Appendix A A-1 Appendix A. Example of Calculation for Fuel-to-Air Ratio Table A-1. Combustion Properties of Hexane Property Value Heat of Combustion, BTU/lb. 19,200 Molecular weight 86.17 Limits of inflammability in air (% by volume) percent:
Lower Upper
1.2
7.4 Flash point -7 °F Boiling point 156 °F Auto-ignition temperature 433 °F Vapor pressure at 70 °F (psia) 2.5 Note: The combustion of hexane and oxygen is written as:
2 𝐶6 𝐻14 + 19 𝑂2 = 14 𝐻2𝑂 + 12 𝐶𝑂2 For every 2 moles of hexane consumed, 19 moles of oxygen are required for complete combustion with no residual oxygen. Thus, 172.34 gm of hexane require 19 x 32.00 = 608 gm of oxygen or 2627.48 gm of air, which is 23.14 percent by weight oxygen.
Hence, the weight of air to weight of hexane required for stoichiometric burning (i.e., complete combustion of hexane with no excess oxygen) is 15.24. A 1.15 fraction of stoichiometric mixture of air and hexane has an air-to-fuel weight ratio of:
2627.48
1.15 × 172.34 = 13.2 Table A-2. Fuel-to-Air Mixtures for Flame Arrestor Tests Condition JP-4 Percent by Volume JP-4 Fuel/Air Mass Ratio Hexane Percent by Volume Hexane Fuel/Air Mass Ratio Lean limit 0.90 0.035 1.3 0.04 Between lean limit and stoichiometric 1.10 0.045 1.7 0.05 Stoichiometric 1.58 0.065 2.2 0.0658
1.15 Stoichiometric 1.82 0.074 2.5 0.07567 Between stoichiometric and rich limit 3.0 0.15 6.3 0.2 Rich limit 6.16 0.23 8.0 0.26
Appendix A A-2 Table A-3. Combustion Properties of Propane Property Value Heat of combustion (298K), kcal/gm-mole 530.6 Flammability limits in air (% by volume), percent Lower Upper
2.2
9.5 Flame temperature (stoichiometric in air, STP), °C 1925 Quenching diameter,* inches 0.11 Minimum spark ignition energy,* millijoules 0.027 Critical velocity gradient for flashback,* sec-1 600 Laminar flame speed,* cm-sec 40 *Applicable to 1.1 stoichiometric propane-to-air at standard temperature and pressure (STP).
Note: The combustion of propane and oxygen is written as:
𝐶3𝐻8 + 5 𝑂2 = 4 𝐻2𝑂 + 3 𝐶𝑂2 For every mole of propane consumed, 5 moles of oxygen are required for complete combustion with no residual oxygen. Thus, 44.09 gm of propane require 5 x 32.00 = 160 gm of oxygen or 691.44 gm of air, which is 23.14 percent by weight oxygen.
Hence, the weight of air to weight of propane required for stoichiometric burning (i.e., complete combustion of propane with no excess oxygen) is 15.7. A 1.15 fraction of stoichiometric mixture of air and propane has an air-to-fuel weight ratio of:
691.44
1.15 × 44.09 = 13.7
Appendix A A-3 Figure A-1. Fuel Tank Vent Flame Arrestor Test Schematic
Not e: This test schematic is not intended to represent an acceptable test configuration for project-specific compliance testing. The test installation, including the length and diameter of downstream vent line and the orientation of the flame arrestor will have a significant impact on the effectiveness of the flame arrestor. Therefore, the test configuration should be representative of the actual airplane installation. Additional surface temperature measurements may be needed if the flame arrestor is installed in a location where ignition of flammable vapors could lead to propagation of the external vent fire into the fuel tank. For example, a vent flame arrestor could be installed adjacent to a fuel surge tank, such that hot surfaces of the flame arrestor or vent system could cause ignition of flammable vapors and propagation of the fire into the fuel tank.
Additional surface temperature data would be needed to demonstrate that surface temperature limits are not exceeded.
Appendix A Figure A-2. Flame Arrestor Surface Temperature at Various Flow Rates and Stoichiometric Mixture Ratios2 500 480 Relative Flow Rate 460
0.550 BOTTOM SURFACE TEMPERATURE (°F) 440 420
• 0.648 400
0.774 380
0.915 360 1 Inch X 0.050 Inch Stainless Steel Arrestor 1.000 340
0.9 1.0 1.1 1.2 1.3 1.4 1.5
FRACTION OF STOICHIOMETRIC 2 FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph 3.3 of this AC).
A-4
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