Appendix
A – Example of Calculation for Fuel-to-Air Ratio
ED Decision 2018/005/R
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
A-2. Flame Arrestor Surface Temperature at Various Flow Rates and Stoichiometric
Mixture Ratios*
* FAA Technical
Report ADS-18, Lightning Protection Measures for Aircraft Fuel Systems (see
paragraph 2.2 of this AMC).
[Amdt 25/21]