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NORA Oilheat Technicians Manual

Fire Tube Return Water Flue Pipe Hot Flame Gas 13-8 Heating Systems first pass, and back-to-front in the second pass before they are transported by the flue pipe to chimney. Some steel boilers include a third pass through fire-tubes for increased efficiency. Some features of these boilers include low mass construction and reduced water storage for lower heat loss. Cast-iron boilers are narrow sections of cast iron that are joined to form the boiler. Each section contains boiler water on the inside, while hot gases pass in channels between the sections. The cast sections are joined together with either metal push nipples or non-metallic “O” rings. Some cast-iron boilers have “wet-legs” or “wet-backs” so that the combustion chamber is partly enclosed by boiler water. Water is supplied to each casting through a common header at the bottom and top of each section. The water flows upward and it is heated by the hot inner surfaces of the cast-iron sections. The heated water leaves the boiler through the outlet fitting and then it is piped to the radiation. Extra attention is needed when assembling or servicing sectional boilers to be sure that there is no way for air to leak into the boiler between the sections. These must be sealed tightly to prevent the entrance of secondary air that lowers operating efficiency. Firebox and combustion chambers The burner is fired into a combustion area that may be lined with a refractory material that reflects radiant heat back to the flame. The reflected heat helps to gases flow inside long tubes surrounded by water. These fire-tubes can be arranged vertically (up-and-down) or horizontally within the boiler water. As the combustion gases rise through the tubes, heat passes through the wall of the tubes to the water. Figure 13-8 shows a vertical fire tube boiler. In horizontal fire-tube boilers, the hot gases travel to the back of the boiler and then pass into the horizontal fire tubes where more heat is transferred to the water. This design is called a “two-pass” boiler because the gases go front-to-back in the Chapter 13 Heating Systems Figure 13-8: Dry base vertical fire tube boiler Flue Gas Barometric Damper Hot Water to House Boiler Water Combustion Oilburner Chamber Cross Section View


NORA Oilheat Technicians Manual
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