The critical distinction between maximum instantaneous thermal demand (peak load) and mean thermal demand over time (average load) in refrigeration system sizing. Equipment rated for average load fails during peak events; equipment sized for peak load handles all conditions—yet the industry consistently sells average-load systems for peak-load applications.
The Sizing Scandal
Refrigeration equipment carries capacity ratings—typically in kilowatts (kW) of cooling. A 4kW system removes 4kW of heat energy per hour under rated conditions. Simple enough.
But thermal loads aren’t constant. Courier operations experience:
| Condition | Thermal Load |
|---|---|
| Steady-state (doors closed, cruising) | 1.5-2.0 kW |
| Door opening event | 4-6 kW spike |
| Recovery period | 3-4 kW sustained |
| Stationary in summer sun | 2.5-3.5 kW |
| Urban traffic (low airflow) | 2.0-3.0 kW |
Average load across 8-hour route: 2.3 kW Peak load at each door opening: 5-6 kW
A system rated for average load (let’s say 3kW with safety margin) cannot physically remove heat fast enough during peak events. Temperature rises. Between stops, the system runs continuously but only partially recovers. Across 15 stops, temperature drifts from -18°C toward -12°C.
The Mathematics of Failure
Consider a 15-stop route with 10-minute intervals:
- Peak thermal intrusion per opening: 400 kJ
- System cooling capacity: 3 kW = 3 kJ/second
- Time to remove 400 kJ at 3 kW: 133 seconds (2.2 minutes)
- Remaining recovery time: 7.8 minutes
- Steady-state load during recovery: 1.8 kW
- Additional heat accumulated: 1.8 × 468 = 842 kJ
- Net heat removed in 10 minutes: (3 × 600) – 400 – 842 = 558 kJ
The system barely keeps pace—and this assumes rated capacity throughout, which altitude and ambient temperature reduce by 20-30% in Johannesburg summer conditions.
Now consider a 6kW system:
- Time to remove 400 kJ at 6 kW: 67 seconds
- Remaining recovery time: 9 minutes
- Net heat removed in 10 minutes: (6 × 600) – 400 – 842 = 2,358 kJ
- Surplus capacity provides actual recovery margin
Why Industry Sells Average-Load Equipment
Smaller systems cost less. Purchase price drives fleet decisions. A 4kW unit costs R15,000 less than a 6kW unit. Across 10 vehicles, that’s R150,000 “savings.”
But undersized equipment runs continuously at maximum capacity, consuming more fuel, wearing faster, and still failing to maintain temperature. The R150,000 “saved” becomes R400,000 in excess fuel and R200,000 in premature equipment replacement over vehicle lifetime.
Suppliers quote average-load equipment because it closes sales. Operators pay peak-load consequences throughout ownership.
Related Terms: Recovery Ratio, Door Openings (Thermal Load), Transport Refrigeration Unit (TRU), Multi-Stop Delivery (Cold Chain)
Related Articles: Door Opening Recovery: The Hidden Capacity Requirement, The Multi-Stop Thermal Load Reality
Related Resources: Technical Formulas Reference