The RL is a horizontal fire-tube with three passes. The combustion chamber is completely cooled by boiler water. It provides a low heat loss from radiation thanks to the insulation of great thickness as well as front and back door insulation. Thermal insulation is composed by a thick layer of low conductivity and high density material, covered by a stainless steel metallic enclosure. Good accessibility for maintenance and checks due to the front door with access to the entire furnace and the front tubular plate. Furthermore, there is total access to the rear tubular plate and smoke box via the rear door.