Infiltration

Detailed Explanation

Infiltration is the uncontrolled entry of outdoor air into a building or space, through leakage or openings. In HVAC and humidity control design, it is generally considered an "unwanted load" because moisture, temperature, particulates, and gas content are uncontrolled. Main sources of infiltration:

1. Door openings — cold storage, loading ramps, dry room airlock crossings 2. Building envelope leaks — wall/roof junctions, cable penetrations, fan blocks, window seals 3. Ventilation imbalance — net negative pressure in the room (exhaust fans > supply fans) 4. Stack effect — natural air circulation in tall spaces due to temperature differences 5. Wind effect — leakage direction driven by pressure differences across the building face

Infiltration load varies seasonally: in summer, outdoor air is hot and humid; in winter, cold and dry; in both cases, it differs from the indoor target and creates a load. For cold storage, summer infiltration brings both sensible and latent loads; particularly the latent load (moisture) can be 40–60% of the total load.

Proper design is not to eliminate infiltration but to bring it under control. Zero infiltration degrades indoor air quality (CO₂ buildup); the HVAC system must provide planned outdoor air, while uncontrolled infiltration is minimized.

Infiltration Calculation

General infiltration load formula:

Qinf = ACH × V × ρ × cp × ΔT (sensible) Minf = ACH × V × ρ × ΔW (latent, moisture load)

Qinf: sensible infiltration load (kW) Minf: latent/moisture infiltration load (kg/h or kg/s) ACH: air changes per hour (1/h) V: space volume (m³) ρ: air density (1.2 kg/m³ standard) cp: air specific heat (1.012 kJ/kg·K) ΔT, ΔW: outdoor-indoor temperature and humidity difference

Door infiltration (Tamm formula, cold storage): Mdoor = 0.221 × Adoor × ρout × √(2 × g × H × (ρout − ρin) / ρout)

Adoor: door opening area (m²) H: door height (m) g: gravitational acceleration (9.81 m/s²) ρout, ρin: outdoor and indoor air densities (kg/m³)

Rule of thumb: a 2×2 m cold storage door open for 30 seconds mixes ~50–100 kg of outdoor air.

Practical Example

Infiltration analysis for a logistics cold storage warehouse:

Warehouse: 1,500 m² × 6 m = 9,000 m³ Indoor temperature: −22°C Summer outdoor: 32°C, 65% RH Door: 3 × 3 m, 80 forklift crossings per day (each crossing ~15 seconds)

Door infiltration (Tamm approximation): ρout (32°C, 65%) ≈ 1.15 kg/m³ ρin (−22°C, saturated) ≈ 1.40 kg/m³ Door area = 9 m² Mdoor ≈ 0.221 × 9 × 1.15 × √(2 × 9.81 × 3 × (1.40−1.15)/1.15) ≈ 5.5 kg/s average (door open) Daily crossing time: 80 × 15 = 1,200 s Daily outdoor air: 5.5 × 1,200 = 6,600 kg

Moisture load: Wout = 19.3 g/kg Win (≈ at −22°C, typical frost point): ~0.7 g/kg Daily moisture ingress: 6,600 × (19.3 − 0.7) / 1,000 = 122.8 kg/day

This moisture buildup accelerates ice formation on cooling evaporators; creates a daily defrost requirement of 4–6 cycles.

Improvement — air curtain + high-speed industrial door + airlock: • Air curtain: 60% reduction • High-speed door: additional 25% reduction • Combined: daily outdoor air 6,600 → 1,500 kg • Annual energy savings: ~85,000 kWh (defrost + cooling + compressor cycle life) • Payback: 14–22 months

Engineering Note

Engineering tools for infiltration control:

• Air curtain — a horizontal jet blown from the top creates a barrier when the door is open. 50–70% infiltration reduction; correct velocity and angle (12–18 m/s, 15–20° tilt) are critical. • High-speed industrial door — reduces door open time to 5–10 seconds (instead of the classic 30+ seconds). Provides 40–60% reduction. • Airlock — two-door transition tunnel; while one door is closed, the other opens. Mandatory in extreme environments (dry room, cold storage). • Strip curtain (PVC) — low investment, 20–30% reduction; can create aesthetic loss and hygiene issues. • Building Tightness Test (Blower Door Test) — standard method for measuring building airtightness (ASTM E779). Mandatory before commissioning in cold storage and dry rooms. • Balanced ventilation — supply = exhaust + planned extract; neutral pressure minimizes passive infiltration.

Climate data usage: • Summer outdoor: ASHRAE 0.4% annual exceedance (for Türkiye: Istanbul 32°C/65% RH, Antalya 35°C/55% RH, Ankara 33°C/40% RH) • Winter outdoor: 99.6% frequency (Istanbul 0°C, Antalya 4°C, Ankara −10°C) • Bin-hour analysis is the standard approach for annual energy consumption estimation

The NKT energy simulation tool automates these calculations; annual infiltration load and the energy benefit of different countermeasures are reported.