单项选择题
A.filled with water B.purged C.cleared of air D.None of the above is true
（） are mounted in pairs to protect the boiler against o...
单项选择题（） are mounted in pairs to protect the boiler against overpressure.
A.Safety valves B.Water tubes C.Steam stop valves D.Blow down valves
In a （） boiler the hot gases from the furnace pass thro...
单项选择题In a （） boiler the hot gases from the furnace pass through the tubes while the water is on the outside.
A.water-tube B.fire-tube C.exhaust gas D.None of the above is true
In a （） boiler the water flows through the inside of th...
单项选择题In a （） boiler the water flows through the inside of the tubes while the hot gases pass around the outside.
A.water-tube B.fire-tube C.exhaust gas D.none of the above
