Pressurized combustion (Radiation) type cylindrical, flame smoke tube steam boilers have 3 passes where first and second passes occur as reverse flame in the large combustion chamber. This constitutes turbulence occurring more efficient air-fuel mixture and good combustion. On the third pass, hot gas passes through the smoke tubes. Temperature of the gas is lowered as much as possible during the flow in the tubes until the flue gas leaves the boiler. Thus high boiler efficiency of 85-90% is obtained. Advantages Has large and corrugated combustion chamber Has relatively small dimensions and requires small installation area Produces steam rapidly Has optimum heat transfer surface (it does not have excessive heat load per until heat transfer surface) Has minimum heat loss on exterior surface Has long life time and high efficiency The combustion chamber and the smoke tubes are easily reachable from the front door and the waterside is easily reachable from the manhole and the handholes)