Glasses- Furnaces

Question 1

Pot furnaces and what they are used for

Answer 1

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

Question 2

Process for pot furnaces

Answer 2

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

Question 3

Main feature of cross-fired furnace

Answer 3

Regenerators transfer heat from hot waste gas to combustion air (and producer gas)

Question 4

Sections of a cross-fired furnace

Answer 4

Doghouse, port, tank, port neck, throat, working end, regenerator, regenerator packing

Question 5

Basic process in cross-fired furnace

Answer 5

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

Question 6

Dimensions and temperatures in CF furnace

Answer 6

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

Question 7

Considerations for temperature in CF furnace

Answer 7

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

Question 8

Operational stats for CF furnace

Answer 8

Operates 24h/day 365day/yr for 10+ years. Many 10s of millions of pounds to build. Output 50-1000ton/day

Question 9

How is temperature monitored in CF furnace

Answer 9

Monitored by thermocouples/radiation pyrometers in crown and controlled by varying fuel input

Question 10

What occurs due to back and front ends of tank being cooler?

Answer 10

Convection currents which aid mixing and stop forward batch pile drift

Question 11

How accurate does temperature have to be in working end?

Answer 11

+/- 0.5°C (1150C)

Question 12

Why can’t a CF furnace be turned off?

Answer 12

It must stay hot or it will crack

Question 13

Fuels used for continuous melting

Answer 13

Used to use wood, coal, heavy fuel oil. Now mostly natural gas. Future use hydrogen or electricity

Question 14

How do burners work in the furnace?

Answer 14

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

Question 15

Electricity for continuous melting

Answer 15

Electrodes in melt boost output by 10-20%. All-electric furnaces for specialist applications, completely different design principles, no waste gas/CO2

Question 16

How to reduce production of NOx

Answer 16

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)

Question 17

What is excess heat in waste gas partly recovered by?

Answer 17

Regenerators and recuperators

Question 18

Regenerators

Answer 18

These are open piles of bricks through which waste and combustion gases pass alternately every 30mins. Achieve air preheats of 1100°C

Question 19

Recuperators

Answer 19

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).

Question 20

What is heat recovery good for?

Answer 20

Improves overall thermal efficiency from 20 to over 50%. Waste heat can also be used to preheat batch or produce electricity/hot water

Question 21

Refractories used

Answer 21

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

Question 22

What is thermal efficiency?

Answer 22

Theoretical heat / actual heat

Decreases with age and varies with load on furnace

Question 23

What is heat used for in theory?

Answer 23

Heat for batch reactions ΔH. Heating batch gases CO2. Heat to raise melt T. Using Culley can save first 2.

Question 24

Why is water added to batch?

Answer 24

To wet it. This reduces demixing/segregation but increases energy cost because it evaporates.

Question 25

Typical furnace energy use per ton

Answer 25

4000MJ per ton so thermal efficiency 60%

Question 26

Principal sources of energy loss

Answer 26

Hot waste gases going up chimney (25%)

Smaller losses through furnace and regenerator chambers walls (15%)

Question 27

Advantages of cullet

Answer 27

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

Question 28

Issues of cullet

Answer 28

Colour balance of available cullet

Contaminants where the wrong types of glass are mixed up

Question 29

Typical residence time for glass in container tank

Answer 29

24h but less for higher operating temperature

Question 30

How can some material short circuit the system or stay in for longer?

Answer 30

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

Question 31

Radiation conductivity

Answer 31

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)

Question 32

Effective thermal conductivities of green and clear glass

Answer 32

Both larger than their true thermal conductivities. All increase with temperature but clear glass much more so

Question 33

Mathematical formulation for radiation conductivity

Answer 33

λR=(16n^2σT^3)/3α
σ is Stefan-Boltzmann constant
α is absorptivity
T is temperature

Question 34

Dependence of heat transfer on iron content of batch

Answer 34

Radiation thermal conductivity decreases exponentially with increasing FeO concentration. Can deliberately dope with Fe to get more consistent (stable) conductivity