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issue: June 2003 APPLIANCE Magazine

Engineering Insulation
Advances in VIP Design for Super Insulation of Domestic Appliances


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by Paolo Manini, Enea Rizzi, Guido Pastore, and Pierattilio Gregorio, SAES Getters

Presented at the 54th International Appliance Technical Conference (IATC), March 10-12, West Lafayette, IN, U.S.

Vacuum insulated panels (VIPs) are very efficient thermal barriers, having thermal conductivity 3 to 10 times lower than conventional insulators.

Therefore, VIPs have great potential for energy savings in a variety of industrial and domestic applications. In spite of this, VIPs have so far found limited use in the insulation of appliances such as refrigerators, freezers, water heaters, or vending machines. The main reasons for this have been the cost and the limitation in the VIP shapes and design options available on the market. This latter aspect is being effectively addressed by the industry through the improvement of the manufacturing technologies, a deep focus on component selection, and extensive VIP testing.

As a result, VIPs are now available in a variety of geometries (e.g. flat, curved, cylindrical, shaped), and with added features (holes, cut-outs), which allow them to nicely fit in a large number of applications, over a broad temperature range.

Standard VIP Design

VIPs are produced by vacuum packaging an open-celled, micro-porous insulating core in a gas barrier bag. Several cores [1,2,3] have been proposed so far, some of their properties being summarized in Table 1.



Table 1. Some properties of currently available cores for VIPs.

Barrier envelopes are available in the form of multi-layered structures, which generally include one or more metallized sheets coupled with high-gas barrier plastic layers. A thin metal foilÑ5 to 8 microns thickÑis added in some barrier designs to further cut gas permeation and to improve mechanical properties. A getter device and/or an engineered amount of desiccant is also generally required in most VIP designs to remove gases and moisture and to ensure long-term projected performances [4].

Most vacuum panels are currently being produced in the shape of flat rectangles or squares, as shown in Figure 1, which can be easily attached to the liner of a refrigerator or a freezer. This design cannot, however, be applied to contour-shaped surfaces like the compressor area of a refrigerator or to cover a round or cylindrical surface, as is required, for example, to insulate a water heater.

Figure 1. Example of standard flat panels.

Curved Panels

To overcome this problem, some novel concepts recently have been proposed. One is based on realizing longitudinal grooves [5], through machining or by mechanical compression, in the panel core.

Once the panel has been evacuated, the applied atmospheric pressure exerts a force on the film, and the presence of the grooves makes the panel bend. This approach has a few drawbacks. The first is the relatively high added cost (preparation of the grooves is labor intensive). The second is the increased level of stress induced on the envelope, which might be responsible for long-term reliability problems. Another approach does not make use of the grooves. Panels are manufactured using the conventional technology, and then curved by means of a special mechanical process, which ensures careful control of the radius of curvature and surface smoothness. A typical result is shown in Figure 2.

Ê
Figure 2. Curved panels. Ê

These panels can be produced using several cores in combination with a variety of high-gas barrier envelopes, once the process parameters are properly adjusted. This design introduces negligible stresses in the film, as checked by specific tests carried out in SAES GettersÕ R&D Laboratories. These tests include thermal conductivity and pressure measurements in panels over an extended period of time, as well as helium permeation rate measurements through film samples taken from the panels before and after the curving process. Due to its small molecular size and high diffusivity in barrier films, helium is an ideal tracer to measure changes in film properties. Its permeation rate is proportional to the density of pin-holes or micro-defects in the film structure. Therefore, an increase in helium permeation rate indicates a larger level of defects in film, which in turn, means that the barrier properties have deteriorated.

All these experiments have consistently confirmed that the film properties of curved panels are well within the typical required specification values for most applications.

Figure 3. Gas inlet rate (mbar l) in flat and curved panels versus time (@25¾C, 50 percent RH). Click Here to see the full-size graphic.

As an example, the pressure rate of rise as a function of time in flat and curved VIPs is given in Figure 3. Pressure was measured over several months by a dedicated spinning rotor gauge sensor attached to the panels [6].

Results show no substantial difference in the pressure rate of rise in the flat and curved panels, thus confirming that film retains full functional integrity after the panel has been curved.

Shaped Panels with Added Design Features

In a variety of appliances including vending machines, water heaters, refrigerators, and freezers, the presence of service pipes, feed-through cables, and wiring poses additional requirements on VIPs. In these cases, shaped vacuum panels with notches and holes would be the ideal solution.

Figure 4. Examples of shaped curved panels with holes.

There are three important issues to solve. One is related to the bag manufacturing and sealing process. The presence of dust and particles in the sealing area, as well as the difficulty of keeping the sealing parameters under full control over a complex area, have proved to be the main critical elements influencing the quality of the final product. Another issue is how to integrate the panel sealing process into the complete VIP manufacturing cycle. Some cores, like silica, polyurethane foams, or glass wool require a bake-out treatment to remove water. After being dried-up, they can be exposed to the environment for a very limited period of time. This aspect had to be taken into consideration when designing the overall panel assembling process.

Figure 5. VIP long-term thermal conductivity increases for some design options (films/getter). Click Here to see the full-size graphic.

Last but not least, being that cost is a critical factor for the effective adoption of vacuum panels, the manufacturing technology must be easily scalable to large volumes without adding substantial costs to the finished product. All of these critical issues have been satisfactorily solved.

In the case of these shaped panels, extensive long-term reliability tests have been carried out measuring pressure and thermal conductivity over time, as described previously. Examples of shaped vacuum panels with controlled curvature and holes are given in Figure 4.

Examples of Application of Curved Panels

Vacuum panels like the ones shown in Figures 2 and 4 have been tested in several electrical and gas-fired water heater models.

In these trials, the panels were applied to the outer surface of the water heater tank, partially replacing the standard polyurethane foam or glass fiber mats. The use of these vacuum panels resulted in much less energy consumption. Improvements in energy efficiency as high as 25 percent have been measured [7]. Typical results are shown in Table 2.



Table 2. Measured energy consumption values in a water heater with and without vacuum panels (reference units). Results are compared with FEA calculation.

Particularly interesting for the replacement market, the use of VIPs in water heaters allows for the design of efficient, ultra-slim units with eye-catching features or the manufacture of standard-size appliances meeting the most stringent energy requirements without increasing the insulation volume.

Due to the relatively high-operating temperature of the water heater (70¡C to 90¡C), the correct selection of VIP components is of the outmost importance in this application to ensure long-term performance and reliability. This aspect is shown in Figure 5, where different design options in terms of films and getter/desiccant have been evaluated.

Provided that the right components are used, long-term performance can be met.
Curved VIPs with the design features described have also been field-tested in a variety of other appliances with good results.

Conclusion

Significant efforts are being made to improve VIP design features. Thanks to these efforts, vacuum panels are now available in a variety of forms and geometries, which allow for an easy fit into a variety of appliances, from water heaters to refrigerators.

This result has been obtained through a careful characterization and selection of the components and the introduction of innovative manufacturing concepts.


References

1. ÒAdvanced Evacuated Thermal Insulations: the State of the Art,Ó Journal of Thermal Insulation, 12, 1989, H. A. Fine.
2. ÒDevelopments of Vacuum Panel Technology Based on Open Celled Polyurethane Foam,Ó Proceeding of the Polyurethanes Expo Õ96, 1996, W. Wacker, A. Christfreund, D. Randall, and N.W. Keane.
3. ÒState of the Art of VIP Usage in Refrigeration Applications,Ó International Appliance Manufacturing, 2001, B. Malone and K. Weir.
4. ÒThe Combogetter as a Key Component in the Vacuum Insulated Panels (VIPs) Technology,Ó Vuoto, Apr/Jun 1997, XXVI, n¡2, P. Manini.
5. ÒMethod for Producing Non Planar Evacuated Insulation Panels,Ó European Patent EP0820565, R. De Vos and G.L. Biesmans.
6. ÒThe Spinning Rotor Gauge,Ó J. Vac. Sci. Technol. A 3(3), May/June 1985, J.K. Fremerey.
7. ÒImproved Vacuum Insulated Panel Design for Water Heaters,Ó 5th Annual Vacuum Insulation Symposium, May 2001, P. Di Gregorio, E. Rizzi, and M. Urbano.

 

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