The usual symptoms of cavitation in a centrifugal pump is （）.

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

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

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

单项选择题A salinity indicator is used to determine the （）.

A.cause of salt contamination
B.location of salt water contamination
C.chemical makeup of feed-water
D.level of alkalinity in condensate