Question:
We have an electrically heated mesh-belt furnace that uses an exothermic gas atmosphere. We normally use the furnace for brazing at temps of 1900ºF (1040ºC) and higher. We'd also like to anneal parts at 1100ºF (590ºC). I was wondering if we'd be violating NFPA 86 guidelines if we ran exothermic gas at furnace temps lower than 1400ºF (760ºC)? The furnace has flame curtains at either end. What do you think?

Answer:
Since you are asking about a safety-related issue, please be aware of our limit of liability that applies to any/all statements we will make. Specifically:

“The generic and practical information presented here is not intended to replace or supplement company policies and procedures; federal, state and local codes; government standards; insurance requirements; or common sense. Nor is it intended to replace or supplement the equipment manufacturers’ instructions and operating and maintenance manuals that should always be thoroughly read, understood and followed. Comprehensive personnel training should be provided by the equipment manufacturer and their sub-suppliers unequivocally to everyone who will be associated with and operating their heat-treating equipment.”

That being said, exothermic gas can be produced in two distinct types – "lean" exothermic gas and "rich" exothermic gas – depending on the air-to-gas ratio being run in the exothermic gas generator. A "lean" exothermic gas is produced at ratios of between approximately 7:1 (read "7 to 1") to 11:1, while "rich" exothermic gas is produced at ratios of between approximately 5.5:1 to 7.8:1 (see article referenced below for typical gas compositions and application uses). Note that these categories do overlap. The key difference is the amount of hydrogen present in the furnace atmosphere. If it is under 3%, the atmosphere is considered non-combustible and the furnace is safe to operate at any temperature.

First of all, the exothermic gas atmosphere must be introduced into the furnace when it is operating above 1400ºF (760ºC). Once the furnace and atmosphere is stable, the furnace temperature can be turned down below 1400ºF (760ºC) provided the furnace is maintained under positive pressure and there is a nitrogen purge available in the case of power outages or other emergency conditions.

All furnaces and generators are subject to NFPA 86 standards and safety recommendations whether the equipment is operating above or below 1400ºF (760ºC). A copy of the specification is available at www.nfpa.org for around $46. Exothermic gas generator requirements are covered in the specification also.

As I interpret the specification, your furnace falls under the category of Class C, Type III (both inlet and outlet ends of the furnace are open and there are no external doors or covers). I interpret it this way because while the furnace most likely has at least one door, it is raised or open at all times to allow work to pass through. Specific atmosphere introduction and removal instructions are provided in the NFPA document. Annex A should be consulted as well. You should review NFPA guidelines concerning scenarios in which the furnace is being operated at below 1400ºF (760ºC) for more specifics.

Please recognize that operating a combustible atmosphere under 1400ºF requires all furnace, generator and safety systems to be functioning properly, and a great deal of attention must be paid to the operation. For example, the furnace should never be left unattended when operating below 1400ºF (760ºC).

Also, atmosphere introduction and removal is especially important, and operator procedures should be carefully checked to ensure this is done safely. Drafts or situations involving air infiltration must be carefully controlled. Door opening is an important consideration as well – the higher the door opening, not only the greater the atmosphere flow required, but the more dangerous the situation with respect to maintaining positive flow and pressure conditions. Proof of pilot ignition must be confirmed at all times. Finally, the design of the furnace also plays an important role. For example, does it have an extended front tunnel (preferred), or is the entrance vestibule relatively short (undesirable)?

Finally, exothermic gas, because it does not contain large percentages of hydrogen, does not burn with a "strong, healthy flame" such as found with endothermic gas. Instead, it burns with a "wispy, anemic flame," so flame curtains and pilots must be properly adjusted and controlled. Differentials in pressure tend to affect exothermic gas more than other types, and this becomes magnified when operating below 1400ºF (760ºC). As it so adequately states in the NFPA document, "Most causes of failures can be traced to human error. The most significant failures include inadequate training of operators, lack of proper maintenance and improper application of equipment."