Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 196 of 331

process by heat application. In more recent years the scientific aspects of heat en- gineering have come to be appreciated. True economy in operation matched by superior results can only be obtained by expert design based on sound engineering followed by quality manufacturing and conscientious installation. The ovens may be on the plant floor or elevated overhead, either inside the plant or outside, on the ground or on the roof. With the ovens elevated the factory floor may be used for manufacturing or storage. The oven panels should be tongue- and-groove, all-welded construction, fabricated of heavy-gauge sheet steel, with a minimum of through metal, which reduces the transmission of heat by conductance. To avoid "insulation sag," which would leave an open space at the top of the wall panels, oven panels should be insulated with mineral wool batts ΒΌ in. greater in thick- ness than the oven panels. Access doors should be fully insulated or of nonsagging construction and equipped with explosion latches. These doors should allow open- ing from inside or outside the oven. The relatively few ovens shown here are merely examples of the many types be- ing built, each representing dozens and even hundreds of similar installations. Many special types are not shown because of limited use. For standard ovens, cus- tom designs, and even economical prefabricated components it pays to consult true experts in heat engineering. Direct versus Indirect Heating Systems In a direct fuel-fired system the products of combustion are allowed to come into contact with the work; the combustion equipment can be located inside or outside the oven. Although gas is generally used with a direct fuel-fired system, modern pro- cessing of fuel oils, along with improved handling and firing equipment, has in- creased their use in this area. An indirect-fired system does not allow products of com- bustion to come into contact with the work. Electric and steam heating are com- mon choices and fuel-fired equipment may be used in conjunction with a heat ex- changer to separate the products of combustion from the oven atmosphere. Fuels The brief outline that follows describes the most popular fuels being used today. You may want to investigate the possibility of using one as an alternative to back up your present system or as a supplement. Gas: This is generally considered a clean, convenient, and easy-to-use fuel. Work loads are commonly heated by the direct-fired method with no adverse effect from the products of combustion; however, because the availability and overall costs have changed it may be to your advantage to check the alternative fuels in your area. Oil:Many of the problems that once plagued the use of oil with direct-fired oven equipment have been eliminated, and indirect-fired systems are easily designed and installed when needed.In either case plan for a safe, convenient storage space. Steam: A very clean source that is simple, easy to control, and reliable for low-to- medium temperature operations, steam can be produced in a number of ways that are most economical. Electricity: This is a clean, simple, and efficient source of heat. High tempera- tures are easily obtained and heat recovery systems are available to economically re- duce operating cost. Combination: Combination fuel systems are very popular with people who have been plagued by fuel price increases and shortages because they allow the ability to switch from one fuel to another without stopping production or adversely affect- ing the product. 195

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