A.insufficient lubricationB.insufficient refrigerantC.e……

单项选择题The unloading system on an air compressor will （）.

A.increase compressor discharge pressure on demand
B.increase compressor operating speed as necessary
C.allow the motor to turn the compressor opposed only by friction
D.reduce the compressor frictional load when starting

单项选择题Which of the listed methods of heat transfer takes place when two substances of different temperatures are in physical contact with each other?（）

A.Radiation
B.Conduction
C.Convection
D.Each of the above

单项选择题Which of the conditions listed will cause a refrigeration compressor to run constantly without simultaneously decreasing the temperature in the refrigerated space?（）

A.Shortage of refrigerant oil
B.Slight shortage of refrigerant
C.Excessive condenser cooling water flow
D.Faulty expansion valve

单项选择题Air compressor cylinder un-loaders enable the compressor to （）.

A.vary their speed according to temperature and load
B.start and come up to speed before air compression begins
C.change speed according to overload demands
D.reduce compressed air charge density

单项选择题The movement of heat within a fluid, caused by the application of thermal energy, is called （）.

A.radiation
B.conduction
C.convection
D.condo-radiation