As with most large jumps in technology, additional improvements or discoveries are uncovered and implemented. We hope to describe some facets of material improvements and the results of these advances in the actual working units now deployed for use in the high-temperature industry.
Recuperative Radiant TubesIn the late ‘70s and early ‘80s, an improvement was made in the method of gas firing radiant tubes. Mostly through research done at the Midlands Research Center of Midlands Gas Ltd. in the U.K., the single-ended recuperative radiant tube was developed. This took the conventional method of firing a radiant tube and added the integral use of a flue-gas recuperator. Prior to this, conventional flue-gas heat recovery required bulky, remote-style heat exchangers that were inordinately expensive for the amount of fuel saved.
Actually, it should be better stated that the amount of fuel or energy-dollars saved could oftentimes not support the purchase of the equipment, at least in the U.S. The disparity of fuel costs made heat recovery almost mandatory from a business standpoint in Europe. In the U.S., however, the fuel costs were still at a low enough level that recuperators were not popular. This, despite the fact that heat recovery has proven to be a significantly effective way of cutting fuels usage. The paybacks were simply not there.
The competition from electricity as a clean source of heat in these applications was difficult since the cost of electric power was also very low in this time frame. The development of the single–ended recuperative radiant tube (SER) brought the gas-fired efficiencies into the realm where they were competitive with electric elements, at least at the pure efficiency level (Fig. 1). Whereas gas efficiency was historically measured at levels as low as 15% for pre-mix fuel burners, the newer recuperative technologies now brought that up to the levels of 70% fuel efficiency.
In the early to mid-‘80s, electric rates began to escalate, and these burner technologies saw widespread use in the conversion market from electric to gas. It was also about that time that the developments in hybrid alloys and ceramic materials began to emerge, and these items began to have an impact on burner and firing-tube design as well. The costs of some of this material were prohibitive at the early stages of development, but the support of several trade groups and universities – as well as some utility-sponsored research facilities – urged the development along. The result of that work has now come fully to light, as these materials are competitive in the marketplace and no longer considered developmental.
Needs Drive DevelopmentAs with most industries, advancement in process techniques and production demands began to drive the need for more efficient high-performance heating devices. And the cost of fuels began to escalate at an alarming rate to the point that fuel – as an operating expense of many heat-treating plants – was 25% or more.
Another factor was looming large over the heat-treating industry. Emission controls and permitting would also affect how equipment was designed. Since that time, energy costs have nearly quadrupled, and in many cases have gone up more than that. As a result, high-performance heating devices were not only desirable, they were almost required in order to stay in business. Enter the technology of the single-ended radiant tube (SER) and the materials of construction that make such burner equipment even more impressive in light of the requirements for today’s processes.
Radiant-Tube HistoryEarly radiant-tube technology utilized some of the best-available materials that had been developed post-war. These were mostly ferrite-based alloys with varying percentages of nickel and chrome added to address temperature or corrosion issues. Most radiant tubes were constructed of materials that had nickel concentrations from 18-40% and chrome from 10-20%.
Then along came the “superalloys” with nickel as high as 65%. Added concentrations of tungsten and other rare metals created metallurgical properties that allowed even higher operating temperatures and burner loading rates. These alloys were expensive but still almost considered a “consumable” by the heat-treating establishment mostly because they did not have sophisticated burner or control technologies that would render long life spans for the alloy. As with fuel, nickel prices began to climb and reached an all-time high (in 2007), resulting in the cost of alloy jumping almost threefold from prior levels. The heat treater is now faced with not only higher-priced fuels but also a huge leap in equipment costs because of the rise in alloy costs.
SiC Improves Cost and Increases Temperature Capability
Fortunately, the development of the aforementioned materials and the integration of these materials into the design of modern-day burners have provided some options to the user of high-temperature gas-fired equipment. One of the biggest developments has been the utilization of silicon carbide (SiC) as a material in burner components. There are several families of SiC and different methods of blending and curing each of the respective chemistries, but this presentation will focus mostly on reaction-bonded SiC and sintered SiC chemistries. Others, such as mullites and nitride-bonded SiC, tend to be specialized in use, have more of a refractory or insulating nature and are not in use in the products described here.
The following are two significant improvements in these materials:
- Manufacturing cost reductions
- High-temperature capabilities under heavy thermal shock
Latest Burner TechnologyThis discussion will center on the development of the Eclipse Auto-Recupe SER V3 burner (Fig. 2). Eclipse V3 is in the fourth generation of burner development with single-ended technology, so comparison of early designs and performance is well documented.
Early designs of SER burners utilized relatively low-momentum flame dispersion down the inside of the inner flame tube. The hot gases would reverse in the capped tube and return along the outside of the inner tube, along the exhaust-path annulus and then enter the heat-exchanger section (Fig. 3). In this area, the incoming combustion air source would pass counter flow to the direction of the flue gases and pick up heat as it converged onto the nozzle. This renders extremely efficient heat transfer, and fuel efficiencies exceeded the 70th percentile. Another unique feature of SER technology is that the temperature uniformity along the tube tends to be very, very even. This aids in heat delivery, furnace-chamber uniformity and minimizes stress and creep on the tube itself (Fig. 4).
SER Design DrawbacksSome of the drawbacks of this kind of design are that the inner tubes are encased, and, therefore, the temperatures of that alloy tube are very elevated. This is the normal area of failure in an SER burner. The inner tube becomes worn, distorts and begins the gradual destruction of the exhaust-path concentricity. This can result in hot spots or bulges that indicate the inner tube is distorting. Because of this, when sizing a radiant tube for use, there were specific limitations on the amount of net heat release (heat flux) that could be rendered. Another characteristic of SER burners is the high preheat temperatures that provide such high fuel efficiencies also elevate the flame temps. With elevated flame temperatures, so also comes elevated NOx formation in the flue product. Levels of 275 PPM of NOx are not uncommon in standard recuperative burners. At higher furnace temps this can actually go higher yet.
Improved SER DesignThe design of the modern-day Auto-Recupe V3 uses a variety of engineering changes that are extremely effective. Firstly, the nozzle design uses staged air to give a cooling effect on the nozzle and to hold down NOx production. Secondly, the nozzle is placed inside a combustor, or outlet nozzle, which increases the flame velocity to more than triple the previous designs, thus allowing for very high scrubbing of the exhaust gases and improving temperature uniformity. Thirdly, the inner tube (and in some cases the outer tube) is made of SiC materials that can withstand the heat generated in the flue-gas stream. Finally, this inner tube is segmented to allow for growth and is designed to allow for flue-gas recirculation. This recirculation, along with the staged-air-design nozzle, reduces the NOx by more than half.
The entire design renders supreme temperature uniformity, oftentimes within 30°C from end to end (Fig. 5). It offers lower NOx that will allow for most permitting to remain compliant given the reduction of fuel. Fuel savings of up to 50% over conventional straight-tube burners are not uncommon, given the 70-74% fuel efficiency delivered by the design. Perhaps most significantly, it allows for the higher net heat delivery over conventional alloy inner-tube designs. This means fewer burners are normally required to give the same net equivalent over conventional burner/tube combinations. Tube life has also shown about a 40% gain in field applications.
The overall effect of these design changes has made the ROI on this equipment drop to the point that the payback on this equipment is less than a year in some instances. Certainly, almost all applications fall within a two-year ROI. One of the designs that has shown the most promise for field applications and the wear and tear associated with this type of furnace duress is the use of high-grade alloy for the outer tube. This method maintains the strength factor of conventional tubes but utilizes SiC components for the extreme-temperature components of the inner tube and other integral components. This renders optimum performance while extending tube life and allowing for recirculation and extremely high velocities. This combination “hybrid” design has found acceptance in over 2,000 applications already, and additional super-high-performance features can be expected in the next few years. IH
For more information: Jim Roberts is the director of strategic accounts for Eclipse, Inc., 1665 Elmwood Road, Rockford, IL 61103; tel: 815-637-7217; fax: 815-623-6321; e-mail: email@example.com; web; www.eclipsenet.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: single-ended recuperative radiant tube, SER, flue gas recuperator, superalloy, silicon carbide, low NOx