Sizing Pressurisation, Car Park and Tunnel Ventilation

Design Objective & Industry Benchmarks

Design normal ventilation and emergency smoke control as separate operating modes. Normal ventilation limits pollutants and heat. Emergency systems protect escape routes or manage smoke movement.

For week 2026-W43, establish the locally adopted code editions before calculating. HVAC means heating, ventilation and air conditioning.

Use these preliminary benchmarks only after checking project criteria:

• High-rise escape routes: 25–50 Pa provides an illustrative pressure study range. Required pressures depend on the enclosure and governing code.
• Car parks: 6 air changes per hour provides an initial normal-ventilation comparison, not universal compliance.
• Industrial spaces: calculate contaminant generation and capture needs rather than copying parking rates.
• Road tunnels: derive smoke-control velocity from the design fire and tunnel geometry. No single velocity suits every tunnel.

Step-by-Step Sizing Methodology

1. Define design scenarios.

Record occupancy, vehicle mix, traffic queues, enclosure geometry and outdoor conditions. Identify escape paths, fire locations, door states and available discharge points.

For tunnels, select the design fire’s heat release rate in megawatts. Include its growth, vehicle restrictions, gradient and evacuation strategy.

2. Calculate pressurisation leakage.

Use:

Q = Cd × Ae × √(2ΔP/ρ)

• Q = leakage airflow, m³/s.
• Cd = dimensionless discharge coefficient.
• Ae = effective leakage area, m².
• ΔP = pressure difference, Pa.
• ρ = air density, kg/m³.

This gives supply airflow needed to replace leakage at the target pressure. Model high-rise floors as connected pressure zones. Include wind and stack effect, the pressure caused by indoor–outdoor temperature differences.

3. Check open doors and opening force.

For each required open doorway:

Qdoor = v × Adoor

Here, v is required doorway velocity in m/s; Adoor is clear area in m². Add leakage through the remaining closed boundaries for that scenario.

For a rectangular door, estimate pressure-related handle force:

Fp = ΔP × Adoor × w/(2r)

Here, w is door width in metres; r is hinge-to-handle distance in metres. Fp is force in newtons. Add closer and latch forces before checking accessibility limits.

Size for the governing scenario, then provide pressure regulation and relief.

4. Size normal car park ventilation.

Calculate dilution airflow for each relevant pollutant:

Q = G/(Clim − Cout)

G is pollutant generation in mg/s. Clim and Cout are indoor limit and outdoor concentration in mg/m³. Convert parts-per-million limits correctly before using this equation.

Compare against any required air-change minimum:

Q = n × V/3600

Here, n is air changes per hour; V is enclosed volume in m³. Select the greater requirement, then check air distribution and short-circuiting.

Size fire-mode extraction separately using the approved smoke-control strategy.

5. Establish tunnel smoke control.

For longitudinal ventilation, calculate critical velocity using an accepted correlation or validated model. Critical velocity limits upstream smoke movement under the selected fire conditions.

Calculate Q = At × v, where At is tunnel area in m². Check whether longitudinal smoke movement supports evacuation for the traffic arrangement.

For extraction systems, calculate smoke capture and replacement airflow instead. Use computational fluid dynamics where simpler methods cannot resolve smoke movement.

6. Select fans against system resistance.

Calculate duct, shaft, fitting, intake and outlet losses. Include wind, tunnel vehicle resistance and fire-related buoyancy where applicable.

Estimate electrical power as P = QΔPfan/(1000η). P is kW; ΔPfan is fan pressure in Pa; η is overall efficiency. Select tunnel jet fans by effective thrust and momentum balance, not airflow alone.

Practical Worked Example

Consider one stair pressurisation zone with 40 Pa target pressure. Assume Cd = 0.65, Ae = 0.12 m² and ρ = 1.2 kg/m³.

Closed-door leakage is:

Q = 0.65 × 0.12 × √(80/1.2) = 0.64 m³/s.

One open door measures 0.9 × 2.1 m. Assume an approved doorway velocity of 0.75 m/s.

Qdoor = 0.75 × 1.89 = 1.42 m³/s.

Remaining closed-boundary leakage is calculated separately as 0.40 m³/s. Required open-door supply becomes 1.82 m³/s, exceeding the closed-door requirement.

Check opening force separately. With r = 0.80 m, pressure contributes approximately 43 N. Add the specified door hardware forces before approval.

Cross-Trade MEP Coordination

Coordinate mechanical, electrical and plumbing systems with structural and architectural layouts. Reserve shafts, fan loads, discharge clearances and fire-rated penetrations. Align emergency power, detection, controls and damper positions with each operating scenario.

Applicable Standards & Codes

Apply the National Building Code of India 2016 to relevant building provisions. Apply NFPA 502, Standard for Road Tunnels, Bridges, and Other Limited Access Highways, within its scope. Confirm local authority requirements; neither standard replaces project-specific engineering.

Core Rules of Thumb for Practice

These rules of thumb require project verification:

• Compare pressure and open-door airflow separately.
• Cross-check parking dilution against air-change requirements.
• Never transfer tunnel velocity between projects without recalculation.