GB2311064A

GB2311064A - Method of cooling glass for tempering

Info

Publication number: GB2311064A
Application number: GB9619767A
Authority: GB
Inventors: James Arthur Albert Hickman; William Eric Ferguson
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-10-02
Filing date: 1996-09-23
Publication date: 1997-09-17
Also published as: GB9619767D0

Abstract

A method of cooling glass which has been heated to tempering temperature, by latent heat of vaporisation. The cooling medium used to extract the thermal energy by phase change may be wet steam sprayed on the glass surfaces.

Description

Title: Method of Cooling Glass for Tempering This invention relates to a method of cooling annealed flat glass sufficiently to set up tempering stresses in the glass.

The tempering of glass and in particular flat glasses used for architectural purposes, is well known and established in the industry. The glass is tempered or toughened by heating the glass to over 600"C and rapidly cooling it to set up the necessary compressive stresses for tempering. This has the effect of increasing the resistance to breakage from impact. Although this process is used primarily for tempering annealed flat glasses for architectural purposes the process can be used similarly for tempering any glass object.

There are many tempering systems throughout the world which use electricity or gas for heating and a glass cooling system using air which quenches the glass by air impingement.

It is well known that there are very critical parameters in the initial cooling in the first few seconds ofthe quenching or cooling stage.

This invention provides a method of cooling or quenching of the glass which is more energy efficient, cost effective and at substantially reduced noise levels compared with air cooled quenching systems.

In this invention there is provided a method of cooling the glass in which the cooling medium is used to extract the thermal energy primarily by a phase change or latent heat exchange process. It should be appreciated that some sensible heat exchange may also occur either before or after the phase change process, or both.

The latent heat of vaporisation of a fine spray of liquid coolant is used to exchange large amounts of thermal energy to set up the tempering stresses. This method can be described as 'film boiling' or 'nucleate boiling' and exhibits a potentially very high surface heat flux of up to 600 kW/m2 when cooling a surface at approximately 6000C.

This can easily match and improve on the cooling capacity of existing air quenching systems.

Preferably the liquid coolant is distributed within a gas or mixture of gases and is directed under pressure towards the glass surfaces. The pressure may be generated with the aid of a fan, or by compression of the gas, vapour or mixture of gases or vapours in a pressure vessel.

However the coolant can be any liquid with the necessary thermal and physical characteristics.

The preferred liquid coolant is water and the preferred mixture of gases is air with water droplets or steam.

Preferably the mixture of water and air is evaporated into steam and dispersed under pressure through an array of nozzles, orifices or slots disposed opposite each surface of the glass.

The array of nozzles, orifices or slots facing each surface of the flat glass sheet should be positioned at appropriate spacing to ensure that surface heat flux is uniform over the entire area of each sheet of glass. Variations in the cooling or quenching can cause incorrect thermal stresses over the surfaces leading to breakage or distortion.

The amount of steam dispersed through the nozzles, orifices or slots, must be controlled, and in accordance with nominal rates of heat extraction shown in Table A.

Nominal Heat Transfer Coefficient. Approximate Heat Glass Thickness (WIm2 C) Extraction Rate mm (kW/m2) 3 658 355 4 498 269 6 347 187 10 223 121 Table A Cooling Requirements for tempering glass of varying thickness A particular aspect of the invention relates to what is generally referred to as flat coated glass. These can be either pyrolytic coatings or vacuum deposited coatings. A number of coated glasses are reflective to reduce solar gain and a number are low emissivity glass specifically to reduce thermal transmittance properties and can be referred to as low emissivity glasses, which reflect longwave thermal radiation.

The tempering or toughening of coated glasses such as low emissivity glasses encounters differential parameters than that experienced in conventional tempering processes for ordinary annealed glass.

A further object of the present invention is to provide an improved method of quenching or cooling by latent heat of vaporisation to provide the required initial tempering stresses in coated glasses or low emissivity glasses.

When low emissivity glasses enter a conventional air quench to set up tempering stresses, an energy imbalance is created due to the opposing flat surfaces having differential radiant properties. Air as a cooling medium is virtually transparent to longwave thermal radiation and the opposing surfaces of low emissivity glass, due to one of the opposing surface bearing the low emissivity coating, will cool at different rates due to imbalance in radiant heat loss.

The differential opposing surface radiant properties of low emissivity glass during air cooling at temperatures in excess of 600"C, are significant. The resultant internal stress parabola will have a skewed distribution resulting in bent, concave or convex glass.

There is a resultant cooling imbalance of typically 7.4% with air cooling, depending on the emissivity parameter for coated surface. To remedy the imbalance on conventional quench systems requires major mechanical adjustments to the air cooling quench.

This can be redressed by the present invention of quenching or cooling by latent heat of vaporisation. Water or wet steam particles encountered in the latent heat of vaporisation process, are highly absorbent of longwave thermal radiation. The normal total emissivity of water at approximately 1000C is 0.963. This is a property which very few substances exhibit.

Consequently, wet steam or atomised hot water is almost totally absorbent of longwave thermal radiation. Thus, this is an effective means of imposing almost equivalent quenching or cooling to opposing surfaces of low emissivity glass.

By using Latent Heat of Vaporisation cooling or quenching for coated glasses there is no significant energy imbalance.

An embodiment of the invention is now described by way of example with reference to the accompanying drawings.

Mode of Operation for the Invention With reference to Fig 1 of the drawings, this is an illustrated cross-section of the flat glass tempering system. The glass (I) on leaving the heating section (A) enters the first cooling section (B) on driven rollers (2) from right to left for the first quench from the steam injection nozzles, orifices or slots (3), to set up the compressive surface stresses by cooling the glass surfaces to below the strain point whilst maintaining the mid-plane temperature above the strain point in the glass (I) then exits by the conveying rollers (2) into the second stage cooling section (C) until the glass (1) has cooled to a safe handling temperature whilst maintaining the compressive and tensile stresses necessary to temper the glass.

The present invention is concerned with the controlled cooling of glass primarily for the purpose of tempering or toughening. To toughen or temper glass it is necessary to heat the glass to higher than 600"C and then cool the surfaces rapidly to below the strain point but maintaining the centre temperature above the strain point initially and thereafter cooling to bring the centre or mid-plane of the sheet of glass below the strain point and the surfaces further cooled to ambient temperature where the surface has compression with centre tension to provide adequate tempering.

In existing tempering systems which use air impingement to quench the glass, the rate of cooling is controlled by varying the delivery pressure of the air andlor the speed of glass movement.

In the present invention the rate of cooling can also be varied by changing the rate of steam impingement on the glass sheet and/or the speed of glass movement.

However in the present invention it is preferable to change the cooling medium at temperatures below 350"C, as can be noted in Fig 2 (nucleate boiling graph), whilst the rate of steam impingement can be varied to amend the rate of cooling. The further use of steam at temperatures below 250"C may cause direct contact of water to glass and may cause localised irregular cooling over the glass surface leading to breakage.

The wet steam in close proximity to the glass surface will dry and may become superheated during glass cooling.

It should be noted that whilst the initial tempering or toughening stresses are provided by the latent heat of vaporisation, a second stage cooling should be provided to maintain the tempering stresses and reduce the temperature of the glass for safe handling.

This can be effected by conveying or indexing the glass sheet by means of rollers to a second stage cooling station. Other glass transport systems may be used e.g. hanging with tongs, air cushions or any other suitable method.

The preferred method is to incorporate air impingement nozzles immediately adjacent to the liquid or steam ejection nozzles to provide the second cooling stage when the initial tempering stresses or heat transfer with the latent heat of vaporisation has occurred.

This can be carried out either by oscillating rollers or by single-pass indexing rollers.

This preferred arrangement provides economy of space and the added advantage of dispersing any moisture surplus to the process.

In certain operating environments it may be advantageous to incorporate a moisture extraction system.

It should be noted that in the first quenching station using the latent heat of vaporisation, it may be necessary under certain conditions to heat the internal surface and interior of the quenching station. This will ensure that surplus steam or liquid coolant ejected from the array of nozzles, orifices or slots, at a temperature of approximately IOOOC, not actually vaporising at the glass surface will not be deposited on the internal surfaces or interior of the quenching station. It may also be advantageous to heat the roller surfaces.

In the majority of tempering systems in use today the width of the conveying rollers are such that the effective inammum width of glass is approximately 2200mm.

However in normal practice a number of panes of various lesser widths are laid on the conveyor to enter the heating station (A) and thereafter to the cooling station or stations. The effective cooling area is approximately 3200mm x 2200mm wide in which there is air impingement over the total area, whilst there may be only a small number of panes of a much lesser area.

To optimise on the use of latent heat of vaporisation in a first quenching station (B), it is preferred that the various sizes of glass panes are recognised by sensors as they index from the heating station to the first quenching station, and that the sizes be relayed to the control panel, which will then wordy activate the steam or liquid coolant nozzles, orifices or slots in immediate opposition to the glass surface, as they are conveyed on the rollers through the first quenching or cooling station. Whilst the rollers in the first quenching station can be oscillated if required it is preferred that the rollers operate on a continuous forward motion as the latent heat of vaporisation can cool the glass surfaces sufficiently in a few seconds to set up the first necessary stresses for tempering.

When the glass surface temperature has been cooled in the first quench station to the required initial stresses it is preferable to continue the cooling of the glass by low pressure air impingement in the second cooling station (5) resulting in glass at a safe handling temperature. However, the requirements of this latter stage cooling are very moderate and may be achieved by fan driven, low pressure air or gas. This method of cooling the glass sheet which uses moderate pressure flow also utilises less primary energy than traditional methods.

When the coolant in the quenching station approaches the glass surface its temperature increases as it passes through the thermal boundary layer creating an area of film boiling, as shown for 100% water by curve E-F in graph Fig 2. As noted the surface heat flux can be very substantial depending on the amount of wet steam or liquid contained in the steam ejected from the array of nozzles, orifices or slots, during the film boiling stage. It is clear that this is an extremely effective cooling principle and complies easily with temperature parameters in cooling glass for tempering purposes.

It will be appreciated that the surface heat flux will be variable depending on the amount of moisture or steam directed towards the glass surfaces.

Any glass conveyor system used in current or existing tempering systems can be used to convey the glass sheet through the various stations of heating and cooling or quenching and can be continuous line or oscillating rollers. Other glass transport techniques may also be used, vertical or horizontal.

The glass sheet enters the first heating station and heats the glass to approximately 630"C or higher. The glass then exits into a cooling station, or an initial quenching station to achieve the initial cooling to below the strain point and then a second cooling station, to continue the cooling at a reduced rate of cooling.

In this invention two stage cooling is preferred, the first cooling or quenching station using the latent heat of vaporisation, and the second cooling station to continue the cooling of the glass at a reduced rate to a safe handling temperature. First and second stage cooling may occur in a single quenching or cooling station with integral first and second stage cooling.

At the typical temperature of over 600"C, surface heat flux can approach 600kW/m2.

It can be seen from Table A that the maximum extraction rate to toughen 3mm glass is approximately 355kW/m2, which is well within the capability of the cooling performance shown in Fig 2.

In accordance with the invention, the heat extraction rate can be controlled by varying the coolant injection rate or volume flow. Also, the heat extraction rate may be controlled by the pattern of nozzles or by adjusting the area of active nozzles.

A hot water heating system or boiler may be employed to preheat the water which will thereafter be ejected from the array of nozzles, orifices or slots towards the glass sheet surfaces.

The water under heat and pressure ejects under pressure towards the glass surfaces either as hot water below 100"C or as wet steam above 100"C. It absorbs heat from the glass and increases in dryness fraction and in certain circumstances may become superheated..

It should be appreciated that the coolant does not wet the glass during film boiling. If the coolant flow is stopped before the glass reaches point E in Fig 2 (example shown is water) then the glass will exit dry from the process.

Cooling below point E in Fig 2 can be carried out with the aid of air, driven by an appropriate fan.

However it should be appreciated that cooling below point E (Fig 2) can be carried out by latent heat transfer albeit at a moderated rate until glass temperature equals the coolant delivery temperature.

The required water/steam flow rate can be calculated as follows It should be appreciated that there are many possible variations to the calculations, but the following relates to the cooling of 4mm thick flat glass using wet steam from a boiler pressurised to approximately 2 Bar (1 5prig). It is luther assumed that all of the wet steam is converted to superheated steam before leaving the quench or cooling station.

For lkg of wet steam at 2 Bar, 120.2 C saturation temperature and a dryness fraction of 10% Specific Enthalpy = 0.1 x 2202 kJ/kg = 202.2 kJ/kg This is expanded to local atmospheric pressure of 1 Bar (approx.) and changes in dryness fraction to (202.2/2258 = 8.9 /e). At this condition the lkg of wet steam has the capability to absorb (0.089 x 2258 = 2010 kJ'kg) of heat from the glass sBlx before becoming superheated.

If the 1 kg of wet steam were distributed over the initial quench period of say 7 seconds the heat flux would be equivalent to (2010/7 = 287 kWlkg) of steam.

For 4mm thick glass (requiring 269 kW/m2), an area of (287/269 = 1.07 m2) could be tempered, with first stage quenching only.

It should be appreciated that the process is not absolute and an additional mass flow of wet steam will be required to make up for scattered atornised particles rebounding off the thermal boundary layer.

The above figures show how the glass can be effectively cooled to provide tempered or toughened glass. This latent heat of vaporisation cooling method provides substantial energy savings over established methods cooling.

It is well known that cooling systems in current tempering plants can use up to 600HP (448kW) motor driven fans to cool the glass with substantial energy costs and high noise emissions. The method of cooling the glass by film boiling or nucleate boiling encompasses all the environmentally friendly aspects associated with low energy costs, reduced noise levels and the use of non-hazardous substances.

Whilst the above describes a method of cooling flat glass, it will be appreciated by those skilled in the art that this cooling method can be applied to any glass object or shape. For example if the flat glass sheet was bent or curved, it could be cooled in a similar manner to provide the necessary tempering stresses by adjustment of the cooling nozzles, orifices or slots to suitable radii, and utilise the latent heat of vaporisation technique as previously described.

It will be appreciated that there are various methods of using latent heat of vaporisation to cool flat glass or glass articles without departing from the spirit of the invention. For example tempering plants today use in the main a system of rollers for conveying the glass from heating station through to the cooling stations, either on a continuous roller system or a shortened system of oscillating rollers to ensure even heating and cooling over the surface for tempering.

In some older glass tempering systems a method of tongs or pincers are still used to hang or suspend the glass and convey it through the heating and cooling stations. This method has been largely discontinued in modern glass tempering systems.

It can readily be seen that using steam, water or other suitably atomised liquid in such systems which currently use direct air impingement, could be utilised. Thus the effective cost of cooling can be substantially reduced. Whilst most systems use motor driven fans or compressed gas or air or combinations of both on an impingement basis it will readily be appreciated that by using the latent heat of vaporisation techniques as described herein, substantial benefits will be achieved. There are various other cooling methods outwith the direct impingement method for example the coolant medium can be drawn or biown across the glass surfaces preferably using turbulation techniques to provide sufficient cooling for tempering purposes.

Modifications and improvements may be incorporated without departing from the scope of the invention

Claims

CLAIMS 1. A method of cooling or quenching primarily by latent heat of vapourisation to create tempering or toughening stresses in an annealed flat glass sheet which has been heated to temperature above the strain point of the glass, whereby the cooling medium is wet steam at approximately boiling condition, depending an local atmospheric conditions. The wet steam is impinged or forced onto the opposing surfaces of the annealed flat glass sheet under pressure by an array of steam ejection devices, close to the opposing surfaces of the flat glass sheet, the steam ejection devices themselves being suitably spaced apart to ensure the heat extraction rate is uniform over the entire area of each opposing surface of the annealed flat glass sheet, at heat extraction rates sufficient to temper or toughen various thicknesses of annealed flat glass sheets, by creating the required tensile and compressive stresses.
2. A method according to claim 1, whereby the wet steam impinging on opposing surfaces of the glass sheet is created from pressurised hot water at approximately the nonnal boiling condition which when released through the ejection devices or nozzles expands into wet steam thereby impinging on the glass surfaces.
3. A method according to claim 1, whereby the heat extraction rate required to provide tempering stresses is created by phase change or latent heat of vapourisation.
4. A method according to claim 1, whereby the pressure of impingement and/or the florate ofthe wet steam is selected to cool or quench the various thicknesses of flat glass.
5. A method according to claim I, whereby the pressure of impingement and/or the flowrate of the wet steam and/or the speed of movement of the glass is selected to cool or quench the various thicknesses of flat glass.
6. A method according to claim 1, whereby the cooling medium is any liquid, with the necessary thermal and physical characteristics to temper or toughen flat glass by latent heat of vapourisation at a temperature sufficient to vapourise and/or atomize from ejection devices under pressure.
7. A method according to claim 1, whereby the steam ejection devices can be nozzles, slots or orifices. These can be any size suitably spaced apart to provide uniform impingement of the cooling medium on opposing surfaces of the flat annealed glass sheet at any combination pressure and volume flow rate to obtain the required heat extraction rates.
8. A method according to claim 1, whereby the glass heated to a temperature above the strain point, can have a surface heat extraction rate up to approximately 600 kW/m2, from the cooling medium, wet steam at the approximate boiling condition.
9. A method according to claim 1, whereby the heated annealed glass sheet to be cooled to provide tempering stresses, can be bent to any radii or other shape.
10. A method according to claim 1, whereby the annealed glass sheet is conveyed on rollers between the steam ejection devices, in a single direction, or oscillated backwards and forwards on the conveying rollers.
11. A method according to claim 1, whereby the heated annealed glass sheet enters the cooling station to a stationary position and the steam ejection devices oscillate over the glass areas to be cooled to provide tempering stresses.
12. A method according to claim 1, whereby the heated annealed glass sheet enters the cooling station to a stationary position and remains stationary whilst the wet steam cooling medium impinges on the glass to provide tempering stresses.
13. A method according to claim 1, whereby the annealed glass sheet is heated to above the strain point, and thereafter cooled or quenched by latent heat of vapourisation to safe handling temperatures.
14. A method according to claim 1, whereby the annealed glass sheet is heated to above the strain point, and thereafter cooled or quenched in two stages. The first stage by phase change cooling and the second stage by low pressure air impingement, to further cool the glass to sate handling temperature.
15. A method according to preceding claims, whereby the secondary cooling is provided by low pressure air ejection devices integral with the first cooling or quenching station, which are fitted at centres adjacent to the steam ejection devices. The air ejection devices can be nozzles, slots or orifices suitably spaced apart to achieve uniform secondary cooling rates.
16. A method according to preceding claims, whereby the annealed glass sheet is a coated glass or low emissivity glass, heated to above the strain point and thereafter cooled by latent heat of vapourisation to provide tempering stresses, the cooling medium being wet steam at approximately boiling condition, which is highly absorbent of longwave thermal radiation and therefore provides a uniform heat flux to each opposing surface of the glass, with no significant energy imbalance, thus preventing the glass sheet becoming concave or convex.
17. A method according to preceding claims, whereby the glass is a glass article, vessel or container.
18. A method according to preceding claims, whereby the pressure for impingement is created by liquid temperature at or in excess ofthe local liquid boiling point.
19. A method according to preceding claims, whereby the impingement pressure is generated by a pump or prime mover.
20. A method according to preceding claims, whereby the impingement pressure is a combination of pressure from the boiling process and pressure from a pump or prime mover.