Hospital HVAC Retrofit in Chicago

Scenario

A 12-story acute-care hospital in Chicago underwent a retrofit of its central AHU serving the ICU and surgical suite. Space constraints prohibited duct resizing, and existing galvanized steel ductwork (aged ~25 years) exhibited visible corrosion and accumulated dust buildup. The project required maintaining ≥99.97% filtration efficiency and strict pressure differentials—demanding precise TESP validation before fan replacement.

Given Data

• Duct Length: 82 m (measured laser-tracked path from AHU to furthest terminal)
• Duct Diameter: 0.42 m (existing round duct, verified with calipers)
• Airflow Rate: 2.3 m³/s (required for 12 air changes/hour in 650 m³ ICU zone)
• Component Pressure Drops: [120 Pa (HEPA filter), 85 Pa (dampers), 65 Pa (coils), 40 Pa (grilles)] → total = 310 Pa
• Duct Roughness: 0.12 mm (measured via profilometer on corroded interior surface)

Calculation

Using the Fan TESP Calculator:

1. Friction loss computed via Colebrook-White + Darcy-Weisbach (embedded in tool): With Re ≈ 245,000 (turbulent), relative roughness ε/D = 0.00012/0.42 ≈ 0.000286 → f ≈ 0.019 → ΔP_friction = f × (L/D) × (½ρv²) ≈ 0.019 × (82/0.42) × (0.6 × 2.3²) ≈ 112.7 Pa
2. Component losses: Sum = 120 + 85 + 65 + 40 = 310 Pa
3. Total TESP = friction loss + component losses = 112.7 + 310 = 422.7 Pa (rounded to 422.70 Pa per tool output)

Result and Decision

The calculated TESP (422.70 Pa) exceeded the original fan’s rated static pressure (380 Pa at design flow), confirming chronic underperformance. A backward-curved centrifugal fan (Model BC-250S) rated for 450 Pa TESP at 2.3 m³/s was selected—with integrated VFD and real-time static pressure feedback. Post-installation field verification measured 418 Pa, validating <1% error.

Lesson

Duct roughness degradation in aging healthcare systems is often underestimated; direct surface measurement—not catalog defaults—is essential for reliable TESP prediction in critical environments.