Improved Glassmaking with Microtherm®
The benefits from the use of a microporous insulation in glass making.
There is a growing global awareness of the impact of industry on our environment. Terms such as “greenhouse gases” and “global warming” are now in every day use by politicians and scientists. Also the phrase “carbon footprint” which is a measure of how our activities based on the consumption of energy created from the combustion of fossil fuels can be related to the amount of carbon dioxide emitted.
Energy is the key to industry and the conservation of energy is a key factor in reducing the environmental impact of industry. It goes without saying that a reduction in the energy consumption of a manufacturing operation will also have a dramatic impact on the profitability of that operation.
Glass making is an energy intensive process with between 75% and 85% of the energy consumed during the melting stages. Any reduction in energy losses in the various stages of glass manufacture will show benefit in two ways. First, energy is the most expensive consumable in the process and energy savings are direct cost savings. Second, by making the process more thermally efficient the viscosity of the molten glass can be controlled more closely. This in turn makes quality control easier resulting in a further direct cost saving through scrap reduction.
Comparison TC chart
In simple terms, energy losses can be controlled by enclosing all hot equipment in an efficient thermal insulation. To optimise the thermal efficiency of the process, the insulation must block all of the modes of heat transfer, particularly infra red radiation which is the means by which most of the heat is transferred at high temperatures. The most efficient insulation currently available for use in hot manufacturing such as glass making is Microtherm® microporous insulation which can be as much as four times more effective than conventional insulation materials. In practical terms this means that a Microtherm® system can be only one quarter of the thickness for equivalent thermal control. This becomes crucially important wherever space is limited.
Heat will move from any region of high temperature to a lower temperature. The rate at which heat flows from hot to cold depends on many factors, but there will always be some flow. A layer of thermal insulation positioned between hot and cold regions will impede the rate of heat transfer. The thermal conductivity of a material is a physical property which describes its ability to transfer heat. The lower the thermal conductivity, the more effectively the material acts as an insulation. Microtherm® maintains an exceptionally low thermal conductivity across its full working temperature range. It is capable of sustained and stable exposure to 1000 °C (1832 °F). It offers even better insulation protection than still air.
The transfer of heat from a region of higher temperature to a region of lower temperature occurs by three basic mechanisms.
- Conduction in a solid, a liquid, or a gas is the movement of heat through a material by the transfer of kinetic energy between atoms or molecules.
- Convection in a gas or a liquid is the bulk movement of fluid caused by the tendency for hot areas to rise due to their lower density.
- Radiation is the dissemination of electromagnetic energy from a source. This does not require any intervening medium and occurs most efficiently through a vacuum.
- Generally, all three mechanisms work simultaneously, combining to produce the overall heat transfer effect.
A microporous insulation is by definition around 90% air, but the air is contained in minute cells, or pockets, that are smaller in average size than the mean free path of an air molecule. In heat transfer, gaseous conduction occurs when gas molecules collide and transfer their kinetic energy. The mean free path is the average distance they need to travel before they hit another molecule. If the collisions are prevented, as in a microporous insulation, heat transfer through the gas is dramatically reduced.
The typical dimension of the mean free path of an air molecule at STP is around 93 nm which is equivalent to 3.66 x 10-6 inches.
The very small size of the pockets of air in microporous insulation also serve to prevent any heat transfer by fluid flow due to convection.
The main constituent of a microporous insulation is usually pyrogenic silica which is present as very small amorphous particles. In Microtherm® these range in size from 5 – 25 nm and the silica has a low intrinsic thermal conductivity of around 1.4 W/m.K, meaning it is a good insulation anyway. By size comparison, the diameter of a human hair can be anything from 1000 to more than 7000 times bigger. These small particles chemically bond to form long particle chains which then tangle around each other as the insulation is mixed and formed.
On a microscopic scale they form long convoluted heat conduction paths through the insulation. Now, solid conduction of heat through a material occurs when adjacent molecules vibrate together and transfer
energy and it is influenced by two separate dimensional factors. The rate of solid conduction is directly proportional to the cross sectional area of the conduction path, and inversely proportional to the length of that conduction path. In Microtherm®, as with other microporous insulations, all the properties of the silica structure combine beneficially to give an exceptionally low rate of solid conduction.
To give Microtherm® integrity for moulding and machining and to give it handling strength it also contains a small percentage of chopped glass filament reinforcement. The insulation is classified by the World Health Organisation as free of respirable fibres as defined in the European Dangerous Substances Directive Amendment, 97/69/EC.
The other more important ingredient is the opacifier which is a fine mineral oxide powder that gives the insulation the ability to block the movement of infrared radiation almost completely. We know from the laws of physics, from the Stephan-Boltzmann fourth power law to be precise, that radiative heat losses from a surface are directly proportional to the fourth power of the temperature difference. At temperatures above 100 °C (around 212 °F) radiation becomes the dominant mode of heat transfer and increases rapidly with further increase in temperature.
Infrared radiation is a form of electromagnetic radiation with a wavelength longer than visible light but shorter than microwave. It is just outside the red end of the visible spectrum and exists over a range of wavelengths divided into “near”, “mid”, and “far” IR. Far infrared waves are thermal and any object which has a temperature above absolute zero will radiate in the infrared.
The small particles of the mineral oxide opacifier are dispersed uniformly throughout the Microtherm® and work by refracting (bending) the IR wave at the particle surface and changing its direction. The particle size is close to the wavelength of the IR to optimise the effect. Repeated scattering of the wave occurs, mostly close to the surface of the Microtherm®. The efficiency with which this scattering occurs means that Microtherm® effectively blocks the transmission of IR radiation and is the reason for its remarkable high temperature performance.
All of these facts present an overview of the nanotechnology scale of physics relating to a microporous insulation. They also demonstrate that a more appropriate generic name for the material would be a nanoporous insulation.
Although glass has the appearance of a solid it is actually a supercooled liquid. At the molecular level, the chains of molecules which form the glass interlink with each other when hot, then, as cooling occurs, the chains cannot untangle themselves in order to reach a stable crystalline structure. Whereas true solids melt sharply at a defined temperature, when glass is heated it progressively softens. This characteristic of glass allows good control of viscosity when it is molten by close control of temperature and by minimizing any temperature gradients across the bulk of the molten glass.
The largest use of glass around the world is in the manufacture of bottles with millions being made every day on automated plants. Tight quality control and consistent product size and weight are of paramount importance. Throughout these plants the control of heat losses will ensure the most efficient and cost effective manufacture.
Microtherm® has potential use at every stage due to the variety of product forms available including both rigid block and panel, flexible panel, and moulded forms for insulation of pipelines and ducts.
The molten glass stream is conditioned for homogeneity in the forehearth. Microtherm® insulation on the cold face of a forehearth structure reduces weight, reduces energy losses, minimises temperature gradients in the molten glass, and reduces the surface temperature of the structure for safe personnel access.
The use of Microtherm® pre tailored insulation in a feeder bowl directly affects the quality and cost of the glass finished product. A more uniform temperature is maintained within the bowl giving an even glass flow through the orifice. The resultant glass gob will have a consistent size and a minimum weight.
| Calculated cold face temperatures and heat flow through different insulation arrangements. |
Glass temp
(°C) |
Refractory bowl (mm) |
Jargal M (mm) |
Ceramic fibre (mm) |
Microtherm® (mm) |
Cold face temp
(°C) |
Heat flow (W/m²) |
| 1120 |
38 |
65 |
- |
- |
435 |
16,537 |
| 1120 |
38 |
52 |
- |
13 |
177 |
2,912 |
| 1120 |
38 |
- |
65 |
- |
171 |
2,751 |
| 1120 |
38 |
- |
52 |
13 |
136 |
1,862 |
| 1120 |
38 |
39 |
- |
26 |
117 |
1,458 |
| 1120 |
38 |
- |
39 |
26 |
104 |
1,206 |
Assumed conditions : Still air, vertical face, emissivity = 0.9, ambient temperature = 20 °C.
Temperature limits: Microtherm® Super G 1000 °C continuous.
|
The use of Microtherm® insulation in feeder bowl covers and around the orifice ring contributes to the accurate control of temperature and glass gob size. Because of the versatility of shape and form in the Microtherm® range, customized components allow fast installation and a precise fit. Standard kits are held for all well known feeder bowl designs. The fast availability of insulation systems in an optimised size and form is critical for minimizing maintenance down time.
Recuperator, Photo courtesy of Perrier.
With a hot air recuperator, it is important that heat loss from the piping system supplying hot air at 750 °C to the burners is kept to an absolute minimum. In the unit above for which all piping was insulated with a combination of Microtherm® Slatted Panel and Microtherm® Moulded Pipe Section, the temperature drop between the recuperator to the final burner was
below 30 °C.
Weight of insulation is important as all piping is supported off the recuperator structure. The Microtherm® system is both thinner and lighter than conventional insulation alternatives.
A further benefit from the Microtherm® was the stability of the insulation over a long period of time with continuous high temperature exposure. During maintenance, after 10 years of continuous operation, the only visible change was a slight breakdown of the woven E-glass covering on the exposed face of the product. The Microtherm® itself suffered no deterioration. A common problem with fibrous insulations and mineral wool is a progressive deterioration of the binder above 400 °C. This can cause a partial collapse of the insulation and a corresponding drop in performance.
With furnace temperature of up to 1550 °C and discharge temperature into the float at around 1100 °C, flat glass manufacture is very energy intensive and control of heat losses is essential to control product quality.
In a typical example with a Microtherm® customer with a glass temperature of 1300 °C, the addition of a 22 mm thick Microtherm® layer around the structure resulted in a drop of surface temperature on the sides from 159 °C down to 114 °C, and on the bottom from 184 °C down to
130 °C. Overall heat loss was reduced by around 45%.
A big furnace can hold 2,000 tonnes of molten glass and the compressibility of the bottom insulation layer becomes critical in order to maintain the thermal performance. Microtherm® insulation has excellent resistance to compressive loads and shows only slight compression without adverse effect on thermal performance in furnace installations. For example, a load of 100 KN/m2 results in just 5% compression.
Because the thermal conductivity of Microtherm® varies only very slightly over a wide density range, the applied load from an operating glass furnace does not result in any noticeable loss of thermal performance of the insulation system.
With modern advances in float bath technology combined with the precise temperature control achieved using efficient thermal insulation it is possible to manufacture flat glass sheet as thin as 0.3 mm and as thick as 25 mm. By controlling heat losses throughout, optimum energy efficiency is targeted for minimum environmental impact.
Approximately 70% of the float glass made globally is for windows in buildings, 10% goes for automotive glass applications, and the remainder goes for mainly high technology use in high definition display screens and other electronics applications. These specialist products demand extremely tight temperature control during manufacture to ensure optimum glass quality and uniformity of thickness. Controlling the thermal efficiency of the process by high performance insulation is the preferred approach and the best solutions available utilise Microtherm® microporous insulation.
The business of the Microtherm Group is based largely around the creation of complete thermal insulation solutions. In the example below, the customer wished to use a furnace for both clear glass and coloured glass. In order to achieve this versatility it was necessary to increase the bottom temperature from 690 °C to 730 °C to promote improved temperature homogenisation.
The solution was to mount Microtherm Panel on lightweight, easily mounted / demounted support boards shown schematically below.
Summary
Throughout glass making processes the careful control of loss of heat ensures optimised energy efficiency and minimum harmful environmental impact.
The most effective way of containing heat is by the use of a microporous insulation system such as Microtherm®. The benefits offered are as follows.
- Lowest thermal conductivity means the most efficient temperature control.
- Thinnest and lightest insulation system.
- No respirable fibres. Totally harmless and environmentally friendly.
- Capable of sustained and stable continuous operation at 1000 °C. Negligible shrinkage at maximum rated temperature. No loss of performance over prolonged use.
- Available in a versatile product range with pre-tailored systems when required.
- Clean to handle, shape, and install.
- The insulation of choice when space is a problem.
- The insulation of choice when tight control of temperature and heat loss is critical.