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Scarfing oxygen exiting from the slot formed by the upper and lower preheat blocks must be uniform across its width and shielded against the aspiration of air into the stream to provide an efficient, uniform, and smooth metal removal. The fuel gas, which exits from the upper and lower preheat blocks, performs this shielding function. The lower preheat block (LPB) fuel gas is the most critical to surface quality since it is the last gas to pass over the molten surface of the steel as scarfing progresses. Any discontinuities in the fuel gas stream from the LPB will cause a disruption in the oxygen stream, which in turn will cause ridges or irregularities on the scarfed surface.
In the standard LPB design, fuel gas exits from the end of the block. The gas from each port is immediately drawn against the oxygen stream exiting the slot, providing the shield that prevents the aspiration of air into the reaction. For a continuous uninterrupted gas shield it is critical that each port remains open throughout the scarfing cycle. If ports become plugged by slag there is a disruption in the shield gas, and therefore in the oxygen stream, which in turn will produce irregularities on the surface of the steel. In the Smooth Surface Lower Preheat Block (SS-LPB) design, the physical end of the block is actually some distance beyond the exit point of the fuel gas ports. This extended baffle creates a "chamber" between it and the slot oxygen stream. As fuel gas is drawn from this chamber towards the steel surface, the oxygen compresses it against the end of the baffle. The result is a gas shield uniformly distributed across the width of the block. This design minimizes the effect of plugged gas ports on the surface quality, because uniform distribution of the gas is less dependent on flow from individual ports.