单项选择题
A.They should always be started with the discharge valve closed
B.They should always be started with the discharge valve opened
C.A priming pump is always required to flood the impeller suction
D.They should always be started with the sealing line valves closed
单项选择题 Excessive oil foaming in the crankcase of a refrigeration compressor at start up can cause（）.
单项选择题 Which of the valves listed is NOT considered to be a hydraulic system directional control valve?（）
单项选择题 Which of the listed refrigeration system components keeps the refrigerant circulating through the system?（）