单项选择题
A.two or more impellers housed together in one casing and mounted on a single shaft
B.a single stepped impeller mounted in a progressively staged casing
C.a large radial clearance between the impeller and casing to prevent overheating due to
D.an increase in the discharge velocity of the liquid with a corresponding decrease in pressure through the stages
单项选择题 If a refrigeration system compressor crankcase is sweating, you should （）.
单项选择题 Crankcase sweating in a refrigeration system is caused by （）.
单项选择题 In an electro-hydraulic steering system, under movement is maintained in close synchronization with the steering wheel position by means of the （）.