In previous months, this series has explored the geometry of a tube, why radiant tubes matter, what happens inside the tube, and radiant tube control systems. For the first three installments, check out Heat Treat Today’s digital editions in November 2022, December 2022, and February 2023. For the month of May, we will continue our discussion of different modes of control for radiant tube burners.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today’s May 2023 Sustainable Heat Treat Technologiesprint edition.
If you have suggestions for radiant tube topics you’d like John to explore for future Technical Tuesdays, please email Bethany@heattreattoday.com.
John B. Clarke Technical Director Helios Electric Corporation Source: Helios Electric Corporation
High/low and on/off controls require different control strategies from a proportional mode of control. In all cases, we assume the temperature control will be provided by a proportional-integral-derivative loop (PID loop). The function can be provided by a stand-alone instrument or a PID function in a programmable or process controller. The PID algorithm looks not only at the temperature of the process as indicated by the control element (thermocouple or RTD) and compares it to the setpoint — but it also considers the offset and rate of change as well. When properly tuned, a PID control loop can provide control accurate enough to match the process (actual) temperature to the setpoint within a degree or two.
For the lay person, another way of describing a PID loop is to consider how a driver regulates the speed of his automobile. Assume you are driving and want to catch up with and follow the car ahead of you — to do so, you need to match that car’s speed and maintain a safe distance. What you don’t do is floor the automobile until you get to the desired following distance and then hit the brakes. What you do is first accelerate to a speed faster than the target car to close the gap, then you instinctively take your foot off the accelerator when you get close, slowing gradually until your speed and position are as you desire. In this example, you have considered your speed, how close you are to the car you are attempting to follow, and the rate at which you are closing the gap. A PID loop is nothing more than a mathematical model of these actions.
The PID control loop provides an output — the format can vary, but it is in essence a percent output. It is a percent of the maximum firing rate the system needs to provide to achieve and maintain the desired furnace temperature. This percent output can be translated directly into a proportional output for proportional control — where the firing rate is proportional to the loop’s output.
On/off or high/low controls require a different approach where a time proportioning output is provided in which the burner fires on and off on a fixed time cycle. In this mode of control, the PID loop’s output is multiplied by the cycle time to determine the on or high fire period and the on or high fire time is subtracted from the cycle time to determine the off or low fire period. Cycle times can run from as little as 30 seconds to as much as a few minutes. Obviously, the shorter the cycle time, the more responsive the control, but also the more wear on the control components. The cycle time should be as long as possible but still meet the needs of the process control.
Don’t confuse these pulses with other control methods that are marketed as pulse firing. When people speak of pulse firing, they often mean a pattern with alternate burners firing to provide greater temperature uniformity and heat transfer. This is a very interesting subject and the topic for another day.
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Heat Treat Today publishes eight print magazines a year and included in each is a letter from the publisher, Doug Glenn. This letter first appeared in Heat Treat Today's March 2023 Aerospace Heat Treating print edition.
Doug Glenn Publisher and Founder Heat Treat Today
Depending on where you live, “green” started to appear outside in March.
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Such was the case this March with Heat Treat Today. Our efforts were “greening up” around here as well. With the push for sustainability and environmental corporate responsibility, we decided to start the industry’s first and only “green” heat treat annual magazine edition and quarterly e newsletter. The Heat Treat Today team has been working on these items for several months now, but we are officially announcing them this month and encouraging you to watch for them both in May.
Whether you’ve been mandated to make your in-house heat treat operation more sustainable, or you want to do it simply because it’s the right thing to do, we’re here to help.
NEW Green Technologies in Heat Treat Annual Print Edition
Heat Treat Today's May print magazine will be the inaugural yearly focus on Green Technologies in North American heat treat. We’ll have articles and special editorial sections focused on sustainable technologies currently or soon-to-be available in the North American heat treat industry. This highly-focused issue will give industry suppliers a chance to shout loud and far about the technologies they have that will help you make your in-house heat treat operation more sustainable and productive. We anticipate topics such as:
Induction heating equipment
Electrical furnaces and ovens, including vacuum furnaces
High-efficiency gas-fired equipment
High-efficiency burners
Efficiency-maximizing control systems
Energy-saving insulating materials
Emission control or capture
Eco-friendly quench media
Economizing cooling systems
Industrial gas economizing systems
High-efficiency radiant tubes
High-efficiency heating elements
Potentially, there will be many other topics added to this list. There should be something for everyone who is interested in making their in-house heat treat operations, or commercial heat treat shop, more sustainable. I hope you look forward to receiving your copy and enjoying the content . . . in May!
NEW Quarterly Heat TreatGreen E-Newsletter
Sustainable technologies come into the market more than once a year, so, Heat Treat Today is launching a new quarterly e-newsletter this May that focuses on sustainable heat treat technologies for the North American marketplace. This e-newsletter, aptly named Heat Treat Green, will also focus on emerging and currently available sustainable technologies and products that will help your heat treat operations reduce environmental waste in a responsible manner. We anticipate that this e-newsletter will be deployed in the months of February, May, August, and November each year.
Do You Have a Green Story To Tell?
In both the annual magazine edition and the quarterly e-newsletter, we’d be interested in publishing your in-house heat treat sustainability story if you have one to tell. Our readers benefit from hearing what other manufacturers are doing to make their heat treat operations more sustainable. Many chief compliance officers or others in your organization responsible for promoting sustainable practices are typically quite interested in telling their sustainability stories. If that’s you or your company, we’d like to help you get the word out to the North American heat treat industry. Please contact our editors at editor@heattreattoday.com, and we’ll be sure to be in touch
Finally, if you’re a supplier to the North American heat treat industry and your product has a sustainability story to tell, you also should contact our editors: editor@heattreattoday.com.
Keep your eyes peeled for Heat Treat Green!
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Let’s discover new tricks and old tips on how to best serve air and atmosphere furnace systems. In this series, Heat Treat Today compiles top tips from experts around the industry for optimal furnace maintenance, inspection, combustion, data recording, testing, and more. Part 3, today's tips, examines AI and record keeping. Look back to Part 1 here for tips on seals and leaks and Part 2 here for burners and combustion tips.
This Technical Tuesday article is compiled from tips in Heat Treat Today's February Air & Atmosphere Furnace Systems print edition. If you have any tips of your own about air and atmosphere furnaces, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own ideas!
1. Use AI To Simplify Your Maintenance
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"cloud view of heat treating operation" Source: NITREX
Simplify your maintenance! Today, using artificial intelligence (AI) software allows the “Cloud” to do the hard work. NITREX has introduced QMULUS, a web-based software solution, with each of its nitriding systems, which examines key parameters to determine if your furnace is having any issues. Gas flows, amperage, motors, and cycles are all monitored for health factors. But QMULUS is so much more than that. It also analyzes input usages and calculates the cost of each run; logs all data relevant to running processes more efficiently; and provides an easy and seamless cloud view of heat treating operations.
This is a very simple tip that is often overlooked when customers are focused on meeting production goals instead of the maintenance of their equipment. It is critical to record the operating settings of their furnace systems when parts are coming out at their best, or simply before issues arise. When something goes awry in the process and troubleshooting is required, service technicians hear all too often that there is no record of what the ideal or correct setpoints are for various systems. Nearly every item on a modern heat treating furnace (or in its control panel) has a setpoint or position that can be recorded or physically marked. Now, clearly some items are more critical than others when it comes to air and atmosphere settings. Below are a few items you’ll want to have setpoint/positioning records of before they require troubleshooting:
Flowmeter setpoints (at the furnace and generator)
Blower/pump/motor VFD setpoints (primarily frequency setpoints and ramp rates)
Manual or actuated damper positions on flues
Regulator setpoint (from pressure gauge or in-line test port)
High/low pressure switch setpoints
Any air/gas/atmosphere ratios for various recipe steps
Burnout frequency and duration (if applicable)
An added incentive to record these settings is the preventative maintenance benefit. The best way to avoid supply chain issues and delivery delays is to fix a problem before it grows into a bigger issue. When a setpoint/setting is correct but product quality begins changing, it is a warning sign that consumables may be approaching end of life (such as nickel catalyst in endothermic gas generators) or components require maintenance (such as air inlet filter replacements).
Let’s discover new tricks and old tips on how to best serve air and atmosphere furnace systems. In this series, Heat Treat Today compiles top tips from experts around the industry for optimal furnace maintenance, inspection, combustion, data recording, testing, and more. Part 2, today's tips, examines burner and flame safety. Look back to Part 1 here for tips on seals and leaks.
This Technical Tuesday article is compiled from tips in Heat Treat Today's February Air & Atmosphere Furnace Systems print edition. If you have any tips of your own about air and atmosphere furnaces, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own ideas!
1. Operating with a Multiple Burner System
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If a furnace or oven has a multiple burner combustion system with only one valve train, a multi-burner combustion safeguard should be used. This ensures that if one burner fails, they all go out.
Source: Bruce Yates, "Ten Tips for Safeguarding Combustion Processes"
#multiburner #combustion #safety
2. Regularly Inspect Retort Alloys
Source: Nitrex
Retort alloys must be inspected on a regular basis. Hot spots can be identified by bulges. Plastic deformation occurs due to overheating, causing the hotter section to bulge because it is surrounded by stronger metal. Inspect your retorts or radiant tubes for deformations. In addition, constant thermal cycling can cause problems with some alloys. Look for cracks in welds or near welds. Some leak detection methods can also detect alloy issues or overheating.
Localized overheating could indicate a problem with the burner or the heating element. Early detection and correction can save you a lot of money on expensive alloys.
Flame supervision may be defined as the detection of the presence or absence of flame. If a flame is present during the intended combustion period, the supervisory system will allow a fuel flow to feed combustion. If the absence of flame is detected, the fuel valves are de-energized.
This basic definition does not consider the hazard potential during startup or ignition, however. A dangerous combustible mixture within a furnace or oven consists of the accumulation of combustibles (gas) mixed with air, in proportions that will result in rapid or uncontrolled combustion (an explosion). It depends on the quantity of gas and the air-to-fuel ratio at the moment of ignition.
Source: Bruce Yates, "Ten Tips for Safeguarding Combustion Processes"
#flamedetection #combustion #valves
4. Remember that Flame Safety Starts with Purging
The sequence for flame safety starts with purging the furnace or oven. Purge time should allow for four air changes.
Fuel valves can — and do — leak gas. The purpose of purging is to remove combustible gases from the combustion chamber before introducing an ignition source. The four air changes in the combustion chamber are based on a worst-case scenario that includes having a burner chamber that is completely filled with gas.
Once airflow for purge is verified, the proof-of-valve closure is confined and safety limits are proven. Then the purge timer — which may or may not be integral to the combustion safeguard — determines the period of time required to evacuate the combustion chamber.
Source: Bruce Yates, "Ten Tips for Safeguarding Combustion Processes"
What process holds a soft spot in your heart? Tempering or annealing? For Valentine's Day, turn up the heat -- errr heat treatments -- with this look at the differences in tempering and annealing! Heat TreatToday has resources for you to spark some thought and learning on these processes.
Sentiments and strong feelings can certainly be heightened this Valentine's Day. While tempering and annealing may not lend themselves easily to the holiday, we hope you enjoy a bit of a nod to the day in our headings below. Make use of the Reader Feedback button, too, and keep us in the loop with questions and comments on what heat treatment you love.
Problem with Annealing? Get to the Heart of the Issue
An automotive parts manufacturer was running into problems with cracking parts. The variable valve timing plates were returning from heat treatment with this problem. To determine why those parts were cracking after the annealing process, an investigation was launched by metallurgists at Paulo.
The presence of nitrogen combining with the aluminum already present in the particular steel being used was forming aluminum nitrides. What could be done? Read more in the case study article below to find out a workable solution that allowed the annealing to create a crack-free product.
Induction, Rapid Air, Oven and Furnace Tempering: Which One do You Love?
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This article gives some perspectives, from experts in the field, on what kinds of tempering are available and for what the processes are used.
Hear from Bill Stuehr of Induction Tooling, Mike Zaharof of Inductoheat, and Mike Grande of Wisconsin Oven with some basics and background information on tempering. Those reasons alone make this resource helpful with information like this: "tempering at higher temperatures results in lower hardness and increased ductility," says Mike Grande, vice president of sales at Wisconsin Oven. "Tempering at lower temperatures provides a harder steel that is less ductile."
More specific in-depth study is presented as well. The Larson-Miller equation is considered, and the importance of temperature uniformity is emphasized. Read more of the perspectives: "Tempering: 4 Perspectives — Which makes sense for you?"
Cast or Wrought Radiant Tubes in Annealing Furnaces - is Cheaper Really What to Fall For?
Marc Glasser, director of Metallurgical Services at Rolled Alloys, takes a look at radiant tubes. He particularly discusses the cast tubes and wrought tubes. For use in continuous annealing furnaces, there are several factors contributing to choice of radiant tube type.
Marc says, "Justification for the higher cost wrought alloy needs to take into consideration initial fabricated tube cost, actual tube life, AND the lost production of each anticipated downtime cycle as these downtime costs are often much more than material costs." He probes into areas that may not be considered when thinking of all the costs involved. Read more of his article "Radiant Tubes: Exploring Your Options."
Tempering Furnaces: Improvements are Thrilling
The expert behind this piece shows the importance of tempering, particularly in automotive fastener production. Tim Donofrio, vice president of sales at CAN-ENG Furnaces International Limited examines what's working in the tempering furnaces. The products are meeting and exceeding expectations.
To wrap up this Technical Tuesday post on tempering and annealing, head over to this additional resource to round out the scope of each process. "What is the Difference: Tempering VS. Annealing" gives a summary perspective on the heat treatments discussed above.
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Over the last several months, the Combustion Corner series has challenged readers to spend some time researching opportunities to improve their use of radiant tubes — their performance, efficiency, and uniformity. So far, the series has explored the geometry of a tube, why radiant tubes matter, and what happens inside the tube. When it comes to radiant tube systems controls, what are your options? Read on to learn about the three modes of control.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today’s February 2023 Air & Atmosphere Furnace Systemsprint edition.
If you have suggestions for savings opportunities you’d like John to explore for future columns, please email Karen@heattreattoday.com.
John B. Clarke Technical Director Helios Electric Corporation Source: Helios Electric Corporation
This month we will discuss the various modes of control that can be applied to radiant tube systems. We will consider three typical modes of control: on/off, high/low, and proportional control.
When a radiant tube is operated in an on/off mode, the burner is fired full on or completely off. Using this mode of control, the burner must be relit at the start of each cycle. The advantage of this mode of control is that the on firing rate can be optimized to provide optimum heat transfer, and when the burner cycle is off, the tube will idle. If the pulses are rapid enough, there is very little cyclical variation in temperature. The heat capacity (stored heat) of the radiant tube provides a flywheel effect to smooth out the temperature swings between on and off periods. The drawback of this mode of control is that the ignition system, most commonly a spark plug, is energized frequently, loading the transformer and wearing material off the spark plug and the valves that control the air and fuel are cycled frequently. If the cycle time is one minute — the burner must relight, and the valves must cycle over 500,000 times a year. Care must be taken to ensure the components used in this system are rated to survive this demand.
Another mode of control is high/low firing. With this mode of control, the burner cycles between the high firing rate and low firing rate, but instead of shutting down completely, the burners are returned to a low firing condition. In this mode of control, care must be taken to ensure the low firing rate does not overheat the firing leg of the radiant tube. Other than that, this mode of control is very similar to on/off control.
The last mode of control is fully proportional. In this mode of control, the burner fires between 0 and 100 percent of the maximum output depending on the burner demand. The air can be adjusted using a proportional valve or by varying the combustion air blower speed using a variable frequency drive, or in some cases, both. The fuel gas is regulated by a proportional valve or a regulator that matches the output pressure to an impulse or control pressure. Using this mode, the burner fires more or less on ratio (with a consistent level of excess air), or some systems will increase the excess air at low fire to ensure clean combustion and to reduce the available heat at low fire. When a burner has higher levels of excess air, more energy is used to heat the air not used to burn the gas; therefore, less energy is available to heat the furnace chamber. This provides greater turndown (the difference between high and low firing).
Which method is best for a given furnace? That is impossible to say without considering the burner type and geometry of the radiant tube used in the furnace. All three methods can provide good uniformity and efficiency, provided it is appropriate for the equipment in question. In fact, there are applications that blend proportional with high/low firing to meet very specific needs. These systems simply alter the maximum — or high — firing rate to better meet the systems’ requirements.
Again, the control approach is a function of the burner, the radiant tube, and the application. There is really no one-size-its-all; each application must be approached with an open mind. The next column will address the role of heat recovery to efficiency in greater detail.
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There are many radiant tube options on the market, so which one is best for your furnace and your budget? In this column that compares radiant tubes in carburizing and continuous annealing furnaces, discover how two major types of radiant tubes stack up.
Marc Glasser, director of Metallurgical Services at Rolled Alloys, investigates more deeply the two choices. This Technical Tuesday discussion on radiant tubes options will be published inHeat Treat Today'sFebruary 2023 Air & Atmosphere Heat Treating Systems digital edition.
Introduction
Marc Glasser Director of Metallurgical Services Rolled Alloys Source: Rolled Alloys
Radiant tubes are used in many types of heat treating furnaces from carburizing furnaces to continuous annealing of steel strip. Generally, a heat treater has three options for radiant tubes: cast tubes, wrought tubes, and ceramic silicon carbide tubes. Silicon carbide tubes are rarely used by heat treaters, so this article will not delve too deeply into this option. Suffice it to say, ceramic materials can often handle much higher temperatures at the expense of ductility; ceramics are more brittle than metals, making them prone to failure from the small impacts, so metal cages are sometimes fabricated to protect them. Most of the tubes being used today are cast radiant tubes. With new casting technology — primarily centrifugal casting — thinner tubes are being cast at a lower cost, which then results in a shorter life.
The primary factors for choosing radiant tube material are tube temperature and carbon potential of the furnace atmosphere. Cost-benefit analysis should also be considered. There are multiple applications for radiant tubes, including carburizing furnaces, continuous annealing furnaces for steel sheet galvanizing, steel reheat furnaces, and aluminum heat treating. This article will explore two of the aforementioned radiant tube options, specifically for carburizing and continuous annealing furnaces.
Radiant Tubes for Carburizing Furnaces
Gas carburization is traditionally performed between 1650°F and 1700°F at a carbon potential of 0.8% approximating the eutectoid composition. In today’s competitive environment, more heat treaters are increasing temperatures to 1750°F and pushing carbon potentials as high as 1.6% to get faster diffusion of carbon while spending less time at temperature. INCONEL® HX (66% Ni, 17% Cr) has been a common cast alloy seen in carburizing furnaces. This alloy is regularly selected for its resistance to oxidation and carburization up to 2100°F. Super 22H is more heavily alloyed than HX and is seeing more use as carbon potentials increase but at a premium price. With advances in centrifugal castings, cast tube wall thicknesses have decreased from 3/8-inch to 1/4-inch. Some heat treaters have shared that this decrease in wall thickness has also led to shorter tube life.
Fabricated and welded radiant tubes in alloys 601 and RA 602 CA® have been tested in industry. When tested, these wrought alloys were fabricated to have a wall thickness of 1/8-inch. At the extremes, tubes fabricated from 601 only lasted 50% as long as cast HX. Historically, HX tubes have been approximately 33% higher in cost than that of 601 and utilize heavier 3/8-inch walls. A little-known fact is that by switching to a thinner wall cast tube, the life drops by 50%. By switching to 1/8-inch wall thickness, RA 602 CA tube life has been extended to eight years or more, while running at 1750°F and up to 1.6% carbon potential, at just a 33% premium over cast HX. Life cycle data are presented in Figure 1.
Figure 1. These life cycle comparisons were done in carburizing furnaces only. In non-carburizing furnaces, justification of alloy selection is dependent on actual operating conditions and each individual operator’s own experience. Source: Rolled Alloys
Radiant Tubes for Continuous Annealing Furnaces
In the area of continuous annealing, the cast alloy of choice is HP/HT (35% Ni, 17% Cr, 1.7% Si, 0.5% C). Here again, this casting has been compared to 601 and RA 602 CA, with the same results. The total life data from these trials are also incorporated into Figure 1. During the collection of this data, there has been no effort to measure the actual tube temperature, so the effect of tube temperature is not clearly defined. In these continuous annealing furnaces, it has been reported that the tubes at the entry end are subject to more heat absorption as burners are firing more due to the continuous introduction of cold material; in trials, the operators have not kept adequate documentation of specific tubes, making justification more diffcult.
Justification for the higher cost wrought alloy needs to take into consideration initial fabricated tube cost, actual tube life, AND the lost production of each anticipated downtime cycle as these downtime costs are often much more than material costs. Only individual fabricators can determine these costs.
The Economics
Table 1 Source: Rolled Alloys
Table 1 above shows the economics of metal alloy choice. To properly interpret, understand that the costs are not actual, but rather relative to 601, so a round number of 1000 was used. With a 30% greater cost of cast tubes, that translates to a relative cost of $1300. The annual cost is the amortized cost over the life of the tube. The total eight-year cost is the relative cost times the number of tubes that would have to be purchased to obtain the life cycle of one tube of the longest-lasting material over its full life cycle.
Missing in this analysis is the additional cost of downtime and lost production. For the replacement of radiant tubes in a carburizing furnace, this typically entails a full week to turn a furnace off, allow it to cool, replace the tubes, and then heat it up again. Many heat treaters do not consider this, and therefore it is a hidden cost. Even without the downtime being considered, by examining the total cost of materials (including replacements) compared to the longest-lasting tube, it turns out that the most expensive tube is the cheapest tube. The obstacle to overcome is whether the heat treater is willing to wait eight years to realize these cost savings.
There can be additional factors to consider. With improvements in the efficiency of casting, the actual costs of the thinner wall casting may be somewhat less, but to match the overall cost of the longest-life material, it would have to be less than half the expected cost. As better, more expensive cast alloys become accepted and actual life data becomes available, these more costly alloys can be added to this table for comparative analysis, too.
This same method of analysis can be applied to radiant tubes for continuous annealing furnaces, but more details will need to be added including furnace position. Different alloy candidates will have to be put to the test in actual operations, carefully document what alloy is in what position or location, and when it gets changed out. This becomes quite cumbersome when annealing furnaces (depending on design and manufacture) can have over 200 radiant tubes.
Conclusion
Currently, cast alloy tubes dominate the market. The concept of total life cycle cost has been introduced as a means of more accurately justifying one’s choice of radiant tube. This comes into play more as processes are pushed beyond traditional process conditions. Cost-benefit analysis must be balanced over acceptable amortization time, of course. However, performing the full analysis as well as the costs saved from downtime may lead some heat treaters to some alternate materials.
About the author: Marc Glasser is the director of Metallurgical Services at Rolled Alloys and is an expert in process metallurgy, heat treatment, materials of construction, and materials science and testing. Marc received his bachelor’s degree in materials engineering from Rensselaer Polytechnic Institute and a master’s degree in material science from Polytechnic University, now known as the NYU School of Engineering. Contact Marc at mglasser@rolledalloys.com
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Last month, we introduced the importance of radiant tubes in the heat treat industry. We explored the “why” of radiant tubes and skimmed the surface, exploring materials, sizing, shapes, longevity, and installation — all topics we’ll deep dive into in future posts. This month, let’s explore what typically occurs inside a radiant tube.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today's December 2022 Medical and Energyprint edition.
If you have suggestions for topics you’d like John to explore in future columns, please email Karen@heattreattoday.com.
John B. Clarke Technical Director Helios Electric Corporation Source: Helios Electrical Corporation
The radiant tube burner combines fuel and an oxidizer (commonly air) in the presence of a source of ignition. Radiant tube burners differ from burners that are fired into an open furnace. They function to distribute heat as uniformly as possible within the interior of the tube to maximize its temperature and heat transfer uniformity. In some applications, a low rate of heat transfer is acceptable (for example, in the holding zone of a continuous furnace). In that same furnace, a much higher heat transfer rate may be required in the front of the furnace. In all cases, higher heat transfer rates result in higher internal tube temperatures. In most cases, the higher the temperature, the greater the stress on the material.
Within the radiant tube in the visual flame region, the energy is transferred to the inner surface of the tube by convection and radiation. The rate of convective transfer has much to do with the mixing characteristics of the burner in question. Once combustion is complete, the heated products of combustion — CO2 , O2 , H2O, and N2 — continue to flow through the radiant tube. They impart heat to the interior surface of the radiant tube through convections and — in the case of the CO2 and H2 — radiation. The non-polar gases (O2 and N2) are effectively transparent to radiation: neither absorbing nor radiating heat. This transparency poses a problem for the performance of radiant tubes because the combustion process is ideally complete some distance before the end of the radiant tube.
There are a few ways to make use of the heat stored in the O2 and N2 . One way is to stir the mixtures to ensure these gases meet the inside walls of the tube and can convectively transfer their energy. Another way is to insert a “core buster” or other device into the exit end of the radiant tube. This device must be able to withstand the peak temperature of the products of combustion at this point, so it is typically constructed of some ceramic material or a composite of ceramics. As the heated gases pass over this “core buster,” the resistance forces higher flows around the perimeter of the tube, increasing convective transfer. The “core buster” also is convectively heated and can then radiate heat to the inner surface of the tube and, finally, the “core buster” increases mixing of the gases to ensure all remaining hydrocarbons and carbon monoxide are brought into contact with oxygen to complete the oxidation process.
The transfer of heat to the inner surface is dependent on the effective surface area. A tube with a nominal inside diameter of four inches may have a much greater effective surface area due to roughness, which resemble very small peaks and valleys. Anyone who has attempted to walk around a small Caribbean island can attest — it takes a lot longer than you would think by looking at the map and really scares your shipmates when they cannot find you. Cast and composite radiant tubes can be fabricated to increase this effective internal surface area. Tubing can also be equipped with internal fins.[blocktext align="left"]No matter what the construction, ultimately it does no good to transfer heat to the interior of the radiant tube if the tube cannot transfer the same quantity of heat through the exterior to the furnace and work being heated.[/blocktext]
Which mode of control is better? High/Low, proportional, or pulsed? Any method can achieve a uniform tube heat release given the correct burner radiant tube combination. The important thing is that the vigor of the mixing is matched to the length and roughness of the radiant tube. Burner X may be perfectly suited to a short radiant tube but lead to non-uniform heating as the tube length is extended. On the other hand, Burner Y, with a relatively lazy flame, may work perfectly on long tubes with lower heat transfer demands but be unsuitable for short tubes where high heat transfer rates are desired.
In the coming months, we will examine many of these areas in greater detail, and this author can make use of his experience of many failures to inform the readers of what not to do. Then, by extension, we’ll learn how to get more from the furnaces by thinking systematically about their radiant tubes, burners, and controls.
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Radiant tubes are prevalent in heat treating applications. They are very simple devices: basically, a pipe that enters and exits the work chamber. Geometrically simple — but the considerations of how they should be applied, the optimal materials for their construction, and the best burner to use present a myriad of challenges and opportunities for improvement. As all heat treaters know, radiant tubes represent a significant expense as well as an opportunity to save on maintenance costs and improve furnace performance.
This column is a Combustion Corner feature written by John Clarke, technical director at Helios Electric Corporation, and appeared in Heat Treat Today's November 2022 Vacuumprint edition.
If you have suggestions for topics you’d like John to explore in future columns, please email Karen@heattreattoday.com.
John B. Clarke Technical Director Helios Electric Corporation Source: Helios Electrical Corporation
In the coming months, I hope to challenge the reader to spend some time researching opportunities to improve their use of radiant tubes — that is to improve their performance, both heating rates and efficiency, as well as to extend their life and perhaps improve the uniformity of the furnace being heated.
I apologize in advance if I sound like an economist — “It is this way, but on the other hand . . .” There are a lot of factors to consider when planning to upgrade your radiant tubes, their associated burners, recuperators, mountings, and supports.
To start, let’s answer a simple question: Why do we use radiant tubes? Two reasons come to mind: to protect the furnace atmosphere from the products of combustion and/or to diffuse the release of heat within the furnace or oven chamber to maximize temperature uniformity. In many heat treating applications, even a very small leak will contaminate the furnace atmosphere, damaging the work being processed.
How do we size radiant tubes? Again, it is obvious that we need to have sufficient heated external surface area to transfer the heat to the furnace chamber. This heat transfer will occur through convection and radiation, with the latter mode being more significant as the furnace temperature rises. The rate of convective heat transfer will depend on mass and velocity of air or atmosphere passing over the tubes. The radiant heat transfer rate is a function of the difference between the tubes’ surface temperature and the temperature of the furnace and work being heated. The good news with radiant heat transfer in closed furnaces is that all surfaces in the furnace participate to a degree with the transfer of heat to the work.
There are many shapes for radiant tubes: U-shaped, W-shaped, three legged, as well as systems where the firing and exhaust occur at the same opening, including P-tubes and single-ended tubes. Each has its advantages and disadvantages, which we’ll discuss in future articles.
How about materials? Again, we have a lot of choices. The tubes can be centrifugally cast, fabricated from sheet, or made of some ceramic or composite material. [blocktext align="center"]The formulation of each material varies greatly, and it is important that the material is suitable for the use temperature and chemical composition of the furnace atmosphere as well as always being compatible with the common products of combustion.[/blocktext]
How are the radiant tubes installed? Are the ends welded to a mounting plate, or perhaps a packing gland is employed to seal the tube while allowing some expansion or contraction? Both methods are commonly applied successfully. Composite tubes may have a flange that is clamped at the mounting location, or they may use a packing gland. The tubes may have internal supports within the furnace to prevent sagging. The tubes can be hung vertically, located to the side of, or placed under and over the work being heated.
How long should my radiant tubes last? Simply answered, for as long as practical. As a young person, I was mortified when I dropped a hammer in a customer’s pusher carburizing furnace, and it broke an alloy tube. When I confessed to the plant metallurgist, he laughed and told me the tube I broke was over twenty years old. Other customers may be satisfied if their tubes last 18 months, so there is no simple answer. That said, there may well be opportunities to extend the life of the radiant tubes in your specific application.
We will revisit many of these discussions in later articles, but hopefully this column has whetted your appetite for the next discussion in December: What typically occurs inside the radiant tube? After all, this is the Combustion Corner.
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Nico Schmitz, Christian Schwotzer, and Herbert Pfeifer with the Department for Industrial Furnaces and Heat Engineering (IOB) in Germany have collaborated on an analysis of metallic recirculating radiant tubes, their purpose in the heat treating process, and their design and installation. In particular, the authors, with access to a furnace-equipped pilot plant operated by IOB, investigate the factors that affect tube productivity and contribute to tube failures. They have reported on these findings in an exclusive paper published at heat processing online, the official publication of the European Committee of Industrial Furnace and Heating Equipment Association (CECOF).
An excerpt:
“It is common to assume a homogeneous temperature distribution for construction calculations. In real operation, inhomogeneous temperature distributions occur. The temperature gradients induce thermal stresses that can substantially influence the lifetime of the tubes. In addition to that, higher furnace temperatures come along with an increasing thermal load.”
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