The Specification That Lies by Omission
Why systems sized for average thermal load guarantee temperature drift—and why peak recovery capacity is the metric nobody discusses
Your refrigeration system is rated for 4 kW. Your calculated thermal load is 3.2 kW. That’s 25% safety margin—comfortable, right?
Wrong.
That 3.2 kW is an average. Averaged across hours of operation. Averaged across steady-state and transient loads. Averaged in a way that makes undersized equipment look adequate on paper.
But thermal loads don’t arrive as averages. They arrive as peaks—sudden, intense, demanding immediate response. Every door opening dumps hundreds of kilojoules of heat into your cargo space in seconds. Your refrigeration system must remove that heat before the next stop or temperature drifts upward.
Average capacity cannot handle peak demand. This is the hidden capacity requirement that specification sheets never mention.
The Peak vs Average Problem
Let’s visualise what happens during a typical multi-stop route.
Thermal load profile across 15 stops:
Load (kW)
│
6 ┤ ▲ ▲ ▲ ▲ ▲ ▲ ▲
│ ╱│╲ ╱│╲ ╱│╲ ╱│╲ ╱│╲ ╱│╲ ╱│╲
4 ┤ ╱ │ ╲ ╱ │ ╲ ╱ │ ╲ ╱ │ ╲ ╱ │ ╲ ╱ │ ╲ ╱ │
│ ╱ │ ╳ │ ╳ │ ╳ │ ╳ │ ╳ │ ╳ │
2 ┤─────────────────────────────────────────── Average (2.3 kW)
│ ╲ │ ╱ │ ╱ │ ╱ │ ╱ │ ╱ │ ╱ │
1 ┤ ╲_│_╱ │_╱ │_╱ │_╱ │_╱ │_╱ │_╱
│
0 ┼────┬────┬────┬────┬────┬────┬────┬────┬────
Stop 1 2 3 4 5 6 7 8
Average load: 2.3 kW. Peak load at each door opening: 5-6 kW.
A system rated for 3 kW handles the average beautifully. It cannot handle the peaks. After each door opening, the system runs at maximum capacity but cannot remove heat fast enough before the next stop. Temperature rises incrementally with each cycle.
The Mathematics of Recovery
When doors open, warm ambient air floods the cargo space. The thermal energy introduced:
Q_door = ρ_air × V_cargo × Cp_air × ΔT × η_exchange
For a 12m³ cargo space in 35°C ambient with -18°C cargo temperature:
Q_door = 0.95 × 12 × 1.005 × 53 × 0.4
Q_door = 243 kJ per opening
Time to remove this heat depends entirely on available capacity above steady-state load.
If your system produces 4 kW and steady-state load consumes 2.5 kW, you have 1.5 kW available for recovery:
Recovery time = Q_door / Available capacity
Recovery time = 243 kJ / 1.5 kW
Recovery time = 162 seconds = 2.7 minutes
That sounds manageable. But consider real-world complications:
Complication 1: Steady-state isn’t steady during recovery
When doors open, you also lose the thermal mass of cold air. The replacement warm air increases steady-state load temporarily. Your 1.5 kW “available” capacity shrinks to perhaps 0.8 kW during the critical recovery period.
Revised recovery time: 243 kJ / 0.8 kW = 304 seconds = 5.1 minutes
Complication 2: Multiple door openings per stop
Courier delivery isn’t one door opening. It’s: open doors, locate packages, remove packages, potentially receive returns, close doors. Often 2-3 openings per stop.
Three openings: 243 kJ × 3 = 729 kJ Recovery time: 729 kJ / 0.8 kW = 911 seconds = 15.2 minutes
Complication 3: Your next stop is in 10 minutes
If recovery takes 15 minutes but your route has 10-minute stop intervals, you never fully recover. Each stop starts warmer than the last.
The Drift Calculation
Let’s trace temperature across a 15-stop route where recovery time exceeds stop interval.
Assumptions:
- Starting temperature: -18°C
- Door opening heat load: 729 kJ (3 openings per stop)
- Available recovery capacity: 0.8 kW
- Time between stops: 10 minutes (600 seconds)
- Heat removed per interval: 0.8 kW × 600s = 480 kJ
- Unrecovered heat per stop: 729 – 480 = 249 kJ
Temperature impact per stop:
ΔT = Q_unrecovered / (m_cargo × Cp_cargo)
For 500 kg frozen product (Cp ≈ 2.0 kJ/kg·K):
ΔT = 249 kJ / (500 × 2.0) = 0.25°C per stop
Cumulative drift across 15 stops:
Stop 1: -18.0°C → -17.75°C
Stop 5: -17.0°C → -16.75°C
Stop 10: -15.5°C → -15.25°C
Stop 15: -14.0°C → -13.75°C
Final temperature: -13.75°C (started at -18°C)
The system never alarmed. It ran continuously at maximum capacity—doing exactly what it was designed to do. But 4.25°C of drift occurred because recovery capacity was insufficient for the duty cycle.
R638 compliance requires -18°C for frozen food. This route failed compliance from stop 8 onwards despite equipment “working perfectly.”
What Adequate Recovery Capacity Looks Like
To prevent drift, recovery must complete before the next stop. Working backwards:
Required recovery capacity:
Available capacity = Q_door / Time_between_stops
Available capacity = 729 kJ / 600 s
Available capacity = 1.22 kW minimum
Total system capacity needed:
Total = Steady-state + Recovery + Safety margin
Total = 2.5 kW + 1.22 kW + 20%
Total = 4.46 kW at altitude
Altitude correction (Johannesburg):
Sea-level rating = 4.46 / 0.79 = 5.65 kW
Compare to the original “adequate” 4 kW system. Proper recovery capacity requires 40% more than average-load sizing suggests.
Why Recovery Capacity Is Hidden
Specification sheets show total capacity. They don’t show:
- How much steady-state load consumes
- What remains for transient recovery
- How long recovery takes at partial capacity
- What happens when recovery exceeds stop interval
This information would reveal that catalogue ratings are inadequate for multi-stop applications. Suppliers benefit from this opacity.
The sales conversation that doesn’t happen:
“This unit is rated 4 kW, but your steady-state load will consume 2.5 kW, leaving 1.5 kW for door opening recovery. With your 15-stop route and 10-minute intervals, you’ll experience 4°C temperature drift by end of route. You need our 6 kW unit, which costs R18,000 more.”
Instead:
“This unit is rated 4 kW, your load is 3.2 kW, you have 25% safety margin. That’ll be R62,000.”
Same physics. Different commercial outcome.
The Recovery Ratio
We use a simple metric to evaluate whether equipment is properly sized for multi-stop operations:
Recovery Ratio = Available Recovery Capacity / Door Opening Load Rate
Where:
- Available Recovery Capacity = Total capacity – Steady-state load (kW)
- Door Opening Load Rate = Energy per stop / Time between stops (kW)
Interpretation:
| Recovery Ratio | Performance |
|---|---|
| > 1.5 | Excellent—full recovery with margin |
| 1.2 – 1.5 | Adequate—full recovery, minimal margin |
| 1.0 – 1.2 | Marginal—recovery possible, no margin for variance |
| 0.8 – 1.0 | Insufficient—gradual drift across route |
| < 0.8 | Failed—significant drift, compliance risk |
Calculating for our example:
Available capacity: 4.0 - 2.5 = 1.5 kW
Door opening rate: 729 kJ / 600 s = 1.22 kW
Recovery Ratio: 1.5 / 1.22 = 1.23
A ratio of 1.23 is marginal—the system can theoretically recover but has no margin for:
- Hotter ambient days
- Longer door openings
- Traffic delays compressing stop intervals
- Equipment degradation over time
Proper specification targets Recovery Ratio > 1.5 to handle real-world variance.
The Compressor Duty Cycle Problem
Even when recovery capacity is theoretically adequate, there’s a physical limit: compressor duty cycle.
Transport refrigeration compressors aren’t designed for 100% continuous operation. They need off-cycles for oil return, thermal management, and component longevity. Typical maximum duty cycle: 70-80%.
What this means for recovery:
If your system must run continuously at 100% to achieve recovery, you’re:
- Exceeding design duty cycle (accelerated wear)
- Causing oil migration (compressor damage)
- Overheating components (reliability issues)
- Operating with zero margin (any variance causes failure)
A properly sized system achieves recovery at 70% duty cycle, leaving headroom for peak demands and equipment longevity.
Revised capacity requirement:
Required capacity (at 70% duty) = Recovery requirement / 0.70
Required capacity = 4.46 kW / 0.70 = 6.37 kW at altitude
Sea-level rating = 6.37 / 0.79 = 8.1 kW
The “adequate” 4 kW system should actually be 8 kW. That’s not 25% safety margin—that’s 100% undersizing.
Real-World Recovery Testing
How do you know if your equipment can actually recover between stops? Test it.
The Recovery Time Test:
- Stabilise cargo space at -20°C (below setpoint)
- Open doors for 90 seconds
- Close doors
- Time how long until -18°C is restored
- Record ambient temperature during test
Benchmark results:
| Recovery time | Assessment |
|---|---|
| < 5 minutes | Excellent recovery capacity |
| 5-8 minutes | Adequate for 12+ minute stop intervals |
| 8-12 minutes | Marginal—suitable for light delivery only |
| 12-20 minutes | Insufficient for multi-stop courier |
| > 20 minutes | Severely undersized |
Critical: Test in summer conditions, not winter. A system recovering in 6 minutes at 20°C ambient may take 15 minutes at 35°C.
The Hidden Cost of Slow Recovery
Slow recovery doesn’t just cause temperature drift. It creates operational constraints that cost money:
- Routing limitations: Routes must be planned with longer intervals between stops. A 15-stop route that could complete in 4 hours with proper equipment takes 6 hours with slow recovery, reducing daily capacity.
- Time-window failures: Customer delivery windows become harder to hit when recovery time constrains route sequencing.
- Fuel waste: Systems running at maximum capacity continuously consume more fuel than systems cycling normally. A 4 kW system running 100% uses more fuel than a 6 kW system running 70%.
- Equipment lifespan: Compressors designed for 70% duty cycle running at 95% duty cycle fail in 3-4 years instead of 7-8 years. The R15,000 saved on equipment costs R45,000 in early replacement.
- Product risk: Even “acceptable” drift to -15°C affects product quality. Ice cream softens. Meat surfaces thaw. Seafood degrades. You’re technically compliant but delivering compromised product.
Specifying for Recovery Capacity
When discussing equipment with suppliers, shift the conversation from total capacity to recovery capacity:
Questions to ask:
- “What steady-state load do you assume for my cargo volume and ambient conditions?”
- “What recovery capacity remains after steady-state load?”
- “How long does your specification assume for recovery between stops?”
- “What’s the assumed door opening frequency?”
- “At what duty cycle percentage does recovery occur?”
Red flags:
- Supplier cannot separate steady-state from recovery capacity
- Recovery time assumptions exceed your actual stop intervals
- Duty cycle assumptions approach 100%
- Door opening frequency assumptions are less than half your reality
Specification requirements:
Include these in your equipment specification:
- Recovery time: “Must restore -18°C within 8 minutes of 90-second door opening at 35°C ambient”
- Recovery capacity: “Minimum 2 kW available above steady-state load”
- Duty cycle: “Recovery achievable at 70% compressor duty cycle maximum”
The System Diagram
Here’s how to visualise whether equipment is properly sized:
Total Rated Capacity (at altitude): 6.0 kW
│
├── Steady-state load: 2.5 kW (42%)
│ ├── Wall transmission: 1.2 kW
│ ├── Floor (hot pavement): 0.6 kW
│ ├── Solar gain: 0.4 kW
│ └── Air infiltration (seals): 0.3 kW
│
├── Recovery capacity: 2.0 kW (33%)
│ └── Available for door opening recovery
│
└── Reserve margin: 1.5 kW (25%)
├── Peak ambient variance
├── Equipment degradation
├── Product temperature variance
└── Operational contingency
If your system diagram shows recovery capacity below 25% of total, or reserve margin below 20%, the equipment is marginal for multi-stop operations.
Conclusion
Average thermal load is a statistical fiction. Refrigeration systems don’t fight averages—they fight peaks. Every door opening creates a peak demand that must be met before the next stop, or temperature drifts upward across the route.
Recovery capacity—the ability to restore temperature between stops—is the critical specification for multi-stop operations. It’s also the specification that never appears on equipment datasheets, never gets discussed in sales conversations, and never gets tested before delivery.
Systems sized for average load will maintain temperature in laboratory conditions with two door openings per day. They will not maintain temperature on a 15-stop courier route with 10-minute stop intervals.
The question isn’t whether your equipment has enough total capacity. It’s whether enough capacity remains after steady-state load to recover from door openings within your actual stop intervals while running at sustainable duty cycles.
That’s a longer question. It’s also the right question. And it’s the question the industry hopes you never ask.
Your specification sheet shows total capacity. What matters is what’s left over. Size for recovery, or watch your temperature drift.
Related Reading:
- The Multi-Stop Thermal Load Reality
- The Product Temperature Question: Physics as Liability Shield
- Technical Formulas Reference
At The Frozen Food Courier, we specify refrigeration systems for recovery capacity, not just total capacity. We test recovery performance before accepting vehicles. We know exactly how long our systems take to restore temperature after door openings—because we’ve measured it across many thousands kilometers of multi-stop operations.
