單項(xiàng)選擇題

In order to properly remove air from the casing of a centrifugal pump when starting, the pump should have a （）.

A.positive head
B.negative suction head
C.mechanical seal
D.vent valve attached to the casing at the top of the volute

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單項(xiàng)選擇題

The valve best suited for throttling gas or liquid flow in a pipeline is the（）.

A.gate valve
B.globe valve
C.check valve
D.plug cock

單項(xiàng)選擇題

Which of the listed statements is a characteristic of the liquid charged power element used with thermostatic expansion valves?（）

A.At the designed operating temperature, the liquid refrigerant charge has changed to a vapor
B.The liquid refrigerant tends to collect at the bellows or diaphragm and reduces the valve sensitivity
C.The sensing bulb is empty of liquid refrigerant charge at the designed operating temperature
D.The sensing bulb is never emptied of liquid refrigerant under normal operating conditions