Pipe freeze calculator
Estimate how long water standing in an insulated pipe takes to cool to 32 °F, and how long until it freezes solid. Choose the pipe, the design conditions and up to three insulation systems, and the results compare them side by side at the same wall thickness.
Cooling curve
Results by system
| System | k | To 32 °F | To fully frozen | Loss at 32 °F, W/LF | Jacket temp |
|---|
How the calculator works Inputs and units, assumptions and a worked example
Inputs and units
Every input starts with a default, so the calculator shows a result as soon as the page loads. Change any value and the results update.
| Input | Unit | Default | What to enter |
|---|---|---|---|
| Material and standard | — | Ductile iron | Sets the pipe’s inside and outside diameter and the conductivity of the pipe wall. Choose ductile iron (AWWA C151), carbon steel (ASTM A53 IPS standard wall), PVC (AWWA C900 DR18) or HDPE (DR11, IPS outside diameter). |
| Nominal size | in (NPS) | 12 | 4 to 36 in for ductile iron and steel. 4 to 24 in for PVC and HDPE. |
| Insulation wall | in | 2 | Insulation thickness: 1, 1½, 2, 2½, 3 or 4 in. Every system is compared at the same wall. |
| Starting water | °F | 45 | Water temperature when flow stops. 32 to 120 °F. |
| Design ambient | °F | 0 | Air temperature around the line. Use the site’s ASHRAE 99.6% heating dry-bulb. −60 to 31 °F. |
| Wind speed | mph | 15 | Steady wind across the pipe. Enter 0 for still air. 0 to 80 mph. |
| Sky temperature | °F | Ambient | Leave blank to use the ambient. A clear night sky is colder than the air and pulls more heat off the surface. |
| Insulation systems | — | Dragon Jacket, fiberglass, bare pipe | Up to three systems. Each uses the conductivity (k) in the table below. Tick “Use my own k” to enter a different value. |
Results
| Result | Unit | Meaning |
|---|---|---|
| Hours to 32 °F | hours | Time for the water to cool from the starting temperature to freezing, for the first system. |
| To fully frozen | hours | Time to 32 °F plus the time to remove the water’s latent heat. |
| Water held | gal per 100 ft | Volume of water in 100 ft of the pipe. |
| Heat loss at 32 °F | W/LF | Heat leaving each linear foot of pipe when the water reaches freezing. |
| Jacket surface | °F | Outer surface temperature when the water reaches freezing. |
Insulation conductivity
| System | k, Btu·in/hr·ft²·°F | Mean temp | Basis |
|---|---|---|---|
| Dragon Jacket | 0.190 | 75 °F | Polyiso core, polyurea shell |
| Polyiso board | 0.190 | 75 °F | ASTM C591 Type IV, design value |
| Fiberglass | 0.230 | 75 °F | ASTM C547 Type I |
| Mineral wool | 0.250 | 100 °F | ASTM C547 |
| Elastomeric | 0.280 | 75 °F | ASTM C534 Type I |
| Cellular glass | 0.290 | 75 °F | ASTM C552 |
| Calcium silicate | 0.344 | 100 °F | ASTM C533 Type I |
Assumptions
A screening calculation for a stopped line. Confirm the result against project conditions and any freeze analysis the design authority requires.
Still water
Flow has stopped and the water starts at one temperature. The water stays mixed, so it is the same temperature throughout the pipe as it cools.
Heat path
Heat passes through the pipe wall and the insulation, then leaves the outer surface by convection to the air and radiation to the sky. The surface temperature is solved by energy balance at each step as the water cools. Inside film resistance is not included.
Layer resistance
R = ln(D2/D1) ÷ 2πk per foot of pipe, for the pipe wall and the insulation in series.
Convection
Churchill–Bernstein for a cylinder in crossflow when the wind is above 0.1 mph. Churchill–Chu for still air. Air properties at the film temperature.
Radiation
q = εσ(Ts4 − Tsky4) per square foot of outer surface, with a surface emittance of 0.90 and the sky temperature entered.
Conductivity
Held constant at the published value for each system. It is not adjusted for temperature.
Water
Density 62.4 lb/ft³, specific heat 1.0 Btu/lb·°F, latent heat of fusion 143.4 Btu/lb.
Freezing solid
Time to fully frozen removes the latent heat at the 32 °F heat-loss rate. It ignores the insulating effect of the ice as it forms, so a real line takes longer to freeze solid. Supercooling is not modeled.
Insulation condition
Dry and intact, with no gaps. Wet or damaged insulation conducts more heat, and the line freezes sooner.
Geometry
A straight, uniformly insulated run in open air. Supports, fittings, valves, joints, ground contact, sunlight and heat trace are not modeled.
Pipe dimensions: AWWA C151 ductile iron, AWWA C900 DR18 PVC, ASTM A53 IPS standard-wall steel and DR11 HDPE. Conductivity values are manufacturers’ published data under the ASTM standard listed.
Worked example
The calculator’s default line: 12 in ductile iron with 2 in of Dragon Jacket, holding 45 °F water, shut in at 0 °F with 15 mph wind.
| Step | Working | Result |
|---|---|---|
| Diameters | AWWA C151 12 in pipe: 12.64 in inside, 13.20 in outside. Add 2 in of insulation on each side. | 17.20 in outside the insulation |
| Water held | π/4 × (12.64/12 ft)² × 62.4 lb/ft³ | 54.4 lb/ft (6.5 gal/ft) |
| Heat to remove before freezing starts | 54.4 lb/ft × 1.0 Btu/lb·°F × (45 − 32) °F | 707 Btu/ft |
| Insulation resistance | ln(17.20 ÷ 13.20) ÷ (2π × 0.190/12). The iron pipe wall adds 0.0002. | 2.66 hr·ft·°F/Btu |
| Heat loss | Solved for wind and radiation at the surface. The jacket stays within 1 °F of the air, so the insulation sets the rate. | 4.86 W/LF at 45 °F 3.46 W/LF at 32 °F |
| Time to 32 °F | Average heat loss is 4.16 W/LF, or 14.2 Btu/hr per foot. 707 Btu/ft ÷ 14.2 Btu/hr·ft ≈ 50 hours. The calculator steps through the cooling in 240 increments. | 50.3 hours |
| Time to fully frozen | 54.4 lb/ft × 143.4 Btu/lb = 7,797 Btu/ft, removed at 3.46 W/LF (11.8 Btu/hr per foot) = 661 hours. Add the 50.3 hours to 32 °F. | 712 hours (30 days) |
Same line, other systems
| 2 in wall | To 32 °F |
|---|---|
| Dragon Jacket | 50.3 hours |
| Fiberglass | 41.7 hours |
| Bare pipe | 72 minutes |
Same line, other walls
| Dragon Jacket | To 32 °F |
|---|---|
| 1 in | 27.4 hours |
| 1½ in | 39.2 hours |
| 2 in | 50.3 hours |
| 3 in | 70.7 hours |
Shorter timelines
Longer lifespan
Send us the line you checked: the size, the length, the fittings and the design conditions. We’ll quote the insulation, and [Q]Core heat trace where the line needs more hold time than insulation alone gives.
