Bleed Air System Sizing

Bleed Air System Sizing (vehicle-design/sizing/bleed-air-system-sizing)

Use when the task is sizing the aircraft pneumatic bleed distribution system of a twin-engine transport at the conceptual level: the bleed air bled from the engine offtakes is distributed to the ECS packs, the wing anti-ice system and the pressurization trim through a bleed manifold and per-engine ducts. This leaf rolls up the total bleed offtake mass flow from the fixed consumer demands, splits the offtake evenly across the two engines, computes the thermal budget the precooler and conditioning system must reject in cooling the bleed from the offtake supply temperature to the consumer supply temperature, and sizes each engine bleed duct diameter from compressible pipe flow at a fixed design Mach number. The pack flows, the wing anti-ice flow and the trim flow are INPUTS here (values computed by the sibling environmental-control and ice-protection leaves); nothing downstream of the offtakes is recomputed. The module implements the model in pure Python, stdlib only. It pairs with vehicle-design/sizing/environmental-control-sizing (pack flow demand and pressurization trim demand) and vehicle-design/sizing/ice-protection-sizing (the anti-ice bleed demand). The regulatory context is FAR 25.863 bleed and flammable fluid plumbing, referenced but not reproduced.

Domain quick reference

• Bleed offtake rollup: m_total = sum(pack flows) + m_anti_ice + m_trim; with two engines the per-engine offtake is m_eng = m_total / 2. The consumer flows are fixed demands computed by the sibling leaves, so the rollup is pure summation.
• Bleed thermal budget: q = m * CP_AIR * (T_bleed - T_supply), the sensible heat the precooler and conditioning system must reject to cool the bleed from the engine offtake total temperature T_bleed to the consumer supply temperature T_supply (288 K sea level standard day by default). CP_AIR = 1005.0 J/(kg K).
• Duct flow state at the fixed design Mach number M: density rho = p / (R_AIR * T_bleed), sonic speed a = sqrt(GAMMA_AIR * R_AIR * T_bleed), duct velocity V = M * a, with R_AIR = 287.0 J/(kg K) and GAMMA_AIR = 1.4.
• Duct geometry: flow area A = m / (rho * V), diameter D = sqrt(4 * A / pi). The duct is sized at the per-engine offtake flow at the nominal duct static pressure (350 kPa default) and the fixed design Mach number (0.30 default).
• Fit verdict: PASS when the sized duct diameter is at or below the nominal duct diameter limit the installation can accommodate, else FAIL.
• Units are SI throughout: kg/s, K, Pa, m, m/s, W.
• FAR 25.863 frames the bleed plumbing safety context (flammable fluid lines); the relations above are standard engineering methodology, summary-only.

Workflow

1. Collect the fixed consumer bleed demands: each ECS pack flow in kg/s (environmental-control-sizing output), the wing anti-ice bleed flow in kg/s (ice-protection-sizing output), and the pressurization trim flow in kg/s.
2. Roll the offtake up with total_bleed_offtake(pack_flows_kg_s, anti_ice_kg_s, trim_kg_s): the total and the per-engine offtake (total split over the two engines).
3. Fix the bleed supply temperature T_bleed (engine offtake total temperature) and the consumer supply temperature T_supply.
4. Compute the precooler rejection load per engine and total with bleed_thermal_budget on the per-engine and total offtake flows.
5. Size each engine duct with bleed_duct_diameter at the per-engine flow, the bleed temperature and the duct static pressure; the design Mach number and pressure default to the module constants.
6. Judge the architecture with bleed_system_summary: pass the consumer flows, T_bleed and the nominal duct diameter limit; read the total and per-engine offtake, the thermal budgets, the duct area and diameter, and the PASS/FAIL fit verdict.
7. Confirm the deterministic checks with the contract test scripts/test_bleed_air_system_sizing.py.

Worked example

Reference twin-engine transport: two ECS packs at 0.80 kg/s each, wing anti-ice 0.0179 kg/s (ice-protection-sizing anchor value), pressurization trim 0.05 kg/s, bleed supply 450 K, duct pressure 350 kPa, design duct Mach 0.30, nominal duct diameter limit 0.06 m.

• Offtake rollup: total = 2 * 0.80 + 0.0179 + 0.05 = 1.6679 kg/s; per-engine = 0.83395 kg/s (total_bleed_offtake real output 1.6679 kg/s total, 0.83395 kg/s per engine).
• Per-engine thermal budget: q = 0.83395 * 1005 * (450 - 288) = 135775.4 W (135.8 kW class); total = 1.6679 * 1005 * 162 = 271550.8 W (271.6 kW class, 271551 W rounded). The total is exactly twice the per-engine value.
• Duct flow state at 350 kPa and 450 K: rho = 350000 / (287 * 450) = 2.7100 kg/m3; a = sqrt(1.4 * 287 * 450) = 425.22 m/s; V = 0.30 * 425.22 = 127.57 m/s (module outputs 2.7100 kg/m3 and 127.57 m/s).
• Duct geometry at the per-engine flow: A = 0.83395 / (2.7100 * 127.57) = 0.002412 m2; D = sqrt(4 * 0.002412 / pi) = 0.0554 m (55.4 mm, module output 0.05542 m).
• Fit verdict at the 0.06 m limit: PASS (0.0554 <= 0.06). At a 0.05 m limit the verdict flips to FAIL.

Pitfalls

• Feeding total flow into the duct sizing: the duct is a per-engine component sized at the per-engine offtake 0.83395 kg/s, not at the total 1.6679 kg/s; sizing at the total flow overstates the diameter by sqrt(2).
• Recomputing consumer demands: the pack flows, anti-ice flow and trim flow are fixed inputs produced by the sibling environmental-control and ice-protection leaves; this leaf rolls them up and never recomputes them from heat loads or icing flux.
• Treating T_bleed as the duct static temperature: the duct flow state uses the bleed supply total temperature for the density and sonic speed at the stated static pressure, consistent with the fixed design Mach number model.
• Confusing the thermal budget with a heat exchanger size: q is the precooler rejection load only; it sets the conditioning duty, not the pack flow demand or any pressurization quantity.
• Sizing at a non-fixed Mach number: the duct Mach number is a fixed design parameter (0.30 default); lowering it at fixed flow and state enlarges the required area.

Verification

• Confirm total_bleed_offtake([0.80, 0.80], 0.0179, 0.05) returns a total of 1.6679 kg/s with 2 * per_engine == total within 1e-9.
• Confirm bleed_thermal_budget(1.6679, 450) returns 271550.8 W, that doubling the mass flow doubles the budget, and that the dict keys are exactly q_w, mass_kg_s, t_bleed_k, t_supply_k.
• Confirm bleed_duct_diameter(0.83395, 450) returns a diameter of 0.0554 m within 1e-3 and that the area round-trips through A = pi * D^2 / 4 within 1e-9.
• Confirm bleed_system_summary verdicts PASS at the 0.06 m limit and FAIL at the 0.05 m limit.
• Confirm every negative flow, every non-positive mass, temperature or pressure, every Mach number outside (0, 1) and every bleed temperature at or below the supply temperature raises ValueError.
• Confirm identical inputs give identical outputs.
• Run the contract test offline: python3 scripts/test_bleed_air_system_sizing.py (35 tests, deterministic).

Related leaves

• vehicle-design/sizing/environmental-control-sizing: the ECS pack flow demand and pressurization trim demand that feed this rollup.
• vehicle-design/sizing/ice-protection-sizing: the wing anti-ice bleed demand input (0.0179 kg/s anchor value).
• vehicle-design/sizing/engine-sizing: main-engine performance sizing that consumes bleed for its thrust bookkeeping.
• vehicle-design/sizing/apu-fuel-burn-sizing: the APU-side offtake and fuel accounting for the auxiliary power installation.
• propulsion/engine-airframe/engine-airframe-integration: bleed as a thrust-loss term at the engine-airframe interface.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_bleed_air_system_sizing.py

The test covers the twin-engine transport worked example (offtake 1.6679 kg/s total, per-engine budget 135775.4 W, duct diameter 0.0554 m inside the 1e-3 bound), the offtake identity 2 * per_engine == total, the duct area round trip through A = pi * D^2 / 4, thermal budget doubling with mass flow, the duct state relations (rho = p/(R T), V = M * a), the summary fit verdicts at the 0.06 m and 0.05 m limits, dict key contracts for every function, output determinism, and ValueError rejection of negative flows, non-positive mass, temperature and pressure, Mach numbers outside (0, 1), and bleed temperatures at or below the supply temperature.

Compliance

• Standards referenced, not reproduced: FAR 25.863 (bleed and other flammable fluid plumbing context) is named for context only; the sizing relations above are standard engineering methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.