power consumption (%) with respect to an air consumption (%) by a compressor is such that the power consumption is linearly decreased, as the air consumption is decreased, in either a suction throttling system (S
1
) or a combination system (S
2
) in which a system for repeating load operation and unload operation by reducing the discharge pressure is combined with a suction throttling system. However, about 30% (in the case of the system S
1
) or 70% (in the case of the system S
2
) of the rated power is consumed even though the air consumption becomes zero.
If a plurality, for example, two, of compressors having the above-mentioned characteristic are automatically operated in parallel and stopped, characteristics S
1
, S
2
shown in
FIG. 2
, are exhibited. As clearly understood from this figure, the operation of two compressors having volumes which are substantially equal to each other is advantageous in view of power saving in comparison with operation of a single compressor having a discharge air volume which is twice as large as that of the former two compressors.
On the contrary, in a system (R
1
) using the above-mentioned variable speed compressor disclosed in the above-mentioned Japanese Laid-Open Patent No. H9-250485, a power consumption (%) with respect to an air consumption (%) has a characteristic which is substantially exhibited by a direct proportional curve, but is deviated slightly therefrom if the air consumption is excessively low. Further, if the air consumption becomes zero, the power consumption is lowered to about 10% of its rated power. Accordingly, in the case of operation of two compressors each having this characteristic in parallel, a characteristic R
1
shown in
FIG. 2
can be obtained, which is excellent in power saving in comparison with those of the systems S
1
, S
2
. Although the variable speed compressor has this advantageous characteristic, a variable speed mechanism such as an inverter is required so as to cause such a disadvantage that the costs of the facility becomes higher.
BRIEF SUMMARY OF THE INVENTION
The present invention is devised in view of the disadvantages inherent to the above-mentioned prior art, and accordingly, an object of the present invention is to reduce the power consumption during parallel operation of a plurality of compressors.
To the end, according to the first aspect of the present invention, there is provided a compressed air production facility comprising at least one constant speed compressor which repeats load operation and unload operation under on-off control, a variable speed compressor driven by a motor incorporating an inverter, and a common pipe line or an air reservoir into which discharge air is led from both constant speed compressor and variable speed compressor, detecting means provided in the common pipe line or the air reservoir, for detecting a discharge pressure of the discharge air, and control means for controlling the discharge pressure to a set pressure with the use of the variable speed compressor, wherein the control means controls the discharge pressure of the variable speed compressor during operation to a set pressure between a discharge pressure of the constant speed compressor during load operation and a discharge pressure of the same during unload operation.
With this first aspect of the present invention, when the consumption of the compressed air is deceased, the variable speed compressor is driven through unload operation at a lower limit speed, and then, if this unload operation continues for a predetermined time, it is desired to stop the operation of the variable speed compressor. Further, if the consumption of the compressed air is decreased, it is desired that the unload operation and load operation are repeated for the variable speed compressor at the lower limit speed, and further, the discharge pressure of the variable speed compressor during load operation and the discharge pressure of the same during unload operation are both controlled to fall in a range between the discharge pressure of the constant speed compressor during load operation and the discharge pressure of the same during unload operation. Further a plurality of constant speed compressors are connected to one another in parallel.
Further, it is desirable that both set pressure at which the variable speed compressor is changed over from the load operation into the unload operation, and set pressure at which the variable speed compressor is changed over from the unload operation into the load operation are controlled so as to allow both set pressures to fall between a set pressure at which all constant speed compressors are changed over from the load operation into the unload operation and a set pressure at which all constant speed compressors are changed over from the unload operation into the load operation. More preferably, set pressures at which the constant speed compressors are changed over from the load operation into the unload operations and set pressures with which the same are changed over from the unload operation into the load operation are made to be different from one another, respectively.
Moreover, there may be provided input means for inputting a set pressure for the variable speed compressor, and setting means for setting the discharge pressure or start pressure of the variable speed compressor during the load operation to a value which is lower than the set pressure, and setting the discharge pressure or the rest pressure during the unload operation to a value which is higher than the set pressure.
Further, the constant speed compressor and the variable speed compressor may be provided thereto with automatic setting means so that their set pressures are automatically set to be between the discharge pressure or start pressure during the load operation and the discharge pressure or rest pressure of the same during the unload operation.
Further, the constant speed compressor and the variable speed compressor may be provided with means for automatically stopping the operation of them if the unload operation continues exceeding a preset unload time. In this arrangement, the set unload time of the variable speed compressor may be set to be longer than the set unload time of the constant speed.
In order to achieve the above-mentioned objects, according to the second aspect of the present invention, in the above-mentioned air production facility, the control means controls the discharge pressure of the variable speed compressor to a set pressure between the discharge pressure of the constant speed compressor during the load operation and the discharge pressure of the same during the unload operation, and there is provided pressure detecting means for detecting the discharge pressure, a storage means for storing a pressure at which the discharge pressure changes from its increasing direction into its decreasing direction when the air consumption is decreased, as an unload opera
Aoki Masakazu
Hase Masakazu
Matsuda Hiroyuki
Antonelli Terry Stout & Kraus LLP
Campbell Thor
Hitachi , Ltd.
Walberg Teresa
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