The Specification Fiction

Why manufacturer sizing assumes you open doors twice a day—and why courier operations blow past that before morning tea

Somewhere in an air-conditioned office, an engineer calculated how to size transport refrigeration units. They assumed a truck leaves a warehouse, drives to a destination, opens doors for unloading, and returns. Maybe two or three door openings per day. Steady driving at highway speeds with good airflow over condensers. Predictable, controlled, manageable.

This describes long-haul transport.

It describes nothing about courier operations.

Yet the same sizing methodology—developed for trucks doing two stops per day—gets applied to vehicles doing fifteen, twenty, or forty stops. The industry knows these are different applications. They sell you the same equipment anyway.

Two Stops vs Twenty: The Numbers

Let’s quantify what “multi-stop” actually means for thermal load.

Long-haul assumptions (what gets specified):

Parameter	Assumption
Door openings per day	2-4
Average speed	80 km/h
Condenser airflow	Ram air at speed
Ambient exposure	Highway, minimal urban
Duty cycle	90% driving, 10% stationary

Courier reality (what actually happens):

Parameter	Reality
Door openings per day	30-60
Average speed	25-35 km/h
Condenser airflow	Minimal (stop-start traffic)
Ambient exposure	Urban heat island, parking lots
Duty cycle	40% driving, 60% stationary/slow

These aren’t minor variations. This is a fundamentally different application.

Calculating the Gap

Using a 12m³ cargo space in Johannesburg summer conditions (35°C ambient):

Door opening thermal load per opening:

Q_door = ρ_air × V_cargo × Cp_air × ΔT × η_exchange
Q_door = 0.95 kg/m³ × 12 m³ × 1.005 kJ/kg·K × 53K × 0.4
Q_door = 243 kJ per opening

Long-haul daily load (4 openings):

Q_total = 243 kJ × 4 = 972 kJ = 0.97 MJ

Courier daily load (40 openings):

Q_total = 243 kJ × 40 = 9,720 kJ = 9.72 MJ

The multiplier: 10× the door opening thermal load.

But it gets worse.

The Compounding Problem

Door openings don’t just add heat. They create a cascade of compounding effects that manufacturer sizing ignores.

Effect 1: Recovery time compression

• Long-haul: 4 hours between stops. System has time to fully recover, stabilise, and build thermal reserve.
• Courier: 8-15 minutes between stops. System must recover before next opening or temperature drifts upward.

Effect 2: Cumulative drift

If recovery takes 12 minutes but your next stop is in 10 minutes, you open doors before reaching setpoint. Each stop starts slightly warmer than the last. Across 15 stops, a system running “fine” on paper drifts from -18°C to -14°C.

Effect 3: Condenser starvation

• Long-haul: Sustained highway speed provides ram air effect over condenser. Heat rejection efficient.
• Courier: Stop-start urban traffic. Condenser relies on fans alone. Heat rejection capacity drops 30-40%. The system that could recover at 80 km/h cannot recover at 0 km/h.

Effect 4: Urban heat island

• Long-haul: Highway ambient temperatures match weather station data.
• Courier: Urban core 5-8°C hotter. Parking lot pavement radiating 60°C. Stationary vehicle absorbing radiant heat from all directions. Actual thermal environment far exceeds “ambient temperature” specification.

What This Means for Capacity

Let’s build a realistic specification for courier duty versus long-haul.

Long-haul specification (standard industry approach):

Steady-state transmission losses: 1.5 kW
Door opening allowance (4/day): 0.1 kW average
Solar/ambient: 0.4 kW
Subtotal: 2.0 kW
Safety margin (20%): 0.4 kW
Sea-level specification: 2.4 kW

Courier specification (reality-based):

Steady-state transmission losses: 1.5 kW
Door opening load (40/day, 6-hour route): 0.45 kW average
Peak recovery requirement: +1.5 kW capacity
Urban heat island addition: 0.4 kW
Reduced condenser efficiency (low speed): +25% capacity needed
Solar/ambient (stationary exposure): 0.6 kW
Subtotal: 4.45 kW + condenser penalty
Effective requirement: 5.6 kW
Altitude correction (Johannesburg): 5.6 / 0.79 = 7.1 kW
Safety margin (25%): 8.9 kW sea-level specification

The gap: 2.4 kW vs 8.9 kW

A courier operation needs 3.7× the refrigeration capacity of a long-haul truck with the same cargo volume. The industry sells you the same equipment for both applications.

Why the Industry Ignores This

The multi-stop thermal load problem is well understood by refrigeration engineers. It’s ignored for commercial reasons:

Reason 1: Price competition

Courier operators shop on price. Quoting 8.9 kW when competitors quote 2.4 kW loses the sale. The operator who understands thermodynamics pays more; the operator who doesn’t buys cheaper and suffers later.

Reason 2: Blame transfer

When undersized equipment fails, suppliers blame operator practices: “too many door openings,” “doors held open too long,” “product loaded too warm.” The specification assumed two stops per day—exceeding that becomes your problem.

Reason 3: Replacement revenue

Equipment running at maximum capacity fails faster. Compressors cycle constantly, condensers strain, components wear. Undersized equipment generates replacement sales and maintenance revenue. Properly sized equipment lasts longer—bad for the supplier’s parts business.

Reason 4: No category exists

Industry doesn’t have a “courier duty cycle” specification category. You get “delivery vehicle” which assumes store-to-store transport, or “long-haul” which assumes warehouse-to-warehouse. Multi-stop last-mile courier fits neither, so suppliers pick whichever lets them quote cheaper.

The Stop Count Inflection Point

At what point does stop count fundamentally change capacity requirements?

Based on thermal modelling and operational data:

Stops per day	Duty cycle	Capacity multiplier vs long-haul
1-4	Long-haul	1.0× (baseline)
5-10	Light delivery	1.5×
11-20	Multi-stop delivery	2.2×
21-40	Intensive courier	3.2×
40+	High-frequency courier	4.0×+

At 15 stops per day—a light day for most courier operations—you need more than double the capacity of a long-haul specification. Yet the industry applies the same sizing to both.

What Proper Courier Specification Looks Like

If a TRU builder genuinely understood multi-stop operations, specification conversations would include:

Duty cycle questions:

• How many stops per typical route?
• What’s your average time between stops?
• What percentage of route is urban vs highway?
• How long are doors typically open per stop?

Environmental questions:

• What’s your operating altitude?
• Do you stage in parking lots or shaded depots?
• What’s your exposure to urban heat island areas?
• What ambient temperature range must you handle?

Capacity philosophy:

• Are we sizing for average load or peak recovery?
• What temperature drift is acceptable across a route?
• How quickly must the system recover between stops?
• What safety margin accounts for equipment aging?

If the conversation starts and ends with cargo volume and target temperature, you’re getting long-haul sizing applied to courier operations.

The Route Simulation Test

Here’s a practical test for whether equipment is properly sized for multi-stop duty:

The 15-stop summer simulation:

1. Pre-cool cargo space to -20°C
2. Load product at -18°C
3. Open doors for 90 seconds
4. Close doors, wait 10 minutes
5. Repeat steps 3-4 fifteen times
6. Measure final cargo temperature

Results interpretation:

Final temperature	Verdict
-18°C or colder	Properly sized for multi-stop
-15°C to -17°C	Marginal—will fail on hot days
-12°C to -14°C	Undersized for courier duty
Above -12°C	Dangerously undersized

Most “delivery vehicle” specifications fail this test by stop 8-10. The system works for long-haul but cannot sustain courier duty cycles.

Protecting Your Operation

Specify for your actual duty cycle:

Don’t accept “delivery vehicle” as a specification category. Provide your actual parameters:

• Typical stop count
• Average time between stops
• Operating altitude
• Urban exposure percentage

Require multi-stop capacity documentation:

Ask suppliers to show their door opening frequency assumption. If it’s less than half your actual stop count, the specification is fiction.

Test before acceptance:

Run the 15-stop simulation on new vehicles before final payment. Equipment that fails during testing will fail during operations.

Factor peak recovery, not average load:

Average thermal load calculations hide the problem. A system handling average load cannot handle the peaks that occur at each stop. Specify for peak recovery capacity.

The Real Cost of Undersizing

Operators often choose cheaper, undersized equipment to reduce capital cost. Here’s what that decision actually costs:

Direct costs:

• Product loss from temperature excursions: R5,000-R15,000 per incident
• Increased fuel consumption (maximum compressor runtime): +25-40%
• Accelerated equipment wear: 30-50% shorter service life
• Emergency repairs: R8,000-R20,000 per breakdown

Indirect costs:

• Customer complaints and contract losses
• Reputation damage from temperature failures
• Staff time managing problems instead of growing business
• Regulatory risk if temperature logs show non-compliance

Lifecycle comparison (5-year ownership):

Item	Undersized (2.4 kW)	Properly sized (8.9 kW)
Equipment cost	R45,000	R95,000
Fuel (5 years)	R280,000	R195,000
Maintenance	R65,000	R35,000
Product losses	R75,000	R5,000
Replacement (year 4)	R45,000	R0
Total	R510,000	R330,000

Saving R50,000 upfront costs R180,000 over five years. The “expensive” properly sized equipment is 35% cheaper to own.

Conclusion

Multi-stop courier operations face thermal loads 3-4× higher than long-haul transport with identical cargo volumes. The industry knows this. Standard sizing methodologies ignore it.

When suppliers quote “delivery vehicle” specifications for courier operations, they’re applying long-haul assumptions to a fundamentally different duty cycle. The result: equipment that works on paper, fails in practice, and generates blame-shifting conversations about operator practices.

Proper specification starts with understanding that courier is not delivery is not long-haul. Each has different thermal profiles requiring different capacity. Anyone selling you the same equipment for all three applications is selling price, not solutions.

The physics of multi-stop thermal loads is unavoidable. The only question is whether you pay for adequate capacity upfront or pay more for inadequate capacity over the vehicle’s lifetime.

Your cargo space doesn’t care about specification categories. It cares about thermal load. Size accordingly.

Related Reading

• Door Opening Recovery: The Hidden Capacity Requirement
• The Product Temperature Question: Physics as Liability Shield

At The Frozen Food Courier, we have learned what equipment actually handles courier duty cycles versus what merely survives long-haul assumptions. Our specifications start with reality, not catalogue ratings.