Heat-treating furnaces can be divided into two main types: batch and continuous. The fundamental difference between these two styles is not in the materials of construction, although there are differences due to inherent design requirements. Instead, the key differences lie in how workloads are positioned in the units and how they interact with the atmosphere within the furnaces.

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
Of all the continuous furnace types, pusher furnaces are the most common.