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

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

单项选择题An increasing head pressure in a refrigeration system, without any corresponding change in the cooling water inlet temperature, would probably be caused by （）.

A.restriction in refrigerant piping
B.air and non-condensable gases in the condenser
C.water in the refrigerant
D.flooding back of liquid refrigerant from the evaporator

单项选择题Which of the following problems could cause the high pressure cutout switch to shut down the compressor in a refrigeration system?（）

A.A shortage of liquid refrigerant
B.Excessive frost on the evaporator
C.Excessive condenser cooling water
D.Insufficient condenser cooling water flow