Furnace

In general a furnace is a structure or device with an arrangement to apply heat to any material. It can be used to melt metal for casting or to heat materials in order to change their shape (rolling, forging, etc.) or their properties (heat treatment). The efficiency of a furnace is the ratio of useful output to heat input. 

Performance assessment test of furnace is done to determine efficiency and specific energy consumption of the furnace and followed by energy balance. There are many factors affecting furnace performance or efficiency, such as capacity utilization of furnaces, excess air ratio, final heating temperature, insulation of furnace etc.

For energy saving tips 

https://www.eec-fncci.org/page/improving-furnace-efficiency