The main types of furnaces include:
Pot furnaces(discontinuous)
Day tanks(semi-continuous)
Recuperative/unit type melters
Cross-fired regenerative furnaces–throat or neck/waist design
End-port fired regenerative furnaces–throat design
Oxygen-fired unit melters
Special(segmented)melter(LoNOx®,Flex®melters)
All-electric furnaces
1.Discontinuous furnace(day tanks and pot furnaces)
The following actions take place(generally in a one-day cycle within discontinuous melting furnaces:
Melting tank or pot is charged with mixed raw material batch
This batch is heated to the desired temperature
The glass is melted,fined,homogenized and subsequently cooled down to the working temperature to allow forming by the craftsman or semi-automatic machines taking portions(gobs)of glass from the glass melt pot
2.Continuous glass furnaces
Usual synonyms for a continuous furnace are glass-melting tank or tank furnace.
These furnaces are applied for:
a.Container glass production
b.Flat glass(Float&Rolled)production
c.Most tableware glass production
d.Fiber&glass wool production
e.Most specialty glass production(tubes,display glass,glass-ceramics,lighting bulbs,..)
These furnaces not applied for:
a.Most hand-made glass
b.Vitreous silica
c.Optical glass fibers
3.Furnaces Continuous glass furnaces Characteristics
Tank of refractory material,continuously charged with mixed batch
Heat transfer from combustion chamber using fossil fuel(mostly natural gas)firing with preheated air or oxygen
All basis process steps in different zones or sections of furnace
Continuous operation,during campaigns 5-15 years
Indefinite number of trajectories from batch charger to exit of furnace(throat or canal).
These furnace types are suitable for the mass production of glass
The furnace melting capacity(glass pull)usually is expressed in the number of(metric)tons of glass melted per day(24 hours)
Depending on the furnace and type of glass produced,the pull can vary from~20 tons per day(TPD)up to>700 TPD
Within the melt,currents(glass melt flow patterns)are being generated,both by pull&by free convection
Extra mixing by the application of bubbling or electrodes
Possibility to boost energy input using electrodes
Electric current in melt will release latent energy
Large number of trajectories of material in tank:wide residence time distribution&quality differences depending on route
Temperature gradients in melt:higher levels(close to the surface)are generally hotter than bottom glass melt
Weirs or dams are optionally applied to bring bottom glass to upper glass melt layers
Using air preheating(regenerators/recuperators)or pure oxygen
A melting furnace consists of:
a.Melting tank(glass melt bath)
b.Superstructure(combustion chamber)
c.Throat as connection between the melting end and the riser that brings the molten glass in the refiner,working end or distributor
d.Neck in case of float glass production,between the melting end and working end
e.Working chamber(working end,gathering end,nose,refiner)
f.Heat exchangers:regenerators or recuperators
4.Continuous glass furnace components
Designations of glass furnace components(tank furnace,cross fired,dimension scale is not meant to be correctly presented).
5.Regenerative furnaces
A regenerator consists of a regenerator chamber in which a checkerwork(or just checkers)of refractory bricks has been stacked.
In one cycle the checker is heated up by flue gases,subsequently in the following stage(20-30 minutes)the heat is transferred to combustion air
These furnaces are provided with 2 or more(an even number)regenerators
In principle the optimum half-cycle time depends on the pull of the melting tank(thermal load)
During the burner reversal,lasting about 30-60 seconds,there are no flames within the furnace.
The reversal period(no-firing interval)should be as short as possible to avoid too much cooling down of the furnace
6.Cross-fired regenerative furnaces
The regenerators are placed on the side of the furnace
The furnace can be equipped on both sides with 4 up to 8 burner ports(per side)depending on furnace size.
The profile of heating(fuel distribution among the burners located along the sidewalls)determines location and size of the hot spot area(primary fining zone)in the glass melt.
7.End port-fired(or U-flame)regenerative furnaces
Burners(2 to 4 burners at each port)and the regenerator chambers are connected at the back wall side of the superstructure.
The combustion of fuel&preheated air from one regenerator chamber takes place:flames starting from the burner nozzles and extending almost over the length of the furnace
Flame/Combustion direction turns at front wall
Less structural heat losses compared to cross fired regenerative furnaces(combustion gases have longer residence time)
8.Recuperative furnaces
Recuperator:heat exchanger in which heat is transferred from the flue gases to the combustion air in co-current or in counter-current flow
Recuperative furnaces are provided with one or two recuperators
Most recuperators are made from high temperature resistant steels,like chrome nickel steel(or chromium-nickel-aluminum steels)
Because heat transmission in this type of recuperators is based mainly on radiation,these heat exchangers are called radiation
Recuperators are used to pre-heat the combustion air
The hot flue gases are send through the recuperator to heat the combustion air
Investment costs are relatively low
No cycle(firing reversal)system,therefore continuous process conditions
Controllable temperature profile along the length,due to the large number of burners which might be controlled independently(5-15 burners per side)
The furnace is easily accessible(also for an end-port fired regenerative furnace the side-walls can easily provided with peepholes)
The combustion chamber has a relatively simple construction and it can be sealed reasonably well(no large burner port)
But:preheating of the combustion air is less efficient than for regenerative furnaces
9.Oxygen-fuel fired furnaces(Oxy-fuel)
The fuel is fired without nitrogen in the applied oxidant(pure oxygen)(lower volumes of flue gases,less diluted)
In general,oxy-fuel glass furnaces have the same basic design as recuperative glass melters,with multiple lateral burners and a limited number of exhaust port(s).
Most oxygen fired glass furnaces hardly utilise heat recovery systems to pre-heat the oxygen supply to the burners(there are some developments in oxygen and natural gas preheating using the heat contents of the flue gases)
Burners positioned in special burner blocks in the sidewalls
Typically only 4 to 6 burners per sidewall are installed.
NB:Burners from opposite sidewalls are preferably not placed in one line.This would lead to instable flame tips influencing each other.
Advantages
a.cheaper furnace designs
b.lower specific NOx emissions(in kg NOx/ton molten glass);
c.smaller flue gas volumes
d.smaller footprints for furnace system
e.reduction in fuel consumption
Drawbacks
1.oxygen costs may exceed the reduction in fuel costs
2.oxygen-firing require higher refractory quality superstructures
10.All-electric melting furnace
The heating is not provided by combustion systems,but by electric energy provided by electrodes plunging in the melt
Below is an example of an all-electric furnace with top electrodes
Melt tank and superstructure construction
Arrows indicate supports of superstructure by steel construction
Downstream glass melting tank
Temperature of the glass melt,flowing from the melting-end through the throat into the riser and then into the distributor/working-end/refiner,is too hot for forming.
Required cooling:by refiner and feeders by 200 to 300oC to approximately the working temperature.
Glass portions or gobs or a continuous flow of glass at this lower temperature level are required for a well performing forming process.
