There are several burner designs available.
atmospheric burner
In atmospheric burners, part of the air required for combustion is premixed with the gas prior to ignition. Air premixed with gas is called primary air. The remaining air is called secondary air and is drawn from the surrounding atmosphere at atmospheric pressure, hence the name atmospheric burner. The pioneer of the atmospheric burner system was the well known Bunsen burner.
To deliver the air-gas mixture to the burner head, the principle of Venturi and Ben production is used (Fig. 39.1). Delivering gas through the orifice (1) in the pressure range of 3-11 inches of water will produce approximately 40-60% primary air (2). Proper combustion can take place as long as sufficient secondary air (3) is present. Venturi mixers (4) connected to burner nozzles (5) or headers became the mainstay of early gas applications. Many such burners exist today.
Wider pattern requirements for distributing the flame by providing ducts with drilled holes (Fig. 39.2). This burner works well, but only at low energy outputs.
electric burner
Once the flame is established, the flame front burns towards the source of the gas mixture.
The front is only stable if the velocity of the gas matches the rate at which the mixture is burning. If the velocity is too high, the flame will lift the port away. If the velocity is too low, the flame will retract towards the source of the gas mixture and will cause flashback. This tendency can be minimized by keeping the port area small. The ignition temperature is also important. It takes about 630°C to ignite natural gas, but only 480°C to ignite propane. The cold metal surrounding the port will extinguish the flame as long as there is enough mass to absorb the heat. This explains the need for a certain port depth.
The key to solving these problems is to reduce the velocity of a portion of the air-gas mixture or limit the capacity of the burner. A Spray Gun nozzle was developed, as shown in Figure 39.3. A portion of the air-gas mixture is diverted into a small protected area (1). The velocity of this portion of the mixture is reduced until the pilot will provide continuous ignition of the main air stream emanating from the large center port (2).
This increases the rate of mixing outside the nozzle, increasing heat output.
This feature is known as flame retention and is inherent to all electric burners. Unlike atmospheric burners, power burners utilize a power source of combustion air.
The flame holding feature was introduced into indwelling burners, such as drill pipe burners. A row of holes is drilled down the center, along the pattern of the flame holding nozzles, and rows of small holes are drilled on each side. The deflector or ignition rail is placed over the two rows of guide holes (Figure 39.4). Figure 39.5 shows a cross-section of a typical borehole line burner widely used in the industry today.
The ribbon burner (Fig. 39.6) was developed by Harold Flynn. Appropriate slots are milled into the casting and the tape stacks are inserted. The velocity of a portion of the air-gas mixture is reduced, establishing guidance along each side of the main row of ports created by the ribbon configuration (Fig. 39.7). This eliminates any external deflectors or rails. Further improvements in stripline burners have made it possible to manufacture various slot widths and strip configurations to produce various flame patterns.
