AppendixAppendix
Appendix AExample 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) per cent: Lower Upper | 1.2 7.4 |
| Flash point | – 22 °C/– 7 °F |
| Boiling point | 69 °C/156 °F |
| Auto-ignition temperature (AIT) | 223 °C/433 °F |
| Vapour pressure at 21 °C/70 °F (Pa/psia) | 17 237/2.5 |
Note: The equation for 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 g of hexane require 19 × 32.00 = 608 g of oxygen or 2 627.48 g of air, which is 23.14 per cent by weight oxygen. Hence, the ratio of the weight of air to the 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:
Table A-2. Fuel-to-Air Mixtures for Flame Arrestor Tests
| Condition | JP-4 Per cent by Volume | JP-4 Fuel-Air Mass Ratio | Hexane Per cent 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 |
Table A-3. Combustion Properties of Propane
| Property | Value |
|---|---|
| Heat of combustion (298 °K), kcal/g-mole | 530.6 |
| Flammability limits in air (% by volume), per cent: Lower Upper | 2.2 9.5 |
| Flame temperature (stoichiometric in air, STP) | 1 925 °C/3 497 °F |
| Quenching diameter,* cm/in | 0.28/0.11 |
| 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 equation for 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 g of propane require 5 × 32.00 = 160 g of oxygen or 691.44 g of air, which is 23.14 per cent 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:
Figure A-1. Fuel Tank Vent Flame Arrestor Test Schematic [Figure or form omitted from this preview — available in the Avioverse workspace library.]
Figure A-2. Flame Arrestor Surface Temperature at Various Flow Rates and Stoichiometric Mixture Ratios* [Figure or form omitted from this preview — available in the Avioverse workspace library.] * FAA Technical Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see paragraph 2.2 of this AMC).
[Amdt 25/21]
APPENDIX · Appendix A — CS-25 · ED Decision 2018/005/R · CS-25 Easy Access Rules · EAR revision 26 Jan 2023