單項(xiàng)選擇題Some vessels are equipped with a water lubricated stern robeWhen at sea, operating under normal conditions, the water service valve from the ships saltwater system to the bearing should be （）.

A.closed, and no leakage permitted across the shaft packing
B.closed, and only slight leakage permitted across the shaft packing
C.open, and no leakage permitted across the shaft packing
D.open, and only slight leakage permitted across the shaft packing


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1.單項(xiàng)選擇題Sub-cooling is a method of reducing the temperature of the liquid refrigerant below its （）.

A.freezing point
B.floc point
C.condensing temperature
D.compression temperature

2.單項(xiàng)選擇題If a bilge pump is able to develop vacuum, but is unable to sufficiently pump out the bilges, you would check for all of the following EXCEPT （）.

A.the circuit breaker
B.for leaks in the suction piping
C.relief valve is not properly seated
D.the suction strainer

3.單項(xiàng)選擇題The principal purpose of sub-cooling liquid refrigerant prior to its entering the expansion valve is to （）.

A.increase the refrigerating effect, by decreasing the amount of flash gas
B.allow the refrigerant to enter the throttling device in a saturated condition
C.increase the refrigerating effect by increasing the amount of flash gas
D.minimize the temperature drop of the liquid as it passes through the solenoid valve to the outlet side

4.單項(xiàng)選擇題Refrigerant is normally sub-cooled in a refrigeration or air conditioning system condenser to （）.

A.maintain adequate coil back pressure
B.prohibit flashing in the liquid line
C.reduce refrigerant volume in the system
D.reduce compressor discharge line loading

5.單項(xiàng)選擇題An advantage of a helical gear pump over a simple gear pump is that the helical gear pump is capable of （）.

A.maintaining a steadier speed
B.delivering liquids over greater distances
C.operating for longer periods of time
D.producing a smoother discharge flow