Warm Edge – equivalent thermal conductivity

Warm Edge – equivalent thermal conductivity


The ift-Rosenheim (Institute for Window-Technology) released the Guideline WA-17/1!

The guideline was written for the determination of the equivalent thermal conductivity of warm edge.

Warm Edge – Determination of the Equivalent Thermal Conductivity Becomes as Easy as Child’s Play!

NETZSCH is well-known for finding measuring solutions that translate well into practice. Based on conversations with customers, this instrument manufacturer has implemented a measurement routine that helps users determine the equivalent thermal conductivity of thermally improved spacers. This is NETZSCH‘s second step into the “glass and window industry” after the introduction of the mobile measurement device for Ug-Value “Uglass”.

Uglass allows for measurement of the U value of double- and triple-glazed insulation glass and can be used either for items in pre-existing inventories or directly following production. NETZSCH has presented this instrument at the Glasstech in Düsseldorf twice and received the Bavarian Energy Award (“Bayerische Energiepreis”) for it in 2016. In 2017, the company introduced another measurement instrument to this industry sector in addition to Uglass.

HFM 446 Lambda Medium

The picture shows a Heat Flow Meter HFM 446 Lambda Medium
Fig. 1: HFM 446 Lambda Medium

With the HFM 446 Lambda Medium – a heat-flow-meter in accordance with ISO 8301 – the insulating industry can meet its requirements for CEconformity within the scope of factory production control for insulating materials. The instrument determines the thermal conductivity after a defined temperature gradient is set and after a stationary heat flow is achieved using the Fourier equation.
The statistical Lambda 90/90 value is calculated in a special report from a multitude of measurement values that are generated during quality assurance. This is the value that has a 90% probability of applying to 90% of the production output. It is the basis for the nominal value of the thermal conductivity declared on the product’s label. Within the scope of evaluation for CE conformity, factory production control is mandatory in the insulating industry and is regularly checked by third parties on volunteer basis (for example KEYMARK).

Heat Flow Meters are used for the Guideline WA-17/1!

Material developers also employ heat-flow measurement instruments for a variety of other materials. The technique is very easy to understand and measurements only take a moderate amount of time. That is why Guideline WA-17/1 of the Rosenheim Institute for Window Technology (“ift-
Richtlinie WA-17/1”) stipulates this method as the basis for determining the equivalent thermal conductivity of thermally improved spacers.

Guideline WA-17/1 by “ift” was developed as a collaborative effort with the “Warm Edge” (“Warme Kante”) work group of the German Federal Flat Glass Association (Bundesverband Flachglas e.V.) and the Technical University of Rosenheim.

Motivation for the Measurement of Equivalent Thermal Conductivity of Warm Edge

The thermal transmission coeffiecient, ψ , of windows is stipulated in ISO 10077-1. To this end, the manufacturer requires the following
values:

  • U value of the frame
  • U value of the glass
  • Linear thermal transmission coefficient Psi [ψ]

Psi depends on the spacer employed and describes the heat loss generated as a result of the insulating glass unit being installed into the frame. The calculation of ψ is carried out in accordance with ISO 10077-2, with knowledge of the exact cross section of the spacer and the thermal conductivity of the materials used.

Some thermal characteristics are included in the corresponding tables from the material manufacturer. For new materials, the thermal conductivity must be determined by measurement. There are numerous techniques accredited bodies are using. NETZSCH has a variety of instruments in its product line for this. Since the measurement methods are not as well suited for some materials as for others, a number of problems have occurred in the work and investigations that have been carried out. It is impossible, however, to determine exactly which techniques are best suited for this purpose, with a low tolerance for the measured values.

The method offered by Guideline WA-17/1 by “ift” for this takes a different, practice-oriented approach. The spacers manufacturer does not individually measure the thermal conductivities of all materials used, but rather the “equivalent thermal conductivity” of the entire spacer system. This approach provides a clear procedure and defines a clear specific sample, allowing reliable values to be obtained by means of a simple technique.

Guideline WA-17/1 states that the determined equivalent thermal conductivity λeq,2B helps calculate the linear thermal transmission coefficient ψ in a given application situation.
To this end, the “replacement spacer” of the two-box model replaces the detailed spacer in the application situation. This minimizes the time required for modelling of the system – errors do not occur. Determination of the ψ-value is carried out in accordance with “ift” Guideline
WA-08/1.

How does the method work?

Determination of the equivalent thermal conductivity is carried out in accordance with DIN EN 12664. It can be equated in this context with the method insulating manufacturers employ in quality assurance.

Two methods are described there:

  • ISO 8301 (heat-flow meter)
  • ISO 8302 (guarded hot plate apparatus)

As mentioned earlier, the method NETZSCH offers is based on ISO 8301 – the principle of stationary heat transfer in the heat-flow meter.

On a horizontally inserted, plate-shaped sample, a temperature gradient is set via the plate temperatures T1 and T2. Two highly precise heat-flow sensors with known surface area, integrated into the heating and cooling plates, determine the stationary heat flow through the sample.
Thus, the thermal conductivity can be determined based on the Fourier equation if the sample thickness is known.
Heat-flow meters are generally calibrated with the help of internationally recognized thermal conductivity reference materials (NIST SRM 1450D or IRMM-440).

The picture shows the working principle of a static heat flow measurement
Fig. 2: The working principle of a static heat flow measurement

Typical equivalent thermal conductivities of thermally improved spares are generally within a range of 0.15 W/m/K and 0.9 W/m/K. This is significantly above the thermal conductivity of internationally available reference materials.

In order to routinely validate the heat-flow meter technique for higher thermal conductivities, NETZSCH offers a reference material made of borosilicate glass. Each customer is provided with a manufacturer’s certificate for this. The reference materials are validated in the in-house
GHP 456 Guarded Hot Plate System. This opens the door for heat-flow meters to be used for the higher thermal conductivities of a WA-17/1 sample.

The Sample

The “ift” guideline stipulates a setup for the sample which can be measured with the most common-sized heat-flow
meter. The sample’s dimension is 30 x 30 cm². The warm edge spacers to be tested are placed between two 4-millimeter-thick glass plates.

The picture shows the prepared sample filled with warm edge spacers
Fig. 3: How to prepare a sample: 2 glass-sheets filled with warm edge spacers

If the warm edge spacers consist of hollow bodies, the user must fill the samples by means of a molecular sieve with pore opening of 3 Å (Ångström) prior to the measurement. Per the guideline, the PHONOSORB 551 material by Grace Davison is to be used here. If it can be shown that no deviation occurs in the measured equivalent thermal conductivity, alternative molecular sieves can also be used.

In order to be able to record the sample’s surface temperatures during the measurement, the guideline recommends the mounting of three diagonally arranged thermocouples per sample side. Measurements with the NETZSCH instrument have shown that one thermocouple per side is sufficient in this case.

The picture shows additional thermocouples
Fig. 4: Additional thermocouples attached on a borosilicate reference material with rubber sheets

 

Evaluation

As a result, the heat-flow meter presents the entire thermal resistance Rges of the sample along with the thermal conductivity. This is important for calculating the thermal resistance of the spacers. To this end, the user subtracts the thermal resistance of the glass panes used from the entire thermal resistance. This value is shown in Guideline WA-17/1 in tabular form.

The picture shows thermal resistance values of glass
Table 1: The thermal resistance reference values from the guideline WA-17/1

The equivalent resistance of the spacer Req is as follows:

The picture shows the equation for equivalent thermal resistance
Equation 1: Equivalent thermal resistance

Furthermore, the heat-flow meter automatically determines the sample thickness. With knowledge of the thicknesses of the glasses in sample dg, the user can determine the measure of the “inter-pane cavity”.

The picture shows the equation for the thickness of the interpane cavity
Equation 2: The thickness of the interpane cavity
The picture shows the reference value for the glass thickness
Table 2: The reference value for the glass thickness from guideline WA-17/1

Based on the equivalent thermal resistance and the calculated inter-pane cavity, the equivalent thermal conductivity of the samples is as follows:

The picture shows the equation for the equivalent thermal conductivity
Equation 3: The equivalent thermal conductivity

The equivalent thermal conductivity must be determined in at least 3 samples. From the individual results, the equivalent thermal conductivity of the thermally improved spacer system λeq,2B can be determined, based on the statistical calculation per ISO 10456 Annex C for a 90% fractile.

 

Reporting

Already upon creating a measurement routine, a product type can be selected for each spacer by means ofthe “product type database” that is integrated into the software. This ensures that at the end, upon evaluation, only measurements on spacers of the same type are
incorporated into the final result. The individual measurements that are saved in the software can now be retrieved and incorporated per
ISO 10456 Annex C by means of the “equivalent thermal conductivity” report type, on the basis of a series of definable filter criteria.
The final result will be issued, at the push of a button, into clearly structured reports in Word or Excel format.

 

Alexander Frenzl has been employed in the Development Department at
NETZSCH Analyzing & Testing since 2005. In 2008, he became Head of the
Mechanical Development Department and, as such, has been involved
in the development of all NETZSCH instruments. Since 2014, Alexander
Frenzl has been the Business Segment Manager for Glass, Ceramics and
Building Materials and serves as an interface between our Development,
Sales and Marketing Departments. One of his focal points is industrial
quality assurance for insulating materials as well as the process optimization
during processing ceramics, especially with respect to new and more
efficient technologies.

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