A commonly overlooked factor in energy efficiency is scheduling and loading of the furnace. “Loading” refers to the amount of material processed through the furnace or oven in a given period of time. It can have a significant effect on the furnace’s energy consumption when measured as energy used per unit of production, (e.g. Btu/lb).

Certain furnace losses (wall, storage, conveyor and radiation) are essentially constant regardless of production volume; therefore, at reduced throughputs, each unit of production has to carry a higher burden of these fixed losses. Flue gas losses, on the other hand, are variable and tend to increase gradually with production volume.

Equally, if the furnace is pushed above its design rating, flue gas losses increase more rapidly, because the furnace must be operated at a higher temperature than normal to keep up with production. Total energy consumption per unit of production will follow a bell curve, with the lowest cost at 100% of furnace capacity and progressively higher costs the further throughput deviates from 100%.

The lesson here is that furnace operating schedules and load sizes should be selected to keep the furnace operating as near to 100% capacity as possible. Partially loaded or overloaded furnaces are less efficient.