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Description  |
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The invention relates to heat exchangers of thermoplastics materials.
The invention provides a heat exchanger of thermoplastics material
comprising two spaced-apart headers interconnected by an extruded board
having a profile comprising a plurality of tubular passages extending from
one end of the board to the other; each tubular header having a
longitudinal groove in its exterior surface and a plurality of holes
spaced along the groove, the holes interconnecting the groove and the
interior of the header; each end of the board being located along the
groove of one of the headers; each header being held in position by a
sealing bead adhering to both the header and the board, and extending
continuously right round the mouth of the groove where covered by the
board, to seal against loss of any fluid flowing between the header and
the tubular passages of the board.
When operating this heat exchanger, the two headers are connected to an
external circuit and the system filled with a suitable fluid, such as
water. The fluid is then caused to flow from the external circuit into one
header, through the holes, and thence into and along the plurality of
tubular passages in the extruded board where it either picks up or loses
heat to the environment. At the other end of the tubular passages, the
fluid flows through the holes into that other header, and eventually out
into the external circuit again. The fluid may suitably be caused to
circulate by a pump, although in some applications, thermo-siphoning
effects due to the temperature difference across the heat exchanger, may
be sufficient.
The heat exchangers may be used singly or a plurality of similar exchangers
may be connected together to form an array. For efficient working, it is
desirable to have uniform flow through all the tubular passages, and this
also applies throughout a full array of such heat exchangers. This uniform
even flow may be achieved in a flat panel by using a diagonal flow pattern
with the fluid entering at one end of one header and leaving from the
opposite end of the other header. Uniform distribution of the fluid to the
various tubular passages along the width of the board may be assisted by
leaving the grooves sufficiently clear to permit free flow of fluid along
them. This may be achieved by mounting the board at the mouth of the
groove. However, it is difficult in practice to extrude the sealing bead
forcibly into contact with both the board and the header while maintaining
their alignment and also while ensuring that the sealing material does not
enter the groove or the ends of the tubular passages, with consequent
restriction (or even complete blockage) of fluid flow during use. These
problems can be avoided by inserting the ends of the board at least
part-way into the header grooves to provide positive location and avoid
ingress of sealer, and such heat exchangers are preferred. Free flow of
fluid along the groove may still be achieved by preventing the board from
reaching the base of the groove (e.g. by using a groove whose width at its
mouth is sufficient to accommodate the end of the board but which narrows
e.g. stepwise or by tapering, to a width insufficient to accommodate the
board at a point part-way between the mouth of the groove and its base, or
by inserting temporary water-soluble spacers at the base of the groove) or
by shaping the end of the board such that part thereof is held clear of
the base of the groove to allow free flow of fluid along the groove, by a
further part thereof which extends to the base of the groove.
In constructing the heat exchanger, it is desirable to avoid restricting
the flow of the fluid through it. There are various reasons for this.
Thus, for example, a restricted flow requires more energy, and indeed,
adequate circulation of the fluid by thermo-siphon effects alone may be
prevented if flow is restricted. Restricted passages are more likely to
become blocked during use, and it would be more difficult to obtain
uniform flow because it would then be necessary to ensure that the
restrictions were all the same for each tubular passage, because without
restrictions or other such adverse factors, the diagonal flow pattern
tends to give a substantially uniform flow pattern across the board.
The total area of the holes spaced along the length of the header groove,
is therefore preferably sufficient to permit fluid to flow at a rate
sufficient for the particular application without substantially
restricting the flow. The holes may be round, formed for example by
drilling, or they may be elongated to form slots where a greater area is
required. However, as there are a plurality of holes according to this
invention, there must necessarily be at least one (and preferably many
more) portion between the holes, bridging between the two sides of the
groove. The purpose of these bridging portions is to carry the stresses
within the headers, rather than transfer them to the relatively thin
sections of the board. Preferred holes are rectangular-sectioned, and
arranged with adjacent sides substantially parallel. The
rectangular-sectioned holes provide a larger flow passage area for any
specific width of bridging portion than corresponding circular holes. For
particularly arduous conditions it is preferred to provide rectangular
holes with one side longer than the other, the holes being aligned with
their longer sides adjacent. For most applications, substantially
square-sectioned holes are suitable, and for lower stress conditions, the
holes may be elongated along the direction of the line of holes.
At the interface between the ends of the tubular passages and the adjacent
surface of the header where the holes emerge, the total area of the flow
passage cross-section will be reduced by the thickness of both the
bridging portions and the webs unless there is complete coincidence
between them, unless the structure is proportioned to avoid that. Hence in
a preferred heat exchanger the bridging portions are narrowed
substantially to a knife edge at their outer surface. The internal width
of the tubular passages from one side of the board to the other is also
preferably less than the width of the holes through the header by an
amount such that the total cross-sectional area of the holes is
substantially equal to or greater than the total cross-sectional area of
the tubular passages. By reducing the thickness of the bridging portions
to a knife edge, there is no significant reduction of the flow passage
area at the interface which might otherwise cause some restriction in
flow. Likewise by broadening the holes, reduction in the flow passage area
may be avoided while allowing adequate bridging material for carrying the
stresses within the header pipe.
The knife edge may be obtained by tapering the holes over the full
thickness of the header wall. However, it is preferred to merely chamfer
the bridging portions at their outer surface so as to provide the maximum
quantity of material in each bridging portion while still avoiding
restriction of the flow. The chamfer may be on one or both sides of the
bridging portion, a unilateral chamfer being preferred.
The boards may be extruded in the manner described in British patent
specification No. 1 042 732, wherein FIGS. 1 to 4 show various board
profiles, each of which comprises tubular passages extending from one end
of the board to the other, and could be used in the present application.
However, as thermoplastics materials are generally of low thermal
conductivity, it is preferable for the board to have a profile which
minimises the amount of conduction required to dissipate the heat over the
surface of the board or to collect heat from all over the surface.
Preferred configurations of the four shown therein are those of FIG. 1
wherein the tubular passages are substantially square in section, and FIG.
4 wherein the passages each have a domed roof and thicker-sectioned webs
between the passages. These configurations are also illustrated in FIGS. 1
and 5 respectively of the accompanying drawings.
The heat exchanger may be used for extracting heat from its environment,
e.g. as a solar energy collector. For this application the headers and
especially the board may be extruded from a thermoplastics material
(preferably a stabilised grade of polypropylene) which is either pigmented
or coated with a solar radiation-absorbing colour, e.g. black. Water
circulated through such a panel exposed to the sun may become warmer, and
be used as a low grade heat supply suitable for heating e.g. swimming
pools (where it may be convenient to use the pool's own pump to circulate
the water being filtered, around the panel or preferably array of panels),
greenhouses or buildings.
The heat exchangers may be used to supply heat to the environment, by
circulating water which is warmer than the environment. The heat exchanger
may, for example, be used as a radiator in a central heating system of a
building, as a greenhouse heater, or even as a direct soil heater when
buried in the soil of a greenhouse or cold frame. The water for such
applications may be warmed by solar energy, collected for example by a
further heat exchanger according to the invention when adapted as a solar
collector as described above.
The invention is illustrated by various specific embodiments, described
hereinafter by way of example with reference to the accompanying drawings
in which
FIG. 1 is a cut-away isometric view of an end portion of a heat exchanger
in which the board extends radially from the header,
FIG. 2 is a transverse section through one end of a further heat exchanger
similar to that shown in FIG. 1,
FIG. 3 is a transverse section through a further heat exchanger having a
face-mounted header,
FIGS. 4 and 5 are cut-away views of two further heat exchangers, and
FIGS. 6 and 7 are mutually perpendicular sections through a still further
heat exchanger.
The embodiment illustrated in FIG. 1 comprises a thick-walled polypropylene
extruded tube 1 which forms the header, and an extruded board 2 extending
radially from the header. The board comprises two spaced-apart sheets 3, 4
connected by a plurality of parallel webs 5 subdividing the space between
the sheets into a plurality of parallel tubular passages 6 of rectangular
section. The boards are extruded from polypropylene with the webs and
sheets being extruded together as integral parts of a common extrudate.
Running longitudinally along the header is a groove 7 milled into its
external surface, the groove having a length and width substantially the
same as the width and thickness respectively of the board. Spaced along
the groove are a plurality of holes 8 interconnecting the groove and the
interior 9 of the header. The end of the board is located along the mouth
of the groove with the tubular passages opening into the groove. Along the
angle formed between the header and the board is a sealing bead 10, which
is fused to both parts, and not only holds them firmly together, but also
seals the joint against escape of fluid. The board continues beyond the
edge which has been sectioned to show the profile of the board, until it
reaches a second header which is parallel to and substantially the same as
that shown.
The heat exchanger is made by first milling the grooves 7 in the two
headers, and drilling the row of holes along the base of each groove,
through to the interior of the header. The headers and board are held in a
jig with the ends of the board lying along the mouths of the two grooves,
and polypropylene (e.g. a low ethylene composition) extruded as a bead
progressively along the joint, into contact with the board and header,
suitable melt temperatures being around 280.degree. C. The extruder tip
may be allowed to touch the header (but not the board) immediately prior
to the bead of molten polymer. The molten polymer melts the surface of
both the board and the header, and on cooling forms a firm weld to each.
In use, the headers are connected into an external circuit as required, and
a fluid circulated through it. The fluid enters one header through an
inlet at one end, passes through the holes into the groove, and thence
into the tubular passages, running along the groove to become distributed
to all the passages. After passing through the passages within the board,
the fluid enters the groove in the second header, and passes through the
holes into the interior of the second header, from which it is returned to
the external circuit via an exit which is diagonally opposite the inlet on
the other header.
A similar heat exchanger is shown in FIG. 2, and like numerals have been
used for like parts. The only differences are that the board has been
inserted into the groove, and a portion 12 has been cut out of the end of
each web to allow the fluid to flow along the groove and distribute itself
uniformly amongst the tubular passages.
The heat exchanger shown in FIG. 3 also comprises a tubular header 31 at
each end of a board 32 extruded with a profile substantially as shown in
FIG. 1. However, instead of the board extending radially from each header,
the latter are mounted on one face 33 of the board. The ends of all the
tubular passages through the board are closed by sealer 34 extruded into
them (an alternative would be to clamp the ends of the sheets together in
a heated press), and the board has a slot 35 cut out of the upper sheet
adjacent the sealed ends, to provide access to the tubular passages.
Mounted over the slots at either ends of the board are the headers 31
extruded with a profile specifically designed for the illustrated
application. Thus while there is again a basic circular-sectioned pipe,
there are also two integral longitudinal ridges 41 which are parallel and
spaced apart sufficiently to provide a groove 42 between them.
Interconnecting the groove and the interior 43 of the header, is a row of
closely-spaced holes 44, and the headers are secured in place using a bead
45 of sealer extruded into place as described above. However, unlike the
embodiments of FIGS. 1 and 2 in which a groove of the required length only
was machined into the surface of the header, the groove of FIG. 3 being
defined by the continuous integral ridges, extends continuously along the
tube, and so extends beyond the edges of the board. The grooves must
therefore be sealed where it so extends, e.g. with the sealer when
securing the headers to the board.
Other fixtures may be secured to the heat exchanger in similar manner. Thus
for example, mounting lugs may be extrusion welded to the boards or to the
headers, if formed of a suitable thermoplastics material.
The heat exchanger shown in FIG. 4 is generally similar to that of FIG. 1,
in comprising a tubular polypropylene header 47 welded to a polypropylene
extruded board 48, by an extruded polypropylene bead 49. The board again
comprises two spaced-apart sheets 50, the space between which is
subdivided by a plurality of webs 51 to provide rectangular-sectioned
tubular passages 52. Down the side of the header is a longitudinal groove
53 with holes 54 through to the interior 55 of the header, the material
between the holes providing bridging portions 56.
The board 48 is inserted into the groove 53 whereby it is positively
located during the welding process. The end of the board lies flush with
the base of the groove, but unlike that shown in FIG. 2, the webs are not
cut away to allow the fluid to flow freely along the groove. Instead, the
rectangular-sectioned holes 54 through the header are closely spaced to
give narrow bridging portions, and the tubular passages are fed directly
with little flow (if any) along the groove.
The heat exchanger of FIG. 5 is similar to that of FIG. 4, except that the
upper sheet 57 of the board is corrugated and provides a domed roof to
each of the tubular passages. The configuration of this board is
substantially as shown in FIG. 4 of British patent specification No. 1 042
732 referred to hereinabove. This configuration of board, however, is
particularly suitable for use in solar energy collectors as the corrugated
upper surface is effective for absorbing radiation as the angle of
incidence varies throughout the day. Furthermore, polypropylene is
degradable unless it contains at least a minimum effective amount of a
suitable stabiliser, and most known stabilisers tend to be very slowly
leached by warm water circulating through the heat exchanger. As this
leaching can occur from both surfaces of the webs but only from one
surface of the sheets, the provision of webs whose minimum thickness is at
least twice that of the material separating the tubular passages from the
surrounding environment, is also advantageous.
The use of thicker webs, however, may lead to poor flow distribution where
flow must be direct from the holes to the passages due to blocking of the
passages by the bridging portions 56. Accordingly a small step 58 is
provided in one side of the groove to hold the board off the base of the
groove during welding. Corresponding steps could equally be formed in both
sides of the groove, but that is not generally much advantage with the
corrugated configuration of board shown.
Alternatively, the bridging portion may be modified in a manner similar to
that of the embodiment shown in FIGS. 6 and 7. That embodiment comprises a
tubular header 61 having a longitudinal groove 62 in which is located the
end of a hollow board 63. The board which is held in place by a bead 64,
comprises two spaced-apart sheets 65 interconnected by integral webs 66.
The webs divide up the space within the hollow board, into parallel
tubular passages 67 of rectangular cross-section, which run from one
header to the other.
The groove has a flat base 68 against which sits the end of the board.
Through the header is a row of rectangular-sectioned holes 69, the
material between the holes providing bridging portions 70. The bridging
portions are chamfered at their outer surfaces to a knife edge 71 and the
holes stretch the full width of the groove into which the board is
inserted. Consequently the internal width of the tubular passages is less
than that of the holes by an amount equal to the thickness of the two
sheets making up the board, and also the clearance allowed either side of
the board. The total cross-sectional area of the holes is substantially
equal to the total cross-sectional area of the tubular passages so that
flow through the passages will not be restricted on passing into the
headers.
The bridging portions are important because they carry any stresses which
may develop between the parts of the header on either side of the board,
rather than the stresses being applied to either side of the relatively
thin-walled board. The thickness of the bridging portions and other
portions of the header wall are made sufficiently strong to withstand the
expected stresses, e.g. the head of the circulating liquid at various
temperatures. In the headers shown in FIGS. 1 and 2, the groove is milled
out of the constant wall thickness, thereby considerably weakening the
header along the line of the groove, and in consequence a thicker overall
section is required. In FIG. 3, however, the header is shaped so that the
groove lies external to the basic cylindrical shape, the thickness need be
no more than that required to withstand the expected load together with
the usual safety tolerances. Similarly, a header is shown in FIG. 6 whose
configuration avoids undue wastage of material. In this header the wall
thickness at the holes is sufficient to provide bridging portions capable
of carrying the stresses developed in use, with a balancing portion 72 of
similar thickness diametrically opposite. The two symmetrically-placed
portions 73 between them are able to be made thinner without reducing
their bursting strength below that at the holes.
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Description |
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