If you have ever noticed that a glass of ice water stays at 0°C long after the ice starts melting — rather than warming up immediately — you have already witnessed latent heat in action. This same thermodynamic principle is the reason that phase change materials (PCMs) are the engine of every effective cold chain packaging system.
Sensible Heat vs Latent Heat: The Core Distinction
When you add heat to a material that does not change phase, its temperature rises — this is sensible heat. A kilogram of water warming from 10°C to 11°C absorbs approximately 4.18 kJ. Latent heat is fundamentally different: when a material undergoes a phase transition — melting, freezing, or sublimating — it absorbs or releases a large amount of energy without any change in temperature. During the phase transition, the material acts as a thermal battery, absorbing heat from the environment while holding its own temperature constant.
Key engineering value: Water absorbs 4.18 kJ/kg/°C as sensible heat. At 0°C, the same kilogram absorbs 334 kJ as it melts — equivalent to warming 79 kg of water by 1°C. This is the foundation of all PCM refrigeration.
Latent Heat Values by Refrigerant Type
| Material | Phase Transition Temp | Latent Heat (kJ/kg) | Common Application |
|---|---|---|---|
| Water / water-gel | 0°C | 334 | Fresh food, 0–8°C pharma |
| Dry ice (solid CO₂) | −78.5°C | 571 | Frozen goods, cryogenic |
| Eutectic −4°C blend | −4°C | ~280 | Chilled pharmaceuticals |
| Eutectic −21°C blend | −21°C | ~200 | Deep-frozen biologics |
| Paraffin wax (~12°C) | 12°C | ~210 | CRT pharmaceuticals (15–25°C) |
Practical Sizing Calculation
Example: Sydney to Brisbane pharmaceutical shipment
Insulated carton — 40mm EPS walls, surface area 0.8 m², ambient 35°C, payload target 5°C, 24-hour transit.
- Heat flux: Q = 0.035 × 0.8 × 30 / 0.040 = 21 W
- Total heat over 24h: 21 × 86,400 = 1,814 kJ
- Gel packs required (334 kJ/kg + 25% margin): 6.8 kg
For this application, gel ice packs conditioned to −18°C are the correct engineering choice. Conditioning to −18°C (rather than 0°C) adds ~37.6 kJ/kg of sensible heat buffer, extending effective transit duration.
Dry Ice: The High-Capacity Frozen Freight Solution
At 571 kJ/kg, dry ice absorbs 71% more heat per kilogram than gel packs. For products that must remain below 0°C — frozen food, cryogenic biologics — dry ice packs are unmatched. The trade-off is the −78.5°C surface temperature, which requires a barrier between dry ice and the payload to prevent freeze damage or cryogenic burns. In a standard EPS shipper at 25°C ambient, dry ice sublimates at approximately 3–5% of mass per hour.
Why Pre-Conditioning Is Non-Negotiable
A gel pack stored at 0°C instead of −18°C has significantly less effective cooling capacity — it is already at its phase transition temperature with no sensible heat buffer remaining. The correct protocol is to freeze at −18°C or below for a minimum of 12–16 hours before dispatch. This is one of the most commonly overlooked sources of cold chain failure in Australian operations.
Conclusion
Choosing the right PCM — at the right transition temperature, in the right quantity, and properly pre-conditioned — is the highest-leverage decision in cold chain packaging design. Explore Dry Chill’s full range of dry ice packs and gel ice packs engineered for Australian cold chain conditions.