![]() ![]() In fuel-fired furnaces, the flame extends over the charge on the hearth, part of the heat reflected by radiation from the roof, which gives the name “reverberatory furnace”. They can also be heated by burning gaseous or liquid fuel, or by electrodes suspended from the roof see Figure 6.9. They are fired by pulverized coal with multiple end-wall burners. Reverberatory furnaces are constructed with dimensions of up to six by thirty meters. Graphite or silicon carbide resistance bars are used instead of metal bars in furnaces in which temperatures can go over 2000 ☌, and these are simple to operate, provided the restrictions on atmosphere control are noted. Furnaces operating above a temperature of 1350 ☌ should do so in an inert or reducing atmosphere. Table VIII gives a range of resistance elements which, in theory, allows a temperature of 2800 ☌ to be reached. The resistance elements of the furnace control the temperature that can be obtained. ![]() These points are made because, for special purposes, the operator might well require the construction of furnaces to a specific design. Because of the danger of a magnetic field interfering with the measurements of certain physical properties of the sample or system, the furnace should be noninductively wound: that is, it should have two similar windings carrying current in opposite directions so that their magnetic fields cancel, and also, the spacing of the windings should be decreased toward the end of the furnace to compensate heat losses. The outside of the furnace is generally well insulated, although this has a bearing on the rate at which the furnace will cool down. The resistance wire is generally coiled around an insulating packing. All instrumentation furnaces are electrically powered (although high-frequency inductive and infrared heating furnaces have been put on the market). One further point about furnace size and shape is that a long narrow furnace will generally give a larger uniform hot zone than a short wide furnace of similar volume. This generally involves a design feature involving both the unit in which the measurements are made and the furnace itself. The sample and reference material in differential measurements must also be subjected to the same temperature change. The smaller the furnace, the easier it is to cool the temperature of the furnace back to ambient. The second point about the size of the furnace is that large furnaces will take considerable time to reach a particular temperature and also take some time to cool down. Small furnaces, however, will not have this uniform range of temperature, and the positioning of the sample in smaller furnaces becomes quite important. If a large furnace is used, then one is going to have a range within the furnace at which a uniform temperature can be recorded. However, there are some general observations that can be made, first, regarding the thermal capacity of the furnace. Furnaces have to be designed for particular applications since they have to be compatible with the measuring system and also with problems associated with each technique, for example, convection currents in TG.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |