Is it an oven or a furnace?

Fig. 1. Ovens use recirculated air to heat the load (courtesy of Wisconsin Oven Corp.).

Fig. 2. Gas-fired furnaces fire the burners directly into the heating chamber (courtesy of Wisconsin Oven Corp.).

A frequently asked question is, "What's the difference between an oven and a furnace?" The fact is that it's more than just terminology. There are significant differences in the construction, technology and function of ovens and furnaces. 

Temperature

Traditionally, ovens operate under 540°C (1000°F) but today are rated to 760°C (1400ºF). Furnaces typically run up to 1100ºC (2000°F) or higher.

Atmosphere

Ovens (unless supplied with a retort) operate in either an air atmosphere or an inert-gas atmosphere. Furnaces can operate in air, an inert atmosphere or a combustible atmosphere (e.g., hydrogen, exothermic or endothermic gas).

Construction

Ovens typically utilize a sheet-metal (18 or 20 gauge) interior and exterior. Insulation is mineral-wool-style, 75 mm (3 inches) thick at lower temperatures and 101 mm (4 inches) or 150 mm (6 inches) thick at temperatures above 260°C (500°F).

Furnaces, on the other hand, have a heavy steel-plate exterior, 5-13 mm (0.1875-0.5 inch) thick. Insulation is 200-305 mm (8-12 inches) thick and either ceramic blanket, ceramic modules or refractory brick. The thicker steel exterior shell helps maintain flatness and prevent buckling due to the higher temperature, especially in areas near through-metal such as the end openings and shell penetrations.

Heat Distribution

In ovens, a system of ductwork distributes heated air onto the load being processed (Fig. 1). The air is ducted to a separate heating chamber containing the recirculation fan and the heat source, such as a burner or heating elements. The heat source can also be steam or hot oil at temperatures below 150°C (300°F). The amount of circulated air is high in order to transfer the heat from the source to the load. The fan is interlocked with the control system so the burner or heaters shut down to prevent damage or fire in case of failure.

In furnaces, the heat is introduced directly into the heating chamber where the product resides (Fig. 2). In gas-fired furnaces, several burners fire above and below the load, which is elevated on high-temperature structural members called piers or furniture. In electric-heated furnaces, heating elements are mounted directly to the interior sidewalls and radiate heat onto the load. For gas-fired furnaces, the combustion fans are interlocked with the control system so the gas is shut off if they fail.

It is typical for a furnace to have several burners on each sidewall above and below the work and a heat input of 10,550,000 kJ/hour (10,000,000 BTU/hour) or more in comparison to ovens, which generally have a lower heat input. Furnace burners use high-velocity combustion fans that inject a large volume of excess air (greater than that required for the combustion process) to mix the burner heat with the internal furnace atmosphere and provide agitation to increase the heat transfer.

Performance

Both ovens and furnaces can provide excellent results in their respective applications and temperature ranges. Ovens perform better than furnaces at lower temperatures, where a greater volume of air is recirculated. Ovens excel at curing of paint and powder, plastic annealing, aluminum aging, composite curing, removing moisture, adhesive drying and many other low-temperature applications where the mass flow of air plays an important role.

Since air expands as the temperature increases, it becomes too light to effectively transfer heat at the higher temperatures at which furnaces operate. Most of the heat transfer in furnaces, therefore, occurs by radiation. Furnaces are very effective at tempering, annealing, carburizing, austempering and normalizing of forgings, castings, heavier parts with thick cross sections and dense baskets of parts

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