0.3 t 0.4 t 0.5 t 1 t 2 t 4 t 6 t 10 t of fuel gas steam boiler is horizontal steam boiler shell type three return, all wet back structure. The fuel in the combustion furnace positive pressure, high-temperature furnace flue gas along the back through the combustion chamber back into the first flue tubes, the oppressive smoke box before turning 180 ° into the second bundle, after the convection heat transfer into the atmosphere. We use the center symmetrical design, the design capacity of the three return structure of a large three-pass steam boiler. The greatest feature of this boiler: All pressure parts and drum central symmetry, boiler all wet back structure, with great inner cylinder and eccentric firebox structure that is conducive to provide adequate radiation heating surface. The convection heating surface with a second backhaul and three return bundles composition. Before returning combustion chamber pipe connected to the board using furnace, combustion chamber back to the tube plate with ellipsoidal head and tube sheet. The second and third backhaul backhaul turn the smoke chamber constitutes a former smoke box, this out tobacco smoke temperature has been below 500 ℃. Therefore, before the smoke box structure fire-resistant insulation materials lighter shaped structure. After the smoke chamber flue gas outlet is disposed along the longitudinal axis of the barrel, not only shorten the length of the furnace; and also avoid chimney in a direction perpendicular to the longitudinal axis of the cylinder, boiler condensate corrosion in the boiler starts to stop. This boiler is designed primarily consider the principle of boiler maintenance and stability. Mixing water in the boiler return means import settings, return to scour smoke tube bundles, then the contact furnace, furnace to improve the return water temperature exposure. Avoid local produce subcooled boiling phenomena. Boiler water temperature in the lower part of the combustion flue gas, low in the upper flow on heat convection, it has been to achieve the best design. Steam boiler has reached the maximum of vaporization space, coupled with the high-temperature surface of the rear bulkhead plus circulating water through the combustion chamber back to make it reasonably uniform heat transfer. Back to the combustion chamber to ensure the safe operation of the tube plate. (B) waveform furnace (according to the user's choice) in this series waveform boiler furnace, both to strengthen the smoke disturbance, but also increases the radiative heat transfer area; both increased stiffness furnace, and effectively weakened expansion stress furnace; and both enhance heat transfer, and the promotion of fuel combustion in the furnace. (C) the smoke pipe and tube sheet welding using the first all-welded and then the pre-expansion, then expanded joint advanced technology, both to prevent the pipe crevice corrosion, but also enhance the weld strength. Ago (iv) symmetry living hinge, door easy to open after smoke, ease of maintenance, cleaning. This series of boiler structure is compact, easy installation and transportation. Hot commissioning before delivery, regardless of the operation rate is still in operation can be stable operation. This series boiler is equipped with imported fill pumps and burners, automatic control, a high degree of automation. Can achieve optimal combustion boiler.