Retrofit of Retail Mall Food Court Exhaust Duct

Case Study 2: Retrofit of Retail Mall Food Court Exhaust Duct

Scenario A 20-year-old shopping mall in Toronto, Canada is upgrading its food court exhaust system to meet updated ASHRAE 170-2021 and local fire code requirements. The existing 1.2 m × 0.4 m rectangular duct (0.48 m²) suffers chronic grease buildup and excessive static pressure (>320 Pa), causing fan motor overloads and inconsistent hood capture. Renovation budget caps duct material costs, and ceiling height is fixed at 2.8 m — limiting maximum duct height to 0.7 m due to sprinkler and lighting obstructions. Rectangular duct is mandated for ease of integration into suspended ceiling plenums.

Given Data

• Airflow rate: 38,500 m³/h (updated load accounting for higher-capacity hoods and stricter dilution rates)
• Velocity: 12.5 m/s (upper limit per ASHRAE for kitchen exhaust to prevent grease separation)
• Duct shape: Rectangular
• Duct height: 0.65 m (max feasible height after accounting for sprinkler clearance)

Calculation

1. Convert airflow: 38,500 m³/h ÷ 3,600 = 10.694 m³/s
2. Required cross-sectional area: A = Q / V = 10.694 ÷ 12.5 = 0.8555 m²
3. For rectangular duct with fixed height h = 0.65 m: width w = A / h = 0.8555 ÷ 0.65 = 1.316 m
4. Standardized to nearest 50-mm increment: 1.35 m width, yielding actual area = 0.65 × 1.35 = 0.8775 m² and resulting velocity = 10.694 ÷ 0.8775 ≈ 12.2 m/s (within limit).

Result and Decision The Duct Sizing Calculator returned:

• Cross-sectional area: 0.86 m²
• Duct width: 1.32 m A 1.35 m × 0.65 m galvanized rectangular duct was installed — matching existing hanger spacing and enabling reuse of 70% of support infrastructure. Static pressure dropped from 320 Pa to 195 Pa, allowing replacement of the oversized 25 kW fan with a 15 kW EC motor unit.

Lesson When retrofitting constrained spaces, fixing one dimension (e.g., height) and solving for the other using the calculator transforms an open-ended design problem into a deterministic, code-compliant solution — accelerating engineering approval and reducing field coordination risk.