In top-supported boilers, the steam drum is securely anchored in place, and the lower portions of the boiler, including the lower drum, are allowed to expand downward when the boiler is operating. In the case of high boilers, there is considerable movement of the lower portion of the boiler as a result of temperature change. This movement of lower drums, waterwall tubes, and other pressure parts must be taken into consideration when furnace walls, stokers, ash hoppers, and other boiler equipment are installed or repaired.
Bottom-supported boilers expand upward, minimizing the problem of sealing the bottom of the furnace to prevent air leaks. This arrangement is used effectively in connection with slag-tapped furnaces and some types of stokers. It is generally more economical to support smaller units from the bottom and larger units from the top.
The upper portion of the boiler is supported by the tubes when theunit is bottom supported. The expansion resulting from increased temperature causes the drums to move upward. This movement must be considered in the design and installation of piping and of flues and ducts that connect to the boiler.
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