U.S. patent number 3,757,855 [Application Number 05/189,509] was granted by the patent office on 1973-09-11 for primary surface heat exchanger.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Leslie C. Kun, James B. Wulf.
United States Patent |
3,757,855 |
Kun , et al. |
September 11, 1973 |
PRIMARY SURFACE HEAT EXCHANGER
Abstract
An all purpose primary-surface heat exchanger, comprising an
array of parallel channels formed and bounded by thin heat
conductive walls, at least one wall of which has on at least a
portion of its surface substantially uniformly disposed
unidirectional truncated conical wall-supporting projections formed
from the wall. The projections are arranged so as to mate with and
to abut supportingly against corresponding wall-supporting
projections of a similar adjacent wall. The walls, so arranged, are
sealed at the wall edges in a manner to form and isolate alternate
enclosed channels from intervening open channels so that the
alternate channels may contain and conduct a first fluid, and the
intervening channels may contain and conduct a second fluid at a
different temperature, thereby effecting heat exchange between the
fluids.
Inventors: |
Kun; Leslie C. (Williamsville,
NY), Wulf; James B. (Williamsville, NY) |
Assignee: |
Union Carbide Corporation (New
York, NY)
|
Family
ID: |
22697629 |
Appl.
No.: |
05/189,509 |
Filed: |
October 15, 1971 |
Current U.S.
Class: |
165/166;
29/890.039; 165/148; 165/152; 165/175; 165/178; 165/DIG.376 |
Current CPC
Class: |
F28D
1/0391 (20130101); F28F 9/02 (20130101); F28F
3/044 (20130101); Y10S 165/376 (20130101); F28F
2001/027 (20130101); F28D 2021/0094 (20130101); Y10T
29/49366 (20150115) |
Current International
Class: |
F28D
1/03 (20060101); F28D 1/02 (20060101); F28f
003/02 () |
Field of
Search: |
;165/152,3,166,167
;29/157.3A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myhre; Charles J.
Assistant Examiner: Streule, Jr.; Theophil W.
Claims
What is claimed is:
1. A primary surface heat exchanger comprising at least one channel
element bound by two thermally conductive walls being spaced by
edge portions, said channel element having an entrance opening at
one end, an exit opening at the opposite end and on at least a
portion of each wall surface substantially uniformly disposed only
outwardly extending truncated conical wall-supporting projections
formed from the wall, said projections having load bearing segments
at their extremities which are shaped for mating with and abutting
against similar type load bearing segments on wall-supporting only
outwardly extending projections of a second channel element; said
wall-supporting outwardly extending truncated conical projections
having a dimensional size and a dimensional relationship
therebetween defined by a H/D ratio of between about 0.05 and about
0.2; a D/d ratio of between about 3 and about 10, a D dimension of
between about 0.2 and 2.5 inches and a wall thickness between about
0.003 and about 0.25 inch; a R/D ratio greater than about 0.075;
and a cone angle .theta. less than 35.degree.; wherein H equals the
maximum height measured perpendicularly from a surface which
contains the extremities of the wall-supporting outwardly extending
projections to the plane containing the undeformed surface of the
wall adjacent the projection; D equals the spacing between the
centers of the closest adjacent wall-supporting outwardly extending
projections on the surface of the wall; d equals the equivalent
diameter defined by the ratio 4a/p whereby a equals the area of the
load bearing segment of the wall-supporting outwardly extending
projection and p equals the perimeter of said load bearing segment;
.theta. equals the acute angle measured between the horizontal
undeformed surface of the wall adjacent the outwardly extending
projection and the substantially straight segment along the sloped
side of the projection; and R equals the radius of curvature of the
surface segments on both sides of the bounding line of intersection
formed by the projection and the undeformed surface of the wall
adjacent the projection.
2. The heat exchanger of claim 1 wherein said thermally conductive
heat exchange element is made from at least one material selected
from the group consisting of metals, metal alloys, metal clads,
plastics and plastic coated metals.
3. The heat exchanger of claim 2 for use in conjunction with
internal combustion engines wherein said H/D ratio is between about
0.05 and about 0.12; wherein said D/d ratio is between about 3 and
about 7; wherein said D dimension is between about 0.2 and about
0.6 inch; wherein said angle .theta. is less than about 35.degree.;
wherein said R/D ratio is greater than about 0.075; wherein said
wall thickness is between about 0.003 and about 0.02 inch; wherein
at least two of said elements are juxtaposed with the mating load
bearing segments on the wall-supporting projections of said
elements in touching relationship to form a first set of passages
defined by said channel elements and a second set of passages
defined by, and between, adjacent channel elements.
4. The heat exchanger of claim 3 wherein the heat exchange element
is made of aluminum having a wall thickness of about 0.008 inch;
said H/D ratio is about 0.08; said D/d ratio is about 4.8 and said
D dimension is about 0.4 inch.
5. The heat exchanger of claim 2 wherein secondary surface heat
dissipating fins are added to said channel element.
6. The heat exchanger of claim 3 wherein secondary surface heat
dissipating fins are added to said channel elements.
Description
FIELD OF THE INVENTION
This invention relates to a thin metal or plastic plate heat
exchange channel element having on a portion of its surface
substantially uniformly disposed unidirectional wall-supporting
truncated conical projections.
BACKGROUND OF THE INVENTION
The need for light weight, inexpensive heat exchange elements for
various heat transfer applications has been in demand by industry
for a long time. The automobile industry has constantly been
searching for a compact, light weight radiator for use in cooling
the internal combustion engine. Various types and styles of
radiators have been designed, such as the individually finned round
tubes, the hexagon-shaped air tubes with water passages between the
tubes, and the flat dimpled water passages with air flow
therebetween. The pre-1942 automobile engines were designed to
deliver between 50 and 125 horsepower and required radiators
operable close to atmospheric pressure. A simple solder joined
finned-copper constructed radiator was therefore sufficient to cool
the low horsepower engine of the automobile without much of a
threat of over-heating. Various copper radiators having cup-like or
frusto-conical surface projections have been designed during this
pre-1942 period but the finned copper radiator proved more
successful and suitable for automobile applications.
The automobile industry, however, in the post-1945 era embarked
upon the design of higher power rated engines while simultaneously
attempting to compact them as much as possible. This dual design
approach coupled to the employment of improved lubricants resulted
in an internal combustion engine capable of operating at high
permissible temperatures. To satisfy the heat transfer requirements
of such compact high power rated engines, and to avoid loss of
coolant, the tube and fin copper radiators were designed to operate
under pressure so as to increase the boiling temperature of the
coolant. However, within the last several years, additional power
operated equipment, such as air conditioners and the like, was
added to the automobile thereby further increasing the demands on
the internal combustion engine and consequently the duty of the
heat rejection system. This has necessitated the designing of
present day radiators to operate at pressures as high as 15 psig to
prevent coolant loss and overheating. The operating temperature of
the automobile engine is anticipated to rise further in the near
future thereby necessitating a heat transfer system operable with
existing coolants under still higher pressure conditions. The
conventional type finned-copper radiator will not perform
satisfactorily in an increased temperature environment due to the
low stress characteristics inherent in soft solder at high
temperatures, such solder being the securing medium between the
tubes and fins of the radiator. In addition, the steady increase in
the price of copper is causing copper to become an undesirable
material for radiator applications from an economical
standpoint.
An alternate solution to the conventional type finned-copper
radiators for heat transfer applications in the automobile, is to
replace the soft soldered copper fins with aluminum fins. Although
aluminum is less expensive than copper, the fusion bonding of
aluminum fins to the tubes of a conventional radiator by brazing
techniques is expensive. In addition, if a corrosive flux is used,
the deposits left by the salt bath of the brazing process must be
meticuously removed. Alternate brazing techniques and methods,
i.e., vacuum brazing, are still in the experimental stage and when
perfected their high cost will probably overwhelm the savings
otherwise gained in the use of aluminum rather than copper for
producing automobile radiators. Other proposals have been advanced,
such as the use of adhesive bonding between the fins and tubes of a
radiator. However, the low thermal conductivity of present day
adhesives renders this approach inefficient for radiator
applications.
In heat exchange applications requiring pressure-bearing walls as
the primary heat exchange surface, the present invention enables
such walls to be fabricated from thinner thermally conductive
material than is presently required of conventional type primary
heat exchangers. In order to utilize relative thin sheet materials,
the walls of conventional type primary heat exchangers have to be
stayed by means of numerous support members so as to reduce stress
in the walls. However, stayed walls are normally not practical
because of the following reasons:
a. high stress concentrations are still produced in the wall at the
point of attachment of the stays;
b. a substantial amount of material is required in the stays, and
in heat exchangers such stays contribute only indirectly if at all
to heat transfer; and
c. the numerous stays are tedious and expensive to install,
particularly in heat exhangers where the spacing between walls is
very small and often inaccessible.
The present invention overcomes the above drawbacks by providing an
all-purpose, primary-surface heat exchange channel element having
on at least a portion of its surface substantially uniform disposed
unidirectional truncated conical wall-supporting projections. The
heat exchange element is economical to fabricate and when employed
in stacked units, they are admirably suited as a heat exchanger for
use with internal combustion engines.
SUMMARY OF THE INVENTION
The primary-surface heat exchanger of this invention basically
comprises at least one channel element formed and bound by at least
one thin walled, thermally conductive metal or plastic material,
such channel element having an entrance opening, an exit opening
and substantially uniformly disposed on a portion of its wall
surface unidirectional wall-supporting truncated conical
projections formed from the wall in a dimensional relationship to
be discussed hereinafter. The wall-supporting truncated conical
projection have load bearing segments at their extremities which
are shaped and arranged so as to mate with and abut against
corresponding load bearing segments or wall-supporting truncated
conical projections on a second surface. At least two such
channels, when aligned in juxtaposed relationship, will form a heat
exchanger having a first set of passages defined by and bound
within the conductive walls of each channel, and a second set of
passages defined by, and disposed between, the juxtaposed channels
so that a first medium can be fed through one set of passages while
a second cooler medium can be fed through the other set of passages
thereby effecting a heat exchange between the mediums without
having the mediums intermix.
The term primary-surface heat exchanger refers to heat exchangers
wherein substantially all the material which conducts heat between
two media comprises the walls separating the two media. In
addition, in heat exchange applications wherein a pressure
difference exists between the two media of the system,
substantially all of the heat exchanger material is stressed
pneumatically. Stated another way, primary-surface heat exchanger
refers to a heat exchanger consisting primarily of plates or sheets
and having no separate or additional internal members, such as
fins, so that the exchanger is constructed of plates or sheets each
side of which is in contact with a different fluid, and heat
transfer is substantially and directly between the plates and the
fluid.
A truncated conical projection from a wall of a thin sheet material
is a protrusion in the wall of the material having a cone angle
.theta. and a radius of curvature R as shown in FIG. 1A. Cone angle
.theta. equals the acute interior angle measured between the
horizontal undeformed surface of the wall adjacent the protrusion
and the substantially straight segment along the sloped side of the
protrusion, and the radius of curvature R equals the radius of the
surface segments on both sides of the bounding line of intersection
formed by the protrusion and the undeformed surface of the wall
adjacent the protrusion. The surface segment at the extremity of
the truncated conical projection is a load bearing segment and is
shaped so as to mate with, and abut against, similar type surface
segments at the extremities of truncated conical projections on a
second wall. For stacking or abutting two or more channel elements
together, the wall-supporting unidirectional truncated conical
projections are disposed in a pre-aligned space relationship on the
surface of each element so that when the walls are juxtaposed, the
load bearing segments at the outer extremities of the
wall-supporting projections hereafter referred to as buttons, will
be in touching relationship. With reference to any adjacent pair of
pressure withholding walls, wherein the buttons of both walls
project inward into the space between the walls, the forces due to
the pressure either external or internal of the pair will be
substantially balanced, i.e., the secured contact between the
buttons will sustain by tension or compression the entire force due
to the pressure and no other structural member will be needed to
absorb the load. Thus the pressure force will be counter-balanced
by a restraining force developed within the pair of walls without
the necessity of any external structure.
With reference to any adjacent pair of pressure witholding walls,
wherein the buttons of both walls project outward from the space
between the wall, the pressure either external or internal of the
pair will not be balanced and a member external of the pair will be
needed on each exposed face of the pair to absorb the load by
supportive contact with the buttons in either tension or
compression. Thus a restraining force will not be developed within
the pair of walls to counterbalance the pressure force. In a
series, stack or array of walls, the member external of the pair
may be yet another wall with buttons matching those of the
juxtaposed surface of the pair.
With reference to any series, stack or array of truncated conical
projected pressure withholding walls wherein the buttons of the two
outermost walls of the stack project inward toward the stack, the
forces due to pressure will be substantially balanced throughout
the stack and no other structural member will be needed to absorb
the pressure load and to restrain the walls from deflecting outward
from the stack.
With reference to any series, stack or array of truncated conical
projected pressure withholding walls wherein the buttons of the two
outermost walls of the stack project outward from the stack, the
forces due to pressure will not be balanced within the stack and a
structural member will be needed juxtaposed in supportive contact
with the buttons of each outermost wall to absorb the pressure load
and restrain the stack.
Since the truncated conical projected wall of the channel element
of this invention is designed as a primary-surface heat exchange
channel element, its wall material need not be highly conductive
and thus can be selected from at least one of the groups consisting
of metals, metal alloys, metal clads, plastics (such as Mylar),
plastic-coated metals and the like. The criteria of the material
selected for the heat exchange channel element is that it be only
sufficiently thermally conductive so that as a hot medium is passed
through the channel, the heat of the medium will be conducted
through the wall of the channel to a cooler medium external of, and
adjacent to, the channel which can absorb the heat thereby
successfully effecting a heat transfer between the mediumswithout
intermixing of said mediums. At least one material selected from
the group consisting of copper, steel, brass, titanium and Mylar is
suitable for this application.
"Substantially uniformly disposed wall-supporting projections" is
intended to be broad enough to include a pattern of wall-supporting
projections having a progressive variation in spacing along at
least one axis of the heat exchange element. In addition, as
hereinafter stated, additional wall-supporting projections can be
provided along the curved portion of the channel which may have a
spacing relationship different from that of wall-supporting
projections occupying the central portion of the heat exchanger
element.
The dimensions of, and the dimensional relationship between, the
wall-supporting truncated conical projections on the wall surface
of the channel are somewhat restrictive depending on the end use
environment of the heat exchange channel. The pattern of
wall-supporting truncated conical projections can be arranged in a
square, diamond, triangle or any other design configuration
depending somewhat on the actual shape of the channel and the
intended differential pressure to which the wall of the channel
will be subjected in its intended environment. To minimize the
resistance to flow and maximize the heat transfer effectiveness of
any defined flow area of a heat exchange channel of this invention,
the wall-supporting truncated conical projections should be
designed and arranged in only such size number and pattern as will
provide the restraint necessary to withstand the maximum
differential pressure for which the channel wall is designed in its
intended environment. This will result in maximum primary-surface
area being available for heat transfer. Once the desired size and
pattern of the wall-supporting truncated conical projections are
determined for efficient heat transfer in an intended end use
pressurized environment, the projections can be imparted to the
surface of a thin-walled thermally conductive sheet of material by
any conventional technique such as pressing, stamping, rolling or
the like.
A thermally conductive truncated conical projected sheet, so
prepared, can be longitudinally folded upon itself with the
projections facing either inwardly or outwardly, and the folded
sheet segments spaced sufficiently apart so as to define a passage
therebetween. When the buttons project inwardly of the passage,
they should match and contact with buttons extending inwardly
across the passage from the opposite wall. The width of the passage
so formed is thereby defined by the projected heights of the
wall-supporting buttons from the undeformed surface of the walls.
Since stress concentration may occur at the bending area of the
sheet in its intended operational environment, additional
wall-supporting truncated conical projections may be disposed
within the vicinity of such area so as to equalize the stresses
throughout the channel structure. The longitudinally mating edges
of the sheet can then be suitably sealed by conventional
techniques, i.e., soldering, brazing, welding or with an adhesive
filled lock-seam joint, to make it leak-tight. This unidirectional
wall-supporting truncated conical projected channel is then ready
for use as a heat exchange element. when a channel is formed with
buttons projecting inwardly and when intended for internal
pressurization, then the button contacting surfaces within the
passages should be bonded together by conventional means as
soldering, brazing or with an adhesive. An array of channels so
formed with the wall-supporting projected buttons in touching
relationship, can then be appropriately assembled to produce a
compact, efficient, primary-surface heat exchanger. When the
wallsupporting truncated conical projections are disposed
outwardly, then the channels can be superimposed in button touching
relationship wherein the heights of the projected buttons from the
undeformed surface of the walls will define the size of the passage
between adjacent channels. When the wall-supporting truncated
conical projections are disposed inwardly, then the channels will
have to be spaced apart by some external structure so as to define
a passage between adjacent channels. A pressurized medium, such as
hot water, could then be passed through the channels while a
coolant medium, such as cool air, could be passed between, and
contact the outer surface of, the channels thereby effecting a
transfer of heat between the mediums. The wall-supporting truncated
conical projected sheet could also be fabricated into a circular or
spiral channel, or any multiple sided channel by appropriate
bending and/or folding techniques. The heat exchange channelized
elements so formed can also be shaped into any curvilinear
configuration and then superimposed one on the other leaving
defined passages therebetween to form a simple or complex geometry
heat exchanger having multiple confined channelized passages and
multiple separate passages defined by, and between, the outer
surfaces of adjacent heat exchange channelized elements. By passing
a medium through the channelized passages defined while directing a
second coolant medium through the passages defined by, and between,
the outer surfaces of adjacent elements, an effective, large,
primary-surface heat exchanger is obtained. In a cross-flow heat
exchange operational mode, the heat exchanger of this invention
will provide a low frontal area and a low external fluid pressure
drop. Frontal area is the area of the projection of the entire
array of heat exchange channels onto a plane normal to the
direction of fluid flow through the channelized passages. Low
external fluid pressure drop is the static pressure drop across the
length of the flow path of the external coolant medium.
The mediums can be fed through their respective passages in a
mutually parallel relationship, a perpendicular relationship or at
any angle relationship therebetween.
DESCRIPTION OF THE DRAWINGS
FIG. 1 -- A graph of applied pressure vs. surface deflection for
30.degree. and 45.degree. truncated conical projections in an
aluminum wall.
FIG. 1A -- Truncated cone surface.
FIG. 2 -- Isometric view of an automobile radiator employing the
heat exchange elements of this invention.
FIG. 2A -- View taken of the longitudinal edges of a heat exchange
element of FIG. 2.
FIG. 2B -- Side view of elements 1 of FIG. 2.
FIG. 2C -- Alternate embodiment of elements 1 of FIG. 2.
FIG. 2D -- Alternate embodiment of the longitudinal edges of
elements 1 of FIG. 2.
FIG. 3 -- Isometric view of an array of truncated conical projected
channels with outwardly projected buttons.
FIG. 3A -- Cross-sectional view of channels in FIG. 3 taken along
line 3A--3A.
FIG. 3B -- Sectional side view of channels in FIG. 3 taken along
line 3B--3B.
FIG. 4 -- Isometric view of an array of truncated conical projected
channels with inwardly projected buttons.
FIG. 4A -- Sectional side view of channels in FIG. 4 taken along
line 4A--4A.
For general heat exchange application, a truncated conical
projected surface, as illustrated in FIG. 1A, having a repeatable
wall-supporting projection spacing D of between about 0.2 and about
2.5 inch; a D/d ratio between about 3 and about 10, a H/D ratio
between about 0.05 and about 0.2, a cone angle .theta. less than
about 35.degree., a R/D ratio of greater than about 0.075; and a
sheet or wall thickness between about 0.003 and about 0.25 inch
will be quite suitable. As used above and as shown in FIGS. 1A, H
equals the maximum height measured perpendicularly from a surface
which contains the extremities of the wall-supporting projections
to the plane containing the undeformed surface of the wall adjacent
the projection; D equals the spacing between the center of the
closest adjacent wall-supporting projections on the surface of the
wall, d is equal to the equivalent diameter of the projection
defined by the ratio 4a/p whereby a equals the area of the load
bearing segment of the wall-supporting projection and p equals the
perimeter of said load bearing segment, i.e., for a circular load
bearing segment d is equal to the diameter of the circle as shown
in FIG. 1A; .theta. is the acute interior angle measured between
the horizontal undeformed surface of the wall adjacent the
projected protrusion and the substantially straight segment along
the sloped side of the protrusion; and R equals the radius of
curvature of the surface segments on both sides of the bounding
line of intersection formed by the protrusion and the undeformed
surface of the wall adjacent the protrusion. The limitation on the
D spacing is imposed because spacing less than 0.2 inch results in
very small clearance passages on the projected side of the wall
thereby being very susceptible to fouling, i.e., trapping of
foreign matter between adjacent wall, which if excessive, would
clog the passages for one of the fluid mediums. A high external
fluid pressure drop per unit length of fluid flow path would also
result. Spacing D above 2.5 inches would result in a small heat
exchange area per cubic foot of heat exchange volume thus resulting
in excessive manufacturing cost and decreased efficiency. Also the
ability for the material to withstand a differential pressure
across its wall thickness would be decreased for a constant wall
thickness because of the large spacing between wall supports.
For D/d ratio of less than 3, the allowable differential pressure
across the wall of a channelized heat exchange element would go up,
but a very large percentage of the surface area would be lost for
heat exchange purposes. On the other hand, a D/d ratio greater than
10 would require tight manufacturing tolerance to insure the mating
of bearing segments (buttons) on abutting walls and would also
localize and concentrate the load at the contact point of the
bearing segments and produce stresses sufficient to cause rupture
or excessive deformation of the walls.
A heat exchanger composed of truncated conical projected channels
with a H/D ratio less than 0.05 would be susceptible to fouling and
have a high external fluid pressure drop per unit length of fluid
flow path. For a H/D ratio of greater than 0.2, a small heat
exchange area per cubic foot of heat exchange volume would result
thereby resulting in excessive manufacturing cost and decreased
efficiency. A R/D ratio lower than 0.075 would result in excessive
stresses along the bounding line of intersection formed by the
conical projection and the undeformed surface of the wall adjacent
the projection. In effect, the bounding line of intersection
becomes a line of high stress concentration.
A heat exchange wall having truncated conical projections with a
cone angle of greater than 35.degree. would result in excessive
deflections of the unsupported areas disposed between the
projections when pressure is applied to the wall. If such
deflections are imposed repeatedly in service, then the material
may be fatigued and crack after a relatively short sevice life.
Additionally, deflections reduce the available space between heat
exchange walls in the lower pressure passages, and result either in
higher fluid pressure drop or in reduced rate of fluid flow.
A material thickness t of less than about 0.003 inch would be
unsuitable due to local imperfections in the metal, produced during
rolling or as a result of pitting (corrosion) or erosion. A
material thickness to above 0.25 inch is not suited to this
invention when employed within the imposed limits of D, H and d,
because full or near-full utilization of the material strength
implies extremely high pressure differentials. Embodiments wherein
pressure forces are not balanced within the channels require
massive external structures to absorb the loads, while
force-balanced embodiments wherein wall-supporting projections are
bonded together and loaded in tension would be characterized by
severe stress concentration in such bonded areas.
To meet the specific heat exchange requirements for radiators of
internal combustion engines, the allowable ranges expressed above
have to be narrowed to the following: a repeatable distance D
between about 0.2 and about 0.6 inch, a D/d ratio of between about
3 and about 7; a H/d ratio of between about 0.05 and about 0.12; an
angle .theta. of less than about 35.degree., preferably less than
about 30.degree., R/D ratio of greater than about 0.075; and a
sheet or wall thickness between about 0.003 and about 0.02 inch.
The preferred dimensions of a truncated concial projected surface
for automobile radiator applications are a repeatable D of about
0.4 inch, a height H of about 0.035 inch; a button dimension width
d of about 0.09; a D/d ratio of about 4.8; a H/D ratio of about
0.08; an angle .theta. less than about 30.degree.; a R/D ratio of
greater than about 0.075; and a sheet or wall thickness of about
0.008 inch.
As an illustration of this invention, the following example will be
directed to an automobile radiator employing the primary-surface
heat exchange elements described above.
The deflection of truncated conical projected surfaces suitable for
automobile radiator applications was investigated by plotting
curves of applied pressure (lb./sq. inch) versus surface deflection
(inches) for 0.008 inch thick aluminum sheets having truncated
conical projections with cone angles of 30.degree. and
45.degree..
The sheets were stamped with wall-supporting truncated conical
projections arranged in a square pattern. The D spacing between the
projected supports was 0.4 inch and the height H was 0.035 inch as
shown in FIG. 1A. Pressure was applied to the surface of the
aluminum sheet of the indented side of the sheet such as to place
the cone material under compression, and the deflection at the
center fo the diagonals of the square pattern was measured. This
data for cone angle surfaces of 30.degree. and 45.degree. is shown
plotted as curves on the graph of FIG. 1.
Deflections of the material tending to distort the wall are
objectionable and should be minimized even though such deflections
may be safely below the buckling point of the material. Moreover,
the material is usually stressed in bending and shear as it
deflects, and when deflections are excessive the material may
experience stresses approaching the yield point in localized areas.
If such deflections are imposed repeatedly in service, the material
may be fatigued and crack after a relatively short service life.
Additionally, deflections reduce the available space between the
heat exchange walls in the lower pressure passages, and result
either in higher fluid pressure drop or in reduced rate of fluid
flow. With reference to FIG. 1, it is seen that the 30.degree. cone
surface used in the tests exhibited a relatively small deflection
at the center of the diagonals of the square pattern of the
truncated conical projections for pressure differentials as high as
35 psi. In contrast, the deflection of the 45.degree. cone surface
was approximately double the deflection of the 30.degree. cone
surface pressure differentials up to about this 35 psi. level. For
environments wherein pressure differentials across a heat exchange
surface will be as high as 35 psi, a surface with 35.degree. cone
angle truncated conical projections, sized and spaced within the
ranges specified above, will be suitable.
In the foregoing tests of the 30.degree. and 45.degree. cone angle
surfaces, the stress in the material was also measured directly be
means of strain gauges at 30 psi differential pressure. The stress
was measured on the diagonal at the point where the inclined
surface of the conical indentations met the flat undeformed segment
of the material, i.e., in the radius R arc. The following data was
taken;
Surface Stress, psi 30.degree. cone 18,400 45.degree. cone
42,000
The data shows the increase in stress resulting from use of the
45.degree. cone surface over the 30.degree. cone surface thus
demonstrating the high stress in the radius R arc of the 45.degree.
cone surface.
It is essential that all the surface area exclusive of the
wall-bearing supports be unrestricted so as to be free to deflect
and therefore be devoid of local mechanical loading. As can be seen
from FIGS. 3A and 4A, if the undeformed areas were bonded together
than the passages between such bonded surfaces would disappear, and
if stays were employed to provide suitable spacing therebetween, a
primary advantage of this type construction would vanish.
Once the dimensions of, and the dimensional relationship between,
the desired wall-supported truncated conical projections of a heat
exchange element are determined, a die or the like can be prepared
by conventional techniques. The die can be used in conventional
type apparatus to impart by pressing, stamping or rolling the
desired truncated conical projections onto a thin-walled thermally
conductive sheet, such as aluminum. For radiator applications, a
rectangular aluminum sheet can be stamped or the like with the
truncated conical projections. If the sheet is to be folded, then
the central folding area shall be left free of the projections. The
sheet, which may have any desired thickness, as specified above,
although about a 0.008 inch thick sheet is preferable, can then be
longitudinally folded at the center forming a flattened tube-like
configuration with the wall-supporting truncated conical
projections facing inward or outward. Instead of preparing one
large sheet and folding it, two sheets may be prepared and formed
appropriately at the longitudinal edges for bonding and then spaced
apart by suitable means to form a flattened tube-like
configuration. If desires, the longitudinal edges of the sheets
could be flared a specific amount so that when said longitudinal
edges of two sheets are juxtaposed in touching relationship, they
will provide the desired spacing within the channel. The edges of
the sheets can be "potted" as with epoxy resin to seal the sheets
leak-tightly together to form tube-like configurations, an array of
which can also be sealed leak-tightly into a header to form a
radiator assembly.
As shown in FIGS. 2, 2A and 2B, flattened tube-like heat exchange
elements 1 can be air-tightly sealed along their edges 2-3 using a
lock-seam joint filled with an adhesive 14, such as a suitable
epoxy type adhesive. The heat exchange elements 1 having an
horizontal surface 4 with spaced apart wall-supporting projections
5, can be superimposed with the surface extremities 17 (buttons) in
touching relationship to form a multiple layer heat exchanger. As
shown in FIG. 2B, the couching projected buttons 17 provide
passages 15 between adjacent heat exchange elements 1 defined by
the horizontal surfaces 4 of the adjacent elements 1, and in
addition, the contacting buttons 17 act as a restraint against
internal pressure in the heat exchange elements 1. The projected
button 5' could be offset or non-symmetrically disposed on opposite
sides of each element 1', as shown in FIG. 2C, thereby altering the
passage area of element 1'. The ends 6 of elements 1 are slightly
depressed, if necessary, to provide a clearance for the teeth 7 of
comb-shaped members 8. Members 8 retain elements 1 in proper
relationship and provide an outer plate segment 9 adaptable for
securing header 10 thereto. In addition, members 8 must also
produce a leak-tight seal to header 10 and to the channel elements
1 so that in the operational mode a fluid fed through the elements
1 via the header 10 will not leak into the space between adjacent
elements 1. As shown, header 10 can be secured to members 8 by
using an adhesive type joint arrangement 11. A suitable resin for
use in adhesive type joints for aluminum is Resin Type EA-914,
manufactured by Hysol Division of Dexter Corporation, California.
However, this resin must be used in conjunction with an Alodine
process for pretreating the surfaces to be bonded. An Alodine
pretreatment process would basically consist of the following
steps:
a. soaking and rubbing the surfaces to be bonded in acetone to
degrease;
b. immersing the surfaces in weak H.sub.3 PO.sub.4 acid for 10-15
seconds at room temperature;
c. washing the surfaces in water;
d. immersing the surfaces in Alodine 1200 at room temperature for 5
to 20 minutes (Alodine 1200 is manufactured by Amchem Products,
Inc., Freemont, California, and contains acidic chromates and
fluorides);
e. washing the surfaces with water; and
f. drying the surfaces.
The dried surfaces can thereafter be bonded with Hysol preferably
within a 4 hour period. Elements 1 can then be retained together by
employing a tension-type channel 12 which can be secured to either
members 8 and/or to a separate structure member 13. Channel 12 must
also be designed rigidly with sufficient cross-sectional moment of
inertial to absorb a bending load and to permit a small expansion
of elements 1. Members 8 and/or 13 can further be secured to a
frame of the automobile for better support. To better illustrate
the dual set of passages of an array of elements of this invention,
FIGS. 3, 3A and 3B show an array of elements 21 with outwardly
protruding wall supports 22. Passages 23 in elements 21 define one
set of confined passages independent of and separate from a second
set of passages 24 formed between adjacent elements 21. One fluid,
shown as solid line arrows, can be fed through passages 23 in
elements 21 while simultaneously a second cooler fluid, shown as
broken line arrows, can be fed through passages 24 to effectively
cause a transfer of heat from the hotter fluid to the cooler fluid
without having them intermixed. For this type embodiment, a rigid
frame or support similar to support 12 of FIG. 2 is required so as
to constrain the stack of elements 21 along the sides. FIGS. 4 and
4A illustrate a similar array of elements 30 except that the
wall-supporting projections 31 are inwardly projected. Passages 32
within elements 31 are independent of and separate from passage 33
formed between adjacent elements 30. One fluid, shown as solid line
arrows, can be fed through passages 32 while simultaneously a
second cooler fluid, shown as broken line arrows, can be fed
through passages 33 to effectively cause a transfer of heat from
the hotter fluid to the cooler fluid without having them
intermixed. For this type of element arrangement, spacers 34 are
required to space the elements 30 sufficiently apart so as to
define passages 33. It is to be understood that the spacer 34 could
be similar to the comb-like structure 8 as shown in FIG. 2, which
in turn could be coupled directly to a header similar to header 10
illustrated also in FIG. 2.
In operational mode of an automobile radiator, as shown in FIGS. 2,
2A and 2B, hot water from an internal combustion engine is fed
through elements 1 while cool air is circulated through the
passages 15 formed between adjacent elements 1. To increase the
efficiency of the heat exchange elements 1, one or both of the
edges 2 and 3 may be extended to provide a secondary surface heat
dissipating fin 16 as shown in FIG. 2D. The fin, which could also
be added to the elements by conventional securing means, can be
provided with dimplings to promote turbulence, or provided with
slots or assume any other desirable geometric configuration which
would enhance the performance of the heat exchange elements. Also
side bars could be used to separate the elements as shown in U. S.
Pat. No. 3,291,206 or edge ribs, as shown in U. S. Pat. No.
3,106,242.
Although the above illustrated was directed to automobile
radiators, the primary-surface heat exchange element of this
invention can be employed in any type heat exchanger wherein a heat
transfer between a heated medium and a coolant medium is to be
accomplished without an intermixing of the media occurring. The
design flexibility of the primary-surface heat exchange elements of
this invention makes them admirably suited for complex type heat
exchanger applications including preheaters for gas turbines and
low grade heat rejectors for atomic power plants.
As used herein, Mylar is a tradename of E. I. DuPont Company, and
Alodine is a tradename of Amchem Products, Inc.
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