TM 5-3895-374-24-2Table 10Gas Burner Orifice Sizing Information (See pages 32, Fig. 33 for side orifice detail.)(A)orifices are not generally used on natural gas for higherratings of On/Off, Low-High-Off, Low-High-Low units orany ratings of modulating units as the butterfly functionsas a variable orifice. Modulating LP. gas units require aproperly sized limiting orifice.(B)Approximate pressure for initial start-up. Final pressureshould be determined after checking actual flow with gasmeter. Stack temperature. CO/2, O/2 and fireboxpressure will help in determining actual input when gasmeter is not available for this unit.Consideration should be given to magnitude of furnace pressure.Furnace pressure must always be added to above orificepressures, which are based on neutral furnace pressure.Burners equipped with optional adjustable premix tubeswill provide flows shown at slightly lower pressures.Most fixed premix tubes will require slightly higherpressures. When available supply pressure is too low toprovide above, orifice may be enlarged or removed andproper adjustment made on gas pressure regulator.(page 3 - 985)
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