Granite Flame Finishing Internal Burner

This invention concerns flame finishing of granite and similar heat-spallable stone which has become popular for producing rough surface finishes for structural and walkway purposes.

In this field, a high-velocity, high-temperature flame is directed against the stone surface to produce spalls of individual as well as multiple grains.

Although oxy-fuel flames are often used for this purpose, their operating costs are high. Compressed air-fuel oil flames may also be used where the reactants are burned within an enclosed combustion chamber with the hot products of combustion expanding through a restricting nozzle to form a jet flame which is directed against the rock surface.

Such an air-fuel burner is described in my U.S. Pat. No. 3,211,242 issued July 23, 1963. Further, this patent discloses that maximum stone removal rates are associated with flame burners of the internal combustor type operating between 3 and 15 psig. Such pressures produce a high-velocity, but subsonic flame jet. Combustion at this low pressure level is difficult and requires large diameter burners to produce useful flows of hot gas.

The present invention produces the equivalent subsonic flame jets with a burner structure of much smaller size per unit of reactants burned. In essence, combustion takes place in a relatively small combustor volume at pressures between 50 psig and 100 psig depending on flame jet requirements and the available compressed pressure. The products of combustion expand through a small diameter nozzle through a pressure drop greater than critical into an enlarged passage for final discharge to the atmosphere at subsonic velocity.

The single FIGURE is a longitudinal sectional view of a subsonic flame finishing internal burner forming a preferred embodiment of the invention.

FIG. 1 discloses an internal burner from which one has ready understanding of the principles of the invention. A burner body 10 includes a closed combustion chamber 11 into which compressed air and fuel are injected via passages 12 and 13 respectively as indicated by the labeled arrows. Ignition is initiated within chamber 11 conventionally by a spark plug (not shown) or by backflash from the nozzle 16 exit whose bore diameter is d.sub.2, larger than d.sub.1. Combustion pressures may vary but are always maintained higher than 30 psig. Gases formed at high pressure during combustion expand through a first nozzle 14 at the outlet of chamber 11 having a bore of diameter d.sub.1 into a downstream low pressure region producing a gas velocity greater than Mach 1. These gases in the form of a jet suddenly reduce their velocity to subsonic within downstream, radially enlarged chamber or duct 15 by shock action. The gases then pass through a second nozzle 16 at the exit end of chamber 15 with a pressure drop less than critical due to the diameter d.sub.2 of the bore of nozzle 16. Duct 15 may be formed by a tapered wall 15' from first nozzle 14 to second nozzle 16, as indicated in dotted line fashion.