Construction Materials
The construction materials used in building combustion chambers for conversion burners should consist of the best grade of insulating firebrick and a good, high-temperature refractory mortar.
Insulating firebricks are available in different standard shapes and sizes for residential, commercial, and industrial applications. Their primary function is to provide a lining capable of withstanding the high temperatures in furnace and boiler combustion chambers, flues, chimneys, stacks, and fireplaces. To function in these high temperature environments, firebricks have upper-limit melting points that range from 2800°F (1540°C) for a brick made of fire- clay to 4000°F (2200°C) for one made of silicon carbide.
Firebrick materials
The materials used in firebricks should make them capable of withstanding not only high temperatures, but also slag chemicals and spalling (i.e., flaking) under temperature changes. Firebricks are made from fire clay or kaolin, silica (silicon car- bide), alumina, and magnesite (magnesia).
• Clay firebricks. Firebricks made from fire clay or kaolin are the most common type, but they are expensive and not good insulators. The principal advantage of the clay firebrick is that it creates a thin, lightweight combustion chamber wall. This results in a quick rise of temperature that produces fuel savings. On the down side, clay firebricks have a relatively low melting point of 1600°F (871°C).
• Silica firebricks. Silica firebricks retain their strength at high temperatures and do not react with ash, which makes them resistant to deposits. On the other hand, they are subject to spalling (flaking). They are also very expensive.
• High alumina firebricks. High alumina firebricks are capable of withstanding both high load deformation levels and high temperatures.
• Magnesite (Magnesia) firebricks. Magnesite firebricks resist the effects of alkalis.
Note:
Modern firebricks are asbestos free. If you have stocks of older firebricks containing asbestos do NOT use them, Contact a local EPA office for instructions about their safe disposal.
Firebricks can be machined or drilled at the factory in custom shapes and sizes to meet the requirements of specific applications. To mention only a few possibilities, custom firebricks are available with machined grooves, tapers, radii, tongue-and-groove edges, and drilled holes.
Insulating firebricks are produced in different grades to meet the requirements of specific applications. The temperature use limits are an important factor in making a selection. Another important factor to consider when selecting a suitable firebrick is its mean temperature; that is to say, the temperature at the midpoint of the firebrick. These and other specifications for each grade of firebrick are provided by the refractory material manufacturer.
Always use the refractory mortar furnished or recommended by the brick manufacturer for cementing the insulating firebrick. The mortar should be thinned to the consistency of a very thick cream before it is used. The usual method for applying the mortar is to dip each brick into it and set the brick in place as you lay each course.
Note:
Additional support is possible by using metal anchors project- ing from the metal casing.
The shape of the bricks is also important, with arch or circled brick where 41⁄2-inch wall thickness is preferred. The mortar-filled joints between the bricks should be not more than 1⁄8 inch.
The insulating firebricks are usually backed by a second, separate insulating layer, which may consist of one of the following materials:
1. Common brick
2. Magnesia block insulation
3. Hard firebrick
4. Expanded mica products (e.g., vermiculite or zonolite)
5. Dry sand
6. Dry sand mixed with an expanded mica product
The spaces between the outer edges of the firebrick should be filled with high-temperature refractory mortar and small pieces of firebrick to obtain firm construction and prevent infiltration of vapors through the wall.