A natural circulation, shockproof, horizontal furnace, combination water tube and fire tube boiler. The water tubes comprise a plurality of membrane-connected, ring-shaped tubes, defining a substantial portion of the primary combustion cylinder for the burner flame, surrounded by a larger diameter, secondary combustion cylinder which defines the remaining portion of the longitudinal combustion zone, and which redirects the combustion gases in a return path back over the outer surface of the water tube-membrane structure. The secondary combustion cylinder is connected only at one end and is free to expand and contract due to temperature changes without stressing the boiler or the combustion cylinder. The combustion gases then reverse direction a second time in a reversal chamber to pass through horizontal fire tubes extending the length of the boiler directly through the large outer shell containing the water to be heated, and then out through the flue gas outlet. This novel boiler unit can be used to generate steam, or for hot water applications with minor modifications.
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