WNS series horizontal internal combustion boiler is a boiler shell type 3 passes wetback oil/gas fired boiler. After fuel is being atomized by burner, flame is full of the wave furnace and transfer heat via furnace wall, this is 1st pass; the high temperature smoke is collected in reversal chamber and then enters 2nd pass which is grooved tubes bundle area; after heat convection, air temperature gradually falls and goes to front smoke box and turn to 3rd pass, which is smooth tubes bundle area, and then enters chimney passing back smoke box and exhausted to the atmosphere.