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Radiant tube concepts

Two main groups of radiant tubeconcepts can differentiated. One groupis called non-recirculating radiant tubesand the other one the recirculatingradiant tubes. As it can be seen in Fig. 1with the exception of the nonrecirculating single ended radiant tube,all the non-recirculating radiant tubeshave a burner at one end of the tube andthe flue gas outlet at the other end of thetube. Usually, slow mixing burners areused to distribute the heat as good aspossible over the length of the tube.Recirculating radiant tubes are usinghigh velocity burners to circulate thecombustion gases within the radianttube thereby distributing the heat evenlyover the radiant tube surface. Thetemperature differences on the tubesurface can be minimize to a fraction,compared to non-recirculating tubes.Non recirculating tubes are still widelyused, especially U-tubes in the heattreating industry and U-and W-tubes inthe steel industry, but within the lastyears, most of the major burnercompanies included recirculating singleended radiant tubes in their productportfolio.

Radiant tube material

In the present time, three differentmaterial classes are used:

1. cast alloy tubes

2. fabricated alloy tubes

3. ceramic tubes

There are various pros and cons for castand fabricated alloy tubes, but in generalboth types can be used under similarconditions. Ceramic tubes gained

importance since the introduction ofsilicon carbide, especially reactionbonded silicon carbide SiSiC. Thesematerials overcame the problem ofthermal shock sensitivity from tubesmade from mullite and others.

The material properties of SiSiC at hightemperatures are superior to alloy,especially regarding strength, creepstress and thermal oxidation, allowingfor net heat fluxes of up to 110 BTU/hrin2 and can be used in furnaces withzone temperatures up to 2250°F. Alloytubes are generally used for heat fluxesof 60 BTU/hr in2 and less and forfurnace temperatures of maximal2000°F. On the other hand, the ceramicproduction process permits only certainshapes and the possibilities for

machining and joining (welding, brazing)are very limited. The mechanicalbrittleness must be also considered.However, proper design and sometraining of the operators can overcomethese difficulties and many thousandceramic radiant tubes are in operationnow, providing reliable high performanceto their users. In strip lines, ceramicradiant tubes open the opportunity forconsiderably increased production of 30% and more. However, the furnace andthe process have to be checkedthouroughly to avoid problems like tubedamage caused by strip breakage andalso problems which can result fromhigher furnace temperatures, which canbe well above the final strip temperature(strip breakage at reduced strip speed,

Flameless oxidation burners for heatingstrip lines

Natural gas is the most widely used fuel for heating strip in furnaces due to itsavailability and clean combustion and is enabling much lower energy costs,compared to electrical heating systems, provided that an efficient system is used.Besides energy costs issues, heating systems play an important role for theperformance of a furnace, especially regarding temperature uniformity / productquality, net heat input / productivity, energy efficiency / operating costs,maintenance / operating costs, tube life / operating costs, flue gas emissions /pollution, furnace downtime, ease of operation and investment costs. Thefollowing reports shows that all these aspects have to be considered to make adecision for a new heat treating furnace or a retrofit project.

Dr.-Ing. Joachim G. WünningWS WärmeprozesstechnikGmbH, Renningen(Germany)Phone: +49 (0)7159 / 1632-0Email:J.G.Wuenning@FLOX.com

Fig. 1: Recirculating and non-recirculating radiant tubes

Fig. 5: Calculated (CFD) temperatures for flame and FLOX® -mode

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rollers, furnace insulation). Alloy tubesremain a good choice for manyapplications due to their greaterflexibility in shape and in applicationswhere mechanical damage cannot beavoided, e.g. strip breakage in a verticalstrip line, strong vibrations, shocks orderailed baskets.

Efficiency

The efficiency of fuel fired systems isoften defined by the term available heat.The available heat is the heat, availableto the furnace and the workload and isequal to the gross input minus the fluegas losses [1]. Rising flue gastemperatures are leading to higher fluegas losses and thereby to loweravailable heat (Fig. 2). For example: atflue gas temperatures of 1800°F, withoutair preheating???=0), only about half ofthe energy, provided in the form of fuel isavailable to heat up the furnace, the restis lost with the flue gases, even when theburners are adjusted properly. Thiseffect is even more important for radiant

tube systems compared to direct firedfurnaces since the flue gas temperaturecan be substantially higher than thefurnace temperature (Fig. 3).

For example: For a furnace temperatureof 1300°F, the tube temperature is1700°F and the internal gas temperature(flue gas temperature prior to an optionalheat exchanger) is 2000°F and more, if aceramic radiant tube with a net heat fluxof 110 BTU/hr in2 is used.

Therefore, without proper heat recoverymethods, efficiency can be very poor(available heat < 50%) even at relativelylow furnace temperature. For evenhigher furnace temperatures, theavailable heat can drop even further,eliminating the cost advantage of naturalgas over electricity completely.

The most effective way to reduce fluegas losses is to use heat recoverysystems to preheat the combustion air.Several heat recovery concepts areused:

• plug in recuperators and central heatexchangers provide moderate airpreheat temperatures

• self recuperative burners providehigher air preheat temperatures byeliminating transport losses since theheat exchanger is integrated into theburner and placed within the furnacewall

• regenerative burners enable largeheat transfer areas in a compactdesign, obtaining air preheattemperatures which are close to theflue gas temperature (prior to the heatexchanger)

Fig. 4: Recuperative burner in flame and FLOX® - mode

Fig. 2: Available heat

Fig. 3: Exhaust temperatures from radiant tubes

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Central heat exchangers and plug inrecuperators are used in combinationwith non- recirculating radiant tubes.Self recuperative burners are generallyused for recirculating radiant tubes.Regenerative burners were used in non-recirculating tubes but the challengescoming from the high air preheattemperatures often lead to difficulties. Anewly developed regenerative burnerdesign in combination with A-typeradiant tubes will overcome thesedifficulties and is ready for the marketintroduction. It will be mainly depend onthe future energy prices how long it willtake until this concept gains aconsiderable market share.

Emissions

For the most common gaseous fuels likenatural gas, NOx-emissions are thegreatest concern. High temperatures,confined flow conditions and high airpreheat temperatures contribute tostrongly rising NOx-emissions.Therefore, very effective low NOx-

combustion concepts must be used.The high velocity concept was a firststep, but not enough as the air qualitystandard started to became moresevere. The introduction of air staged,high velocity combustion allowed for afurther reduction but future air qualitystandard will require even furtherreduction. The development of selfrecuperative and regenerative burnersoperating in the so called FLOX® -mode(Flameless Oxidation) enabled lowemissions even at highest air preheattemperatures. In FLOX® mode, largeamounts of combustion products are re-entrained into burner jet before thecombustion takes place. Thereby, peaktemperatures are avoided, minimizingNOx-emissions and reducing thethermal stress on the burner [2] (Fig. 4and 5).

Controls and Flame Safety

Many non-recirculating radiant tubes areproportionally or high low controlled.The main reason is cost savings since

Fig. 7: Horizontal strip line

one air and gas valve can be used tocontrol a whole temperature controlzone. But there are some things toconsider. The performance of a radianttube can be optimised only for one pointof operation. Operating with higher orlower input will compromise, especiallythe temperature uniformity and NOx-emissions. Another important fact isflame safety. If flame safety is applied orhas to be applied, the advantage ofhaving only one air and gas valve perzone would led to the loss of a wholetemperature control zone, if only oneburner fails. On/Off or pulse firingsystems (Fig. 6) are suiting the highperformance radiant tubes the best.Each radiant tube is an independent unitwhat makes burner adjustment verysimple. Often, flame safety is mandatoryfor ceramic radiant tubes.

Applications

Energy efficient radiant tube systems areused in a wide variety of applications.Recirculating type radiant tubes can beinstalled wherever non-recirculatingtubes are in operation. In addition, SiSiCsingle ended radiant tubes can beinstalled in furnace atmospheres andtemperatures where expensive electricheating was the only option. There aretens of thousand recirculating radianttube systems operating in furnacesincluding:

• batch furnaces

• pusher furnaces

• roller heart furnaces

• rotary hearth furnaces

• vertical and horizontal strip lines

and many others.

Fig. 7 is showing a silicon steel strip line.In the fall of 2001, more than twohundred 8"-ceramic single endedradiant tubes (Fig. 8) where installed to

Fig. 6: On/Off - pulse firing

Fig. 8: Ceramic single ended radiant tube

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increase production in an existing line. Inthese works, a total of more than twothousand single ended, recuperativeradiant tubes are installed. Also in the fall

of 2001, a galvanizing line with a verticalfurnace, containing about 200 alloydouble-P-type radiant tubes was startedup (Fig. 9 and 10).

Conclusions

Efficient reliable radiant tube systemsare available on the market. Newdesigns and material, especially SiSiCceramic improved the performance andlife of radiant tubes considerably.However, rising internal temperaturesrequire proper design and adaptation ofburner and radiant tube. Recuperatorsand regenerators are exposed to hightemperatures and must be made fromceramic material as well. WS specializedon these type of burners and sold morethan one thousand ceramic radiant tubesystems in 2001 alone. At the currentenergy price level, additional expensesfor efficient radiant tube systems can berecovered in a few years. Counting theother advantages, especially increasedproduction, but also less downtime,better temperature uniformity, flamesafety and better tube life can reduce thepayback period for these systemsconsiderably. In many cases, whenceramic radiant tube allow for smallerfurnaces or increased production,investment costs and operating costscan be reduced at the same time.

Literature

[1] Heat Balances – Determining the HeatNeeds of Furnaces and Ovens, IndustrialHeating Dec. 2001, page 52

[2] Wünning J.A., Wünning J.G., Ten Years ofFlameless Oxidation – TechnicalApplications and Potentials, 4th HTAC2001, Rome, Italy, November 2001

Fig. 9: Double P-tubes in a vertical galvanizing strip furnace

Fig. 10: Double-P-tube with recuperaive burner