Tubular heater with an overload safety means

A tubular heater in which a safety fuse or a PTC-element, a sensor for an electronic circuit or a thermostat is disposed as an overload safety means in the non-heated end region of the tubular heater.

The invention relates to a tubular heater comprising a heating coil of 
resistance wire, which is disposed in a casing tube, being embedded in 
insulating material, with an overload safety means. 
DE-OS (German laid-open application) No. 21 01 062 discloses an electrical 
tubular heater with temperature limiting means, wherein the switching 
member of the temperature limiting means, together with the heating 
conductor, is embedded in the compacted insulating material of the heater 
body. In that arrangement, the switching member may be a fusible solder 
safety device or a bimetal safety device. Arranging the safety device 
within the heating region was found not to be a practicable proposition 
as, in the necessary operation of compacting the tubular heater, the 
safety means is compressed and thereby suffers damage. The rolling forces 
which are applied in the operation of compacting a tubular heater are so 
high that for example even solid pins which are embedded into the 
insulating material are reduced in diameter. Hitherto, no safety device 
which could withstand such rolling forces has yet been found. 
Arranging the safety means within the heated region gives rise to the 
further disadvantage that the safety means also responds when the tubular 
heater suffers from calcification, which is generally unavoidable, as the 
interior of the tubular heater heats up due to the reduction in the amount 
of energy which is given off. As however tubular heaters must be so 
designed that they can still be operated when suffering from the 
calcification which occurs in normal operation, the safety means would 
have to have a high response threshold. However, having regard to the 
large production tolerances which are due to mass production, it would 
then not be possible to ensure that all tubular heaters are adequately 
protected. 
DE-OS (German laid-open application) No. 24 42 717 discloses an electrical 
heating plate for electric stoves having a means for protection against 
overheating, wherein a cold conductor which is incorporated into the 
electrical circuit, that is to say, an element which is referred to as a 
PTC-element, the cut-out temperature of which is considerably below the 
upper working temperature of the heating plate, is arranged underneath the 
plate at a location at which the temperature level in operation of the 
heating plate is substantially lower than that of the plate. That 
arrangement makes it possible for the heating plate to reach a higher 
temperature for a brief period of time, without the cold conductor 
responding. The problem of brief overheating is not an acute one, in 
tubular heaters. In contrast, the safety means should respond as quickly 
as possible if for example the tubular heater begins to go incandescent, 
for example as a result of the arrangement running dry. 
The object of the present invention is to provide a tubular heater with 
overload safety means, which provides for an improved response 
characteristic in respect of the overload safety means. 
The manner in which that object is achieved is set forth in the 
characterising features of the main claim. The subsidiary claims set forth 
preferred embodiments. 
In accordance with the invention therefore, the safety means is arranged in 
the unheated portion within the tubular heater. That arrangement, at the 
connecting end of the tubular heater, makes it possible for the tubular 
heater to be compacted and for the safety means to be only subsequently 
fitted in position. That in principle avoids damage to the safety means, 
in a simple fashion. 
As the unheated portions of the tubular heater do not suffer from 
calcification and are also colder than the heated portions, the safety 
means registers only increases in the temperature of the heating 
conductor, with respect to the relatively cold unheated end of the tubular 
heater, and irrespective of calcification. That means that the safety 
means can be given a cut-off value which also takes account of production 
tolerances and which protects the tubular heater from overloading. 
The novel design of the tubular heater gives a number of advantages. As the 
tubular heater itself has a precisely adjusted safety fuse, a series of 
uncertainty factors is no longer encountered. In particular, the 
arrangement ensures that the response value of the overload safety means 
must necessarily always remain the same, independently of the operating 
conditions and the operating time of the piece of equipment which is 
fitted with the tubular heater. Even if additional temperature switches 
and controllers are used, the arrangement ensures in any case that, in the 
event of failure of such temperature regulators, the overload safety means 
responds when a given limit temperature is exceeded.

A heating coil 2 is fitted by means of its ends onto a connecting element 
which comprises a cylindrical body 4, a stepped cylindrical portion 4a of 
smaller diameter and an elongate conical end portion 5. Now, some windings 
are welded or soldered to the cylindrical portion 4a, at 6, as shown in 
FIG. 2, by means of a contact-less soldering or welding process, in 
particular a laser beam welding process. The resulting assembly is now 
introduced in the usual manner into a casing tube 1 which is filled with 
insulating material, for example magnesium oxide. When that is done, the 
connecting element is partially embedded into the insulating material, 
that is to say, embedded therein at a maximum as far as an annular groove 
18 in the cylindrical body 4. 
Then, as shown in FIG. 4, the resulting intermediate product is pressed for 
example by being passed through a pair of rolls 30, for the purposes of 
compacting the insulating material 3. Thereupon, as shown in FIG. 5, the 
portion 19 of the connecting element, which projects beyond the annular 
groove 18, is broken off so that a free space is formed in the end of the 
tubular heater. By virtue of the annular groove 18, the connecting element 
is broken off without forming a burr or fin, so that a sleeve 20 can be 
pushed onto the cylindrical portion 4 of the connecting element. In that 
connection, the sleeve 20 may be slitted along a generatrix, to enhance 
its resilient pinching force. 
A safety fuse 9 which has a connecting wire 11 is now fitted into the 
sleeve 20. The safety fuse 9 is fixed and sealed in position by means of 
an insulating bush member 22. 
In the embodiment illustrated in FIG. 8, a connecting tube 7 is fitted over 
the cylindrical portion of the connecting pin 4 and the weld location 6, 
and is pressed against the weld location 6. The connecting tube 7 is held 
concentrically in the casing tube 1 by a sleeve-like insulating bead 8, 
the end of the connecting tube 7 projecting beyond the end of the 
insulating bead 8. 
A substantially cylindrical safety fuse 9 is now fitted into the connecting 
tube 7, and is secured to a connecting wire portion 11. An insulating 
sheath 12 is drawn over the connecting wire portion 11. 
The length of the connecting tube 7, which is in the casing tube 1, is 
embedded into the insulating material. The tubular heater can thus be 
rolled to compact the insulating material 3 over its full length, the 
connecting tube 7 being suitably fixed in positin. In that operation, the 
elongate conical end portion 5 of the connecting pin 4 makes it possible 
for the pressing rolls to be suitably set by an automatic control means, 
with the roll gap being increased but the pressing force being maintained 
at a constant value. 
Therefore, the tubular heater has a double connection, the end of the 
connecting tube 7 which projects beyond the insulating bead 8 having a 
direct electrical connection to the heating coil 2, while the projecting 
end of the connecting wire 11 permits electrical connection to heating 
coil 2 by way of the safety fuse 9. 
The tubular heater can therefore be subjected for example in the factory 
where it is manufactured, for test purposes, to a voltage which goes 
beyond the capacity of the safety fuse 9. In addition, it is possible to 
ascertain, by way of the above-mentioned connecting end, whether the 
heating coil including its weld location 6 on the connecting pin 4 is 
intact. Furthermore, the end of the connecting tube 7 can be utilised for 
operating or monitoring conditions which do not require a safety means, 
for example monitoring or checking the temperature of the heating coil by 
a resistance measurement operation, performing a running check on the 
leakage current, and the like. 
The safety fuse 9 can be easily replaced by being drawn out of the 
connecting tube 7. 
The embodiment illustrated in FIG. 9 differs from the embodiment 
illustrated in FIG. 7, in that the connecting pin 4 is omitted. This 
arrangement has a connecting tube 13, the end 14 thereof, which is 
disposed in the tubular heater, being of an elongate conical 
configuration. The heating coil 2 is welded or soldered to the conical end 
portion 14, at 15. The safety fuse 9 has a correspondingly shaped 
projection portion 16 which is fitted into the end 14. This embodiment has 
the advantage that a saving is made in respect of the connecting pin 4 and 
that the safety fuse 9 is better fixed in position by suitably clamping 
the end 16. Contact-making is also more reliable. 
The use of a contact-less soldering or welding process, for example using 
laser beams, for securing the heating coil 2 to the connecting pin 4 or 
the connecting tube 13 is of particular significance, particularly in the 
embodiment illustrated in FIG. 9. In the case of the previous welding 
process in which the heating coil had to be pressed against, by means of 
welding electrodes, in the welding operation, there is the danger that the 
conically tapering inner end portion 14 of the connecting tube 13 may be 
pressed in. If, to avoid that danger, the connecting tube 13 were made of 
greater wall thickness, then, in a tubular heater of the usual sizes, the 
thickness of the layer of insulating material 3 between the casing tube 1 
and the connecting tube 13 would be too small, thereby detrimentally 
affecting the dielectric strength of the tubular heater. 
In the embodiment illustrated in FIG. 10, a tube 27 is fitted to the 
connecting pins of the tubular heater. Disposed in the tube 27 is a safety 
fuse 9 which is pressed into position by a spring 28 in such a way as to 
ensure good electrical contact of the inner connection 29. 
The embodiment illustrated in FIG. 11 is similar to that shown in FIG. 8. 
However, instead of a safety fuse 9, this embodiment uses PTC-discs or 
plates 23 which are set in place with the interposition of an insulating 
film or foil 24. The PTC-members 23 comprise a resistance material having 
a positive temperature gradient. Accordingly, the flow of current from the 
connecting wire 11 to the connecting pin 4 falls with increasing heating 
of the PTC-members 23. If therefore, as a result of overheating of the 
tubular member, heat is correspondingly conducted by the connecting tube 7 
from the interior of the tubular heater to the PTC-elements 23, then that 
gives a corresponding drop in the flow of current due to the increase in 
resistance, and accordingly causes the heating output to be cut down. 
However, under conditions of higher power consumption on the part of the 
tubular heater, the entire flow of current will not be passed by way of 
the PTC-elements 23. An auxiliary voltage may be set up between the 
connecting wire portion 11 and the connecting tube 7, through a suitable 
relay circuit 25. In that case, in the event of a fall in the flow of 
current through the PTC-elements 23, as a result of an inadmissible rise 
in temperature, the main current supply is switched off, through the 
relay. It will be appreciated that, when using circuit arrangements of 
that kind, it is also possible for the reverse procedure to be followed, 
that is to say, instead of the PTC-elements 23, it is also possible to use 
elements having a negative temperature coefficient, in which case the 
arrangement is cut out, with increasing current flow. 
FIG. 12 shows that it may be advantageous for a corresponding 
temperature-sensitive assembly or unit 26, to be disposed outside the 
casing tube 1 of the tubular heater. In that case, the assembly 26 which 
is shown in diagrammatic form, may comprise the described PTC-elements, 
but it may also comprise other temperature-sensitive switching members 
such as bimetal switches or an electrical switching means. Because the 
temperature-sensitive assembly 26 is disposed outside the casing 1 and is 
cooled, the response point thereof may be set close to the melting point 
of the heating coil or of the casing tube 1 of the tubular heater. On the 
other hand however, the assembly is caused to respond when the current 
flow rises in an inadmissible fashion, for example when the tubular heater 
begins to burn through or blow due to the formation of an arc from the 
heating coil 2 to the casing tube 1. 
As the overload safety means is disposed in the non-heated end of the 
tubular heater and no calcium depositing occurs on the tubular heater 
casing at that point, that arrangement ensures that, in operation, at a 
given level of electrical power consumption, the same temperature always 
obtains in the vicinity of the safety means. The safety means can 
therefore be set to a temperature which is for example only 50.degree. C. 
above that temperature around the safety means. That ensures reliable 
response long before there is a danger of fire occurring, so that it is 
also possible to equip, with the tubular heater electrical equipment with 
burnable plastic casings. The elongate conical end portion 5, 14 of the 
connecting pin 5 or the connecting tube 13 conducts the heat which occurs 
in the interior of the tubular heater to the safety element so that the 
safety element responds if the heating coil 2 attains an excessively high 
temperature, for example due to the tubular heater burning through or 
running dry due to a failure in the water supply, for example in washing 
machines or dishwashing machines. The length of the conical end portion 5, 
from the weld location 6, which projects into the tubular heater, should 
correspond to at least twice the outside diameter of the tubular heater.