The directional movement and circulation of air within refrigerated space, determining whether cargo experiences consistent -18°C or sees ±8-12°C temperature variation that compromises product quality. Airflow pattern is fundamental to refrigeration effectiveness—yet transport refrigeration manufacturers ignore convection physics because proper airflow design costs money.
Why Airflow Matters for Frozen Delivery
Research consistently demonstrates that temperature homogeneity in refrigerated transport is directly governed by airflow patterns. Poorly designed airflow creates temperature variations of ±8-12°C between different locations in the same cargo space—meaning product near the evaporator might be at -25°C while product at the rear sits at -10°C, above safe frozen food thresholds.
The Physics Problem
Cold air is denser than warm air and naturally sinks, creating thermal stratification where warm air accumulates near the ceiling while cold air pools at floor level. Industry-standard evaporator placement at the front ceiling ignores this basic convection physics. The supplied air jet follows the Coanda effect along the ceiling, potentially short-circuiting back to the return without ever reaching rear cargo or lower layers.
Studies show that warmer temperatures consistently occur at the rear of trailers, on the passenger side, and in lower pallet layers—precisely the areas standard airflow designs neglect.
South African Operational Context
In multi-stop delivery operations with 15-40 door openings per route, each opening disrupts established airflow patterns and introduces warm ambient air. At Johannesburg’s 35°C+ summer temperatures, recovering uniform airflow after door openings becomes critical—yet most courier TRUs lack the ducted air distribution systems that research proves necessary for temperature uniformity.
Professional Approach
Effective airflow management requires supply air ducts directing cold air to rear and lower cargo areas, adequate gaps between cargo and walls (minimum 50mm), and proper loading patterns that don’t obstruct circulation. The centerline cargo arrangement method achieves the best temperature uniformity by enhancing airflow across cargo gaps and reducing stagnant zones.
The 90-Degree Problem
Walk into any warehouse cold room and observe the evaporator: fans blow directly through coil face in a straight line. This works because stationary installations allow airflow engineering.
Examine any small form factor transport refrigeration unit: fans mounted perpendicular to coils, air making 90-degree turns, no transition ducting. This fundamental design flaw exists because it’s cheaper to manufacture—not because it works well.
A 90-degree turn in airflow:
- Reduces effective air velocity by 30-40%
- Creates turbulence that prevents laminar flow through coil fins
- Wastes fan energy on direction change rather than useful airflow
- Results in 40% capacity loss versus properly aligned fans
Convection and Stratification
Cold air sinks. Warm air rises. In a cargo space with inadequate circulation:
- Floor level: -22°C (cold air pooling)
- Mid-height: -18°C (target temperature)
- Ceiling level: -12°C (warm air stratification)
This 10°C vertical variation means product at different heights experiences different temperatures. Top-loaded cargo partially thaws while floor-loaded cargo over-freezes.
Proper airflow pattern requirements:
- Evaporator positioned to draw warm air from ceiling zone
- Discharge directed along floor, walls, and ceiling surfaces
- Air velocity sufficient to reach all cargo space extremities
- Return path ensuring circulation through cargo, not around it
The Load Configuration Reality
Airflow patterns designed for empty cargo spaces fail when loaded. Product blocks intended air paths. Palletized cargo creates dead zones. Partial loads concentrate in areas of convenience, not airflow optimization.
Professional cold chain operations train staff in loading patterns that maintain airflow channels. Amateur operations stack product against evaporator discharge (blocking circulation) and wonder why temperatures vary.
Temperature Uniformity Targets
| Application | Acceptable Variation |
|---|---|
| Pharmaceutical cold chain | ±1.5°C |
| Premium frozen food | ±3°C |
| Standard transport refrigeration | ±8-12°C (actual) |
| What the industry claims | ±4°C |
The gap between claimed and actual performance reflects systematic airflow design failure across the transport refrigeration industry.
Related Terms: Evaporator, Temperature Uniformity, Thermal Stratification
Related Articles: The 90-Degree Delusion: Why Small Freezer Manufacturers Are Ignoring Basic Airflow Physics