High-Emissivity CoatingsEmissivity is an indicator of how much heat a material can absorb, and if conditions are right reradiate to a cooler body. The emissivity scale ranges from 0 to 1, where 1 is a theoretical black body that absorbs 100% of the energy to which it is exposed and 0 is a perfect reflector that absorbs no energy. The coatings used in the examples reported here had emissivities of 0.85 to 0.92 at use temperature, which can be over two times greater than the high-temperature emissivities of most refractory and metal substrates. The amount of heat reradiated from a high-emissivity coating is predicted by the following equation:
Q = Ew • σ • (TC4–TL4) Where: Q = reradiated energy Ew = emissivity of the coating σ = Stefan-Boltzmann constant TC = coating temperature TL = load temperature
The application of a high-emissivity coating to refractory increases the emissivity (Ew) of the furnace lining, which increases the amount of energy radiated from the refractory (Q). As long as the temperature of the ceramics being fired, the ingots being reheated or the metal being annealed or heat treated (TL) is less that the temperature of the coated refractory (TC), Q will be positive and the energy absorbed by the coating will be reradiated to and absorbed by the furnace load. It is important to note that the reradiation of the absorbed energy is possible only when a gap between the coating and the absorbing load exists. High-emissivity coatings are not effective when they are in contact with molten metal or slag or in processes where the coating will become covered with slag, mill scale, dust or process materials. These coatings are not insulating and should never be applied between layers of refractory in multi-component refractory linings.
Case HistoriesHigh-Temperature Tunnel Kiln
A high-emissivity coating compatible with the fireclay refractory lining was sprayed in the preheat and soak zones of this kiln used to fire refractories at 1400-1520°C (2550-2770°F). The refractory was cracked and spalled in some places, but the surface was otherwise strong and not friable. This is not an unusual condition for linings in refractory kilns. Figure 1 shows brick that have been successfully coated, even though they are spalled, cracked and corroded. Since the coating is applied at a thickness of only 8 mils, it will not fill cracks or repair any other damage to the lining. Loose dust and debris were brushed from the refractory before applying the coating. The EMISSHIELD® coating was sprayed on the walls and crown above the kiln car deck elevation from the first burner in the preheat zone to the last burner in the soak zone. The cooling zone was not sprayed since the brick in this zone are hotter than the furnace walls and no benefit from the coating would be expected.
When the kiln was put back into service, the radiant heat increase due to the coating resulted in the kiln load being overfired by 20-35°C, as indicated by pyrometric cones placed on the brick hacks. The burners were turned down to return the load to the desired temperature, which resulted in lower gas usage. Initially, the coated kiln was run at the maximum push rate in order to replace depleted inventory. For this month a 22% fuel savings was achieved. In subsequent months as lower temperature burns and slower push rates were used, fuel savings of at least 8% were reported. The average fuel savings for the first year of operation was 16%.
Before the coating was applied, it was not unusual to have about 5% of the kiln load under-fired. The high-emissivity coating turned the entire kiln lining into a heat radiator, and a more even temperature distribution was achieved. Underburning was eliminated, and productivity was effectively raised by 5%.
The preheat and soak zones of a tunnel kiln firing face brick at 1000-1050°C (1832-1922°F) was sprayed with EMISSHIELD®. The refractory was in excellent condition and dust removal was the only surface preparation required (Fig. 2). When put back into service, the kiln operator noted that the crown refractory expansion was slower and less than expected. This is common in kilns coated with high-emissivity coatings since the coating is directing more heat into the furnace load rather than being absorbed by the refractory.
Rather than turning down the burners to accommodate the increased heat being reradiated from the coating, the kiln operator chose to increase the car push by up to 14% to gain higher production. Faster car pushes beyond this level had an adverse effect on the brick color. Even at this higher production rate, the burners could be turned down slightly so that an additional 8-10% natural-gas reduction was experienced.
An EMISSHIELD® product designed to adhere to refractory ceramic fiber (RCF) was applied to a periodic kiln firing face brick. In batch-type kilns, most of the energy expended is during the heat-up portion of the firing schedule where the load and furnace refractory are absorbing heat. The refractory absorption can be reduced considerably by lining the furnace with low mass RCF, as had been done with this kiln, but the application of a high-emissivity coating to the hot face of the RCF presents additional opportunities to save energy. After the coating was applied, the time to reach the first hold temperature of 800°C (1472°F) was reduced by over 22%. The ramp time between this hold and the top hold temperature of 1100°C (2012°F) was reduced by 7%. These time reductions resulted in an overall reduction in cycle time of 10-13%, increasing furnace availability and reducing fuel cost.
A furnace used to heat treat high-alloy castings at 1220°C (2230°F) was sprayed with EMISSHIELD® on all interior refractory surfaces except the hearth (Fig. 5). After the coating was applied, the burner cycling dropped from 25-30% active to 5-10% active – a 15% fuel savings. When inserting a cold load into the hot furnace, the top temperature recovered 30% faster, which allowed an additional one to two heats per day to be run. After coating, the temperature of the furnace shell was 15-20°C cooler than before the coating was applied.
Ideally, radiant tubes should be coated on the inside and outside surfaces (Fig. 7). The coating on the inside absorbs energy from the burner flame. Because there is no cooler body within the tube to absorb reradiated energy, the energy absorbed by the coating is conducted through the tube and into the coating on the outside of the tube, where it is radiated to the cooler furnace load. The increased absorption by the coating within the tube and the increased radiation on the outer surface coating increases the heat flux through the tube wall compared to an uncoated tube. This increases heat flow through the tube by 7-15%. A high-emissivity coating recommended for radiant tubes – EMISSHIELD® M-1 – has the added advantage of firing to a pinhole-free glaze that protects the tube from corrosion and prevents insulating oxidation from forming.
ConclusionsHigh-emissivity coatings can lower fuel costs, increase productivity and improve quality in many firing and heat-treat furnaces. Care must be taken to assure that a thermal gradient is maintained between the coated refractory and the furnace load and that process materials do not adhere to the coating. IH
For more information: Thomas Kleeb is Manager – Product Technology for ANH Refractories Company, 400 Fairway Drive, Moon Twp., PA 15108; tel: 412-375-6901; fax: 412-375-6783; e-mail: firstname.lastname@example.org. John Olver is president of Wessex Incorporated, 2000 Kraft Drive, Suite 2600, Blacksburg, VA 24060; tel: 540-961-0999; fax: 54-961-5808; e-mail: email@example.com
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