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Pipe & Duct Insulation Thickness – What this Calculator Does, How it Works, and How to Use It
This guide explains the physics behind the calculator, shows you how to use it (SI/Imperial), and walks through a complete real-world example—from blank inputs to final thickness, heat-loss, and cost.
What problem does it solve?
Two big reasons we insulate pipes/ducts are to prevent condensation on cold services and to limit heat loss/gain on hot/cold services. The right thickness protects against dripping, corrosion-under-insulation (CUI), wasted energy, and comfort problems.
How the calculator works (under the hood)
-
1) Physics model
We use 1-D steady conduction through a cylindrical (pipe) or plane (duct) layer plus outside film coefficient. Internal film and metal wall are neglected for simplicity.
Pipes: Rcond = ln(r₂/r₁)/(2πk), Rconv = 1/(heff·2πr₂), q′ = ΔT/(Rcond + Rconv).
- 2) Condensation check Dew point via Magnus formula; surface temperature Ts = Tamb + q′·Rconv. For anti-condensation, we size so Ts ≥ Tdew + margin.
- 3) Effective film coefficient External h includes a linearized radiation term: heff ≈ hconv + 4σ·ε·T³ (about ambient).
- 4) Solving thickness Thickness is solved by bisection until either heat-loss target or anti-condensation surface temperature is met; then an optional safety factor inflates the recommendation.
- 5) Costing Material cost is scaled by surface area and thickness relative to 25 mm reference rates (India/US/UK), with optional wastage and markup.
Methodology & formulas (compact)
Pipe (per meter): Rcond=ln(r₂/r₁)/(2πk), Rconv=1/(heff·2πr₂), q′ = ΔT/(Rcond+Rconv), Ts=Tamb+q′Rconv.
Duct (per meter): A=2(W+H), Rcond=t/(kA), Rconv=1/(heffA), q′ = ΔT/(Rcond+Rconv).
Magnus dew point: γ = aT/(b+T) + ln(RH/100), Tdew= bγ/(a−γ), with a=17.62, b=243.12 °C.
Linearized radiation: hrad ≈ 4σ·ε·T³ (T in K).
How to use the calculator (quick start)
- Pick Units — SI (mm, °C, W/m·K) or Imperial (in, °F, Btu·in/h·ft²·°F). The calculator converts on the fly.
- Choose a Standard (optional) — e.g., ASHRAE, CIBSE, ISO 12241, SMACNA. Defaults for ho, emissivity, and safety factor auto-populate (you can edit).
- Add Segment(s) — For each pipe or duct: enter service temp, ambient temp & RH, size (pipe OD / duct W×H), length, and insulation material (or k).
- Select Mode — Given Thickness → Heat loss, Target Heat Loss → Thickness, or Anti-Condensation → Thickness.
- Costing (optional) — Pick region (India/US/UK) and adjust wastage/markup. Rates are per m² at 25 mm and scale linearly with thickness.
- Review Results — KPIs (segments, total heat loss, cost, savings), a results table, and a quick chart of q′ vs thickness. Download a clean HTML report anytime.
Sample Project — Chilled Water Pipe (Full Calculation)
Goal: Prevent condensation and also limit heat gain to ≤ 10 W/m on a 2″ chilled water line indoors.
Given
| Fluid / Service Temp (Thot) | 7 °C (chilled water) |
|---|---|
| Ambient (Tamb), RH | 26 °C, 65 % RH (indoor) |
| Pipe Size (OD) | 60.3 mm (2″ nominal) |
| Length | 30 m |
| Insulation Material | Elastomeric foam (NBR), k = 0.035 W/m·K |
| Surface Emissivity (ε) | 0.9 (typical black jacket) |
| External Convection hconv | 8.0 W/m²·K (still air, indoor) |
| Safety Factor | 1.10 (10 % on thickness) |
| Anti-Condensation Margin | +2 K above dew point |
Step 1 — Dew Point
Using Magnus: with T=26 °C and RH=65 %, Tdew ≈ 18.91 °C. We want the outer surface ≥ 20.91 °C.
Step 2 — Effective h
Linearized radiation about ambient (σ=5.67×10⁻⁸, ε=0.9, T≈299.15 K): hrad ≈ 5.46 W/m²·K. So heff = 8.0 + 5.46 ≈ 13.46 W/m²·K.
Step 3 — Anti-Condensation Thickness
We solve for t so the surface equals the target: Ts=Tamb+q′Rconv=20.91 °C (bisection on the cylindrical model).
| Result | Value |
|---|---|
| tanti-cond (no safety) | 6.45 mm |
| t with 10% safety | 7.10 mm |
| q′ at t=7.1 mm | ≈ 15.8 W/m (heat gain into pipe) |
| Ts at t=7.1 mm | ≈ 21.0 °C |
Step 4 — Energy Target Thickness
Target |q′| ≤ 10 W/m. Solving for thickness gives:
| Target q′ (W/m) | Required t (mm) |
|---|---|
| 25 | 2.78 |
| 20 | 4.31 |
| 15 | 6.99 |
| 10 | 12.96 |
Step 5 — Final Performance at 13 mm
| q′ (per meter) | ≈ 9.98 W/m (vs uninsulated ≈ 48.46 W/m) |
|---|---|
| Surface Temperature | ≈ 23.27 °C (above 20.91 °C target) |
| Total Heat Gain (30 m) | ≈ 299 W (saved ≈ 1.155 kW vs uninsulated) |
Step 6 — Material Cost (reference rates per m² @ 25 mm)
Surface area at 13 mm: A ≈ 8.13 m² for 30 m run (based on outer perimeter). Costs scale linearly with thickness (t/25 mm) with 5% wastage and 10% markup.
| Region | Rate @ 25 mm | Material Cost (before W/MU) | Est. Total |
|---|---|---|---|
| India | ₹ 650 / m² | ₹ 2,749 | ₹ 3,175 |
| USA | $ 11.5 / m² | $ 48.64 | $ 56.18 |
| UK | £ 10.0 / m² | £ 42.29 | £ 48.85 |
Good practice, standards & next steps
Citations (verify editions/clauses in your project):
ASHRAE Fundamentals— surface coefficients & calculation methods.CIBSE Guide C— convective coefficients and typical data.ISO 12241— calculation rules for equipment & piping insulation.SMACNA— HVAC duct design considerations and insulation practice.
“Values derived from <Standard>, clause <xx> (user must verify).”
