Heat exchanger having fusion bonded plastic tubes/support plate

Process for permanently fastening the end or ends of at least one rod, made at least partially of a thermoplastic material and having at least one continuous cavity, in at least one opening of a support plate. The opening surrounds at least one rod, with each rod - in order to be fastened - positioned in its designated opening in the support plate. Each rod cavity exhibits a hydraulic diameter which corresponds to at least 10% of the minimum outside cross-sectional dimension of the rod, and the rod walls may exhibit pores which take up a maximum of 20% of the wall volume. The rods are positioned with the support plate in such a manner that the ends are essentially arranged vertically upward and the support plate essentially horizontally, and, subsequently, sufficient heat is applied to the rod ends from above, so that the rod ends will soften at least to some extent and the outside walls of the rod ends will expand to such an extent that they touch the wall surfaces of the support plate openings and adhere or fuse to thereto. Also disclosed is a heat exchanger which may be formed by this process.

FIELD OF THE INVENTION 
The invention concerns a process for permanently fastening the end or ends 
of at least one rod made of a thermoplastic material, each rod having at 
least one continuous cavity, in at least one opening of a support plate. 
The opening surrounds at least one rod, with each rod positioned in its 
designated opening in the support plate. Each rod cavity exhibits a 
hydraulic diameter which is at least 10% of the minimum wall thickness of 
the rod, and the rod walls may exhibit pores which take up no more than 
20% of the wall volume. The support plate, at least in the area of the 
openings, is made of a thermoplastic material, in particular a material 
suitable for the manufacture of heat exchangers. 
The invention also concerns a heat exchanger, comprising a number of 
thermoplastic rods, each exhibiting at least one continuous cavity, whose 
ends are permanently joined to at least one support plate, with the 
cavities enlarged toward the ends of the thermoplastic rods. 
The term "heat exchanger", as used in this specification, also covers 
substance exchangers, such as for gases, as well as sound absorbers for 
gas or liquid jets. 
BACKGROUND OF THE INVENTION 
A process of the same class and a heat exchanger of the same class are 
known from German Offenlegungsschrift No. 33 38 157. In the known process, 
the tubes are introduced into a tube sheet in the form of a perforated 
sheet. Subsequently, the tube ends are welded to the tube sheet by means 
of an ultrasound head. The production of an ultrasonic horn (sonotrode) 
which is suitable for use as an ultrasound head is very costly. By means 
of the known process, each tube end must either be welded to the tube 
sheet individually, which is very inefficient, or one needs a 
specially-designed ultrasonic horn for each desired tube arrangement or 
number of tubes, respectively. Moreover, if the tube ends are welded 
individually, some previously-welded tube ends could become partially 
loose again, due to the thermal effect, while adjacent tube ends are being 
welded. The outer contour of the ultrasonic horns may be selected to be 
conical. If such an ultrasonic horn is used for welding the tube ends to 
the tube sheet, each tube end on the tube sheet of the finished heat 
exchanger will be conically-enlarged toward its end. This enlargement 
facilitates the admission and discharge of the medium flowing in the heat 
exchanger. Through these enlargements one achieves a reduction in the 
pressure drop during admission of the medium into the tubes, which is not 
necessarily desirable, for instance if the heat exchanger manufactured by 
this process is to be used as a vaporizer. The heat exchanger manufactured 
according to the known process exhibits an annular bulge at each tube exit 
on the front surface of the tube sheet. In addition, a distinct boundary 
between the tube material and the tube sheet material is visible on the 
front surface of the tube sheet. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an especially simple 
process of the aforementioned type, in particular for the manufacture of a 
heat exchanger, which does not require any expensive tools and which can 
also be utilized independent of the desired arrangement of the rods. 
Moreover, it is an object of the present invention to provide an 
especially inexpensive heat exchanger. Preferably, there should be a 
seamless transition between the support plate material and the rod 
material, i.e., there should not be a visible boundary between the support 
plate material and the rod material. Another object is to provide a heat 
exchanger which is also suitable for use as a vaporizer.

DESCRIPTION OF THE INVENTION 
The above and other objects of the invention are achieved by positioning 
the rods in relation to the support plate in such a manner that the ends 
of the rods essentially extend vertically upward, and the support plate is 
arranged essentially horizontally. Subsequently, from above, a sufficient 
amount of heat is applied to the rod ends to cause them to soften at least 
partially and to cause the outside walls of the rod to expand to such an 
extent that they touch the walls of the support plate openings and adhere 
to same. To facilitate this, the support plate, at least in the area of 
the openings, must comprise a material to which the utilized rods will 
adhere when this process is used. 
In this connection, it was found that, when the rods are arranged nearly 
vertically, the top ends of the rods will open up outwardly as soon as the 
thermoplastic material of the rods has reached a doughy consistency. This 
phenomenon is exploited by the process covered by this invention. Due to 
the heat radiation, the rod ends open out, touch the inner surfaces of the 
support plate openings and adhere to same as more heat is applied. As 
already mentioned, heat may be applied in the form of heat radiation, for 
instance by a heating plate. However, it could also be supplied by sound 
waves, light, electromagnetic waves, hot gases, etc. In special cases, the 
thermal treatment may take place in a vacuum or may be gas-shielded. The 
amount of heat required for the process as per this invention can be 
determined by means of simple tests. As a rule, the thermal treatment is 
applied in accordance with the following conditions: 
##EQU1## 
where Q is the amount of heat applied per surface (front surface) and time 
in J/m.sup.2 s 
e is the linear expansion coefficient of the rod material in 1/K, 
t is the time duration of the thermal effect in sec., 
.zeta. is the density of the rod material in kg/m.sup.3, 
C.sub.P is the specific heat of the rod material in J/kg K, and 
.lambda. is the thermal conductivity of the rod material in J/msK. 
The degree of enlargement of the rod ends is best controlled by using rods 
of a thermoplastic material which has a mean linear expansion coefficient 
between 100.times.10.sup.-6 and 400.times.10.sup.-6 1/K in the range 
between room temperature and melting temperature. 
It is especially favorable if the support plate, at least in the area of 
the openings, is made of thermoplastic material and if sufficient heat is 
applied to the rod ends to cause them to fuse with the support plate. The 
insertion of the rods into the openings provided in the support plate is 
facilitated by utilizing support plates whose openings, at least in a 
cross-sectional dimension, are larger than the corresponding outside 
cross-sectional dimension of the rod by an amount that equals at least 20% 
of the wall thickness of the rod. It has also been found that this has an 
especially favorable effect on the strength of the joint between the rod 
and the support plate. 
The process according to this invention will have excellent results with 
rods (which may have a porosity up to 10% of the wall volume) having 
cavities with a hydraulic diameter of between 0.3 and 15 mm, preferably 
between 0.5 and 7.5 mm, and having a wall thickness, at the thinnest 
point, between 5 and 25% of the hydraulic diameter, preferably between 7.5 
and 17.5% of the hydraulic diameter. It is known that the hydraulic 
diameter is defined as the quotient of four times the inside 
cross-sectional area of one channel of the rod divided by the wetted 
inside perimeter (in this connection, refer to Dubbel, Taschenbuch fuer 
den Maschinenbau, 13th edition, 1970, page 314, paragraph 4): 
##EQU2## 
It is preferable to use support plates with openings whose cross sectional 
areas are enlarged, at least toward the front surface, in relation to the 
outside cross section of the utilized rods or rod groups, respectively. 
The rods should be positioned in such a manner that the rod ends extend 
into the support plate openings at least as far as the enlargements. The 
manner in which the cross-sectional area is enlarged or expanded, 
respectively, toward the front surface will have an influence on the 
inside contour of the rod after the heat treatment. If the cross sectional 
enlargement of the support plate openings toward the front surface is 
gradual, the rod inside cross sections will exhibit a constriction--in the 
direction of the inside of the rod--whose area is about 10 to 60% of the 
area of the inside cross section and which subsequently, toward the 
inside, changes-over to the rod inside cross section. If the cross 
sectional enlargement of the support plate openings toward the front 
surface is abrupt, the inside of the rod end will not exhibit any or only 
a hardly perceptible constriction of its inside cross section. 
It has been found to be particularly advantageous, prior to the thermal 
treatment, to position the rods in the support plate openings in such a 
manner that the rod ends extend 1 to 2 mm beyond the front surface of the 
support plate. 
The strength of the joint between the rod and the support plate can be 
enhanced--subsequent to the positioning but prior to the thermal 
treatment--by filling the clearance, if any, between the outer surface of 
the rods and the support plate openings at least partially with a polymer 
powder. Preferably, the utilized polymer powder should have a melting 
temperature which is about 5.degree. to 50.degree. C., preferably 
20.degree. to 50.degree. C. below the melting temperature of the rod 
material. A polymer powder with an average particle size of 20 to 100 
.mu.m, preferably 40 to 100 .mu.m is preferred. A particle size of over 
100 .mu.m, in many cases, will result in an undesirable, non-homogeneous 
surface. 
The process according to the invention is particularly successful if the 
support plate, the rods and possibly the polymer powder as well, are made 
of thermoplastic polymers which, according to their basic structure, 
belong in the same group of polymers and/or have the same or nearly the 
same melting point or melting range, respectively. 
The heat exchanger manufactured in accordance with the process of this 
invention exhibits a number of features which differ from those of the 
previously manufactured heat exchangers. For instance, if a heat exchanger 
is manufactured in accordance with the process known from DE-OS 33 38 157, 
supra, the front surface of the support plate or tube sheet, respectively, 
remains smooth; there is a clearly visible border line between the support 
plate material and the rod material, not only due to the annular bulge on 
the front surface of the support plate; and the insides of the rod ends 
show evidence of the use of contact ultrasonic horns. The front surface of 
a heat exchanger manufactured according to the process of this invention 
has the appearance of the typical thermoplastic hardened from the molten 
state, which might be compared to the surface of a crepe rubber shoe sole: 
the surface appears to be somewhat bubbly. 
In a heat exchanger manufactured according to the process of this 
invention, even a cut through the plate and the tube will not reveal any 
transition between the support plate material and the rod material. Also, 
the surface of the support plate material exhibits the typical shine, 
recognized by the expert, of a thermoplastic material hardened from the 
molten state. 
Objects of this invention, therefore, are also met by a heat exchanger 
comprising a number of thermoplastic rods, each hving at least one 
continuous cavity, whose ends are permanently joined to at least one 
support plate, wherein the cavities are enlarged toward the end of the 
thermoplastic rods, and which can be manufactured according to the process 
of this invention. 
As a rule, the heat exchanger according to this invention is also 
distinguished in that each rod end is fused to the support plate at least 
to a depth which corresponds to the minimum wall thickness of the rod, 
viewed from the front surface of the support plate. As already mentioned 
above, the heat exchanger according to this invention, as a rule, is 
distinguished in that the rod material is fused with the support plate 
material in such a manner that there is a gradual transition between the 
two materials. 
The rods of the heat exchanger may be embodied in the form of tubes which 
have one or several continuous cavities and whose hydraulic diameter (of 
the individual cavity) is between 0.5 and 15 mm and whose wall thickness, 
at the thinnest point, is between 5 and 25% of the hydraulic diameter. The 
heat exchangers according to this invention may also contain rods which 
comprise groups of tubes. These tube groups may have the outward shape of 
a plate, for instance if the tubes are lined up side by side with their 
axes parallel and extending along a straight line. However, they may also 
take the form of a shaped or spiral plate. In the case of a spiral plate, 
the tubes are lined up side by side in a group with their axes extending 
along an imaginary spiral line. Such a group of tubes may comprise 2 to 
100,000 tubes, preferably 2 to 20,000. 
If the heat exchangers according to this invention are used as vaporizers, 
each enlargement of the cross sections of the rod ends narrows toward the 
inside of the rod, to a cross section whose area is about 10 to 60% of the 
area of the inside cross section, with this constriction subsequently, 
toward the inside, changing over to the rod inside cross section. 
Preferably, the constricted cross section area equals about 20 to 50% of 
the area of the inside cross section. 
The heat exchangers according to this invention may be made at least 
partially of a thermoplastic polymer which is one of the fluorine 
polymers. However, they may also be at least partially made of 
polyethylene or polypropylene. 
The advantages of this invention, among other things, also lie in the fact 
that the joint between the rod and the support plate is established 
without the application of any mechanical force, and, therefore, the rod 
ends do not exhibit any signs of, for instance, a punch used to apply 
force. Such tools are not required in the use of the process of this 
invention. In the process according to this invention, the contact 
pressure of the rods against the walls of the support plate openings is 
strictly the result of the expansion of the rod ends caused by the thermal 
treatment. In addition, the cross sectional area of the heat source is 
dependent only on the outside diameter of the support plate, i.e., the 
same heat source may be used independent of the type, number and 
arrangement of the rods, as long as the cross sectional area of the heat 
source is adequately dimensioned. The process according to this invention 
does not involve any contact, so that the heat exchanger will not adhere 
to the heat source. Therefore, the front surface of the heat exchanger 
does not exhibit the hairy surface which is typical in the use of contact 
units. 
FIG. 1 is a schematic representation showing a cross section through a heat 
exchanger in which, according to the process of the invention, the tubes 
(rods) 1 are introduced on the top side into a tube sheet (support plate) 
2 in such a manner that they extend about 1.5 mm beyond the front surface 
with the conical enlargements (top side). The gaps, formed between the 
ends of the tubes 1 and the conical enlargements, have already been filled 
with polymer powder 5. The heat exchanger prepared in this manner was 
already placed, by means of a housing 3, below a heating plate 6--which 
delivers the heat radiation for the application of the process according 
to this invention--in such a way that the ends of the tubes are arranged 
vertically upward and the support plate is oriented essentially 
horizontally. After a sufficiently long period of exposure to the heat 
radiation, the tube ends are solidly joined to the tube sheet. The joint 
between the tube and tube sheet is shown in cross section in FIG. 2. The 
end of the tube 1 is arranged in the tube sheet 2, with the tube end in 
the tube sheet exhibiting a conical enlargement 7 and an annular bulge 8 
toward the smallest cross section of this conical enlargement. The end of 
the tube 1 is solidly fused to the tube sheet 2 in the area of the conical 
enlargement 7 and the annular bulge 8. 
The conical enlargements of the front surface may be produced simply by 
countersinking the holes in the tube sheet. The joint between the tube and 
tube sheet will be especially strong if the tube sheet is also made of a 
thermoplastic material, particularly the same thermoplastic material as 
the tubes. The boundary line between the tube end and the tube sheet, as 
shown in FIG. 2, will no longer be visible, if the tube sheet 2--in the 
area of the openings--is made of thermoplastic material and if a 
sufficient amount of heat is applied to the tube end to cause the tube end 
7 to fuse with the tube sheet 2. 
FIG. 3 is a representation in perspective of an arrangement of flat rod 
ends 9 in a support plate 10. Each rod end 9 has six penetrating openings 
11. The rod ends 9 are positioned in the support plate 10 in such a manner 
that they extend beyond the front surface of the support plate. The 
openings of the support plate 10 (not shown) exhibit enlargements 12 
toward the front surface, which enlargements are shaped like an elongated, 
truncated pyramid. 
FIG. 4 is another representation in perspective of a possible arrangement 
of flat rods 13 whose ends have the contour of a circle segment, and which 
are arranged in a suitable support plate 14. These rods 13 also exhibit 
several continuous passages 16. For the thermal treatment, a heating rod 
16, positioned as shown in FIG. 4, is especially useful. 
The openings of the support plates 17 may also, as shown in FIG. 5, be 
developed in such a manner that they are extended in the direction of the 
front surface by means of webs 18 which project beyond the body of the 
support plate 17. Support plates of this design ensure an especially 
strong joint between the rod ends and the support plate. This applies as 
well to the design of an opening in the support plate 19, shown in FIG. 6. 
In this case, the support plate 19 exhibits an enlargement 20 in the 
direction of the front surface which, in the case of round rods, may be 
called conical and, in the case of flat rods, may be said to have the form 
of a truncated pyramid. Moreover, within the enlargements 20, there are 
webs 21, which guarantee that the fused bond with the rod ends will be 
especially intimate and strong. During the thermal treatment, the webs 21 
will widen together with the rod ends so that the webs 21 will touch the 
enlargements 20. Subsequent to the thermal treatment, the webs 21 will be 
fused with the support plate 19, and the tube ends with the webs 21. 
FIGS. 7 through 13 show examples of possible embodiments of enlargements of 
the support plate openings in the direction of the front surface. 
In FIG. 7, a rod end 24 is inserted into the support plate 23 in such a 
manner that the rod end 24 extends beyond the support plate 23. The 
support plate opening (not numbered) exhibits a gradual enlargement 22. In 
the case of round rods 24, the thermal treatment will result in the 
configuration shown in FIG. 2. 
FIGS. 8, 10, 11 and 12 show abrupt enlargements 25, 31, 34 and 37, 
respectively of the openings 27, 33, 36 and 39, respectively, in the 
support plates 26, 32, 35 and 38, respectively. FIGS. 9 and 13 illustrate 
gradual enlargements 28 and 41, respectively, of the openings 30 and 44, 
respectively, in the support plates 29 and 42, respectively. In FIG. 13, 
the rod end 43 extends into the enlargement 41 of the support plate 42, 
however without projecting beyond the front surface of the support plate 
42. 
The following examples will serve to explain preferred embodiments of the 
invention in detail. They are intended for clarification, and not for 
limitation. 
TABLE 
__________________________________________________________________________ 
Determination PVDF PVDF 
Property according to 
Unit type I 
type II 
PP 
__________________________________________________________________________ 
density ASTM D 792 
g/cm.sup.3 
1.78 1.78 0.905 
melt flow 
ASTM D 1238 
index 
-230.degree. C. 10 kg 
g/10 min 
50 13 -- 
-230.degree. C., 5 kg 
g/10 min 
18 4 9 
-230.degree. C., 2.16 kg 
g/10 min 
6 1 2 
crystalline .degree.C. 
177 177 160-166 
melting point 
linear ASTM D 696 
K.sup.-1 
106 .times. 10.sup.-6 
128 .times. 10.sup.-6 
150 .times. 10.sup.-6 
expansion 
coefficient 
heat ASTM C 177 
W/m .times. K 
0.19 0.19 0.22 
conductivity 
specific heat J/kg .times. K 
960 960 1700 
between 
0 and 100.degree. C. 
tensile ASTM D 638 
MPa 57 54 35 
strength 
5 mm/min 
elongation 
ASTM D 638 
% 12 80 700 
5 mm/min 
modulus at 
ASTM D 638 
MPa 2600 2400 1300 
1 mm/min 
__________________________________________________________________________ 
EXAMPLE 1 
A plate of PVDF type I (see Table), whose outside dimensions are 101 
mm.times.50 mm.times.15 mm is provided with 28 penetrating slots which are 
vertical in relation to the top surface (101 mm.times.50 mm). Each slot is 
40 mm long and 2 mm wide. The slots are parallel to each other and extend 
parallel in relation to the side of the plate which measures 50 mm. The 
distance between the slots is 3.5 mm. Beginning about 1 mm below the top 
surface--i.e., the future front surface--up to the top surface, the slots 
are widened on all sides, with the contour of the widening of opposite 
sides enclosing an angle at 60.degree.. As a result, on the front surface, 
each slot has an opening of about 41.2 mm.times.3.2 mm. 
Rods of PVDF type I, each 1 m long, 45 mm wide and 2 mm thick are inserted 
into the slots. Each rod has 20 continuous cavities with a hydraulic 
diameter of 1.6 mm, with a center-to-center distance of 2 mm between two 
adjacent, continuous cavities. The smallest wall thickness between a 
cavity and the outside of the rod is 0.2 mm. The rods are positioned in 
such a manner that the ends of the rods extend 1.5 mm beyond the front 
surface of the plate, that the rods as such are vertically arranged, and 
that the enlargements of the slots, and thus the front surface, are 
pointing up. 
Now, a flat heating plate with a surface of 50 cm.times.50 cm, an emission 
coefficient of 0.9 and a temperature of 430.degree. C., is directed toward 
the PVDF plate for two minutes at a distance of one centimeter, and is 
then removed. 
An inspection of the front surface shows that now the end of each rod is 
bent outward and is adhering to the enlarged areas of the slots. A cut 
through such a rod end, parallel to the axis, reveals that, in the area of 
the enlargement of the rod, the rod has become partially fused with the 
wall of the slots over a length of 1.5 mm. 
EXAMPLE 2 
This example uses the same plate and the same rods as in Example 1, but 
this time, the rods are 39.5 mm wide. Again, the rods are inserted and 
positioned in the slots in the plate in such a manner that they extend 
about 1.5 mm beyond the front surface of the plate. The wedge-shaped gap, 
formed between the outer wall of the rods and the surface of the enlarged 
areas of the slots, is now partially filled with PVDF powder type I, whose 
mean particle size is 0.06 mm. Subsequent to the thermal treatment, as 
described in Example 1, the powder has melted. A cut, parallel to the 
longitudinal axis of the rod, shows that the rod has become evenly fused 
with the surface of the slot over a depth of 1.5 mm. 
EXAMPLE 3 
A cylindrical plate, made of PVDF type II, has a diameter of 150 mm and a 
thickness of 30 mm. The plate exhibits 1440 penetrating bores whose 
diameter is 2 mm. On the one surface, measuring 150 mm in diameter, of the 
plate--i.e., the front surface--all bores are abruptly enlarged. These 
abrupt enlargements have a diameter of 3 mm and a depth of 0.8 mm. A tube 
of 2 mm outside diameter and 0.15 mm wall thickness is inserted into each 
bore, and positioned in such a manner that the ends of the tube extend by 
1.5 mm beyond the front surface. The tubes are made of PVDF type I. 
The tubes are arranged vertically so that the front side of the plate 
points up. Polymer powder of type I, having a mean particle size of 0.06 
mm, is sprinkled onto the plate and the slots formed by the tubes and the 
enlargements. Subsequently, the heating plate described in Example I is 
directed toward the front surface at a distance of 1 cm. The temperature 
of the heating plate is 430.degree. C. After 2 minutes, the heating plate 
is removed. Then the plate, with the tubes permanently joined to it, is 
allowed to cool to room temperature. 
EXAMPLE 4 
Example 3 is repeated. Now, however, a plate is used whose enlargements are 
conical. The largest diameter of the conical enlargements, which lies on 
the front surface of the plate, is 3 mm while the center angle of the cone 
of the conical enlargement is 85.degree.. After the thermal treatment, 
each tube exhibits an annular constriction at that point in the plate, 
where the transition between the cylindrical bore and the conical 
enlargement is located. 
EXAMPLE 5 
The manufacture of a heat exchanger, as described in Example 1, may proceed 
under the same process conditions if the material chosen for the plate and 
the rods is polypropylene (PP) as shown in the chart.