Temperature uniformity is influenced by factors such as flame geometry, burner placement and how the airstream mixes with the burner products of combustion. Knowing these elements is essential in the optimization of heat transfer throughout an industrial oven, dryer or furnace. A defined temperature uniformity survey (TUS) is required for the heat treatment of aerospace products; however, improving temperature uniformity will benefit other processes as well.
In high capacity systems, the energy source is primarily the combustion of fuel. Three burner platforms commonly are used in air-heating systems: in-duct, nozzle-mix (or forward-flame) and lean premix burners.
In-duct burners spread the heat release across a section of the duct (figure 1). They require a uniform inlet process stream flow and may require profile plates to achieve the required differential pressure drop across the burner grid. Pass-through connections for ignition and flame detection also are needed. In some applications, multiple flame detectors may be necessary to meet the specific requirements outlined in NFPA 86-2019 Standard for Ovens and Furnaces. In this case, clause 8.10.6 requires a flame to be proven at the far end (opposite the ignition end) for a burner longer than 3 linear feet.
Also known as forward-flame burners, nozzle-mix burners typically are used in applications where the burner can be located on an elbow or corner in the ductwork (figure 2). They require a flame-protection tube, typically a cylinder, to limit flame quenching and promote complete combustion. With the process flow coming in perpendicular to the flame-protection tube, the flame may be deflected and result in overheating due to the momentum of the process flow stream. Overheating of the tube can result in distortion, early failure and frequent need of replacement. Upstream and downstream flow conditioning may be necessary to achieve the desired temperature uniformity downstream.
FIGURE 2. Also known as forward-flame burners, nozzle-mix burners typically are used in applications where the burner can be located on an elbow or corner in the ductwork. Image provided by Fives North American Combustion Inc.
The uncertainty of flame deflection creates difficulty when predicting localized temperatures. This can hamper the design and material selection for the flame-protection tube. Target baffles, mixing baffles, flow straighteners and heat shields internal to the ductwork may be needed. Larger ovens and dryers with input between 15 and 100 million BTU/hr often require ultra-low NOX burner systems to meet air permit requirements.
Lean Premix Burners
FIGURE 3. Lean premix burners typically operate with 60 to 70 percent excess air and can achieve single-digit NOX ppm levels. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
Lean premix technology is a burner technology capable of meeting ultra-low NOX and CO emissions limits, particularly in low temperature applications (figures 3 and 4). Lean premix burners typically operate with 60 to 70 percent excess air and can achieve single-digit NOX ppm levels. When implemented with an extended reaction chamber — the ultimate flame protector — they can simultaneously achieve low CO even at low process temperatures (less than 1000°F [538°C]).
FIGURE 4. Lean premix technology is a a burner technology capable of meeting ultra-low NOX and CO emissions limits, particularly in low temperature applications. Image provided by Fives North American Combustion Inc.
Lean premix burner technology also produces compact flame envelopes such that combustion can be completed within an extended reaction chamber. To illustrate this point, figure 5 is an example of burner and reaction chamber with an input of 112 million BTU/hr. Its footprint is 11 feet long by 5 feet diameter.
FIGURE 5. Lean premix burner technology produces compact flame envelopes such that combustion can be completed within an extended reaction chamber. This burner and reaction chamber have an input of 112 million BTU/hr yet have a compact 11 foot by 5 foot diameter footprint. Image provided by Fives North American Combustion Inc. (Click to enlarge image)
The extended reaction chamber is similar to the flame-protection tube, but it is refractory lined and ensures the completion of combustion by preventing any interaction with the process stream. The refractory lining increases the total weight, however.
The velocity of the products of combustion exiting the extended reaction chamber can be designed to optimize mixing with the process stream. In order to achieve the desired temperature uniformity for a specific process, it is necessary to select the an appropriate technology for mixing of the products of combustion (POC) with the process stream to be heated.
The temperature uniformity resulting from mixing the products of combustion and the process stream is directly linked to the mixing technology selected and the amount of mixing length required. Each technology has trade-offs that affect the required mixing length, initial costs and maintenance costs. Four mixing systems of interest — in order of increasing complexity and decreasing mixing length — are duct, target and baffles, static mixers and dilute jet.
Lean premix burners with extended reaction chambers provide an effective heat source for mixing (figure 6). Despite the duct-mixing system’s simple construction and extremely low process stream pressure drop, temperature uniformity may be more difficult to predict due to less efficient mixing and extremely long mixing distances.
Duct-mixing systems are the simplest of all the mixing systems, containing a burner located on the elbow section of the duct (figure 6). Turning vanes could be added to the incoming process stream, but they have no effect on mixing. Essentially, this system does not have any mixing elements, and the mixing occurs due to turbulence as the fluids travel down the duct. Changes in the duct geometry and direction can increase mixing, but such changes do not have an appreciable effect.
FIGURE 6. Duct-mixing systems are the simplest of all of the mixing systems. The inlet process stream flow will deflect the hot burner products of combustion stream up against the duct wall. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
Typically, with this style system, the inlet process stream flow will deflect the hot burner products of combustion stream up against the duct wall. (In this design, the duct wall acts as an elbow to the process stream.) This will result in a portion of the duct wall getting hotter and having a nonuniform circumferential temperature distribution. Due to the hot products of combustion impinging on the duct wall, a higher-grade alloy is necessary for construction to prevent rapid oxidation and corrosion. Additionally, care will need to be taken to avoid allowing the duct wall’s thermal nonuniformity to distort the chamber.
Target and Mixing Baffles
Mixing systems consisting of target and mixing baffles (figure 7) improve upon the mixing distance but do little else to improve upon the detriments in the duct-mixing system. While the addition of the baffles or bluff bodies promotes mixing, they are prone to failure and frequent replacement. The baffles also require an appropriate material to handle the high temperatures they will experience as the hot products of combustion impinge upon them.
FIGURE 7. Mixing systems such as target and mixing baffles promote mixing but are prone to failure and frequent replacement. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
Static mixers replace the baffle-style system with a mixing element design to swirl the flow in multiple directions to enhance mixing (figure 8). The static mixers are placed far enough downstream of the burner outlet to ensure the hot products of combustion mix with the process stream to prevent overheating and damage to the static mixer.
FIGURE 8. Static mixers replace the baffle-style system with a mixing element design. The sets of vanes swirl the process stream flow in opposite directions to enhance mixing. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
The static mixer uses sets of vanes to swirl the process stream flow in opposite directions, which promotes mixing. For example, the inner set of vanes swirl the flow clockwise while the outer set swirl the flow counterclockwise. This swirl creates enough turbulence to provide a predicable mixing result thermally. Thus, static mixers can shorten the length of duct needed to achieve the desired temperature uniformity.
Dilute Jet Mixing Systems
Dilution jet mixing eliminates the need for physical objects in the hot products of combustion flow path to promote mixing by the turbulence they create (figures 9 and 10). Instead, dilution jet mixing employs cross-jet mixing.
FIGURE 9. Dilution jet mixing eliminates the need for physical objects in the hot products of combustion flow path. A dilute jet mixing system for process outlet temperatures up to 800°F (427°C) is shown. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
This style of mixing directs jets of the process stream into the hot products of combustion such that the process stream jets penetrate to the desired position inside the hot POC stream to maximize mixing. This technology requires the proper balance of the hot products of combustion momentum to the process stream jets momentum.
FIGURE 10. Dilution jet mixing eliminates the need for physical objects in the hot products of combustion flow path. A dilute jet mixing system for process outlet temperatures between 800 and 1200°F (427 and 649°C) is shown. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
By employing dilute jet mixing, the distance to achieve a uniform stream is greatly reduced. By directing the process stream through these cross-jet holes, turbulent mixing is created by the process stream. Thus, the need for physical objects that are subject to thermal stress — like the baffles and static mixers previously mentioned — are eliminated.
Dilute Jet Mixing Case Study
Empirical data has been obtained to confirm the performance of the dilute jet mixing system. A case study was done on a unit installed in a spray dryer. The uniformity achieved on this unit (figure 11) at the spray dryer temperature measurement rake — located at three length/diameter units downstream of the process stream outlet — had a peak-to-peak temperature difference of just 44°F (24.4°C). This surpassed the requirement of 815°F (435°C) ±25°F (13.9°C) thermal uniformity.
FIGURE 11. The temperature uniformity provided by a dilute jet mixing system is shown. Image provided by Fives North American Combustion Inc. (Click image to enlarge)
Finite element analysis (FEA) is performed on the dilution jet mixing system to ensure that it is sufficiently strong enough to hold the weight of the reaction chamber. FEA also is used to examine the loading on the legs of the heater assembly and predict overall growth of the heater assembly. The heater typically uses a thin shell thickness to keep weight down, similar to the shell thickness used for the ductwork. This results in the unit behaving as a thin-walled structure with the ability to flex. The FEA looks at how the unit will flex while ensuring that no areas of the unit are overstressed at service temperatures.
In conclusion, optimizing temperature uniformity can improve product quality and production rate and reduce maintenance outages and costs. Conditioning of the inlet process stream and controlling how it mixes with the burner products of combustion are critical parameters to achieving a uniform temperature in the process stream exiting the heater and entering the process.
Dilute jet mixing appreciably shortens the mixing distance (length) and eliminates flame deflection that can create localized hot spots encountered with the other mixing methods that require physical objects to create turbulence and mixing. Protection of the burner flame is critical to controlling burner attributed NOX and CO emissions and to the design of the mixing system. Together, these elements provide enhanced temperature uniformity in process dryers, furnaces and ovens.
Ted Jablkowski is the Northeast regional sales manager and David Hausen is the lead design engineer with Fives North American Combustion Inc. The Cleveland-based company can be reached at 800-626-3477 or visit combustion.fivesgroup.com.