Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to -the manufacture of glass and in par-
ticular to a method o~ conditioning molten glass and a glass mel-t~
ing tank for carryillg out such conclitioning.
~ n a known method of manufacturing glass in a continuous pro-
ce~s, raw materia~s are fed in at one end o~ a glass mel-ting ~ank
to ~orm a blanket floa-ting on an existing ba-th of molten glass.
The~ rate of ~eeding is su~ficient to maintain a constant glass
depth ln the tank whilst mol-ten glass progressively flows towards
the opposite end of the tank known as the working end, from which
molten glass is taken away for use in a forming process. The
blanket of raw materials is converted to molten glass as it passes
through a melting zone at one end of the tank by heat which may
come for example from burning fuel supplied from burners situated
at spaced intervals ill the side walls above the glass level or ~rom
electrical heating devices~ The molten glass passes ~rom the
melting zone into a refining zone where heat is also applied above
the molten glassO In the refining zone bubbles oE gas still re-
maining in the glass are encouraged to escape or go into solution
in ^the glass. The glass passes from the re~ining zone into a
conditioning zone ad~acen-k the working end o~ the tankO In the -
conditioning zone the glass is homogenised and brought to a suit-
able thermal condition for use in the forming processO Normally
a canal leads from the working end of -the tank to a ~orming pro-
cessc
From -the above 5 it can be seen that certain regions of the
tank are defined as melting, re~ining and condi-tioning zones. As
regards the molten glass passing from one zone to another, all the
glass leaving any one zone may no-t necessarily have reached a
final state for -that operation eOg. a ~ully ~efined state as it
enters the condi-tioning zone, Some refining can s-till occur in
the conditioning zone, and conditioning may start -to some extent
in the relining region. Hence the zoned regions are defined -to
show -the areas in which the grea-ter part or all of a par-ticular
operation is carried out in a -tank, and enables the man prac-tised
in the art to identify the temperature conditions required ill these
zones. '
Conventionally flat glass melting tanks are designed to con-
taLn a large bocly of molten glass and are norrnally of substan-tially
; uniform depth througllout the melting, refining and conditioning
zones. Convec-tive flow currents set up within the molten glass
aid in mixing the glass to achieve homogeneity of temperature and
composition. At -the same time colder return flows of glass which
, OCCUi~ in the lower regions of the tank from the conditioning zone
I back towards the melting zone protect the refractories at the bot-
tom of the tank from -the wear which might otherwise occur if sub~
~ected to the higher glass temperatures used in the melting and
refining zonesO
The above process for the manufacture of glass is however
wasteful of heating energy since the returning colder glass in -the
lower layers of the tank must be reheated each time it passes back
, through the glass tank, It has been found that the amount of
¦ 20 glass which circulates and returns ~rom the conditioning zone to
the melting zone is dependent on the depth of the molten glass as
well as the temperature gradient between the two ends o~ the tank
, and the output from -the tank~ It is possible to choose -the con-
i ditions so that all the glass flows in a downs-tream direction
i towards the outlet end with ro re-turn flow~ There are however
dif~iculties in achieving satisfactory homogeneity of temperature
and composition if the flow within the tank is entirely in one
direction with no return flow. Fur-thermore~ in the conditioning
zone it is necessar~ to lo~er the tempera-ture of the glass but any
3o excessive surface cooling in -the conditioning zone is likely to
cause unacceptable inhomogenei-ty in the mo:lten glass, Fur-ther
more~ it is desirable to avoid an excessively long conditioning
zone.
~ 3
,
Conditioning as a process can ~ary from achie~ing subs-tantial
thermal and physical homogeneity in glass when it leaves the con-
ditioning zone to achieving a par-ticular temperature gradient in
the glass. Conven-tional methods of achieving a suitably condi-
tloned glass are normally based on supplying cooling air to the
sur~ace of the ~lass as it flows to a ~orming process. However
as the unit outpu-t and hence -throughput of glass is increased,
using conventional air cooling systems it has been necessary to
increase the size of the conditioning area and provide more accur-
ate control so as to avoid causing steep -temperature gradients in
the glass due -to using large volumes of cooling air on the glass
sur~ace. There have been other proposals for removing heat from
the bottom of the conditiQning zone by use of cooling air and in
some cases cooling pipes have been positioned in the glass for the
removal of unwanted heat from the glass. However these previous
uroposals have not in~Jolved selective removal of heat from within
the body of the glass adjace~lt the inlet of the conditioning ~one
to achieve a controlled temperature profile -through the glass.
It is an object of the present invention -to provide an improv~
2~ ed method and apparatus for conditioning molten glass and achieving
a desired tempera-ture pro~ile through the glass ~hile all the glass
flows in one direction.
The present invention provides a me-thod o~ conditioning mol-
ten glass to achieve a desired thermal dis-tribution in -the glass
suitable for feeding to a forming process, which me-thod comprises
feeding mol-ten glass through a conditioning zone in a container
adapted to contain molten glass5 causing all the ~,lass ~lowing
through the zone -to flow in a direc-tion from an inlet to the zone
towards an outlet frorn the zone, selectively cooling the molten
glass at or near the inlet to the conditioni~lg zone to achieve a
desired temperature profile through a -transverse cross-section o~
the glass adjacent the :inlet end such -that on flowing throug~h the
.~
remainder of the conditioning zone the further conditioning com~
pletes the transformation of the glass to a state suitable for fe~d-
ing to a forming process, said cooling being effected by passing
cooling flui~ through means located in the fo~ard flowing body
of the glass~
Preferably the means through which cooling fluid is passed is
located in the forward flowing body of the molten glass at a posi~
tion selec-ted in dependence on the temperature distribution within
the glass and the required temperature profiler
The cooling fluid may be passed through a plurality of fluid
cooled stirrers and preferably the cooling flui.d comprises water.
Cooling fluid may alterna-tively or additionally be passed
through one or more pipes located in the glass.
The cooling at or near the inlet to the conditioni.ng zone may
be carried out upstream or downstream of the inlet to the condi-
tioning zone.
When cooling by -the use o~ pipes the said cooling may comprise
cooling the lower part of the molten glass by one or more fluid
cooled pipes located within the molten glass and extending across
.- 20 the base of the inlet to the conditioning zone and further cooling
ths molten glass at or near the inlet to the conditioning 7.0ne by
at least one fluid cooled pipe located in the forward flowing body
of molten glass between the upper and lower boundaries of the mol
ten glass,~
Preferably the method includes detecti.ng -the temperature dis-
tribution ~ithin the molten glass at or near -the entrance to the
condi~ioning zone and positioning at leas-t one of the fluid cooled
pipes in dependence on the detected temperature distribution~ ..
, The -temperature distribu-tion downstream of -the cooling means
.1 30 may be also monitored to check on -the correctness o~ the posi
tioning of the cooling meansO
Preferably cooling liquid is circulated within the fluid cool
ed pipes9
. .. . . . . ..
` : :
d
The molten glass may also be treated by homogenizing means
at or near the inlet to the conditioning zone. The method may
include the controlled application of heat or cooling to the
molten glass in the conditioning zone.
The upper surface of the molten glass in the conditioning
zone may be cooled by use of cooling blasts of air.
In the aforesaid method of manufacturing mol-ten glass
wi.thin a glass melting tank, the depth of molten glass in the
refining zone may be greater than the depth of glass within the
conditioning zone whereby some recirculation of the molten glass ~;
occurs in -the refining zone.
The invention also provides a glass melting tank comprising
an elo~gated tank for containing molten glass, said tank having
a melting region into which glass forming material is fed, means
for heating and thereby melting the contents of the tank in the
melting region, a refining region downstream of the melting re-
gion in which the molten glass is refined, and a conditioning
region having an inlet adjacent the refining reglon and an out-
let at a working end of the tank from which the molten glass is
removed, the conditioning region being shallower than the refin-
ing region whereby all the molten glass flowing through the con-
ditioning region flows in a downstream direction towards the
working end, and cooling means for cooling the glass in the con-
ditioning region, the cooling means comprising at least one
fluid cooled pipe extending across the base of the inlet to the
conditioning region and at least one additional fluid cooled
pipe at or near the inlet to the conditioning region and located
in the body of the forward flowing glass at an adjustable posi-
tion so as to achieve a desired temperature profile at or near
the inlet to the conditioning region.
The or each said additional fluid cooled pipe is preferably
located in the body of forward flowing glass above the lower
boundary oE the molten glass and below the upper boundary of the
molten glass.
- 6 -
`:
Preferably one or more temperature de-tectors is provided for
detecting the tempera-ture distri.bu-tion within the molten glass at
or near the :inlet tu the conditioning zone, so that -the posi-tion
of at least one of the ~luid cooled pipes may be selected i.n depen-
dence on the detected tempera-ture di.stribution. The temperature
detectors may comprise an array of thermocouples or other ternpera-
ture de~ectorsO
Pre~erably the fluid cooled pipe at -the base o~ the inle-t to
the conditioning zone is located a-t the top of a step in the base
of the tank, the step being at the junction of the refining and
conditioning zones. Preferably the pipe has upstanding sidc arrns
which extend up opposite side walls of the tank adjacent to the
inlet to the conditioning zone~ Preferably the pipe is substan-
tially U-shaped. The height o.~ the pipe may be ad~ustable~
The additlonal ~luid cooled pipe may be located in the con-
ditioning zone downstream of the inletn Alternatively the addi-
tional cooling means may be located within the refining zone imme-
diately upstream of the inlet to the conditioning zone. In some
cases it may be desirable to provide further cooling means both
upstream and downstream of the inlet to the conditioning zone.
Pre~erably the or each additional fluid cooled pipe is also
adjustable in height and preferably comprises a U shaped water
cooled pipe.
The glass melting tank may also incorporate one or more s-tir~
~ers, preferably water cooled~ Such s-tirrers may be loca-ted
upstream, at, or downstream of the inlet -to the conditioning zoneO
Pre~erably the fluid cooled pipe ex-tending across the base of
.
-the ir~et to the conditioni.ng region extends transverse to the
length of the tank and extends across the entire wi.dth of the tank~
3o The additional fluid cooled pipe or pipes may on -the o-ther hand
ex-tend only part way across the width of the tank and be loca-ted
centrally across the w.id-th~ -
. ~ 7
.:
1~ 7~1L~Lg3'7
The inven~ion also provides a glass melting tank comprising
an el.ongated tank for containing molten glass, said tank having a
melting region into which glass forming ma-terial is fed, means for
heating and thereby melting the contents of the tank in the meltirlg
re~ion, a refining region do~nstream of the melting region in which
the mol.-ten glass i.s refined, and a conditioning regi.on ~laving an
inlet adjacent the refining region and an outlet at a working end
of the tank from which the molten glass is removed, the condition-
ing region being shallower than the refining region whereby all
the molten gl.ass flowing through the conditioning region may flow
in a downstream direction towards the working end9 and cooling
means for cooling the glass on passing from the refining region to
the working end, said cooling means cooprising a plurality of fluid
cooled stirrers located in the body of forward flo~ing glass and at
least one fluid cooled pipe located in the body of molten glass .in
the conditioning region arranged to achieve a desired temperature
pro~ile in the molten glass.
By selective use of the fluid cooled pipe or pipes and/or
stirrers it is possible to achieve satisfactory cooling o~ the mol~
2~ ten glas~ in -the conditioning zone and a desi.rèd temperature pro-
file while all the glass flows in one direction without the need
~or an excessively long conditioning zone~ By adjusting the
exact height and location of at least one fluid cooled pipe it is
possible to achieve optimum ternperature profile conditions within
the conditioning zoneO
As is already known, any inhomogeneity in the conditioning
zone tends -to ~orm into thin horizontal layers in the molten glass
each having a sligh-tly differen-t composition from the immediately
adjacént layerO Generally the layers are so thin and -the dif-
ferences of composition are so small that provided the layers
rema.in substantially parallel to the major surfaces in the final
glass product, no adverse affect is observed~ If however -these
, . ;, .
layers are caused to deviate from the parallel condition, op-tical
faults in the glass may resultO By use of the present invention,
it is possible to reduce the likelihood of such faults occurring.
Some embodiments of the invention will now be described by
way of example and with reference to -the accompanying drawings,
in which:-
Figure 1 is a side eleva-tion of a glass melting tank
- in accordance wi-th the invention,
Figure 2 is an enlarged view of part of the -tank
shown in Figure 1,
Figure 3 is a plan view of the part of -the -tank
shown in Figure 2,
Figure 4 is a view similar to Figure 2 showing an
alternative embodiment,
Figure 5 shows yet a further view similar to
Figure 4 showing a further embodiment, and
Figure 6 shows in plan view an alternative -to the
arrangement shown in Figure 3, shown with Figure 3.
Figure 1 shows a glass melting tank 11 having a filling end
12 into which is fed raw material for glass manufacture. The
raw materials float on the previously mel-ted glass ln the form of
a blanket 17. The blanket melts progressively in a melting zone
13 adjacen-t the filling end of the tank. The molten glass passes r
progressively in a downstream direction through a refining zone 14
to a conditioning zone 15 adjacent the working end of the tank.
An outlet 16 is provided at the working end ~rom which the glass
is removed for use in a subsequent forming process. Gas or oil
heating devices are located along the sides of the body of the
tank downstream of the filling end 12 for heating the molten glass
3~ via heating por-ts 18. ~Jaste gases pass through regenera-tor ports
in the sides of the furnace, the regenerator ports leading to a
furnace chimney,
,_ g _
In the refining zone 14, the molten glass circulates with the
glass in the upper layers flowing in a downstrear~ directlon while
the glass nearer the bottom o~ the tank forms a return flow marked
by the arrows 19 leading back towards the filling end of the tallkO
In the refining zone undissolved gases are released to the atmos-
phere. In the conditioning zone 15 the glass is conditioned so
as to achieve th~ desired thermal condition and composition homo-
geneity ready for the subsequen-t glass forming pxocess.
In each of the zones in the tank~ it is possible to achieve
some circulation of the glass with a return flow towards the fil--
ling end of the tank. The amount of .eturn flow, if any, is
dependent on the depth of the molten glass in the zone, the output
from the tank, and also the temperature gradient between the begin-
ning and end of the zone. In the examples shown, the melting zone
13 and ~he refining zone 14 are the deepest zones in the tank and
the base of the tank has an upward step 21 at the junction of the
refining and coilditioning zones so that the conditioning zone is *
substantially shallower than the melting and re~ining zonesO The
conditions in the refining zone are such that a degree of return
~low 19 occurs. Substantially all the glas~ flow in the condi-
tioning zone is away from the filling end of the tank, the depth
of glass being selected to achieve -this conditionD
Although the return or recirculating flow in the melting and
... .
re~ining zones of the tar~ improves homogeneity, glass quality is
not necessarily sufficiently improved~ particularly at high outputs
from the tank. To improve this 3 in this embodiment of the inven~
tion9 stirrers 22 are introduced through the roo~ 23 of the furnace
immediately upstream of the inlet to the conditioning zone. The
stirrers are arranged to act on the ~ol~ard flow of glass only and
0 cause attenuation of the glass layers wi-thou-t substantial distur-
! bance frorn their normal horizontal sta~e. As can be seen from
Figure 3, the refining zone 14 is of greater ~lidth than the condi~
tioning zone 15 and four s-tirrers are arranged s-ide by side ln a
~ 10 -
.
t~ .
row extending ~ransversely across -the width of the reIining zone of . :
-the tank~ Ad~aGent s-tirrers are arranged to rotate in opposite
directions. The stirrexs are pre~erably formed ~rom hollow tubes
through which cooling~ water is circulated to remove heat more rap-
ldly from the body of glass ~lowing fo~rards at the downstream end
o~ the re~ini.ng zone and a-t the same t:ime equalising the tempera~
ture distribution across the w.id-th of -the tanl~ at the entrance to
the conditioning zone 15.
To achieve cooling in the conditioning zone 15, surface cool-
ing occurs by directi.ng cooling air towards the surface of -the
molten glass. In addi-tion, means for achieving additional selec--
tive cooling are provided in -the ~orm of water cooled pi.pes 24 and
25 located adjacent the inle-t to the conditioning zone 15. The
pipe 24 comprises a straight horizontal section 26 extending across
the full width o~ the conditioning zone transverse to its length
: and the section 26 is located in a rectangular recess 27 formed in
.l the top of the step 210 The pipe 24 has two upstanding side arms
28 and 29 which extend vertically up opposite side walls o~ the
tank~ In this way the pipe 24 is of rectangular U-shape. The
1 20 pipe has an inlet at the top of the arm 28 and an outlet at the top -'
~ o~ the arm 29, the inlet and outlet are connected -through the roof
~¦ of the tan~ to a circuit in which cooling water is circulated. By
¦ locating the pipe 24 immediately adjacent the inlet to the condi~
¦ tioning zone~ the pi.pe removes some heat ~rom the lower regi.ons
~¦ of the glass entering the conditioning zone and furthermore pro~
tects the refractory corners o~ the step 21 from erosion by the
I accelera-ting glass passing from the refining to the condi-tioning
.~ zone. In order to remove additional heat from the body o~ the
glass in the condi-tioning zone and achieve a desired temperature
pro~ile, a second ~Jater cooled p.ipe 25 is loca-ted adjacen-t the
inlet to -the conditioning zone immediately do~rnstream of -the pipe
24 The pipe 25 is also o~ rec-tangular U-shape having a base
.~ .
~!LE37 ~
section 30 and -two upstanding side arrns 31 and 32 providing inlet
and ou-tle-t passages for cooling water~ The pipe 25 is arranged 50
tha-t -the horizontal section 30 1.s located in the body of ^the glass
betwe~en the upper and lower boundaries o~ the molten glass i.n the
condi.tioning zone~ The wi.dth of the pipe 25 is approximately half
the width of the conditioning zone~ The pipe 25 Is adjus-table in
position both vertically and transversely and as shown i.n the draw
ings the pipe is loca.ted substantially centrally across the width
o~ the zone, with -the section 30 substantially mid-way bet~ieen the
upper and lower boundaries of the molten ~lass. To allo~r the
adjus-tability in position~ the pipe 25 is fixed -to a mounting arm
33 which extends to one side of the tank and adjus-tably engages a
support mernber 3~. The arm 33 can be moved both vertically and
transverse to the length of the tank, on the support member 340
The ta~ also has an array o~ thermocouples 35 near the inlet to
.the conditioning zone. The thermocouples 35 are all transversely
aligned and are spaced across the base of the tank. The thermo-
couples are mounted in refractory sheaths closed a-t the top 9 each
sheath contains a r.umber of thermocouples at varying heigh-ts so as
to ensure adequate monitoring throughout the glass depth. By
projecting into -the molten glass the thermocouples m.easure the
temperature distribution ~ithin the molten glass. The position
o~ the pipe 25 is adjusted in dependence on the measured -tempera-
ture distribution wi-thin the glass so that the cooling ~ffected
by the water pipes in the conditioning zone causes the g~ass to
have a desired temperature profile at or near the entrance to the
conditioning zone~ This temperature profile is selected such ~Ghat
the further conditioning which occurs as the glass flows along the
condi-tioning zone results in the molten glass having -the desired
tempera-ture conditions ~or the subsequent ~orming process on leav~
ing the conditioning zone -through the out~let 16~
It has been found that cooling -the glass adJacent the bottom
- 12 ~-
of the conditioning zone improves the glass stability in the upper
layers in the conditioning zone and reduces the likelihood of glass
f`rom the upper surfaces flowing downwards towards -the bot-torn of
the conditioning zone. By locating the additional pipe 25 in -th~
hotter area of glass within the conditioning zone it is possible
to achie~e the desired -temperature profile with a more uniform
temperature clistribution throughout the glass wi-thin the condition
ing zoneO It also allows quicker cooling to be achieved in the
conditioning zone as a whole ~ithout the disadvan~age o~ unstable
flow in the conditioning zone. By extracting the heat at a great-
er rate, it is possible to use a much shorter conditioning zone.
In this example, both the pipes 21~ and 25 are ad;justable in
height although the pipe 24 is normally located such that -the hori-
zontal section 26 is totally within the recess 27, More than olle
pipe 25 may be provided, and so~e pipes 25 may be located upstream
of the pipe 24 (that is at the end of the refining zone) if neces-
sary. In this case it is necessary to ensure tha-t any pipe 25 -'
located in the refining zone upstream of the pipe 24, is so posi~
tioned that it does not substantially enter the return flow and
affect that return flow. The position of the pipe5 25 iS adJUSt-
; able both to allow optimum operating conditions to be achieved and
to cope with changes in conditions during operation. The adjust~
ment in position of the pipes may be made, as previosly described,
as a result of signals derived from thermocouples irnmersed in the
glass in f`ixed positions~ Al-ternatively depth temperature surveys
in the glass may be carried out by passing -thermocouples verti-
cally -through the roof of the tank and observing values at fixed
increments of depth through the glass. These results can -then be
used to determine the desired positions for the pipes~
Figure 4 shows an alternative arrangemen-t adjacent the junc~
tions of the refining and conditioning zones. Sirnilar numera]s
have been used for similar par-ts. In this case the stirrer 22
. .
. ~ .
L07
is removed from the refining zone and located adjacent the inlet
to the conditioning zone between the pipe 24 and the pipe 25.
Figure 5 shows a further al.-ternative arrangement adjacent the junc~
tions of the refining and conditioning zones. In this case t,he
step between the two zones has a sloping surface 36 interconnecting
the step 21 with the floor of the conditioning zone.
The roof of the refining zone is prov~ded with a downwardly
pro~ecting wall 37 which forms a barrier extending down into the
molten glass adjacent the junction of the refin.ing and conditioning
zones. The lower horizontal edge of the wall 37 is provided with
a horizontal cooling pipe 38 through which cooling water circul~
tes via vertical inlet and outlet pipes 39 connected to opposite
ends of the pipe 38. In this example -the pipe 25 is provided as
~previously described and the stirrer 22 is located u.pstream o~ the
pipe 25 as already described with reference to Fig~re 4~
. In the arrangement shown in Figure 2, the stirrers 22 may be
. water cooled al-though the water pipes 24 and 25 are arranged in
~: dependence on the temperature distribution within the glass and the
~ required temperature p~ofi.le. However9 in an alterna-tive construc-
20 tion to that shown in Figure 2, one or both of the wate.r cooled
pipes 24 and 25 may be omitted~ In such an arrangement, the water
cooled stirrers 22 are positioned and arranged to produce the
desired temperature profile through a transverse cross-section of
the glass adjacent the inlet end of the cond.itioning zone, In
such an alternative arrangement where the water coo~ed stirrers are
used to produce the desired tempera-ture profile a-t the inlet end
of the conditioning zone, further cooling in the conditioning zone
can be achieved by one or more water pipes extending through the
mol-ten glass, the or each water pipe being positioned at any desured
3~ location along the leng-th of the conditioning zone and at any
desired position bet~een the upper and lower boundaries of -the mol
ten glass.
, . ' 'I
In the arrangement shown in Figure 3, the refining zone 14
i5 arranged to feed a single conditioning zone, -the conditioning
zone belng narrower than the refining zoneO It i5 however possi-
ble to feed two or more conditioning zones in parallel and one
such arrangement is shown in Figure 6. In this arrangement, -~o
narrow tank portions 40 and 41 extend towards the outlet end of
the tank from the ma.in body portion providing -the refining zone 140
Each of -the narrow cha}mels 40 and 41 provides a separate condi
tioning zone 15 similar to that previously described with reference
to Figure lo The depth of molten glass in each of the narrow
channels 40 and 41 is arranged so that the glass flow through each
of the channels is in the di.rection o~ the outle-t onl-~. Each
channel has a water pipe 24 lobated at the top of a step 21 at the
en-trance to the conditioning zone as previously described. A
; further water cooled pipe 25 is positioned slightly downstream
: from the pipe 24 and an array of thermocouples 35 are provided
both upstream and downsiream of the cooling pipe 25. The operation
of -the modifica-tion shown in Figure 6 is generally the same as
tha-t previously described with reference to Figures 1, 2 and 3.
The invention i.s not limited to the details of the foregoing
examplesO For instance, -the tank may have a waist adjacent th~
~unction of the refining and conditioning zones so that the molten
glass passes through a narrow region at this junctionr
Further, as indicated above, in achieving coo.ing in the con-
ditioning zone, surface cooling is achieved by directing cooling
air toward the surface of -the molten glass, in some instances
additional or al-terna-tive cooling or heating means may be required
in order to achieve a desired -temperature profile within the
designed limits of the equipment. One such case is where the
3o form of the invention is such -tha.t homogenising and cooling means
are provided before -the .inle-t to the conditioning zone, in this
case we find it convenient to provide fur-ther cooling means a-t any
- ~.5 ~
poin-t along ~he length of the conditioning zone in the ~orward
flowing molten glass as it passes -through the conditioning zoneO
Suc~l an arrangement may enable the con-tinued use of a relatively
short condi-tioni.ng zone despite an inc,rease in loarl on the tankD
In addition or alternatively burners rnay be provided in the side
walls of the conditioning zone if additional heat is required.
i ~ 16 ~
