The Getter Device

As discussed in the previous sections, gas desorption from the surfaces exposed to the vacuum (filler and barrier) and gas permeation through the bag contribute to increase the pressure in a VIP during its life. 

For given temperature and environmental conditions, the pressure build-up depends on the specific size of the panel, and in particular on the ratio between the perimeter and the thickness, the lower the latter the higher the pressure increase. A barrier film incorporating an aluminium foil and a panel seal-off pressure of 5 Pa have been here considered. In this example, water has been assumed to be completely absorbed by a proper amount of desiccant and not to contribute to the pressure increase. 

In the case of a typical 50 x 50 x 2 cm size VIP, the pressure build-up exceeds 100 Pa after a few years and even not considering water, the total pressure after 20 years exceeds 250 Pa. 

Due to the nature of the thermal conductivity versus pressure curve, the deterioration of the X factor is negligible at the beginning but increases steadily with time. 

In some applications, such as in domestic refrigerators and freezers, an additional gas burden may be also generated in the panel during the appliance manufacturing process or by the appliance operating conditions, which may even accelerate the deterioration of the thermal conductivity. 

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The getter device does not need to be heat activated or pre-treated before being used in VIP and can be handled in air for a reasonable period of time (several minutes), during the panel manufacturing process, without affecting its sorption capacity. 

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