单项选择题

The control mode in which the final control element is moved from one of two flexed positions to the other is known as （）.

A.dead band action
B.neutral zone action
C.range
D.on-off action

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单项选择题

Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.

A.cavitation
B.electrolysis
C.abrasion
D.corrosion

单项选择题

If the head pressure of a reciprocating refrigeration compressor is excessive, （）.

A.the relief valve should open before the high pressure cutout
B.the relief valve should open and allow the excess refrigerant to flow to the receiver
C.the high pressure cutout switch should operate before the relief valve opens
D.you should close in on the suction valve