GB1452771A - Apparatus for controlling centrifugal compressors - Google Patents

Abstract

1452771 Centrifugal compressors OWENS-ILLINOIS Inc 20 Sept 1973 44173/73 Heading F1C [Also in Division G3] An electrically driven centrifugal compressor 48 has an inlet valve 46 positioned by a control signal which is normally the difference between the actual output pressure and a desired value but which is modified if the motor current approaches a maximum value in order that that value is not exceeded. As shown, the control signals are pneumatic, but they may alternatively be electric. Output pressure is applied via line 64 to a device 66 which modulates the pressure from a control air source 60 via a manifold 62. If this source fails a valve 116 admits air from line 64 for control purposes. The output of 66 is led to a comparator 72 where it is compared with the desired pressure, the resultant error signal being boosted at 84 to control the inlet valve 46. The motor current is measured at 80, the electrical signal being converted into a pressure signal at 78 where it is also compared with the set maximum value and the difference used to modify the output of comparator 72 if necessary. The system also has means for preventing surges; this comprises a second motor current detector 98 whose electrical signal is converted to pneumatic pressure at 96 for operation of a controller 102. This normally provides a high output to keep a blow-off valve 56 closed, but as the motor current drops towards a predetermined critical value the controller 102 progressively opens 56 until it is fully open just before the surge point. The pneumatic control elements are not described in detail, but Figs. 1 and 3 (not shown), illustrate electrical relay circuits for starting the compressor from rest (which includes a number of safety interlocks related to parameters such as oil flow and temperature) and also for continuing stable operation during short power failures, e.g. 5 seconds, after which full control is automatically resumed. During such failure a control by-pass 89, 90 is operable.