... Wet back shell boiler High efficiency WNS series automatic Oil&Gas-fired boiler is horizontal, three-pass, wet back shell boiler. The ... structure includes: Boiler shell, Bellows furnace, Returns burns room, Threaded gas pipe, etc. The shell and the furnace are made by ... , burning rapidly, mix evenly of air and gas and adjusting scale of fuel gas and air. The burning efficiency is over 99 ...