YQL series gas-fired thermal oil boilers have a three-pass round coil structure. A conical coil is used for the effective protection of the furnace wall at the boiler end. This product is equipped with an advanced combustion device and is fully automated in operation. After the burner ignites the fuel, flame fills the round coil tubes and transfers heat by radiation through the tube wall (first pass). The high temperature flue gas gathers at the back door and then turns to the convection tubes section (second pass), where heat transfer takes place by convection. The flue gas gradually cools down here and flows to the front door, where it is reversed and goes into the third pass. After that, it enters into the chimney through the economizer and is released into the atmosphere.