The olefins convection tubes are arranged in a triangular pattern between rows of
tubes, or in a triangular pitch, to maximize heat transfer to the tubes. In a triangular
pattern, end tubes in alternating rows have a larger gap between the tube and the end wall.
These larger gaps are filled with refractory corbels to keep the same distance between end
tubes and end walls in all rows of tubes, as shown in Figure 9.12. The corbels near the end
tubes in each row break up flow and minimize flue gas channeling, thus maximizing
efficiency.