Working Toward the Ideal Radiant-Tube Performance
Optimized production and minimized maintenance requirements are desirable conditions in any heat-treating operation. For applications depending on radiant-tube heat, following a few fundamental tips can help you achieve those goals.
For a wide variety of applications where indirect heat is required, as opposed to direct firing, radiant tubes can provide a dependable answer. This indirect heating approach allows many sensitive materials to be processed in the absence of the products of combustion and, in many cases, in the presence of a protective or reactive atmosphere to impart changes to the properties of the heated product. Functioning by means of firing a burner into a tube structure, which then transfers the heat to the application, this method is deployed for processes such as case hardening, galvanizing steel and more.
But like any industrial heating technique, radiant-tube applications require proper selection, calibration and tuning in order to maximize production rates, minimize maintenance and optimize energy use and consumption.
Realizing the operational ideal for radiant-tube applications requires knowing the fundamentals, the ability to recognize common issues, and taking advantage of different methods and available technologies. This article will explore techniques for making the most of your radiant-tube heating applications.
Reviewing the Fundamentals of Radiant Tubes
First, understanding the common types of radiant tubes is critical to knowing how to operate them properly.
Radiant tubes can have many shapes, but for the currently installed industrial base, “W” and “U” tube systems are the most common and will be the primary focus here. A practical starting point to understanding their functionality is to review the tube temperature distribution of a W-type radiant tube (Fig. 1), where a burner is located on one end of the tube and an exhaust system on the other (along with an optional energy recuperator (Fig. 2) – more on that later).
In an ideal state, a radiant tube generates perfectly uniform temperature. This uniformity would result in the optimum use of the tube to transfer heat to the process while respecting the maximum-use temperature of the radiant-tube material. In common practice, however, this isn’t the case for a variety of reasons.
Resulting from the combustion and heat-transfer processes, physics dictates that the tube temperature will vary along its length. The temperature profile is characterized by an increase in temperature from the region near the burner tip to a maximum temperature that is typically located in the firing leg or at the first return bend in U and W tubes. If the tube is long enough, the temperature will decay along the tube’s length and is at its minimum at the exhaust end of the tube.
Tube shape, diameter, length, location in the furnace, firing rate and tuning all have an impact on the temperature distribution and fuel efficiency, as does burner type and energy-recovery method. Understanding the influence of these parameters and optimizing their application and use result in optimized operation of the radiant-tube-heated furnace.
Obtaining Better Production, Longevity and Efficiency Through Tuning
Improvements in temperature uniformity can double or triple the service life of a conventional tube of the same material, and that can have a major impact on your operation. So why accept the status quo?
One way to ensure the best-possible performance and longevity of radiant tubes is through diligence in tuning (Fig. 3). Just like any regular maintenance task in a manufacturing environment, tuning helps reduce breakdowns, can improve your production rate and can ensure you are not losing dollars due to inefficiency. In an average radiant-tube furnace, a poorly tuned system using up higher rates of fuel and more regularly requiring failed-tube replacement can cost an organization hundreds of thousands of dollars per year.
When is it time to tune? Some common indicators can include:
- A drop in your rate of production
- An increase in furnace energy consumption
- Increased failures of your radiant tubes
- “It’s been a while…”
Of course, there are a wide variety of factors that can limit the rate of production on a typical heat-treating furnace. But in many applications, production rate is determined primarily by the rate at which heat can be delivered to the furnace, and that is determined by your radiant tubes.
There are a couple of ways that regular tuning can impact your production rate. First, your available heat can be dramatically reduced when your tubes are operating at a less-than-optimized fuel-to-air ratio. Available heat is defined as the percentage of the fuel energy that is available to the process – the total energy input to the tube minus the exhaust losses.
When comparing systems, it is important to reference the available heat to a standard. For most applications in North America, the available heat is commonly referenced to the fuel’s higher heating value, whereas Europe and much of the rest of the world references available heat to the lower heating value.
Take a look at the example table, which uses a 1750°F tube exhaust with 725°F combustion air preheat. As oxygen and excess air rise, available heat output to your furnace drops. Consider the impact on production if you’re running at 63% available heat versus 57%. It can be significant, wasting fuel not to mention potential production losses for heat-limited products.
It’s a case of “little things mean a lot” with tuning. To understand the magnitude of fuel required to heat the furnace, a simple equation can be utilized:
Consider the implications, from Fig. 3, of an increase in oxygen from 2.1% to 3.8%. The difference in fuel consumption is (61.9/59.7-1) x 100 = 3.6% extra fuel required for the same heat delivered to the furnace. It is not uncommon to find radiant tubes currently operating with much higher excess-air rates.
It’s not just production and fuel consumption. Tuning can have a big impact on the life of your tubes. Regular tuning can help you avoid the common-but-inadvisable practice of operating tubes above the maximum material temperature, as described earlier.
One common practice is not reducing the radiant-tube input in the higher-temperature zones, such as within the last heating and soaking zones of continuous-processing furnaces. As the product temperature (or receiver) increases with the same burner input, the tube temperature rises. Figure 4 shows how neglecting input by zone will result in a portion of the tube operating over the material temperature limit. Understanding this reality will allow tailoring the input to the furnace demand. Most burner and furnace suppliers should be able to provide analysis and support to optimize the input profile to your furnace and reduce tube maintenance issues.
It’s also important to keep your eyes on the furnace wall when it comes to ideal temperatures. In vertical furnaces, such as for steel strip annealing, there are some tubes that have product on both sides of the tubes and others that have a furnace wall on one side and product on the other. The side of the tube facing the wall will receive re-radiation from the wall, resulting in a higher tube temperature. When analyzed, the fuel input to these tubes should be reduced to 60-70% of the neighboring tubes to maintain a similar tube temperature and prevent overheating.
As a final consideration, care should be exercised with turndown on zones with a zone ratio control system. Maintaining even flow distribution to many burners becomes more challenging as the input to the zone is reduced since the pressure is dropping with the square of the input reduction. Very quickly there is not sufficient pressure to evenly distribute the flow, and the ratio control on each tube suffers as a result. This becomes increasingly complicated if energy recuperation is employed. On/Off firing is often employed as a workaround to maintain distribution at turndown on these systems. However, this should only be exercised in conjunction with energy recovery since operating tubes at maximum capacity eliminates the tube efficiency gains as the input is reduced by turning the system down.
There are many other potential interactions with radiant tubes that could be mentioned, but they are beyond the scope of this piece. In order to better understand your process, get in touch with your combustion-systems expert.
So, how often should you be tuning? There isn’t a one-size-fits-all answer. It can depend on available manpower, your furnace’s ratio control system, type and resolution, and other reasons. But you should be tuning once per year at an absolute minimum. Better yet, do it twice per year – even if you aren’t noticing any immediate issues.
A simplified tuning process might look like the following:
Step 1: Set your maximum fuel rate to the burner according to design conditions.
Step 2: Adjust combustion airflow rates by oxygen concentration. Typically, this will range between 2-4%. Set at hot if there is no air-temperature compensation.
Step 3: If you’re working with a modulating system, reduce it to the minimum setting. Be sure to watch the minimum pressure to your tubes for distribution. Also, try to set limits according to your process requirements.
Step 4: Adjust your system flow/pressure to deliver target oxygen at turndown. Typically, this will range from 4-6% to maintain even distribution. Lower is better if your ratio control system is up to the task.
Improving Performance Through Technology
Beyond tuning best practices, any heat-treating application can provide additional improvements through the application of newer technologies and combustion styles.
One of the easiest improvements to implement (as well as to economically justify) is the addition of a plug-in recuperator in the exhaust leg. When applied with burners utilizing flue-gas recirculation, both efficiency improvements and tube temperature profile improvements are achievable.
Perhaps the most effective technologies are compact regenerative burners, which can be installed directly onto existing single-pass “U” and “W” tubes. In this system, one burner will fire into the radiant tube while the companion burner at the other end operates in exhaust mode, collecting energy in a regenerative heat-storage bed.
The firing/exhaust mode of each burner is reversed at regular intervals, thereby recovering energy previously stored in the exhaust cycle and delivering it as preheated combustion air to the firing burner of the radiant tube. Regenerative systems typically deliver available heat around 85%, but the greatest benefit is tube temperature uniformity, potentially increasing production rates and helping promote the longevity of your entire system.
Another advanced technology for existing straight tubes is a variant of the classical single-ended recuperative radiant tube. With this system, a burner and center tube are installed into one end of an existing (or new) straight-through radiant tube, with the other end capped. A high-efficiency recuperative burner initiates combustion in the annulus between the inner tube and the radiant tube to transfer energy directly to the outer tube, which radiates into the furnace. This method, combined with integral flue-gas recirculation, provides enhanced temperature uniformity and enables greater average heat input.
If you’re looking to make a more significant investment, another combustion style involves complete replacement of the single-pass tubes with recirculating radiant tubes, such as “P” or double “P” forms. These replacement tubes utilize high-velocity burners to recirculate high volumes of spent combustion products through the tube, helping to dilute the combustion temperature and increase the temperature uniformity.
No matter your system, there are a number of best practices that should be followed to maximize your efficiency, improve your furnace production and avoid costly breakdowns and downtime.
For more information: Contact Dennis Quinn, burner and blower products engineering manager, Fives North American Combustion, Inc., 4455 E 71st St, Cleveland, OH 44105; tel: 216-271-6000; e-mail: firstname.lastname@example.org; web: combustion.fivesgroup.com/.
Advanced Radiant-Tube Products from Fives North American Combustion
Fives North American Combustion offers a range of radiant-tube products spanning several product classifications.
The 4723 family of ambient and preheated air burners has spanned decades of service across most radiant-tube application markets. External flame-length adjustments allow tailoring of the flame to the radiant tube, whatever the geometry. In recent years, passive-progressive flue-gas recirculation has been incorporated into the 4723LNx (Fig. 6), an easy solution to lower emissions and improve radiant-tube uniformity.
The 8480-series (Fig. 7) plug-in recuperator perfectly complements the 4723-series burner family. This robust design incorporating high-efficiency, high-grade stainless cast heat-transfer sections has proven longevity while achieving enhanced heat recovery, saving energy while simultaneously eliminating costly repairs of existing equipment.
Depending on requirements, the company’s TBRT III (Fig. 8) compact TwinBed® regenerative radiant-tube burner offers increased production, significantly reduced fuel consumption and unequaled radiant-tube life, benefits realized by the users of over 3,000 current and earlier-generation TBRT burners. This high-efficiency bed typically yields energy savings of up to 60%.
Other radiant-tube burner products, air- and fuel-control systems and standard and custom combustion controls round out a full equipment offering for radiant-tube applications.