U.S. patent number 3,927,300 [Application Number 05/447,513] was granted by the patent office on 1975-12-16 for electric fluid heater and resistance heating element therefor.
This patent grant is currently assigned to NGK Insulators, Ltd.. Invention is credited to Shigetaka Wada, Noboru Yamamoto.
United States Patent |
3,927,300 |
Wada , et al. |
December 16, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electric fluid heater and resistance heating element therefor
Abstract
An electric fluid heating element comprises a column-shaped
honeycomb structural body of an electrically conductive cermaic
material having a positive temperature coefficient of resistance.
The body has a multiplicity of substantially uniform parallel
channels extending therethrough with each of the channels being
bounded by a partition wall which is substantially uniform in
thickness. The body has a surface to volume ratio in the range of
10 to 60 cm.sup.2 /cm.sup.3 and is selfcontrolling without the
necessity of a safety device such as a fuse or thermostat. Ohmic
electrodes are mounted on oppsoite surfaces of the body to supply
heating current thereto. The heating element can be used as the
heating means in an air heater, humidifier or liquid heater.
Inventors: |
Wada; Shigetaka (Kuwana,
JA), Yamamoto; Noboru (Nagoya, JA) |
Assignee: |
NGK Insulators, Ltd. (Nagoya,
JA)
|
Family
ID: |
12206971 |
Appl.
No.: |
05/447,513 |
Filed: |
March 4, 1974 |
Foreign Application Priority Data
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Mar 9, 1973 [JA] |
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48-26934 |
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Current U.S.
Class: |
392/502; 338/55;
338/22R; 392/362; 392/383; 392/403; 392/485 |
Current CPC
Class: |
F24H
3/0417 (20130101); H01C 1/1406 (20130101); H05B
3/141 (20130101); F24H 9/1872 (20130101); F24H
1/00 (20130101); A45D 20/12 (20130101); H01C
7/022 (20130101) |
Current International
Class: |
A45D
20/00 (20060101); A45D 20/12 (20060101); H01C
1/14 (20060101); H01C 7/02 (20060101); D06F
58/26 (20060101); F24H 1/00 (20060101); F24H
3/04 (20060101); D06F 58/20 (20060101); H05B
3/14 (20060101); H05B 003/14 (); H01C 007/02 ();
F24H 001/10 (); F24H 003/04 () |
Field of
Search: |
;219/381,382,374-376,300,307,338,319,553 ;338/22,23,362,53,55,58
;252/518,520 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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512,667 |
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Oct 1920 |
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FR |
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932,558 |
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Jul 1963 |
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UK |
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499,074 |
|
Jan 1939 |
|
UK |
|
Primary Examiner: Bartis; A.
Claims
What is claimed is:
1. A heating element consisting essentially of:
a. a column-shaped honeycomb structural body of electrically
conductive ceramic material, said body having a multiplicity of
substantially uniform parallel channels extending therethrough with
each of said channels being bounded by a partition wall which is
substantially uniform in thickness, said structural body also
having a surface-to-volume ratio in the range of 10 to 60 cm.sup.2
/cm.sup.3 and having a positive temperature coefficient of
electrical resistance;
b. a pair of ohmic electrodes mounted on the opposite surfaces of
the body and in electrical contact therewith;
c. Means for feeding fluid through said channels.
2. The heating element of claim 1 in which the ohmic electrodes are
mounted on opposite surfaces substantially perpendicular to the
axial direction of the channels.
3. The heating element of claim 1 in which the surface to volume
ratios are 10 to 40 cm.sup.2 /cm.sup.3.
4. The heating element of claim 3 in which the surface to volume
ratios are 12 to 30 cm.sup.2 /cm.sup.3.
5. A fluid heater which comprises:
a. a column-shaped body of electrically conductive ceramic material
said ceramic body having a multiplicity of substantially uniforms
parallel channels extending therethrough with each of said channels
being bounded by a partition wall which is substantially uniform in
thickness, said body having a surface to volume ratio in the range
of 10 to 60 cm.sup.2 /cm.sup.3, said ceramic material having a
positive temperature coefficient of electrical resistance;
b. A pair of ohmic electrodes mounted on the opposite surface of
the body;
c. Means for feeding fluid through said channels.
6. The heater of claim 5 in which the ohmic electrodes are mounted
on the opposite end surfaces substantially parallel to the axial
direction of the channels.
7. The heater of claim 5 in which the surface to volume ratios are
10 to 40 cm.sup.2 /cm.sup.3.
8. The heater of claim 7 in which the surface to volume ratios are
12 to 30 cm.sup.2 /cm.sup.3.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to heating elements, and more particularly
to a heating element comprising a ceramic article having a positive
temperature coefficient of electric resistance (hereinafter
referred to as PTC ceramic article). The element is composed of a
honeycomb structural body and includes a pair of ohmic electrodes.
The element is adapted to generate a large amount of heat radiation
with a small volume without any risk of overheating and breaking.
The invention also includes a heater comprising this heating
element which is particularly useful as an air heater, dryers
inclusive of a hair dryer, air towel, liquid heater, humidifier,
volatilizer and the like.
In the in the remainder of the specification, the term honeycomb
structural body shall be understood to mean a structural body
having a multiplicity of channels extending therethrough and
generally parallel to each other and having high surface-to-volume
ratios.
2. Description of the Prior Art
Heretofore, it has been the common practice to use metals and
ceramics having a negative or a positive temperature coefficient of
electric resistance.
Metal has a small specific resistance and hence is commonly used in
linear form in the heating element. If such heating element is
used, for example, in a hair dryer and a fan is used to feed air
therethrough, and suction opening is clogged, the metal
constituting the heating element becomes overheated, causing a
fire. Eventually there is a risk of the metal being oxidized and
broken.
The ceramic article having a negative temperature coefficient of
electric resistance, is commonly formed into a rod having a pair of
ohmic electrodes mounted thereon and used as a heating element. A
ceramic article for example, silicon carbide, in order to generate
a given amount of heat radiation without overheating the heating
element per se, a number of these heating elements must be used
simultaneously. Alternatively temperature control means must be
used, and as a result, the assembly becomes complex in construction
and/or there is a risk of the heating element being broken. Thus,
such a heating element is not suitable for use with domestic
heaters.
In addition, a PTC ceramic article made of semiconductive barium
titanate has commonly been formed into a disc-shaped pellet on
which are mounted a pair of ohmic electrodes for use as a heating
element. Such kind of heating element, however, can only generate
the amount of heat radiation corresponding to a few watts from one
heating element. As a result, in order to cause it to generate the
amount of heat radiation corresponding to several watts, a large
heat radiating plate must be added thereto. Thus, this kind of
heating element is subjected to restrictions in its construction.
Moreover, if it is desired to obtain a large amount of heat
radiation with the aid of a high electric power of, for example,
more than 1 KW, a number of heating elements each including the
above described heat radiating plate are required. The use of the
measures described is extremely uneconomical and results in
considerable constructional disadvantage as that it could not be
applied to domestic heat radiators.
In addition, a heating element comprising a cylindrical PTC ceramic
article which is provided at its inner and outer surfaces with
ohmic electrodes has been known. If this heating element has its
surface area of about 1,000 cm.sup.2 in order to obtain a
sufficiently large amount of heat radiation, for a the diameter of
5 cm it is necessary to make the length longer than 30 cm. As a
result, the volume of the heating element becomes too large as
compared with the volume of the customary type of heating element.
Thus, the mechanical strength of the heating element becomes
lowered and this type of heating element has not achieved any
importance in actual practice.
SUMMARY OF THE INVENTION
The invention is based upon recognition of the fact that a PTC
ceramic article composed of a honeycomb structural body provides a
heating element which is small in volume and which can generate a
large amount of heat radiation without overheating and
breaking.
The invention thus provides a heater comprising the above described
heating element and particularly useful in air heater, dryers
inclusive of a hair dryer, air towel, liquid heater, humidifer,
volatilizer and the like.
The principal object of the invention, therefore, is to provide a
heating element comprising a PTC ceramic article composed of a
honeycomb structural body.
Another object of the invention is to provide a heater, which
comprises a heating element comprising a PTC ceramic article
composed of a honeycomb structural body and particularly useful as
an air heater, dryers inclusive of a hair dryer, air towel, liquid
heater, humidifer, volatilizer and the like.
A further object of the invention is to provide various types of
heaters each having a preferable construction.
A still further object of the invention is to provide a method of
producing a heating element comprising a PTC ceramic article
composed of a honeycomb structural body and to provide a method of
forming ohmic electrodes therefor.
Other objects, advantages and capabilities of the present invention
will become apparent from the following description taken in
conjunction with the accompanying drawings showing only a preferred
embodiment of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a preferred embodiment of the
heating element according to the invention;
FIG. 2 is a section along line II--II in FIG. 1;
FIG. 3 is a perspective view of another embodiment of the heating
element according to the invention;
FIG. 4 is a section along line IV--IV in FIG. 3;
FIG. 5 is a graph which illustrates the relation between the amount
of heat radiated from the heating element according to the
invention and the amount of air fed to the heating element;
FIG. 6 is a perspective view of a preferred embodiment of an air
heater employing the heating element according to the invention, a
part being broken away for clarity;
FIG. 7 is a perspective view of a preferred embodiment of a liquid
heater employing the heating element according to the invention, a
part being broken away for clarity; and
FIG. 8 is a perspective view of a preferred embodiment of a
humidifier employing the heating element according to the
invention, a part being broken away for clarity.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiments of the invention will now be described in
greater detail with reference to the accompanying drawings.
In FIGS. 1 and 2 a heating element 1 is shown according to the
invention which is composed of a honeycomb structural body provided
with a multiplicity of channels 2 extending therethrough and
generally parallel to each other. Each of these channels 2 is
bounded by a partition wall 3 which is substantially uniform in
thickness. The honeycomb structural body constructed as above
described insures extremely high surface-to-volume ratios. The
partition walls 3 are provided at their opposed end surfaces
substantially perpendicular to the axial direction of the channels
2 with a pair of ohmic electrodes 4, 5, respectively. The thickness
of both the partition wall 3 and ohmic electrodes 4, 5 is shown in
enlarged scale in FIGS. 1 and 2 for clarity.
A preferred method of producing the heating element according to
the invention will now be described.
BaCo.sub.3, TiO.sub.2, SiO.sub.2 and La.sub.2 O.sub.3 were weighed
with mole ratios of 1.00:1.02:0.02:0.003, respectively, mixed in a
ball mill with rubber lining for 12 hours, dried and calcined at
1,100.degree.C for 3 hours. The calcined raw material was roughly
pulverized by a double roll crusher with alumina rolls and then
finely pulverized in a ball mill with rubber lining for 6 hours.
The finely pulverized particles were passed through a screen with a
mesh of 149 microns and then dried. To 100 parts by weight of the
dried powders were added 4 parts by weight of methyl cellulose,
16.5 cc of 12% polyvinyl alcohol water solution, 3 parts by weight
of polyethylene glycol and 8.5 parts by weight of water. The
mixture was well blended in a kneader and then subjeted to
de-airing while blending with the aid of a de-airing pug mill. The
product thus obtained was extruded from the de-airing pug mill
through a honeycomb structural body forming nozzle so as to form a
green body composed of a honeycomb structural body. The green body
thus obtained was dried by means of a freeze drying method using
dry ice and then fired in a standard electric furnace using silicon
carbide heaters at 1,350.degree. C for 2 hours. A column shaped
honeycomb structural body having a diameter of 4 cm and a thickness
of 1 cm (length of channels) and provided with a multiplicity of
channels each square in section having each side of 0.195 cm and
bounded by a partition wall having a thickness of 0.03 cm was
thereby obtained. The honeycomb structural body has
surface-to-volume ratios of about 16 cm.sup.2 /cm.sup.3 and total
surface area of about 200 cm.sup.2. Ohmic electrodes were provided
for each end surface substantially perpendicular to the axial
direction of the channel, thus completing a heating element.
The ceramic compositions are not restricted to the above described
ceramic composition and any other customary compositions which can
obtain a positive temperature coefficient of electric resistance
may also be used. Examples of such ceramic compositions are those
described in U.S. Pat. Application Ser. No. 431,397 filed on Jan.
7, 1974 which is a continuation-in-part of Application Ser. No.
256,368 filed on May 24, 1972; now abandoned and those described in
U.S. Pat. No. 2,981,699, No. 3,373,120 and No. 3,441,517,
respectively.
In addition, the honeycomb structural body may be formed by any
methods other than the above described extrusion method. Examples
of these methods are a press method, a method of producing a bundle
of a number of green ceramic pipes and then firing the bundle, or a
method of forming a honeycomb structural body comprising coating
ceramic raw material particles suspended in an organic binder on
the surface of sheet of paper and the like, corrugating the coated
sheet of paper and the like, accumulating the corrugated sheet of
paper and the like and firing the accumulated sheet of paper and
the like as disclosed in U.S. Pat. No. 3,112,184.
In addition, the sectional configuration of the channels may be
formed into any configurations other than square as shown in FIGS.
1 and 3, such, for example, as triangular, hexagonal or any other
polygonal, circular and the like sectional configurations (not
shown).
The outer configuration of the honeycomb structural body
constituting the essential part of the heating element is shown as
columnar in shape in FIG. 1 and rectangular parallelepipe in shape
in FIG. 3, but if necessary the outer configuration of the
honeycomb structural body may be of cube, polygonal columnar in
shape (not shown).
In the embodiment shown in FIGS. 1 and 2, the honeycomb structural
body is provided at its each end surface substantially
perpendicular to the axial direction of the channels with the ohmic
electrode. But, these ohmic electrodes may be mounted on any
positions other than end surfaces of the honeycomb structural
body.
For example, the honeycomb structural body may be provided at its
each side surface 6, 7 which is substantially parallel to the axial
direction of the channels and opposed to each other with a pair of
electrodes 8, 9 as shown in FIGS. 3 and 4. Whether the ohmic
electrodes should be mounted on the surfaces substantially
perpendicular to the axial direction of the channels or mounted on
the outer surfaces of the honeycomb structural body substantially
parallel to the axial direction of the channels may be determined
depending on the outer configuration of the honeycomb structural
body its mechanical strength, the purpose to which the honeycomb
structural body is used, the amount of heat radiation, the
restriction in design of the heater into which the honeycomb
structural body is incorporated as a heating element and any other
various conditions.
In FIGS. 2 and 4, the thickness of the partition wall 3 and ohmic
electrodes 8, 9 is shown in enlarged scale for clarity.
The ohmic electrodes may be formed on the outer surfaces of
honeycomb structural body by means of such a common method as those
described, for example, in U.S. Pat. No. 3,676,211, British Pat.
No. 1,252,490, "Electrodes for Ceramic Barium Titanate Type
Semiconductors" by H. M. Landis, Journal of Applied Physics, 1965,
Vol. 36, Nos. 6, pages 2,000 to 2,001 and the like. But, it is
preferable to form the ohmic electrodes on the outer surfaces of
the honeycomb structural body by means of the following method in
order to produce the heating element according to the
invention.
When forming the ohmic electrodes on the surfaces substantially
perpendicular to the axial direction of the channels as shown in
FIGS. 1 and 2, it is preferable to use the silver paste baking
method, the aluminum hot spraying method or electroless nickel
plating method. When forming the ohmic electrodes on the outer
surfaces of the honeycomb structural body substantially parallel to
the axial direction of the channels as shown in FIGS. 3 and 4, it
is preferable to use the aluminum hot spraying method. These
methods will now be described, respectively.
The silver paste baking method comprises coating silver paste on
the ceramic article by means of a screen printing method and baking
the coated surfaces. However, silver paste having the typical
composition used with a customary condenser, for example, Silver
Paste 7095 made by E. I. Du Pont de Nemours and Company, could not
provide the ohmic electrodes which are well suited for the PTC
ceramic article. Such a disadvantage can be obviated by the use of
a silver paste containing indium. On the other hand, such indium
containing silver paste is expensive.
The silver paste developed by the invention consists of silver and
zinc with a range of weight ratios of 2:1 to 40:1, glass powder,
and an organic solvent. This silver paste after coating is baked at
a temperature of 420.degree. to 550.degree.C.
It is preferable to apply such silver paste baking method to a PTC
ceramic article whose Curie Temperature (Tc) is lower than a
temperature of about 150.degree.C. This is because of the fact that
silver is subjected to surface diffusion at a temperature higher
than 200.degree.C so that there is a risk of the silver coated
surfaces being shortcircuited to each other at a temperature higher
than 200.degree.C after long use.
The aluminum hot spraying method is capable of providing electrodes
which can eliminate the above described difficult problem which has
been encountered with the silver paste baking method. In the
aluminum hot spraying method, it has been the common practice to
spray aluminum against surfaces of a ceramic article in a direction
substantially perpendicular thereto so as to expedite adherence of
aluminum to those surfaces, as described in the U.S. Pat. No.
3,676,211. If aluminum is sprayed from a direction perpendicular to
the surfaces which is substantially perpendicular to the axial
direction of the channels of the honeycomb structural body, that
is, from the direction substantially parallel to the axial
direction of the channels, aluminum becomes adhered to the inner
wall surfaces of all of the channels, thereby shortcircuiting the
sprayed surfaces. In order to obviate such disadvantage, the
invention provides an improved method wherein aluminum is sprayed
in a direction inclined from the surfaces which is substantially
perpendicular to the axial direction of the channels by an angle of
10.degree. to 60.degree., preferably, 15.degree. to 45.degree..
In the electroless nickel plating method, the ceramic article is
plated with nickel as a whole. Such method, therefore, could not be
applied to the invention. Thus, the invention provides an improved
electroless nickel plating method by which nickel is plated on
surfaces only substantially perpendicular to the axial direction of
the channels and which comprises immersing the honeycomb structural
body into silicon resin or wax, for example, to mask the total body
with the resin or wax, grinding the surfaces substantially
perpendicular to the axial direction of the channels to remove the
mask, subjecting activating treatment only to the mask removed
surface, and immersing the body into nickel salts solution to form
ohmic electrodes on the surfaces only substantially perpendicular
to the axial direction of the channels. In this method, it is
important to mask the honeycomb structural body with the resin or
wax such that each channel is made round in shape at its corner
edges. These masked round corner edge portions can prevent a
penetration of the activating treating liquid into the channels due
to surface tension and hence prevent an electroless plating against
the inner walls of the channels, thereby preventing shortcircuit
across the electrodes.
A heater employing the heating element according to the invention
will now be described in greater detail.
As an example, use is made of a heating element comprising a PTC
ceramic article composed of a honeycomb structural body which is
provided with a multiplicity of channels each square in cross
section with a side of 0.125 cm and bounded by partition walls
whose thickness is 0.02 cm. The particulars of the PTC ceramic
article are as follows. The diameter is 4 cm, thickness (length of
each channel) 1 cm, Tc=190.degree.C, the surface-to-volume ratios
about 24 cm.sup.2 /cm.sup.3 and the total surface area about 300
cm.sup.2. The heating element is provided at its surface
substantially perpendicular to the axial direction of each channel
with ohmic electrodes.
In FIG. 5, a curve 101 graphically illustrates the relation between
the amount of air Q (m.sup.3 /min) fed into the channels of the
heating element at 20.degree.C and the amount of heat P (watt)
radiated from the above mentioned heating element. As seen from the
curve 101, the amount of heat radiation is about 230 watts when the
amount of air fed is 0.1 m.sup.3 /min, whereas if the amount of air
fed is increased to 0.5 m.sup.3 /min which is 5 times larger than
0.1 m.sup.3 /min, the amount of heat radiation becomes also
increased to about 450 watts. If the amount of air fed is
substantially zero, that is, if the forced draft is stopped and use
is made of natural convection air caused by the heat radiated from
the heating element, the amount of heat radiation becomes only 20
watts.
As described above, when various kinds of fluid inclusive of air
pass through a multiplicity of channels of the heating element
according to invention, the heating element with a small volume can
radiate a large amount of heat. Thus, a heater comprising the
heating element according to the invention and means for feeding
fluid through the heating element such, for example, as a fan,
pump, water pressure of city water and the like can prevent an
overheating of the heating element when the fluid feeding means
becomes stopped without intentionally providing a safety device
such as a temperature fuse, thermostat and the like.
In addition, in the heater of this invention (when use in
combination with means for feeding fluid through the heating
element) is capable of changing the amount of heat radiation by
varying the amount of fluid. It is possible, therefore to control a
large electric power with the aid of a small electric power.
For example, if use is made of air as fluid, the amount of air on
the order of 0.2 m.sup.3 /min can be fed by means of a fan driven
by an electric motor of about 20 watts. As a result, as seen from
FIG. 5, the control of the 20 Watts motor in its rotating speed
ensures a control of the amount of heat radiation corresponding to
electric power of 300 Watts which is about 15 times larger than 20
Watts.
In the heating element according to the invention, it is important
to make the surface-to-volume ratios of the honeycomb structural
body large. Even when the amount of passing air per unit volume of
the heating element is the same, the larger the surface-to-volume
ratio the larger the amount of heat radiation. For comparison, use
is made of another heating element comprising a PTC ceramic article
composed of a honeycomb structural body which is provided with a
multiplicity of channels each square in cross section with a side
of 0.195 cm and bounded by partition walls whose thickness is 0.03
cm. The particulars of the PTC ceramic article are as follows. The
diameter is 4 cm, thickness (length of each channel) 1 cm,
Tc=190.degree.C, the surface-to-volume ratios about 16 cm.sup.2
/cm.sup.3 and the total surface area about 200 cm.sup.2. This
element is made of the same ceramic composition as those of the
body from which the curve 101 was derived. The heating element is
provided at its surface, substantially perpendicular to the axial
direction of each channel, with ohmic electrodes. In FIG. 5, a
curve 102 graphically illustrates the relation between the amount
of air Q (m.sup.3 /min) fed into the channels of the above
mentioned heating element at 20.degree.C and the amount of heat P
(watt) radiated from the heating element. A comparison between the
curves 101 and 102 clearly shows that the amount of heat radiation
P (watt) shown by the curve 101 which is plotted when the
surface-to-volume ratios are about 24 cm.sup.2 /cm.sup.3, is larger
than that shown by the curve 102, which is plotted in the case of
the surface-to-volume ratios are about 16 cm.sup.2 /cm.sup.3.
The frictional resistance of the fluid passing through a
multiplicity of channels against the honeycomb structural body is
proportional to the surface-to-volume ratios under the same
configuration of the channels, so that the surface-to-volume ratios
should be determined in association with the kinds of fluid and
means for feeding fluid through the heating element. It is
preferable to determine the surface-to-volume ratios to a range
from 10 to 60 cm.sup.2 /cm.sup.3 with respect to the honeycomb
structural body of the heating element according to the invention.
Any structural body having some holes made, for example, by boring
could neither make the thickness of the partition will bounding the
holes substantially uniform, nor obtain the surface-to-volume
ratios in the range of 10 to 60 cm.sup.2 /cm.sup.3.
The Curie Temperature (Tc) of the PTC ceramic article of the
heating element according to the invention is determined according
to what purposes the heating element is used. If the heating
element is used for a handy hair dryer, the heating element is
restricted in its size. As a result, the Curie Temperature (Tc) of
the PTC ceramic article is made relatively high that a heating
element small in volume can radiate a desired amount of heat. The
amount of heat radiation is associated not only with the Curie
Temperature (Tc) of the PTC ceramic article, but also with the
surface-to-volume ratios of the honeycomb structural body. It is
preferable to make the Curie Temperature (Tc) 150.degree. to
200.degree.C for the handy hair dryer.
As will be described later, if the heating element according to the
invention is used for a heater, a plurality of heating elements may
be used according to the desired amount of heat radiation and to
the other designs. In this case, a customary heating element made
of nickel-chrome wire, for example, may be included in a plurality
of heating elements.
Though these heating elements may electrically connected in series
and/or in parallel with each other by techniques known in the art,
it is found to be preferable to connect in parallel from our
investigation.
In FIG. 6 is shown a preferred embodiment of the heating element
according to the invention as applied to an air heater.
Referring to FIG. 6, an air heater 10 comprising a heating element
1, and means for feeding fluid through the heating element 1 such
as a fan 12 driven by a motor 11. Such means can feed air into a
multiplicity of channels of the heating element 1. Means for
feeding fluid through the heating element 1 is settled to be
aligned with the channels of the honeycomb structural body of the
heating element 1 and the assembly is enclosed in a housing 13. In
the present embodiment, arrangement and electrical connections are
so designed that the heating element 1 is provided at its opposed
surfaces substantially perpendicular to the axial direction of the
channels with ohmic electrodes, and that the heating element 1 is
sandwiched between a pair of terminal plates 14 and 15. It is a
matter of course that insulating spacers (not shown) are inserted
between these terminal plates 14, 15 and the housing 13 so as to
electrically insulate the former from the latter.
The air heater 10 shown in FIG. 6 will operate as follows. If a
switch 16 is turned ON, the fan 12 is rotated to suck air through
an inlet opening 17 adapted to control the amount of air passing
therethrough into the heating element 1. At the same time, electric
current is supplied to the heating element 1 to bring it into a
heat radiating condition, and as a result, the air passing through
the channels is heated and blown out of the air heater 10.
Between ON and OFF, a state which the fan 12 continues its air
feeding operation and the heating element 1 stops its heating
operation may be employed.
During heating of the heating element 1 by the current supplied
thereto, even if the fan 12 becomes stopped or the air inlet
opening 17 gets clogged by a towel, a curtain or the like, to cause
no air to pass through the multiplicity of channels of the heating
element 1, the amount of heat radiated from the heating element 1
becomes small in the manner as described above. As a result, there
is no risk of the heating element 1 being overheated or of a fire
being caused without intentionally providing a safety device such
as a temperature fuse, thermostat and the like. The air heater
shown in FIG. 6 with or without any modification may be used as a
hair dryer, domestic or industrial dryers, air towel, room heater
and the like.
In FIG. 7 is shown another preferred embodiment of the heating
element according to the invention as applied to a liquid
heater.
Referring to FIG. 7, a liquid heater 18 which comprises a heating
element 1 comprising a PTC ceramic article composed of a
rectangular parallelepiped honeycomb structural body provided with
a multiplicity of channels each square in cross section with a side
of 0.18 cm and bounded by partition walls whose thickness is 0.04
cm, the honeycomb structural body having a side of 4 cm, a
thickness (length of each channel) of 6 cm, Curie Temperature (Tc)
of 120.degree.C, surface-to-volume ratios of about 15 cm.sup.2
/cm.sup.3 and total surface area of about 1,500 cm.sup.2, and ohmic
electrodes provided on the side surfaces of the honeycomb
structural body which is substantially parallel to the axial
direction of each channel, and heat insulating material 21 covering
the heating element 1 and enclosed in a housing 22. In this
embodiment, a pipe 20 is also provided for feeding water to the
channels. The inlet pipe 19 is connected to, for example, a city
water faucet (not shown) to feed water into the pipe 20, so that
the water passing through the multiplicity of channels is heated
and flows out of an outlet pipe 23.
The liquid heater 18 shown in FIG. 7 will operate as follows. If
the switch 16 is turned ON, heat is radiated from the heating
element 1 and hot water flows out of the outlet pipe 23.
If the liquid heater 18 was fed with city water at 20.degree.C at a
rate of about 1 l/min and at the same time the pair of electrodes
were applied with voltage of 240 volts, the city water heated to
about 60.degree.C flowed from the outlet pipe 23. In this case, the
amount of heat radiation was 2.9 kilowatts. The amount of heat
radiated when the water supply was stopped became smaller than 100
watts, thereby involving no danger due to overheating of the
heating element 1.
The electrical insulation of both the electrodes against an
electric conductive liquid such as water may be effected by coating
the heating element 1 as a whole, exclusive of the lead wires led
out of the electrodes, with corrosion resistant materials such as
fluorine resin (Teflon made by E. I. Du Pont de Nemours and
Company, Polyflon, Daiflon and Neoflon made by Daikin Kogyo Co.,
Ltd. Osaka, Japan and the like), fluorine rubber (Viton made by Du
Pont), silicone resin, silicone rubber, silicone varnish and the
like.
In addition, such coating can protect the heating element 1 from
being corroded by corrosive liquid such as acids, alkalis and the
like.
Alternatively, as means for preventing the heating element 1 from
being corroded by the corrosive fluid, provision may be made for a
pipe having a small diameter and made of anti-corrosion material
such as stainless steel and this pipe may be inserted into each of
the channels of the heating element whereby the corrosive fluid can
pass through the pipe without making contact with the inner surface
of each of the channels.
The liquid heater shown in FIG. 7 is not provided with means for
feeding fluid through the heating element, such as provided for the
air heater shown in FIG. 6, but the city water pressure or gravity
serves as means for feeding liquid through the heating element. In
addition, the liquid heater shown in FIG. 7 is connected to the
city water faucet and adapted to heat the city water. But, the
liquid heater shown in FIG. 7 may also be used as a heater for
drinks such as Japanese Sake and the like, and an oil preheater for
heating liquid fuel and the like.
In FIG. 8 a further preferred embodiment of the heating element
according to the invention is shown as applied to a humidifier.
Referring to FIG. 8, a humidifier 24 comprising a housing 26
enclosing water 25 contained therein and a heating element 1. In
the present embodiment, it is important that one of the surfaces
substantially perpendicular to the axial direction of the channels
of the honeycomb structural body is always in contact with the
water 25. In order to make one of the surfaces substantially
perpendicular to the axial direction of the channels of the
honeycomb structural body always contact with the water 25,
provision is made for a water absorbing mat 27 which is supported
in a space above the water 25 and on which is disposed the heating
element 1. The water absorbing mat 27 is provided at its periphery
with a bundle of fibers 28, such as woven cloth secured at its one
end to the mat 27, the other end being immersed into the water 27.
The fiber bundle 28 and mat 27 permit the water to be supplied to
the surface substantially perpendicular to the axial direction of
the channels of the honeycomb structural body with the aid of
capillary phenomenon of the fiber bundle 28 and the mat 27.
The humidifier shown in FIG. 8 will operate as follows. If the
switch 16 is turned ON, the water in contact with one of the
surfaces substantially perpendicular to the axial direction of the
channels of the honeycomb structural body of the heating element 1
is heated by the heating element 1 and converted into steam which
is exhausted from an opening 29 provided at the top of the housing
26 and arranged above the heating element 1.
In the present embodiment, use was made of a heating element
comprising a PTC ceramic article composed of a column shaped
honeycomb structural body provided with a multiplicity of channels
each square in cross section with a side of 0.125 cm and bounded by
partition walls whose thickness is 0.02 cm, the honeycomb
structural body having a diameter of 3.5 cm, a thickness (length of
each channel) of 1 cm, Curie Temperature (Tc) of 190.degree.C,
surface-to-volume ratios of about 24 cm.sup.2 /cm.sup.3, and total
surface area of about 240 cm.sup.2, and ohmic electrodes provided
on both surfaces substantially perpendicular to the axial direction
of each channel. Water at 20.degree.C was treated by applying
voltage of 100 volts across the ohmic electrodes. The water was
evaporated at a rate of 4 cc/min. The housing 26 is provided at its
side wall with a window 30 through which can peep the change in
liquid level and provided at its cover with an opening 31 through
which is added liquid when the liquid level becomes lower than a
desired level.
The humidifier shown in FIG. 8 with or without modifications may be
used as a volatilizer, distillator, fractionator and the like for
use with domestic or industrial water and any other liquids such as
oil.
While several examples have been herein disclosed, it is obvious
that various changes can be made without departing from the spirit
and scope of the invention as set forth in the appended claims.
Further, it is to be understood that all matter hereinbefore set
forth is to be interpreted as illustrative and not in a limiting
sense.
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