Process control system

DETAILED DESCRIPTION That which is claimed is: 1.
Apparatus comprising: compressor means; an expander means; a secondary driving means; a shaft connecting said expander means, said secondary driving means and said compressor means; means for supplying a gas to the suction inlet of said compressor means; means for supplying compressed gas from the discharge outlet of said compressor means; means for supplying a driving force to said secondary driving means; means for supplying a process gas to said expander means; means for bypassing at least a part of said process gas around said expander means; first control valve means for manipulating the flow of said process gas to said expander means; second control valve means for manipulating the bypassing of at least a part of said process gas around said expander means; means for establishing a first signal representative of the actual driving force being applied to said secondary driving means; means for establishing a second signal representative of a low limit on the driving force being applied to said secondary driving means; means for comparing said first signal and said second signal and for establishing a third signal responsive to the difference between said first signal and said second signal; means for establishing a fourth signal representative of the actual position of said second control valve means; means for establishing a fifth signal representative of a low limit on the position of said second control valve means; means for comparing said fourth signal and said fifth signal and for establishing a sixth signal responsive to the difference between said fourth signal and said fifth signal; means for comparing said third signal and said sixth signal and for establishing a seventh signal representative of the lower one of said third and sixth signals; means for establishing an eighth signal representative of the actual position of said first control valve means; means for comparing said seventh signal and said eighth signal and for establishing a ninth signal responsive to the difference between said seventh signal and said eighth signal; and means for manipulating said first control valve means in response to said ninth signal