Heating energy sources: natural gas, liquefied petroleum gas (LPG). An imported famous-brand automatic burner or gas burner is adopted, ensuring high heating performance, low noise and zero-discharge of fume, meeting the environmental protection requirements. After heating, the hot furnace gas is blown back to the upper mouth of the crucible through the flue; or preheated the combustion air from the flue heater ex-changer to improve the heating efficiency and reduce fuel consumption. Multiple safety protection modes are adopted, such as gas under-pressure, assisted gas under-pressure, flameout alarm and so on. Melting rate in a cold furnace: 3.5h Melting rate in a heated furnace: 2h