Cold-chain heat load worksheet

What the worksheet calculates

The calculator is a cold-chain heat load worksheet. It takes box dimensions in inches, computes outer surface area in square feet, and estimates heat to absorb in BTU over the transit period. It uses the positive ambient to target temperature difference, effective total insulation R-value, and engineering allowance. It then reports theoretical dry ice mass, water-ice-equivalent mass, and whole gel packs by heat only. These are steady-state estimates, not a loading plan or compliance determination.

Dimensions and insulation

Enter length, width, and height in inches for the box surface you intend to represent. Area is 2 x (LW + LH + WH) / 144, with 144 converting square inches to square feet. The output is labeled outer surface area, so use external dimensions when that is the surface you mean. The R-value is an effective total insulation value in hour square foot degree Fahrenheit per BTU. It is not derived from wall thickness or material type. A higher entered R-value lowers calculated heat leak; a lower value raises it. Seams, corners, and insulation defects are not modeled separately.

Temperature gap and transit hours

Heat is calculated only when ambient temperature exceeds the target; a zero or negative difference contributes no heat in this model. If ambient is at or below target, the gap is zero and heat, dry ice mass, water-ice-equivalent mass, and whole gel pack count all become zero. Transit hours scale the load linearly: doubling hours doubles heat before allowance, and halving hours halves it. The model does not add solar load, door openings, air exchange, or payload conditioning. Those omissions matter when the box is sun exposed, handled repeatedly, or packed with warm goods.

Allowance and coolant mass outputs

The engineering allowance is a percentage uplift applied after base heat leak. An allowance of 30 increases heat by 30 percent; zero leaves base heat unchanged. The uplift flows to theoretical dry ice mass, water-ice-equivalent mass, and gel pack count because those outputs derive from adjusted heat. Theoretical dry ice mass divides heat by 246 BTU per lb, the latent heat of sublimation used here. Water-ice-equivalent mass divides heat by 144 BTU per lb. These are mass equivalents for the entered heat load, not loading instructions, and they omit coolant placement and phase-change curves. The allowance is an entered uplift, not a certification or guarantee.

Gel pack count and pack weight

Whole gel packs by heat only divides water-ice-equivalent mass by entered water-based gel pack weight and rounds up to the next whole pack. Pack weight affects only this count. Changing pack weight does not change estimated heat, theoretical dry ice mass, or water-ice-equivalent mass. A heavier pack represents more water-ice-equivalent mass, so the whole pack count can fall or stay unchanged after rounding up. The count is a heat-only screening number. It does not reserve space, account for gaps, or verify that ordinary water-based gel packs can hold the target temperature. The worksheet notes they do not maintain a subfreezing target.

Worked examples

Starting scenario: a 12 by 10 by 8 in box with R-4, ambient 85 F, target 39 F, 30 transit hours, 30 percent allowance, and 1 lb gel packs. The worksheet reports 4.11 ft2 outer surface area, 1843.83 BTU estimated heat to absorb, 7.5 lb theoretical dry ice mass, 12.8 lb water-ice-equivalent mass, and 13 whole gel packs by heat only. The 30 percent allowance raises the base heat before the mass conversions, so the dry ice and water-ice figures include that uplift.

Changed scenario: a 12 by 12 by 12 in box with R-4, ambient 80 F, target 40 F, 24 transit hours, 0 percent allowance, and 1 lb gel packs. The worksheet reports 6 ft2 outer surface area, 1440 BTU estimated heat to absorb, 5.85 lb theoretical dry ice mass, 10 lb water-ice-equivalent mass, and 10 whole gel packs by heat only. Compared with the starting case, the larger box increases area, but the smaller temperature gap, shorter transit time, and zero allowance reduce the heat load and coolant masses.

Edge cases and interpretation limits

If target is warmer than ambient, the positive gap is zero, so the worksheet returns zero heat and zero coolant masses. That is a model result, not proof of no thermal risk. If allowance is zero, heat is the unadjusted steady-state value. If pack weight is very small, the whole pack count can become large because each pack carries little water-ice-equivalent mass. The model is a steady-state heat balance only. It omits payload conditioning, seams, air exchange, solar load, coolant placement, and actual phase-change curves. It cannot establish food or medicine safety or shipping compliance, and it should be treated as a screening checklist rather than a guarantee. Dry ice is not interchangeable with ordinary water-based gel packs. The notes state that ordinary water-based gel packs do not maintain a subfreezing target, and dry ice may overcool refrigerated goods and requires a compatible, vented shipping system. Dry ice shipments can trigger transport requirements under 49 CFR 173.217, and carrier and mode approval remain necessary. The calculator does not determine packaging approval, venting adequacy, quantity limits, labeling, or legal status.

Questions about cold-chain heat load worksheet