[0001] The invention relates to an electronic, microcontroller-controlled apparatus whereby,
par unit of time, a specific amount of steam is procuced. Depending on the type of
the apparatus, this invention enables controlling the pressure and the humidity of
the egressing steam.
[0002] The present state of the art regarding the production of steam by means of electricity
for, for instance, steam cleaners or saunas is described hereinbelow. By means of
mechanical thermostats mounted on the outer side or by temperature sensors connected
to an electronic circuit, an electric through-flow heater is approximately brought
to a desired temperature. When the desired temperature is reached, the apparatus becomes
ready for use. By means of a hand-operated switch or by switching an electronic output,
a pump or valve is activated, causing water to be pumped into the through-flow heater.
In the first part of the passage through the through-flow heater, the inflowing water
is heated up to the boiling point, whereupon the water over the greater part of the
passage is converted into steam by means of the energy supplied. When the passing
water is not completely evaporated, the non-evaporated water is entrained in the water
vapor to the outlet of the steam generator. The water/steam ratio determines the humidity.
There are also steam generators consisting of a boiler having a content of from half
a liter to a few liters. At the bottom of the boiler, an electric heating element
is located. With these steam generators, the energy supply to the heating element
is switched off by means of a pressure sensor when the pressure of the boiler exceeds
a particular value or when the temperature becomes too high. Most types involve an
open communication with the outside air.
[0003] The above-mentioned installations according to the present state of the art have
the following drawbacks:
1. Present-day installations and systems in which steam is to be generated involve
the use of often rather unwieldy electric through-flow heaters which, in particular
if they have been developed to operate at a power supply from a 230V mains connection,
are subject to a long warming-up time. This applies to a great extent to the above-mentioned
boiler types where all the water must first be brought to the boil.
2. In addition, these steam generators use thermostats mounted on the outer side of
the through-flow heater. These thermostats have the drawback that they often respond
slowly, due to the mass and the specific heat of the thermostat itself and the heat
resistance between the through-flow heater and the thermostat, which largely determine
the response time.
3. Further, there are great tolerances in the measuring accuracies and a large hysteresis
of these thermostats.
4. In addition, the switching of these thermostats is accompanied by sparking, as
a consequence of which the lifetime of these parts is limited.
5. Due to the relatively slow warming-up of the heating elements, layers of scale
deposit, which may result in a poor heat transfer to the water. This may also lead
to stoppages. In the present steam generators, this effect is retarded through the
use of filters in the water supply. By descaling periodically, the lifetime can be
prolonged.
[0004] The invention provides a solution to the above-mentioned drawbacks by using a through-flow
heater having a small volume and a great electric power density for transferring the
required energy to the water in the form of heat.
[0005] The mass of the through-flow heater is small, so that little energy is required for
heating up the through-flow heater itself to the required temperature. The resistance
in which the electric energy supplied is converted into heat is so arranged that the
available heat can be transferred to the water very directly. To obtain a very fast
and accurate control, use is made of the positive temperature coefficient of said
resistance for determining the temperature. On the basis of the Figures, the principles
of the steam generator are elucidated and applications are explained.
[0006] Fig. 1 shows the principle of an electric through-flow heater based on a heating
element utilizing a dissipating resistance designed in thick-film technology, enabling
the heat generated to be transferred to the rest of the element very directly.
[0007] Fig. 2 shows the block diagram of an electronic circuit with which the principle
of temperature measurement and the operation of the control of the steam generator
will be explained.
[0008] Fig. 3 shows the setup of a steam generator which draws the water to be evaporated
from a pressureless reservoir.
[0009] Fig. 4 shows a circuit wherein the feed to the heating element is switched off in
the event of a failure of the microcontroller or the electric control of the heating
element is interrupted if an error occurs in said circuit.
[0010] Fig. 5 shows the side of a heating element on which a resistance track is provided
between thin insulating and protective layers.
[0011] Fig. 6 shows a spiral-shaped labyrinth in which the water flows while it is being
heated up by the underlying heating element.
[0012] Fig. 7 shows the side of a heating element on which a resistance track is provided
between thin insulating and protective layers as in Fig. 5, but here, the resistance
track is spiral-shaped.
[0013] Fig. 8 shows a spiral-shaped labyrinth in which the water flows while it is being
heated up by the underlying heating element which, in contrast with Fig. 6, is spiral-shaped.
[0014] Fig. 9 shows graphs of the temperature distribution of the water in the through-flow
heater at four different pumping rates as a function of the position in the spiral-shaped
labyrinth.
[0015] Fig. 10 shows a graph of the increase in resistance relative to the resistance of
a specific heating element at 0°C as a function of the optimal flow (F) at which all
the water pumped in is precisely entirely evaporated at the outlet of the through-flow
heater.
[0016] To realize a steam generator in which an electric heating track can be used as temperature
sensor, it is necessary that a lowest possible thermal resistance and thermal capacity
be present between the heating track and the water to be heated, so that a short response
time is present. In Fig. 1, a heating element (2) on the basis of thick-film technology
is used. With this type of heating elements, it is presently possible to dissipate
powers up to about 60 watt per cm
2.
[0017] By a thin electric insulator, the thick-film resistance (23) (Fig. 5) is thermally
and mechanically coupled to a slightly spherical support (2) (Fig. 1), which may be
of stainless steel (SS) design. The thickness of the SS is about 1 mm. The electric
connection to the heating element is effected by means of an adapter (6) (Fig. 1),
which establishes the connection to the ends of the heating track (24) (Fig. 5) by
means of spring contacts. The supply voltage is connected to the terminals (7) (Fig.
1). The through-flow heater (Fig. 1) is created by coupling a spherical cover (3),
provided with a water inlet (1) and a steam outlet (5), to the heating element, the
lower spherical segment (2), by means of a welded joint (4). Depending on the application,
said spherical segments (2+3) can also be joined together by a circular V-shaped clamp
having a layer of elastic plastic or rubber suitable for higher temperatures. Due
to the fast heating up of the SS, calcium layers which may be present on the water
side break up into small pieces. This breaking up is caused by the formation of mechanical
stresses between the SS and the calcium deposit, in that the heating element expands
sooner and faster by heating up. This renders the use of a water inflow filter unnecessary.
[0018] For a better operation of the through-flow heater shown in Fig. 1, a spiral-shaped
object of a high melting point and a high thermal resistance and a low specific heat
can be fitted in this space, which object provides that the water supplied at (1)
(see also 26 (Fig. 6) or 28 (Fig. 8)) flows along the outer side of this spiral in
a spiral-shaped labyrinth towards the center and can leave the outlet (5) (see also
27 (Fig. 6) or 29 (Fig. 8)) in the form of steam and together with any unevaporated
water. An advantage of the use of a spiral is that the through-flow heater can also
be used at an angle of inclination of more than 45° without there being formed unduly
hot spots on the heating element. This is due to the spiral which provides that the
water cannot drop back to one side of the through-flow heater. The spiral-shaped labyrinth
(25) (Fig. 6) mentioned may form an integral part of a plastic cover for forming the
through-flow heater (Fig. 1) together with the heating element (2). In Fig. 2, the
current through the heating element (15) is measured by converting the current through
the low-ohmic measuring resistance (14) into a voltage. This measured AC voltage is
fed to the electronic circuit (12) which amplifies the measured voltage slightly and
subsequently converts it into a DC voltage which is proportional to the AC current
through the measuring resistance (14). At a constant supply voltage (11), the current
through measuring resistance (14) is influenced by the temperature variation over
the heating track (11) of the heating element (15), which is the result of the positive
temperature coefficient of the resistance material of the heating track.
[0019] Calculations demonstrate that with a 1500 watt heating element, a voltage change
of about 5 millivolt per degree Kelvin can be created across the measuring resistance
(14). As the measuring resistance (14) has a resistance value which is smaller than
the track resistance of the heating element (15) by a factor of 100-200, here, only
a measuring resistance suitable for a low power is needed. Since in practice, the
supply voltage (11) may vary considerably, there is provided an electronic and software-mediated
error correction. To enable carrying out said correction in the microcontroller (8),
a DC voltage is presented co the microcontroller (8) via the electronic circuit (9),
which DC voltage is proportional to the prevailing voltage on the heating element.
Circuit (10) provides for the feed of the electronic circuits of said control. By
means of the potentiometer, (13) (or by means of a value stored in the microcontroller),
driven by a reference voltage (V
ref), the desired temperature can be set and, by the control, be maintained. By means
of a user interface consisting of a few keys and a display, which are connected to
the microcontroller (not shown), specific settings (such as pressure, steam humidity
and flow) can be effected and it can be checked, via the display, what is the status
of the apparatus.
[0020] When the desired calculated temperature starts to fall undesirably, this points to
the supply of too much water. However, the microcontroller (8) can maintain the temperature
by controlling the pump capacity to a lower value. Due to the above-mentioned control
properties of the invention, the system recognizes within a few tenths of a second
that the water supply stagnates due to a defective pump or pump control, or simply
because the water has run out. Upon the consequently detected rise of the temperature,
the microcontroller will stop the current supply to the heating element (15) by means
of triac Trl. Hence, this provides a very fast boiling-dry protection. In the control
system described so far, the temperature cannot be measured after switching off of
the heating element (15). By reheating after a short interval, while the pump remains
driven, it can be detected whether the temperature has dropped. If this is not the
case, the intervals will have to be prolonged. It is always possible, depending on
the application, to couple an external temperature sensor to the through-flow heater,
whereby the microcontroller can measure the temperature variation (with a longer response
time) without necessitating energy supply to the heating track. By means of, for instance,
an optic triac Tr2 (preferably with zero-passage detection against line interference),
the microcontroller (8) can switch the water pump (16) on and off in fast alternation
to control the water flow at the inlet (1) (Fig. 1). By means of the software, triac
Tr1 (and also triac Tr2) is controlled by the microcontroller (8) in such a manner
that after a control interruption, a positive half period of the sine voltage (0-180°)
is succeeded by a negative half period (180-360°) and vice versa, so that line interference
is prevented. During said control of the supplied electric power to the heating element,
the measured voltage on the measuring resistance (14) will not be an uninterrupted
sine shape. However, as circuit (12) uses the amplitude of the voltage, the temperature
measurement can be continued normally, with the understanding that due to the time
constant for smoothing the signal from the rectifier of circuit (12), this output
voltage will decrease, which will have to be corrected in the software. Accordingly,
in the software, allowance is made for a measuring error caused by the change of the
measured mains voltage, through a changed load, which is transmitted by circuit (9)
to the microcontroller (8). Fig. 3 schematically represents a possible application
of the invention, which could be used in a steam cleaner, sauna or any other apparatus
requiring a stable supply of steam for a proper operation. The apparatus provides
a pump (18) controlled by the electronics (21) and capable of building up sufficient
pressure for supplying water also if the steam pressure rises, the through-flow heater
(19) and, when this is desired in the apparatus, an electronically settable excess
pressure valve (20), whereby it is achieved that through a further rise of temperature,
the pressure in the through-flow heater (19) will increase to a value equal to the
set pressure value of the excess pressure valve, as a result of which the excess pressure
valve will open and steam will egress at an increased pressure. Because of the distribution
in the resistance values of the different heating elements produced, this invention
provides a system for automatically performing a calibration of the temperature during
the operative mode of the apparatus, for instance each time at the startup. For this,
use is made of the boiling point of water under atmospheric conditions. To this end,
the excess pressure valve (20), if present in the apparatus, is fully opened electronically.
[0021] The pump (18) is set in operation by the electronics (21) for so long that the through-flow
heater of Fig. 1 is filled with water for more than 50%, so that the hollow heating
element is entirely filled. After that, electric energy is transmitted at maximal
power to the through-flow heater (19). By the manner of temperature measuring described,
it will be detected that the temperature of the heating element (2) (Fig. 1) will
rise.
[0022] This rising will continue until the water in the through-flow heater reaches its
boiling point. After that, the temperature of the water will remain stable (100°C)
until all the water has evaporated. The boiling point may then be slightly higher
because of the pressure rise due to the resistance experienced by the steam through
the passage through the outlet (5) and the hoses connected thereto. The temperature
of the heating track (23) (Fig. 5) on the heating element (2) will be slightly higher
due to the heat resistance between the heating track and the water to be evaporated.
After that, calibration can be effected very precisely by adjusting back the power
to the heating element so far that said temperature difference becomes minimal and
the water is still boiling. The derived measuring value is stored in a nonvolatile
memory and can thereafter be used as reference point to enable measuring other temperatures
as well through calculation, since the properties of the heating element are known
after that. Because said calibration is also performed during the testing of the apparatus
in the factory, a temperature determination can already be performed within one second
after the heating element has been switched on. The result of this measurement is
a good approximation for the water temperature in the reservoir (17) (Fig. 3). Because
the energy required for heating 1 gram of water from 20°C to 100°C is only 334 Joule
(
~12.9%), while said heating and evaporating (at 100°C) of this quantity requires 2590
Joule of energy, an error of 10°C in the determination of water temperature in the
reservoir (17) (Fig. 3) would lead to an error in terms of percentage, in the determination
of the amount of water which will evaporate upon supply of a given amount of energy,
of maximally

[0023] Due to this calibration, during the use of the electronically controlled excess pressure
valve, or during the intentional or unintentional closing of the steam outlet, it
is possible to calculate the pressure in the through-flow heater (19) (Fig. 3). When
the pressure increases, the boiling point will also be higher. At a boiling point
of 100°C, the absolute pressure is 101.3.10
3 Pa. When for instance the maximal pressure may be three times as high (304.10
3 Pa), the new boiling temperature may maximally be 133.8°C. Depending on the rate
at which the temperature rises, it can be determined whether the absence of water
is involved or whether an unduly high pressure is involved. In the latter case, the
electronics will be able to guarantee the safety by switching off the current to the
heating element by means of triac Tr1.
[0024] In principle, said temperature measurement/pressure determination is also possible
with conventional heating elements in a boiler (see present state of the art), in
respect of which allowance should be made for the longer response times and, accordingly,
the possible occurrence of dangerous situations. As mentioned, with said through-flow
heater and electronics, it is possible to generate steam of a predetermined mass per
unit of time, a predetermined humidity and a predetermined pressure. During the control
of said quantities, the pump (16 (Fig. 2) and 18 (Fig. 3)) will pump a specific amount
of water per minute into the through-flow heater. This water will be passed via the
spiral-shaped labyrinth (Figs. 6 and 8) to the center of the heating element. At a
specific set power to the heating element, it is calculated in the microcontroller
how much water can, at a specific pressure, be converted into steam per minute. The
desired pressure is set by the electronic reducing valve (if present in the apparatus).
Figs. 5 and 7 show two embodiments of the thick-film resistances on the heating element.
The advantage of the heating element in Figs. 7 and 8, because the heating track co-extends
precisely under the spiral-shaped labyrinth, is that the temperature variation in
the water from the inlet (28) to the outlet (29) rises equally with the temperature
of the resistance track, so that this track has no great temperature transitions,
while in Figs. 5 and 6, the resistance track (23) extends along spots where the temperature
of the water may differ substantially. As a result, the track of the heating element
of Fig. 7 has a longer lifetime. If the pump (16 (Fig. 2) and 18 (Fig. 3)) at a given
power pumps too little water into the through-flow heater, all the water will already
be converted into steam before reaching the outlet (see 5 (Fig. 1), 27 (Fig. 6) and
29 (Fig. 8)). Because the heat transfer to steam is much poorer than to water, the
temperature of the heating element will rise substantially at those locations. The
resistance of the heating track is in fact determined by three temperature areas (Fig.
9):
1. The first part of the spiral labyrinth, where the water is heated from the input
temperature TIN to the boiling point.
2. The portion where the energy which is available at that location is used for evaporating
the boiling water.
3. The final part of the spiral labyrinth, where the temperature of the heating element
may be considerably higher than the boiling temperature at the prevailing pressure.
[0025] In Fig. 9, the variation of the temperature is represented graphically. Graph B is
the graph where the amount of water pumped into the through-flow heater (Fig. 1) has
precisely evaporated when reaching the outlet (5). At an inflow temperature of 20°C
of the water at the inlet of the through-flow heater (1) and a boiling point of 100°C,
the heating up to this boiling point takes 12.9% of the available power. This power
is used in the first 13% of the passage through the spiral labyrinth. In graph A in
Fig. 9, the flow is lower by a factor of 2, so that the path in the labyrinth in which
the water boils is reduced to half the path in graph B.
[0026] As a result, just after half the path (about 56% of the path through the spiral),
all the water has evaporated. Consequently, the temperature in this area rises substantially.
Graphs C and D respectively show a flow which is two and about five times as great
as in graph B. In graphs B through D, the steam humidity becomes greater and greater.
In graph D, for instance, only half of the water has evaporated. The purpose of graph
E is to demonstrate that at a higher pressure, more time is needed for bringing the
water to the higher boiling point, in which case the water has to flow further into
the labyrinth to absorb the required energy. However, the flat portion (where the
water boils and is converted into steam) of graph E is shorter than the flat portion
in graph A, because the evaporation value at a higher pressure is lower.
[0027] The resistance of the heating element is the result of the sum of the resistances
of the three temperature areas mentioned. Fig. 10 shows how the resistance, relative
to the resistance at 0°C, represented by the vertical axis, depends on the flow and
the prevailing pressure (at atmospheric pressure or during the use of an electronically
controlled excess pressure valve (20) (Fig. 3)), at a constant power. When a heating
element according to Figs. 7 and 8 is started from, the heating track at a given position
is at approximately the same temperature as the water flowing past on the other side
of the heating element. For any pressure, a graph as shown in Fig. 10 can be derived.
On the horizontal axis, 'optimal flow (F)' is understood to mean the through-flow
at which all the supplied water, during its complete course through the spiral labyrinth,
has precisely evaporated completely. The bending point in the graph of Fig. 10, at
F=1, is a reference point for the control. In the case of an open connection (atmospheric
pressure) with the outside air, the resistance calculated by the microcontroller can
be related to the steam humidity of 0% at a boiling point associated with the prevailing
atmospheric pressure. With this method, by determining the resistance at a 'bend',
at the prevailing pressure, a fast method for determining the above-mentioned optimal
flow (F) is available. From the determination of this bending point in the graph,
the microcontroller can increase the steam humidity by increasing the pump power,
or by reducing the power to the heating element. When the power is reduced, less water
can be converted into steam, as a result of which the control is no longer at a point
of optimal flow of a graph. For instance a halving of the supplied power to the heating
element will shift the working point of the control to the point 2x the optimal flow
(F). Deviations relative to the theory here described in a specific embodiment of
an apparatus, caused by the differences in resistance occurring in the hoses and accessories
connected to the outlet (5) (Fig. 1), are corrected by software per type of apparatus
by means of measurements of the properties of the relevant type of apparatus.
[0028] Because in Fig. 2, the triac Tr1 has a particular chance of becoming defective, so
that it would constantly remain conductive, normally an external safety thermostat
is needed which switches off the voltage when a maximum temperature is being exceeded.
In apparatuses utilizing the invention described herein and an extension of the electronics
according to Fig. 4 described hereinbelow, the provision of a safety thermostat is
not necessary.
[0029] Relay RY1 (Fig. 4) has a make-contact whereby, in cases of failure, the heating element
(15) (Fig. 2) can be switched off.
[0030] The relay will de-energize if the program in the microcontroller is not or no longer
properly performed. In that case, the lower terminal pin of capacitor C1 will no longer
be provided with a block-shaped signal. Accordingly, transistor T3 will start to block,
so that C2 will start to charge itself to such an extent that the base current from
transistor T1 becomes so low that this transistor starts to block as well. As a result,
the relay RY1 will de-energize and contact ryl is opened.
[0031] Thus, the voltage provision to the heating element is broken. The proper operation
of the safety circuit of Fig. 4 is tested very regularly by the microcontroller. The
voltages on the collectors of all transistors are checked by the microcontroller interface
(22) for a variety of combinations of the two control signals from the microcontroller
interface to the transistors T2 and T3. Any deviation found will lead to the switching
off of relay RY1 and triac Tr1. In addition, as already mentioned, the total system
is so designed that also at a particular limiting rising pressure, at an unduly high
temperature (hence also in the case of boiling dry) it will interrupt the voltage
provision to the heating element very fast. Each time after switching off, the proper
operation of triac Trl is tested by checking whether no current passes through the
measuring resistance (14) anymore.
[0032] Through the use of a microcontroller and suitable software, it has become possible
to generate steam in a controlled and integrally safe manner, in the above-mentioned
manner.
1. A microcontroller-controlled steam generator provided with an electric heating element,
characterized in that for controlling the properties of the steam, use is made of the measured current
through the heating element and the measured voltage across the heating element, as
a result of which the use of separate sensors is not necessary.
2. An apparatus according to claim 1, characterized in that the apparatus is designed so that the influence on the measuring result, caused by
fluctuations in the voltage, is compensated by the program performed in the microcontroller,
by using the voltage measured across the heating element.
3. An apparatus according to one or more of the above claims, characterized in that the apparatus is designed so that measuring errors caused by mutual differences in
the resistance of heating elements produced can be prevented by causing a short, automatic
calibration to be performed periodically, wherein water under atmospheric pressure
is brought to the boil in the through-flow heater and the current through the heating
element is measured.
4. An apparatus according to one or more of the above claims, characterized in that the temperature of the inflowing water is determined in the best possible approximation
by filling the through-flow heater with a sufficient quantity of water and immediately
performing a measurement when the voltage across the heating element is switched on.
5. An apparatus according to one or more of the above claims, characterized by a through-flow heater having a small mass and low specific heat and a high power
per unit area and a relatively small content, to realize a control system which can
respond rapidly to fluctuations, so that the set, desired values can efficiently be
maintained.
6. An apparatus according to one or more of the above claims, characterized in that via a spiral-shaped or differently shaped labyrinth, the water is passed from the
water inlet to the steam outlet, so that the water and the steam flow through the
through-flow heater via a defined route, so that at a specific power and flow, the
temperature at any point can be theoretically derived, said temperature variation
determining the total resistance of the heating track of the through-flow heater.
7. An apparatus according to one or more of the above claims, characterized in that the apparatus is designed so that during the performance of the measurements by the
microcontroller the temperature of the water flowing past at any position can be determined.
8. An apparatus according to one or more of the above claims, characterized in that the microcontroller is designed for determining at a given power supplied, by changing
the flow, a point at which all the water, after the passage through the through-flow
heater, has precisely been converted into steam, referred to as the optimal flow (F),
which is detected by a stronger decrease of the current than normal, due to the rise
of the temperature in the area of the steam outlet of the through-flow heater, when
the flow is caused to decrease further.
9. An apparatus according to one or more of the above claims, characterized in that the microcontroller is designed for influencing the power supplied to the through-flow
heater, at a specific unchanging water through-flow, enabling the steam humidity to
be set at a desired value.
10. An apparatus according to one or more of the above claims, characterized in that the microcontroller is designed for influencing the pump capacity, at a specific
unchanging electric power, enabling the steam humidity to be set at a desired value.
11. An apparatus according to one or more of the above claims, characterized in that through the use of a labyrinth in the through-flow heater, the steam generator can
be used at angles of inclination up to about 45°.
12. An apparatus according to one or more of the above claims, characterized by means which in the case of dry-boiling or undue heeling over of the through-flow
heater, detect the resistance which rapidly increases due to the rapidly increasing
temperature, whereupon the electronics directly provide for the switching off of the
voltage of the heating element.
13. An apparatus according to one or more of the above claims, characterized in that upon the closing of the steam outlet, the microcontroller signals the resulting increase
of the pressure and the consequent rise of the resistance of the heating element,
whereupon the current supply to the heating element is switched off.
14. An apparatus according to one or more of the above claims, characterized in that the heating track is substantially identical in shape to the labyrinth, so that no
great differences in temperature and resulting mechanical stresses are caused in the
heating track.
15. An apparatus according to one or more of the above claims, characterized in that heating elements on the basis of thick-film technology are used, so that the application
of a supply filter is not necessary, since due to the fast heating up of the SS, and
the fast local expansion of the SS, the calcium layers which may be present on the
water side break up into small pieces, said small pieces being entrained by the steam
to the outside.
16. An apparatus according to one or more of the above claims, characterized in that during normal functioning, the electronics monitor the pressure and/or temperature,
so that for guaranteeing the safety, no separate pressure sensor or over-temperature
sensor is needed, while in the event of a failure in the electronics themselves, an
error in the program run in the microcontroller will be detected by the rest of the
electronics, while an error in the last-mentioned part of the electronics will be
detected in that the microcontroller is programmed to perform periodical tests which
will lead to the switching off of the current supply to the through-flow heater.
17. An electric circuit arranged for use in an apparatus according to any one of the preceding
claims.
18. A microcontroller programmed for use in an apparatus according to any one of the preceding
claims.
19. A through-flow heater for use in an apparatus according to any one of the preceding
claims, characterized by two spherical segments, mounted one onto the other, having a water inlet and a steam
outlet and a heating element designed as a resistance formed in thick-film technology.
20. A through-flow heater for use in an apparatus according to any one of claims 1-18
or according to claim 19, characterized by an approximately spiral-shaped labyrinth enclosed between the spherical segments.