High-velocity burners

Figure 23.15 shows the potential of NO reduction of the flameless oxidation. The values are in a logarithmic scale as the emissions are very much temperature dependent. As reference values, the emissions from air-staged high velocity burners are reported. The NO, values from burners without NO countermeasures and with preheated air are clearly much higher. 

With the exception of the burner near-field, the temperature distribution in a furnace is similar to the one onset by high velocity burners. 

Fig. 1.8. Rotary hearth furnace, donut type, sectioned plan view. (Disk type has no hole in the middle.) Short-flame burners fire from its outer periphery. Burners also are sometimes fired from the inner wall outward. Long-flame burners are sometimes fired through a sawtooth roof, but not through the sidewalls because they tend to overheat the opposite wall and ends of load pieces. R, regenerative burner E, enhanced heating high-velocity burner. (See also fig. 6.7.)...

For medium or low temperature furnaces/ovens/dryers operating below about 1400 F (760 C), a forced recirculation furnace or recirculating oven delivers better temperature uniformity and better fuel economy. The recirculation can be by a fan and duct arrangement, by ceiling plug fans, or by the jet momentum of burners (especially type H high-velocity burners—fig. 6.2). 

Fig. 1.18 Car-hearth heat treat furnace with piers for better exposure of bottom side of loads. The spaces between the piers can be used for enhanced heating with small high-velocity burners. (See chap. 7.) Automatic furnace pressure control allows roof flues without nonuniformity problems and without high fuel cost.

Solid material that is heated in industrial furnaces is not necessarily continuous. Very often, the charge consists of coiled strip material or separate pieces piled to various depths or close side by side. In such cases, heat only can fiow from one piece to the adjacent piece through small contact points on their surfaces, or through gas-filled spaces—the thermal conductivity of which is very small. A pile of crankshafts is an example of low overall conductance, but high-velocity burners may be able to blow some gases between the pieces. 

In batch-type furnaces, reducing underload clearance, reducing triatomic gas concentrations, and using high-velocity burners to inspirate furnace gases for increased mass flow under the load has reduced cross-wise load-bottom temperature differentials to less than 15°F (8°C). It is important to remember that the high-velocity underpass gases do not exit the furnace at the end of their pass, but circulate around the load(s) several times, and that they enhance radiation and convection in other parts of the furnace. 

Use enhanced heating Operate with very high velocity burners to inspirate great quantities of furnace gas into the tunnels between the piers. With this high mass flow of gas between the piers and between the load and the hearth, the burner poc temperature is nearly uniform, resulting in a more uniform load temperature (reflecting the more uniform poc temperature). 

Figure 3.5 shows a 40 ft (12.2 m) long car-hearth in a 17.5 ft (5.3 m) high fiber-lined furnace with high-velocity burners at top and between the piers. Automatic furnace pressure control makes it possible to use top flues. Drilled square air manifolds shoot curtains of air across the flue exits as throttleable air curtain dampers for furnace pressure control. 

Example 3,2 Heat a load of three steel rounds, 24" (0.61 m) diameter, for forging in a furnace 8.5 ft (2.6 m) wide x 6 ft (1.83 m) high inside. Loads are on piers with centerlines 3.2 ft (0.98 m) apart. High-velocity burners fire through alleys between the pieces-enhanced heating). The center piece is the most difficult to heat because outer pieces shield it from side radiation and convection thus, it will govern the heating time required. 

Most cover annealers are single stack furnaces, but there are some multistack annealers with three, four, six, or eight stacks, each with a bell cover, all within one rectangular furnace. (Radiant tubes were used in addition to the inner covers in the past because of poor heating between the inner covers.) Now, type H high-velocity burners are fired down or up between the inner covers. 

To assure minimum bottom temperature difference across the furnace width of the load, two T-sensors should be installed, one on each side of the furnace (arrows 3 and 4 in fig. 3.26). The 4 T-sensors should be positioned 1 to 3 in. (25 to 75 mm) above the pier top in the wall opposite the high-velocity burners, controlling the fuel input (with combustion air flow held constant). The 3 T-sensor should be at the same elevation as the 4 sensor, on the same side as the high-velocity burners. In a heavily loaded furnace at forging temperature, the two opposite lower sensors should be within 6°F (3.3°C) of one another. 

Control the furnace bottom temperatures with many small, high-velocity burners firing with constant air to hold the same temperatures below the load as above it. Install fuel meters on each zone. When the fuel flows reach minimum in all zones, hold for several hours, then remove the load from the furnace for processing. The benefits will accrue from shorter cycles, many times by 25% because uniformity of zone temperatures is held from charge-to-draw requiring minimum soak time. 

Provide main regenerative burners in zones 1 and 2, with enhanced heating in the form of small, high-velocity burners directed down at 10° to 25° to move the gases in the alleys between the pieces. The exposure increase will provide a remedy for delay problems, plus improved heat transfer in zone 1. 

Replace large burners in the center (doughnut hole) of large rotary hearth furnaces with high-velocity burners for better crosswise gas and temperature distribution. 

Many small, high-velocity burners might improve heat transfer if installed to fire between the tubes and the refractory walls. 

Side-fired box and car-bottom furnaces are ideally fired with main burners on 2.5-ft to 4.5-ft (0.6 m to 1.4 m) centers along the top on one side, and small pumping high-velocity burners on the opposite bottom side. (See fig. 6.1.) The main burners should have ATP technology so that the temperature can be controlled to a flat profile with the T-sensors located at the level of the top of the load through each of the two long sidewalls. 

The loads should be on piers so that small, high-velocity burners can be fired underneath. For practically constant temperature under the loads, the base pier height should be 5" to 9" (0.13 to 0.23 m) and the burners fired with constant air. Uniform temperature will result from the fact that the thin gas blanket will transfer only about one-third as much heat as above the load, so the blanket temperature will fall very slowly as it moves under the load. Therefore, load temperature profile across the furnace and below the load as well as above will be practically flat, leading to less than 10°F ( 5°C) temperature differential throughout the load. 

With thick loads, the pieces should be on piers with high-velocity burners located in rows near the bottoms of both sidewalls, alternating on 4-ft (1.22 m) centers. With this arrangement, flues can be in the roof. One important point In batch operations, do not pass the poc gases of any zone through another zone because that will result in loss of temperature control for the second zone. 

Rg. 6.5. Sectional view of a rotary hearth furnace (such as fig. 1.8) with enhanced heating. This also could be a car-hearth batch furnace or in-and-out batch-box furnace. In many cases, the higher velocity burners would be smaller (relative to the main burners above) than they appear in this drawing. In other than rotary hearth furnaces, the high-velocity burners should fire between piers and opposite the main burners—to further enhance circulation. 

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