Glasses- Furnaces Flashcards
Pot furnaces and what they are used for
Made of pre-fired high-quality clay (or Pt alloy for special compositions) and reused without cooling. 24h or 48h working schedule. Now for small quantities (1 ton) and special compositions. Artistic glass- hand worked, blowing and moulding. Optical glass- melting in pot and cooled, pot can be broken open if clay
Process for pot furnaces
Heat to 1350°C (liquidus 1000C) and for 16hours melt, refine and homogenise. Cool to 1100C and spend 2+ hours refining, homogenising, conditioning to working temperature
Main feature of cross-fired furnace
Regenerators transfer heat from hot waste gas to combustion air (and producer gas)
Sections of a cross-fired furnace
Doghouse, port, tank, port neck, throat, working end, regenerator, regenerator packing
Basic process in cross-fired furnace
Batch input through the doghouse at one end. Batch piles float forward slowly heated by flames above. Flames fire either across furnace or in U from back wall. Need time-temperature profile for melting, refining, homogenising. Melting in main tank. Cooled in working end before shaping. Glass residence time 24-48h. Melting end divided from working end by bridge wall, glass flows through a sunken throat under water-cooled pipes beneath surface. From working end glass taken along narrow channels (forehearths) to forming machinery
Dimensions and temperatures in CF furnace
Width 3-10m, length 7-50m, depth 1.25-1.75m. Main tank top temperature 1550C, bottom temperature 1300C. Cooled to 1150C in working end. Crown may reach 1620C at hot spot 2/3 along tank
Considerations for temperature in CF furnace
If too high in main tank there is excessive refractory wear and volatilisation from melt. If temperatures too low in working end there can be crystallisation
Operational stats for CF furnace
Operates 24h/day 365day/yr for 10+ years. Many 10s of millions of pounds to build. Output 50-1000ton/day
How is temperature monitored in CF furnace
Monitored by thermocouples/radiation pyrometers in crown and controlled by varying fuel input
What occurs due to back and front ends of tank being cooler?
Convection currents which aid mixing and stop forward batch pile drift
How accurate does temperature have to be in working end?
+/- 0.5°C (1150C)
Why can’t a CF furnace be turned off?
It must stay hot or it will crack
Fuels used for continuous melting
Used to use wood, coal, heavy fuel oil. Now mostly natural gas. Future use hydrogen or electricity
How do burners work in the furnace?
Combustion air enters through 3-8 ports along wall. Burners under or at sides of ports. Waste combustion gases exit ports opposite at 1650C and remove large quantities of energy
Electricity for continuous melting
Electrodes in melt boost output by 10-20%. All-electric furnaces for specialist applications, completely different design principles, no waste gas/CO2
How to reduce production of NOx
Oxyfuel firing uses pure oxygen in place of air to reduce N2 in flame. Does raise furnace temperatures and costs more. Means greatly reduced volume of waste gas and heat loss up chimney so regenerators not needed (major saving)
What is excess heat in waste gas partly recovered by?
Regenerators and recuperators
Regenerators
These are open piles of bricks through which waste and combustion gases pass alternately every 30mins. Achieve air preheats of 1100°C
Recuperators
Pass heat continuously from waste gas to air (tube within tube) across a dividing wall (metal or refractory). Achieve air preheats of 600°C (metal tubes) to 800°C (refractory tubes).
What is heat recovery good for?
Improves overall thermal efficiency from 20 to over 50%. Waste heat can also be used to preheat batch or produce electricity/hot water
Refractories used
Silica crown (single phase, high melting point, cheap).
AZS fused cast (alumina zirconia silica) has excellent resistance to attack by glass melt but is expensive (need to melt using electric furnaces to make).
Zircon paving (base which is cooler) made sintering zircon sand.
Aluminosilicates in cooler regions
MgO in regenerators to resist attack by Na2O in waste gas.
Insulating bricks over almost all structure
What is thermal efficiency?
Theoretical heat / actual heat
Decreases with age and varies with load on furnace
What is heat used for in theory?
Heat for batch reactions ΔH. Heating batch gases CO2. Heat to raise melt T. Using Culley can save first 2.
Why is water added to batch?
To wet it. This reduces demixing/segregation but increases energy cost because it evaporates.
Typical furnace energy use per ton
4000MJ per ton so thermal efficiency 60%
Principal sources of energy loss
Hot waste gases going up chimney (25%)
Smaller losses through furnace and regenerator chambers walls (15%)
Advantages of cullet
Environmentally friendly. 1% increase in cullet saves 0.25% energy (heat of reaction). Allows lower furnace temperature, saves batch, reduces water input and CO2 input. At over 60% cullet may also avoid need for/cost of electrostatic precipitators to trap particulate emissions
Issues of cullet
Colour balance of available cullet
Contaminants where the wrong types of glass are mixed up
Typical residence time for glass in container tank
24h but less for higher operating temperature
How can some material short circuit the system or stay in for longer?
Glass melt has many different flow paths as a result of convection currents and some return flow back through the throat measurable. Short circuit may only be in for 8h but some take 100h. Aids homogenisation but delays a colour change
Radiation conductivity
Glasses are semi-transparent to IR radiation meaning it can let through some of the energy. Means don’t just consider normal thermal conductivity (small) but also radiation conductivity (can be large)
Effective thermal conductivities of green and clear glass
Both larger than their true thermal conductivities. All increase with temperature but clear glass much more so
Mathematical formulation for radiation conductivity
λR=(16n^2σT^3)/3α
σ is Stefan-Boltzmann constant
α is absorptivity
T is temperature
Dependence of heat transfer on iron content of batch
Radiation thermal conductivity decreases exponentially with increasing FeO concentration. Can deliberately dope with Fe to get more consistent (stable) conductivity
