单项选择题
A.the circuit breaker
B.for leaks in the suction piping
C.relief valve is not properly seated
D.the suction strainer
单项选择题 The principal purpose of sub-cooling liquid refrigerant prior to its entering the expansion valve is to （）.
单项选择题 Refrigerant is normally sub-cooled in a refrigeration or air conditioning system condenser to （）.
单项选择题 An advantage of a helical gear pump over a simple gear pump is that the helical gear pump is capable of （）.