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Air Cavity Calculator (Tool)
The Air Cavity Calculator is used as a tool for rapid calculation of equivalent heat conductivity of small air cavities and air gaps based on the input of their dimensions and heat stream direction.
To calculate equivalent heat conductivity (lambda) the application uses formulas defining calculation of heat transfer resistance of air spaces described in EN ISO 6946:2005 Appendix B - please be aware of limitations related to its application (e.g. small, closed air cavities).
Calculation regulation are geared to European norms EN ISO 6946 and EN ISO 10077-2 and cover two dimensional calculations.
| Heat Stream direction | Definition of heat stream direction:
|
| Thickness (stream dir.) | The thickness of the cavity measured in the heat stream direction. Remark: Thickness and width can be swapped via a context menu. |
| Width | The width of the cavity (perpendicular to the thickness) Remark: Thickness and width can be swapped via a context menu. |
| Temperature difference dT | The difference of temperature between cavity boundaries in the heat stream direction. |
| Equiv. Resistance | Equivalent heat transfer resistance of particular air cavity |
| Equiv. Lambda | Equivalent heat conductivity of particular air cavity |
| Result | Calculation result is replaced by the actual value. |
Remark: According to normative regulations one shall set the temperature difference, if exact value is not known, accordingly. Buttons "5K (EN 6946)" and "10K (EN 10077) set this value to one of magnitudes required in these respective regulations.
For the very usual case of non rectangular air cavities normative regulations define following procedure to account for:
An equivalent conductivity is to be calculated for a fictive rectangular cavity. Such fictive cavity shall be constructed as follows:
- create a rectangle surrounding the cavity of interest.
- the area of a fictive cavity rectangle shall be equal to the area original (non rectangular) cavity.
- the ratio of width to the height of the fictive rectangle cavity shall be the same as of the rectangle surrounding the original.
The conductivity calculated for such fictive rectangular cavity is the used instead for the (non rectangular) original air cavity.
| Heat Stream direction | Definition of heat stream direction:
|
| Thickness (stream dir.) | The thickness of the rectangle surrounding the cavity measured in the heat stream direction. Remark: Thickness and width can be swapped via a context menu. |
| Width | The width of the rectangle surrounding the cavity (perpendicular to the thickness) Remark: Thickness and width can be swapped via a context menu. |
| Area | The actual are of the real (non rectangular) air cavity |
| Temperature difference dT | The difference of temperature between cavity boundaries in the heat stream direction. |
| Equiv. Lambda | Equivalent heat conductivity of particular air cavity |
| Result | Calculation result is replaced by the actual value. |
Remark: For this procedure the most sophisticated it is to calculate the real are of the non rectangular cavity itself.
Remark: The exact direction of stream flow is known in advance within multi dimensional calculations. This means, that the requirement to decide the correct choice of the "width" and "thickness" for the cavity cannot be fulfilled easily. It is user's decision to estimate the right heat stream direction by some heuristics based on his experience („along the x- or y-axis“).