The recurring problem

Cleanrooms are designed on paper for a target ISO class and cascade, but reality adds heat loads, equipment wakes, door operations, and operator motion. The result is dead zones, short-circuiting from supply to extract, and backflow across grades. You need visual evidence (smoke/CFD) plus numbers (recovery time, age of air, pressurization balance) to prove the design is robust.

When to go beyond calculations only

• Stay calc-only for small, low-risk rooms with uniform HEPA layout and simple cascades.
• Use CFD-lite (recommended default) for Grade B/C rooms, RABS/C-RABS around filling/lyo lines, or any non-uniform SA/EA.
• Go full CFD for complex layouts (isolators, lyo loading/unloading, hot equipment), or when audits have previously flagged smoke failures.

What good looks like

• Unidirectional zones show uniform downward velocity with minimal lateral drift to the working plane.
• Non-UD zones maintain clean sweep from SA to EA without recirculating pockets near critical operations.
• Pressure cascade holds with doors closed and leaks modeled: e.g., +15 / +10 / +5 Pa (URS-driven).
• Recovery time and age of air meet URS/qualification targets.
• Smoke pathlines match CFD vectors—no surprises in the room.

Brief theory bite (keep it practical)

• Pressurization balance: (ΣSA − ΣEA) ≈ leakage flow required to hold ΔP; distribute leakage across doors, pass-boxes, cracks.
• Short-circuit ratio (SCR): mass flow reaching exhaust without sweeping the working zone. Lower is better.
• Age of air: time since air last entered; younger air at the work plane implies better flushing.
• Particle transport: model as passive scalars for non-viable; add Lagrangian tracking for “what-if” release points.

Setup (numbers you can use)

Geometry

• Include equipment envelopes, guards, RABS/C-RABS gaps, under-cart volumes, pass-boxes, and door under-cuts.
• Represent HEPA modules as velocity/flow inlets with uniformity ±10% (or measured data if available).

Mesh

• Hex/Poly-hex with near-wall refinement around HEPA faces, operator zone, equipment wakes, and exhaust grilles.
• Target cell size in working region: 20–50 mm, finer near HEPA and edges. Prism layers near solid surfaces if thermal buoyancy included.

Physics

• Turbulence: k-ω SST (robust on separation) or RNG k-ε (fast and stable); keep y+ in wall-function range where used.
• Thermal: Boussinesq approximation (density vs temperature) if heat loads matter.
• Species/Scalar: passive scalar for contamination; set scalar = 1 at a release and monitor decay.
• Boundary conditions:
  • SA (H14 HEPA): specified flow or face velocity (per URS).
  • EA: pressure outlets at grille locations.
  • Leakage: pressure jump or porous slit on doors/transfer hatches.
  • Cascade setpoints: verify via (ΣSA − ΣEA) and leakage curve.

Scenarios to run

• Nominal operation (doors closed)
• One door cracked/open transient (15–30 s)
• Equipment heat-on vs heat-off
• RABS glove-port intervention (if applicable)

Evaluate (pass/fail diagnostics)

• Velocity at working plane: stable and directional; no strong lateral jets into critical zones.
• Vectors & streamlines: SA should sweep the work area before reaching EA; observe and reduce recirculation pockets.
• Pressure: cascade holds at setpoints; local drops near large EA not pulling in external air.
• Short-circuit ratio (SCR): trend ↓ with diffuser re-aiming or grille relocation.
• Recovery time: scalar decays from 1 → 0.1 within URS limit after a release stops.
• Age of air: younger at critical locations than bulk average.
• Smoke test parity: your CFD streamlines should match the camera view—use the same injection points.

Drop-in workflow (tool-agnostic, ANSYS-friendly)

1. Define URS & acceptance: ISO class, cascade targets, recovery time, velocity windows, smoke test points.
2. Model & mesh: include leaks; validate SA/EA totals against schedules.
3. Solve steady state: nominal case; then run key “what-ifs”.
4. Post-process: vectors at work plane, pathlines, pressure map, scalar decay curves, age of air.
5. Iterate: move/resize grilles, tweak SA fractions, add baffles or skirts; re-run targeted cases.
6. Handoff: CFD snapshots matched to smoke angle shots, setpoint table, SA/EA schedule, and commissioning checklist.

Quick fixes we see often

• Cold air plunge from HEPA → Add perforated canopy or diffuser plate; trim face velocity.
• Corner eddy near equipment → Insert small return near the recirculation or redirect a nearby HEPA.
• Cascade collapses when a door opens → Increase make-up or add pressure control loop dead-band; reduce leakage elsewhere.
• Smoke hugs the ceiling then exits → Re-aim diffusers or reduce ceiling-to-grille short-circuit path; add side skirts.
• Operator wake contaminates work area → Shift SA to upstream of operator, add low-level return downstream.

Scorecard (choose your path)

Commissioning & audit checklist

• SA/EA totals match schedules (±3%).
• Cascade holds in CFD and on BMS logs for nominal case.
• Vector maps at work plane show sweep through critical areas.
• Recovery time plots meet URS at defined points.
• Smoke test plan uses the same injection points as CFD; camera angles noted.
• Setpoint table (Pa, l/s) and diffuser/grille IDs issued to commissioning.
• Change log: what moved, why, and effect on KPIs.

Where Shirsh fits

We build validation-friendly models, reconcile them with ISO 14644 classification and your URS, then deliver vectors/pathlines, cascade stability, recovery time, and smoke-matching visuals. You’ll also get a commissioning pack (setpoints, SA/EA schedule, and checklist) so field teams can reproduce the results and auditors see a clear line from design to evidence.

Have a Grade B room, RABS/C-RABS, or lyo loading area to check? Share your layout, SA/EA schedule, and URS—we’ll return a scoped plan with timelines and deliverables.