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
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A.radiation
B.conduction
C.convection
D.condo-radiation -
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A gate valve installed in a pipeline should be （）.
A.used in either the fully dosed or fully opened position
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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
