The primary sources of energy to heat the equipment are (natural) gas and electricity. Alternative energy sources, such as oil and other hydrocarbon fuels, are also used.
Heat-treating furnace equipment can further be divided into furnaces and ovens. Today, oven construction can be used in temperature applications up to 1400ºF (760ºC), although 1000ºF (538ºC) is a traditional upper limit. Oven technology utilizes convection heating, that is, the circulation of air, products of combustion or an inert gas as the primary means to heat a workload to temperature. Oven construction also varies considerably from furnace construction.
Furnaces can be classified in a number of ways as summarized in Table 1.
Batch units tend to involve large, heavy workloads processed for long periods of time. In a batch unit, the work charge is typically stationary so that interactions with changes in the furnace atmosphere are performed in near equilibrium conditions. Batch furnace types include: bell furnaces, box furnaces, car-bottom furnaces, elevating hearth furnaces, fluidized-bed furnaces, gantry furnaces, mechanized box furnaces (also called sealed-quench, integral-quench or in-out furnaces), pit furnaces, salt-pot furnaces, split or wrap-around furnaces, tip-up furnaces and vacuum furnaces.
Of all the batch furnace types, integral-quench furnaces are the most common. Continuous furnaces are characterized by the movement of the workload in some manner, and the environment surrounding the workload changes dramatically as a function of the position of the work charge.
Continuous furnace types include:
- Cast link-belt furnaces
- Humpback furnaces
- Mesh-belt furnaces
- Monorail furnaces
- Pusher furnaces
- Roller-hearth furnaces
- Rotary-drum (rotary-retort) furnaces
- Rotary-hearth furnaces
- Shaker-hearth furnaces
- Vacuum furnaces
- Walking-beam furnaces