A.the mechanical seal
B.packed stuffing boxes
C.either A or B
D.neither A nor B
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As for a hydraulic vane pump, which of the following is in contact with the vanes during working: （）
(1)the rotor
(2)the pump shaft
(3)the external surface of the housing
A.(1) only
B.(2) only
C.(1) and (2)
D.(1),(2)and(3)
A.circular
B.elliptical
C.rectangular
D.hexagon
A.suction
B.exhaust
C.pressure
D.discharge
A.draw the liquid in；force the liquid out
B.force the liquid out；draw the liquid in
C.suction；draw
D.deliver；outlet
A.throttling the pressure line
B.changing the speed of the rotor
C.either A or B
D.None of the above is tree
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The general purpose of a heat exchanger is to （）.
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Low compressor head pressure in a refrigeration system can be caused by（）.
If the suction pressure at the refrigeration compressor is below normal, the cause can be（）.
Thermostatic expansion valves can be adjusted for （）.
A salinity indicator is used to determine the （）.
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