Heat-treating atmosphere furnaces are heated using electric heating and fuel- fired radiant tubes. Natural gas is the most widely used fuel due to its availability and clean combustion and is enabling much lower energy costs compared with electrical heating systems, provided that an efficient system is used.
Besides energy cost issues, heating systems play an important role in the performance of a heat treating furnace, especially regarding:
Temperature uniformity and product quality
- Net heat input and productivity
- Energy efficiency and operating costs
- Maintenance and operating costs
- Tube life and operating costs
- Flue gas emissions and pollution
- Furnace downtime
- Ease of operation
- Investment costs
All of these aspects have to be considered when making a decision about a new heat treating furnace or a retrofit project.
Two main groups of radiant-tube concepts are nonrecirculating and recirculating radiant tubes. With the exception of the nonrecirculating single-ended radiant tube, all nonrecirculating radiant tubes have a burner at one end of the tube and the flue gas outlet at the other end of the tube as shown in Fig.1. Usually, slow-mixing burners are used to distribute the heat as well as possible over the length of the tube.
Recirculating radiant tubes use high-velocity burners to circulate the combustion gases within the radiant tube, thereby distributing the heat evenly over the radiant tube surface. The temperature differences on the tube surface can be minimized to a fraction of those for nonrecirculating tubes.
Nonrecirculating tubes are still widely used, especially U-tubes in the heat-treating industry and U- and W-tubes in the steel industry. However, most of the major burner companies have included recirculating single-ended radiant tubes in their product portfolio within the past few years.
Three material types currently used for radiant-tube construction are:
- Cast alloy tubes
- Fabricated alloy tubes
- Ceramic tubes
There are pros and cons for cast and fabricated alloy tubes, but, in general, both types can be used under similar conditions. Ceramic tubes have gained in importance since the introduction of silicon-carbide (SiC) ceramic material, especially reaction-bonded silicon carbide, or SiSiC. These materials overcame the problem of thermal shock sensitivity of tubes made of mullite and other ceramic materials.
The mechanical properties of SiSiC at high temperatures are superior to those of heat-resistant alloys. This especially is the case regarding strength, creep stress and thermal oxidation, allowing for net heat fluxes of up to 110 Btu/hr in2 (50 kW/m2). The tubes can be used in furnaces having zone temperatures to 2250F (1230C). Alloy tubes, by comparison, generally are used for heat fluxes of 60 Btu/hr in2 (27 kW/m2) and less and have a maximum furnace use temperature of around 2000F (1095C).
The ceramic production process allows only certain shapes to be manufactured, and machining and joining (welding and brazing) ceramics are very limited. Also, ceramics are brittle and subject to damage from mechanical impact. However, these difficulties have been overcome through the use of proper design and some operator training, and thousands of ceramic radiant tubes are currently in operation, providing reliable high performance.
Alloy tubes remain a good choice in many applications due to their greater flexibility in possible shapes and where mechanical damage cannot be avoided, such as from strong vibrations, shocks and derailed baskets in batch heat-treating furnaces and steel-strip breakage in a vertical steel-strip processing line, for example.
The efficiency of fuel-fired systems often is defined by the term "available heat." This is the heat available to the furnace and the workload and is equal to the gross input minus the flue-gas losses. Rising flue-gas temperatures are leading to higher flue-gas losses and, thereby, to lower available heat (Fig. 2).
For example, at a flue-gas temperature of 1800F (980C) without air preheating (_ = 0), only about half of the energy provided in the form of fuel is available to heat up the furnace. The rest is lost with the flue gases, even when the burners are adjusted properly.
Compared with direct-fired furnaces, this effect is even more important for radiant-tube systems because the flue-gas temperature can be substantially higher than the furnace temperature (Fig. 3). For example, at a furnace temperature of 1300F (705C), the tube temperature is 1700F (930C) and the internal gas temperature (flue-gas temperature prior to an optional heat exchanger) is 2000F (1095C) or higher if a ceramic radiant tube having a net heat flux of 110 Btu/hr in2 (50 kW/m2) is used.
Therefore, without using proper heat-recovery methods, efficiency can be very poor (available heat <50%) even at relatively low furnace temperature. At higher furnace temperatures, the available heat can drop even further, eliminating the cost advantage of natural gas over electricity completely.
The most effective way to reduce flue gas losses is to use heat-recovery systems to preheat the combustion air. Several heat recovery concepts are used including:
- Plug-in recuperators and central heat exchangers to provide moderate air preheat temperatures
- Self-recuperative burners to provide higher air preheat temperatures by eliminating transport losses because the heat exchanger is integrated into the burner and placed within the furnace wall (Fig. 4)
- Regenerative burners to enable large heat-transfer areas in a compact design, obtaining air preheat temperatures that are close to the flue-gas temperature (prior to the heat exchanger)
Central heat exchangers and plug-in recuperators are used in combination with nonrecirculating radiant tubes. Self-recuperative burners generally are used for recirculating radiant tubes. Regenerative burners were used in nonrecirculating tubes, but the challenges coming from the high air preheat temperatures often lead to difficulties. A newly developed regenerative burner design in combination with A-type radiant tubes can overcome these difficulties. However, when this concept will gain considerable market share will depend on future energy prices.
For the most common gaseous fuels like natural gas, NOx emissions are of the greatest concern. High temperatures, confined flow conditions and high air preheat temperatures contribute strongly to increasing NOx emissions.
Therefore, very effective low NOx combustion concepts must be used. Use of a high-velocity concept was a first step, but was not enough to meet increasingly more severe air quality standards. The introduction of air-staged, high-velocity combustion allowed for a further reduction in NOx emissions, but future air quality standards will require even further reductions.
The development of self-recuperative and regenerative burners operating in the so-called FLOXR, or flameless-oxidation, mode enables low emissions even at the highest air-preheat temperatures. In the FLOXR mode, large amounts of combustion products are reentrained into the burner jet before combustion takes place. Therefore, peak temperatures are avoided, minimizing NOx emissions and reducing the thermal stress on the burner.
Controls and flame safety
Many nonrecirculating radiant tubes are proportionally, or high/low, controlled. The main reason for this is cost savings because one air and gas valve can be used to control an entire temperature control zone.
However, there still are factors to consider. The performance of a radiant tube can be optimized only for one point of operation. Operating with higher or lower input especially compromises temperature uniformity and NOx emissions. Another important factor is flame safety. If flame safety is applied or needs to be applied, the advantage of having only one air and gas valve per zone would lead to the loss of an entire temperature control zone if only one burner fails.
On/off, or pulse, firing systems are best suited for high-performance radiant tubes. Each radiant tube is an independent unit, which makes burner adjustment very simple. Often, flame safety is mandatory for ceramic radiant tubes.
Energy-efficient radiant-tube systems are used in a wide variety of applications. Recirculating type radiant tubes can be installed wherever nonrecirculating tubes are in operation. In addition, SiSiC single-ended radiant tubes can be installed in furnace atmospheres and temperatures where expensive electric heating was the only option. There are tens of thousand recirculating radiant tube systems operating in furnaces including:
- Batch furnaces
- Pusher furnaces
- Roller-hearth furnaces
- Rotary-hearth furnaces
- Vertical and horizontal strip lines and many others
For example, Fig. 5 shows the performance of a brazing furnace using a 100% hydrogen atmosphere. The performance of ceramic radiant tubes with respect to NOx and efficiency is superior to many radiant tubes operating in low-temperature applications. The diagrams show efficiency (available heat) and NOx data for burners operating in flame mode and in FLOXR mode.
Figure 6 shows a silicon steel-strip line, where more than 200 8-in. (200-mm) diameter ceramic single-ended radiant tubes were installed in the fall of 2001 to increase production in an existing line. A galvanizing line having a vertical furnace containing about 200 alloy double-P-type radiant tubes was started up at the same time.
Several years of operation have shown that even very tough conditions can be managed, such as ceramic radiant tubes installed in a cantilever construction in a rotary hearth furnace used to heat turbine blades for forging, running at temperatures to 2250F (1230C).
Many types of efficient, reliable radiant-tube systems are currently available. New designs and material, especially SiSiC ceramic, have improved radiant-tube performance and life considerably. However, rising internal temperatures require proper design and adaptation of the burner and radiant tube. Recuperators and regenerators exposed to high temperatures also must be made of ceramic material.
At current energy prices, the additional expense for efficient ceramic radiant tube systems can be recovered in a few years. Additional advantages, such as less downtime, better temperature uniformity, flame safety and better tube life, can reduce the payback period for these systems even more. In many cases where ceramic radiant tubes allow the use of smaller furnaces or increased production, investment costs and operating costs can be reduced at the same time.