Crucible Furnaces – Improving Efficiency with Emisshield Coatings

Ever increasing fuel costs are in turn increasing the operating costs for crucible furnace operators. In addition to high fuel costs, crucible maintenance and replacement expenses contribute to expenditure, reducing profitability. EMISSHIELD family of high emissivity coatings that reduce fuel costs as well as crucible maintenance and replacement costs are therefore an ideal solution for crucible furnace operators.

EMISSHIELD

EMISSHIELD high emissivity coatings are manufactured by Wessex, Inc. EMISSHIELD technology was developed and patented by NASA for use in space vehicles (Figure 1). Combining this technology with its own patented binder systems, Wessex manufactures high emissivity coatings for industrial applications. EMISSHIELD coatings adhere well to dense refractories, refractory ceramic fibre, insulating fire brick and many metals. Coating the crucibles with EMISSHIELD results in even heating, longer crucible life, fuel savings and increased productivity.

Figure 1. X-33 Orbiter

Coating Crucible Furnace Refractories

Generally, dense refractories used in crucible furnaces (Figure 2) reduce heat loss, but also act as a heat sink, and lead to loss of valuable energy during furnace operation.

Figure 2. Crucible furnace

With a high emissivity EMISSHIELD coating (Figure 3), heat energy radiated from hot furnace gases and the burners is absorbed by the coating and re-radiated back to the crucible, which is cooler. The crucible absorbs this re-radiated heat and conducts it to the load placed in it. Consequently, the refractory lining is cooler and contains less thermal energy.

Figure 3. Crucible furnace with EMISSHIELD coating applied to the refractory hot face.

If T_C is the temperature of the EMISSHIELD coating and T_L that of the crucible and its contents, then the amount of heat transferred by re-radiation from the coating is given by the equation:

where Q = re-radiated heat absorbed by the furnace load
        E_w = emissivity of the coating and
         ó = Stefan-Boltzmann constant

As the temperatures are raised to the fourth power, it is evident that the coating would absorb and re-radiate maximum heat energy when the temperature difference between the coating and the load is maximum. It has to be noted that EMISSHIELD can be effective only if the temperature of the coating surface is greater than that of the furnace load.

By coating the exterior of the crucible with EMISSHIELD, it is possible to make the crucible absorb more radiant energy (Figure 4). The coating does absorb most of the heat energy radiated from the coated refractory, but there is no cooler body to which this energy can be transferred from the coated crucible. As a result, the additional heat energy absorbed by the coated crucible will be conducted by the crucible and the coating to the furnace load placed in the crucible.

Figure 4. Exterior of the crucible coated with EMISSHIELD

Coating Metallic Crucibles

In the absence of any coating, the amount of heat energy conducted by the crucible wall depends on the thermal conductivity of the crucible alloy (Figure 5). The magnitude of heat flux transferred is determined by the difference between the temperature of the hot face of the crucible and that of the cold face and the load.

Figure 5. Heat flux and thermal transfer in an uncoated crucible.

In a crucible furnace, oxidation may often cause insulating scales to be formed on the hot face of the crucible. These scales reduce the crucible’s ability to absorb heat energy and transfer it to load. Removal of scale is essential for the efficient operation of the furnace. However, repeated removal results in considerable maintenance expenses and production loss. Besides, it also causes progressive wear of the crucible and shortens its service life.

When EMISSHIELD M-series coatings are fired above 1500°F, they form ceramic glazes free from pinholes, thus protecting the crucible from the adverse effects of oxidation. These coatings increase the amount of heat energy absorbed at the hot face, thereby increasing the heat flux as well as the amount of heat energy transferred to the crucible’s contents (Figure 6). Similar benefits can be achieved when EMISSHIELD ST-series coatings are used in ceramic and refractory crucibles.

Figure 6. Heat flux and thermal transfer in a crucible coated with EMISSHIELD

EMISSHIELD Use in Crucible Furnaces

The benefits obtained by using EMISSHIELD greatly depend upon the design and operating parameters of the crucible furnace. The emissivity (E_w) of an uncoated refractory lies between 0.3 and 0.5, at standard operating temperatures of the furnace. Applying EMISSHIELD to a refractory increases the refractory’s emissivity to nearly 0.9. With the coating absorbing and re-radiating close to 90% of the heat energy, the amount of radiant heat that is conducted to the load and absorbed by it is determined primarily by the emissivity of the crucible.

From the equation for radiation heat transfer, it can be readily seen that the heat (Q) absorbed by the furnace load can be increased significantly by increasing the emissivity (E_w) of the refractory. This means that after EMISSHIELD is applied to the refractory, the crucible will reach a higher temperature with the same fuel usage, thereby resulting in improved productivity. If a higher crucible temperature is not required, the temperature can be reduced by turning down the burners in order to achieve the same heat energy that was earlier attained when there was no coating. This allows very high production levels to be achieved with low fuel consumption.

As described, furnace efficiency can be enhanced by applying EMISSHIELD coatings, which are designed for metals, to the crucible itself. In addition to further reducing fuel usage, EMISSHIELD coatings applied on crucibles shorten cycle times.

Expected Benefits of EMISSHIELD Use in Crucible Furnaces

EMISSHIELD coatings on crucible furnace refractories absorb more heat from hot furnace gases and burner flames compared to uncoated refractories. Heat absorbed by the coating is re-radiated to the crucible, making more heat energy available in the crucible. The flue gas temperature rises faster, resulting in significant energy savings.

When the crucible furnace is operated in batch mode, the reduction in absorbed heat energy by the refractories allows quicker heat-ups as well as cool-downs. When operated continuously, the contents of the crucible reach the operating temperature faster as a result of EMISSHIELD use. The recovery time of the furnace can be expected to be shorter. Furnace downtime and cycle time can be expected to decrease as well.

As less heat is absorbed by the refractory lining and stored, refractory materials stay at a relatively lower temperature. Consequently, they are subjected to less thermally-induced stress and thermal shock. As a result, longer refractory life can be expected.

When EMISSHIELD is applied to crucibles in addition to refractories, more productivity and fuel savings can be expected. Moreover, non-operational cost savings including that associated with crucible procurement and maintenance can be gained.

This information has been sourced, reviewed and adapted from materials provided by ANH Refractories Europe.

For more information on this source please visit ANH Refractories Europe.