Direct fire, similar to what one might see with a gas stove-top or propane grill, directs the flame and combustion directly into the chamber where products are being processed.  Because all of the energy is being transferred into the production chamber or furnace, direct heating processes are often seen as more efficient and capable of reaching higher temperatures.  However, direct fire heating has the downside of introducing the products of combustion (CO2, H2O and potential amounts of CO, NOx, and sulfides).  While some processes can tolerate these combustion products, many processes require a controlled atmosphere within the furnace and must rely on indirect heating.

 

Indirect heating is when heating fuel (i.e. natural gas, oil, etc.) is burned inside of a “heat exchanging element” (i.e. radiant tube, muffle, etc.), suspended within the furnace where product is being processed.  The products of combustion are isolated from the atmosphere of the furnace.  The energy released by this combustion must then be transferred, typically through a mixture of convection and radiation, from the heat exchanging element into the furnace chamber and product the furnace is processing.  While this process decreases the overall efficiency of the process, it is necessary to control the furnaces’ atmosphere to ensure product quality.