IGMA February 2015
By Bill Lingell
Nothing in between
By Bill Lingell
The Insulating Glass Manufacturers Alliance (IGMA) is preparing to
release an informational paper giving an extensive overview on the
subject of vacuum insulating glass (VIG).
The Insulating Glass Manufacturers Alliance (IGMA) is preparing to release an informational paper giving an extensive overview on the subject of vacuum insulating glass (VIG). One of the IGMA technical committees, Emerging Technologies & Innovations, has been working on the document for over a year and has collected some very interesting and educational information and data on this topic.
There are many contributors and experts within the IGMA membership working on this effort and it looks like the finished product will assist those in the glass industry – designers, architects, specifiers, engineers and end users of VIG – to understand the product along with the technology and science that goes into this type of insulating glass unit.
This article is intended to give an outline of a few topics that will be covered as VIG becomes of viable use to the glass and glazing industry. Developing VIG knowledge has been part of this association’s efforts to provide energy efficient products, and advance the use of glass. The paper begins with a brief history of the concept that had a patent described for VIG as early as 1913 with today’s versions similar to the original patent. The VIG unit normally consists of two pieces of glass with an evacuated space between each layer and sealed to keep the vacuum in the space. The principle is to virtually eliminate conductive and convective heat transfer. The gap between the two pieces of glass is very narrow (usually around one millimeter) as compared to a conventional IGU that uses air or an inert gas (usually six to 13 mm) to assist in minimizing conductive heat transfer. Due to the vacuum in the space, the VIG unit requires pillars, spacers, or stanchions to maintain the gap and keep the two glass lites from touching. These spacers are carefully engineered and placed so that the glass stresses are managed properly due to the loading from the atmospheric pressure and vacuum in the space.
The control of radiant heat transfer can be minimized by placing a transparent low-E coating on one or both of the internal surfaces of the VIG. There has been extensive research, development, testing, and commercialization of VIG in the past 20 years with efforts supported by industry and governments to attain the energy benefits from this technology.
A direction for future VIG research would be to comprehend some of the heat transfer characteristics as compared to the conventional IGU. The basic concept of both VIG and IGU is to improve the overall heat transfer coefficient or lower the U-factor of the product as compared to a single lite of glass. Both VIG and IGU do this by having two lites of glass and a sealed air gap or gas cavity, further reducing conductive and convective heat transfer. There will be a small amount of energy transferred by conduction around the edge seal and pillars (in the case of VIG). An important property of VIG is that these units significantly reduce conduction and convection in the space between the lites because the residual gas between the lites with the high vacuum is such that the volume of residual gas remaining is approaching zero. When there is a vacuum in the space there are very few molecules which will in turn reduce the energy transfer between the lites. This condition will be illustrated in the paper.
The level of vacuum attained will influence the residual conduction of air within the VIGU and researchers have found that the lites need to be evacuated to a pressure low enough that the distance between molecular collisions is about the same as the distance over which the gas is contained. Obviously, another important issue with the VIG system is that once the vacuum is attained the port or method through which the vacuum was pumped out must be sealed in a manner to keep the vacuum locked in the VIGU. We will have more on these topics as the paper advances through the IGMA process.
Bill Lingnell has over 46 years of experience in the technical field of glass and architectural products. He holds three Masters of Science degrees in engineering: civil, mechanical and engineering science. Lingnell is the technical consultant for the Insulating Glass Manufacturers Alliance.