单项选择题
Which features of a centrifugal pump reduce the need for renewing worn impellers and pump casings?（）
A.Close radial clearance between impeller hub and casing
B.Low rotational speed of impeller
C.Removable end plate
D.Replaceable impeller and casing wearing rings
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单项选择题
When securing a flash-type evaporator for an extended period of time, you should （）.
A.fill the unit with saltwater
B.fill the unit with de-scaling compound
C.completely drain the unit
D.tightly seal the unit to exclude air -
单项选择题
A distinguishing feature of an educator, when compared to other pumps, is the （）.
A.dischargeendbeingsmallerthanthesuctionend
B.smallsizeofimpeller
C.lackofmovingparts
D.easeatwhichthewearingringsmaybechanged -
单项选择题
The heat required to change a substance from a liquid to a gas without experiencing a temperature change is defined as the latent heat of （）.
A.fusion
B.vaporization
C.sublimation
D.condensation
