Heavy wear in a particular area of the inner circumference of a packing ring may be caused by （）.

单项选择题A pressure drop through the refrigerant liquid line may cause （）.

A.solenoid valve to seize
B.compressor to hunt
C.formation of flash gas in the liquid line
D.expansion valve to freeze open

单项选择题Low compressor head pressure in a refrigeration system can be caused by（）.

A.insufficient condenser cooling water
B.excessive condenser cooling water
C.air in the refrigeration system
D.excessive refrigerant in the system

单项选择题Which of the problems listed would came the discharge temperature of an R-12 refrigeration compressor to increase?（）

A.Thermal expansion valve frozen open
B.Suction gas heat exchanger bypassed
C.Condenser clogged or fouled
D.High pressure cutout switch inoperative

单项选择题The usual symptoms of cavitation in a centrifugal pump is （）.

A.noise and vibration
B.an increase in discharge pressure
C.an increase in suction pressure
D.lifting of the relief valve

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