單項(xiàng)選擇題Although lube oils used in the main lubricating service systems should have a relatively high flash point to avoid ignition, they can create smoke and fire hazards when they（）

A.are exposed to a vacuum
B.come in contact with extremely hot surfaces
C.are reduced in temperature to just above the pour point
D.become extremely agitated or aerated


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1.單項(xiàng)選擇題Compensated flow control or constant flow valves are used in hydraulic systems to（）

A.compensate for major leaks in the system
B.maintain the original fluid viscosity
C.allow for changes in pressure and temperature within the system
D.assure constant fluid temperature

2.單項(xiàng)選擇題To avoid damaging the components of a printed circuit board when testing it with a DC volt-ohmmeter, you should （）

A.ground the board
B.avoid reversing the polarity of the leads
C.isolate sensitive components with heat sinks
D.all of the above

3.單項(xiàng)選擇題The heated feedwater entering any flash chamber of a flash-type evaporator will（）

A.vaporize, with the unflashed water remaining at the temperature at which it entered the flash chamber
B.vaporize, with the unflashed water equalizing to the saturation conditions existing in the flash chamber
C.vaporize, with the remaining water at the temperature greater than it entered the flash chamber
D.boil, allowing steam bubbles to rise through the brine at the bottom of the flash chamber

4.單項(xiàng)選擇題In a refrigeration system, which of the following statements describes the actions occurring when a solenoid valves, used in conjunction with an unloader head is energized?（）

A.The solenoid valves directs high pressure gas to the valve lifting device raising the suction valve off its seat and causing one specific cylinder to become unloaded
B.The solenoid valves directs high pressure gas to the underside of the unloading piston, causing it to rise and allowing the recirculating ports to be opened
C.When the solenoid valve becomes energized, the valve ports become closed, allowing discharge pressure to build up above the piston, shifting it and causing it to unload
D.The solenoid valve allows the refrigerant to pass from the suction chamber to the top of the unloader piston, causing the piston to lift and unload the cylinder

5.單項(xiàng)選擇題The efficiency and output capacity of a low pressure distilling plant will be reduced if the last effect shell （）

A.pressure does not fluctuate rapidly
B.pressure remains steady and unchanged
C.brine density does not exceed 1~5
D.brine density is too low