Process Control & Instrumentation: Radiant Tubes for Continuous Strip Processing Furnaces
Worldwide competition is forcing companies to produce more steel strip product using fewer work staff. Uptime of the lines must increase while the number of furnace operators and maintenance staff is reduced. In addition, emission requirements are getting tighter and future possible carbon-dioxide tariffs are unknown. All of these challenges are also related to the radiant tube heating systems of the strip line furnaces. Strip lines with outdated technology will not be competitive in the future. On the other hand, a lot of effort was put into the development of new radiant tube designs, and there is also new technology out on the market, which can be adapted for strip line furnaces.
Heat transfer in continuous strip line furnacesGalvanizing strip processing lines include a vertical or horizontal radiant tube-heated furnace in which steel strip is heated to temperatures in the range of 720 to 860°C (1330 to 1580°F), depending on the material type. Furnace zone temperatures of 900°C (1650°F) are typical. Heat transfer is dominantly by radiation and largely influenced by the emissivity of the strip and the temperature of the radiant tubes surface (Fig. 1). The emissivity varies with wave length, cleanliness, oxidation state and temperature. Usually, heat-up calculations are based on an emissivity (e) of ± 0.3.
The maximum production rate of a strip line is limited by the maximum line speed or the installed heating capacity, and in most cases by the maximum zone (radiant tube surface) temperature. The following example illustrates the influence of radiant tube temperature on production:
Calculations of this kind can be performed quickly, and they are a good tool to estimate strip-line performance and to evaluate the influence of planned changes to a furnace. For more detailed investigations, computer simulations like computational fluid dynamics (CFD) or finite element modeling (FEM) are finding there way from academics to practical applications. These simulations can address various questions such as what is the influence of tube spacing on the temperature distribution; what are the thermal stresses in radiant tubes; and what is the NOx formation. However, it is not yet possible to perform a complete detailed simulation of an entire strip line furnace.
Energy efficiencyWaste heat recovery has the greatest influence on the efficiency of a radiant tube-heated strip line. Several different strategies for waste heat recovery are used. In some plants, waste heat is used to preheat the strip or used in other areas of the plant. A better way to save energy is to use the waste heat for combustion air preheating through recuperative or regenerative heat exchangers. U- and W-tubes can be equipped or retrofitted with external or plug-in recuperators. Difficulties can arise from high air preheat temperatures regarding NOx emissions and high temperatures at sealing surfaces and flanges. The hot air piping requires thorough insulation to prevent heat losses and unpleasant high temperatures around the furnace. The integration of a counter-flow heat exchanger into the radiant tube eliminates these needs. One example of an integrated design is the self recuperative burner.
Regenerative systems will become more important as energy costs remain high and when the learning curves of these systems level off. The additional complexity of regenerative systems is counterbalanced by outstanding fuel efficiency, temperature uniformity and almost cold exhaust gases.
Another factor influencing efficiency is the fuel/air ratio, which is affected by the burner and furnace control, as well as burner tuning and maintenance.
The following examples illustrate the differences of common systems and provide some rules of thumb for a first evaluation of a furnace (Note that some numbers are rounded).
The examples show that fuel costs for a large strip line can vary by several hundred thousand dollars depending on the combustion system.
Tube materialBoth fabricated and cast tubes made of different alloy grades are used for radiant tubes. For horizontal lines, ceramic single-ended tubes have become more popular (Fig. 2). As shown above, the heating rate of a strip line depends largely on the radiant tube surface temperature. Allowable tube temperatures are often a limiting factor for production. However, this is not the case for ceramic radiant tubes, which could be operated at tube surface temperatures exceeding 1250°C (2280°F), higher than is needed in a galvanizing line. The tube life of ceramic radiant tubes is not limited by thermal aging as with metal tubes, but more by breakage during handling or from broken strip. However, it has been demonstrated with dozens of strip line furnaces with thousands of tubes installed that these situations can be managed.
Temperature uniformityTemperature uniformity is one of the main goals in radiant tube development, not just for an even strip temperature profile, but also for tube life. Hot spots cause tube failures by burning holes into the tube or because they cause high thermal stresses. A radiant tube with an even temperature distribution can provide more heat to the furnace and has a longer tube life. Figure 3 shows temperature uniformity of different radiant tube designs.
Low NOx combustionLow NOx burners are now in every burner manufacturer's portfolio. Especially when the combustion air is preheated, NOx emissions can be extremely high if no measures are taken. Exhaust gas treatment is extremely expensive and, therefore, low NOx burners are always preferable. Techniques used to reduce thermal NOx formation, which is the predominant source for NOx when burning natural gas include:
- High velocity combustion
- Air staging
- Fuel staging
- External recirculation
- Internal recirculation
Most low-NOx burners incorporate one or more of these techniques. Other methods like reburning and steam injection are applied in large capacity firings like boilers and gas turbines only.
Recirculation of exhaust gases has proven to be very effective. External recirculation can be retrofitted to U- and W-tube systems. A drawback of external recirculation is that recirculated gases are passed through the heat exchanger, lowering the efficiency or causing a need for a larger heat exchanger.
Internal recirculation is a part of the concept of recirculating radiant tube designs. A special form of internal recirculation leads to a special form of combustion, called flameless oxidation, or FLOX®. In contrast to the 20-40% recirculation in common systems, FLOX recirculation rates of well over 100% lead to drastic reduction of NOx emissions even for very high air preheat. FLOX technology is a key technology to keep NOx in check, especially in regenerative systems.
Control conceptsRadiant tubes in strip lines are operated as pull, push or push-pull systems. Pull systems provide a negative tube pressure, which prevents combustion products from leaking into the furnace in case of a cracked tube. For pull-only systems, there is no combustion air piping, which makes this system inexpensive, but burner tuning often is a challenge, especially on older lines or when several radiant tubes are out of service. Push or push-pull systems are preferred now because they allow for better fuel/air ratio control.
The heat input can be controlled as proportional, high/low or on/off. The division of a furnace into zones is either done by the piping arrangement in the case of proportional zone-controlled furnaces or it is done electrically for pulse-fired systems. In pulse-fired systems, the burners are fired on/off in a staggered pattern, which prevents pressure impulses in the air and gas manifolds. The increasing use of field-bus communication fits well with the introduction of pulse firing and individually controlled and supervised burners. There is a trend to package radiant tube systems complete with burner, heat exchanger, controls and flame safety. This makes furnace design, start up and maintenance significantly easier.
Furnace models are generally based on zone temperature measurements. It is not easy to define a zone temperature particularly in vertical furnaces. Well-defined positioning of the thermocouples is essential for reliable control. Bad measurements or undefined thermocouple positions can lead to premature radiant tube failures. An alternative to systems based on zone temperatures is a furnace model based on heat input.
Radiant tube designsRadiant tube designs can be distinguished between recirculating and nonrecirculating systems (Fig. 4). While nonrecirculating tubes are still widely used, recirculating systems are becoming more popular due to their temperature uniformity, NOx performance, integration of waste heat recovery and easier sealing.
Nonrecirculating tubes are often proportionally controlled. The burner design aims for a flame that is stretched over the first tube leg. Heat exchangers can be plug-in recuperators or attached types. The tube temperature uniformity of these designs is rather poor, but can be improved with regenerative firing.
Recirculating tubes should be operated with on/off operation and with high velocity burners. P-and double-P-tubes (Fig. 5) provide good temperature uniformity, but they also depend on even temperatures to keep the thermal stress between the center and return leg low. The combination of a recirculating tube design and regenerative air preheating results in an A-tube design. The regenerators are arranged in both legs of the tube with flow directions changing every ten seconds. Besides minimizing exhaust gas losses, this tube design has superior temperature uniformity and extremely low NOx emissions when operating in FLOX mode.
ConclusionsThere are many different radiant tube designs on the market and in operation leading to an observation of the following technology trends:
- Heat recovery
- Plug in recuperators and self
- recuperative systems
- Self-regenerative systems
- Low NOx
- Internal and external recirculation
- Flameless oxidation (FLOX)
- Ceramic single-ended radiant tubes for horizontal lines
- Push or push-pull systems
- Pulse-firing systems
- Direct spark-ignited on/off systems
- Flame safety
- Field bus communication
- Tube design
- Recirculating radiant tubes
- Packaged systems
It is expected that energy conservation will gain further importance in the future. NOx emissions have to be kept in mind, and advanced low-NOx burner technology is available. Modern computer tools will enable further progress, but this will require significant R&D, as well as a close cooperation of users and suppliers.