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

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

单项选择题Which of the following modes of heat transfer does NOT require any physical contact between a warmer and a cooler substance?（）

A.Radiation
B.Conduction
C.Natural convection
D.All of the above

单项选择题Which of the valves listed should be used either in the fully opened or the fully closed position?（）

A.A gate valve
B.A globe valve
C.Any check valve
D.Any needle valve