It is important to accept that a burner is a device for producing a flame. A burner does not heat the work; it is the flame that does it. From this it follows that the term torch brazing is the incorrect term to use when talking about a
TABLE 4.8
Some Useful Figures Related to Fuel Gases
Fuel Gas
Gas-Oxygen Flame Temperature
(ºC) Calorific Values (Btu/ft3)
Burning Velocity (ft/sec) Fuel
gas-oxygen Gas Gas-air Gas-Oxygen
Natural gas 2850 1000 95 330 11
Propane 2850 2500 96 416 16
Acetylene 3200 1440 110 411 32
Propylene 2878 2200 — 397 13
MAPPa 2917 2200 — 407 15
Hydrogen 2950 330 94 220 30
aMAPP = Methyl acetylene propadiene stabilized.
brazing process that is undertaken with a flame. This explains why the correct term for the process is flame brazing.
Many different types of burners are available, each having different charac-teristics and uses. Two typical examples are shown if Figure 4.7. Because of the huge range available, one might think that it would be quite difficult to pick the correct one for a particular job. Once it is established what type of flame is needed and there is an understanding of the fundamental principles by which burners operate, it will be easy to establish which is the right burner for the job.
Earlier in this chapter it was mentioned that an oxygen-acetylene mixture has a high burning velocity. A beneficial effect of this fact is that an oxygen-acetylene flame will remain attached to the nozzle of the torch over a wide range of mixture velocity. This is not true when, for example, the fuel gas is either natural gas, propane or butane. These gases each provide a flame that tends to lift from the nozzle of the burner at quite low mixture velocities.
This is a problem in those cases where brazing trials show that the job being studied needs slow, even heating, yet because of its mass it is established that a high mixture velocity is necessary to achieve this objective. In these circumstances, the phrase used by astronauts, “Gentlemen, we have liftoff,”
takes on quite a different meaning.
Astronauts depend on a liftoff for success. In brazing, liftoff spells F-A-I-L-U-R-E. Fortunately, the liftoff problem with gases that have a relatively low burning velocity can be handled by using a burner that employs a means of providing pilotage.
(a)
(b) FIGURE 4.7
(a) A piloted burner in operation. Note the fine definition of the shape of the flames. (b) A piloted ribbon burner in operation. The fuel-gas being used is compressed air-natural gas. This is a small example of this type — burners with an overall length up to 1.5 m have been built.
(Photos courtesy of BFT Limited, Stalybridge, Cheshire, U.K.).
2112_book.fm Page 98 Tuesday, November 4, 2003 1:07 PM
4.5 Pilotage
If a gas mixture at high velocity is fed to a burner, there are burner designs that provide for a small portion of the high-velocity gas to have its speed reduced. When this low-velocity gas stream is burned, the resultant flame remains anchored to the burner as a pilot light. This anchored pilot flame ignites the remainder of the gas as it leaves the burner. As a result, it becomes possible to use mixture velocities that are substantially greater than the velocity that would result in liftoff if the pilot light were not present. The principle of the mode of operation of a piloted burner is illustrated in Figure 4.8.
Burners of the type illustrated in Figure 4.8 are generally more expensive, but substantially more efficient than simple single-point flame burners. The reason for this is clearly demonstrated by Figure 4.9. The capacity of the burner is determined by the size of the flame port (hole) of the burner, the flame stability that it can support and the mixture pressure being fed to it.
These burners are simple to construct. Their advantages and disadvantages are outlined in Table 4.9.
Due to their lower cost when compared with piloted burners, single-point flame burners are probably the most widely used type of burner in mecha-nized systems. They are functional across a wide range of applications, but are not suitable for everything. This has to be remembered when burner selection for a particular job is being made, and some of the reasons for their unsuitability in some applications are illustrated in Figure 4.9.
Contrast the length of the inner cone and the size of the working zone in a single-point burner with those of a multihole piloted burner and it becomes clear why the latter is very popular and widely used in mechanized brazing systems (see Figure 4.10).
FIGURE 4.8
A cross section through a typical piloted burner.
Pilot supply hole (gas at low pressure!)
Pilot flame
Main flam
Pilot flame
Pilot supply hole (gas at low pressure!)
Main flame
Gas mixture
TABLE 4.9
The Advantages and Disadvantages of Single-Point Burners
Advantages Disadvantages
Simple design Narrow working width
Easily cleaned Short working zone
Long inner cone Working zone located at the point of maximum flame temperature & heat transfer
Low cost Turbulent flame
Low burner head pressure resistance Not particularly flexible in use
FIGURE 4.9 Single-point burner.
FIGURE 4.10
Multihole piloted burner.
TABLE 4.10
The Advantages and Disadvantages of Multihole Piloted Burners.
Advantages Disadvantages
Wider working zones Complex design is reflected in cost Produce laminar flames Higher burner head resistance
Numerous designs Higher maintenance costs
Long inner cone Point of maximum heat transfer
Working zone
Narrow flame width
Short inner cone
Pilot flames Point of maximum
heat transfer
F
Main flames
Flame Working zone Width 2112_book.fm Page 100 Tuesday, November 4, 2003 1:07 PM
The number and size of burner ports, the flame stability that can be achieved, and the pressure of the gas mixture being supplied to the burner determine its capacity. This multihole piloted burner is substantially more complex than the single-point burner, but is also more expensive. Its advan-tages and disadvanadvan-tages are listed in Table 4.10.