DE102007028976B9 - Float bath tub and process for the production of flat glass - Google Patents

Abstract

A process for the production of flat glass by the float process, in particular TFT glass, in which a molten glass is applied to a bath (2) of molten metal located in a float bath tub (1) extending longitudinally of the tub into float bath sections (Bays ) (101-108),
the glass melt is continuously formed into a glass ribbon (20), and
the glass ribbon (20) in the last Floatbadabschnitt (108) is lifted out of the metal bath (2),
wherein the glass ribbon temperature is measured and the glass ribbon (20) is heated from above, characterized in that the last float bath portion (108) is subdivided into at least three heating zones (50),
that a predetermined heating power profile (HLP) is set over the glass band width, in which the heating power P _{N of} the heating zones (50) is set to a fixed ratio to the heating power P _{R of} a reference heating zone H _R and
the heating power P _{R of} the reference heating zone H _{R is} regulated as a function of the glass band temperature measured in the reference heating zone H _R.

Description

The The invention relates to a method for producing flat glass according to the float process, in particular of TFT glass, in which a glass melt is applied to a bath of molten metal, according to the preamble of claim 1. The invention also relates to a float bath tub for the production of flat glass according to the preamble of claim 8th.

The production of flat glass by the float process, known as float glass, has been known since the last century and is essentially based on the fundamental intellectual property rights of Pilkington ( US 3,083,551 A . DE 147 19 50 A ).

At the Float method leaves one liquid glass, which is brought about by means of a gutter from the working trough, on a bath of molten metal, generally tin flow. Of the Volume flow of the glass is over a movable slider regulated, with its setting under other things, the glass thickness is set. In the flow direction of Glases seen behind the slider is the pouring lip, from the glass melt continuously on the in the Floatbadwanne located metal bath flows. The glass melt becomes dimensionally stable in the float bath Glass band is formed and then the solidified glass ribbon removed from the metal bath and further processed.

The Float glasses made in this way, usually a thickness of less than 1.5 mm are used as thin glass substrates, among others used for the production of flat screens, z. B. of plasma screens (PDP = Plasma Display Panel), field emission screens (FED = Field Emission Display), TFT liquid crystal displays (TFT = Thin Film Transistor), STN Liquid Crystal Monitors (STN = Super Twisted Nematic), plasma-assisted liquid crystal displays (PALC Plasma Assisted Liquid Crystal), electro-luminescence displays (EL) and the like or for the production of thin-film solar cells.

at The flat screens will vary depending on the type of display two glass panes either a thin layer of a liquid crystal compound introduced or it will be on the front and back of the back or front disc each applied dielectric layers, from which cells are formed in which phosphors are housed.

It Here it is important that the layer thickness of the liquid crystal layer or the thickness of the dielectric layer is strictly adhered to, especially with large ones Dimensions of a screen no annoying color distortions or similarity deviations occur. As the layer thicknesses, currently about 30 microns, getting smaller and the screens always grow, This condition is becoming increasingly important.

The Float bath tub, which is located in a Floatbadgehäuse, is usually longitudinal the tub is divided into float bath sections, called bays, in which run different process steps and accordingly different Process conditions prevail. The room above the float bath sections is usually by means of curtains divided to special atmospheric conditions in the Floatbadabschnitten too create. As a rule, the float bath tub is in seven or eight Float bath sections divided, the Floatbadabschnitte of Be counted section where the molten glass is poured.

in the last float bath section, d. H. in the 7th or 8th float bath section, the glass ribbon is lifted off the metal bath and a so-called Drossbox supplied. Here, the glass ribbon is subjected to a certain radius of curvature, wherein the distance between the end-side end wall of the Floatbadwanne and the detachment line the glass ribbon is called a take-off area.

In the glass band center, this distance is usually 40 mm and takes to the two edges of the glass band to values of up to 90 mm. This enlargement of the Take-off area on the glass band edges is characterized by a bulge of the border area caused by the glass ribbon, because the glass has already cooled too much. In this area, a special temperature control is necessary to avoid stress in the glass band and avoid any glass breakage. If the Distance of the detachment line over to the front wall the entire glass band width could be kept almost the same size, the Risk of glass breakage going to zero. By heaters in the End of the float bath are arranged on the Floatbaddecke and on the glass ribbon from above heat will try to adjust the distance in the border area to values under 90 mm.

to Control of this heater above the glass ribbon are in Prior art different solutions offered.

From the DE 2055479 A For example, a float method and apparatus is known in which electrical resistance heaters spaced from the ceiling structure of the float bath housing are spaced above the metal bath. The temperature of the glass ribbon is measured and compared with a predetermined temperature value. The Power supply to the heating elements is controlled in accordance with the detected temperature differences.

The DE 1596421 A is based on the observation that in practical operation it often happens that the glass ribbon breaks during or immediately after leaving the Zinnbadbehälters. The risk of breakage is particularly high at this point, because the temperature of the glass ribbon must be as low as possible in order to avoid damage to the glass ribbon surface. A breakage of the glass ribbon at this point usually means a complete interruption of production with extremely costly and detrimental consequences. To solve this problem, it is proposed not to act on all along the tin bath arranged heating elements but only to control the power supply to the heating elements at the end of the tin bath. This is to ensure that the temperatures are kept constant at the end of the tin bath at the most endangered area. As a solution to the problem, it is proposed to control the supply of energy to the heating or cooling elements controlling the temperature of the glass ribbon as a function of the amount of glass supplied to the tin bath.

From the EP 1 475 355 A1 It is known to set the temperature of the glass ribbon at the float bath end to Tg -50 ° C to Tg + 30 ° C, where Tg is the glass transition temperature of the float glass to reduce stresses. However, it is not communicated how the regulation of the heating elements installed there is to be carried out.

It has been shown that the known controls the problem of Do not loosen the glass breakage. For this reason, it has been transferred to the Heaters over Manually control the glass band in the end area of the float bath. at Temperature fluctuations but it takes too long to the optimal Heating the borders of the glass ribbon is achieved.

It is therefore an object of the invention, a float bath tub and a method to provide with the risk of glass breakage in the end of the float bath tub can be reduced.

The object is achieved by means of a method for the production of flat glass, in particular of TFT glass, according to the float method, in which the last Floatbadabschnitt is divided into at least three heating zones, on the glass band width a predetermined Heizleistungsprofil (HLP) is set, in which the heating power P _{N of} the heating zones is set to a fixed ratio to the heating power P _{R of} a reference heating zone H _R and the heating power P _{R of} the reference heating zone is regulated as a function of the glass ribbon temperature measured in the reference heating zone.

It has been shown that the measurement error of the temperature measurements different measuring points on the glass ribbon can be different sizes, so that the regulation of the assigned heaters in the course of the Time to changing heating power profiles over the glass bandwidth lead with the consequence that the desired Degradation of stresses in the glass ribbon is not always achieved.

It has been surprising proved that better results with a fixed set heating profile over the Glass bandwidth can be achieved, preferably only at the Temperature measurement at a measuring point of the glass ribbon in the last Float bath section of the float bath oriented.

If at the preferably one measuring point a deviation from the target temperature occurs, the heating power of all heating zones in the end the float bath in the same ratio changed to the performance of the reference heating zone. Once set Heating power profile over The glass bandwidth will also change with temperature and changes the heating power maintained.

The Invention is based on the knowledge that at a temperature change at the glass band surface at a given measuring point in the same ratio also temperature changes occur in other places of the glass band.

Of the The advantage of this fixed coupling to a reference heating zone has the Advantage that with temperature changes the heat outputs of all heating zones are changed simultaneously, so that there are no delays in the adjustment the desired Temperature or heating power can come.

It has been shown that in float glass, the process of the invention produced, a smaller glass breakage occurs than in the process According to the prior art is the case.

Preferably the glass ribbon temperature is only measured at a single measuring point. It has been shown that a single measuring point for the investigation the glass ribbon temperature is sufficient. This has proved to be more preferable Place exposed a central area of the glass band width. Especially the glass ribbon temperature is measured in the middle of the glass ribbon width.

The Reference heating zone is also preferably in a middle Area of the glass ribbon laid, especially if there too the glass ribbon temperature is measured.

The Results can be further improved by the fact that the free surface of the Tin bath heated next to the glass band becomes. This heating has the advantage that by heating the Zinns also a heat input takes place in the edge region of the glass ribbon.

At the brew the molten glass on the metal bath automatically sets an equilibrium thickness a, which is usually at 6 mm. Here, the border has the Same thickness as the other areas of the glass band. Since the molten glass pulled apart to a glass ribbon is, the glass thickness is reduced, resulting in the edge area to a thickening leads the glass ribbon. This means that the thinner the glass is pulled out, the edge area becomes thicker in relation to Middle of the glass band. This thickening of the glass band edge leads to curvature the detachment line. The targeted heating in the edge area can reduce this curvature become.

• – Herstellung eines Glasbandes und Entfernen des Glasbandes im letzten Floatbadabschnitt,
• – Einstellung identischer Heizleistungen in allen Heizzonen,
• – Bestimmung des Abstandes der Ablöselinie Glas/Metallbad zur endseitigen Stirnwand der Floatbadwanne,
• – Veränderung der Heizleistungen in den Heizzonen des Glasbandrandes,
• – Bestimmung der Verschiebung des Abstandes der Ablöselinie an den Glasbandrändern,
• – Ermittlung der Heizleistungsverhältnisse in den Heizzonen bezogen auf die Referenzheizzone, sobald die Kontaktlinie sich um 5–10 mm verschoben hat.

In order to determine the heating power profile, the following method steps are therefore carried out according to a first embodiment:

• Manufacture of a glass ribbon and removal of the glass ribbon in the last float bath section,
• - setting of identical heating capacities in all heating zones,
• Determination of the distance of the glass / metal bath detachment line to the end wall of the float bath tub,
• Change in heating power in the heating zones of the edge of the glass band,
• Determination of the displacement of the distance of the detachment line at the edges of the glass band,
• - Determination of the heating power conditions in the heating zones based on the reference heating zone, as soon as the contact line has shifted by 5-10 mm.

It has been surprising pointed out that it is not necessary, the heating power so to increase greatly that the curvature the detachment line Completely will be annulled. It is completely sufficient, through targeted heating in the area of the glass band edges a shift in the detachment line from only 5-10 mm to significantly reduce the risk of glass breakage. If this shift the detachment line which is determined by the different heat outputs in the heating zones resulting heating power profile for the further production process maintained.

• – Herstellung eines Glasbandes und Entfernen des Glasbandes im letzten Floatbadabschnitt,
• – Einstellung identischer Heizleistungen in allen Heizzonen,
• – Bestimmung der Glasbandtemperatur, wobei die Temperaturen mindestens an den Glasbandrändern gemessen werden,
• – Veränderung der Heizleistungen in den Heizzonen des Glasbandrandes,
• – Bestimmung der Temperaturänderung an den Glasbandrändern, und
• – Ermittlung der Heizleistungsverhältnisse in den Heizzonen bezogen auf die Referenzheizzone, sobald die Temperatur an den Glasbandrändern sich um mindestens 2 K verändert hat.

According to a second embodiment, the following method steps can be carried out to determine the heating power profile HLP:

• Manufacture of a glass ribbon and removal of the glass ribbon in the last float bath section,
• - setting of identical heating capacities in all heating zones,
• Determination of the glass ribbon temperature, the temperatures being measured at least at the edges of the glass ribbon,
• Change in heating power in the heating zones of the edge of the glass band,
• - Determination of the temperature change at the glass band edges, and
• - Determination of the heating power conditions in the heating zones based on the reference heating zone, as soon as the temperature at the glass band edges has changed by at least 2 K.

preferred temperature changes are 2-3 K, Max. 10 K.

The Process is particularly suitable for the production of borosilicate glasses, alkali-free glasses, Alumosilikatgläsern, Alumolithiumsilikatgläsern and Precursor glasses for glass ceramic.

Particularly suitable is the method for producing borosilicate glass, z. For fire protection applications, having a composition of (all of the following in terms of weight percent based on oxide):
SiO ₂ 70-85, B ₂ O ₃ 7-13, Na ₂ O + K ₂ O + Li ₂ O 3-8, MgO + CaO + SrO 0-3, Al ₂ O ₃ 2-7,
for the preparation of alkali-free alumino (boro) silicate glass with a composition of
SiO ₂ 50-70, B ₂ O ₃ ≤15, Al ₂ O ₃ 10-25, MgO 0-10, CaO 0-12, SrO 0-12, BaO 0-15, with MgO + CaO + SiO + BaO 8 -26, ZnO 0-10, ZrO ₂ 0-5, TiO ₂ 0-5, SnO ₂ 0-2,
z. B. for the production of display glass, in particular with a composition of
SiO ₂ > 55-65, B ₂ O ₃ 5-11, Al ₂ O ₃ > 14-25, MgO 0-8, CaO 0-8, SrO 0-8, BaO ≤ 10 with MgO + CaO + SrO + BaO 8-21, ZnO 0-5, ZrO ₂ 0-2, TiO ₂ 0-3, SnO ₂ 0-2,
in particular SiO ₂ > 58-65, B ₂ O ₃ > 6-10.5, Al ₂ O ₃ > 14-25, MgO 0 - <3, CaO 0-9, BaO> 3-8 with MgO + CaO + BaO 8-18, ZnO 0 - <2, As ₂ O ₃ -free, Sb ₂ O ₃ -free,
preferably Zn-oxide, Ce-oxide, Zr-oxide, Ti-oxide-free.

It is also particularly suitable for the production of various green glasses for glass-ceramic, such. B. with
SiO ₂ 55-69, Al ₂ O ₃ 19-25, Li ₂ O 3-5, Na ₂ O 0-1.5, K ₂ O 0-1.5, Σ Na ₂ O + K ₂ O 0.2 -2, MgO 0.1-2.2, CaO 0-15, SrO 0-1.5, BaO 0-2.5, Σ MgO + CaO + SrO + BaO below 6, ZnO 0-1.5, TiO ₂ 1-5, ZrO ₂ 1-2.5, SnO ₂ 0 to less than 1, Σ TiO ₂ + SrO + SnO _{2 2} 2.5-5, P ₂ O ₅ 0-3
or a glass-ceramic precursor glass having a composition of
SiO ₂ 55-75, Al ₂ O ₃ 15-30, Li ₂ O 2.5-6, Σ Na ₂ O + K ₂ O less than 6, Σ MgO + CaO + SrO + BaO less than 6, B ₂ O ₃ 0 to less than 4, ΣTiO ₂ + ZrO ₂ less than 2
or a glass-ceramic precursor glass having a composition of
SiO ₂ 60-72, Al ₂ O ₃ 18-28, Li ₂ O 3-6, Σ Na ₂ O + K ₂ O 0.2-2, Σ MgO + CaO + SrO + BaO less than 6, ZnO 0-1.5, B ₂ O ₃ 0 to less than 4, SnO 0.1-1.5, ΣTiO ₂ + ZrO ₂ less than 2, P ₂ O ₅ 0-3, Σ 0-2.

The float bath tub according to the invention is characterized in that the last Floatbadabschnitt is divided in the transverse direction of the Floatbadwanne in at least three heating zones, in which the heaters are each connected together to form a heater, each heater group has at least one heater. The heating power P _{N of} the heater groups are in a predetermined fixed ratio to the heating power P _{R of} the arranged in a reference heating zone H _R heater group. The temperature sensor is arranged in the reference heating zone and the heating power P _{R of} the heater group in the reference heating zone can be regulated as a function of the temperature measured by means of the temperature sensor.

The change The heating power in each heater group can according to a first embodiment be achieved by the heating power of the individual heaters the relevant heater groups is changed.

According to one second embodiment can the heaters located in each heater groups either switched on or off. While according to the first embodiment a continuous change the heating power of the individual heating zones is made possible according to the second embodiment a gradual change the heating power performed in the heating zones.

It is possible, too in the individual heating zones a different number of heaters be provided, preferably in the heating zones on the glass band area more heating devices are arranged than this in the middle the glass band is the case. In this case, an increase in the Heat output in the glass band alone achieved by all heaters emit the same heat output.

Preferably the reference heating zone is centered, i. H. the reference heating zone is in the middle of the glass band.

Preferably have the outside heating zones, in particular the heating zones, above the Glass band edge are arranged, at least three heating devices on.

The Heaters are preferably offset from one another, to ensure even heating throughout the heating zone.

At least in a further heating zone at least one temperature sensor is arranged. The Provision of further temperature sensors is advantageous when the heating power profile over the change the temperature in the glass ribbon area is determined.

Preferably are five Heating zones provided.

Preferably In each heating zone, a temperature sensor is arranged.

exemplary embodiments The invention will be explained in more detail with reference to the drawings.

It demonstrate:

1 a top view of a float bath,

2 an enlarged view of the last Floatbadabschnitts,

3 a cut through the in 1 shown float bath along the line II-II

4 a cut through the in 1 shown float bath along the line III-III and

5 a diagram with a heating power profile.

In the 1 is a float bath tub 1 shown in plan view, with liquid metal to form a metal bath 2 is filled. The float bath tub 1 is through sidewalls 4 and 5 , a front end wall 6 and an end-face end wall 7 limited. The float bath tub 1 is in eight float bath sections 101 - 108 divided, with only the sections 101 . 102 and 108 are drawn. In the area of the front bulkhead 6 is the hot glass melt on the metal bath 2 poured continuously, from the later a glass band 20 is formed.

Im in the direction of movement of the glass ribbon 20 seen last Floatbadabschnitt 108 the float bath tub 1 becomes the glass band 20 by means of rollers 41 . 42 in the so-called Drossbox 40 are arranged, lifted off. The glass band 20 is in this float bath section 108 already solidified that way from the metal bath 2 can be removed.

The line where the glass band 20 from the metal bath 2 is replaced as dashed line 9 drawn and becomes as a detachment line 9 designated. In the center of the glass band is the distance to the rear end wall 7 the float bath tub 1 A _m and widens towards the edges 21 . 22 of the glass band 20 to the value A _B.

This area, which is characterized by the distances A _M and A _B , is called the take-off area. The greater the difference between A _B and A _M , the greater the risk of glass breakage, because the curvature of the line 9 inevitably with a bulge around the edges 21 . 22 of the glass band 20 connected, indicating different temperatures of the glass ribbon 20 which can lead to tensions and thus to glass breakage.

The float bath section 108 is transverse to the glass band 20 in heating zones 50 divided. Overall, in the illustration shown here are five heating zones 50 provided, the two outer heating zones 50 are arranged above the free surface. The middle three heating zones 50 are arranged distributed over the glass ribbon, the middle heating zone 50 the reference heating zone H _R forms. The two heating zones adjacent to the reference heating zone H _R extend over the respective edge of the glass band 21 . 22 out.

In every heating zone 50 is a group 60 arranged by heaters. Representing the individual heater groups 60 are heating devices 30 . 31a and 31b located. In addition, in the reference heating zone H _{R is} still a temperature sensor 32 shown. The heaters are preferably radiant heaters, which are attached to the Floatbaddach.

The heating capacities in the heating zones 50 are in a fixed ratio to the heating power of the heating zone H _R.

If from the temperature measuring device 32 a deviation from the target temperature is determined, the heating power P _{R of} the heaters 30 adjusted accordingly in the reference heating zone H _R , wherein also the heating powers of the other heating devices 31a , b in the heating zones 50 be adjusted according to the predetermined ratio to the heating zone H _R.

In the 2 is the float bath section 108 with the heating zones 50 shown enlarged. The individual heating zones have different widths, wherein the largest width has the reference heating zone H _R. Within each heating zone 50 are the heaters 30 . 31a . 31b staggered to each other. All heaters 30 . 31a . 31b each form a heater group 60 ,

The heaters 30 . 31a . 31b within each heating zone 50 and thus within each heater group 60 can be individually regulated with regard to their heating capacity. Also it is possible to use the individual heating devices 30 . 31a to turn b on and off. Both the control of the heating power and the switching on and off of the individual heaters 30 . 31a , b can be combined with each other.

In the reference heating zone H _R as well as in the two external heating zones 50 are pyrometers 32 arranged for measuring the temperature of the glass ribbon. By way of example only, two are the temperature sensors 32 and two heaters 31b with a control device 34 connected.

The heating capacities of the respective outer heating zones 50 are in a fixed relationship to the middle heating zone 50 which forms the reference heating zone H _R. In the case of a deviation of the setpoint temperature, that with the middle pyrometer 32 is determined, the heating power in the heating zone H _{R is} changed. Due to the specified heating power profile, the heating capacities in the remaining heating zones also become within the specified ratio 50 changed.

In the 3 is a section through the float bath tub 1 along the line II-II in 1 shown. The take-off area between the detachment line 9 and the edge of the rear bulkhead 7 is marked by the distance A _M. Above the glass band 20 the heaters are arranged, wherein in the sectional view shown here only the heaters 30 the reference heating zone H _R can be seen. The heaters are designed as radiant heaters, their heating power down to the top of the glass ribbon 20 radiate. Alternatively to the arrangement in 1 , is the temperature sensor 32 in the direction of movement behind the heater 30 arranged. The heaters 30 and the temperature sensor 32 are on the ceiling wall 11 arranged.

In the 4 is a section along the line III-III shown, including the roof 10 the float bath tub 1 is drawn. The roof 10 consists of the ceiling wall 11 and sidewalls 12 and 13 , At the bottom of the ceiling wall 11 are the heaters 30 . 31a , b suspended, each to a power supply 33 are connected. This power supply device has a control device 34 on, to which the temperature sensor 32 connected. About the control device 34 For adjusting a desired heating power profile, the heating power ratios of the heating devices can be selected 31a , I'm referring to the reference heaters 30 be entered in the reference heating zone H _R.

In the 5 a heating power profile HLP of the heating zones H ₁ , H ₂ , H ₃ , H ₄ and the reference heating zone H _{R is} indicated.

The reference heating zone H _R is set to a reference heating power P _R , to which the heating track tions of the heating zones H ₁ , H ₂ , H ₃ and H _{4 are} obtained.

In the production of float glass with thicknesses of up to 2 mm, it is necessary to emit a larger amount of heat in the border area than in the middle area of the glass ribbon. In the example shown, the curve HLP starts at a heating power of 1.2 · P _R for the heating zone H ₁ and drops towards the middle. In the heating zones H ₃ and H ₄ , the heating power increases.

The illustrated heating power profile is maintained with changes in the reference heating power P _R.

1

Floatbadwanne

2

metal

4

Side wall

5

Side wall

6

front bulkhead

7

end-side bulkhead

8th

end the float bath

9

separation line

10

Floatbaddach

11

ceiling wall

12

Side wall

13

Side wall

20

glass tape

21

edge of the glass band

22

edge of the glass band

30

heater

31a, b

heater

32

Temperature measuring device

33

Power supply means

34

control device

40

Drossbox

41

role

42

role

50

heating zone

60

heater group

101-108

float bath (Bay)

Claims (16)

Process for the production of flat glass by the float process, in particular of TFT glass, in which a glass melt is applied to a bath ( 2 ) is applied from molten metal which is in a float bath tub ( 1 ) in the longitudinal direction of the tub in Floatbadabschnitte (Bays) ( 101 - 108 ), the molten glass continuously to a glass ribbon ( 20 ), and the glass ribbon ( 20 ) in the last float bath section ( 108 ) from the metal bath ( 2 ), whereby the glass ribbon temperature is measured and the glass ribbon ( 20 ) is heated from above, characterized in that the last Floatbadabschnitt ( 108 ) in at least three heating zones ( 50 ) is divided over the glass bandwidth, a predetermined Heizleistungsprofil (HLP) is set, in which the heating power P _{N of} the heating zones ( 50 ) Of a Referenzheizzone H _R is set to a fixed ratio to the heating power P _R and that the heating power P _R is regulated the Referenzheizzone _R H as a function of the measured _R H in the Referenzheizzone glass ribbon temperature. Method according to claim 1, characterized in that that the glass ribbon temperature is only measured at one measuring point. A method according to claim 1 or 2, characterized in that the glass ribbon temperature in a central region of the glass ribbon ( 20 ) is measured. Method according to one of claims 1 to 3, characterized in that the reference heating zone H _R in a central region of the glass ribbon ( 20 ) is placed. Method according to one of claims 1 to 4, characterized in that in addition the free surface of the metal bath ( 2 ) next to the glass ribbon ( 20 ) is heated. Method according to one of claims 1 to 5, characterized in that for determining the heating power profile HLP the following process steps are carried out: - Production of a glass ribbon ( 20 ) and removing the glass ribbon ( 20 ) in the last float bath section ( 108 ), - setting identical heat outputs P _N in all heating zones ( 50 ), - determination of the distance of the detachment line ( 9 ) Glass / metal bath to the end wall ( 7 ) of the float bath tub ( 1 ), - change in heat outputs P _N in the heating zones ( 50 ) of the glass band edges ( 21 . 22 ), - determination of the displacement of the distance of the detachment line ( 9 ) on the glass band edges ( 21 . 22 ), - determination of the heating power conditions in the heating zones ( 50 ) relative to the reference heating zone H _R as soon as the detachment line ( 9 ) has shifted by 5-10 mm. Method according to one of claims 1 to 5, characterized in that for determining the heating power profile HLP the following process steps are carried out: - Production of a glass ribbon ( 20 ) and removing the glass ribbon in the last float bath section ( 108 ), - setting identical heat outputs P _N in all heating zones ( 50 ) Determination of the glass ribbon temperature, the temperatures being at least at the glass ribbon edges ( 21 . 22 ), - change in heat outputs P _N in the heating zones ( 50 ) of the glass band edges ( 21 . 22 ), - determination of the temperature change at the glass band edges ( 21 . 22 ), and - determination of the heating power ratios P _N in the heating zones ( 50 ) relative to the reference heating zone H _R , as soon as the temperature at the glass ribbon edges has changed by at least 2 K. Float bath tub ( 1 ) for the production of flat glass, which is in the longitudinal direction of the tub ( 1 ) in float bath sections (bays) ( 101 - 108 ), with heaters arranged above the float bath tub ( 30 . 31a , b) which release their heat downwards, and at least one temperature sensor ( 30 ), characterized in that the last float bath section ( 108 ) in the transverse direction of the float bath tub ( 1 ) in at least three heating zones ( 50 ), in which the heating devices ( 31a , b) each to a heater group ( 60 ), each heater group ( 60 ) at least one heating device ( 30 . 31a , b) that the heat outputs P _{N of} the heater groups ( 60 ) in a predetermined fixed ratio to the heating power P _{R of} the arranged in a reference heating zone H _R heater group ( 60 ), that the temperature sensor ( 32 ) is arranged in the reference heating zone H _R and that the heating power P _{R of} the heater group ( 60 ) in the reference heating zone H _R as a function of the temperature sensor ( 32 ) measured temperature is adjustable. Float bath tub according to claim 8, characterized in that the heating power P _{R of} each heater group ( 60 ) by the heating power of the heating devices ( 31a , b) is adjustable. Float bath tub according to claim 8 or 9, characterized in that the heating power P _{N of} each heater group ( 60 ) by switching on and off heating devices ( 31a , b) is adjustable. Float bath tub according to one of claims 8 to 10, characterized in that the reference heating zone H _{R is} arranged centrally. Float bath tub according to one of claims 8 to 11, characterized in that at least the outer heating zones ( 50 ) at least three heating devices ( 31a , b). Float bath tub according to one of claims 8 to 12, characterized in that the heating devices ( 30 . 31a , b) are offset from one another. Float bath tub according to one of claims 8 to 13, characterized in that in at least one further heating zone ( 50 ) at least one temperature sensor ( 32 ) is arranged. Float bath tub according to one of claims 8 to 14, characterized in that five heating zones ( 50 ) are provided. Float bath tub according to one of claims 8 to 15, characterized in that in each heating zone ( 50 ) a temperature sensor ( 32 ) is arranged.