The source of heat may be derived from combustion of fuel or electricity. Heat is transferred to the work primarily by natural gravity, forced convection or by radiant sources if the temperature is high enough. As stated earlier, oven construction can be used in temperature applications up to 1400ºF (760ºC), although temperature ratings of 1250ºF (675ºC) or 1000ºF (538ºC) are more common.
Selection of the type of oven involves the careful consideration of several variables, including:
- Quantity of material to be processed
- Uniformity in size and shape of the product
- Lot size
- Temperature tolerances
- Effluent evolution, if any
There are several design criteria for oven construction, which include:
- Operating temperature
- Heating method
- Thermal expansion of materials
- Airflow patterns
Several distinct changes occur in oven construction as the temperature increases. Problems with expansion and interior (heat and atmosphere) sealing become much more significant at higher temperatures. For example, an oven designed for operation at 400ºF (205ºC) will have mineral wool insulation 4 inches (100 mm) thick. By contrast, for a 700ºF operating temperature, a thickness of 7 inches (175 mm) is required. Thermal expansion in large ovens is generally compensated for by the use of telescoping panel joints in the walls, ceiling and floor. Door construction must incorporate similar expansion joints.
The type and quantity of airflow is important. For example, ovens designed for handling explosive volatiles, such as paint drying or solvent extraction, have special considerations including large airflow volumes to dilute the volatile, explosion-relief hatches, purge cycles, powered exhausters, airflow safety switches and fresh-air dampers.
Several different patterns of airflow can be used depending on the workload configuration. These are horizontal, vertical and combination (uniflow).
The method of heating an oven often depends not only on the availability of a particular fuel but on the process itself. Many processes cannot tolerate products of combustion from direct-fired systems, so indirect (radiant-tube) firing or alternate energy sources need to be considered. In addition, some means of heat transfer, such as microwave heating, is severely limited in the type of product that can be processed. Ovens are commonly heated by fuel (natural gas or other hydrocarbons), steam or electricity. Infrared heating and microwave (radio frequency) can also be used.