Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1999-05-18
2001-01-09
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C065S162000, C065S273000, C065S350000
Reexamination Certificate
active
06172336
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of heating glass in a tempering furnace provided with rollers, in which method the glass is heated from above with upper resistors and from below with lower resistors.
The invention also relates to an equipment for heating glass in a tempering furnace provided with rollers, the equipment comprising upper resistors for heating glass from above and lower resistors for heating glass from below.
BACKGROUND
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. This 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 centre of the glass. Examples of oscillating roller furnaces are disclosed in Fl patents 83,072 and 86,407.
When heating glass in a tempering furnace, where the glass is oscillated in a reciprocating manner upon rollers during the entire heating process and the glass is heated with resistors located above and below the glass, heat transfer to the glass is difficult to control. A reason for this is that heat transfer from massive rollers to glass, for example, is predominant particularly at the initial heating stage. In this case t e lower surface of the glass is subjected to a greater thermal current (than) above, even if the heat transfer of the upper surface had been intensified e.g. by utilizing forced convection. This makes the edges of the glass bend upwards and the contact surface between the glass and the rollers becoming quite indefinite. Furthermore, the surface pressure at the point of contact where the glass touches the roller becomes high enough to subject the glass to optical faults, i.e. white marks and scratches, breaking the surface of the glass. Furthermore, the conditions in the furnace change during the heating period. The temperature of the glass changes relative to time and particularly the heat transfer from the rollers diminishes as the temperature of the glass approaches the temperature of the rollers and as the temperature of the rollers falls at the initial stage of the heating period when the thermal current taken up by the glass is at its highest. This causes the problem of keeping the heating of the upper and lower parts of the furnace equalized during the entire heating period.
At the start of heating, a power equal to the combined power of the resistors is connected to the furnace, i.e. each resistor is switched on at maximum heating power. In this case it is impossible to adjust the furnace, and it is impossible to control the actual relative power of the upper and lower sides because of resistor delays. In this case unevenly arranged glass loads in the furnace cause the furnace to become cooled in places comprising most glass, even though the need for heating is greatest in these places.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an equipment for avoiding the above drawbacks.
The method of the invention is characterized in that the heating resistors are dimensioned such that their combined power is greater than the maximum heating power needed by the furnace, and that the resistors are controlled such that the maximum amount of resistor power being used simultaneously corresponds to the maximum heating power needed by the furnace.
The equipment of the invention is characterized in that the combined power of the heating resistors is greater than the maximum heating power needed by the furnace, and that the resistors are coupled to be controlled such that the maximum amount of resistor power being used simultaneously corresponds to the maximum heating power needed by the furnace.
It is an essential idea of the invention that the tempering furnace is heated with heating resistors dimensioned such that their combined power is higher than the heating power needed by the furnace, and that the resistors are controlled such that the maximum amount of resistor power being used simultaneously corresponds to the maximum heating power needed by the furnace. It is the idea of a preferred embodiment that the resistors are divided into separate control groups. It is the idea of another preferred embodiment that the connected electric load of the furnace is smaller than the combined power of the resistors.
It is an advantage of the invention that heat may be directed at a desired place in the furnace also at the initial stage of heating. Furthermore, a single resistor is efficient and fast, allowing heat to be directed quickly at places where it is needed. If needed, strong heat direction bands may also be achieved. Furthermore, owing to limited power, the furnace cannot be overheated and left uncontrolled. Furthermore, the investment cost of electric distribution boards does not rise unreasonably high as the connected load remains at a normal level. Still further, the customer may make the decision on the size of the connected load in accordance with the supply of energy. On the whole, the glass load can be heated as uniformly as possible over the entire surface, the entire load being within a small temperature tolerance at the final heating stages. This way the average outlet temperature of the load may be kept as low as possible.
In connection with the present invention, the term highest heating power needed by a furnace refers to an power 2,5 times the average theoretical power needed by glass for raising the temperature of the glass from 20° C. to 620° C. during a heating period of 40 seconds per thickness millimetre of the glass. In practical glass production it has been found that it is not worth using a temperature exceeding said maximum power needed by the furnace, as in practice the glass cannot initially be heated faster than a preset speed, owing to the poor thermal conductivity of glass. Problems arise particularly when heating thick glass which breaks in the furnace if heated too rapidly, i.e. using too high a heating power. An example for a 4mm thick glass per glass square yields:
P
=
Q
d
×
40
s
=
m
×
c
(
T
)
×
[
T
(
end
)
-
T
(
start
)
]
4
×
40
=
41
kW
/
m
2
,
where P=average power required by glass
Q=energy needed to raise temperature
d=thickness of glass, mm
m=1 m
2
glass mass=10 kg
c(T)=specific heat power of glass, dependent on the temperature of the glass, range of variation about 820 to 1260 J/kgK
T(end)=final temperature of glass=620° C.
T(start)=initial temperature of glass=20° C.
In this case the maximum power demand is 100 to 108 kW per glass square. Practically, the maximum power demand does not depend on the thickness of the glass. The maximum power demand includes loss through the furnace insulants, etc.
REFERENCES:
patent: 3744985 (1973-07-01), Peternel
patent: 4390359 (1983-06-01), Reunamaki
patent: 4529380 (1985-07-01), McMaster
patent: 4601743 (1986-07-01), Canfield
patent: 4807144 (1989-02-01), Joehlin et al.
patent: 4824464 (1989-04-01), Perin et al.
patent: 5028250 (1991-07-01), Deb et al.
patent: 5122180 (1992-06-01), Mathivat et al.
patent: 5337393 (1994-08-01), Reunamaki
patent: 5368624 (1994-11-01), Lehto et al.
patent: 0416332 (1991-03-01), None
patent: 62043 (1982-07-01), None
patent: 96043 (1982-07-01), None
patent: 83072 (1991-02-01), None
patent: 86407 (1992-05-01), None
patent: 93719 (1995-02-01), None
patent: 97378 (1996-08-01), None
patent: 9803439 (1998-01-01), None
Ladas and Parry
Pelham Joseph
Uniglass Engineering Oy
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