單項(xiàng)選擇題An un-loader is installed on an air compressor to （）.

A.bypass the high pressure stage to the low pressure stage on 100% of the air compressors in service
B.prevent excessive inter-stage pressure buildup
C.control compressor discharge pressure
D.remove all but the frictional load during starting


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1.單項(xiàng)選擇題After installing a new hydraulic pump in a system, what special attention should be given to the hydraulic system?（）

A.The relief valves in the system should be readjusted
B.The filters and strainers should be checked frequently
C.All system pressure should be readjusted
D.The system should be drained and renewed with a fluid of different operating characteristics

3.單項(xiàng)選擇題One of the main functions of wearing rings, as used in a centrifugal pump, is to （）.

A.maintain radial alignment between the pump impeller and casing
B.absorb all impeller shaft end thrust
C.prevent water leakage to the atmosphere
D.prevent wear of the pump casing and/or impeller

4.單項(xiàng)選擇題Brine density in a distilling plant is measured with a/an （）.

A.psychrometer
B.salinometer
C.anemometer
D.sphygmomanometer

5.單項(xiàng)選擇題Which of the following statements is correct regarding an oil with a high viscosity index?（）

A.A large change of viscosity occurs with a minor change in temperature
B.No change in viscosity occurs with any change in temperature
C.Very little change in viscosity occurs with a significant change in temperature
D.The viscosity of the oil increases with an increase in temperature