The cumulative heat energy introduced into refrigerated cargo space through air exchange each time vehicle doors open during delivery operations, representing the single largest variable thermal load in multi-stop courier delivery and the factor most systematically underestimated in transport refrigeration equipment sizing.
The Physics of Door Opening Heat Infiltration
When refrigerated vehicle doors open, buoyancy-driven air exchange occurs:
- Cold dense air (-18°C) falls out of cargo space
- Warm ambient air (25-40°C) rushes in to replace it
- Temperature differential drives continuous exchange until doors close
- Each opening introduces 150-400 kJ depending on conditions
Research demonstrates that heat infiltration during door opening can reach 50% of total thermal load in frequent-stop operations—yet manufacturer sizing guides assume 2-3 openings per day typical of long-haul transport.
Quantifying Door Opening Impact
Our Technical Formulas Reference provides the calculation:
Q_door = ρ_ambient × V_cargo × Cp_air × (T_ambient - T_cargo) × η_exchangeWorked Example – Johannesburg Summer Route:
- Cargo volume: 12 m³
- Ambient temperature: 35°C
- Cargo target: -18°C (ΔT = 53K)
- Air density at altitude: 0.95 kg/m³
- Air exchange efficiency: 40% per opening
- Door openings: 30 per route (15 deliveries)
Single opening heat load: ~243 kJ Daily route total: 7,290 kJ = 7.29 MJ Average continuous load: 337W additional
Multi-Stop vs Long-Haul Reality
| Operation Type | Daily Door Openings | Infiltration Load |
|---|---|---|
| Long-haul transport | 2-4 | 5-10% of total |
| Wholesale delivery | 8-12 | 15-25% of total |
| Retail multi-stop | 15-25 | 25-40% of total |
| Courier operations | 30-50 | 40-60% of total |
Transport refrigeration equipment sized for long-haul assumptions systematically fails in courier applications. A system adequate for 4 door openings cannot recover from 40 openings—the thermal arithmetic doesn’t work.
Recovery Time Requirements
Between stops, refrigeration must:
- Remove infiltrated heat
- Cool any remaining warm air
- Restore cargo temperature to target
- Build thermal buffer before next opening
With inadequate refrigeration capacity, each stop causes incremental temperature drift. By route end, products may have experienced cumulative temperature excursions even though vehicle arrived “cold.”
South African Compounding Factors
Door opening thermal loads intensify in South African conditions:
- Johannesburg altitude: Lower air density = higher temperature differential per exchange
- Summer ambient 35-40°C: Temperature differential reaches 53-58K
- Urban heat island: Pavement temperatures 55-65°C radiating additional heat
- Security delays: Extended waits at gated communities with doors opening
Mitigation Strategies
- Specify refrigeration 50-100% oversized for multi-stop operations
- Use side doors or compartmentalized access when possible
- Minimize opening duration through efficient delivery procedures
- Pre-stage deliveries to reduce search time with doors open
- Route planning to minimize total stops per thermal load
Related Terms: Multi-Stop Delivery (Cold Chain), Temperature Excursion, Thermal Load