Richard Edward Collins

Vacuum glazing—A new component for insulating windows

Abstract The substantial progress in the science and technology of vacuum glazing that has occurred over the past few years is reviewed. Vacuum glazing up to 1 m × 1 m in size has been produced with an air-to-air, mid-device thermal conductance as low as 0.90 W m−2 K−1, compared with 1.3 W m−2 K−1 for high performance double glazing. The mechanical tensile stresses in vacuum glazing due to pressure and temperature differentials are well understood, and appear to be tolerably small. The internal vacuum is high, and extremely stable over long times at moderate temperatures. The likely cost of volume production of vacuum glazing should be only slightly greater than for high performance double glazing.

HEAT CONDUCTION THROUGH THE SUPPORT PILLARS IN VACUUM GLAZING

Abstract Vacuum glazing consists of two glass sheets with a narrow internal evacuated space. The separation of the sheets under the influence of atmospheric pressure is maintained by an array of small support pillars. The thermal resistances associated with the heat flow through individual pillars, and through the pillar array, are calculated using a simple analytic method, and by more complex finite element models. The results of both approaches are in very good agreement, and are validated by comparison with experimental data. It is shown that, for many purposes, the amount of heat which flows through the pillars can be determined without incurring significant errors by assuming that the heat flow is uniformly distributed over the area of the glass. Finite element modelling, and a superposition method, are used to determine the temperature distribution on the external surfaces of the glass sheets due to pillar conduction. Again the results obtained with both approaches are in very good agreement. An approximate method is described for calculating the magnitude of these temperature non- uniformities for all practical glazing parameters.

Thermal outgassing of vacuum glazing

Vacuum glazing consists of two sheets of glass separated by a narrow evacuated space. In this article we describe investigations of the causes of thermally induced degradation of vacuum glazing. It is shown that the rate of increase of pressure within the glazing during high temperature aging is mainly determined by the diffusion of water out of near-surface regions in the glass. In addition, adsorption of water molecules onto the internal surfaces influences the temperature dependence of the pressure in a degraded glazing. A prediction is made of the likely upper limit of the thermally induced degradation of vacuum glazing, and hence its properties as a thermal insulator, for a given production schedule and under conditions representative of practical applications.

Architectural glazings: Design standards and failure models

Abstract Architects and glass designers often refer to published standards when selecting glass thicknesses and areas for glazings in buildings. Glass design standards generally are based upon the results of experiments involving the breakage of standard sized glass sheets under carefully controlled conditions. More recently, some design standards have incorporated mathematical failure models which contain empirical parameters whose values have been estimated from these same experimental studies. In this paper, the technical basis for the recommendations within current architectural glass design standards is discussed. Existing glass failure prediction models are reviewed and it is shown that significant discrepancies exist between their recommendations and those of the design standards. A modified crack growth model is proposed which predicts failure probabilities for both short and long term stresses which are consistent with established design practice.

Stresses and fracture probability in evacuated glazing

Abstract Evacuated glazing consists of two plane sheets of glass which are hermetically sealed around the edges and which contain a narrow evacuated space. An array of support pillars maintains the separation of the glass sheets under the influence of atmospheric pressure. The vacuum effectively eliminates gaseous heat transport through the structure, and radiative heat transport can be reduced to a low level by the incorporation of transparent, low emittance coatings on one or both of the inner glass surfaces. The application of evacuated glazing is in insulating windows. Evacuated glazing contains steady or long term stresses as a result of the combined effects of atmospheric pressure and temperature differentials. A method is developed for determining the magnitude of such stresses due to these separate influences. The stresses are calculated analytically, and by finite element modelling. The results of the calculations are validated by experimental measurements. These stresses are used with a model of delayed fracture in glass to estimate the probability of mechanical failure of evacuated glazing over long periods of time. It is shown that evacuated glazing which is well designed, manufactured and installed should be no more susceptible to mechanical failure than conventional double glazing, for similar operating conditions.

Manufacture and cost of vacuum glazing

The vacuum glazing project at the University of Sydney has progressed to the point where the main features of the vacuum glazing design are determined well. Over 500 glazings with areas up to one square meter have been formed. The stresses to which these glazings are or may be exposed have been studied extensively. The durability of the glazing structure and the internal vacuum has been demonstrated. Vacuum glazing of the type designed and formed at the University of Sydney has a center-of-glazing thermal conductance as low as 0.85 and 1.2 Wm−2K−1, for glazings with two and one internal low emittance coatings, respectively. A method for the manufacture of the vacuum glazing is outlined from which the cost to manufacture the glazing can be estimated. A cost at the factory of about

Temperature-induced stresses in vacuum glazing : Modelling and experimental validation

40 ± 7 m−2 for vacuum glazing using two sheets of low-e glass and about

Bakeable, all-metal demountable vacuum seal to a flat glass surface

32 ± 6 m−2 for glazing using one sheet of low-e glass is obtained, when production volume is approx. 105 m2yr−1 and is partially automated. This is about 25% higher than the estimated manufacturing cost of the high thermal resistance, argon filled, double glazing utilizing low-e glass, which are currently in production and being sold in the United States, Europe and Japan. These glazings typically have center-of-glazing thermal conductances of about 1.1 Wm−2K−1 or more.

Vacuum glazing: Development, design challenges and commercialisation

A temperature difference across a sample of vacuum glazing causes differential expansion of one glass sheet relative to the other. In vacuum glazing with a fused edge seal, this results in tensile and compressive stresses in the glass sheets, and bending of the structure. The physical origins of these stresses and deflections are discussed, and a finite element model is used to determine their magnitude. The model has been validated by comparison with experimental data for a well-characterised sample of vacuum glazing under accurately defined external conditions. Modelling data are presented for two glazing designs which have properties that are characteristic of the extremes of performance of this type of glazing. It is shown that mechanical edge constraints can profoundly alter the spatial distribution of stresses in the glazing.

Design and validation of guarded hot plate instruments for measuring heat flow between evacuated plane-parallel glass surfaces

A technique is described for making a demountable vacuum seal of reasonably good quality between an all-metal evacuation cup and a flat glass surface. The seal is bakeable to temperatures close to the softening point of glass. The stainless steel evacuation cup has two concentric and coplanar sealing surfaces. The regions defined by these surfaces are differentially pumped. Pressures typically below 1 Torr are achieved in the annular space between the two sealing surfaces using a conventional rotary pump. The center region inside the inner sealing surface is typically evacuated to below 10−4 Torr using conventional diffusion or turbomolecular pumps.