Brick refractory walls

Many older heaters have massive brick refractory walls. The weight of these walls greatly exceeds the weight of the heater s tubes. The massive brick refractory walls store a great deal of heat. This creates several operating problems. One such problem is that it takes many hours to bring such a heater up to its normal operating temperature. However, a far more serious problem occurs when the process flow to a heater is interrupted. 

Modern technology has come a long way in mitigating such problems. In new heaters, lightweight ceramic tiles, rather than massive brick refractory walls, are the norm. These ceramic tiles do not store very much heat. Hence, when the heater process flow is reduced or lost, as long as the fuel flow is quickly curtailed, the tubes tend not to overheat. 

The predicted effect of load emissivity, combustion space size, and refractory emissivity for a particular furnace and load is shown in Fig. 18.46 [197], A furnace 5 m long by 1 m high by 1 m wide was loaded with a 0.15-m-thick sheet of iron, while the refractory walls were constructed of 0.5-m-thick red clay brick. The methane burners fired at a rate of 500 kW during operation. Additional process parameters and thermophysical properties are listed in [197]. 

Monolithic refractories have lower thermal expansion than most refractory bricks. Whatever small expansion does occur can usually be absorbed by the supports. Therefore, unlike refractory bricks, monolithic refractory walls do not require clearances for thermal expansion. Clearances required for brick construction may allow passage for furnace gas leaks out or air into a furnace. The superior sealing capability and reduced expansion of monolithic refractories make them suitable for higher furnace... 

Newer heaters typically have thin reflective tiles, rather than massive refractory brick walls. Such newer heaters will heat up more rapidly. Also, the process fluid outlet temperature responds more rapidly to changes in the firing rate. This improves the heater outlet temperature control. Perhaps for this reason, it seems that heaters with reflective refractory walls are less subject to process tube coking and shortened heater run lengths. There is also a process, called "alonizing," that increases the reflectivity of older brick refractory heater walls. 

Corrosion of refractories by melts takes place in reduction pots and the cast house. The melt of liquid metal or the bath contacts the surface of the refractory wall, made from bricks or from castables. The chemical interaction between the constituents of the melt and the constituents of the refractory takes place, and all chemical principles of interaction between liquid and sohd reactants should be taken into account. The chemical nature of reactants (acid-base) may also be a factor. 

Incinerator designers have used 70% AI2O3 brick for years in the primary and secondary combustion chambers of the kilns. For a long time, the 70% AI2O3 bricks would experience rapid wear in the fireball regions of the secondary combustion chamber where conditions reach in excess of 1750°C. This is also an area where corrosive lime-alumina-iron oxide-silica slags coat the refractory walls. 

The heating walls have traditionally been constructed of silica brick refractories (see Chapter 6). Silica is the refractory of choice primarily because, at normal coke battery operating temperatures, silica refractories are subject to minimal creep. Also, since nearly all of the expansion of silica brick takes place below about 650°C, during normal operation of a battery, the moderate temperature fluctuations of the walls have no effect on the volume stability of the refractory comprising the wall. 

Therefore, the thermocouple was measuring the relatively cool zone inside the refractory wall, rather than the far hotter zone in the combustion chamber. 1 should have paid closer attention to the physical appearance of the chamber. Bricks glowing bright red are radiating heat at about 1500°F. Bricks glowing a dazzling white are radiating heat above 2800 F (see Table 25.1). 

A typical large three-phase ferroalloy furnace using prebaked carbon electrodes is shown in Eigure 4. The hearth and lower walls where molten materials come in contact with refractories are usually composed of carbon blocks backed by safety courses of brick. In the upper section, where the refractories are not exposed to the higher temperatures, superduty or regular firebrick may be used. The walls of the shell also may be water-cooled for extended life. Usually, the furnace shell is elevated and supported on beams or on concrete piers to allow ventilation of the bottom. When normal ventilation is insufficient, blowers are added to remove the heat more rapidly. The shell also may rest on a turntable so that it can be oscillated slightly more than 120° at a speed equivalent to 0.25—1 revolution per day in order to equalize refractory erosion or bottom buildup. 

The refractory used to constmct the hearth can be in the form of bricks, preformed shapes, or monolithic. Often a furnace design utilizes all three. Openings or passageways through the walls are fashioned in the same manner as windows in a brick building. 

Furnace Design. Modem carbide furnaces have capacities ranging from 45,000 t/yr (20 MW) to 180,000 t/yr (70 MW). A cross-section of a 40 MW furnace, constmcted in 1981, having a 300 t/d capacity is shown in Figure 2. The shell consists of reinforced steel side walls and bottom. Shell diameter is about 9 m and the height to diameter ratio is shallow at 0.25 1.0. The walls have a refractory lining of 0.7 m and the bottom has a 1-m layer of brick topped by a 1.5-m layer of prebaked carbon blocks. The steel shell is supported on concrete piers and cooling air is blown across the shell bottom. A taphole to withdraw the Hquid carbide is located at the top of the carbon blocks. 

Refractories. Its low coefficient of expansion, high thermal conductivity, and general chemical and physical stabihty make sihcon carbide a valuable material for refractory use. Suitable apphcations for sihcon carbide refractory shapes include boiler furnace walls, checker bricks, mufflers, kiln furniture, furnace skid rails, trays for zinc purification plants, etc (see Refractories). 

In North America, a special, high conductivity, low permeability, "hot-pressed" carbon brick is utilized almost exclusively for hearth walls. Because of their relatively small size and special, heat setting resin cement, and because the brick is installed tightly against the cooled jacket or stave, differential thermal expansion can be accommodated without refractory cracking and effective cooling can be maintained. Additionally, the wall thickness is generally smaller than 1 m, which promotes the easy formation of a protective skull of frozen materials on its hot face. Thus hearth wall problems and breakouts because of carbon wall refractory failure are virtually nonexistent. 

The basic construction consists of a rectangular or cylindrical steel chamber, lined with refractory bricks. Tubes are arranged around the wall, in either horizontal or vertical banks. The fluid to be heated flows through the tubes. Typical layouts are shown in Figure 12.69a, b and c. A more detailed diagram of a pyrolysis furnace is given in Figure 12.70. 

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