Engineering guide
The WAKO module calculates the heat transmission through a multi-layer plane wall with up to three layers and checks whether condensation forms on the warm wall surface. From the layer data, the heat transfer coefficients on both sides and the temperatures, the module determines the overall heat transfer coefficient, the heat flux and the wall surface temperatures.
Typical applications are cold store and cold room walls, insulated vessel walls and enclosures in process engineering: wherever a cold wall borders humid ambient air, the surface temperature must stay above the dew point temperature of the ambient air, otherwise water condenses out. To check this, the module compares the calculated wall temperature with the dew point determined from ambient pressure and relative humidity, and reports the humidity and enthalpy differences between ambient air and wall.
A thermal bridging factor allows the increased heat flow through profiles, fasteners and panel joints to be accounted for globally — in practice, it is often precisely the thermal bridge that decides whether condensate occurs locally, even though the undisturbed wall calculates as dry.
Typical workflow
Define the wall construction
The number of layers (up to three) is chosen; for each layer, thickness and thermal conductivity are specified — for example the facing sheet, insulation core and facing sheet of a sandwich panel.
Set the boundary conditions on both sides
For both sides of the wall, temperature, ambient pressure, relative humidity and heat transfer coefficient are entered; the thermal bridging factor captures additional loss paths through the construction.
Calculate the overall heat transfer coefficient and heat flux
From the series connection of the surface resistances 1/α and the conduction resistances s/λ of the layers, the overall heat transfer coefficient k follows; with the temperature difference, the heat flux q is obtained.
Determine the wall temperatures
Via the surface resistances, the temperatures of both wall surfaces are calculated — for the condensation check, the surface on the warm, humid side is decisive.
Condensation check
From air temperature, ambient pressure and relative humidity, the dew point temperature of the ambient air is determined. If the wall temperature falls below it, the module reports condensation; the humidity difference and the enthalpy difference between ambient air and wall quantify the condensation potential.
Frequently asked questions
On which side of the wall does condensation form?
Condensate forms on the surface whose temperature is below the dew point temperature of the adjacent air — for cold stores, that is on the outside, the warm and humid ambient side. The module checks both sides against their respective air states, which is why temperature, humidity and ambient pressure are entered separately for each side.
What role does the heat transfer coefficient play in the condensation check?
The key quantity is the ratio of surface resistance to total resistance. In stagnant corners and behind claddings, the heat transfer coefficient α is small and the wall temperature approaches the core temperature — those are the locations where condensate appears first. For verification calculations, conservatively small α values are therefore used.
What is the thermal bridging factor for?
Sandwich panels, substructures and fasteners conduct heat much better than the insulation core. The thermal bridging factor raises the area-related heat flow globally without modeling each bridge individually. For the local condensation assessment at a specific thermal bridge (e.g. a steel profile), the global factor is not sufficient — a detailed two- or three-dimensional analysis is required there.
Does the calculation also apply to transient conditions?
No, the calculation is steady-state. Short-term conditions such as defrost phases, door openings or humid weather spells can cause temporary condensation even though the steady-state calculation indicates a dry wall. For design purposes, the most unfavorable sustained conditions to be assumed (highest outside humidity, lowest room temperature) should therefore be checked.
Worked example
A cold store wall made of sandwich panels separates a deep-freeze room (−20 °C) from a hall at 25 °C and 60 % relative humidity. Calculate the overall heat transfer coefficient, the heat flux and the wall temperature on the warm side, and check for condensation (without thermal bridging allowance) — a worked example of a multi-layer wall heat transmission calculation.
Given
| Layer 1: steel sheet | 1 mm, λ = 50 W/(m·K) |
|---|---|
| Layer 2: PUR insulation core | 100 mm, λ = 0.025 W/(m·K) |
| Layer 3: steel sheet | 1 mm, λ = 50 W/(m·K) |
| Heat transfer coefficient warm side αa | 8 W/(m²·K) |
| Heat transfer coefficient cold side αi | 8 W/(m²·K) |
| Air temperature warm / cold | 25 °C / −20 °C |
| Relative humidity warm side | 60 % |
Solution
Overall heat transfer coefficient
1/k = 1/αa + s₁/λ₁ + s₂/λ₂ + s₃/λ₃ + 1/αi
1/k = 1/8 + 0.001/50 + 0.100/0.025 + 0.001/50 + 1/8 = 0.125 + 0.00002 + 4.000 + 0.00002 + 0.125 = 4.250 m²·K/W
k = 0.235 W/(m²·K) — the steel sheets are thermally negligible; the insulation core dominates.
Heat flux
q = k · (ϑwarm − ϑcold) = 0.235 · (25 − (−20)) = 0.235 · 45 = 10.6 W/m²
Wall temperature on the warm side
ϑW = ϑwarm − q/αa = 25 − 10.6/8 = 23.7 °C
Dew point and condensation check
At 25 °C the saturation vapor pressure is about 31.6 mbar; at 60 % relative humidity the water vapor partial pressure is 0.60 · 31.6 = 19.0 mbar. From this (Magnus formula) a dew point temperature of ϑdew ≈ 16.7 °C follows.
Since ϑW = 23.7 °C > 16.7 °C: no condensation on the undisturbed wall.
Result
| Overall heat transfer coefficient k | 0.235 W/(m²·K) |
|---|---|
| Heat flux q | 10.6 W/m² |
| Wall temperature warm side | 23.7 °C |
| Dew point temperature (25 °C, 60 %) | 16.7 °C |
| Condensation | no |
Technical content source: LV-Solved-Problems-Content 628d313247e7