Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~g7~7
Radf~nt h t~U~i~
The invention relates to a radiant heat unit
compri~ing a heating element, a heating el~ment support
and a thermal lnsulation located below the heating
element support.
Such radiant heat units are known from
DE-A-2165569, DE-A-2551137 and the corre~ponding
US-A-4161648, DE-~-2339768 and the corre~ponding
GB-A-1433478, DE-C-2760339 llnd EP-A-204185 and the
: 10 corresponding US-A-4713527.
~ DE-A-2165569 describes supports for the heating ele%lents,
: which cover part of the surface or the entire surface and
a thermal Lnsulation, located i~ appropriat~ below the
~upport, consi~ting of mineral wool and/or gla~ wool
arranged in layers, to allow a sprung bedding for the
support.
DE-A-2551137, DE-~-2339768, DE-C-2730339 and EP-A-204185
each describo supports which cover the entire surface for
the heating ele~ents, it being pos3ible, i appropriate,
for the heating elament to be fixed by being laid,
; clamped or or glued in or on rece~ses or projections
provided for this purpo~e, and a thsrm~l i.n~ulation,
located belo~ the support, con~i~ting of a micropor~u~
thermal insulator.
Since a material which i~ relatively strong
m~chanically and at the 3~me tLme ha3 a high tel~per~ture
resi~tance is needed for the heating el~ment support3,
~uch material~ have a low thermal in~ulatin~ capacity,
which mean~ that design~ of the heating element supports,
which cover the entire surface, coun~eract high
efficiancies.
: Th~ ob~ect of the i~vention i~ to provide a
radia~t heat unit which is easy to handle during a~sembly
and allow~ high efficiencies.
The inven ion relat~ to a radiant heat unit
compri~ing a heating element, a heating elem~nt support
and a thennal insulation loca~ed below the heating
element support, wherein the haating element support is
2~2~
~,
constructed no~ to cover the enti:re ~urface of the
tharmal in~ulatlon locateA below 1~ and the thermal
in~ulatloll con~ s o~ a microporou3 thennal in~ulator.
The total energy con3umption in a radiant heat
unit accordi~g to the invention is reduced by up to 10 ~
relati~a to a radlant heater in which the heating element
i~ bedded on a heatlng ~l~m~rlt support which cover~ the
entire surface.
The as~embly of the radiant heat unit accordln~
to the in~ention i~ made considerably ea~ier ~ince the
heating element can be introduced in~o the heatlng
element support out~ide the radiant heat unit and the
final assembly to gi~e the complete radiant heat unit can
sub~equently be carried ouk. This ls of advantage in
particular because of the low mechanical. stahility o~ the
thermal insulation COnBiSting of a microporous thsrmal
in~ulator.
According to the i~ention, the heat~ng element
support does not cov~r the entire surface of the thermal
insulation located below it. It preferably co~ers 5 to
70 %, ln particular 10 to 20 %, of the sur~ace of the
thermal insulation located below it.
: A mechanically strong material which ha6 resist-
ance to change~ in tempsrature, high temperature re8i8t
ance, low temperaturo-dependent expansion and shrinkage
and/or a high electrical resistance is required for the
heating element support. Furthenmore, no alkali should
be pre~ent, in ord~r to avoid destruction of the haating
; ele~ent~.
Since the heating element ~uppoxt can have
various shapes, particular requirements are additionally
to be Lmposad on the ~upport material in respect of
shaping, which is why ceramic materials are usually used.
Such matexial~ ar~ known and are de~cribed, for
example, in Singer, Industrielle Reramlk ~Indu~trial
Ceramics), Yolu~e three, SpringarYerlag, 1966, in
particular on pages 126 156.
Preferred support m~terials are clay6, such as
: China clay or kaolin, bentonite, quartz, feld3par,
2~72~7
-- 3 --
Cornish ~tone and chamotte from earthenwara, porcela~n,
fireclay, ~llimanite and magne~it~. Smaller amounts of
granite, hsaalt, porphyry, chalk, calcite, dolomite,
magneYite, corundum, graphlke, talc:, rutlle, baryt~,
gyp~um, zirconium oxide, ~ill~nanite, cobalt ore, chrom-
ium ore, molten quartz, ~ilLcon carbide, ferrosilicon,
clinoenstatite, forsterite, plagiocla~e, nepheline and
cordierlte can also be pre~nt.
Aluminum oxide and rutile are furthermore used.
Kaolins, ball clays, pota3h feldspars, pegna-
tlte~, flint, quartz ~and, cry~talline quartz, chalk,
talc and expressed oils are preferably used.
Aluminum oxide ls used i~ particular.
The heating element supports are shaped by
~haping processes customary in ceramics, ~uch aq ~lip
ca~ting, and in thi~ ca~e hollow ca~ting for complicated
and thin-walled pieces, such as solid castiny for thick-
walled pieces and casting under pxes~ure.
Other proca~ses are manual brushing-in of compo
sitions of coarse~grained chamotte; a6 well a~ pressing
out, extrusion, pre~ing of ~ine-grainad compositionc in
the plastic state and hydro~t~tic pressing.
The shaped article~ thus produced are baked at tempsra-
tures in the range f rom 400 to 1700C.
Preferrad shape~ for the heating element support~
are ray or star shapes, the individual rays bein~ con
structed in the form of strip3. Other sh~pe~ are circle~
inside one another, connected by crosspieces, as well as
rectangular or oval figures and/or any combînation~ of
the geo~etric figure mentioned. The strip~ can be
square, rectanglllar, triangular, circular or oval in
cross-section or in a combined fo~m.
On the upp~r side of the heatiny element ~upports
are u ually hooked, circular, o~al, cyli~drical or
pyramidal, preferably hooked or ~awtooth like~ pro~ec-
tions, in or on which the heating el~men ~ are fixed. The
hooksd pro~ections can have the ~hape of an inverted "L~,
and ~hey c~n remain in their basic shape or be bent
slightly with re~pect to one another. If the pro~ections
2 0 ~ 7 2 g3 ~
are ~eformed, thay are usually defo~ed al~ernately to
one or other side, that L~ to say they point aither
inward or outward. The pro~ectlons can be opposite one
another or else located diagonally. ~he decisive featu~e
here i~ that the heating element can be embedded irmly
into the a~choring according to it~ shape.
It is often suficient for the upper side o~ the heating
element support to have reces~e~ which partly imitate the
shape of the heating alement. The di~tances between tha
pro~ection~ or th~ di~meter~ oE the reces~es here ~hould
be at least the same a~ but E~referably omewhat smaller
than the diameters of the heating element.
In addition to these fixings on the heatiny elemenk
support, the heating elaments can also be fixed by means
of adhe3ives which are known per ~e, such a~ are
described, for example, in EP-A-130 629l which i~
expre~sly referred to in thi~ connection.
Prefsrred adhesive~ are water-gla3ses, silica ~ol~ and
ceramic adhe3ives.
Since according to the~e arrangements the heating
el~ments largely li~ free, that i~ to say contack with
the pro~ection~ or reces~e~ i8 small, they radiate toward
all side~, which mean that the affi~iency is increa~ed
con~iderably compared wi~h radiant heater~ haviny heatlng
element ~upport3 which cover the antire ~urf ace .
The radiant heat unit~ ~re u~ually circular in
3hape, but any de~ired ~hape~ o~ haated surface~ can be
ropresented by mean~ of the heating element supports u ed
according to the invention.
The heating ele~ents themsalve~ are usually
meandering, spiral or ~traight in shape and are usually
operated by means of elec~ric current.
During a qe~bly, the heating element is u~ually
inserted into the heating element suppor~ by gentle
pressure or by twi~ting and i8 therefore fixed but not
rigidly clamped. In this wag, temperature-dependent
expansion of the heatlng re~istance i~ not impeded.
The radiant he2t units can be operated by con~rol rods
with electronic and also con~entional ~mperature
- 5 ~ v~ ~3 ~
monitorinq.
The the.rmal lnsulatlon corlsi3tiny of a micro~
porou~ thermal ln~ulator pr~ferably ha~ the following
compo~ltian.
30 - 100 ~ by waight of fine;L~ dlvided metal oxide,
0 - 50 ~ by weight of opac:ifying agent,
0 - 50 ~ by waight of fiber material and
0 - lS ~ by weight of inor~anic binder material.
Preferred composition~ contain:
30 ~ 89 % by weight of finely divided metal oxids,
10 - S0 % by weight of opacifyin~ agent,
1 ~ 50 ~ by weight o fi~er matexial and
0 - 5 % by weight of inorgcinia bindsr material.
Par~icularly good re~ults are achieYed with the
following compo~ition~:
50 - 89 % by weight of finely di~ided metal o~ide,
20 - 40 ~ by waight of opacifying agent,
5 - 70 % by weight o~ fiber materi~l and
0.5 - 2 % by weight of inor~anic binder m~terial.
Examples of finely dlvided metal oxide ar~
p~rogenicall~ produced ~ilicic acid~, including arced
silicic acids r low-alkali precipitated ~ilicic acids,
alumin~m oxide, titanium o~ide and zirconi~m oxid*
prepared analogou~ly, and mixtureY thexeof. Pyrogenically
produced 8ilicic acid, aluminum oxide or a mixture
thereof are pre~erably u~ed. The finely di~ided metal
oxida~ hav~ specific suxface area~ o~ pre~erably
50 - 700 m2/g, in partlcular 70 - 400 m2Jg.
Examples of opacifyi~g agent~ are iLm~nite,
titanium dio~ide, silico~ carbide, iron(II)Jiron(III)
mixed oxide, chromium dioxide, zirconium oxide, mangane~e
dioxide, iron oxide, silicon dioxida, al~minum oxide and
zirronium 3ilicata, and mixture~ thereof. Ilmeni~e and
zirconium silicat~ are preferably used. Th opacifying
agent~ advantageously have an ab~orption maxLmum in the
infraxed re~gion between 1.5 and 10 ~m.
~ Exz~ples of fiber material arc gla~s wool, rock
: wool, ba~alt wool, slag wool, ceramic fiber~t such a~ are
obtained from melt3 of alumi~um o~ide and/or silicon
w 6 ~ 2 ~ 6 r~
oxide, and as~e~to3 fibers, and mixtures thereof. Fibers
obtained from a melt of aluminum oxide and/or ~ilicon
oxide are preferably used.
Inorganic binder~ which can be u~ed are all ~he
binder~ which are known for u~e in microporou~, pres~ed
thermal in~ulator~. Examples of such binders are di~-
closed, for example, in EP-A-29227, which i~ expressly
referred to in this connection. Borides of aluminum, of
titanium, of ~irconium or o~ calcium, silicides ~uch a-
~
calcium siliclde and calcium/aluminum silicide, and inparticular boron carbide are pre~erably employed. Ex-
amples of other constltuents are ba~ic oxide~, in par-
ticular magnesium o~ide, calcium oxids or barium oxide.
The productlon of the thermal insulation consi~t-
ing of a microporous, pressed thermal insulator preferab-
ly comprise the following process ~teps:
a) preliminary compaction of the thermal insulating
mixture based on ~inely divided metal oxide under pres-
sures of 1 to 5 bar, in particular 2 ~ar or approxLmately
2 bar;
; b) pressing of the precompacted material into the desired
shape under final pres3ures o~ 8 to 20 bar, the thickness
of the re~ulting shapes preferably being 1 to 25 mm, in
particular 2 to 10 mm; and
c) if appropriate heating of the pressed article at
temperature of 100 to 900C.
The gases enclo~ed in the bulk material should be
able to escape during the prel ~inary comp~ction a~d
pre~sing. The compaction and pressing are therefore
pre~erably carried out while applying reduced pres~ure.
The dega~sing can al50 already be carried out before the
compactio~ or pressing.
A pressing tool which ha8 prominences or depres-
~ion~ which image the geo~etric shape of the heating
element support can be used during the pressing opera-
tion. Firm anchoring of the heating element support in
the thermal insulation i~ achievad in this way. The
hea~ing element support can also additionally be flxed in
or on the thermal insulation by clamping or gluing.
2 ~
-- 7 --
Radiant heat unit~ according to the l~ventlon are
us~d for heating a plate, in particular a glas~ cex~nic
plate, in radiant heaters of ovens, i.n particular bake~y
ovan~, in radLant heater~ or in halogen radiators.
Radiant heaters have a resistance wire a~ the source of
heat, and halogen radiators a halogen lamp. H~logen and
xesi~tance wire heaters combined in a radian~ heater have
recently been disclosed.
Preferred embodiments of the invention are shown in
Figures 1 and 2. In these figures:
1 denot2s a microporou~ thermal insulator
2 denotes a ceramic heating element ~upport
3 denotQs a heating coi:L
4 denotes a sawtooth~like retaining cone
Figure 2 repr0sent~ an enlarged longikudirlal
section of Figure 1.
Example 1
Clay was made into a pa~te with water to give a
plastic, ~asily shaped mass. This mas~ wa~ rolled out to
a thickneæs o~ 5 ~m.
An eight-rayed ~tar cross which was additionally ~tabil-
i~ed by a square on the tip wa~ shaped from thi~ using a
blade. The ray width -~a~ 11 mm. The diameter of the
entire holder wa~ 178 mm~ Sawtooth-like holding cone~ of
tha clay mass ~erQ positioned on the rays ~uch that they
imitated a spiral geometry. ~hi~ heating element ~upport
was baked at a temperature of 1050C.
A thermal insulatio~ con~i~ting of
62.5 ~ by weight o~ pyrogeni~ally produced silicic
acid,
31.7 ~ by weight of zirconium silicate,
5 ~ by weight of aluminum ~ilicate fiber and
: 0.8 ~ by weight of boron carb~de
wa~ pre~3ad, using a pre~sure ram which imagad th~
geomstry of the heating elemQnt ~upport, to a molding
having a diameter of 198 mm, edge height o~ 10 mm,
o~erall he:ight 3f 31 mm and basa heigh~ of 16 mmy and ~he
molding wa,~ treated in an oven at ~00C for 1 hour.
A heating coil ~diameter 5 mm, wire thickness
2 ~ l~ 12 ~3 7
a
0.8 mm) wa~ laid .in the heating element ~upport. The
heating element support equipped in this way was laid in
the thermal insulstion, the heatlng element support being
fixed in the thenmal insulation by 2 metal clamp~.
S The radiant heat unit having an ouput of
1800 watt was sub~ected to a fatigue te~t lasting
100 hour~ with on and off cycles of lS minute~.
~xample 2
The procedure accerding to Example 1 ~as repeat-
ed, with the modification that the ceramic holder was
~quare in construction. The ray width was al50 11 mm
here, and the edge length of t~le square wa~ 240 mm. The
appearance of the ceramic holder corresponded to
Example 1, but the rays were lengthened 60 that they fill
a square of edge length 240 mm. The ray ends were
stabilized by ~ square fram0 of the 8ame ceramic.
A thermal in~ulation con~i~ting of
62.5 ~ by weight of pyroganically produced siliciG
acid,
37.7 % by weight of zi~conium silicate3,
5.0 % by weight of aluminum 3ilicate fiber~ and
0.8 % by weight of boron carbide
was preQsed by mean~ of a pre~sure ram which imaged the
geometry o~ the heating element support to give a molding
having tho following dimen~ion~:
external length o~ square 300 mm,
edge width 30 mm,
e~dge height 14 mm,
b~se height 16 mm,
overall height 30 mm.
The heating element ~upport with heating coil
according to Ex~mple 1 was laid in thi~ moldin0 and fixed
with 5 holding clamp8. This unit wa~ subjected to the
f atigue te~t according to Example 1 .
Example 3
The procedure accordi ng to Example 1 wa~ repeat-
ed, with the modification that th~ resi~ance wire wa~
not in the shape of a coil but wai constructed a~ a level
f lattened wire .
