The formation of distinct horizontal temperature layers within refrigerated cargo space, with warm air accumulating at ceiling level and cold air pooling at floor level. Thermal stratification creates 8-15°C temperature differences top-to-bottom—completely negating the temperature uniformity required for frozen food quality—yet evaporator placement and airflow design in standard transport refrigeration actively worsens this natural phenomenon.
The Physics
Cold air is denser than warm air. Without forced circulation, temperature layers form naturally:
- Ceiling zone: Warmest (heat rises)
- Middle zone: Intermediate
- Floor zone: Coldest (cold sinks)
In still conditions, stratification reaches equilibrium within 15-20 minutes. A 10m³ cargo space can develop 15°C temperature differential top-to-bottom without adequate air mixing.
How Standard Equipment Worsens Stratification
Logic suggests evaporators should draw warm air from the ceiling (hottest zone) and discharge cold air along the floor and walls. This would fight stratification through continuous mixing.
Standard transport refrigeration installation:
- Evaporator mounted high on front wall
- Discharge blows horizontally across cargo space
- Return air drawn from mid-height in front of evaporator
- Ceiling warm zone ignored
- Floor cold zone undisturbed
This configuration creates a circulation pattern that shorts-circuits: cold air exits evaporator, travels partway across cargo space, and returns to evaporator intake without ever reaching rear corners or mixing ceiling warm air. The warmest and coldest zones remain stratified.
Measured Stratification Effects
In testing with industry-standard equipment, setpoint -18°C:
| Location | Temperature | Deviation |
|---|---|---|
| Evaporator discharge | -24°C | -6°C |
| Mid-height center | -17°C | +1°C |
| Rear upper corner | -8°C | +10°C |
| Floor center | -22°C | -4°C |
Total variation: 16°C despite nominal “maintenance of -18°C”
Mitigation Strategies
- Ceiling-level return air intake: Draw warmest air to evaporator first
- Floor-level supplementary circulation: Destratification fans or ducted discharge
- Load configuration: Leave airflow channels at ceiling and floor
- Circulation velocity: Higher evaporator fan speed improves mixing
- Continuous operation: Don’t let circulation stop during compressor-off periods
These strategies add cost and complexity. Standard equipment ignores all of them because stratification isn’t a sales problem—it’s an operating problem.
Related Terms: Airflow Pattern, Temperature Uniformity, Evaporator