The unloading system on an air compressor will （）.

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

单项选择题A gate valve installed in a pipeline should be （）.

A.used in either the fully dosed or fully opened position
B.installed with the stem down
C.used for steam service only
D.used to throttle or regulate the flow of liquid