Understanding burners for heat-treating furnaces
Heat treating is an industrial process that alters the physical properties of metal by heating parts or components to extreme temperatures to achieve a desired hardness. According to the book, "Atmosphere Heat Treatment," by Dan Herring, president of The Herring Group Inc., "Heat treatment of semi-finished goods takes place in box, pit, mechanized box, and custom-designed equipment being either batch type or continuous throughput designs, which are primarily direct or indirect-fired. Processes include annealing, brazing, case hardening (carburizing/carbonitriding, nitriding/nitrocarburizing), hardening, normalizing, sintering, stress relief, and tempering to name a few."
Typically, ovens operate at temperatures below 800°F to 1,000°F and furnaces operate above this level. Ovens and furnaces used for heat treating can be heated using electrical heating elements or gas-fired burners. This article focuses on burners for gas-fired heat-treating furnaces.
Heat-treating furnaces typically have multiple burners that can either heat the furnace atmosphere directly or through a network of radiant tubes. Increasingly, furnaces have sophisticated digital controls for temperature management, process control, and for assurance of safety through flame detection and stabilization.
Efficiency is more important than ever
Most heat-treating furnace projects have an energy-efficient burner design incorporated into new project requirements. "As recently as the 1970s, it was not uncommon to find burner efficiencies in the range of 25% to 35% as the best available technology," said Jim Roberts, specialist-application professional, ETO (engineered to order) at Honeywell Thermal Solutions. Then came fuel shortages and rising costs. "All of a sudden, it was critical to contain the process costs associated with heat treating, and to become fuel efficient. This spurred burner companies to develop burners that recover some of the waste heat they generate, and to return it to the process. The most efficient way to do this is to preheat the air used in the combustion of fuel gases. When the air is preheated before being mixed with the gas, it can render fantastic fuel efficiency gains if done correctly." Roberts estimates that fuel efficiencies around 60% for high-temperature burners are now possible.
According to Michael Cochran, marketing engineer, combustion systems at Bloom Engineering Company Inc., while there are some efficiency enhancements due to improved materials or insulators, the biggest improvements have been made in control and process advances. "With a careful engineering analysis, it often is possible to obtain more efficiency by optimizing either process or system control. As an added benefit, in many cases, such optimization does not require substantial physical hardware upgrades."
Jerry Last, vice president of Furnace Solutions Inc., says, "Every furnace style and every application present different inefficiency issues to overcome. In regard to heat treating, a common issue is getting efficiency out of a tube-fired burner. For many years, that efficiency was limited to the tube itself, because the tube could only shed so much heat per square inch. Significant strides have come with the tube material and design that assists in that. Additionally, over the past 15 years or so, design advances in how the burner fires within the tube allows the tube to be fired uniformly along its entire length as well as more efficient "scrubbing" of the tube, which together allow for much higher tube loading."
Recuperative versus regenerative burners
In order to boost efficiency, many industrial processes use heat recovery systems that will strip heat out of the waste gases and deliver it back to the process. Cochran explains that recuperative systems affect this heating by using an external (usually metallic) heat exchanger where the waste gases flow through the hot side (thus cooling off) and the combustion air flows through the cold side (thus accepting heat to return to the process). So, recuperative burners recover heat from the tube exhaust and use it to preheat fuel gases. "For a regenerator, the waste gas and air alternately flow through a common case of heat storing (often ceramic) material. As the waste gas passes through, it gives up heat to the media, and when the air passes through later, it retrieves the heat and brings it back into the process," Cochran said.
Regenerative burners are alternately fired in opposite directions and discharge exhaust through a refractory bed or case, which captures a large portion of the heat. When the refractory is heated, the flow is reversed and the opposite end of the tube collects exhaust heat. The goal of both regenerative and recuperative designs is to capture heat energy that would otherwise be wasted.
Last says that regeneration is extremely efficient and will cut most fuel bills in half. "Regeneration is relatively costly, difficult to incorporate in a retrofit, difficult to incorporate in smaller furnaces, and often more impactful is the amount of additional maintenance that is required. Recuperation is simply using a heat exchanger in the waste gas stream. The combustion air passes through the heat exchanger (recuperator), allowing the combustion air to preheat. Recuperation is very simple, less expensive, smaller footprint, easier to meet temperature uniformity at lower temperatures, easy to incorporate in a retrofit, and often will provide a fuel reduction of 30%."
According to Roberts, the Eclipse SER V5 recuperative radiant tube burners from Honeywell Thermal Solutions are well suited to retrofit burners and external recuperators in existing furnaces. The SER V5 can be mounted in horizontal or vertical configurations and is suitable for either continuous or batch type furnaces with a variety of atmospheres. For the direct fired side of heat treating, Roberts said, the Eclipse TJSR V5 is a direct fired, self-recuperative burner with a space saving, integral eductor that pulls the furnace exhaust through an internal ceramic recuperator. The recuperator preheats the incoming combustion air to very high levels, which improves furnace operating efficiency to reduce fuel usage by as much as 50% over typical ambient air burners.
Cochran says, "While the physical burner hardware (rightly) receives quite a bit of attention, Bloom is making important contributions to the control of the system. One of our most innovative recent developments has been to reinvent the control of a regenerative system. By fundamentally changing some of the key components (physical and conceptual) in regenerative system control, we have been able to increase fuel efficiency, boost productivity, and cut yield loss. We have always been at the forefront of emissions reduction research, and many of our burner products make use of technologies to reduce NOX emissions. In particular, our line of radiant tube products, regenerative burners, and high thermal release (flat-flame) burners are some of the most advanced in terms of emissions mitigations."
In the most general of terms, industrial heat-treating chambers, which can be furnaces, ovens, or kilns, there are two types: batch and continuous. Cochran explains the differences: "Batch furnaces take a stationary load of material and put it through a thermal cycle. A continuous process takes a load and physically moves it through a heating cycle. In the broadest terms, often batch processes, such as aluminum melting furnaces, and forge furnaces, are good candidates for regenerative systems. However, recuperative systems are common for many continuous operations, such as steel reheat furnaces. In actual application, the distinction is not so clear-cut. Most applications, with proper engineering, can generally accommodate most types of combustion systems."
Controlling burner operation
Controlling burners is actually done by controlling the ratio of fuel and air to them. While a thorough definition of burner control can be extensive, Cochran provides a brief explanation: "A burner control system provides the proper amounts of air and fuel for good combustion. Fundamentally, there are two main ways, (with many variations) of controlling the air and fuel flows. First, a technique generally called pressure balance modulates the flow of air, and then uses a pressure regulator to permit a corresponding flow of fuel. The fuel flow always follows the air flow. The other major type of system allows for independent control of air and fuel flows. This system uses an algorithm to determine the flows of each, meaning that they can function somewhat independently of one another. The flexibility of such a system means that it is more versatile and can handle a wide range of process requirements."
Last adds that recuperative and regenerative burners can be controlled in any manner that cold air burners are controlled. As with cold air burners, the control style is determined by the application. Last offers the following control application examples:
- Tight temperature uniformity, large temperature control ranges, and narrow firing lanes are some reasons to consider pulse firing. Note that pulse firing consists of either high/low or on/off firing. The decision between those two is primarily determined by the operating temperature range.
- Lower temperatures or incineration requirements are some reasons to consider fuel-only control.
- Higher temperatures are typically on-ratio. Very efficient and simple air primary control via a modulated valve on the air with cross-connected regulators on the gas work well. Additionally, single-zone applications often can incorporate variable frequency drives (VFDs) on the combustion blower removing the valve/actuator setup and adding another level of efficiency.
- Tube fired burners would be high/low, on/off, and possibly pulse fired.
Keeping an eye on combustion safety
Because combustion necessarily involves igniting natural gas, safety is always a concern. "Some systems, such as atmosphere furnaces, require more built-in safety, and some systems, such as tube-fired burners that have explosion-resistant tubes require less," Last said. "In addition, each application requires specific items, such as self-checking ultra-violet (UV) scanners on systems operating more than 24 hours without shutting down.
"Generally, there is a strainer/drip leg to make certain the supply gas to the system is clean," continues Last. "There often is a pressure-reducing regulator to not only reduce the incoming gas pressure but also to maintain a more stable pressure to the system. There is a gas train that makes certain the gas pressures are within the high and low limits of the components and burner firing capability. The gas train has both automatic and manual shut-off valves. The manual valves are for when the system is down and for leak testing. The automatic valves are interlocked with the flame supervision, which allows gas and ignition during a trial-for-ignition period. After the trial-for-ignition period, if there is no flame proven in the burner, the gas is shut off. If the flame is proven at the end of the trial-for-ignition period, the gas remains on and the system is released to control.
"The combustion blower also is tied into the safeties by a motor-starter contact, low air-pressure switch, and typically a high proof-of-flow switch for the purge. The purge is done prior to the trial-for-ignition to remove the potential for any combustible substance in the system. The purge is typically done with the burners at 100% output and the system doors closed. The trial-for-ignition typically is done with the burners at 0% output and the system doors open. There are excess temperature controllers required to make certain there is not an out-of-control temperature situation in the furnace. Also, there is a remote shut-off valve located away from the equipment that would shut off the fuel to the system in case of an emergency."
Take initiative in finding savings
Additional information on improving furnace and burner process efficiency is available on the U.S. Department of Energy website, and from the various furnace manufacturers. Many furnaces in industrial use today that have not been recently upgraded are operating at less than optimum efficiency, and would benefit from a professional review, with a view to upgrading or replacement.
This article originally appeared in the Gas Technology Spring 2018 issue.