SZL series Double-drum horizontal coal-fired steam boiler, the steam output of boiler include 4, 6, 8, 10, 15, 20t/h 6 specs and the steam pressure is range from 1.25 to 2.5Mpa. The boiler is packaged assembly, including upper and lower parts. The upper part is the heating surface of boiler, in which the cindering chamber consists of surrounding water-cooling wall pipe, lower header and steam drum, and the back of furnace consists of steam drum, water drum and the convection tube bank between two drums. The lower part is combustion equipment with traveling grate layer combustion mode. Under the grate there are seven independent draft compartments structure, allowing draft from two sides with draft adjusting device respectively. Above the grate the bricked front and back arches form whole furnace, and it can set up economizer and air preheater in the heating surface on the rear part of boiler according to the coal quality and power of boiler. The smoke shall enter cindering chamber through furnace, and transversely blow the convection tube bank as S type, and being discharged to the air through economizer or air preheater, deduster, draught fan and chimney after blowing rear smoke pass. Laying out more convection heating surface and radiance heating surface, increasing the thermal efficiency than the same kind of others area products.