U.S. patent number 3,664,928 [Application Number 04/885,116] was granted by the patent office on 1972-05-23 for dimpled heat transfer walls for distillation apparatus.
This patent grant is currently assigned to Aerojet-General Corporation. Invention is credited to Ernest Roth Roberts.
United States Patent |
3,664,928 |
Roberts |
May 23, 1972 |
DIMPLED HEAT TRANSFER WALLS FOR DISTILLATION APPARATUS
Abstract
According to the present disclosure, a heat transfer wall of a
distillation apparatus is dimpled so that a plurality of dimples
protrude from the evaporating surface of the heat transfer wall.
The dimples are preferrably arranged so that tortuous flow paths
are formed between the dimples.
Inventors: |
Roberts; Ernest Roth
(Claremont, CA) |
Assignee: |
Aerojet-General Corporation (El
Monte, CA)
|
Family
ID: |
25386171 |
Appl.
No.: |
04/885,116 |
Filed: |
December 15, 1969 |
Current U.S.
Class: |
202/236; 165/111;
165/179; 159/13.1; 165/166; 203/10; 138/38; 159/28.1; 165/165;
203/89 |
Current CPC
Class: |
C02F
1/08 (20130101); F28F 3/04 (20130101); F28D
3/00 (20130101); Y02A 20/124 (20180101); Y02A
20/128 (20180101) |
Current International
Class: |
C02F
1/08 (20060101); F28F 3/04 (20060101); F28D
3/00 (20060101); F28F 3/00 (20060101); B01d
003/08 (); C02b 001/04 (); B01d 003/28 (); B01d
001/22 (); B01d 003/00 (); B01d 001/00 () |
Field of
Search: |
;202/185,190,234,236
;203/10,11,27,89 ;165/80,82,83,115,165,166 ;159/13,13X,15,24,28
;138/38 ;122/182R,182S,182T |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
205,057 |
|
Mar 1955 |
|
AU |
|
23,394 |
|
Jan 1883 |
|
DD |
|
25,926 |
|
Apr 1883 |
|
DD |
|
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Sofer; J.
Claims
What is claimed is:
1. In a distillation apparatus, the improvement comprising: a
bundle of vertically disposed rigid, metallic heat transfer tubes,
each providing an evaporating surface on one side of the metallic
tubular wall and a condensing surface on the other side, an array
of a plurality of dimples in said tubular wall and protruding thru
the outer surface of the wall to form liquid flow paths thereon
between said dimples, said dimples being structurally free of
contact with adjacent tubes and their dimples and suitable for the
formation of thin liquid films thereover and acting to improve the
heat transfer capability of the metallic wall with the wall
thickness t.sub. d of the dimpled portions of said heat transfer
wall being less than the wall thickness t.sub.u of the undimpled
portions of said heat transfer wall, a first means for supplying a
thin film of a liquid undergoing evaporation to the evaporating
surface of said wall, and a second means for supplying a
heat-transferring vapor to the condensing side of said wall, and
wherein the thickness t.sub.u of the undimpled portions of said
wall is between about 0.020 and 0.650 inch and the thickness
t.sub.d of the dimpled portion of said wall is between about 0.015
and 0.610 inch.
2. Apparatus according to claim 1 wherein said dimples are so
disposed and arranged in said array that said flow paths are
tortuous in a vertical direction.
3. Apparatus according to claim 1 wherein said dimples protrude
from said condensing surface of said wall.
4. Apparatus according to claim 1 wherein said dimples protrude
from said evaporating surface of said wall.
5. Apparatus according to claim 4 further including a second array
of a plurality of second dimples in said wall and protruding thru
the inner surface.
6. Apparatus according to claim 1 wherein the inner surface has an
array of a second plurality of dimples protruding inwardly
therefrom to form second flow paths on said opposite surface
between the second dimples.
7. Apparatus according to claim 6 wherein said second plurality of
dimples are so disposed and arranged that the flow paths
therebetween are tortuous in a vertical direction.
8. In a distillation apparatus, the improvement comprising: a
bundle or rigid, metallic heat transfer tubes, each providing an
evaporating surface on one side of the tubular wall and a
condensing surface on the other side, an array of a plurality of
dimples in said tubular wall and protruding thru the outer surface
to form liquid flow paths thereon between said dimples, said
dimples being structurally free from contact with adjacent tubes
and suitable for the formation of thin liquid films thereover and
acting to improve the heat transfer capability of the wall with the
wall thickness t.sub.d of the dimpled portions of said heat
transfer wall being less than the wall thickness t.sub.u of the
undimpled portions of said heat transfer wall and wherein t.sub.d
is approximately equal to
where y is the inside sagitta of the dimples, r is the radius of
the dimpling, and K is a constant.
9. In a distillation apparatus, the improvement comprising: a
bundle of rigid, metallic heat transfer tubes, each providing an
evaporating surface on one side of the tubular walls and a
condensing surface on the other side, an array of a plurality of
dimples in said tubular wall and protruding thru the exterior
surface to form liquid flow paths thereon, said dimples being
structurally free from contact with adjacent tubes and suitable for
the formation of thin liquid films thereover and acting to improve
the heat transfer capability of the wall with the wall thickness
t.sub.d of the dimpled portions of said heat transfer wall being
less than the wall thickness t.sub.u of the undimpled portions of
said heat transfer wall and wherein the smallest pitch P between
adjacent dimples is between about 0.1875 and 1.250 inch, the inside
sagitta y of each dimple is between about 0.0084 and 0.3875 inch,
and the dimpling radius r is between about 0.0625 and 0.3875
inch.
10. Apparatus according to claim 9 wherein the thickness t.sub.u of
the undimpled portions of said wall is between about 0.020 and
0.650 inch and the thickness t.sub.d of the dimpled portion of said
wall is between about 0.015 and 0.610 inch.
11. In a distillation apparatus, the improvement comprising: a
bundle of rigid, metallic heat transfer tubes, each providing an
evaporating surface on one side of the tubular wall and a
condensing surface on the other side, an array of a plurality of
dimples in said tubular wall and protruding radially outwardly from
the exterior surface to form liquid flow paths thereon, said
dimples being structurally free of contact with adjacent tubes and
their dimples and suitable for the formation of thin liquid films
thereover and acting to improve the heat transfer capability of the
wall with the surface opposite the first surface having an array of
a second plurality of dimples protruding radially inwardly
therefrom to form second flow paths on said opposite surface
between the second dimples and wherein the thickness of t.sub.d of
the dimpled portions of the heat transfer wall is less than the
wall thickness t.sub.u of the undimpled portions of said wall and
wherein t.sub.d is approximately equal to
where y is the inside sagitta of the dimples, r is the radius of
the dimpling and K is a constant.
12. In a distillation apparatus the improvement comprising: a
bundle of rigid, metallic heat transfer tubes providing an
evaporating surface on one side of the tubular wall and a
condensing surface on the other side, an array of a plurality of
dimples in said tubular wall and protruding radially outwardly from
the outer surface to form liquid flow paths thereon between said
dimples, said dimples being structurally free from contact with
adjacent tubes and their dimples and suitable for the formation of
thin liquid films thereover and acting to improve the heat transfer
capability of the wall and wherein the inner surface has an array
of a second plurality of dimples protruding radially inwardly
therefrom to form second flow paths on said inner surface between
the second dimples and wherein the smallest pitch P between
adjacent dimples is between about 0.1875 and 1.250 inch, the inside
sagitta y of each dimple is between about 0.0084 and 0.3875 inch,
and the dimpling radius r is between about 0.0625 and 0.3875
inch.
13. In a distillation apparatus the improvement comprising: a
bundle of rigid, metallic heat transfer tubes, each providing an
evaporating surface on one side of the tubular wall and a
condensing surface on the other side, an array of a plurality of
dimples in said tubular wall and protruding radially outwardly from
outer surface to form liquid flow paths thereon between said
dimples, said dimples being structurally free of contact with
adjacent tubes and their dimples and suitable for the formation of
thin liquid films thereover and acting to improve the heat transfer
capability of the wall and wherein the interior surface has an
array of a second plurality of dimples protruding radially inwardly
therefrom to form second flow paths on said interior surface
between the second dimples and wherein the thickness t.sub.u of the
undimpled portions of the wall is between about 0.020 and 0.650
inch and the thickness t.sub.d of the dimpled portion of the wall
is between about 0.015 and 0.610 inch, t.sub.d being less than
t.sub.u.
Description
This invention relates to dimpled heat transfer surfaces, and
particularly to distillation apparatus having dimpled heat transfer
walls.
A distillation process is one whereby an impure liquid may be
purified by vaporizing the liquid and thereafter condensing the
vapors to obtain a condensate and a concentrate. For example, fresh
water may be separated from saline water in a distillation process
by bringing thin films of saline water into contact with a hot
surface to vaporize part of the water and separate it from the salt
or brine. The vaporized water is then condensed on a cool surface
and is recovered as fresh water. Ordinarily, a heat transfer wall
separates the saline water from a source of heating fluid, such as
steam.
One factor relating to the effectiveness of such distillation
apparatus resides in the rate at which the saline water is
vaporized per unit area of the heat transfer wall. The rate of
vaporization of liquid is dependant, in part, upon the rate at
which the heat is transferred to the saline water which in turn is
dependant upon the thermal resistance of the heat transfer wall,
and the thermal resistance of the layer of saline water on one side
of the wall. It is desirable to construct the heat transfer wall
from a suitable thermally conductive material, such as copper, and
it is desirable to increase the surface area of the wall so that
the condensation area and vaporization area are as large as
practical.
One problem associated with distillation processes for saline water
resided in the fact that the thermal resistance of water is
relatively high, and is usually higher than that of the heat
transfer wall. Since relatively thin films of liquid transfer their
heat more readily than thicker films, it is desirable to maintain
both the condensate and the saline water in layers as thin as
possible on the heat transfer wall. Heretofore, heat transfer walls
for distillation apparatus have been enhanced by providing
continuous fins or grooves on one or both sides of the heat
transfer wall. These walls, often referred to as "fluted" or
corrugated walls, provided continuous flow paths for condensing and
evaporating liquids so that the liquids would develop into streams
which run down the surface of the wall. However, prior heat
transfer walls providing continuous flow paths for liquid have not
been altogether effective for distillation apparatus.
It is an object of the present invention to provide heat transfer
walls for distillation apparatus which is more effective than prior
walls providing continuous fluid paths.
In accordance with the present invention, distillation apparatus is
provided with dimpled heat transfer walls. The dimpled heat
transfer walls enhance the overall effective heat transfer surface
of the walls. Furthermore, when liquids are distributed on the
dimpled surface in sufficient quantities to flood, most of the
liquid flows through the low areas between the dimples so that
extremely thin films of water are formed over the dimpled portion
due to the surface tension of the flood. It is believed that fluid
disposed in thin films over the dimpled portion more readily
transfers heat than thicker films of liquid thereby achieving
condensation of the heating fluid and evaporization of the saline
water at a greater rate than heretofore provided by other types of
walls. Furthermore, there is no preferential flow path for moisture
in a dimpled wall surface so that the film is in a turbulent flow
over substantially the entire wall surface of the heat transfer
wall. The turbulent flow mixes the concentration of brine which
might otherwise occur as a result of evaporation and increases
convective heat transfer.
One optional and desirable feature of the present invention resides
in the fact that the dimples may be of any configuration, such as
spherical or even teardrop, and may be arranged in any desirable
pattern.
Another optional and desirable feature of the present invention
resides in the use of a shedder or baffle in connection with the
dimpled wall to remove excess condensed fluids therefrom.
Another optional and desirable feature of the present invention
resides in the arrangement of the dimples so that the flow paths of
liquid over the wall surface are tortuous in a vertical
direction.
The above and other features of this invention will be more fully
understood from the following detailed description and the
accompanying drawings, in which:
FIG. 1 is a side view elevation in cutaway cross section of a
simplified distillation apparatus in accordance with the present
invention;
FIG. 2 is a side view elevation of a dimpled heat transfer wall for
use in the apparatus illustrated in FIG. 1;
FIG. 3 is a side view elevation in cutaway cross section of a
portion of the dimpled heat transfer wall illustrated in FIG.
2;
FIG. 4 is a side view elevation in cutaway cross section of a
modification of the wall illustrated in FIG. 2;
FIGS. 5A-5C are top view elevations of various dimple
configurations for heat transfer walls in accordance with the
present invention;
FIGS. 6A-6C, 7 and 9 are top view elevations of various dimple
patterns for dimpled heat transfer walls in accordance with the
present invention; and
FIG. 8 is a section view taken at line 8--8 in FIG. 7.
Referring to the drawings, and particularly to FIG. 1, there is
illustrated a housing 10 separated by walls 11 and 12 into chambers
13, 14 and 15. Inlet conduit 16 is provided through a wall of
housing 10 to admit saline water into upper chamber 13. Conduit 17
is provided through a wall of housing 10 to admit a heating fluid,
such as steam, into chamber 14. By way of example, conduit 16 may
be connected to any source of heated liquid or vapor, such as a
boiler or the exhaust of a turbine. Outlet conduit 18 permits
removal of condensed steam from chamber 14, and outlet conduit 22
permits removal of steam vapor from chamber 14.
Saline water to be distilled is admitted through inlet conduit 16
and permitted to flow, in thin films, down tubes 19, 19a and into
lower chamber 15. Tubes 19, 19a, which may be arranged in a bundle,
are constructed of suitable heat transfer material. The tubes pass
through chamber 14. Outlet conduits 20 and 21 are associated with
chamber 15 to remove concentrate (enriched liquid) and evaporate
(vaporized water) from chamber 15, respectively. The bundle of
tubes may include any number of tubes, the two tubes being shown
for sake of clarity.
In FIG. 2 there is illustrated a portion of a heat transfer wall 30
in accordance with the presently preferred embodiment of the
present invention. Heat transfer wall 30 may be used for tubes 19,
19a in the distillation apparatus illustrated in FIG. 1. Heat
transfer wall 30 is constructed of a suitable heat conductive
material, such as copper, copper-nickel allow, copper-iron alloy,
or aluminum-brass alloy, the particular material used being
governed by such factors as durability, thermal conductivity in the
range of temperatures contemplated, and availability.
Wall 30 includes a plurality of dimples 31 which are illustrated in
greater detail in FIG. 3. Dimples 31, may, for example, be formed
in the configuration of a portion of a sphere. In the case of a
semi-spherical dimple, the dimple is generated from a center point
29 and has a radius r to the inside surface of the dimple. The
dimple has a diameter d across the inside thereof between opposite
points where the dimple joins surface 32 of wall 30. Dimension d
will be greater than the radius r and less than 2r. Angle .alpha.
is the angle between opposite portions of the cone generated by
radius r as it traces about the circumference of the dimple. It is
preferred that angle .alpha. be between 60.degree. and 180.degree..
The dimple has an inside height sagitta y from an extension of
surface 32 of wall 30. Dimension y is less than or equal to radius
r. The thickness of wall 30 in the undimpled portion thereof is
represented by dimension t.sub.u, and the thickness of the dimpled
portion of wall 30 is represented by dimension t.sub.d. As will be
observed from an examination of FIG. 3, t.sub.d is less than
dimension t.sub.u.
The thickness t.sub.d of the dimple is proportional to the product
of the thickness of the wall t.sub.u and the ratio of the projected
area of the surface to the actual area of the dimple. Hence, the
thickness t.sub.d of the wall forming the dimple can be
approximated by the following formula;
where K is a constant. It can therefore be understood as the
surface area of the dimple is made larger (and y is made larger)
the thickness of the wall forming the dimple becomes thinner.
The dimpled heat transfer walls are preferrably arranged so that
the vaporizing surface of tubes 19, 19a has dimples protruding
therefrom to form noncontinuous or tortuous flow paths thereon in a
vertical direction for both the condensate and the vaporizing
liquid. The dimples preferrably protrude into tubes 19, 19c, but it
is to be understood that the dimples may protrude outwardly
instead, or a combination of inwardly and outwardly protruding
dimples may be used.
In operation of the distillation apparatus having heat transfer
walls in accordance with the present invention, steam is admitted
through conduit 17 and contacts the outside or condensing surface
of heat transfer tubes 19, 19a. Some of the steam gives up its
latent heat of condensation and condenses on the surface of heat
transfer tubes 19, 19a at a temperature T.sub.1 (See FIG. 4). The
force of gravity on the condensed steam on the outside of the tubes
causes the condensed steam to run down the outside walls of the
tubes to be discharged through conduit 22 from chamber 14.
Saline water admitted through conduit 16 flows, in a thin film,
down the inside wall, or vaporizing surface, of tubes 19, 19a. The
temperature of the saline water is at some temperature T.sub.2
below the temperature T.sub.1 of the condensing steam. (See FIG.
4). The saline water is heated and water is vaporized therefrom.
The concentrated salt solution or brine continues down the inside
of the tube due to the force of gravity and is collected at the
bottom of chamber 15 where it is discharged through conduit 20.
The heat transfer capabilities of a wall constructed in accordance
with the present invention are significantly greater than
heretofore achieved in connection with other types of heat transfer
walls for distillation apparatus. It is theorized that when a heat
transfer wall is provided with corrugations or flutes in the form
of continuous parallel or spiraling grooves, there is a continuous
laminar flow of fluid through the grooves and the fluid tends to
stratify in the groove and act as an insulator between the heat
transfer wall and the bulk vapor. Hence, as steam condenses and as
saline water is distributed onto continuous groove-type heat
transfer walls, the saline water and the condensate flow in laminar
films in the grooves to impose a significantly greater heat
transfer resistance between the wall and the vapor. It is theorized
that the raised portion of the dimple area is covered with a
significantly thinner film of fluid because the surface tension of
the fluid pulls the fluid into the depressed portions between the
dimples causing it to flow in the depressed paths between the
dimples. Hence, any thick flow occuring on the wall will occur only
between the dimples, and the dimples cause a turbulent flow. Also,
if the dimples are positioned to prevent continuous vertical flow
of fluid on the wall, any collection of fluid is divided by dimples
downstream, or below the region of thick film formation. Hence, the
fluid flow is maintained turbulent.
If the dimples protrude outwardly from the tube, the condensate
accumulates and flows through the lower regions of the surface. At
the same time, the vaporizing fluid forms thin films on the
internal raised portion opposite the depressed portion of the
dimples and turbulent films form in the depressions opposite the
outwardly protruding dimples. If the dimples protrude inwardly, the
thin film is formed on the dimple by the vaporizing liquid, and the
condensate forms thin films on the raised portion opposite the
inside depressed portions. As illustrated in FIG. 4, some dimples
may protrude inwardly while some protrude outwardly so that the
advantages of both may be obtained.
The fluid on the raised portion of the dimples is so thin that heat
transfer resistance of the fluid on the dimples is relatively low,
thereby permitting rapid release of latent heat of condensation by
the condensing vapor and rapid absorption of latent heat of
vaporization by the evaporating liquid. Furthermore, due to the
thinner wall thickness of the dimpled portions of the wall, the
heat transfer resistance of the wall is lower in the dimpled
portions than the other portions. Since the wall thinning occurs at
the same position as where the film of liquid is the thinnest, the
heat transfer resistance is minimized and heat transfer capability
is maximized. Furthermore, since the depressed portions of the
wall, between the dimples, is tortuous, some condensing fluid will
shed and fall free when a sufficient flow is established. Hence,
when water vapor is condensed onto the outside surface of tubes 19,
19a, excessive condensate falls free from the wall to expose the
wall and lower the heat transfer resistance.
When the tubes 19, 19a are constructed in the manner illustrated in
FIG. 4, steam is directed to surface 33 of the wall and a layer of
condensed steam is condensed thereon at a temperature T.sub.1. A
thin layer 35 of saline water to be vaporized is directed onto
surface 36 of the heat transfer wall at a temperature T.sub.2. The
dimpled portions 37 of the wall are covered with a thinner film of
saline water 35 than the non-dimpled portions of the wall. Some
dimples 38 are raised on surface 33 of the wall to aid in
collection removal of condensed steam from surface 33 of the wall
so that the surface tension of the condensate draws the condensate
into depressed portions on the wall to develop regions of reduced
heat transfer resistance where the condensate film will be
relatively thin. Also, baffle 39 may be provided to shed condensed
steam from the surface 33 of the wall.
When the tubes 19, 19a are constructed in the manner illustrated in
FIG. 4, condensed steam forms on surface 33 as the layer
illustrated in 34. When a sufficient quantity of steam has
condensed on surface 33 to initiate flooding, it is removed by
means of shedder or baffle 39. Likewise, the saline water
preferrably flows between dimples 37 on surface 36 of the wall in a
tortuous path to induce turbulent flow.
As illustrated in FIGS. 5A, 5B and 5C, the dimples may be in any
desired shape. For example, in FIG. 5A dimple 40 comprises a
substantially spherical portion whereas in FIG. 5B dimple 41 is
somewhat teardropped shaped. In FIG. 5C, dimple 42 is in a shape of
a two-way teardrop which is substantially semi-spherical with
teardropped tongues at opposite ends. Preferrably, the elongated
tongues at opposite ends of the two-way teardrop illustrated in
FIG. 5C are arranged in line with the force of gravity along arrow
44.
FIGS. 6A, 6B and 6C illustrate various patterns of dimples. In FIG.
6A the dimples 43 are arranged in a square or rectangular
configuration and are separated by pitch distance P. However, the
configuration is off-set from a horizontal line by angle .THETA..
The flow of fluid under the influence of gravity is illustrated by
arrow 44. FIG. 6B illustrates a different dimple configuration
wherein dimples 45 are arranged in a substantially equilateral
triangular grid each separated from the next dimple by a pitch
distance P. Like the grid illustrated in FIG. 6A, the grid
illustrated in FIG. 6B is off-set from the horizon by angle
.THETA.. In FIG. 6C another triangular grid of dimples 46 is
illustrated except that the triangular grid is in a form of an
isosceles triangle wherein one side P.sub.2 is shorter than the
other two sides P.sub.1 of the triangle. Like the grids illustrated
in FIGS. 6A and 6B, it is preferred that the grid be off-set from
the horizon by some angle .THETA..
One reason for off-setting the grid from the horizon by angle
.THETA. is to prevent the existence of continuous vertical flow
paths for fluid. Thus, by off-setting the grid pattern from the
horizon, the dimples become arranged, in a somewhat irregular
pattern to a vertical flow path in a direction of arrow 44 so that
fluid under the influence of gravity is diverted by the various
dimples through a tortuous path between the dimples. For this
reason, angle .THETA. may vary between 0.degree. and 90.degree.,
depending upon the configuration.
FIG. 7 illustrated another type of grid pattern wherein the dimples
are substantially diamond-shaped dimples 47 having flow paths 48
formed between them. FIG. 8 illustrates a cross section of the
dimple pattern illustrated in FIG. 7 wherein flow passages 48 are
formed in a substantially diamond grid. FIG. 9 is a top view
elevation of another type of diamond-shaped grid of dimples 49
having flow channels disposed 45.degree. from the vertical flow
path of fluid.
Ordinarily, the shortest pitch P of any arrangement of dimples, as
measured between the centers of adjacent dimples, is between about
0.1875 and 1.250 inch. The diameter d across the dimples ordinarily
is about 0.125 to 0.750 inch. The inside height y of the dimples is
ordinarily between about 0.0084 and 0.3875 inch while the radius r
of dimpling is ordinarily between about 0.0625 and 0.3875 inch.
Angle .alpha. is ordinarily between about 60.degree. and
180.degree.. The thickness t.sub.u of an undimpled wall portion is
ordinarily between about 0.020 and 0.650 inch and the thickness
t.sub.d in the dimpled area is ordinarily between about 0.015 and
0.610 inch. The pitch-to-diameter ratio P/d for the smallest pitch
in any arrangement is between about 1.06 and 1.66 and the ratio of
dimple height to diameter y/d is between about 0.134 and 0.50.
Distillation apparatus having heat transfer walls in accordance
with the present invention are more effective in operation than
heat transfer walls heretofore used in distillation apparatus and
they provide effective maintenance-free operation of the
distillation apparatus. The heat transfer walls in accordance with
the present invention are easily fabricated and used and are
durable.
This invention is not to be limited by the embodiment shown in the
drawings and described in the description, which are given by way
of example and not of limitation.
* * * * *