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


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1.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

2.單項(xiàng)選擇題In most pumps and pump motors overpacking the ball bearings full of grease will result in （）

A.moisture emulsification of the bearing grease
B.insufficient circulation and overheating of the bearing
C.proper grease circulation to cool the bearing
D.sliding friction between balls and races

3.單項(xiàng)選擇題Aqueous Film Forming Foam (AFFF), commonly known as ‘light water’, is especially suitable for fighting （）

A.oil fires in the engine room bilges
B.class B fires in paint lockers
C.any class D fire
D.all of the above

4.單項(xiàng)選擇題Which of the following statements concerning the water tube and fire tube boilers is not true?（）

A.natural circulation of the water in the water tube boiler is better than that in the fire tube boiler
B.for the same output the water tube boiler will carry less mass of water compared with the fire tube boiler
C.the water tube boiler will cost shorter time for steam raising
D.the water tube boiler require lower skill in operating and maintaining

5.單項(xiàng)選擇題Which one of following statement is not true concerning the suitable sites of the direct acting relief valve and the pilot operated relieve valve?（）

A.the direct acting relief valve is used for controlling lower flows and pressure
B.the pilot operated relief valve is used for controlling higher flows and pressure
C.the pilot operated relief valve is used for controlling lower flows and pressure
D.the controlling accuracy of direct acting relief valve is worse than that of the pilot operated relief valve