[0001] The present invention relates to a method as defined in the preamble of claim 1 for
manufacturing a thermal element, a thermal element as defined in the preamble of claim
5 and an electric sauna stove as defined in the preamble of claim 12 for the use of
the thermal element.
[0002] Traditionally, various electrically operated thermal elements are manufactured using
tubular resistors. In all practical tubular resistor applications, the worst problem
are the manufacturing costs of the tubular resistor. Depending on where they are used,
tubular resistors also have other drawbacks. If they are used without insulation and
e.g. only covered with stones as in electric sauna stoves, the glowing resistors cause
a deterioration of steam quality, as is well known. On the other hand, in heat storing
water tanks, tubular resistors are placed in the water space, i.e. they are used in
conditions subject to corrosion, and in this case, too, the installation of the resistors
is a considerable cost factor. Tubular resistors are also used in various direct and
storing heater units and thermal elements, such as radiators. A problem with all these
applications is how to install the resistors safely and so that they will not cause
a risk to health or a fire risk even when aglow.
[0003] The object of the invention is to eliminate the drawbacks referred to above. A specific
object of the invention is to disclose a new type of thermal element and a method
for the manufacture thereof as well as new applications for the use of said element,
thus to render different uses and applications of thermal elements both safe and structurally
simple and economic to implement.
[0004] As for the features characteristic of the invention, reference is made to the claims.
[0005] In the method of the invention for the manufacture of a thermal element, a first
layer of pourable mass is cast into a desired mould, the shape of which is not defined
in any way by the invention. While the layer of pourable mass is still fresh, an uninsulated
sheet-like resistance element cut from a metal sheet is placed upon it, whereupon
a second layer of pourable mass is poured onto the resistance element in the mould
so that only the ends of the resistance element remain visible. Thus, as the resistance
element is a sheet-like and flat piece lying in one plane, it is easy to mount on
the layer of pourable mass and it sets tightly in the mass, unlike the various helical
resistors and other clearly three-dimensional resistor structures used in prior art,
which generally have had to be placed in bores and similar cavities provided in the
mass.
[0006] In an embodiment of the invention, the first and/or the second layer of pourable
mass are/is cast as two separate sub-layers, with a metallic grounding network placed
between them, said grounding network being then grounded in the conventional manner.
[0007] In the method of the invention, the pourable mass used may be any of various kinds
of concrete, other fire resistant mass or e.g. ceramic paste. In an embodiment of
the invention, a core consisting of two layers of a medium capable of withstanding
high temperatures is cast in the middle of the thermal element, in which case the
resistance element is placed between the two layers of temperature resistant medium
and the actual heat storing and heat releasing frame of the thermal element, including
the outer surface of the frame, is cast around the core, on both sides of it, e.g.
from a cheaper material or a material with a better heat storing capacity. Thus the
core part will withstand the high temperatures of the resistor and release heat uniformly
and at a lower temperature to the frame of the thermal element, the frame being less
temperature resistant but having a good heat storing capacity.
[0008] The thermal element of the invention comprises heat storing pourable mass and a resistance
element embedded in the pourable mass. According to the invention, the resistance
element is a sheet-like and flat element lying in one plane, machined from an uninsulated
metal sheet and placed in the mass in conjunction with the casting of the pourable
mass.
[0009] In a preferred embodiment, the thermal element comprises a grounding network or grounding
conductor placed inside the pourable mass and having no galvanic connection with the
resistance element. In an embodiment of the invention, the thermal element comprises
a layer of a medium capable of withstanding high temperatures, placed around the resistance
element to prevent direct contact between the hot resistance element and the less
temperature resistant components of the thermal element.
[0010] In an embodiment of the invention, the thermal element is implemented as a plate-like
radiator which, having a good heat storing capacity, can preferably be arranged to
use night-time electricity.
[0011] In another embodiment of the invention, the thermal element forms the heating element
of a heat storing water tank. In this case, the heating element may be placed in the
water space of the heat storing water tank, but preferably the thermal element is
mounted on the outer surface of a metallic shell of the heat storing water tank. Thus,
the shell and the thermal element are surrounded by a heat insulating layer and all
of the heat generated by the thermal element can be passed through the shell of the
heat storing water tank into the water space inside. This construction provides a
very simple and cheap solution for manufacturing heat storing water tanks because
it obviates the need for building electric connections inside the reservoir while
the structure of the thermal element is also as simple as possible.
[0012] A preferred embodiment of the invention is an electric sauna stove. In the electric
sauna stove of the invention, an electric resistor is placed inside a casting made
of fire resistant pourable mass. The casting has preferably been cast around the electric
resistor. Thus, the electric resistor is completely hidden inside the casting and
cannot be affected by the steam water or come into direct contact with it. Therefore,
the whole electric sauna stove may consist of only one casting of suitable shape,
the electric resistor inside it being connected to an electrical power network.
[0013] The invention does not define the shape of the casting as such, but if the electric
sauna stove consists of only one coherent casting, then it is preferably provided
with cup-like water spaces with their open sides upward so that steam water can gather
in them and be gradually evaporated.
[0014] In an embodiment of the invention, the casting in the electric sauna stove is e.g.
a plate-like structure or a structure with a planar surface at least on its upper
side, allowing loose sauna stove stones to be placed on top of the casting to give
the structure an appearance that is more like a traditional sauna stove. At the same
time, the stones heated upon the casting prevent the steam water thrown onto the stove
from getting into direct contact with the casting, thus increasing its durability
and prolonging its service life. Such a casting with loose sauna stove stones placed
upon it is preferably provided with vertically extending holes to permit hot air to
rise up through them so that heat is effectively conveyed from the casting to the
sauna stove stones.
[0015] In an embodiment of the invention, the electric sauna stove comprises a shell, i.e.
a vertical cage-like envelope delimiting a space inside it, which traditionally has
been used to house glowing tubular resistors. In the structure of the invention, this
space contains one or more castings, i.e. thermal elements as provided by the invention,
which are preferably covered with conventional loose sauna stove stones. The castings
may be plates placed in an upright position and side by side in a suitable manner,
at a distance from each other. Of course, it is possible to use thermal elements of
other shapes as well. As the electric resistors are embedded in the pourable mass,
a heated sauna stove will not have any surfaces aglow and steam water thrown on the
stove cannot come into contact with the resistors. Thus, the formation of harmful
ions and ozone is avoided, agreeable sauna steam is obtained and durable structures
with highly economic manufacturing costs are achieved.
[0016] The electric resistors used in the casting for an electric sauna stove are made from
mere uninsulated metal wire of suitable material or from some other suitable uninsulated
resistive material. For instance, the resistor may be made by cutting a piece from
an uninsulated metal sheet and machining it so as to form a sheet-like and flat resistance
element, which is placed inside the casting in conjunction with the moulding thereof.
In this way, a heating element for an electric sauna stove which is very simple and
cheap to manufacture is achieved.
[0017] As described in detail above in conjunction with different embodiments, the method
and structure of the invention have several significant advantages as compared with
prior art. However, the greatest advantage provided by the invention can be considered
as consisting in the fact that it replaces well-known and relatively expensive structures
with a simple, cheap and durable construction which will bring the profitability of
the manufacture of e.g. traditional electric sauna stoves to a completely new level.
[0018] In the following, the invention will be described in detail with reference to the
attached drawings, wherein
Fig. 1 presents a thermal element according to the invention, layer by layer.
Fig. 2 presents a first embodiment of the invention designed to be used as a sauna
stove,
Fig. 3 presents a second embodiment of the invention designed to be used as a sauna
stove,
Fig. 4 presents a third embodiment of the invention designed to be used as a sauna
stove,
Fig. 5 presents a cross-section through a heat storing water tank according to the
invention, and
Fig. 6 presents an embodiment of a thermal element according to the invention.
[0019] Fig. 1 illustrates a thermal element according to the invention and a method for
the manufacture thereof. In this embodiment, a first layer of pourable mass 1 is first
cast as two sub-layers 1a and 1b, placing a grounding network 4 between them so that
only a suitable projection 15 for the connection of a grounding wire is left outside
the first layer of pourable mass 1. A relatively thin medium layer 5a of a mass capable
of withstanding high temperatures is cast upon the first layer of pourable mass 1,
and a sheet-like and relatively flat resistance element 2 is placed upon the medium
layer 5a. A second medium layer 5b of corresponding mass capable of withstanding high
temperatures is cast upon the resistance element, which is thus completely embedded
inside the heat-resistant mass while only the ends 16 of the resistance element are
left outside to permit the connection of conductors. After this, a first sub-layer
3a of a second layer of pourable mass 3 is cast upon the second medium layer 5b. Upon
the first sub-layer 3a is placed a grounding network 4, which is covered with a second
sub-layer 3b. In this way, a thermal element has been formed which has an inner part
capable of withstanding high temperatures and outer layers with a good heat storing
efficiency and a slow heat releasing property and which, or whose different embodiments,
can be used in a plurality of possible applications.
[0020] Fig. 2 illustrates an embodiment of the thermal element of the invention used as
a sauna stove. In this embodiment, the thermal element is a casting 10 of a substantially
square shape, attached in a horizontal position to a wall by means of a bracket 17.
The casting is provided with vertical holes 18 disposed at a distance from each other
substantially in the entire area of the casting. Heaped on the casting are a number
of conventional loose sauna stove stones 12. In addition, a switching and control
device 19 as is generally known and used in conjunction with sauna stoves is connected
to the casting for the operation of the sauna stove. Thus, a very simple and compact
new sauna stove structure is formed in which the thermal element 10 is first heated
by means of the electric resistor inside it and from which the heat rises both by
direct transmission and by air flow via the holes 18, heating the stones 12 and the
air space in the sauna.
[0021] Fig. 3 illustrates another embodiment of the invention for application in an electric
sauna stove. In this case, instead of expensive tubular resistors as are exclusively
used so far, castings 10, i.e. plate-like thermal elements are mounted in the interior
space 14 delimited by a cage-like envelope 13 made of sheet metal. This embodiment
comprises three plate-like thermal elements placed side by side in an upright position
and at a distance from each other, leaving clear vertical air spaces both between
themselves and between the thermal elements and the envelope of the sauna stove.
[0022] On top of the thermal elements, it is possible to set conventional sauna stove stones
or equivalent in the traditional manner, which will be heated by the heavy upward
heat flow from the thermal elements. The essential point about this sauna stove application,
too, is that no red-hot metal parts with which the steam water or the oxygen in the
air would get into contact are formed in the sauna stove.
[0023] The thermal elements used in the sauna stove applications are preferably entirely
cast from previously known pourable masses capable of withstanding high temperatures.
The resulting elements will endure the throwing of steam water onto them, although
in the embodiments in Fig. 2 and 3 the sauna stove stones placed on the thermal elements
minimise the chance of steam water getting into contact with the surface of the thermal
elements.
[0024] Fig. 4 again presents an embodiment for application in a sauna stove corresponding
to Fig. 2 but without the use of separate sauna stove stones. In this case, an electric
sauna stove formed by a mere plate-like thermal element designed to be mounted in
a horizontal position, which can be attached to a wall, floor even a ceiling with
fixtures placed at suitable distances. The plate-like element comprises a number of
holes 18 to create an intensive upward air flow through the element so as to produce
an effective heat transfer from the thermal element to the sauna space. Moreover,
the thermal element has a number of cup-like, i.e. upward-opening water spaces 11.
When steam water is thrown onto the plate, some of the water will evaporate immediately
and some of it will boil slowly in the water spaces 11. Thus, thanks to the water
spaces, agreeable and soft steam is produced and splashing and flow of steam water
outside the sauna stove is prevented.
[0025] Fig. 5 presents a sectional view of a heat storing water tank application of the
invention. The heat storing water tank 7 comprises a metallic shell 8, i.e. a cylindrical
mantle, which is generally mounted in a horizontal position for use. The electric
resistor of the heat storing water tank consists of a thermal element 6 according
to the invention, whose surface has a shape corresponding to the curvature of the
shell 8 and is disposed below the shell 8 against its outer surface, i.e. at the bottom
of the heat storing water tank. In addition, the entire shell as well as the thermal
element 6 are surrounded by a layer 9 of insulating material of sufficient thickness.
In this way, a very simple heat storing water tank is formed, in which heat is conducted
from the thermal element through the shell 8 to the water inside. Thus, the heat storing
water tank need not be provided with any mounting bosses for resistors or with expensive
resistors designed to endure the conditions in a water tank.
[0026] Fig. 6 presents a preferred embodiment of the thermal element of the invention. The
thermal element consists of a hollow tube manufactured by casting a first layer of
pourable mass 1 around a suitable cylindrical mould. After this, an uninsulated band-like
resistance element 2 has been wound in a spiralling fashion around the first layer
of pourable mass 1. Finally, a second layer of pourable mass 3 has been cast upon
the first one, covering the resistance element so that only its ends are left visible
to allow the required connections to be made. To give a clearer illustration of the
structure, the figure shows only a part of the second layer of pourable mass.
[0027] The element presented in Fig. 6 is especially applicable for use in electric sauna
stoves in place of the traditional resistor. For example, when mounted in an upright
position, the structure allows an effective air circulation both inside and outside
the tube, thus accelerating the heating of the sauna, yet the resistors are completely
hidden in pourable mass, so they cannot come into contact with the steam water. In
addition, the element can be manufactured by a simple and fast process.
[0028] In the foregoing, the invention has been described by way of example with reference
to the attached drawings while many variations of the invention are possible within
the scope of the inventive idea defined in the claims.
1. Method for manufacturing a thermal element, characterised in that a first layer of pourable mass (1) is cast into a mould, an uninsulated sheet-like
resistance element (2) cut from a metal sheet is placed upon the fresh layer of pourable
mass and a second layer of pourable mass (3) is cast into the mould so that only the
ends of the resistance element remain visible.
2. Method as defined in claim 1, characterised in that the first and/or the second layer of pourable mass (1, 3) are/is cast as
two sub-layers (la, b, 3a, 3b) and a metallic grounding network (4) is placed between
the sub-layers.
3. Method as defined in claim 1 or 2, characterised in that the pourable mass used consists of concrete, fire resistant mass and/or ceramic
mass.
4. Method as defined in claim 1, characterised in that a layer (5a, 5b) of a medium capable of resisting high temperatures is cast
in two parts between the first and the second layers of pourable mass and the resistance
element (2) is placed inside said layer (5a, 5b) .
5. Thermal element comprising heat storing pourable mass (1, 3) and a resistance element
(2) embedded in it, characterised in that the resistance element is a substantially sheet-like element machined from
an uninsulated metal sheet.
6. Thermal element as defined in claim 5, characterised in that it comprises a grounding network (4) placed inside the pourable mass (1,
3) so that it is not in contact with the resistance element (2).
7. Thermal element as defined in claim 5, characterised in that the resistance element (2) is surrounded by a layer (5a, 5b) of a medium
capable of resisting high temperatures, outside which the layers of pourable mass
(1, 3) are disposed.
8. Thermal element as defined in claim 7, characterised in that the thermal element is a plate-like radiator, which is preferably arranged
to use night-time electricity.
9. Thermal element as defined in any one of claims 5 - 7, characterised in that the thermal element (6) is the heating element of a heat storing water tank
(7).
10. Thermal element as defined in claim 9, characterised in that the thermal element (6), having a shape corresponding to the shape of the
surface of the shell (8) of the heat storing water tank, is placed on the outer surface
of the shell, inside an insulating layer (9).
11. Thermal element as defined in claim 9, characterised in that the heating element is placed in the water space of the heat storing water
tank.
12. Electric sauna stove comprising an electric resistor, characterised in that an electric resistor is placed inside a casting (10) manufactured from fire-resistant
pourable mass.
13. Electric sauna stove as defined in claim 12, characterised in that the casting has been cast around the electric resistor.
14. Electric sauna stove as defined in claim 12, characterised in that the casting (10) comprises one or more cup-like water spaces (11) open at
their upper side.
15. Electric sauna stove as defined in any one of claims 12 - 14, characterised in that it comprises loose sauna stove stones (12) placed upon the casting (10) to
prevent steam water from getting to the surface of the casting.
16. Electric sauna stove as defined in any one of claims 12 - 15, characterised in that the casting comprises holes (18) extending vertically through it.
17. Electric sauna stove as defined in claim 12, characterised in that the electric sauna stove comprises a substantially upright cage-like envelope
(13) with one or more castings (10) mounted in the space (14) delimited by it.
18. Electric sauna stove as defined in claim 17, characterised in that it comprises loose sauna stove stones placed upon the castings, above the
space delimited by the envelope.
19. Electric sauna stove as defined in claim 17 or 18, characterised in that the castings (10) are plates placed in an upright position and side by side
at a distance from each other.
20. Electric sauna stove as defined in claim 17 or 18, characterised in that the casting is a tube, the electric resistor being placed inside the tube
wall.
21. Electric sauna stove as defined in any one of claims 12 - 20, characterised in that the electric resistor consists of an uninsulated metal wire placed inside
the pourable mass.
22. Electric sauna stove as defined in any one of claims 12 - 20, characterised in that the electric resistor consists of a substantially sheet-like resistance element
machined from an uninsulated metal sheet and mounted inside the pourable mass.