A gate valve installed in a pipeline should be （）.

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

单项选择题If a refrigeration compressor is running continuously without lowering the temperature in the refrigerated space, the trouble may be （）.

A.a shortage of compressor oil
B.warm food in the refrigerator
C.excessive condenser cooling water
D.a shortage of refrigerant

单项选择题In the design of hydraulic piping and equipment consideration is given to minimize turbulence in the hydraulic fluid, as this will cause （）.

A.molecular fluid vibration
B.energy losses
C.wide pressure variations
D.mechanical damage to control valves

单项选择题Which of the conditions listed represents the greatest effect of excess frost accumulation on the evaporator coils?（）

A.Keeps the refrigerated space cooler
B.Reduces the efficiency of the plant
C.Takes the load off the compressor
D.Has no affect on the system