Static structures (e.g. – buildings) – Composite prefabricated panel including adjunctive means – Sandwich or hollow with sheet-like facing members
Patent
1994-07-28
1997-09-09
Safavi, Michael
Static structures (e.g., buildings)
Composite prefabricated panel including adjunctive means
Sandwich or hollow with sheet-like facing members
156109, 428 34, E06B 366, C03C 2700
Patent
active
056643950
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention concerns improvements to thermally insulating evacuated glass panels; these panels may be used for windows.
BACKGROUND ART
Thermally insulating glass panels 1 typically comprise two spaced apart sheets of glass, 2 and 3, enclosing a low pressure space; refer to FIGS. 1, 2 and 3 (not shown to scale). These sheets are interconnected by a peripheral joint of fused solder glass 4 and an array of pillars 5.
The pillars 5 ensure sufficient structural strength to withstand the forces imposed by atmospheric pressure, and maintain the sheets of glass spaced-apart. The pillars 5 comprise a preform 6 made of glass, ceramic, metal or other materials completely coated with a layer of solder glass 7. The preform 6 is usually made from the same material as the glass sheets in order that the thermal expansion coefficients of the preforms, the sheets and the solder glass should match. The purpose of the preform is to maintain the separation of the glass sheets during the fusion operation when the solder glass has little mechanical strength.
A pump-out tube 8 is incorporated into the panel and is used during the construction of the panel to evacuate the space between the glass sheets. The pump-out tube 8 is hermetically sealed by fused solder glass 9 to a hole 10 which passes from an interior face of glass sheet 2 to the bottom of a recess 11 in the exterior face of glass sheet 2. The recess 11 allows the pump-out tube 8 to be melted and sealed leaving a stub which does not protrude beyond the plane of the exterior face of the glass sheet 2.
A chemical getter 12 is often included within a machined recess 13 in one of the sheets of glass in the panel to counteract any rise in pressure due to outgassing from the glass.
A low emittance coating may be provided on the interior surface of one or both sheets of glass. The emittance of the coatings is usually between 0.05 to 0.2 to ensure a thermal conductance due to radiation of approximately 0.15 to 0.6 Wm.sup.-2 K.sup.-1 or less.
The internal pressure of the panel is usually below 10.sup.-2 torr and sometimes below 10.sup.-3 torr.
PILLAR DESIGN
The design of the pillar array is a trade-off between reducing heat flow through the pillars (which imposes a requirement to reduce the number and size of the pillars) and reducing stresses within the panel and pillars (which requires more, and larger pillars).
A first sight, it would appear that the use of metal pillars would result in large heat transfer rates through the panel. Indeed, the thermal conductance of metal pillars is very large, due to the high thermal conductivity of the metal.
In the published literature, it is shown that a short circular contact between two bodies gives rise to a finite thermal impedance, equal to 2 Ka, where K is the thermal conductivity of the bodies and a is the radius of the contact. The existence of this finite thermal impedance arises because of the "spreading resistance" for heat flow within the bodies themselves. The thermal conductance values which apply to glass pillars of zero height also apply to metal pillars, and it has surprisingly been found that the actual heat flow through a metal pillar is quite low; it is almost exactly equal to the thermal conductance of a glass pillar of zero height.
It has therefore been realised that it is possible to design a pillar array using metal pillars, which has adequately low thermal conductance, for which external mechanical stresses are low, and for which internal fracture near the pillars is unlikely to occur.
Accordingly, a first aspect of the present invention, as currently envisaged, provides a thermally insulating glass panel comprising:
two spaced apart sheets of glass enclosing a low pressure space and interconnected by a peripheral joint of fused solder glass and an array of pillars, wherein at least some of the pillars are made entirely of metal.
Preferably the diameter of the metal pillars is 0.2 millimeters or less, for instance 0.1 millimeters. The stress within the panel is affected by the glass thickne
REFERENCES:
patent: 2303897 (1942-12-01), Smith
patent: 2749579 (1956-06-01), Shaw
patent: 3914000 (1975-10-01), Beckerman et al.
patent: 3990201 (1976-11-01), Falbel
patent: 4683154 (1987-07-01), Benson et al.
patent: 4786344 (1988-11-01), Beuther
patent: 5124185 (1992-06-01), Kerr et al.
Collins Richard Edward
Tang Jian Zheng
Safavi Michael
The University of Sydney
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