Patent Publication Number: US-2015068018-A1

Title: Vacuum insulated glass units system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/062,014 that was filed Oct. 24, 2013, which claims the benefit of priority of the following U.S. provisional applications: Ser. No. 61/718,406, filed on Oct. 25, 2012, and entitled “Vacuum Insulating Window with Multiple Stage Edge Seals;” Ser. No. 61/725,110, filed on Nov. 12, 2012, and entitled “Vacuum Insulating Window with Integral Vacuum Pump;” Ser. No. 61/732,577, filed on Dec. 3, 2012, and entitled “Vacuum Insulating Window Baked-Out after Installation and Method for Same;” Ser. No. 61/751,891, filed on Jan. 13, 2013, and entitled “Flapper Valve for Vacuum Insulating Window;” Ser. No. 61/760,854, filed on Feb. 5, 2013, and entitled “Hollow Cylindrical Spacer for Vacuum Insulating Window;” Ser. No. 61/767,379, filed on Feb. 21, 2013, and entitled “Cross Shaped Spacer for Vacuum Insulating Window;” Ser. No. 61/775,637, filed on Mar. 10, 2013, and entitled “Cross Shaped Polymer Spacer and Large Vacuum Gap for Vacuum Insulating Window;” Ser. No. 61/802,527, filed on Mar. 16, 2013, and entitled “Cross Shaped Polymer Spacer, Large Vacuum Gap, and Imbedded Edge Seal Spring for Vacuum Insulating Window;” Ser. No. 61/804,688, filed on Mar. 24, 2013, and entitled “Fluid Joint and Seal for Vacuum Tubing;” Ser. No. 61/863,639, filed on Aug. 8, 2013, and entitled “Polymer Spacer for Vacuum Insulated Glass Formed in Two Shot Injection Molding Operation;” Ser. No. 61/866,590, filed on Aug. 16, 2013, and entitled “Optically Clear Mat Polymer Spacer with Very Low Outgassing and Water Absorption for Vacuum Insulated Glass Unit;” the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     A vacuum insulated glass (VIG) unit comprises two sheets of glass with at least one vacuum space in between them that is at a pressure less than atmospheric pressure. Separation of the glass sheets and maintenance of the vacuum space is accomplished by some sort of physical spacer system in between the glass sheets that resists the compressive load of atmospheric pressure and contributes to the prevention of collapse of the vacuum space. Around the periphery of the unit is at least one edge seal that seals the vacuum space from the atmosphere. The vacuum space reduces heat conduction and convection between the glass sheets. The spacer system, which conducts an amount of heat between the glass sheets that depends in part on the thermal conductivity of its materials, can tend to negate the high insulating value of the vacuum. A rigid or somewhat inflexible edge seal such as glass frit or metal may result in unacceptably high stress in a VIG unit if the temperature difference between the inner and outer glass sheets becomes too large as might occur in cold climates, so that the use of these materials for an edge seal can limit acceptable climates, the size of a unit, or both. 
     Current state of the art VIG units are evacuated once and permanently sealed at time of assembly. Those that utilize glass frit edge seals and discrete metal spacers maintain their service pressures only because the glass sheets, glass edge seals, and metal spacers can achieve extremely low outgassing rates in a high temperature bakeout at time of assembly. 
     SUMMARY OF THE INVENTION 
     Various embodiments of this invention relate to vacuum systems comprising, without limitation, VIG units whose vacuum spaces are connected to one another by conduit and to one or more vacuum pumps that operate during the service lives of the VIG units. A conduit herein is any enclosure capable of allowing gas flow. By way of example only, and without limiting the scope of this invention, a conduit may comprise tubing, ducts, pipes, valves, pumps, and interconnections and fittings such as tees, flanges, and manifolds. The vacuum pumps maintain most of the vacuum spaces at service pressures for a time period of indefinite duration or for an indefinite number of time periods of indefinite duration for the purpose of reducing heat conduction and convection through the residual gasses in the vacuum spaces. 
     By pumping a vacuum system comprising VIG units while the VIG units are in service, materials with relatively high outgassing rates, such as polymers, may be used in both edge seals and spacer systems instead of metals and glasses. Pumping would remove the outgassing products that continuously desorb from polymers so that vacuum service pressures may first be established and then maintained. Without removal, these outgassing products would cause the pressure in the vacuum space of a VIG unit to quickly exceed service pressures. 
     Edge seals comprising polymers would be less expensive than metal or glass and would cause significantly less stress than metal or glass seals. Polymer spacers with their very much lower thermal conductivity than metal spacers would greatly improve energy efficiency. 
     The lowest attainable pressure in a VIG vacuum space that is part of a pumped vacuum system comprising VIG units depends in part on the gas load (Torr·liter/second); molecular conductance (liter/second) of the vacuum system, which includes the effective molecular conductance of the VIG vacuum spaces; the pumping speed (liter/second) of the vacuum pump; and the integral leak rate of the vacuum pump (Torr·liter/second). 
     What follows is a discussion of gas load, outgassing, outgassing rates, molecular flow, molecular conductance, and the inverse power law nature of the reduction in outgassing rates, and how the conventional wisdom, based on these parameters, may explain why vacuum engineers have not, to the inventor&#39;s knowledge, previously developed a vacuum system comprising VIG units that are pumped while in service. 
     All vacuum systems, including VIG, are subject to a gas load or quantity of gas entering the vacuum per unit of time, frequently given as (Torr·liter/second), where Torr is a pressure unit equivalent to 1/760 of atmospheric pressure. At a given temperature, a gas load of one (Torr·liter/second) means that each second a quantity of gas that would occupy a volume of one liter at a pressure of one Torr at the given temperature enters the vacuum. 
     Gas load will cause a rise in vacuum pressure unless it is removed, which may be by pumping. Some of the gas load may be the result of leaks in the system or seal imperfections, but generally it is to a far greater extent a result of outgassing. Jousten (1999, p. 111) states (references cited: other publications): 
     Outgassing means usually two things ( FIG. 1 ):
         1. Molecules diffusing through the bulk of material of a vacuum chamber, entering the surface and desorbing from it [known as gas permeation where the higher the permeability of a material the higher the rate of gas permeation].   2. Molecules which have been absorbed previously, usually during venting of the vacuum chamber, that desorb again, when the chamber is pumped to vacuum.   Both effects have the same consequences: they limit the lowest achievable pressure in a vacuum chamber, they considerably extend the time for a high or ultrahigh vacuum to be reached, and the outgassing molecules are a source of impurities in a vacuum chamber . . .   A further source of outgassing is the vaporization or sublimation of atoms or molecules from a material with a vapour pressure higher or comparable to the vacuum pressure to be applied to a chamber.       

     In the first type of outgassing Jousten (1999) describes above, which is known as gas permeation, gas molecules are first absorbed on outer surfaces of a vacuum chamber, they then diffuse through the bulk solid materials of the vacuum chamber and finally desorb from the inner surfaces of the vacuum chamber and enter the vacuum. Gas permeability is a measure of a material&#39;s ability to allow permeation, the higher the permeability of a material the higher will be the rate at which gas permeates through it. 
     Glasses and metals generally have very much lower gas permeabilities and outgassing rates than polymers.
     Roth (1994, p. 6-7) states (references cited: other publications):
       Gases have the possibility to flow through solids even if the openings present are not large enough to permit a regular flow [leak]. The passage of a gas into, through and out of a solid barrier having no holes large enough to permit more than a small fraction of the gas to pass through any one hole is known as permeation (American Vacuum Society 26 ). The steady state rate of flow in these conditions is the permeability coefficient or simply the permeability. This is usually expressed in cubic centimeters of a gas at STP [standard temperature and pressure or 273.15° K (0° C., 32° F.) and 100 KPa (14.504 psi, 0.987 atm, 1 bar, 750.061 Torr)] flowing per second through a square centimetre of cross section, per millimetre of wall thickness and 1 torr of pressure drop across the barrier [cm 3 (STP)mm/cm 2 ·sec·Torr].   
       

     Although he does not specifically state it, the second type of outgassing that Jousten (1999) describes also includes the release of gas and water molecules adhering in layers to the inner surfaces of a vacuum chamber with the greatest percentage of these molecules generally being water molecules. For the second type of outgassing described by Jousten (1999), the outgassing rate is given in terms of the quantity of gas evolving from a unit surface area per unit of time at a given temperature. Common units are (Torr·liter/cm 2  second). 
     For the vacuum space of a VIG unit to be insulating, that is to reduce heat conduction and convection through the residual gases in the vacuum space to an insignificant level, the mean free path of a gas molecule must be very much larger than the distance between obstructions, for example, and without limitation, the distance between the glass sheets of a VIG unit or the dimensions of the pore spaces of fumed silica or aerogel spacer mats. In physics, the mean free path is the average distance travelled by a moving particle (such as an atom, a molecule, or a photon) between successive impacts with other particles, which modify its direction or energy or other particle properties. 
     When the mean free path is very much larger than the smallest cross dimension of a vacuum system, the gas flow regime is molecular flow. Molecular flow is distinctly different from viscous flow. Everyone has experiential knowledge of viscous flow. However, very few people outside the fields of vacuum engineering and the physical sciences are aware of molecular flow and its probabilistic nature. Viscous flow is represented by fans, vacuum cleaners, and air escaping a balloon where gas molecules and atoms collide with each other and are moved by local pressure differences created by those collisions. 
     In molecular flow, gas molecules for the most part do not collide with each other but instead travel unimpeded directly between obstructions where they are absorbed on a surface and then may desorb in any given direction, including a direction opposite that of a vacuum pump, with nearly equal likelihood. 
     If a gas molecule enters the vacuum space of a VIG unit by permeating through a polymer edge seal or by desorbing from the surface of a polymer spacer it must be conducted, or make its way, to a vacuum pump by a sort of random walk of repeated absorptions and desorptions that is governed by probability and the size and 3D geometry of the vacuum system. Jousten (2008, p 81) states (references cited: other publications):
         In the case of molecular flow, the individual gas particles travel back and forth between the walls of the tube with thermal velocity. A particle&#39;s direction after hitting the wall is (nearly) independent of its direction prior to the collision. Thus, a zigzag route develops ( FIG. 4.2   a ). The geometry of the tube determines the resulting velocity of flow.       

     Concerning the three different types of flow regimes, including molecular flow, Jousten (2008, p 79-81) states (references cited: other publications):
         Depending on the pressure and the cross dimensions of a tube, three types of flow can be differentiated:   1. For sufficiently low pressure, the mean free path of a gas particle is high, compared to the cross dimensions of the tube. Hardly any mutual particle collisions occur. Each gas particle travels through the tube due to its thermal motion, independent of other particles. However, frequent collisions with the tube walls cause a zigzag route. On average, the paths of many individual particles combine to form the macroscopic flow behavior. This situation is referred to as single-particle motion or molecular flow.   2. Under high pressure, the mean free path of gas particles is much lower than the cross dimensions of the tube. The particles experience frequent mutual collisions, thereby exchanging momentum and energy continuously. Even a small volume contains many frequent colliding particles. Thus, the gas behaves as a continuum. A flow is the result of local pressure gradients. This situation is referred to as continuum flow or viscous flow.   3. The medium-pressure range is characterized by a transition between molecular and viscous flow. In this transition, collisions of gas particles with the wall occur just about as often as mutual collisions amongst gas particles. This situation is referred to as transitional flow or Knudsen flow.       

     The ability of a system of enclosures to allow gas to flow in the molecular flow regime is known as molecular conductance. The higher the molecular conductance of a vacuum system in (liters/second) at a given temperature, the more probable it is that a gas molecule will reach the pump and the lower will be the attainable pressure. The longer, narrower, and more obstructed the passages leading to a pump, the lower the molecular conductance and the less likely it will be that a gas molecule will reach the vacuum pump, if ever. 
     To satisfy physical, practical, economic, and architectural constraints, a vacuum system comprising pumped VIG units must tend to be long with small cross sections, including the vacuum spaces between the glass sheets, and must therefore tend to have very low molecular conductance. This calls into question whether sufficiently low pressures can be established and maintained in the vacuum spaces of a system of connected VIG units with polymer edge seals or spacers with relatively high permeability and high outgassing rates. 
     Molecular conductance may impose a practical lower limit on the pressure in a vacuum system regardless of pumping speed. Jousten (2008, p 87-89) derives the relationships between pressure in the vacuum vessel, pressure at the pump, pumping speed, and conductance that establish the importance of low conductance for attaining low vacuum pressures (references cited: other publications). Jousten (2008, p 89) states (references cited: other publications):
         If the conductance is considerably below the pumping speed, the effective pumping speed is determined largely by the conductance and hardly by the pumping speed of the pump. Thus, any larger pump would not increase the effective pumping speed. Consequently, when installations are planned, tubes with maximum possible conductance should be selected (short tubes with large cross section).       

     The effective molecular conductance of a VIG vacuum space can be increased by increasing the distance between the glass sheets, or vacuum space gap, but in general, for any given VIG vacuum system with discrete spacers there will be a gap size beyond which any further increase quickly becomes unworkable. This is because the larger the gap size becomes, the lower the pressure must be for the mean free path to remain at least one order of magnitude larger than the gap size, which is necessary to make heat convection negligibly small. 
     Although increasing the gap size between two glass sheets of a VIG unit increases the effective molecular conductance of the vacuum space it also requires increasing the size of discrete spacers, which not only increases their outgassing area but also decreases molecular conductance by creating bigger obstructions between the glass sheets. This becomes particularly problematic for polymer spacers with their high outgassing rates and even more problematic for a polymer inclusive edge seal where the polymer contacts the vacuum. If discrete polymer spacers are used, going from a gap of 0.04 inch (1 mm) to 0.32 inch (8 mm) only makes it more difficult to achieve service pressures because the increase in the vacuum space effective molecular conductance due to the increase in gap size is more than offset by the increase in gas load from the increase in spacer surface area. Furthermore, the increased spacer size obstructs glass flow, which also tends to offset any gain in conductance from increased gap size. If in addition there is a 0.32 inch wide ribbon of elastomer edge seal ringing and in contact with the vacuum space this offset is compounded. 
     One can always increase the pumping speed of the pump (liter/second), but as discussed above, without sufficiently high molecular conductance of the vacuum system the lowest obtainable pressure is determined not by pumping speed but by the molecular conductance of the system, so that at some point increasing the pumping speed has no practical effect on the lowest attainable pressure at the furthest region from the pump, which in this case would be the furthest VIG vacuum space region. 
     Another consideration that would have naturally discouraged professionals practiced in the art of vacuum technology from pursuing a vacuum system comprising in-service pumped VIG units is the manner in which outgassing rates decrease once the pump starts. After pumping of a vacuum system has begun and after the flow regime becomes molecular, generally the net desorption rate or net outgassing rate will decrease according to an inverse power law such as j des =Kt −n , where j des  is net desorption rate, K is a constant, t is time, and n is a constant. 
     Within a vacuum system, as the outgassing rates for the various gas species decrease so does the pressure. It therefore follows that pressure decrease also follows an inverse power law as molecules adhering to a vacuum chamber&#39;s surfaces and absorbed in its walls release, desorb, and are removed during pumping. 
     Because reduction in net outgassing rate and pressure follow inverse power laws, the rate at which pressure drops diminishes rapidly with time, so that if there were no ways to speed up this process, high vacuums (1E-3 to 1E-6 Torr) would be unachievable within any practical time frame for many applications. As time goes by the pressure in a continuously pumped vacuum vessel becomes asymptotic and reaches what is effectively a minimum value. 
     The process known as a “high temperature bakeout,” which is described below, is one way to speed up the process of outgassing and to attain low pressures within practical time frames. Unfortunately, as will be discussed below, this is not an option for a vacuum system comprising VIG units with polymer edge seals or spacers. 
     Raising the temperature of a vacuum chamber increases outgassing rates and speeds up the removal of absorbed and adhering gas molecules. This occurs because the added energy overcomes binding energies, increases diffusion rates, imparts energy to liberated molecules, and increases molecular conductance. When the chamber cools and returns to ambient temperatures, outgassing rates may be very much lower than when the process began. If the temperature increase during this outgassing process is high, the procedure is called a high temperature bakeout. 
     Without a high temperature bakeout many high vacuum (1E-3 to 1E-6 Torr) systems would never achieve their low pressures in any reasonable amount of time. The same holds for current state of the art VIG. The permanently sealed commercially available VIG units with glass frit edge seals typically undergo a 200 to 350° C. (400 to 660° F.) high temperature bakeout at time of manufacture. 
     Raising the temperature of a current state of the art VIG unit to between 200 and 350° C. (400 to 660° F.) while evacuating with a vacuum pump attached directly to the unit with very little tubing distance between the vacuum space and pump can, within an hour, cause the glass sheets and metal spacers to outgas sufficiently such that when the unit returns to ambient temperatures the overall outgassing rate within the vacuum space is so low that the unit will maintain service pressures (1E-4 to 1E-3 Torr) for decades. 
     It is not practical or even possible to outgas a VIG unit with polymer edge seals or spacers in a high temperature bakeout operation at the time of its fabrication as is done with current state of the art VIG units. Even if it were possible to perform a high temperature bakeout of a VIG unit with polymer seals and spacers without damaging these materials, it is not practical because during the process of installation in a building and connection to a system of tubing the vacuum space of a VIG unit will be opened to the atmosphere (vented) causing re-absorption of gas and water molecules and layers of water molecules to redeposit on the internal surfaces of the glass sheets, spacers, and edge seal. 
     Furthermore, if there is a breach in a system of pumped VIG units causing the atmosphere to enter the system (venting), in order not to have to scrap those units, which may be prohibitively expensive, it must be possible to reestablish service pressures by pumping through the connecting tubing system at ambient temperatures. 
     For at least the reasons described above, a workable system of pumped VIG units with polymer edge seals or spacers must be able to achieve vacuum service pressures from an initially non-outgassed state while being pumped after installation at ambient or close to ambient temperatures. 
     To the inventor&#39;s knowledge, there is no published data indicating that the outgassing rate of a pumped ambient temperature vacuum system comprising VIG units with polymer edge seals or spacers could ever decrease to a point where required vacuum space service pressures could be achieved let alone maintained. 
     A vacuum system which is an embodiment of this invention may comprise, without limitation, valves, frit screens, temperature sensors, pressure sensors, air compressors, compressed air lines and pneumatically actuated devices, relays, solenoids, electrical cable, batteries, electric power generators, pumps, backup pumps, automated control systems, pump controllers, active and passive noise reduction systems, computers, computer cables, and computer programs. In part, and without limitation, a pump may contribute to maintaining the vacuum pressures of multiple VIG vacuum spaces by removing gases and gas species that permeate from the atmosphere into the vacuum spaces through the materials comprising the vacuum system including those comprising the VIG units. In part, and without limitation, a pump may contribute to maintaining the vacuum pressures of multiple VIG vacuum spaces by removing materials, gases, and gas species that evolve and outgas from the materials and surfaces comprising the vacuum system including those comprising the VIG units. In part, and without limitation, a pump may contribute to maintaining the vacuum pressures of multiple VIG vacuum spaces by removing materials, gases, and gas species that enter the vacuum system including the VIG units through leaks or less than perfect seals. In some embodiments, the vacuum systems comprise at least 10 VIG units. This includes embodiments in which the vacuum systems comprise at least 50 VIG units, at least 100 VIG units, or at least 250 VIG units. 
     It may be preferable for the pump(s) to run continuously, rather than to cycle on and off according to some time schedule or high and low pressure set points that maintain a VIG vacuum space pressure range in a manner similar to a furnace cycling on and off to maintain a comfortable temperature range. Thus, a pump may be considered to be configured to run continuously if that pump is configured to continue pumping until there is a system failure, an anticipated or increased likelihood of a system failure, an event that could precipitate a system failure, or an anticipated event that could precipitate a system failure. 
     A system failure may be any event that causes or could cause the atmosphere to enter the vacuum system at a rate faster than nominal outgassing and leak rates. By way of example only, and without limitation, a system failure may be a power failure, pump failure, valve failure, leak, physical damage that may result in the atmosphere entering the vacuum system, a need to service a pump or other vacuum system component, seal failure, relay failure, pressure sensor failure, computer failure, or battery failure. Events that may precipitate a system failure are, without limitation, a meteorological storm, a fire or suspected fire, human threat of damage, or a seismic event. For example, the pumps may run continuously for at least a month, at least 6 months, at least a year, at least 5 years, at least 10 years or at least 20 years, while the VIG units are in service. 
     Glass sheet herein may comprise, without limitation, laminated glass, such as, for example, glass sheets bonded together by a polymer. Glass sheet herein also comprises without limitation any glass object that is preponderantly flat with substantially even thickness but which may also have raised or contoured areas in regions that may function to maintain a space and separation between the otherwise flat and even thickness regions of two glass sheets. Though not detailed herein, this invention contemplates that glass sheets with raised contours or bumps may be used in some embodiments. Glass sheet herein also comprises any glass object that is preponderantly flat with substantially even thickness but which may also have recessed regions whose purpose may include containing a viscous material. A glass sheet herein may have coatings applied. A glass sheet herein may have active and or passive devices or components imbedded within it or attached to a surface and those devices or components may, without limitation, generate electricity when exposed to light. A glass sheet herein may comprise electrochromic or photochromic glass. 
     A vacuum insulated glass (VIG) unit herein means, without limitation, any insulating glass unit comprising at least two glass sheets with at least one vacuum space in between them at a pressure less than atmospheric pressure. In between the two glass sheets there may be multiple vacuum spaces and or additional glass sheets with some sort of a spacer system that resists collapse of the vacuum space under the compressive load of atmospheric pressure. 
     A spacer system herein means, without limitation, any physical element or number of elements that contribute to resisting the collapse of a vacuum space of a VIG unit under the total or partial compressive load of atmospheric pressure. By way of example only, and without limiting the scope of this invention, a spacer system may comprise discrete spacers of any size or shape arranged in any pattern in between glass sheets or a mat of a material such as fumed silica or aerogel in between glass sheets; metals, polymers, ceramics, or glasses; bumps or raised portions on glass sheets themselves and being formed of glass sheets or from glass sheets; spheres or rollers; and polymer mats with raised areas, which may be totally or partially optically clear. 
     Service pressure for a VIG vacuum space herein may mean without limitation any gas pressure that significantly reduces heat conduction and convection through a gas or a mixture of gases such as air within the vacuum space and may depend on the dimensions of the vacuum space, which may include without limitation the dimensions between the glass sheets of a VIG unit and/or the elements of a spacer system that may comprise materials such as fumed silica or aerogel or any other material within or defining a vacuum space. 
     The gas load for a VIG unit may have multiple sources that may include without limitation: atmospheric gas and water vapor permeation through the edge seal where gasses are absorbed on the surface of the seal, diffuse through the materials of the seal, and then desorb into the vacuum space; outgassing of gases that have been absorbed in the materials surrounding the vacuum space, or materials in communication with the vacuum space, including absorbed water and air molecules and water and gas molecules forming layers on surfaces; evolution of gas species and materials generated by the glass sheets and the materials of which the seal and spacers are made and by any materials in communication with the a vacuum space; and leaks. The largest source of outgassing may not be gas permeation through an edge seal comprising polymers but instead outgassing from the glass sheets and to an even greater extent outgassing from polymer spacers or edge seals. Gas permeation through glass is negligibly small and may not be a factor. 
     The term polymer as used herein is given its broadest meaning and therefore includes elastomers. An elastomer is a polymer with viscoelasticity (colloquially “elasticity”), generally having low Young&#39;s modulus and high failure strain compared with other materials. 
     By way of example only, and without limiting the scope of this invention, an edge seal comprising a polymer herein may include any of the edge seal technologies currently used for inert gas filled insulating glass units and may include composite, foam, and thermoplastic types of seals. By way of example only and not meant to be exhaustive in scope, or limit the scope of this invention, these seal types along with specific examples and manufactures are discussed in Van Den Bergh et al (2013, references cited: other publications) the entire disclosure of which is incorporated herein by reference. A polymer edge seal may comprise a viscous polymer. 
     Discrete spacers for a VIG unit herein includes any portion of a spacer system comprising individual spacers that, excluding any connections to a glass sheet, are unconnected and that are arranged in some pattern in between the glass sheets of a VIG unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a plan view schematic of a vacuum system comprising VIG units, vacuum valves, vacuum pumps, and a system of tubes and tubing stubs according to one embodiment of this invention. 
         FIG. 2  shows a plan view schematic of a vacuum system comprising VIG units connected to vacuum pumps through a system of tubes with a different configuration than that shown in  FIG. 1  to illustrate that a system of tubing may have a number of configurations and layouts according to this invention. 
         FIG. 3  shows a plan schematic detail of a VIG unit and its connection to a piping system that runs to a vacuum pump or pumps and other VIG units according to one embodiment of this invention. 
         FIG. 4  shows a Section “A” of a highly reliable and simple vacuum valve that can serve as a self-actuating shockwave arrestor valve according to one embodiment of this invention. 
         FIG. 5  shows a Section “B” of the valve shown in  FIG. 4 . 
         FIG. 6  shows a Detail “A” of Section “C” shown in  FIG. 5 . 
         FIG. 7  shows a Section “E” of a highly reliable and simple vacuum valve that can serve as a self-actuating shockwave arrestor valve according to one embodiment of this invention. 
         FIG. 8  shows a Section “D” of the valve shown in  FIG. 7 . 
         FIG. 9  shows a cross section of a fluid joint and seal for vacuum tubing according to one embodiment of this invention. 
         FIG. 10  shows a cross section of a portion of a VIG unit with cross shaped spacers according to one embodiment of this invention. 
         FIG. 11  shows a plan and profile detail of a cross shaped spacer for a VIG unit according to one embodiment of this invention. 
         FIG. 12  shows a plan and profile detail of a cross shaped spacer for a VIG unit according to one embodiment of this invention. 
         FIG. 13  shows a cross section of a portion of a VIG unit with cross shaped spacers according to one embodiment of this invention. 
         FIG. 14  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. 
         FIG. 15  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. 
         FIG. 16  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. 
         FIG. 17  shows a cross section of a VIG unit with a polymer spacer in between the two glass sheets of a VIG unit according to one embodiment of this invention. 
         FIG. 18  is a section through the spacer shown in  FIG. 17 . 
         FIG. 19  depicts a schematic layout of a vacuum system according to one embodiment of this invention. 
         FIG. 20  depicts a schematic layout of a vacuum system suitable for the analysis of the vacuum system of  FIG. 19 . 
         FIG. 21  shows a revised schematic of  FIG. 19  with an additional tubing run and turbomolecular pump according to one embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THIS INVENTION 
       FIG. 1  shows a plan view schematic of a vacuum system comprising VIG units, vacuum valves, vacuum pumps, and a system of tubes and tubing stubs according to one embodiment of this invention. This might represent the layout on any given floor of a building, such as a skyscraper. The vacuum spaces of VIG units  1  are connected by piping stubs  2  to piping system  3 . Piping system  3  is connected to a vacuum pump  4  or multiple pumps such as  4  and  5 . There may be one or more vacuum valves such as vacuum gate valves  6  and  7  that can be closed to isolate the vacuum system from the atmosphere. Closing valves  6  and  7  will maintain low pressures in the vacuum spaces of VIG units  1  for some period of time, allowing pumps  4  and  5  to be turned off for service or removed for replacement. Vacuum valves  6  and  7  may be closed as a safety measure if there is a likelihood of a power failure. There may be one or more vacuum valves such as  8  and  9  between the vacuum spaces of VIG units  1  and a tubing system  3 . Any valves between the vacuum spaces of VIG units  1  and tubing system  3  may be connected to or form a part of stubs  2 . By way of example only, and without limitation, valves  9  may be mini-gate valves. By way of example only, and without limitation, valves  9  may be closed if there is a likelihood of a power failure or of damage to any of the windows, if service needs to be performed on the system, there is a system failure, an anticipated or increased likelihood of a system failure, an event that could precipitate a system failure, or an anticipated event that could precipitate a system failure. Closing valves  9  would isolate all the VIG units from the system and prevent the atmosphere from flooding the system if any of the units  1  were damaged and breached. By way of example only, and without limitation, valves  9  may be remotely controlled. By way of example only, and without limitation, valves  9  may be actuated to close and or open manually, using compressed air, electrical energy (solenoid, motor), spring, or combination of these methods. There may be pressure sensors within the vacuum system that initiate a signal to close valves  9  if the pressure begins to rise faster than a set rate or exceeds a set value. There may be valves  10  and  11  that may be closed to isolate pumps  4  and  5  from the atmosphere. There may be multiple valves between the atmosphere and pumps  4  and  5  such as valves  10 ,  11 ,  12 , and  13 . Any valve within a vacuum system comprising VIG units may be closed while others remain open. Any number or combination of valves within a vacuum system comprising VIG units may be closed while others remain open. Any valve within a vacuum system of VIG units may be actuated to close or open manually, remotely, or automatically as desired or according to some system parameter such as vacuum pressure or temperature. A vacuum valve in a vacuum system comprising VIG units may be any commercially available vacuum valve or valve known in the art. Any valve within a vacuum system comprising VIG units may be connected to a computer and or the Internet using hard wiring or wireless connections or both. Any pump within a vacuum system comprising VIG units may be connected to a computer and or the Internet using hard wiring or wireless connections or both. Any sensor within a vacuum system comprising VIG units may be connected to a computer and or the Internet using hard wiring or wireless connections or both. Any vacuum system component such as and without limitation valves and pumps, any technologies, any processes, or any methods any of which are known in the art of vacuum engineering and vacuum systems may be employed in a vacuum system comprising VIG units. 
       FIG. 2  shows a plan view schematic of a vacuum system comprising VIG units connected to vacuum pumps through a system of tubes with a different configuration than that shown in  FIG. 1  to illustrate that a system of piping may have a number of configurations and layouts according to this invention. In  FIG. 2  the vacuum spaces of VIG units  21  are connected by piping stubs  22  to piping system  23 . Piping system  23  is connected to vacuum pumps  24 ,  25 ,  26  and  27 . 
     A vacuum system of interconnected VIG units may comprise units on multiple floors or different levels of a building and the VIG units may be connected to one or more pumps by tubing that runs between floors or levels. A system of interconnected VIG units may be in different buildings or wings of the same building. Tubing that connects a VIG unit&#39;s vacuum space to a pump may tap the vacuum space at any location on the VIG unit. 
     The tubes or piping that connect a system of VIG units to a pump or pumps may be of any shape or size and may vary in size and shape and material and flexibility. 
       FIG. 3  is a plan schematic detail of a VIG unit and its connection to a piping system that runs to a vacuum pump or pumps and other VIG units. A vacuum space of VIG unit  31  may be connected to a fast actuating vacuum valve  32  that may be connected to a slower actuating vacuum valve  33  that is connected to tubing  34  that connects to pump  35 , other valves, and VIG units. Slower actuating valve  33  may be valve  9  in  FIG. 1 . Fast actuating valve  32  may be a two way valve that can close to limit air from entering tubing  34  or to limit air from entering VIG unit  31 . For example, if VIG unit  31  fails, creating a breach to the atmosphere, air will rush into the system reaching sonic speed (speed of sound) and a shock wave will form. In order to limit the pressure rise in the VIG units connected to VIG unit  31  and to protect the pumps, valve  32  is desirably able to close within milliseconds of such a breach. A fast actuating valve such as valve  32  may be any valve as shown if  FIG. 1 , which includes valves  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12 , and  13 . There is no limitation on where a fast actuating valve may be located in a vacuum system comprising VIG units. Also there is no limitation on where a slower actuating valve may be located in a vacuum system comprising VIG units. Still referring to  FIG. 3 , there may be screens  36 ,  37 ,  38 , and  39  within the vacuum system that allow gas flow but block and stop debris that might enter the system or originate within the system and thus the screens may protect system components such as valves and pumps. Such screens may be located anywhere within a vacuum system comprising VIG units. 
       FIGS. 4 ,  5 , and  6  depict a highly reliable and simple vacuum valve  41  that can serve as the fast actuating vacuum valve  32  in  FIG. 3  or as a fast actuating valve at any location in a vacuum system comprising VIG units. Valve  41  may be called a self-actuating shock wave arrestor valve. Valve  41  may be forced closed within a fraction of a millisecond by the arrival and action of an atmospheric wave front. Valve  41  may be attached to glass sheet  42  of a VIG unit and be in open communication with a vacuum space  43  through a port  44 . Within valve  41  may be a port  45  with raised annular rings  46  and  47  on either side. Lightweight disk  48  with raised tab  49  pivots on low friction pin  50 . Lightweight disk  51  with raised tab  52  pivots on low friction pin  53 . If either or both glass sheets  42  and  54  break, allowing an atmospheric wave front to enter the vacuum system, disk  48  will pivot  55  about pin  50  and may be forced closed by air pressure acting on its backside  55  and on tab  49 . Disk  48  may seat and seal against annular ring  46  and may be held in place by atmospheric pressure or partial atmospheric pressure. 
     Still referring to  FIGS. 4 ,  5 , and  6 . If an atmospheric wave front does not originate in a breach of glass sheets  42  or  54 , but instead elsewhere in the system, then disk  51  will pivot  56  about pin  53  and may be forced closed by air pressure acting on its backside  57  and on tab  52 . Disk  51  will seat and seal against annular ring  47  and may be held in place by atmospheric pressure or partial atmospheric pressure and may thereby seal vacuum space  43  from the atmosphere. 
     Still referring to  FIGS. 4 ,  5 , and  6 . The body of valve  41  may be a polymer and may be a low outgassing polymer. The polymer may have low gas permeability. The body of valve  41  may be baked and outgassed at an elevated temperature before having a coating of a metal applied to its exterior and or interior. The metal may be aluminum and may be vacuum deposited. The metal or other coating may be created by atomic layer deposition (ALD). A metal coating would reduce the permeability and outgassing rate of valve  41 . The body of valve  41  may be metal such as stainless steel or aluminum. Disks  48  and  51  may comprise any suitable lightweight materials, which may include without limitation polymers, carbon fibers, ceramics, metals, or composites. There may be an elastomeric coating on face  60  of disk  48  that seats and seals against annular ring  46 . There may be an elastomeric coating on face  61  of disk  51  that seats and seals against annular ring  47 . Disks  48  and  51  may be baked, outgassed, and coated with a metal that may be vacuum deposited aluminum or a coating created by ALD. 
     Still referring  FIGS. 4 ,  5 , and  6 . Annular rings  46  and  47  may be metal and inserted into valve  41  if valve  41  is of a different material such as a polymer. Annular rings  46  and  47  may be made of a stainless steel. Annular rings  46  and  47  may be annular knife edges that may be polished. Rings  46  and or  47  may have a coating of vacuum grease applied to them. Face  60  of disk  48  may have a coating of vacuum grease applied to it and face  61  of disk  51  may have a coating of vacuum grease applied to it. 
     Still referring to  FIGS. 4 ,  5 , and  6 .  FIG. 6  depicts a detail “A” of section “C”. Pin  50  about which disk  48  pivots may be loosely set in a notched opening  70  so that disk  48  has freedom to align itself and seat flush against annular ring  46  shown in  FIG. 4 . Pin  53  of disk  51  may be situated in a notched opening similar to that of pin  50 . 
     Referring to  FIG. 4 , a magnet  72  may keep disk  48  open until valve  41  and the VIG unit to which it is attached have been installed and the pump(s) started at which time magnet  72  may be removed. Flange or connection  75  may connect to another valve or to tubing, which then connects through other system components to a pump or pumps. 
       FIGS. 7 and 8  depict a highly reliable and simple vacuum valve  101  that can serve as the fast actuating vacuum valve  32  in  FIG. 3 , or self-actuating shock wave arrestor valve. Valve  101  may be forced closed within a fraction of a millisecond by the arrival and action of an atmospheric wave front. Valve  101  may be attached to glass sheet  102  of a VIG unit and be in open communication with a vacuum space  103  through a port  104 . Within valve  101  may be a port  105  with annular seats  106  and  107  on either side. If either or both glass sheets  102  and  109  break, allowing an atmospheric wave front to enter the vacuum system, sphere  108  will be forced by air pressure into annular seat  106  thereby sealing the vacuum system from the atmosphere. Sphere  108  is constrained by a cage comprising members  110 . 
     Still referring to  FIGS. 7 and 8 . If an atmospheric wave front does not originate in a breach of glass sheets  102  and or  109 , but instead elsewhere in the vacuum system, then sphere  112  may be forced by air pressure into annular seat  107  thereby sealing the vacuum space  103  from the atmosphere. Sphere  112  may be constrained by a cage comprising members  114  in the same manner as sphere  108 . Annular seats  106  and  107  may comprise a soft elastomeric material chosen for its low gas permeability and low outgassing. Annular seats  106  and or  107  may be coated with vacuum grease. 
     Referring to  FIG. 7 , flange or connection  116  may connect to another valve or to tubing, which then connects through other system components to a pump or pumps. 
     It is important that any system of tubing that connects a system of VIG units to a pump or pumps be able to withstand expansion and contraction and building movements. This becomes increasing critical as the building becomes taller because lateral sway excursions of the upper reaches of skyscrapers can exceed several feet. It may also be necessary for a system of tubing that connects VIG units to be able to withstand seismic events. Described herein is a fluid joint and seal for vacuum tubing that is suitable for connecting tubing sections of a vacuum system comprising VIG units. The fluid joint and seal compresses, expands, and rotates to accommodate lengthwise and rotational movement of the two tube ends that it joins while maintaining a seal against the atmosphere. 
       FIG. 9  shows a cross section of a fluid joint and seal for vacuum tubing according to one embodiment of this invention. 
     Referring to  FIG. 9 , tubes  201  and  202 , which are to contain and maintain a vacuum at a pressure less than atmospheric pressure, are separated by a partially compressed elastomeric gasket  203 . An inner sleeve  204  runs inside both tubes  201  and  202  and inside gasket  203 . An outer sleeve  205  runs over tubes  201  and  202  and over gasket  203 . A thin gap between sleeve  205  and tubes  201  and  202  and gasket  203  is filled with a low gas permeability fluid  206 . Capping the ends of outer sleeve  205  are elastomeric gaskets  207  and  208 . There may be two additional outer sleeves  209  and  210  that pass over outer sleeve  205 . Outer sleeve  209  caps one end of sleeve  205  and partially compresses gaskets  207  and  217 . Outer sleeve  210  caps the other end of sleeve  205  and partially compresses gaskets  208  and  218 . Sleeve  209  may have a flange  211  and sleeve  210  may have a flange  212 . Flanges  211  and  212  may be bolted  213  together. As tubes  201  and  202  move along their length, which may be the result of thermal expansion and contraction of tubes  201  and  202  or their supporting framework, elastomeric gaskets  203 ,  207 ,  208 ,  217 , and  218  expand or contract and keep fluid  206  trapped between sleeve  205  and tubes  201  and  202 . Fluid  206  presents a low gas permeability barrier between the moving elements of the joint, allowing them to move with respect to each other while providing a fluid seal that cannot be broken. Gaskets  203 ,  207 ,  208 ,  217 , and  218  may have higher gas permeabilities than fluid  206 . 
     Still referring to  FIG. 9 , after assembly, a vacuum pump may be attached at port  214 , and while a port at  215  is closed, a vacuum may be established in the gap between sleeve  205  and tubes  201  and  202 . After a vacuum has been established, port  214  may be sealed and fluid  206  pumped through port  215  into the gap between sleeve  205  and tubes  201  and  202 . Port  215  may then be sealed with a flexible seal. When tubes  201  and  202  move, causing the compression or expansion of gaskets  203 ,  207 ,  208 ,  217  and  218 , fluid  206  flows between tubes  201  and  202 , gasket  203 , and outer sleeve  205 . Fluid  206  may move into and out of reservoir  216  that may be capped with a flexible seal at port  215  in order to accommodate thermal expansion and contraction of fluid  206  and as well as other joint elements. 
     Tubes  201  and  202  and sleeves  204  and  205  may be metal and have very low gas permeabilities and very low outgassing. For example, tubes  201  and  202  and sleeves  204  and  205  may be an aluminum alloy such as 6061-T 6 or stainless steel. If the tubing is aluminum it may have a thin oxide layer to minimize outgassing.    
     Prior to pumping fluid  206  into the gap between tubes  201  and  202  and sleeve  205 , tubes  201  and  202  and the gap between tubes  201  and  202  and sleeve  205  may be evacuated and the entire system may be baked out at an elevated temperature in order to outgas the system. 
     A vacuum system of which tubes  201  and  202  may be a part may be under continuous vacuum pumping or intermittent vacuum pumping. 
     Tubes  201  and  202  may be part of a vacuum system for VIG units in a building where multiple VIG units are connected through tubes such as  201  and  202  to a pumping station. 
     One end of gasket  203  may be adhered or cemented to tube  201  and the other end of gasket  203  may be adhered or cemented to tube  202 . 
     One end of gasket  207  may be adhered or cemented to tube  201  and the other end of gasket  207  may be adhered or cemented to sleeve  205 . 
     One end of gasket  208  may be adhered or cemented to tube  202  and the other end of gasket  208  may be adhered or cemented to sleeve  205 . 
     Fluid  206  may be a low molecular weight polyisobutene (PIB) or a perfluoropolyether. 
     Gaskets  203 ,  207 ,  208 ,  217 , and  218  may comprise for example, and without limiting the scope of this invention, silicone, Viton produced by DuPont, or Kalrez also produced by DuPont. 
     There may be a vacuum grease between inner sleeve  204  and tubes  201  and  202 . 
     A cross shaped polymer spacer that allows a considerably larger vacuum gap than disclosed or practicable by prior art for VIG may be more suitable for VIG units with edge seals comprising polymers and vacuum spaces maintained by vacuum pumping while in service. 
     Referring to  FIG. 10 , a VIG unit  301  comprises glass sheets  302  and  303  separated by a vacuum gap  304  that is at a pressure less than atmospheric pressure. Cross shaped spacer  305  contributes to maintaining separation of glass sheets  302  and  303  by resisting the compressive load of atmospheric pressure. 
     Referring to  FIG. 11 , cross shaped spacer  309  may have a dimension A of 0.32 inch, a dimension B of 0.32 inch, a dimension C of 0.32 inch, and dimensions D and E of 0.047 inch. If spacer  309  is made of a polyimide such as DuPont Vespel TP-8005, which has a thermal conductivity of 0.13 W/(mK) (Watts/meter·degree Kelvin), and if the distance between adjacent spacers in between the glass sheets of a VIG unit is two inches, then that VIG unit can achieve an overall R-value of greater than R-28 (hr ° F. ft 2 )/Btu with a compressive pressure per cross shaped spacer of only 2000 (lbs/in 2 ). This is well below the 21,000 (lbs/in t ) pressure at 10% strain for TP-8005. The cross shape of the spacer allows dimension A to be increased well above the typical 0.02 inch thickness of disk shaped spacers known in the art while minimizing the cross sectional area of the spacer in a plane parallel to the glass sheets while at the same time maintaining dimensions B and C equal to dimension A for stability. This is because the cross shape resists localized buckling. Increasing dimension A while keeping the cross sectional area and cross sectional dimensions at the minimum values necessary to resist pressure and maintain stability minimizes thermal conduction through the spacer. This cannot be achieved with a solid disk spacer. The best R-value that can currently be attained using metal disk spacers separated by two inches is R-10 (hr ° F. ft 2 )/Btu. If the thickness of a disk spacer is doubled then the diameter of the disk must be doubled to maintain stability of the spacer between the glass sheets. But if the diameter is doubled then the area is quadrupled and the net effect of doubling the thickness and quadrupling the area is to double heat conduction through the spacer. 
     Referring to  FIG. 12 , cross shaped spacer  312  may have additional members  314  that assist in resisting localized buckling. 
     A polymer spacer may be outgassed at a temperature greater than ambient temperatures and given a thin coating that may comprise, without limitation, a metal or oxide. A coating may be applied while a spacer is under vacuum and before assembly in a VIG unit. A coating may be a low emissivity coating and or low permeability coating. Coating methods may include, without limitation, vacuum deposition and atomic layer deposition (ALD). 
     Referring to  FIG. 13 , glass sheets  320  and  321  are separated by cross shaped spacers, of which  322  and  323  are representative. As a result of atmospheric pressure, glass sheets  320  and  321  will flex  324  and  325  as shown between spacers  322  and  323 . Furthermore glass sheets  320  and  321  will flex  326 ,  327 ,  328 , and  329  over spacers  322  and  323 . To equalize the pressure over spacers  322  and  323 , the ends  331  of spacers  322  and  323  may be formed with a suitable curve that conforms to the flexure shape of the glass when in service. 
       FIG. 14  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. Glass sheets  341  and  342  are separated by a vacuum space  343 . The distance between sheets  341  and  342 , or gap size, will be at least the size of the spacers such as those disclosed in  FIGS. 10 and 11 . Therefore the thickness of an edge seal, dimension E, that is in between the glass sheets may be 0.32 inch, which again is very much larger than the typical 0.02 inch gap between the glass sheets of VIG units as disclosed by prior art, and very much larger than the gap size contemplated by the prior art. 
     Still referring to  FIG. 14 , elastomeric seals  344  and  345  constrain low permeability fluid or viscous material  346 . Imbedded within viscous material  346  is a very low permeability barrier  347  that reduces the cross sectional area of viscous material  346  through which gas can permeate into the vacuum space  343 . Barrier  347  may be glass, metal, polymer, or a composite material. Between elastomeric seal  345  and vacuum space  343  there may be a low permeability coating or barrier  348  that may be an oxide coating or metal foil. Barrier  348  further reduces permeation of atmospheric gases into the vacuum space and reduces the amount of outgassing products from seal  345  that enter vacuum space  343 . A low permeability coating or layer such as metal foil  349  may be applied to the outside of seal  344  to reduce permeability of air into vacuum space  343 . 
       FIG. 15  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. Glass sheets  351  and  352  are separated by a vacuum space  353 . The distance between sheets  351  and  352 , or gap size, will be at least the size of the spacers such as those disclosed in  FIGS. 10 and 11 . Therefore the thickness of an edge seal, dimension F, that is in between the glass sheets may be 0.32 inch, which again is very much larger than the typical 0.02 inch gap between the glass sheets of VIG units as disclosed by prior art, and very much larger than the gap size contemplated by the prior art. 
     Still referring to  FIG. 15 , elastomeric seals  354  and  355  constrain low permeability fluid or viscous material  356 . Imbedded within viscous material  356  is a very low permeability barrier  357  that reduces the cross sectional area of viscous material  356  through which gas can permeate into the vacuum space  353 . Barrier  347  may be glass, metal, polymer, or a composite material and may not be, and need not be, a solid shape to accomplish its purpose of reducing the permeable cross sectional area of viscous material  356 . 
     Because the disclosed polymer spacer design results in a very large vacuum gap size, the use of elastomeric seals  354  and  355  is allowed because the percent elongations within the elastomer that will result from differential thermal expansion and contraction of glass sheets  351  and  352  will be well within what suitable elastomers can withstand and will meet structural load and resistance factor design (LRFD) standards for structural glazing. If the vacuum gap was the typical 0.02 inch as disclosed and contemplated by the prior art, the percent elongation would exceed that which suitable elastomers can withstand and the elastomer would fail. 
       FIG. 16  shows a cross section of an edge seal of a VIG unit with cross shaped spacers and large vacuum gap according to one embodiment of this invention. Glass sheets  361  and  362  are separated by a vacuum space  363 . The distance between sheets  361  and  362 , or gap size, will be at least the size of the spacers such as those disclosed in  FIGS. 10 and 11 . Therefore the thickness of an edge seal, dimension G, that is in between the glass sheets may be 0.32 inch, which again is very much larger than the typical 0.02 inch gap between the glass sheets of VIG units as disclosed by prior art, and very much larger than the gap size contemplated by the prior art. 
     Still referring to  FIG. 16 , elastomeric seals  364  and  365  constrain low permeability fluid or viscous material  366 . Imbedded within viscous material  366  is a very low permeability barrier  367  that reduces the cross sectional area of viscous material  366  through which gas can permeate into the vacuum space  363 . Barrier  367  may be essentially U-shaped and may comprise a metal. Barrier  367  may comprise stainless steel and the U-shape may flex like a spring. Flexible U-shaped barrier  367  may comprise a polymer with a very thin metal cladding that faces glass sheets  361  and  362 . The spring behavior may keep barrier  367  pressed against the inside of glass sheets  361  and  362 . There may be small studs  369  on the outside of barrier  367  that get pressed against glass sheets  361  and  362  and maintain a gap  370  between barrier  367  and glass sheet  361  and a gap  371  between barrier  367  and glass sheet  362 . The spring action of barrier  367  may not be sufficient to affect dimension G. Spacer  368  may maintain separation of glass sheets  361  and  362  and maintain a fixed dimension G. 
     Still referring to  FIG. 16 , elastomeric seals  364  and  365  may adhere to glass sheets  361  and  362  and may be a structural silicone adhesive sealant or other adhesive sealant. 
     It may be desirable that a polymer spacer for VIG be formed using a two shot or multiple shot injection molding process. 
       FIG. 17  shows a cross section of a VIG unit with a cross shaped polymer spacer in between the two glass sheets of a VIG unit according to one embodiment of this invention. 
       FIG. 18  is a section through the spacer shown in  FIG. 17 . 
     Referring to  FIG. 17 , polymer spacer  501  is in between glass sheets  502  and  503  of a VIG unit. Spacer  501  contributes to the maintenance of vacuum space  504  between glass sheets  502  and  503  by resisting the compressive load of atmospheric pressure. Spacer  501  may comprise two or more regions that may comprise different polymers or polymer formulations that may include fillers. As an example only, and without limiting the scope of the invention, spacer  501  may have two distinct regions  505  and  506  within its volume each with a polymer or fillers that are different from the other region. Region  506  may be the polymer polyetherimide whereas region  505  may be polyetherimide with a filler comprising molybdenum disulfide powder. Spacer  501  may be adhered to glass sheet  503  with an adhesive that may comprise epoxy or acrylic. Region  505  of spacer  501  may be in contact with glass sheet  501 . Because of the inclusion of molybdenum disulfide in region  505  the coefficient of friction of region  505  will be lower than region  506 . The lower coefficient of friction for region  505  reduces the frictional forces between region  505  and glass sheet  502 . Inclusion of molybdenum disulfide increases the thermal conductivity of polyetherimide. By having two regions, the bulk of the spacer may have lower thermal conductivity than the lower coefficient of friction region proximate to and in contact with glass sheet  502 . 
     Still referring to  FIG. 17 , spacer  501  and its two regions  505  and  506  may be formed in a two shot injection molding operation where one shot contains polyetherimide and the other contains polyetherimide with a molybdenum disulfide filler. In general a polymer spacer may be formed in a two shot or multiple shot injection molding operation to produce a spacer comprising regions of different composition. The different compositions may be chosen for any number of reasons to enhance the performance of the spacer with coefficient of friction representing just one such characteristic. By way of example only, and without limiting the scope of this invention, other characteristics may include wear resistance, compression resistance, resistance to UV radiation, emissivity, color, reflectivity, transparency, tensile strength, compression strength, bonding strength, outgassing characteristics, solubility of water, and elastic modulus. 
     Still referring to  FIG. 17 , region  507  may be a metal or other non-polymer material that is molded into spacer  501  as part of the molding operation. This material may be chosen based on its ability to be bonded to glass. 
       FIG. 19  depicts a schematic layout of a particular embodiment of this invention. Seventy-four VIG units  701  (typical) that are each eight feet wide and 12 feet high enclose one floor of a building with rectangular dimensions of 100 feet by 200 feet. Each VIG unit  701  is connected by a tubing stub  702  (typical) to a tubing system  703  that is connected to a turbomolecular pumping station  704  comprising one turbomolecular pump with a pumping speed of 33 liters/second and a backing pump. The size of pumping station  704  may be approximately a one foot cube. The internal diameter of the port connecting each VIG vacuum space to a stub  702  is one inch. The piping system  703  comprises four inch internal diameter 6061-T6 aluminum tubing with a ⅛ inch wall thickness and a thin internal oxide layer. Sections of tubing are connected in a manner according to  FIG. 9  with fluid joints and seals. The edge seals of the VIG units are as shown in  FIG. 16  with a U-shaped plastic element clad with 0.001 inch thick stainless steel on the surfaces of the plastic elements facing the glass sheets. The U-shaped plastic element clad in stainless steel is embedded in polyisobutene with a number average molecular weight of 1100 gram/mole. The U-shaped plastic elements maintain two 0.0015 inch gaps between the stainless steel cladding and the glass sheets. Each of these 0.0015 inch gaps filled with polyisobutene is two inches long. The distance between the glass sheets or gap size of the VIG units  701  is 0.32 inch. The spacers are the polymer polyimide with a thermal conductivity of 0.13 W/mK (Watts/meter·degree Kelvin). They are cross shaped as shown in  FIG. 11  with a cross sectional area of 0.028 inch. The spacers are distributed between the glass sheets of the VIG units  701  in a square two inch by two inch array. 
     To quantitatively analyze the vacuum system in  FIG. 19  it will be assumed that the system is at a state of equilibrium such that the total gas load entering the system is equal to the gas load removed by the pumping station. Furthermore the gas load from the tubing system and valve leak rates and outgassing is taken as negligible compared to the gas load evolved in the VIG vacuum spaces and is therefore ignored. Furthermore it is assumed that all of the seventy-four VIG units  701  (typical) connect through their own stub  702  (typical) to the piping system  703  at the furthest distance in the system from pumping station  704 . This is depicted schematically in  FIG. 20 , which for clarity shows only one of the seventy-four VIG units  701  and its stub  702  that connect to tubing system  703  at the shown furthest location from the pumping station. The arrangement of  FIG. 20  is more stringent than that of  FIG. 19  such that the achievable vacuum space pressures of the VIG units in  FIG. 19  will be lower than the pressures of the VIG units of  FIG. 20 , so that if service pressures can be achieved for the system of  FIG. 20  then they can be achieved for the system of  FIG. 19 . 
     The basic formula used for the analysis is (gas load)=C[P 1 −P 2 ]. Gas load is given in (Torr·liter/second). C is molecular conductance and has the units of (liter/second). Pressure is given as P in units of Torr. For a section of tubing, or other vacuum enclosure such as the vacuum space of a VIG unit, the gas load that will be conducted through that tubing is equal to the conductance C of that section of tubing multiplied by the difference between the high pressure P1 at one end of the tubing and the low pressure P2 at the other end. 
     Using the basic formula gives (gas load VIG )=C VIG [P 1 −P 2 ] where (gas load VIG ) is the rate at which gas enters a VIG vacuum space, C VIG  is the effective molecular conductance of the VIG unit vacuum space, P 1  is the highest pressure inside the vacuum space, and P 2  is the pressure at a port  44  in  FIG. 4  in the VIG vacuum space that connects to a stub  702  in  FIG. 20 . 
     Because the system is at equilibrium, the gas load entering a stub  702  will be equal to (gas load VIG ), which is equal to C VIG [P 1 −P 2 ], and which must equal the gas load conducted by a stub  702 , which is C stub (P 2 −P 3 ), where C stub  is the molecular conductance of the stub, P 2  is the pressure at port  44  in  FIG. 4  in the glass sheet that connects to stub  702  in  FIG. 20 , and P 3  is the pressure at the end of a stub that connects to tubing system  703  in  FIG. 20 . 
     If the number of VIG units is n, the total gas load will be n[Gas load VIG ]=nC VIG [P 1 −P 2 ]=nC stub (P 2 −P 3 ). Because the system is at equilibrium, the total gas load entering the tubing system  703  in  FIG. 20  will be n[gas load w ]=nC VIG [P 1 −P 2 ]=nC stub (P 2 −P 3 ), which must equal the gas conducted by tubing system  703  in  FIG. 20 , which is C tubing (P 3 −P 4 ), where C tubing  is the molecular conductance of tubing system  703 , P 3  is the pressure at the connection of all the stubs  702  in  FIG. 20  to tubing system  703 , and P 4  is the pressure at the inlet to the turbomolecular pumping station  704  in  FIG. 20 . The gas conducted by the tubing system  703  must be equal to the gas load removed by the pumping station minus the gas load at the pump resulting from gas backflow through the pump known as integral leak rate given in units of (Torr·liter/second). The gas load removed by the pump is given as P 4  (pumping speed of pump) and has units of (Torr·liter/second). 
     Stringing all these relationships together gives: n[gas load VIG ]=nC VIG [P 1 −P 2 ]=nC stub (P 2 −P 3 )=C tubing (P 3 −P 4 )=P 4 (pumping speed of pump)−(integral leak rate). This results in a system of four linear equations which can be represented in matrix form as: 
     
       
         
           
             
               
                 [ 
                 
                   
                     
                       
                         nC 
                         VIG 
                       
                     
                     
                       
                         - 
                         
                           nC 
                           VIG 
                         
                       
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       
                         - 
                         
                           nC 
                           VIG 
                         
                       
                     
                     
                       
                         
                           
                             
                               
                                 nC 
                                 VIG 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               nC 
                               stub 
                             
                           
                         
                       
                     
                     
                       
                         - 
                         
                           nC 
                           stub 
                         
                       
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       
                         - 
                         
                           nC 
                           stub 
                         
                       
                     
                     
                       
                         
                           
                             
                               
                                 nC 
                                 stub 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               C 
                               tubing 
                             
                           
                         
                       
                     
                     
                       
                         - 
                         
                           C 
                           tubing 
                         
                       
                     
                   
                   
                     
                       0 
                     
                     
                       0 
                     
                     
                       
                         - 
                         
                           C 
                           tubing 
                         
                       
                     
                     
                       
                         
                           
                             
                               
                                 C 
                                 tubing 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               pumping 
                                
                               
                                   
                               
                                
                               speed 
                             
                           
                         
                       
                     
                   
                 
                 ] 
               
                
               
                 [ 
                 
                   
                     
                       
                         P 
                         1 
                       
                     
                   
                   
                     
                       
                         P 
                         2 
                       
                     
                   
                   
                     
                       
                         P 
                         3 
                       
                     
                   
                   
                     
                       
                         P 
                         4 
                       
                     
                   
                 
                 ] 
               
             
              
             
                 
               
                 = 
                 
                   [ 
                   
                     
                       
                         
                           n 
                            
                           
                             ( 
                             
                               gas 
                                
                               
                                   
                               
                                
                               
                                 load 
                                 VIG 
                               
                             
                             ) 
                           
                         
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                     
                     
                       
                         
                           integral 
                            
                           
                               
                           
                            
                           leak 
                            
                           
                               
                           
                            
                           rate 
                         
                       
                     
                   
                   ] 
                 
               
             
           
         
       
     
     Given the system of equations above and knowing (gas load VIG ), C VIG , C stub , C tubing , pumping speed, and integral leak rate allows the determination of P 1 , P 2 , P 3 , and P 4 , where P 1  is the highest pressure in a VIG vacuum space. 
     Based on empirically determined outgassing rates at ambient temperatures, empirically determined decreasing outgassing rates with time at ambient temperatures, permeabilties for the materials specified, and conductances for the piping and vacuum spaces for the described vacuum system depicted in  FIG. 20 , after outgassing for five to nine months with a turbomolecular pumping station running continuously at a pumping speed of 33 liters/second and at a temperature of 23 degrees Celsius, the pressure P 1  in a VIG unit vacuum space within the system shown in  FIG. 20  is estimated to reach 9E-5 Torr, which at a vacuum gap size of 0.32 inch is service pressure. 
     To demonstrate the insensitivity of the system of  FIG. 20  to pumping speed, if the system is allowed to outgas for five to nine months with the turbomolecular pumping station  704  running continuously at a pumping speed of 33 liters/second at 23 degrees Celsius and the pump is then run continuously at a reduced pumping speed of 3 liters/second, it is estimated that at 23 degrees Celsius the vacuum space of a VIG unit can be maintained at a pressure P 1  of 1.6E-4 Torr, which could be considered service pressure at a vacuum space gap size of 0.32 inch. 
     If there was one building floor above and below that shown in  FIG. 20  with identical VIG vacuum systems and with tubing from those systems connected directly to the pumping station of  FIG. 20 , then at 23 degrees Celsius the VIG units on all three floors could be maintained at a pressure P 1  of 3E-4 Torr with a turbomolecular pumping station  704  running continuously at 3 liters/second. 
     It may be necessary to run a pumping station continuously at full speed, say 33 liters/second, to outgas a VIG vacuum system in the least amount of time, but after that the speed may be reduced to say 3 liters/second while still running continuously to reduce pumping station power consumption, which at 3 liters/second may end being less than 150 Watts. 
       FIG. 21  shows a revised schematic of  FIG. 19  with an additional tubing run and turbomolecular pump according to one embodiment of this invention.  FIG. 21  depicts Seventy-four VIG units  901  (typical) that are each eight feet wide and 12 feet high that enclose one floor of a building with rectangular dimensions of 100 feet by 200 feet. Each VIG unit  901  is connected by a tubing stub  902  (typical) to a tubing system  903  that is connected to a turbomolecular pumping station  904  comprising one turbomolecular pump with a pumping speed of 33 liters/second and one backing pump. There is an additional tubing run  905  with a turbomolecular pump  906  in line as shown. Turbomolecular pump  906  does not need a backing pump or gate valve. Gate valve  907  is sufficient to isolate the vacuum system from the atmosphere because the system is open to the atmosphere only through pumping station  904 . 
     For a VIG unit as specified for  FIGS. 19 and 20 , the whole window thermal transfer coefficient would be U w =0.2 W/(m 2 K) (0.2 Watt/meter 2 ·degree Kelvin) or U w =0.035 Btu/(hr ° F. ft 2 ) (0.035 British thermal Units/hou·degree Fahrenheit·foot 2 ) or R-28 (hr ° F. ft 2 )/Btu. 
     At best, triple pane inert gas filled windows with low emissivity coatings on all four internal surfaces can attain U w =0.6 W/m 2 K or R-10 (hr ° F. ft 2 )/Btu. A whole window thermal transfer coefficient of U w =0.6 W/m 2 K or R-10 (hr ° F. ft 2 )/Btu is also the best that two pane VIG units with metal spacers can achieve. 
     R-28 (hr ° F. ft 2 )/Btu exceeds the U.S. Department of Energy&#39;s recommended wall insulating R-value for new residential frame construction in Alaska, which is R-26 (hr ° F. ft 2 )/Btu. 
     The permeability of the PIB within the edge seals specified for  FIGS. 19 and 20  will vary with its temperature. Its permeability will increase with increasing temperature. During cold seasons less gas will permeate through the edge seal than during warm seasons. In addition, the outgassing rates of the glass and spacers will increase with increasing temperatures. Therefore during colder seasons the pressure within the VIG vacuum spaces may be lower than during warmer seasons. There may be times when the pressure in a VIG vacuum space exceeds what would be considered a service pressure. These instances are more likely to occur during warm seasons when high insulating values of the VIG units are not as critical as during cold weather. Sun exposure will cause the VIG units to heat up, which may push a vacuum space pressure to temporarily exceed service pressures. So there may be seasonal as well as daily pressure fluctuations and some of those fluctuations in some of the windows may exceed service pressures. These fluctuations may result in higher pressures for newer VIG units with decreasing pressures over time as the VIG units continue to slowly outgas over what may be periods of years. A vacuum system comprising VIG units may still fulfill a specified design requirement for building energy efficiency even if pumping only maintains service pressures in most of the VIG units for a period of indefinite duration or for an indefinite number of periods of indefinite duration. The extent to which pressure fluctuations that exceed service pressures can be tolerated may depend on many factors that may include without limitation climate, physical sun exposure patterns, temporal sun exposure patterns, building orientation, shading from adjacent buildings, cost benefit analyses, other energy efficiency measures, and building generated renewable energy to offset building energy use. 
     REFERENCES CITED: OTHER PUBLICATIONS 
     
         
         Fang Y, Hyde T, Eames PC, Hewitt N. 2009. Theoretical and experimental analysis of the vacuum pressure in a vacuum glazing after extreme thermal cycling. Solar Energy 83(9):1723-1730 
         Jensen K I, Schultz J M, Kristiansen F H. 2004. Development of windows based on highly insulating aerogel glazings. Journal of Non-Crystalline Solids 350:351-357 
         Jousten K, editor. 2008. Handbook of vacuum technology. Weinheim, Germany: Wiley-VCH; 1002 p. 
         Jousten K, Author. Thermal outgassing. CAS-CERN Accelerator School:Vacuum Technology; 1999 May 28-Jun. 3; Snekersten, Denmark. pp. 111-125 
         Koebel M M, Manz H, Mayerhofer K E, Keller B. 2010. Service life limitations in vacuum glazing: a transient pressure balance model. Solar Energy Materials and Solar Cells 94(6):1015-1024 
         Lenzen M, Turner G M, Collins R E. 1999. Thermal outgassing of vacuum glazing. Journal of Vacuum Science &amp; Technology A 17(3):1002-1017 
         Ng N, Collins R E. 2000. Evacuation and outgassing of vacuum glazing. Journal of Vacuum Science &amp; Technology A 18(5):2549-2562 
       
    
     O&#39;Hanlon J. F. 2003. A user&#39;s guide to vacuum technology, 3 rd  Ed. Hoboken, N.J.: John Wiley &amp; Sons. 516 p.
     Roth A. 1994. Vacuum sealing techniques. Woodbury, NY: American Institute of Physics. 845 p.   Van Den Bergh S, Hart R, Petter Jelle B, Gustaysen A. 2013. Window spacers and edge seals in insulating glass units: a state-of-the-art review and future perspectives. Energy and Buildings 58 (2013):263-280