Glass Manufacturing

Furnace refractory limitations: Glass melting furnaces are lined with special refractory bricks (silica, alumina-zirconia-silica, fused-cast refractories) withstanding 1600°C molten glass. These refractories are gradually eroded by molten glass (corrosion rate 1-8 mm/day depending on glass type and temperature). Thermal cycling damage: Repeatedly heating furnace from ambient to 1600°C and cooling causes catastrophic thermal shock cracking refractories. Refractory repairs require complete cooldown (7-14 days), rebuild (30-90 days), and reheat (7-14 days) = 1.5-4 months downtime costing ₹5-20 crore lost production. Campaign operation: Furnaces run continuously for "campaign" of 5-15 years (float glass 10-15 years, container glass 5-8 years) until refractory erosion necessitates rebuild. During campaign, production never stops—not even maintenance (hot repairs performed). Economic drivers: Avoiding shutdowns justifies significant redundancy and preventive maintenance. Example: Container glass plant installs duplicate fans for every critical application (annealing, cooling) accepting 2× capital cost to eliminate shutdown risk. Scheduled shutdown: Cold repair (furnace rebuild) planned years in advance, coordinated with market demand lulls. During rebuild, entire facility stops—opportunity for major maintenance on all equipment. Operational philosophy: Glass manufacturing operates 24/7/365 because stopping is economically catastrophic. All support systems (air handling, utilities, controls) must achieve same reliability.

Annealing relieves internal stresses in glass caused by rapid surface cooling vs slower interior cooling. Stress mechanism: When glass cools through transition temperature (~550-600°C for soda-lime glass), it solidifies. If surface solidifies while interior still molten, differential contraction creates permanent stress. Stressed glass weakens and shatters spontaneously or from minor impact. Annealing process: (1) Soak zone: Heat glass to annealing point (545-565°C for soda-lime) holding entire cross-section at uniform temperature. Viscosity low enough for stress relaxation via molecular flow. (2) Slow cooling zone: Cool at controlled rate (1-10°C/min depending on thickness) through strain point (~510°C) where glass becomes rigid. Rate must be slow enough that temperature remains uniform across glass thickness (thin glass cools faster than thick). (3) Fast cooling zone: Below strain point, glass is rigid—rapid cooling acceptable (saves lehr length). Cool to <100°C for safe handling. Airflow control: Annealing lehrs use forced convection (fans blowing air through electric or gas burners, then over glass) to control cooling rate precisely. Critical parameters: Airflow uniformity ±5% across lehr width (uneven flow causes temperature gradients = stress). Temperature control ±3°C (tighter than oven because stress is exponentially sensitive to temperature). Consequences of poor annealing: Container glass: Spontaneous breakage (bottle explodes on filling line or shelf). Flat glass: Cracks when cut or tempered. Optical glass: Birefringence (stress-induced optical distortion). System design: Multiple small fans distributed along lehr vs single large fan ensuring uniform flow, VFD control for precise airflow adjustment, redundant fans (annealing cannot stop during production).

Particulate matter: Batch carryover—raw materials (sand, soda ash) entrained in flue gas before melting. Concentration 100-500 mg/Nm³ uncontrolled. Composition: sodium sulfate (from soda ash decomposition), silica, volatilized borates. Regulatory limit <50-100 mg/Nm³ requiring ESP or bag filter. Nitrogen oxides (NOx): Formed at >1300°C from nitrogen and oxygen in combustion air. Glass furnaces operate 1400-1600°C producing 1,500-4,000 mg/Nm³ NOx uncontrolled. Regulatory limits 500-800 mg/Nm³ requiring combustion modifications (oxygen-fuel firing, low-NOx burners) or SCR (Selective Catalytic Reduction). Sulfur dioxide (SO2): From sulfate in batch (sodium sulfate clarifying agent) decomposing to SO2 + Na2O. Concentration 200-1,000 mg/Nm³. Limits 200-500 mg/Nm³ may require wet scrubber. Hydrogen fluoride (HF): Fluoride compounds used as refining agents volatilize as HF gas. Highly corrosive, toxic. Limits <5 mg/Nm³ requiring wet scrubber with caustic (NaOH). Volatile heavy metals: Lead, arsenic, selenium (used in specialty glasses) volatilize requiring high-efficiency particulate control. Control technologies: ESP (Electrostatic Precipitator): 95-99% particulate removal, no SO2/NOx/HF removal. Capital ₹2-5 crore, operating cost low. Wet scrubber: Removes particles + acid gases (HF, SO2). Capital ₹3-8 crore, operating cost high (water, chemicals, sludge disposal). SCR for NOx: Ammonia or urea injection over catalyst at 300-400°C reducing NOx to N2. Capital ₹5-15 crore. Oxy-fuel furnaces: Use pure oxygen vs air for combustion eliminating 75% of flue gas and reducing NOx 80-90%. Emerging technology.

Radiant heat from furnaces: Glass melting furnace at 1600°C radiates 150-300 kW/m² from crown (roof) and walls. Workers near furnace exposed to heat flux 5-15 kW/m² causing heat stress without cooling. Product heat release: Molten glass (1100-1400°C) transferring to forming section releases enormous heat. Float glass ribbon: 400 TPD glass leaving furnace at 1100°C cooling to 600°C (annealing input) releases ~10 MW thermal energy radiating into workspace. Container glass: Gobs (molten glass chunks) at 1100°C formed in molds, each bottle releases heat cooling from 600°C to 100°C. Workspace temperatures: Without cooling, areas near furnaces reach 50-65°C ambient making work impossible. Target <35°C for sustainable work. Cooling strategies: (1) Roof exhaust: Large propeller fans (10,000-50,000 CFM each, 10-20 fans per facility) exhausting hot air rising to roof by natural convection. Removes 40-60% of heat load. (2) Evaporative cooling: Coolers dropping air temperature 8-15°C through water evaporation. Effective in dry climates. (3) Spot cooling: High-velocity air jets directed at workers providing personal cooling without cooling entire space (energy efficient). (4) Radiant barriers: Reflective shields blocking radiant heat from furnaces. (5) Process heat recovery: Capture process heat for beneficial use (preheating combustion air, space heating in winter, absorption chillers for cooling). Energy balance: Glass plant 400 TPD consumes ~200 GJ/hr fuel energy. 60-70% absorbed by glass melting, 20-25% lost in flue gas (recovered by regenerators), 10-15% radiated to workspace = 20-30 MW requiring removal. Cooling fans + evaporative coolers consume 200-500 kW electrical power.

Stones (solid inclusions): Unmelted batch particles, refractory debris, or crystallized glass (devitrification) appearing as solid spots in glass. Air system cause: Inadequate particulate control on recirculated air allowing dust to settle on molten glass surface, getting trapped during forming. Seeds (gas bubbles): Tiny bubbles trapped in glass from incomplete fining (gas removal during melting). Air system cause: Combustion instability from poor air-fuel ratio control causing temperature fluctuations disrupting fining process. Cords (composition variations): Striae or streaks of different refractive index caused by incomplete homogenization of molten glass. Air system cause: Furnace temperature imbalances from uneven combustion air distribution creating unmixed regions. Thermal shock cracks: Glass cracking during cooling from excessive temperature differentials. Air system cause: Non-uniform cooling airflow creating hot/cold spots, uneven annealing air distribution, sudden airflow changes from fan hunting or control instability. Checks (surface cracks): Fine surface cracks from too-rapid cooling. Air system cause: Excessive cooling air velocity, cooling air temperature too low, improper cooling curve from poor control. Blister (subsurface bubbles): Blisters forming from volatilization of contaminants. Air system cause: Oil or dust contamination from dirty air systems depositing on glass. Prevention: (1) High-efficiency filtration on all process air (HEPA for critical applications like LCD glass). (2) Precise control of combustion air (±2% flow, ±1% O2) preventing temperature excursions. (3) Uniform annealing air distribution (±5% across width). (4) Stable controls preventing hunting or sudden changes. (5) Clean construction (stainless steel, no rust or flaking paint contaminating air). (6) Regular maintenance (duct cleaning, filter changes, sensor calibration). Quality impact: Single defect per ton of glass can cause entire batch rejection. Food/pharma glass: Zero defects tolerance. Auto glass: Strict optical quality. Premium on reliable, clean air systems.

Combustion air (FD fan): 400 TPD float glass furnace requires 250,000 Nm³/hr combustion air. At ambient: ~300,000 m³/hr at 150 mmWC = 450 kW motor. Annual consumption = 450 kW × 8,500 hrs (97% uptime) = 3.8 million kWh/year. At ₹7/kWh = ₹2.66 crore/year. Flue gas (ID fan): 280,000 m³/hr (at 500°C after heat recovery) at 200 mmWC = 600 kW. Annual = ₹3.57 crore. Annealing lehr circulation: 5 zones × 40,000 m³/hr × 80 mmWC = 5 × 35 kW = 175 kW total. Annual = ₹1.04 crore. Product cooling: 4 cooling stations × 50,000 CFM × 50 mmWC = 4 × 60 kW = 240 kW. Annual = ₹1.43 crore. Workspace cooling/exhaust: 15 roof fans × 25,000 CFM × 15 mmWC = 15 × 15 kW = 225 kW. Annual = ₹1.34 crore. Total fan power: 1,690 kW continuous consuming ₹10 crore/year electricity for 400 TPD plant. Percentage of facility: Glass furnace consumes ~200 GJ/hr thermal = 55 MW thermal equivalent. Fans 1.7 MW electrical = 9-12 GJ/hr. Fans represent 5-6% of total energy vs 10-15% in other industries (low because thermal energy dominates glass making). Optimization opportunities: (1) Oxygen-fuel conversion eliminating 75% of flue gas volume → 70% reduction in ID fan power (saves ₹2.5 crore/year) but requires ₹20-40 crore capital for oxygen plant. Payback 8-15 years. (2) VFD on combustion air fans with O2 trim control: Reduce excess air from 15% to 5-8% → 10-15% fan power reduction (saves ₹40-60 lakh/year). Investment ₹30-50 lakh, payback <1 year. (3) High-efficiency impellers: Backward-curved replacing radial designs → 8-15% power reduction (₹80 lakh-₹1.5 crore/year). Premium cost ₹1-2 crore, payback 1-3 years. (4) Waste heat recovery from cooling air powering absorption chillers offsetting workspace cooling loads. Bottom line: Fan energy is significant—careful optimization justified.

Extreme heat: Molten glass 1100-1600°C causes severe burns. Radiant heat exposure causing heat stress, heat stroke. Air system role: Adequate workspace cooling maintaining tolerable temperatures, local exhaust capturing radiant heat, emergency cooling for heat stress. Explosion risk: Natural gas or oil fuel for furnaces. Leak accumulation in ductwork → explosion if ignited. Prevention: Gas detection, proper ventilation preventing accumulation, purge cycles before furnace startup clearing combustibles. Oxygen-enriched atmospheres: Oxy-fuel furnaces use pure O2. Elevated oxygen concentration (>23.5%) makes fires burn intensely. Oil-soaked clothing in O2 atmosphere → flash fire from spark. Prevention: No combustibles (oil, grease, organics) in O2 areas, leak detection, proper system design preventing backflow. Acid gas exposure: HF (hydrogen fluoride) from batch extremely corrosive and toxic. 30-50 ppm exposure for minutes → lung damage. Chronic exposure → fluorosis (bone/teeth damage). Control: Enclosed systems, fume extraction, wet scrubber capturing HF, respirators if maintenance requires entry. Particulate exposure: Silica sand (SiO2) used in batch → silicosis risk from dust. Soda ash dust → respiratory and skin irritation. Control: Enclosed handling, dust collection, proper housekeeping. Hot glass projection: Molten glass splashing or glass breaking explosively from thermal shock. Protection: Safety glasses, face shields, protective clothing, guards. Confined spaces: Entry into regenerator chambers (refractory checkerwork for heat recovery), flues, dust collectors for cleaning. Oxygen-deficient atmospheres, heat, toxic residues. Protocol: Permit system, atmospheric testing, forced ventilation, standby rescue. Air system contribution to safety: (1) General ventilation diluting fugitive gas leaks. (2) Fume capture preventing worker exposure. (3) Cooling preventing heat stress. (4) Make-up air preventing negative pressure pulling combustion products into workspace. (5) Proper design (gas-tight ductwork, explosion venting) preventing accidents.

Float glass (architectural/automotive flat glass) invented 1950s by Pilkington revolutionized industry. Process: (1) Batch preparation: Mix silica sand (70%), soda ash (13%), limestone (9%), dolomite, plus cullet (recycled glass) creating homogeneous mixture. (2) Melting furnace: Load batch into furnace at 1600°C. Raw materials melt forming molten glass, reactions releasing CO2/SO2. Fining (gas removal) bubbling glass to release trapped gases. Conditioning reducing temperature to 1100°C achieving correct viscosity for forming. (3) Float bath: Molten glass (1100°C, specific gravity 2.5) poured onto molten tin bath (232°C melting point, density 7.0). Glass floats on tin surface naturally forming perfectly flat, parallel surfaces with brilliant fire finish (both sides). Controlled atmosphere (N2+H2) preventing tin oxidation. Glass ribbon 3-25mm thickness × 3-4 meters wide advancing at 8-15 meters/minute. Thickness controlled by adjusting glass flow rate and ribbon pulling speed (thinner glass = faster pull). (4) Annealing lehr: 100+ meter long tunnel with controlled temperature zones gradually cooling glass from 600°C to <100°C. Prevents thermal stress. (5) Cutting and inspection: Automated cutting to size (jumbo sheets 3.21 × 6 meters), quality inspection detecting optical distortions, stones, scratches. Accept/reject. Stack on racks. Air handling requirements: Float bath atmosphere control: Slight positive pressure of N2+5%H2 preventing air ingress (would oxidize tin). Recirculation fans 20,000-40,000 m³/hr. Tin vapor exhaust capturing evaporated tin (condenses as fine powder, explosive hazard if accumulated). Lehr cooling: Multiple cooling zones each with 30,000-100,000 m³/hr fans providing precise temperature control ±3°C. Furnace combustion: 200,000-350,000 Nm³/hr air for oxy-gas or air-gas burners. Why float process dominant: Produces optically perfect glass (flat within 0.5mm over 3+ meters), both surfaces fire-polished (no grinding needed), high throughput (400-900 TPD per line), economics of scale. >90% of flat glass worldwide made by float process.