U.S. patent number 3,720,988 [Application Number 05/182,038] was granted by the patent office on 1973-03-20 for method of making a heat pipe.
This patent grant is currently assigned to McDonnell Douglas Corporation. Invention is credited to Elmer Dale Waters.
United States Patent |
3,720,988 |
Waters |
March 20, 1973 |
METHOD OF MAKING A HEAT PIPE
Abstract
A heat pipe wick structure including a homogenous central wick
is fabricated by forming a plurality of laterally disposed
longitudinal pleats in a sheet of wire screen, and compressing the
formed pleats laterally together and inserting the same
longitudinally into a tubular container of a heat pipe. The plates
can be formed in a laterally central portion of the sheet with a
flat screen portion on each side thereof, and the side portions are
turned back over opposite sides of the formed central wick to serve
as wall screens in the tubular container. Reservoir screens having
a generally U-shaped cross section normally engaged by the formed
pleats can be additionally installed on both side of the central
wick in the tubular container.
Inventors: |
Waters; Elmer Dale (Richland,
WA) |
Assignee: |
McDonnell Douglas Corporation
(Santa Monica, CA)
|
Family
ID: |
22666838 |
Appl.
No.: |
05/182,038 |
Filed: |
September 20, 1971 |
Current U.S.
Class: |
29/890.032;
165/104.26 |
Current CPC
Class: |
B21C
37/151 (20130101); F28D 15/046 (20130101); B21D
53/02 (20130101); Y10T 29/49353 (20150115) |
Current International
Class: |
F28D
15/04 (20060101); B21C 37/15 (20060101); B21D
53/02 (20060101); B21d 053/02 () |
Field of
Search: |
;29/157.3 ;165/105 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lanham; Charles W.
Assistant Examiner: Reiley, III; D. C.
Claims
I claim:
1. A method of making a wick structure for a heat pipe, which
comprises the steps of:
forming a plurality of laterally disposed longitudinal pleats in a
first sheet of wire screen of Predetermined length and width;
and
compressing said formed pleats laterally together and inserting the
same longitudinally into an open end of a tubular container, said
pleats being secured in position by elastic expansion thereof in
said tubular container and consequent engagement therewith.
2. The invention as defined in claim 1 wherein said plurality of
laterally disposed longitudinal pleats are formed in a laterally
central portion of said first sheet with a flat screen portion on
each side thereof, and further comprising the step of cutting said
first sheet longitudinally at a selected lateral width within said
central portion to provide a strip including a number of formed
pleats from said plurality of pleats, said number of formed pleats
of said strip being compressed together and inserted longitudinally
into the open end of said tubular container.
3. The invention as defined in claim 2 further comprising the steps
of forming at least one reservoir screen from a second sheet of
wire screen of predetermined length and width, and installing said
formed reservoir screen longitudinally in a fixed position in said
tubular container.
4. The invention as defined in claim 3 wherein said second sheet is
longitudinally formed into a reservoir screen having a generally
U-shaped cross section, and said formed reservoir screen is fixedly
installed in said tubular container with the U-shaped cross section
open ends of said reservoir screen engaged by the compressed pleats
of said strip in said tubular container.
5. The invention as defined in claim 1 wherein said plurality of
laterally disposed longitudinal pleats are formed in a laterally
central portion of said first sheet with a flat screen portion of
predetermined width on each side thereof, and further comprising
the step of turning said side portions back over opposite sides of
said central portion of formed pleats prior to compressing said
formed pleats laterally together and inserting the same with said
turned side portions longitudinally into the open end of said
tubular container, said turned side portions being positioned
therein as wall screens.
6. The invention as defined in claim 5 further comprising the steps
of forming at least one reservoir screen from a second sheet of
wire screen of predetermined length and width, and installing said
formed reservoir screen longitudinally in a fixed position in said
tubular container between said central portion of formed pleats and
one of said wall screens.
7. The invention as defined in claim 6 wherein said second sheet is
longitudinally formed into a reservoir screen having a generally
U-shaped cross section, and said formed reservoir screen is fixedly
installed in said tubular container with the U-shaped cross section
open ends of said reservoir screen engaged by the compressed pleats
of said strip in said tubular container.
Description
BACKGROUND OF THE INVENTION
My present invention pertains generally to the field of heat pipes
and, more particularly, to a novel heat pipe wick and method of
making the same.
A great deal of trouble has been encountered in making and
installing a porous central wick in a heat pipe. The earlier porous
central wicks were fabricated form foamed metals in a generally
rectangular cross sectional configuration and longitudinally
installed in the tubular container of a heat pipe. For relatively
long heat pipes, however, the producers of porous (foamed) metals
have not been able to make long and uniform wick structures which
can be feasibly installed in a heat pipe. The foamed metals are
quite fragile and, moreover, their effective porosity or
permeability to liquid flow is frequently not as good as desired or
required. Of course, it is impossible to install a foamed metal
wick in an angularly formed or bent heat pipe, or to form or bend
the tubular container having a foamed metal wick installed therein
without seriously damaging the wick. A better and more durable
porous central wick was obviously desirable and clearly needed for
heat pipes.
SUMMARY OF THE INVENTION
Briefly, and in general terms, my invention is preferably
accomplished by providing a heat pipe wick structure including a
homogeneous central wick which is fabricated by forming a plurality
of laterally disposed longitudinal pleats in a sheet of wire screen
of predetermined length and width, and compressing the formed
pleats laterally together and inserting the same longitudinally
into an open end of the tubular container of a heat pipe. The
central wick is secured diametrically in position by the elastic
expansion of the formed pleats in the tubular container and
consequent engagement therewith. The pleats are arranged in a stack
providing a plurality of longitudinal passageways with small
diameter pores or openings. Compression of the stack reduces the
pore diameter of the passageways.
The plurality of pleats can be formed in a laterally central
portion of a sheet of wire screen with a flat screen portion on
each side of the central portion. The side portions can be turned
back over opposite sides of the formed central wick and, after
installation of the central wick and turned side portions in the
tubular container of a heat pipe, the turned side portions are
positioned therein as wall screens for conveying and spreading
liquid to the tubular container wall and any circumferential or
longitudinal grooves thereof.
Reservoir screens to take up excess liquid and prevent possible
blockage of a cooler (condenser) portion of the heat pipe can also
be installed therein. The reservoir screens are preferably formed
from respective sheets of wire screen of predetermined length and
width, and are longitudinally installed on both sides of the formed
central wick. Each reservoir screen preferably has a generally
U-shaped cross section wherein the open ends are engaged by the
compressed pleats of the central wick and are thus held in a fixed
position in the heat pipe.
BRIEF DESCRIPTION OF THE DRAWING
My invention will be more fully understood, and other features and
advantages thereof will become apparent, from the following
description of an exemplary embodiment and method of making the
invention. The description is to be taken in conjunction with the
accompanying drawing, in which:
FIG. 1 is an elevational end view of a heat pipe having a sintered,
homogeneous central wick;
FIG. 2 is an elevational end view of a wick being schematically
formed according to this invention for installation in the tubular
container of a heat pipe;
FIG. 3 is an elevational end view of a heat pipe including a wick
structure fabricated in accordance with my invention; and
FIG. 4 is a fragmentary perspective view, shown partially in
schematic and diagrammatic form, of illustrative apparatus for
fabricating the wick structure and installing it in the tubular
container of a heat pipe.
DESCRIPTION OF THE PRESENT EMBODIMENT AND METHOD
In the following description of an exemplary embodiment and method
of making my invention, some specific dimensions and types of
materials are disclosed. It is to be understood, of course, that
such dimensions and types of materials are given as examples only
and are not intended to limit the scope of this invention in any
manner.
FIG. 1 is an elevational end view of an open (uncapped) heat pipe
10 including an elongated tubular container 12 and a central
(diametrical) homogeneous wick 14. The container 12 has a generally
circular cross section, and the wick 14 has a generally rectangular
cross section with slightly rounded ends. The container 12 is made
of a suitable metallic tube having a relatively thin wall
thickness. The wick 14 is fabricated by stacking many layers of
fine wire cloth in a stack and then vacuum sintering the stack into
an integral assembly which is press-fitted into the container 12. A
dense homogeneous wick of fine pore size capable of pumping against
high gravity heads is thus produced. The assembly of wire cloth
layers and sintering processes are, however, very time-consuming
and another method of making a porous, central wick structure was
developed.
FIG. 2 is an elevational end view of a wick 16 being schematically
formed according to this invention. The basic construction and
features of the wick 16 are illustrated in this figure. The wick 16
is constructed in a long continuous length of fine woven wire
screen 18 folded lengthwise back and forth along its width into a
vertical stack or core 20 of horizontal pleats or convolutions 22.
The lower and upper end portions S and y of the screen 18 are
formed into semicircular cylindrical wall screens as indicated by
the arrows 24 and 26, and the entire structure is inserted into an
elongated tubular container preferably of a circular cross
sectional chamber. The stack 20 is vertically compressed together
as indicated by arrows 28 and 30 as it is inserted and installed in
the tubular container. It is noted that the end portions S and y
can be omitted from the wick 16, particularly when a fine screw
thread (not shown) has been tapped in the internal wall surface of
the tubular container to spread the working fluid (liquid) therein.
If only a number of formed pleats is desired, these can be cut out
of the stack 20.
It can be easily seen that the wick 16 is readily self-supporting
within a heat pipe extrusion (tubular container) and creates its
own spring force, both in the stack 20 and in the end portions S
and y, to ensure good diametrical and circumferential wall
contacts, respectively. The folded screen wick structure clearly
has good stability and is not fragile. It can be tailored to
achieve whatever pore size desired. When the screen 18 was folded a
number of times into the stack 20, small individual passageways 32
are created which act as longitudinal pores or flow passages to
give high axial conductance of the working fluid (liquid). If a
smaller pore size is needed for pumping or refilling against a
gravity head, then more folds of the screen 18 are provided and the
stack 20 pressed more firmly together into the tubular container.
Nevertheless, the wick 16 (central stack 20) has an effective
permeability to liquid flow of the order of 100 times that for an
equivalent multilayered sintered wick 14 (FIG. 1). The wick 16 can
also be fabricated very inexpensively as compared to the sintered
wick 14. While the stack 20 preferably has a generally rectangular
cross section, it can be of a different cross sectional
configuration such as an oval one or an non-uniform one wherein the
pleats 22 are varied in their horizontal widths vertically along
the stack as may be desired or required.
FIG. 3 is an elevational end view of a heat pipe 34 including a
tubular container 36 having wick 16 installed therein together with
two excess liquid reservoir screens 38 and 40. The wick 16 and
reservoir screens 38 and 40 preferably extend substantially over
the full internal length of the tubular container 36 which is
hermetically closed (sealed) after it has been properly charged
with a predetermined amount of a suitable working fluid. The
reservoir screens 38 and 40 are provided to take up excess liquid
rather than have it accumulate at the cooler (condenser) end
portion of the heat pipe 34 and block up such portion thereof.
Thus, the reservoir screens 38 and 40 provide static storage spaces
to accommodate any volume increases of the working fluid. These
screens 38 and 40 can also help maintain the semicircular end
portions S and y (wall screens) in firm contact with the internal
wall surface of the tubular container 36 so that the wall screens
can better bring liquid to the container wall surface and/or any
circumferential screw thread (helical) grooves or longitudinal
grooves therein. Of course, the reservoir screens 38 and 40 can be
radially shorter and need not contact the wall screens at all.
The wick 16 and the reservoir screens 38 and 40 are quite flexible
and this allows the heat pipe 34 to be angularly bent or otherwise
formed essentially without damage in any respect. Of course, the
central portion (stack 20) of wick 16 retains its high porosity
without any damage since it is also flexible but not fragile.
Illustratively, for a 1/2 inch pipe with a 0.42 inch inner
diameter, the stack 20 of wick 16 can have a lateral thickness
(pleat or convolution width) of 0.1 to 0.12 inch and a pleat
spacing gap (curved end portions or passages) of approximately
0.004 inch, for example. The wick 16 and the reservoir screens 38
and 40 can be fabricated of either a 100 mesh .times. 0.003 inch
diameter wire screen or a 200 mesh .times. 0.002 inch diameter wire
screen. The working fluid can be, for example, anhydrous ammonia.
Of course, various other working fluids including water can be used
as may be desired or required.
FIG. 4 is a fragmentary perspective view, shown partially in
simplified and diagrammatic form, of exemplary apparatus for
fabricating the wick 16 and screens 38 and 40, and assembling them
together for direct installation in the tubular container 36 (FIG.
3). It is noted that the elevational end view of wick 16 and
screens 38 and 40 as shown in FIG. 3 is of one end of the heat pipe
34 whereas the generally similar end view of such elements as shown
in FIG. 4 is of the other end (rotated 90.degree.) of the heat
pipe. The wick 16 is made by passing fine woven wire screen
material 42 through, for example, four successive sets of forming
rolls 44, 46, 48 and 50. The first three sets of rolls 44, 46 and
48 are preferably followed by respective sets of combs 52, 54 and
56. The sets of rolls 44, 46 and 48, and their respective sets of
combs 52, 54 and 56 have pleat forming teeth profile angles of
120.degree., 90.degree. and 60.degree., for example. The last set
of rolls 50 has a pleat forming teeth profile angle of, for
example, 30.degree.. Thus, as the screen material 42 is passed from
one set of rolls (and combs) to the next, the pleated shape is
progressively formed until it becomes ready for insertion into the
tubular container 36. The last set of rolls 50 need not be followed
by a set of combs because of the fineness of the pleats formed by
that time.
Simultaneously with the formation of the pleated material 42,
reservoir screen materials 58 and 60 are being formed into U-shaped
reservoir screens 38 and 40 by respective sets of rolls 62 and 64.
The unpleated end portions or side flanges S and y of the screen
material 42 can be turned over by a wick turning guide 66 to form
such side flanges to the inner diameter of tubular container 36 for
wall screens. The guide 66 includes lower and upper sections 68 and
70 which are rigidly secured to fixed structure or are fixedly
mounted within a suitable housing (not shown) attached to fixed
structure. The guide 66 is preferably tapered conically to flare
radially outwards a small amount from front to back in order to
facilitate insertion and turning of the pleated screen material 42.
The reservoir screens 38 and 40 are, of course, inserted into the
guide 66 at the same time as the screen material 42 which is formed
into a wick 16 including central stack 20 and wall screens S and
y.
It is noted that in the end view configuration shown in FIG. 4, the
upper reservoir screen 38 is laterally spread very slightly wider
at its open end (cross sectionally viewed) than is the lower
reservoir screen 40 and both screens are, in this instance,
illustratively shown radially shorter than as indicated in the
other end view of FIG. 3. These are very minor and inconsequential
differences since the main purpose of the reservoir screens 38 and
40 is to provide static storage spaces to take up excess liquid
that would otherwise accumulate in the vapor spaces at the
condenser end of the (closed) tubular container 36. The pleats of
the central stack 20 merely engage the open ends of the reservoir
screens 38 and 40 to secure them structurally.
For greater structural stability, the reservoir screens 38 and 40
are preferably secured at a middle section of the central stack 20
and are long enough radially so that their outer ends contact and
engage the wall screens S and y as illustrated in FIG. 3. This is,
of course, not absolutely necessary and the screens 38 and 40 can
be radially shorter as shown in FIG. 4. Also, the radial open ends
of the screens 38 and 40 can be secured to sections other than the
middle section of central stack 20, either both to the same section
or separately to respectively different sections of the central
stack. In fact, if the reservoir screens 38 and 40 were
structurally secured by other means than by the (pleats of) central
stack 20, the open ends of the screens 38 and 40 can be closed and
they can be positioned in any location in the spaces between the
central stack and the wall screens S and y.
Referring further to FIG. 3, the radially inner open ends of the
reservoir screens 38 and 40 are secured between pleats at each side
of a middle section of the central stack 20. The radially open ends
of the screens 38 and 40 can be spaced as close together as
possible so long as they can be structurally secured firmly by the
pleats of the stack 20. The radially open ends of the screens 38
and 40 should not, however, be secured excessively spread apart by
a middle section (of the stack 20) which is more than, for example,
about one-sixteenth of the stack's diametrical length.
Liquid normally enters the static storage space of a reservoir
screen through its longitudinally (passageway) open end. In FIG. 3,
this is perpendicular to the plane of the paper into the passageway
area generally enclosed by a reservoir screen. While the
longitudinally open end of a reservoir screen normally abuts
against the plane of the closing end cap, there are abutting gaps
which are large compared to the screen pores to provide preferred
(lower resistance) passage of liquid therethrough and entry into
the reservoir screen through its longitudinally open end. Of
course, where the cooler (condenser) portion of the heat pipe 34 is
located in a longitudinally middle part of the tubular container
36, excess liquid from the liquid in the stack 20 enters the
reservoir screens 38 and 40 through the radially open ends thereof.
In this instance, the reservoir screens 38 and 40 are preferably
secured to the middle section of stack 20, where the liquid
normally tends to concentrate.
It can be seen from FIG. 3 that the reservoir screens 38 and 40,
together with the stack 20, divide the tubular container 36 cross
sectionally into six passageways with three on each side of the
diametrical stack. The passageways of the screens 38 and 40 must
have a greater capillary pumping action than their adjacent
passageways (areas exterior thereto) in order to fill with liquid
first. Capillary pumping pressure of a passageway is broadly
dependent upon its effective capillary pumping radius or, very
generally, proportional to the sum of the reciprocals of the cross
sectional width and length of the passageway. Thus, the radially
open ends of the screens 38 and 40 cannot be spread too far apart
at the stack 20 since the screens' effective capillary pumping
radii are then excessively increased, such that their pumping
pressures would be so reduced that they would not be filled first
with surplus liquid, and blockage of the condenser end may
occur.
The reservoir screens 38 and 40 preferably extend over the full
internal length of the heat pipe 34. The screens 38 and 40 can,
however, extend over the entire pipe 34 length except for the
evaporator region thereof, or the screens can cover only the
condenser region of the heat pipe. The reservoir screens 38 and 40
are preferably provided in the heat pipe 34 where it is to be used
in a zero gravity environment, and are preferably omitted where it
is to be used in a gravity field which normally serves to return
liquid to the evaporator region. The wall screens S and y and/or
suitable liquid spreading grooves in the internal wall surface of
the tubular container 36 are desirably used in both zero and normal
gravity environments to provide proper spreading and resupply of
liquid to the walls in the heat pipe 34 under dynamic operating
conditions.
In summary, a simple and efficient method has been disclosed for
making a wick structure in long lengths and assembling it in a heat
pipe without requiring expensive tooling. The wick structure can be
fabricated without the need for special furnaces employing
relatively difficult sintering processes. A wick structure is
produced which can be tailored to meet the desired permeability
requirements of any particular application. The wick structure can
be made with a high porosity (low effective density) and yet not be
fragile. The wick structure is also highly flexible and allows a
heat pipe containing it to be angularly bent or otherwise formed to
a considerable degree without damage to either part.
While an exemplary embodiment and method of my invention have been
described above and shown in the accompanying drawing, it is to be
understood that the particular embodiment and method described are
merely illustrative of, and not restrictive on, the broad invention
and that various changes in design, structure and arrangement may
be made in the invention without departing from the spirit and
scope of the appended claims.
* * * * *