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309 Views, 5 Replies

02-19-2014 08:16 PM

Hi,

We are doing thermal analysis using Ecotect software to select suitable materials for building envelope based on the heat load. For this, we have planned to use various materials as well as we know the following details.

**Width**

This defines the width of each layer in millimeters

**Density**

The density of each layer in kg/m³.

**Sp. Heat**

The specific heat of each layer in Joules per kilogram degrees Celsius (J/kg°C).

**Conduct.**

The conductivity of each layer in Watts per meter degrees Celsius (W/m°C).

If possible, I would like to know the method to calculate U – Value from the above specifications.

After that there is one option in Ecotect software. Please refer below for definition.

*Hatch*

*Defines the displayed hatch pattern of the layer. Clicking the left mouse in this box will display a menu from which this value can be selected. The density of the hatch pattern can also be set by changing the second digit in the hatch code*

* *

What is it mean? How to use it for thermal analysis?

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07-08-2014 12:42 PM in reply to:
swaminathan.sekar

Hello swaminathan.sek,

Sorry for the long delayed response, I have been going through the users post to answer unanswered questions...

Ecotect calcultion of the U-value is pretty straight forward:

U-value = 1 / R, where R = the resistance = (thickness (m) / thermal conductivity (W/mk)) + internal air resistance (0.12 m2K/W) + external air resistance (0.06 m2k/W).

To calculate the Admittance of the material is more complex; Ecotect uses the square root of the product of the thermal conductivity, density, specific heat capacity, and the angular velocity, where the angular velocity = 2*(3.14/24).

The Hatch option is mostly for graphic representation, except when using a hatch that includes air gap (air gap or stud work), whereas Ecotect will adjust the thermal conductivity of the assembly to account for the air gap. Be careful to double check that your thermal conductivity number is correct, then press the "Calculate Thermal Properties" button to re-calculate the assembly's properties.

I hope that clarifies things a bit.

Let me know if you have more questions on this topic, otherwise please accept as a solution so that others can benefit from this information.

Cheers,

Olivier A. PENNETIER

Autodesk Ecotect Support Specialist

Autodesk Ecotect Support Specialist

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07-08-2014 09:29 PM in reply to:
Pennetier1

Hi,

Based on the formula you given, Manually I have calculated the U Value &

admittance for single material. But the manual result is not matching with

the software result.

Please help..

Based on the formula you given, Manually I have calculated the U Value &

admittance for single material. But the manual result is not matching with

the software result.

Please help..

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07-09-2014 11:44 AM in reply to:
swaminathan.sekar

Hi Swaminathan.sek,

The formula is correct - note that this is for homogeneous material assemblies. If not homogeneous, you have to add the U-values for each material layers as U-values simply add up. Start testing with an assembly that has only one material layer.

I am attaching a quick spreadsheet for you here.

Hope that helps.

Cheers

Olivier A. PENNETIER

Autodesk Ecotect Support Specialist

Autodesk Ecotect Support Specialist

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07-17-2014 05:40 AM in reply to:
Pennetier1

Hi,

Based on your calculation sheet, Manually i have calculated the U Value for material which has combined with some layers. But it is not matching with software U Value.

Please refer the attachment.

Note; For individual material, software u value almost matching with manual u value.

Please clarify & help.

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07-17-2014 01:13 PM in reply to:
swaminathan.sekar

Hi swaminathan.sek,

You're almost there... a few things here:

Indeed, non-homogeneous assemblies are more tricky, and adding air spaces complicates things a bit.

1. You are counting the inside and outside air resistance for each layers - you should only count once the outside and inside air resitance for the entire assembly. Try an assembly with NO AIR GAP, sum up the R values for each layers then add the outside and outside air resistance and you should get the correct U-value for the whole assembly.

2. When you add an air gap into the assembly, things get a bit more complicated because an air gap has its own resistance that cannot be calculated like the other materials (it is due to the convective flows going on in the air gap, dependant on the air gap orientation and other factors).

So when you add an air gap in your assembly, calculate its R-value like the other materials, and add 0.17 m2K/W + interior and exterior air resistance to the overall R-value of the assembly. You will get the correct U-value.

I hope it clarifies things.

Let me know if you have more questions on this topic, otherwise please accept as a solution so that others can benefit from this information.

Cheers,

Olivier A. PENNETIER

Autodesk Ecotect Support Specialist

Autodesk Ecotect Support Specialist

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