Unlike gel ice packs, which can be refrozen and reused between shipments, dry ice sublimates continuously from the moment it is manufactured. Every hour it sits in your cold room, in a staging area, or inside a shipping box, it is losing mass. The rate at which it does so — the sublimation rate — determines exactly how much dry ice you need per shipment, how much to order for a week’s dispatch volume, and how long you can hold stock before it becomes inadequate for your shortest transit.
Understanding dry ice sublimation is not just an academic exercise. Over-ordering wastes money. Under-ordering causes cold chain failures. Getting the calculation right is a direct input to your operations budget.
The Physics of Sublimation
Dry ice is solid carbon dioxide (CO₂) at atmospheric pressure. Unlike water, CO₂ has no liquid phase at atmospheric pressure — it transitions directly from solid to gas at −78.5°C via sublimation. The energy required for this phase transition is the enthalpy of sublimation: approximately 571 kJ per kilogram of dry ice.
Sublimation occurs because the surface of the dry ice is continuously absorbing heat from the environment — through conduction from any surface it contacts, convection from the surrounding air, and radiation from nearby warmer surfaces. The rate of sublimation is therefore directly proportional to the rate of heat ingress to the dry ice surface.
Key relationship: Sublimation rate (kg/h) = Heat ingress to dry ice surface (W) ÷ Enthalpy of sublimation (J/kg)
= Heat ingress (W) ÷ 571,000 (J/kg) × 3,600 (seconds/hour)
Sublimation Rates in Different Storage Conditions
The sublimation rate of dry ice varies significantly depending on how and where it is stored. The following data represents approximate sublimation rates under typical conditions:
| Storage Condition | Ambient Temp | Approx. Sublimation Rate | Notes |
|---|---|---|---|
| Insulated EPS shipper (38mm walls, sealed) | 25°C | 3–5% of mass per hour | Standard transit condition |
| Insulated EPS shipper (38mm walls, sealed) | 35°C | 5–7% of mass per hour | Australian summer transit |
| Polystyrene storage chest (unsealed lid) | 20°C | 8–12% of mass per hour | Staging area; lid not sealed |
| Open air, no insulation | 25°C | 12–18% of mass per hour | Extremely rapid loss |
| Commercial dry ice storage chest (insulated) | 20°C | 4–6% of mass per hour | Better than EPS due to thicker walls |
| Walk-in cool room (refrigerated) | 4°C | 2–3% of mass per hour | Low ΔT reduces sublimation rate |
Important: Never store dry ice in a sealed airtight container. As CO₂ gas pressure builds, it can cause container rupture or explosion. Always store dry ice in ventilated containers or with lids that allow gas venting.
Calculating How Much Dry Ice You Need Per Shipment
Dry ice quantity per shipment is calculated in two parts: (1) the quantity consumed during transit to absorb heat ingress through the packaging walls, and (2) the quantity lost to sublimation in storage between receipt of your dry ice order and dispatch of the shipment.
Part 1: Transit Consumption
Using our thermal load calculation framework:
For a standard EPS carton (40mm walls, surface area 0.74 m², k = 0.035 W/m·K) at 35°C ambient carrying a frozen payload at −18°C (ΔT = 53°C), over a 24-hour transit:
- Heat ingress rate: Q = 0.035 × 0.74 × 53 / 0.040 = 34.2 W
- Total heat over 24h: 34.2 × 86,400 = 2,956 kJ
- Dry ice consumed (571 kJ/kg): 2,956 / 571 = 5.17 kg
- With 25% safety margin: 6.5 kg
Part 2: Storage Loss Before Dispatch
If your dry ice delivery arrives Monday and you dispatch Tuesday evening (approximately 30 hours of storage in a commercial insulated chest at 20°C, sublimation rate ~5% per hour), you will lose approximately 5% × 30h = 150% of one kilogram for each kilogram held — meaning if you ordered 6.5 kg for Tuesday’s dispatch, you need to have received approximately 16 kg to have 6.5 kg remaining after 30 hours of storage (6.5 kg ÷ (1 − 0.05 × 30)).
This calculation highlights an important operational point: dry ice must be ordered as close to dispatch time as possible. Holding dry ice for more than 12–24 hours in staging involves significant mass loss that must be accounted for in your ordering quantity.
Practical Ordering Formula
For weekly dispatch operations:
- Calculate per-shipment transit quantity (using thermal load calculation + 25% margin)
- Calculate staging loss based on your typical hold time between delivery and dispatch
- Add staging loss to transit quantity to get total dry ice required per shipment
- Multiply by number of shipments per week to get weekly order quantity
- Add 10% buffer for order quantity variation and unexpected delays
Example: 50 shipments/week, 6.5 kg transit quantity, 24h average staging time in commercial chest (5%/h loss rate) → staging loss = 0.05 × 24 × 6.5 = 7.8 kg per shipment → order quantity = (6.5 + 7.8) × 50 × 1.10 = 786 kg per week
CO₂ Gas Management: Ventilation Requirements
As dry ice sublimates in storage areas and during packing operations, CO₂ gas accumulates. The occupational exposure limit for CO₂ is 5,000 ppm (0.5%) for an 8-hour workday. Sublimation of just 1 kg of dry ice produces approximately 0.55 m³ of CO₂ gas at atmospheric conditions.
For operations handling more than 5–10 kg of dry ice at a time, ensure: adequate ventilation (10–20 air changes per hour in the packing area), CO₂ monitoring with alarm capability (fixed or portable sensors), and trained staff who understand the asphyxiation risk in confined spaces.
Tips to Reduce Dry Ice Waste
- Order on the day of dispatch or the evening before — minimise staging time
- Use well-insulated storage — a quality commercial dry ice chest reduces staging loss significantly vs an EPS box with an ill-fitting lid
- Keep staging area cool — even a cool room at 15°C instead of 25°C ambient reduces sublimation rate by approximately 30%
- Work efficiently on pack days — every hour dry ice sits open on the packing bench is 12–18% mass loss
- Right-size your orders — ordering excess “just in case” means paying for sublimation you never use
Conclusion
Dry ice sublimation is a continuous, unavoidable physical process. Managing it well — through accurate transit quantity calculations, disciplined ordering schedules, and proper staging procedures — is the difference between a profitable dry ice cold chain and one that bleeds cost through over-ordering and waste. Explore Dry Chill’s range of dry ice packs engineered for consistent sublimation performance and reliable transit hold times in Australian conditions.