When Kanthal set out to further develop its gas burner systems, its criteria were clearly defined, including high efficiency, low maintenance, low energy costs, reduced NOx emissions and adaptable design.

Fig. 9. Typical furnace installation of Ecothal SER burners


Fig. 1. Running cost comparison shows 35% savings

Meeting the criteria has led to the extensive development and introduction of the new Ecothal® SER gas burner. Its high efficiency means reduced CO2 emissions and, hence, significantly reduced greenhouse gases.

The system for use on furnaces combines high efficiency, low operating costs, reliability and low emissions by using high-temperature materials and innovative burner design to create an optimum air, gas and exhaust flow. A single-ended recuperative burner, Ecothal SER, is the product of an extensive research and development program, including operational testing, carried out by the company, in addition to independent testing and evaluation by customers. All tests confirm the system to be efficient enough to achieve energy savings in excess of 35% (Fig. 1). It offers efficiencies in excess of 80% – a figure independently verified by the German TÜV standards organization, which is 10–20% higher than conventional systems – and is extremely cost effective compared with other recuperative systems.

Because the operating system optimizes the air/gas and exhaust flow, it also significantly reduces the NOx emissions. Extensive testing of the burner shows that a NOx level of less than 50ppm is achieved as standard, compared to around 200ppm with conventional systems (Fig. 3). This means a NOx-emission reduction potential of approximately 75%, which is well below the existing TA-Luft standard regulation of 170-270ppm. It is also possible to reduce the number of burners used on a furnace, typically 12 down to 10, in addition to the burner being able to run at a lower temperature because of operational efficiency.

Fig. 2. Cut-away clearly shows the burner position within the radiating tube and the flame tube.

Burner Description and Performance

Ecothal SER burner design incorporates a glow-plug ignition mounted in the exhaust outlet for the entry model with a spark-ignition model also available. The UV model, which often requires a redesign of the existing electrical and gas supply system, is available where the requirement is for more sophisticated control, regulation and monitoring.

The single-ended recuperative gas-burner system, or SER system, is sometimes compared with regenerative burner systems. They both transfer energy from outgoing exhaust gases back into incoming combustion air. The main difference is how this is done, however, which also has a big influence on heat loss via thermal conductivity through the furnace walls and roof.

In the case of the SER system, a counterflow heat exchanger positioned concentrically in the protective radiant tube in the furnace insulation passage transfers the energy of the outgoing exhaust gases outside the recuperator back through the recuperator wall, where it is picked up by the incoming combustion air. The major advantage of the SER system over a regenerative system is that it only requires access to the furnace from one side, which allows it to facilitate horizontal as well as vertical heating systems.

An additional benefit of the SER system is that it can replace not only straight-through burner designs but also old SER and regenerative systems on existing furnaces as a result of its ability to work at higher energy densities.

Fig. 3. Typical NOx emissions of different burner systems. Ecothal SER burner is well below 350 mg/m³.

The burner system is based on a recuperator taking care of energy recovery from the exhaust/flue gas leaving the furnace chamber. The recuperator ends up in the flame holder/nozzle, and the flame is thrown into the inner flame tube in such a way as to ensure a uniform temperature profile throughout the length of the tube. At the far end of the inner flame tube, the flame is transferred between the inner and outer tube back into the recuperator area.

Up to this stage, the operation is similar to any SER system, but the development has been to bring down the peak temperatures during the combustion phase, thereby reducing the formation of NOx. In parallel with this, the delay in oxidation of CO has been minimized. This results in a burner capable of running at much lower levels of air/oxygen, which in turn directly contributes to the fuel efficiency of the system. If the excess of air is increased with this system there is only a small increase in the formation of NOx.

As a result of this achievement, the burner system is extremely forgiving in operation. It accommodates a big turndown ratio, which in effect means it easily handles variations in gas composition as well as in gas and air supply pressures.

A burner system that does not need design alterations for different fuel/gas mixtures is also less sensitive to normal gas quality variations as well as local variations inherited by the design of the existing gas and air train at the furnace. These can affect emissions, flame stability and position. Having a burner system that has a stable operation under all normal fluctuations and incorporates a large turndown ratio in order to accommodate one design allows flexibility for adjusting power levels without additional costs.

Radiant Tube Material

Over 20 years ago, a significant develop-ment took place with the introduction of a new tube material by Kanthal using a powder metallic (PM) Fe-Cr-Al alloy. PM radiant tubes mean the system is capable of far higher operating temperatures than if FeNiCr tubes were used. They are produced by an advanced PM process combining extrusion and dispersion strengthening. This makes them acceptable for use in a wide range of temperatures, power densities and atmospheres, covering a comprehensive range of processes and applications. The performance of this material encouraged further material development, resulting in the introduction of a new alloy with primary focus on tube applications. By adding molybdenum, among other alloys, the creep strength and time to rupture were increased significantly without any major reduction in oxidation or corrosion properties of the tubes, which are referred to as Kanthal APMT tubes (Fig 4).

Fig. 5. Comparison of carbon buildup

The tubes work well at temperature up to 2282°F (1250°C), are very form stable, resistant to bending and other forms of creep deformation, and do not exhibit the dramatic decrease in strength that FeNiCr tubes show at such elevated temperatures. Even at this high temperature, oxidation is still not a big concern due to the more stable aluminum-oxide layer formed on the tube surface. It is non-spalling, which means no scaling and no resultant contamination of the heat-treated goods within the furnace in addition to reduced risk of burner clogging.

Besides oxidation, the aluminum oxide provides excellent corrosion resistance to carburizing and sulphidizing atmospheres and also outperforms FeNiCr alloys in high carbon-potential atmospheres where coking, carbon buildup and metal dusting can be problematic (Fig. 5).

Capable of far-higher operating temperatures than FeNiCr tubes, the Kanthal PM radiant tubes allow system manufacturers to exploit the higher outputs of modern heating designs, making it possible to dissipate the same power input with fewer tubes or utilize the loading potential for productivity increases.

Fig. 7. The burner is produced according to customer specifications and available for (a.) horizontal or (b.) vertical installation.

For example, a gas-fired SER system using PM tubes could operate at twice the power of a similar system using FeNiCr tubes. The tubes for SER applications are supplied ready-to-install with the flange and bottom welded to the outer tube and distance spacers to the inner tube.

The inherent strength of the tubes means that the burner assembly can be installed either vertically or horizontally within the furnace (Fig. 7). In this form they are able to withstand all the rigors of manual handling and remain fully operational even if the tubes are dropped on the shop floor during installation. Although not recommended, it does clearly demonstrate the substantial quality of the complete assembly.

It is this strength that allows the tube to maintain its shape in use at elevated temperatures and over long periods of time. This is particularly the case when the burner is installed horizontally within a furnace. There is little or no sign of bowing even after extended operational use. When compared to a standard SRP burner tube, the bowing would not only be clearly visible, it would be substantial (Fig. 6).

Fig. 6. Sagging test results graph shows APM and APMT tube’s resistance to sagging.

Reliability and Easy Upgrade

Their adaptability means the burners can be used to upgrade existing furnaces with the latest technology, bringing operational savings, while just as easily being featuring in new furnace designs.

Of a modular design, Ecothal SER burners make installation easy and significantly reduce maintenance due to their robust metal construction, which also makes for ease of handling. Typically, lead time for a total system is around four to six weeks, while upgrading a typical heat-treatment furnace takes around two to three days. Payback on the system can be achieved within one year. In addition, pollution is minimized and, hence, your carbon footprint is reduced.

Applications include the use on furnaces in heat-treatment and hardening shops, toolrooms, automotive-component manufacturers, continuous-annealing and galvanizing lines and anywhere utilizing furnace technology (Fig. 9).

Reduced Noise Level

Ecothal also reduces the operational noise level by a factor of up to 8 when compared to other systems. At less than 85dBA, levels of 75–80 dBA are achievable where required.

Maintenance

The robust design and all-metallic construction of the system ensure reliable and virtually maintenance-free operation. Handling is easy with no risk of damage, and the PM-tube system components significantly reduce the requirement for tuning and cleaning stops when compared to conventional NiCr-tube-based systems (Fig. 10).

Fig. 8. Temperature distribution of the burner

Operational Performance

Bringing the Ecothal SER burner to market after considerable trials and operational testing means we have significant data on energy savings, reduced NOx emissions and maintenance routines that can be clearly demonstrated to potential customers.

Companies that have worked with us and carried out product trials have subsequently ordered and installed Ecothal SER burners, having been impressed with the operational performance. At Volkswagen in Germany, fuel savings of around 37% and reduced maintenance downtime have been reported. Similarly at Stihl, three furnaces are running efficiently using the system, and a fourth is being refurbished to incorporate the burners.

Fig. 10. Higher temperature capability of the APM tubes means increased life and cost efficiency.

Conclusion

Whatever the requirement for furnace heating, the introduction of the Ecothal SER burner system provides the user with the opportunity to increase productivity while saving fuel costs. As one of the cleanest SER burners available with its efficient combustion and flue-gas recirculation, it not only fulfills today’s environmental standards but is just as well prepared for tomorrow’s standards, which will be even stricter.

Designed to be totally adaptable as a result of its modular construction, the system allows the user to upgrade existing furnaces to the very latest burner technology easily and cost-effectively. IH

For more information:
Contact Joe Merta, sales and marketing manager, Kanthal – Heating Systems, 119 Wooster St., Bethel, CT 06801; tel: 203-744-1440 x8318; e-mail: joe.merta@kanthal.com. Kanthal has its own in-house demonstration and research center with the ability to show dedicated set-ups and resultant outputs to potential customers. Full details of the new burners are available on the website: www.kathal.com.

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: NOx emissions, single-ended recuperative burner, radiant tube, powder metal