Glass manufacturing – Processes – Glass preform treating
Reexamination Certificate
1999-01-19
2001-09-04
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Glass preform treating
C065S273000, C065S350000, C065S351000, C065S355000
Reexamination Certificate
active
06282923
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of heating glass sheets in a tempering furnace provided with rollers, in which method the glass sheets are led on a roller conveyer through the tempering furnace, the glass sheets being heated from above and from below, and the glass sheets being oscillated in a reciprocating manner on the rollers, and the lower part of the tempering furnace being cooled during the heating period.
The invention also relates to equipment for heating glass sheets in a tempering furnace provided with rollers, the equipment comprising a tempering furnace, means for heating the glass sheets from above and from below, horizontal rollers inside the tempering furnace which support a horizontal glass sheet and form its conveyor, the rollers being adapted to oscillate the glass sheet in a reciprocating manner during heating, and means for cooling the lower part of the tempering furnace.
BACKGROUND AND PRIOR ART
Current glass tempering machines employ what are known as oscillating roller furnaces in which glass is heated mainly by radiation. In the tempering process the temperature of the glass is increased above the softening point of glass in order to enable the glass to be tempered. Said temperature is between 610 and 625° C. depending on the thickness of the glass. The glass is then cooled at desired speed typically using forced convection whereby air jets are blown at the glass from above and from below. This method enables high heat-transfer coefficients, necessary when thin glass is concerned in order to achieve a sufficient temperature difference between the surface and center of the glass. Examples of oscillating roller furnaces are disclosed in Fl patents 83,072 and 86,407.
A problem with roller furnaces is that heat transmission from massive rollers to glass is predominant particularly at the initial heating stage. The glass is supported by the rollers all the time, and particularly at the initial heating stage, with the temperature difference between the hot rollers and the glass being considerable, heat transmission from the hot rollers to the glass by conduction is abundant. This makes the edges of the glass bend upwards, the contact surface between the glass and the rollers becoming quite indefinite. In this case, uniform heating of glass is almost impossible as the contact point where the glass touches the roller becomes heated at the initial stage significantly faster than the rest of the glass surface. This easily causes curving, making uniform heating with normal heating times almost impossible. Further, the surface pressure at the point of contact of the glass touching the roller becomes high enough to subject the glass to optical faults, i.e. white marks and scratches, marring the surface of the glass.
Furthermore, the conditions in the furnace change during the heating period in tempering furnaces provided with rollers. The temperature of the glass changes relative to time and, particularly, heat transmission from the rollers diminishes as the temperature of the glass approaches the temperature of the rollers. On the other hand, the temperature of the rollers falls at the initial stage of the heating period when the thermal transfer to the glass is at its highest. This causes the problem of keeping the heating of the upper and lower parts of the furnace balanced during the entire heating period.
Fl patent 62,043 discloses a method and device for preventing the curving of glass sheets in a furnace provided with rollers in a horizontal tempering plant. In accordance with the cited publication, an air current is generated on the upper surface of a glass sheet in order to intensify the convection heat effect directed at the upper surface of the glass sheet. The blowing serves to compensate for the active heat transmission caused by the hot rollers to the lower surface of the glass sheet at the initial heating period, but said blowing does not completely equalize the differences in the heat currents affecting the upper and lower surfaces, as the lower surface of the glass gets a stronger heat current from above, particularly during initial heating, causing the above mentioned problems.
Fl patent 83,072 also discloses a method and a device for preventing the curving of glass sheets in a furnace provided with rollers in a horizontal tempering plant. It is known from said publication to direct an expanded convection heat effect at the upper surface of a glass sheet by blowing air at the furnace from blowpipes disposed in the vicinity of the upper surface of the glass sheet. Said publication further discloses that the furnace space below the glass sheet is cooled by leading the air to be blown into the furnace via heat exchange pipes provided underneath the glass sheet to blowpipes located on the upper side. Said arrangement is, however, quite complex and does not provide control of the equalization of heat in the tempering furnace during the entire heating period.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a method and equipment for preventing the curving of glass sheets in a tempering furnace provided with rollers, avoiding the above mentioned drawbacks and providing good control of the equalization of heat during the entire heating period.
The method of the invention is characterized in that at the final stage of the heating period the lower surfaces of the glass sheets are heated by forced convection.
The equipment of the invention is characterized in that the equipment comprises means for heating the lower surfaces of the glass sheets by forced convection during the heating period.
It is an essential concept of the invention that, at the initial stage of the heating period, the lower part of the tempering furnace is cooled, and at the final stage of the heating period the lower surfaces of the glass sheets are heated by forced convection. Furthermore, in a preferred embodiment the lower part of the tempering furnace is cooled with cooling pipes arranged in every second roller gap, and the lower surfaces of the glass sheets are heated by forced convection similarly by heating pipes arranged in every second roller gap. In still another preferred embodiment the cooling pipes are provided with holes from which air jets at room temperature are blown at the rollers for cooling the surface temperature of the rollers and the lower part of the furnace. In a third embodiment the cooling is adjusted by changing the speed of the air passing in the pipe.
It is an advantage of the invention that the glass is prevented from curving, resulting in glass with optimal planarity. It is also an advantage that the thermal shock from the rollers to the glass can be reduced at the initial heating stage. It is a further advantage that heat equalization in the tempering furnace can be very well adjusted, and furnace power can be increased.
REFERENCES:
patent: 4390359 (1983-06-01), Reunamaki
patent: 4505671 (1985-03-01), McMaster
patent: 4529380 (1985-07-01), McMaster
patent: 4681616 (1987-07-01), McMaster
patent: 5032162 (1991-07-01), Reunamaki
patent: 5057138 (1991-10-01), Vehmas et al.
patent: 5078774 (1992-01-01), Vehmas et al.
patent: 5368624 (1994-11-01), Lehto et al.
patent: 5647882 (1997-07-01), Thiessen
patent: 5735924 (1998-04-01), Kormanyos
patent: 0416332 (1991-03-01), None
patent: 62043 (1982-07-01), None
patent: 83072 (1991-02-01), None
patent: 86407 (1992-05-01), None
patent: 97378 (1996-08-01), None
patent: 2159145 (1985-11-01), None
patent: 9803439 (1998-01-01), None
Ladas & Parry
Uniglass Engineering Oy
Vincent Sean
LandOfFree
Heating glass sheets in tempering furnace does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Heating glass sheets in tempering furnace, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Heating glass sheets in tempering furnace will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2547097