Construction of furnaces

On similar principles great attention is required in selecting the materials employed in the construction of furnaces. To prevent the escape of the caloric, and thus (o economise fuel, the hearth and walls of the furnace are invariably compounded of some non-conducting substance, as Stourbridge clay or powdered brick, mixed with a large quantity of powdered charcoal or coke. For the same reason, boilers are imbedded in bricks or day, and in many boilers the fire is literally enclosed within them, that none of the heat may be lost, but that all may be rendered available for raising the temperature of Hie water,... 

The most widely manufactured carbide is SiC which has been in use for a long time as an abrasive because of its extreme hardness, either in powdered form or in that of tools bound by inorganic or organic binders. Electrical heating elements are made of SiC with a small excess of silicon which facilitates sintering and reacts with the tar binder producing secondary SiC. Materials bound with oxide or nitride bonds (see below) are used as refractory structural materials in the construction of furnaces... 

Figure 10.11 Construction of furnace insulation from rigid board insulation material. Source Reprinted from Technical literature of Fiber Material Inc., Biddeford, Maine, USA.

In practice other factors than the conductivity of the material are important for the construction of furnaces. Thus zirconia is sensitive to mechanical and thermal shock. For instance its elastic modulus is only 15.000 kg/mm at 1500 K and its fractiire strength 5 kg/mm at the same temperature. 

The development of integral furnace boilers during the 1930s allowed walls constructed of banks of vertical tubes welded together to form a continuous membrane (membrane walls or water walls), which provided for simultaneous heat transfer from the furnace and a furnace water cooling surface. (Earlier WT boilers had completely separate cooling systems.)... 

Thermal conductivity and heat capacity In practical applications, refractory materials processing high thermal capacity as well as low thermal conductivity are required, depending upon (of course) the functional requirements. In most situations, a refractory that serves as a furnace wall should have a low thermal conductivity in order to retain as much as heat as possible. However, a refractory used in the construction of the walls of muffles or retorts or coke ovens should have a high thermal conductivity in order to transmit as much heat as possible to the interior. The charge remains separated from flame in these specific examples of installations. 

The temperature of the furnace is measured by a thermometer placed alongside the glass tube inside the furnace. The temperature should be 330-340° except in the regions about 10 cm. from each end of the furnace here the temperatures will be 300-340°, depending on the construction of the furnace. 

The FPL vertical wall furnace used in our study was described in some detail by Brenden and Chamberlain (6). This furnace is normally used to evaluate the fire endurance of wall assemblies. The basic guidelines for the furnace test method are given in the ASTM E-119 standard (5). The method was designed to evaluate the ability of a structure to withstand a standard fire exposure that simulates a fully developed fire. The furnace is gas fired, and its temperature is controlled to follow a standard time-temperature curve. A load may be applied to the assembly. The failure criterion can be taken as time at burnthrough, structural failure, or a specified temperature rise on the unexposed side of the wall—whichever comes first. The construction of the furnace is not specified in the ASTM E-119 standard. 

As far as the equipment is concerned, the miniaturization of many components has lead to construction of smaller apparatus without sacrificing the accuracy, efficiency or versatility. Of importance for this development was also the availability of better and new construction materials, e.g. for measuring heads, thermocouples, furnaces etc. 

Apparently a vault had been constructed around the superstructure of the furnace. It was 4.5 x 6 m in cross section and 4 m high. It was constructed of 20-cm-wide flange beams on 0.8 m (front) and 1 m (sides) centers faced on both sides with 10-mm steel plates and filled with dry sand. The explosion displaced the walls about 6 cm. The top of the vault which was supported by a 30-cm beam on top of a 20-cm beam was deflected about 8 cm. Steel doors mounted to slide in angle-iron tracks were blown off. A number of 1.9-cm bolts holding the door tracks were sheared cleanly. The furnace was driven down into the concrete floor and a penetration of some 10 cm was measured. 

Its high melting point also makes it useful for metal electrodes in glassmaking furnaces. Molybdenum s high resistance to electricity makes it useful in high-temperature filament wires and in the construction of parts for missiles, spacecrafts, and nuclear power generators. 

Platinum metal and its alloys have numerous applications. As a precious metal it is used extensively in jewelry. Other important applications include construction of laboratory crucibles and high temperature electric furnaces in instruments as thermocouple elements as wire for electrical contacts as electrodes in dentistry in cigarette lighters and for coating missile and jet engine parts. 

In the introduction to his German translation of P. M. de Respour s Special Experiments on die Mineral Spirit, Henckel mentioned in 1743 diat In our smelting furnaces at Freyberg we have obtained the essence of zinc [zinckische Wesen] in power but not in form (48). He believed that dieir failure to obtain "corporal [metallic] zinc must have been due to die complex nature of dieir ore, to the construction of their furnaces, and to die long-continued heating, which made it impossible for the phoenix, even when resurrected from its ash, to withstand die fire (48). 

Granular materials are shipped raw or calcined and usually have been ground to a specified screen size or size distribution. The additives depend on the application and service conditions. These materials are used in construction, repair, or maintenance of furnaces and vessels. Refractory mortars are used to lay brick of the same composition. These are manufactured wet premixed or dry. 

The temperature of the furnace is measured by a thermocouple which can be moved to various positions in the thermocouple well. The catalyst temperature should be maintained at 325-345°, although it may be as low as 320° at the ends of the catalyst zone, depending unon the construction of the furnace. 

The construction of DTA apparatus is simple and consists of a furnace, differential thermocouple, temperature thermocouple, specimen holders, temperature programmer and recorder. The schematic of a typical DTA apparatus is shown in Figure 3.5. 

The higher boiling phenols, present in considerable amounts in CVR and low temperature tars, are corrosive to mild steel, especially above 300°C. Cast iron, chrome steel, and stainless steel are more resistant. Furnace tubes, the insides of fractionating columns, and the rotors of pumps handling hot pitch and base tar are generally constructed of these metals. Nevertheless, to ensure satisfactory furnace tube life, particularly in plants processing CVR or low temperature tars, the tube temperature should be kept to a minimum. 

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