The most common question from businesses setting up a cold chain for the first time: how many packs do I need? Buying too few means your shipment fails. Buying too many wastes cost on every delivery. The answer lies in a straightforward thermal load calculation that any logistics manager can perform with basic information about their packaging and transit conditions.
The Core Principle
Refrigerant capacity (kJ) must be greater than or equal to Total heat ingress (kJ) multiplied by a Safety margin of 1.25 to 1.30
Step 1: Your Four Inputs
- Target internal temperature — For TGA pharmaceutical cold chain: 2–8°C
- Worst-case ambient temperature — Australian summer exposed freight: 38–42°C
- Transit duration (hours) — Add 20% to expected time to account for delays
- Packaging surface area and wall thickness
Step 2: Temperature Differential
ΔT = Ambient minus Target. Example: Brisbane summer 38°C, payload target 5°C → ΔT = 33°C.
Step 3: Heat Ingress Rate (Fourier’s Law)
Q (watts) = k × A × ΔT / d
Standard 38mm EPS carton (k=0.035, A=0.74m², d=0.038m, ΔT=33°C): Q = 22.5 W
Over 24 hours: 22.5 × 86,400 = 1,944 kJ total heat ingress.
Step 4: Convert to Refrigerant Mass
Gel ice packs (334 kJ/kg): 1,944 ÷ 334 = 5.82 kg
Dry ice packs (571 kJ/kg): 1,944 ÷ 571 = 3.40 kg
Step 5: Apply Safety Margin (+25%)
Gel packs: 5.82 × 1.25 = 7.3 kg | Dry ice: 3.40 × 1.25 = 4.25 kg
Reference Table by Transit Duration (38°C ambient, 38mm EPS carton)
| Transit Duration | Gel Packs Needed | Dry Ice Needed |
|---|---|---|
| 12 hours | 3.7 kg | 2.1 kg |
| 24 hours | 7.3 kg | 4.2 kg |
| 36 hours | 10.9 kg | 6.4 kg |
| 48 hours | 14.6 kg | 8.5 kg |
Australian Climate Zone Adjustments
| Region / Season | Baseline Ambient | Adjustment vs 35°C |
|---|---|---|
| Melbourne, May–Sep | 22°C | −35% refrigerant |
| Sydney / Melbourne, Nov–Mar | 35°C | Baseline |
| Brisbane, Nov–Mar | 38°C | +10% |
| Perth, Dec–Feb | 40°C | +18% |
| Darwin / North QLD, Oct–Apr | 42°C | +25% |
Conclusion
Sizing your refrigerant correctly is a straightforward engineering calculation. The cost of under-packing — a failed pharmaceutical shipment or spoiled consignment — far exceeds the cost of carrying slightly more refrigerant than the minimum. Browse Dry Chill’s dry ice packs and gel ice packs to find the right refrigerant for your application.