Thermal characteristics of refractories in channel induction furnaces.

The fuel crunch of the late 1970s had most foundrymen conserving energy and minimizing power plant operation costs. One area of improvement was to minimize heat loss by increasing refractory linings in channel induction furnaces.

Since then, foundrymen have learned that simply adding more insulation to the backup layers of a lining design may actually increase overall furnace costs. It soon became evident that the lining and operating parameters must be carefully considered to achieve consistent refractory efficiencies.

The intent of this article is to provide a relative comparison of the most common lining designs for a 30-ton vertical channel induction furnace that will minimize heat loss and maximize refractory performance.

The furnace operating parameters assumed for the calculations included a 275OF hot-face temperature, zero wind velocity and an 8OF ambient temperature for the atmosphere surrounding the furnace.

As one or several of these parameters change, so will the thermal characteristics of the refractory. However, it must be stressed that the refractory designs illustrated may not apply to all furnaces. As with all refractory applications, the channel furnace operator must continually work with the refractory and furnace suppliers for optimum lining design configurations. The "K" Factor

Most materials can be tested to determine their thermal characteristics. The unit of measure assigned for this role is known as "K," or an approximation of mean heat flow. The unit of measure for K is referred to as the coefficient of thermal conductivity of the material. in the U.S., the unit K is generally measured as Btu in./hr ft[sup.2[deg.] F. This value will change for any given material as temperature changes.

The K factor of refractories and other materials can be measured by several different test methods, but since each test method yields minor differences in their results, the value of K must be an approximation. The refractories industry is working to standardize one method as its official test.

Not only will different test methods yield minimal differences for thermal conductivity, but the physical properties of the refractory will have an effect. For example, since the density of a refractory greatly affects its thermal conductivity, it is easy to understand that a laboratory test specimen of a dry vibratable or castable refractory may have a different K value from the same material installed to a different density in another unit. Thermal Conductivity of Refractories

Along with the physical properties of a refractory, raw materials also will affect thermal conductivity. Therefore, because the refractories that comprise a channel induction furnace lining vary greatly in density and chemistry, it is obvious that they will differ widely relative to thermal conductivity.

Figure 1 compares the various refractories generally used as portions of channel furnace backup linings. These materials are better insulators with lower K factors than the hot-face refractories graphed in Fig. 2. Alumina-chrome refractories are not represented because their K factors are only negligibly different from 90% alumina materials of the same refractory types. Lining Design Cautions

Foundrymen today are as serious about conserving power by minimizing heat loss through a refractory lining as were foundrymen during the previous energy crisis. Adding insulation to the backup layers of a refractory lining will lower heat loss, effectively control the temperature of the metal, and increase melt rates and possibly refractory life due to improved consistency of the furnace atmosphere.

Caution should be exercised, however, against over insulating a furnace lining. increasing the mean temperature of a hot-face refractory beyond an optimum point will generally cause one or more detrimental reactions to occur.

With hot-face refractory brick, raising the mean temperature by adding insulation to the backup lining may increase the fluidity of the slag. The resultant deeper penetration of slag and metal into the refractory hot face will cause increased corrosion and spalling problems.

These failure modes may be more pronounced in dry vibratables and calcium-aluminate-bonded castables due to the bond mechanisms of these materials fluxing the refractory at higher temperatures.

Certain insulating materials are not suited for channel induction furnace construction. Primarily through experience, refractory suppliers have learned the limits of some insulating materials. This is due to the relatively high compressibility of these materials. Felts and blanket insulations are not suggested as backup to sidewalls or floors because they are highly compressible and, consequently, weaken the structure of the hot-face refractory.

In addition, a maximum 1-in. thickness of dense insulating board should be used for sidewalls and floors because experience has shown that refractory structural failures may occur with greater thicknesses. Basic Refractory Design

Uninsulated refractory sidewalls generally yield a furnace shell temperature of about 65OF for a new lining. Comparatively, new linings with common thicknesses of insulation show sidewall shell temperatures ranging from 450-550F. As the hot-face refractory wears, or is penetrated by metal and slag, shell temperatures will gradually increase.

Experienced foundrymen can usually gauge the deterioration of a channel furnace lining by monitoring the change of shell temperature throughout a refractory lining campaign.

When designing a refractory lining and considering heat loss, most refractory suppliers prefer that the freeze plane of the metal remain in the hot-face material. However, it should be remembered that the freeze plane depth will change with time, as does shell temperature, due to the hot-face refractory deteriorating or penetrating.

Six common lining designs are featured in Figs. 3-8. These designs can be compared regarding heat flow characteristics. Included are three wall, three floor and two roof designs that use popular hot-face refractories and varying backup refractory options. A qualified refractory supplier is capable of reviewing many other design possibilities for a particular channel induction furnace application. Optimum Furnace Lining

It is often difficult to arrive at the "best" refractory and lining design for a channel induction furnace because so many variables must be considered. Accurate records of furnace refractory histories should be maintained to use as guides when considering new refractory designs. Important information will include: * operating temperatures and changes; *slag chemistries; * furnace deslagging frequency; * bath additives; * back charge of treated metal; * refractory maintenance programs; * previously used refractories and lining designs; * production process changes; * in-house or contracted lining construction; * personal preference of workers.

Combining this information along with the referencing refractory experiences of other foundries, a supplier can suggest the lining material and design best suited to a particular application.