WNS series horizontal internal combustion boiler is classic 3-pass wetback oil/gas fired boiler. After fuel is atomized by burner, flame is full of the wave furnace and transfer heat via furnace wall, called 1st pass; the high temperature flue gasses are collected in reversal chamber and then enter 2nd pass, which is grooved tube bundle area through heat convection, air temperature gradually falls and flows to front smoke box and enter 3rd pass, which is done in the smooth tube bundle area, and then through back smoke box lead to chimney and exhausted to the atmosphere.