Formation of distinct horizontal temperature layers within refrigerated space due to natural convection, where warm air accumulates at ceiling level and cold air settles at floor, creating significant vertical temperature gradients (often 8-15°C difference top to bottom in poorly designed systems). Why It Matters: Thermal stratification represents the primary temperature control failure in transport refrigeration. Product at ceiling level can be 10-12°C warmer than floor product in the same vehicle, with temperature sensors typically mounted at mid-height reporting misleading average values. Standard industry practice worsens stratification by installing roof-mounted evaporators that pull air from below the unit (already the coldest zone) while completely ignoring the warm air accumulating at ceiling level where physics demands it rises. This is not an equipment limitation – it’s a design choice prioritizing bodybuilder installation convenience over thermodynamic performance.
The Physics Problem:
- Warm air density: ~1.10 kg/m³ at +10°C
- Cold air density: ~1.35 kg/m³ at -18°C
- Density difference drives buoyancy: warm air rises, cold sinks
- Without forced circulation addressing ceiling zone, warm air remains trapped
- Door openings introduce warm ambient air which immediately rises to ceiling
- Evaporator operating at floor level never encounters this warm ceiling air
- System reports good temperatures (measured at mid-height) while ceiling product warms
Altitude Impact: Johannesburg’s 1,750m elevation reduces air density 19%, weakening natural convection forces and altering stratification patterns compared to sea-level behavior that evaporator designs assume.
Related Terms: Airflow Pattern, Temperature Uniformity, Convection (Natural vs Forced), Evaporator, Heat Transfer (Thermal), Cold Spot / Hot Spot