Fire tube boiler, also known as shell boiler, refers to the flue gas generated after fuel combustion flows through the tube or flue, and heating the water, steam or steam-water mixture outside the tube or flue. The basic structure is a small cylinder, a pipe (called "fire tube") or other shaped shell (called "boiler furnace") is installed in a big cylinder (boiler shell). The fire tube is isolated from the big cylinder and forms a jacket, which contains water, while the fire tube and furnace are used as the combustion chamber and flue to contain flame or flue gas, and the heat generated by combustion or flue gas is transferred to water through the wall of the fire tube.
The thermal efficiency of ZOZEN WNS series gas/oil horizontal internal combustion fire tube boiler can reach over 98%, the NOX emission is as low as 30mg/Nm³.