Fig. 1. Radiant-tube concepts


Regenerative air preheating is accepted as the most effective way to increase energy efficiency for high-temperature process heating, but it was seen as too complex and expensive for heating small and medium size heat-treating furnaces.

Radiant-Tube Designs

Gas-fired radiant tubes can be characterized in different ways. One important feature of the radiant-tube type is whether the tube allows internal recirculation or not. Figure 1 shows the most common radiant tubes. On the left side are the non-recirculating radiant tubes. The simplest form is the straight tube with the burner on one end of the tube and the exhaust outlet on the other. Straight tubes can be found on older furnaces but are rarely used for new installations because of their poor temperature uniformity, the difficulty in sealing and the absence of heat recovery.

U- and W-tubes have larger surface areas per tube and the burner and exhaust outlet are on the same side of the furnace. That makes sealing of the furnace easier because the radiant tube can freely expand inside the furnace.

Recirculating radiant tubes allow internal recirculation for improved temperature uniformity and the application of effective NOx-reducing combustion technologies.

A widely used radiant tube is the single-ended type. Temperature uniformity is far superior in comparison to non-recirculating tubes. The sealing of the tube is easy because there is only one round flange per tube. To guide the flow inside the tube, an inner tube is necessary. Most inner tubes are made out of ceramic materials due to the high internal operating temperatures. For the outer tubes, the usage of SiSiC (siliconized silicon carbide) has also gained popularity since this material allows approximately twice the heat transfer rate and higher furnace temperatures than alloy tubes.

If larger surface areas per tube are required, P-, double-P- and A-tube designs provide comparable surface areas to U- and W-tubes with the advantage of better temperature uniformity and low-NOx technology like air-staged high-velocity combustion or flameless oxidation.

Fig. 2. W-tube with plug-in recuperator

Heat Recovery for Radiant Tubes

For radiant tubes, decentralized heat recovery is preferable. Central heat exchangers – common for large direct-fired furnaces – are not practical for radiant-tube-fired systems because there is no central exhaust outlet on the furnace. The hot exhaust gases would have to be transported to the heat exchanger in costly insulated ducts, and then the hot air would have to be distributed back to the individual radiant tubes. The different radiant-tube designs require different strategies for heat recovery.

In straight-through tubes, heat recovery is very rare. For U- or W-tubes, the most common way to preheat the combustion air is to use plug-in recuperators (Fig. 2). To enhance the air preheat, external recuperators are also possible. The limitation for air preheat is due to the necessity to guide the hot air from the exhaust leg to the burner and also from the coflow heat-exchanger design.

Higher air preheat temperatures and thereby higher efficiency can be achieved with regenerative burner systems in U-, W- and A-tubes. Two burners per tube are firing alternately. The regenerative systems allow air preheat temperatures close to the furnace temperature. Energy savings of more than 30% compared to systems with plug-in recuperators are typical. Besides energy savings, the temperature uniformity of the tubes is much better due to the alternating flow direction in the tube.

Fig. 4. Double-P-tube with self-recuperative burner

Single-ended, P- and double-P-tubes are usually fired with self-recuperative burners. The counterflow heat exchanger – placed inside the furnace wall – allows high air preheat temperatures, and there is no hot air piping required outside the furnace. For high temperatures, self-recuperative burners with ceramic heat exchangers are available. Air preheat temperatures in the range of 1000-1300°C (1832-2372°F) are typical. Figure 4 shows a double-P-tube with a self-recuperative burner.

To combine the advantages of regenerative systems and self-recuperative burners, a self-regenerative burner for radiant tubes was developed.

Fig. 5. Self-regenerative burner principle

Self-Regenerative Burner

Self-regenerative burners combine a regenerative system and the burner into one compact unit. All switching valves and controls can be integrated into the burner housing. Figure 5 shows how the exhaust and air flow are directed through the burner. The heat of the exhaust is stored in ceramic honeycomb regenerators and transferred to the combustion air when the flow direction switches. Cycle times on the order of 10 seconds provide even conditions. The fuel supply is constant and does not need to be switched unlike in regenerative burner-pair designs. At high furnace temperatures, the FLOX® – flameless oxidation combustion mode – ensures low NOx emissions in spite of very high combustion-air preheat temperatures.

Fig. 6. Self-regenerative burner REGEMAT®

Figure 6 shows a self-regenerative burner that could be used for direct firing and for heating of recirculating radiant tubes. The self-regenerative burner is used in combination with a pulse-firing system, meaning the burner is on/off controlled. A local burner-control unit handles all the logic for regenerative switching, flame safety, ignition and valve operation. That makes the installation, start-up and maintenance as easy as with self-recuperative burners. The tube temperature uniformity is excellent due to alternating flow directions and the internal recirculation, and NOx emissions are low due to flameless oxidation.

Fig. 7. Radiant-tube alternatives

To keep the number of radiant tubes and burners at a minimum, radiant tubes with a large tube diameter should be used. With double-P-tubes, it is possible to heat a furnace with a fraction of the burners as compared to a system with small-diameter straight tubes. Figure 7 shows different tube configurations that all provide the same heat input into the furnace.

Conclusions

There are many options of radiant-tube systems on the market. To fight the challenges of rising energy costs and environmental regulations, a close cooperation of the end user, the furnace builder and the burner manufacturer is necessary to choose the best possible configuration with respect to:
  • Radiant-tube performance
  • Energy efficiency
  • Low emissions
  • Low maintenance
And of course investment-cost optimization is always an important consideration. IH

FLOX® and REGEMAT® are trademarks of WS Inc.

For more information: Dr. Joachim G. Wuenning is the president of WS Thermal Process Technology, Inc., 719 Sugar Ln., Elyria, OH 44035 ; tel : 440-365-8029 ; fax : 440-365-9452 ; e-mail : WSInc@flox.com; web : www.flox.com

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