单项选择题
One cause of high head pressure occurring in a refrigeration system can be （）.
A.insufficient cooling water flow to the condenser
B.a low refrigerant charge in the system
C.the liquid valve is open too much
D.high evaporator superheat
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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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Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.
A.cavitation
B.electrolysis
C.abrasion
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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
