ovens and furnaces

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Safety through design is a critical consideration in oven and furnace manufacturing. This slideshow takes a look at industry standards governing their design and identifies common safety oversights.


  • 1.Fires and Explosions in Ovens and Furnaces By: John Holecek

2. What are Industrial Ovens and Furnaces? We are not talking about home cooking and home heating systems. The terms oven, furnace, and kiln are used somewhat interchangeably. Often vary as much by industry than inherent differences in design. The range of ovens, furnaces and applied heat transfer devices used in industry is quite large. Lets look at some examples. 3. Dryers Dryers are used to evaporate a liquid, often water, from a product. Dry Kilns are used to dry water and volatiles from green lumber. Grain dryers are used to dry grain before storage. 4. Baking Oven Baking Ovens are used to heat products and thereby cause a physical change in the product. They will also dry products. Paint Baking ovens cure coatings by a heat catalyzed chemical reaction. Food Baking Ovens are used to cook foods such as chicken and bread. 5. Kilns Kilns are essentially high temperature ovens, usually operating at 1000F+. Brick Kilns are used to fire bricks, typically at approximately 2500F. Rotary lime kilns create lime from limestone by a heat catalyzed chemical reaction. 6. Furnaces Furnaces are also used to heat products, often at higher temperatures, and thereby cause a physical change in the product. Heat treating furnaces change the properties of metal. Furnaces come in many different sizes, from small units shown above to large units as shown on the left. 7. Autoclaves Autoclaves: Ovens that operate at substantially non- atmospheric pressures either pressurized or at vacuum. They may also have specially constituted atmospheres. 8. Thermal Oxidizers Thermal oxidizers: Used to thermally incinerate undesireable or hazardous compounds into less problematic components. May also use catalysts to assist in breaking down the compounds. 9. Other Heating Devices Many applied heat transfer machines use some of the same design elements as industrial ovens. Thermal oil heaters. Heated parts washers share some safety design elements with ovens. 10. Other Heating Devices Thermal Oxidizer Water Heater Biomass Fired Boiler 11. What types of losses occur with ovens and furnaces? Explosions: From fuel supply From product being processed Fires: From product being processed From combustible residues Workers Compensation: Injuries from fires and explosions Electrocutions Burns from hot surfaces Loss of Business Income Failure to Perform 12. What defines the correct way to design, build, and operate ovens and furnaces? Codes and Standards: Codes have been written in a format suitable for adoption into law independent of other codes and standards. An example is the International Fire Code. Standards are documents containing mandatory provisions using the word shall to indicate a requirement. They are written in a form such that their provisions become mandatory when referenced by another code or standard. An example is NPFA 86: Standard for Ovens and Furnaces. Good Engineering Practice Design Guides Normal Practices / State of the Art 13. Codes and Standards that directly reference oven design: International Fire Code (IFC): Has a whole chapter on industrial ovens. Incorporates compliance with NFPA 86 by reference. The IFC is legally adopted in 42 states. NPFA 86: Standard for Ovens and Furnaces: The textbook of safe oven design. NFPA 13: Standard for the Installation of Sprinkler Systems: Gives information on installing sprinklers in ovens. 14. Other codes, standards and guides that relate to safe oven design: NFPA 54, National Fuel Gas Code, Gives proper methods for fuel gas piping. NFPA 31, Standard for the Installation of Oil- Burning Equipment, Gives proper methods for fuel oil piping. NFPA 58, Liquefied Petroleum Gas Code, Gives proper methods for storage, handling, transportation, and use of LP-Gas NFPA 70, National Electrical Code, Gives proper methods for electrical wiring of ovens. NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Noncombustible Particulate Solids, Gives proper methods of exhausting ovens. NFPA 68, Guide for Venting of Deflagrations, Gives proper methods for explosion venting. FM Data sheet 6-9: gives excellent guidance on the proper design of ovens. 15. What are the requirements for permitting ovens and furnaces? Requirements vary by location. Different states, counties, and cities can have different requirements. Where the International Fire Code is enforced, fire code officials are authorized to require building and operational permits for installing or operating an industrial oven. Most states issue specific modifications to the IFC which may define if permits for ovens are required or may give discretion to the permitting official. 16. Ovens are separated by class (per NFPA 86): Class A Furnace. An oven or furnace that has heat utilization equipment operating at approximately atmospheric pressure wherein there is a potential explosion or fire hazard that could be occasioned by the presence of flammable volatiles or combustible materials processed or heated in the furnace. (Translation) Ovens that are not under pressure or vacuum and process combustible products or dry / cure flammable coatings. Examples: Many paint baking ovens, lumber kilns. Class B Furnace. An oven or furnace that has heat utilization equipment operating at approximately atmospheric pressure wherein there are no flammable volatiles or combustible materials being heated. (Translation) Ovens that are not under pressure or vacuum and process noncombustible products and do not dry / cure flammable coatings. Examples: Brick kilns, lime kilns, dryers for non flammable liquids on non combustible products 17. More oven classifications: Class C Furnace. An oven or furnace that has a potential hazard due to a flammable or other special atmosphere being used for treatment of material in process. (Translation) Ovens and furnaces with potentially flammable or hazardous atmospheres. Example: Metal heat treating ovens with flammable gas atmospheres, integral quench furnaces and molten salt bath furnaces Class D Furnace. An oven or furnace that operates at temperatures above ambient to over 5000F (2760C) and at pressures from vacuum to several atmospheres during heating using any type of heating system. These furnaces can include the use of special processing atmospheres. (Translation) Ovens that operate under substantial vacuum or pressure. Example: Autoclaves for curing composite materials. 18. Design Requirements We will look at the following elements of oven design: Housing Construction and Ductwork Ventilation Fire Protection Heating Systems Control Systems Operational Support 19. Housing Construction Noncombustible housing materials required No wooden structures No plastic foam insulation Provide clearance to combustible surroundings Keep adjacent materials below 160F Must provide clearance on all sides and adequate maintenance access Class D ovens with pressurized housings above 15psi must be designed to the ASME Boiler and Pressure Vessel Code 20. Housing Construction Explosion Relief: Specially designed provisions that allow for freely relieving internal explosion pressures. Prevents an exploding oven from becoming a giant hand grenade. Required on all fuel fired ovens with the following exceptions: Indirect fired ovens with demonstrated low levels of combustible / flammable vapors Class D ovens (Autoclaves) or Thermal Oxidizers Certain high temperature furnaces that are made of minimum 3/16 thick steel, structurally reinforced, and refractory lined. Certain low oxygen type furnaces 21. Housing Construction Explosion Relief: 1 ft2 of relief area is required for every 15 ft3 of oven volume Often explosion relief is accomplished by leaving some roof panels simply laid in place without significant restraint Openings, and doors with listed hardware are included in the relief area Basis of design should be that panels relieve explosive pressure before the ovens safe design internal pressure limit is exceeded Heavy materials should not be placed on relief panels Extensions to the oven such as heater houses and ductwork should be included in the oven volume amount when calculating relief area 22. Ductwork Ducts must be made of noncombustible materials Include provisions for cleaning out ducts, especially when products generate combustible dusts or residues. Generally this requires clean out doors. Guard or insulate surfaces over 160F. When vapors are likely to have heavy concentrations of condensable gasses, the duct should be insulated. Otherwise a heavy buildup of combustible residue will form in the duct and present a fire hazard. Keep hot surfaces of ducts away from combustible surroundings. This is frequently improperly done at the exhaust duct penetration through the building roof. NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Noncombustible Particulate Solids, has additional requirements 23. An example of a correct penetration of an exhaust duct through a building roof Duct Bar Joists Rain Skirt Air Gap Per Code Combustible Roof Material Steel Support 24. Fire Protection Ovens and furnaces are frequently equipped with fire protection systems. These systems include wet pipe, dry pipe, deluge systems. Some ovens are installed with special fire protection such as Clean Agent or Water Mist systems. The most common practice is to install a wet pipe system with the supply piping located outside the oven. The International Fire Code and NFPA 86 currently differ somewhat in the requirements for fire protection systems. Lets look at what they say. 25. 2012 International Fire Code on Oven Fire Protection SECTION 2106 FIRE PROTECTION 2106.1 Required protection. Class A and B ovens which contain, or are utilized for the processing of, combustible materials shall be protected by an approved automatic fire-extinguishi


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