Dew point calculator for pipe and duct insulation
Enter the medium, the surrounding room and the pipe, and the tool tells you straight away whether the surface stays dry, what the minimum insulation thickness is, and how much the air warms up or cools down along the run.
Steady-state one-dimensional model of a cylindrical wall following EN ISO 12241. Everything is recalculated as you type; nothing is sent anywhere.
Rez potrubím
Teploty na rozhraniach; farba prstenca = stav.
Povrchová teplota vs. hrúbka izolácie
Hrúbka, pri ktorej modrá krivka prekročí čiaru rosného bodu, je minimum. Červené pole = kondenzácia.
Zmena teploty média po dĺžke úseku
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Podrobné výsledky teplotný profil · hranica vlhkosti · tepelný tok
Teplotný profil v reze
Priebeh teploty od média cez stenu a izoláciu do okolia vrátane prestupových filmov.
Prevádzková hranica vlhkosti
Do akej relatívnej vlhkosti okolia zostane povrch suchý pri zadanej hrúbke.
Tepelný tok
Tepelný zisk/strata na metre potrubia v závislosti od hrúbky steny.
Výpočtové medzivýsledky prestupové súčinitele, tepelné odpory, prietok
Tabuľka hodnôt priebeh podľa hrúbky — na kopírovanie do dokumentácie
Metodika a obmedzenia použité vzťahy a čo model neuvažuje
- Rosný bod — rozšírená Magnusova rovnica (Alduchov & Eskridge 1996) nad vodou, s presnosťou lepšou ako 0,4 % v rozsahu −40 až +60 °C; nad 60 °C odchýlka rastie. Pre teploty pod 0 °C ide o rosný bod nad prechladenou vodou, nie o bod námrazy.
- Vedenie tepla — stacionárny 1D radiálny model valcovej steny: R = ln(d2/d1) / (2π·λ), séria odporov médium → stena → izolácia → okolie.
- Vonkajší prestup — voľná konvekcia podľa Churchill–Chu (vodorovný valec), nútená podľa Churchill–Bernstein, zložená ako h = (hfree³ + hforced³)1/3. Sálanie hr = ε·σ·(Ts²+To²)(Ts+To).
- Vnútorný prestup — pri výpočte povrchovej teploty sa zanedbáva, teda vnútorná stena má teplotu média. Ide o konzervatívny predpoklad podľa EN ISO 12241, na ktorom sú postavené aj tabuľky výrobcov izolácií, a jediný správny pre vodu a glykol. Zohľadnenie filmu by povrchovú teplotu nadhodnotilo a vyšla by tenšia izolácia, než je bezpečné. Film sa počíta iba na kontrolu kondenzácie na vnútornej stene — podľa Gnielinského (turbulentné), resp. Nu = 3,66 (laminárne), a platí len pre vzduch. Vlastnosti vzduchu zo Sutherlandovho vzťahu pri strednej teplote filmu.
- Neuvažuje sa: difúzia vodnej pary do izolácie (vnútorná kondenzácia — pri chladných potrubiach s paropriepustnou izoláciou treba parozábranu a posúdenie podľa μ / sd), tepelné mosty na podperách a armatúrach, stlačenie izolácie, dvojrozmerné efekty na kolenách a odbočkách.
- Množstvo kondenzátu je odhad podľa Lewisovej analógie (Le ≈ 1) a slúži len na predstavu o rozsahu, nie na dimenzovanie odvodu.
- Minimálna hrúbka uvedená vyššie je hrúbka proti kondenzácii. Hrúbku izolácie často určuje až druhé, náročnejšie kritérium — prípustná tepelná strata či zisk podľa energetických požiadaviek (vyhláška MH SR 364/2012 Z. z., resp. EN 12828). Porovnajte obe a použite vyššiu.
- Výsledok je návrhová pomôcka. Pre záväzný návrh použite údaje λ a μ z technického listu konkrétneho výrobku a platnú normu.
What the calculator works out
A cold duct running through a warm, humid room cools its own surface. Once that surface drops below the dew point of the room air, water condenses on it — it drips onto the ceiling, soaks the insulation and, over time, corrodes the duct. The job of the insulation is to keep the surface warm enough for that never to happen.
The tool assembles the thermal resistances in series — medium, pipe wall, insulation, outdoor film — and iterates the surface temperature, because the thermal conductivity of the insulation and the external heat transfer coefficient both depend on it. Free convection follows Churchill–Chu for a horizontal cylinder, forced convection Churchill–Bernstein, and radiation is added through the surface emissivity.
The second question it answers is thermal rather than hygric: how far the transported air drifts towards the ambient temperature along the run. That matters for a cooling distribution routed through a hot roof space, where a thinly insulated duct can give away most of its cooling capacity before it reaches the room.
What you need to know
- Insulation thickness is driven by two criteria — condensation and permissible heat loss or gain. Take the greater of the two.
- Bright metal cladding makes the surface colder, not warmer, so it needs thicker insulation.
- Still air in a suspended ceiling is the least favourable case; a draught warms the surface and lowers the risk.
- EPP duct is a single load-bearing and insulating layer, with no metal wall underneath.
- On cold pipes with vapour-permeable insulation a vapour barrier is still required; this tool assesses the surface, not vapour diffusion.
Frequently asked questions
When does condensation form on a duct or pipe?
Condensation forms as soon as the outer surface of the insulation drops below the dew point of the surrounding air. The surface temperature always lies between the medium and the ambient air, so if the ambient dew point is below the medium temperature, condensation cannot occur at any insulation thickness.
How thick does the insulation have to be?
Thick enough to keep the surface above the ambient dew point, usually with a safety margin of 1 K, in critical rooms 2 to 3 K. The calculator solves that thickness for the entered conditions and rounds it up to the nearest commercially available size. Bear in mind that the energy requirement on permissible heat loss or gain often calls for a greater thickness than condensation control does.
Why does bright metal cladding increase the risk?
A bright aluminium jacket has an emissivity of about 0.05 against roughly 0.90 for a bare insulation surface. It therefore absorbs far less heat by radiation from the surroundings and stays colder, so a clad cold pipe needs noticeably thicker insulation than an unclad one under the same conditions.
How much does the air warm up along the duct?
The temperature approaches the ambient value exponentially: t(x) = t_ambient + (t_inlet - t_ambient) * e^(-x / (m * cp * R)), where m is the mass flow, cp the specific heat capacity and R the thermal resistance per metre. A thinly insulated cooling duct in a hot roof space can pick up well over ten kelvin across thirty metres.
Is an EPP duct calculated differently?
Yes. EPP duct is a single layer that is load-bearing and insulating at the same time, so there is no separate metal wall underneath. The wall thickness field disappears and the EPP thickness alone carries the whole thermal resistance, so the outer diameter is the inner diameter plus twice the EPP thickness.
What does the calculator not cover?
Water vapour diffusion into the insulation, which governs interstitial condensation on cold pipes with vapour-permeable insulation and calls for a vapour barrier, thermal bridges at supports and fittings, compression of the insulation, two-dimensional effects at bends, and the latent heat released once the medium cools below its dew point.
The result is a design aid. For a binding design use the thermal conductivity and vapour resistance figures from the technical data sheet of the specific product together with the applicable standard.