Specifiers Guide to Architectural Glass
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SPECIFIERS GUIDE TO ARCHITECTURAL GLASS Introduction The Glass Association of North America (GANA) provides the organizational structure for addressing the needs of a diverse membership. Comprised of six Divisions and an Affiliate classification, GANA puts members in regular contact with their peers, customers, and suppliers in an environment that facilitates the nurturing of an understanding essential to strong, effective working relationships. GANA provides members with premiere educational programs, publications, networking opportunities, meetings, and conventions. The Flat Glass Manufacturing Division (FGMD) is comprised of manufacturers of flat glass maintaining float glass plants in North America and associate members supplying goods and services to the flat glass industry. GANA has developed this Guide to assist those involved in the specification, selection, and use of glass in architectural applications. Additional GANA reference manuals and informational bulletins should be a part of every fenestration product reference library. See page 16 for a list of additional resources. Table of Contents Chapter One: Float Glass Manufacturing Process Page 2 Chapter Two: Types of Glass Page 4 Chapter Three: Physical Properties and Glass Strength Page 8 Chapter Four: Thermal and Solar Optical Properties of Float Glass Page 10 Appendix 1: GANA Flat Glass Manufacturing Division Member Companies Page 14 Appendix 2: Referenced Standards Page 15 Appendix 3: Technical Resources Page 16 Appendix 4: Referenced Organizations Page 17 Disclaimer The Glass Association of North America (GANA) has produced this Specifiers Guide to Architectural Glass solely to provide general descriptions concerning the basics of glass. It is the responsibility of the user of this Guide to ensure that glass is selected and installed by competent professionals in compliance with all relevant laws, rules, regulations, standards and other requirements. GANA disclaims any liability for any loss or damage of any kind arising out of the use of this Guide and all those using it agree, as a condition of its use, to release GANA from any and all liability, loss or damage of any kind or nature arising out of or relating in any way to its use. Users of this Guide understand that GANA is not responsible for any errors or omissions of any kind contained in this Guide and that GANA does not design, develop, manufacture, guarantee or make any express or implied representations or warranties as to fitness, merchantability, patent infringements or any other matter respecting any products, processes or equipment referred to in this Guide. GANA does not guarantee any results of any kind relating to the use of this Guide. GANA expressly reserves the right, in its sole discretion, to revise, amend, or otherwise modify the Guide from time to time as it sees fit and to do so without notice to prior recipients of the Guide. The standards referenced in the GANA Specifiers Guide to Architectural Glass are under the jurisdiction of a number of organizations and agencies and are continuously being revised. The documents referenced in this Guide are those in effect as of July 2004. The most recent standards should be referenced. Full names of reference standards and publishing entities are listed in Appendices 2 and 4. © GANA 1 2005 SPECIFIERS GUIDE TO ARCHITECTURAL GLASS Chapter One: Float Glass Manufacturing Process The majority of glass used today in architectural applications is float glass. Float glass is manufactured in a continuous process by melting glass batch (i.e., soda, lime, silica sand, and other materials) and, when melted, floating the glass on a bath of molten tin. The glass is slowly and carefully cooled in an annealing lehr to produce glass with exceptionally parallel surfaces, high optical quality, and fire-finished surface brilliance. Float glass may be used as produced in a wide variety of applications, or it may serve as the base material for other products, such as fully tempered glass, heat-treated glass, laminated glass, insulated glass, mirrored and decorative glass. Non-architectural end-users include the automotive, aircraft, and transportation industries. Float Glass Manufacturing Facilities Float glass manufacturing facilities operate continuously, 24 hours per day, 365 days per year. Steady- state operations characterize the float glass industry; no hourly or seasonal variations are expected under normal conditions. The manufacturing process starts with the arrival of raw materials in bulk quantities by rail and/or truck. The raw materials are unloaded and stored in a batch house where they are weighed and mixed with broken glass called cullet. The mixed batch is then conveyed to a furnace where the raw materials are melted. Once melted, the glass flows from the furnace into a tin bath. The molten glass is drawn through the tin bath and then enters an annealing lehr where it is carefully cooled in preparation for cutting. Cut sizes of glass are placed on racks or boxes before being shipped. Unused glass is crushed and returned to the batch house to be reused as cullet. Batch House The most common raw materials used to make float glass are silica sand, soda ash, limestone, dolomite, salt cake, rouge, charcoal, and cullet. In the batch house, raw materials are dumped into hoppers and fed by conveyor to a bucket elevator where they are discharged into storage bins. Each raw material is weighed individually on highly accurate industrial scales and then checked on a totalizer scale prior to discharge into a mixer. After the raw materials are thoroughly mixed, a measured amount of wetting agent is added, cullet is blended into the mix, and the batch is conveyed to a large storage hopper over the melting furnace feeder. Cullet improves the melting characteristics of the raw materials, decreases the energy consumption of the furnace, improves the quality of air emissions, and increases production yields. Furnace A typical float glass furnace has a melting capacity of between 300 and 600 tons per day. The batch is fed, in blanket form, into the furnace. A very accurate level detector, located near the furnace frontwall, controls the operation of the feeder. Flames, emanating from ports in the sidewalls of the melting furnace, melt the batch. Float glass furnaces typically operate at a temperature of 2900 degrees Fahrenheit. The batch is gradually refined in the melting furnace; by the time the batch reaches the furnace backwall, it is degassed, uniform in composition, and free of unmelted batch. This homogeneous blend is then delivered to the tin bath in a constant pouring action. Tin Bath The tin bath is an electrically heated forming oven. A continuous ribbon of molten glass flows onto and across the surface of a pool of molten tin in the bath at approximately 1900 degrees Fahrenheit and is transported along the length of the tin bath. At the tin bath exit, the glass, although still approximately 1100 degrees, is solid enough to be removed from the tin bath without marking the glass surface. © GANA 2 2005 An inert nitrogen atmosphere, made slightly reducing by the addition of small amounts of hydrogen, carefully seals and maintains the tin bath chamber under positive pressure. This, in turn, maintains a clean, pristine surface on the tin that would otherwise rapidly oxidize in the air. The glass, when flowing over the tin, forms a ribbon with virtually flat, parallel surfaces. The glass is drawn through the tin bath at different speeds depending upon the thickness of the glass being manufactured. Lehr The annealing lehr cools the glass ribbon from approximately 1100 degrees Fahrenheit to approximately 200 degrees in a precise, uniform manner to prevent residual stresses. The lehr uses small amounts of electric heat to keep the edges of the glass ribbon from cooling at a faster rate than the center. The lehr requires special rolls and drive systems as well as a sophisticated temperature control system to accurately maintain the required cooling rate. The glass emerges from the lehr in a continuous ribbon slightly above room temperature. Cutting and Packing After exiting the lehr, the glass is ready to be inspected, cut, packed, and shipped. First, the glass is inspected for defects. Glass containing defects is crushed and returned to the batch house to be reused as cullet. The glass ribbon is cut to the required dimensions using computer controlled high-speed cutters. The edge trim is removed, crushed, and returned to the batch house for reuse as cullet. The glass is placed in boxes or racks for shipment to the market. Recycling Float glass manufacturers typically recycle all of their own cullet generated during the float glass manufacturing process. Typically, during any production period, 15 to 30% of the total batch consists of cullet. Because of the high quality requirements, the float glass manufacturing process does not lend itself to the introduction of scrap glass from offsite sources. The possibility of contaminants and varying colors and densities are a significant deterrent. Proprietary company mixtures, tinting, and glass coatings further complicate the acceptance of scrap glass from offsite sources. However, there are glass recycling programs for scrap glass that is unacceptable for use as cullet. To see a short seven-minute film on the float glass manufacturing process, visit http://www.glasswebsite.com/video.html. © GANA 3 2005 Chapter Two: Types of Glass The Design Professional's Responsibility In the design and use of architectural glass, the responsible design professional must carefully consider the performance characteristics of glass as they relate to construction requirements. Safety glazing laws and local municipal building codes may set minimum requirements that do not relate to the specifier’s initial design criteria (i.e., wind loads, thermal stresses, solar or optical properties, and aesthetic considerations). In addition, other issues may affect the design criteria, such as break patterns, fall-out characteristics, acoustical insulation, and security demands.