When thermal energy is supplied directly to a process stream with a fired heater, the
most important consideration is the cost of the fuel. However, the efficiency of the
furnace is also important; for example, if the furnace has an 80% thermal efficiency, then
the amount of fuel that is needed is 1.25 times the process heat load. In some cases, an
energy efficiency project can affect the efficiency of a furnace. For example, increasing
feed preheat ahead of a charge furnace may increase the stack gas temperature, which in
turn reduces the furnace efficiency. This change in efficiency should be included in the
evaluation of the energy savings.