There are several burner designs to choose from.
atmospheric burner
In atmospheric gas 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, called secondary air, is drawn from the surrounding atmosphere at atmospheric pressure, hence the name atmospheric burner. The predecessor of the atmospheric burner system is the famous Bunsen burner.
To deliver the air-gas mixture to the burner head, the principle developed by Venturi and Bunsen is utilized (Fig. 39.1). Gas is delivered through hole (1) at a pressure of 3 to 11 inches. The water column will produce approximately 40% to 60% of the required primary air (2). Proper combustion is possible as long as enough secondary air (3) is present. Venturi mixers (4) and burner nozzles (5) or heads became the mainstay of early gas applications. Many varieties of this burner exist today.
The need to distribute the flame in a wider pattern is addressed by providing pipes with drilled holes (Figure 39.2). This burner works well, but only at low energy outputs.
power burner
Once the flame has established, the flame front burns towards the source of the air-gas mixture. A front is only stable if the velocity of the air-gas matches the velocity at which the mixture burns. If the speed is too high, the flame will rise from the port. If the velocity is too low, the flame will retract to the source of the gas mixture and flashback will result. 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 around 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 gun nozzle was developed, as shown in Figure 39.3. A portion of the air-gas mixture is diverted to a small protected area (1). This portion of the velocity reduces the mixture until the pilot will provide sustained ignition for the main air-flow firing from the large center port (2).
This allows to increase the mixing speed of the nozzle, increasing the heat output. This characteristic, called flame holding, is inherent to all power burners. Unlike atmospheric burners, power burners utilize a power source of combustion air.
The flame holding feature was introduced for indwelling burners such as drill pipe burners. A single row of ports is drilled down the center, with rows of small holes drilled on each side in the pattern of flame holding nozzles. The deflector or ignition rail is placed above the two rows of pilot holes (Figure 39.4). Figure 39.5 shows a cross-section of a typical bore-port line burner, which is widely used in industry today.
The Ribbon Burner (Fig. 39.6) was developed by Harold Flynn. A suitable slot is milled out of the cast and inserted into the ribbon stack. The velocity of a portion of the air-gas mixture is reduced, establishing pilotage along each side of the main port row created by the ribbon configuration (Fig. 39.7). This eliminates any external deflectors or rails. Further refinement of the functional area of the line burner makes it possible to manufacture various slot widths and ribbon configurations to produce a variety of flame patterns.
