... Description Horizontal Steam boiler(gas fired or oil fired)WNS series is the boiler with tri-backhauling internal combustion designed by ... to the second backhauling, such as whorl flue pipe bundle area; after heat exchange by convection, the smog temperature gradually lowers ... turns to the third backhauling, for example, bare pipe bundle area, and then passes through rear smog chest to chimney, and ...