Design of Support Pillar Arrays in Flat Evacuated Windows

Abstract

Flat evacuated glazing consists of two plane sheets of glass separated by a narrow evacuated space. These structures must incorporate an array of support pillars in order to maintain the separation of the glass plates under the influence of atmospheric pressure forces. A design procedure is outlined for determining the dimensions of this pillar array. Two important constraints in the design process are the mechanical tensile stress on the outside of the glass plates near the pillar, and the thermal conductance through the array of support pillars. A third constraint arises because of stress concentration near the pillars on the inside of the window. Evacuated windows having usefully low values of thermal conductance through the pillar array and tolerably small levels of exterior tensile stress can only be produced if large stresses exist on the inside of the structure in the region of the glass plates near the support pillars. The implications of these stresses are discussed. It is concluded that it is possible to design a pillar array for which the localised tensile stresses and overall thermal conductance have usefully small values.