U.S. patent application number 12/056016 was filed with the patent office on 2008-10-02 for finned tube with indentations.
This patent application is currently assigned to Wolverine Tube, Inc.. Invention is credited to Jianying Cao, Zhong Luo, Yalin Qiu, Jian Wu.
Application Number | 20080236803 12/056016 |
Document ID | / |
Family ID | 39792271 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080236803 |
Kind Code |
A1 |
Cao; Jianying ; et
al. |
October 2, 2008 |
FINNED TUBE WITH INDENTATIONS
Abstract
A tube used for heat transfer has adjacent fins extending from
an outer surface of the tube with a channel between the fins. The
fins include a roof formed over the channel, and holes penetrate
the roof into the channel. The fin, including the roof, is
monolithic with the tube body. Helical ridges are formed on a tube
inner surface, and the tube body includes an indentation in the
outer surface which extends the tube body inner surface towards a
tube axis.
Inventors: |
Cao; Jianying; (Shanghai,
CN) ; Luo; Zhong; (Shanghai, CN) ; Qiu;
Yalin; (Shanghai, CN) ; Wu; Jian; (Shanghai,
CN) |
Correspondence
Address: |
BRADLEY ARANT ROSE & WHITE LLP
200 CLINTON AVE. WEST, SUITE 900
HUNTSVILLE
AL
35801
US
|
Assignee: |
Wolverine Tube, Inc.
Huntsville
AL
|
Family ID: |
39792271 |
Appl. No.: |
12/056016 |
Filed: |
March 26, 2008 |
Current U.S.
Class: |
165/179 |
Current CPC
Class: |
F28F 1/422 20130101;
F28F 1/42 20130101; F28F 1/36 20130101; F28F 1/426 20130101; F28F
13/187 20130101 |
Class at
Publication: |
165/179 |
International
Class: |
F28F 1/42 20060101
F28F001/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2007 |
CN |
200720068218 |
Mar 27, 2007 |
CN |
ZL200720068218.6 |
Claims
1. A finned tube comprising: a tube body having an outer surface,
an inner surface, and an axis; at least one monolithic fin defined
on the tube body outer surface such that, in the axial direction,
the tube outer surface has adjacent fins, wherein a channel is
defined between adjacent fins, and wherein the fin has an upper
portion deformed to form a roof over the channel, and wherein the
roof defines holes penetrating in to the channel; a plurality of
helical ridges defined on the tube body inner surface; at least one
helical indentation defined in the tube body outer surface such
that the indentation extends the tube body inner surface towards
the tube axis.
2. A finned tube comprising: a tube body having an inner surface; a
helical ridge defined on the tube body inner surface; and an
indentation defined in the tube body, the indentation looping
around the tube.
3. The finned tube of claim 2 wherein the indentation is helical,
and wherein the indentation spirals around the tube in the opposite
direction of the ridge.
4. The finned tube of claim 2 wherein the indentation is helical,
and wherein the indentation spirals around the tube in the same
direction as the ridge.
5. The finned tube of claim 2 wherein the indentation further
comprises a plurality of indentations forming successive rings
around the tube.
6. The finned tube of claim 2 wherein the indentation further
comprises a plurality of helical indentations spiraling around the
tube body in opposite directions.
7. The finned tube of claim 2 wherein the tube body includes an
outer surface, the finned tube further comprising a monolithic
helical fin defined on the tube body outer surface such that the
tube body outer surface includes adjacent fins when viewed axially,
wherein a channel is defined between adjacent fins, and wherein the
fin includes an upper portion deformed to form a roof over the
channel.
8. The finned tube of claim 7 wherein the tube body outer surface,
adjacent fins, and the roof define a boiling cavity, and wherein
the roof defines a plurality of roof holes penetrating into the
boiling cavity.
9. The finned tube of claim 8 wherein the fins include a fin top
having notches, and the notches define the roof holes.
10. The finned tube of claim 2 wherein the tube is comprised of
copper.
11. A finned tube comprising: a tube body having an outer surface;
a monolithic fin defined on the outer surface such that the outer
surface includes adjacent fins viewed in the axial direction,
wherein a channel is defined between adjacent fins, and wherein the
fins include an upper portion deformed into a roof over the
channel; and an indentation defined in the tube body, the
indentation looping around the tube.
12. The finned tube of claim 11 wherein the indentation is
helical.
13. The finned tube of claim 11 wherein the indentation further
comprises a plurality of helical indentations spiraling around the
tube in opposite directions.
14. The finned tube of claim 11 wherein the indentation further
comprises a plurality of indentations forming successive rings
around the tube body.
15. The finned tube of claim 11 wherein the tube body further
includes an inner surface, the finned tube further comprising a
helical ridge defined on the tube body inner surface.
16. The finned tube of claim 15 wherein the indentation is helical
and the indentation spirals around the tube in the opposite
direction as the ridge.
17. The finned tube of claim 15 wherein the indentation is helical
and the indentation spirals around the tube in the same direction
as the ridge.
18. The finned tube of claim 15 wherein the tube body outer
surface, adjacent fins, and the roof define a boiling cavity, and
the roof defines a plurality of holes penetrating into the boiling
cavity.
19. The finned tube of claim 18 wherein the indentation interrupts
the boiling cavity.
20. The finned tube of claim 11 wherein the tube is comprised of
copper.
Description
[0001] This invention claims priority to Chinese Patent Application
Number ZL200720068218.6, which was filed on Mar. 27, 2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The current invention describes finned tubes used for heat
transfer, such as the tubes used in shell and tube heat
exchangers.
[0004] 2. Description of the Related Art
[0005] Finned tubes have been used for heat transfer for many
years. Heat flows from hot to cold, so heat transfer is
accomplished by conducting heat from a warmer material to a cooler
material. There is also heat given off when a material condenses
from a vapor to a liquid, and heat is absorbed when a liquid
vaporizes or evaporates from a liquid to a vapor. When finned tubes
are used for heat transfer, the warmer material is on either the
inside or the outside of the tube and the cooler material is on the
other side. Usually the tube allows for the transfer of heat
without mixing the warmer and cooler materials.
[0006] For cooling purposes, a cooling medium can be a liquid such
as cooling water flowing through a shell and tube heat exchanger,
or it can be a gas such as air blown over a finned tube. Similarly,
a heating medium is usually either a liquid or a gas. Finned tubes
are sometimes used instead of relatively smooth tubes because
finned tubes tend to increase the rate of heat transfer. Therefore,
a smaller heat exchanger with finned tubes may be able to transfer
as much heat in a given application as a larger heat exchanger with
relatively smooth tubes. The design of finned tubes affects the
rate of heat transfer and sometimes the tubes are designed
differently for specific heat transfer applications. For example,
finned tubes used for condensation tend to have different designs
than finned tubes used for evaporation.
[0007] Examples of the prior art include finned tubes with helical
fins formed on an outer surface of the tube. The tops of the fins
have at least one groove to divide the fin top into at least two
parts, thus forming a "Y" shape along the length of the fin. The
fins can also be notched across the fin, and then helical
indentations are formed by pressing into the tube outer surface.
The fins are broken where the indentations are formed, and beads
are formed on a tube inner surface where the fin is pressed down
into the tube body. There are a plurality of beads formed on the
inner surface along an imaginary line corresponding to where the
indentation in the tube is formed.
[0008] Finned tubes also include fins formed to promote boiling on
the outer surface. The fins are deformed at the top to essentially
close off the channels defined between adjacent fins, except the
closed off channels are open to the outside through pores
penetrating into the channels. The pores can be of varying sizes,
and there can be more than one sized pore on a single tube. There
are a wide variety of finned tubes for evaporation which include
various permutations of closed off channels between adjacent fins,
with some sort of hole or pore penetrating into the closed off
channels.
[0009] Some finned tubes are produced by attaching fin material to
a relatively smooth tube so the fins are not formed from the
material of the tube body. This increases the area available for
heat transfer, which does improve heat transfer rates, but the
interface between the fin and the tube does cause some resistance
to heat flow. The fins attached to the tube can extend radially
from a tube axis so they stand straight up from the tube, but they
can also be curved or bent in various ways to improve heat
transfer.
[0010] Finned tubes are often used in evaporators, such as those
used in air conditioners. Most air conditioning evaporators are a
flooded type of evaporator, where the finned tube is submerged in a
pool of liquid refrigerant. The surface of a tube in a flooded
evaporator is constantly wet with the refrigerant, and evaporated
gas bubbles through the liquid pool to escape. It is also possible
to use a drip evaporator, where the liquid refrigerant is
circulated and dripped or spayed on top of the tubes, and the tubes
are not immersed in liquid. This allows for the use of less
refrigerant, and tends to be more energy efficient. Keeping all
areas of the heat transfer tube wet improves the boiling efficiency
of a drip evaporator, and tube designs which improve the wetted
area of a tube are beneficial.
[0011] There are many designs of finned tubes in existence, but
changes which improve heat transfer are still possible.
BRIEF SUMMARY OF THE INVENTION
[0012] A tube used for heat transfer has adjacent fins extending
from an outer surface of the tube with a channel between the fins.
The fins are formed from the material of the tube outer surface, so
the fins are monolithic with the tube body. The fins include an
upper portion which is deformed into a roof over the channel, and
there are holes penetrating the roof into the channel. Helical
ridges are formed on a tube inner surface, and an indentation is
defined in the tube body outer surface that extends the tube body
inner surface towards a tube axis.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0013] FIG. 1 is a side view of a tube with indentations.
[0014] FIG. 2 is the enlarged portion from the circle labeled "2"
in FIG. 1. FIG. 2 shows a side sectional view of a tube with
indentations with the details of the fins and ridges not shown.
[0015] FIG. 3 is the enlarged portion from the rectangle labeled
"3" in FIG. 2. FIG. 3 shows a side sectional view of a finned
tube.
[0016] FIG. 4 is a side sectional view of a finned tube.
[0017] FIG. 5 is a perspective view of a portion of a finned
tube.
[0018] FIG. 6 is a side partial sectional view of a finned tube
with indentations.
[0019] FIG. 7 is a side view of one embodiment of a tube with an
indentation.
[0020] FIG. 8 is a side view of a second embodiment of a tube with
indentations.
[0021] FIG. 9 is a side view of a third embodiment of a tube with
indentations.
[0022] FIG. 10 is a side partial sectional view of heat
exchanger.
[0023] FIG. 11 is a side partial sectional view of an arbor forming
a finned tube with material from a fin in front of a tooth on the
notching disc.
[0024] FIG. 12 is a side partial section view of an arbor forming a
finned tube showing a notching disc tooth forming a notch in a
fin.
DETAILED DESCRIPTION
[0025] The finned tube of the current invention is intended to be
used for heat transfer, and primarily for phase change on the tube
outer surface. This tube is designed to especially enhance boiling
or evaporation, but it could also be used for condensation or
non-phase change heat transfer. The tube is designed to promote
boiling on the tube outer surface with a heating medium, such as a
liquid, flowing inside the tube. The heating medium is cooled by
the evaporation on the tube outer surface. The tube is often
utilized in the construction of shell and tube heat exchangers, but
other uses are possible.
Heat Transfer Principles
[0026] The following discussion is directed towards evaporation,
and particularly towards evaporation using a drip type evaporator.
The current invention could also be used for other heat transfer
applications as well, and this discussion is not intended to limit
the scope of the invention.
[0027] When heat is transferred from one material to another
material, the larger the temperature difference between the two
materials, the faster the rate of heat transfer. This basically
means if you want to heat something up faster, put it against a
hotter surface, and if you want to cool something off faster, put
it against a colder surface. This is true for a conductive material
or an insulating material.
[0028] A refrigerant on the tube exterior absorbs the heat of
vaporization as it changes from a liquid to a gaseous state, and
this heat of vaporization is ultimately absorbed from the liquid
flowing inside the tube. This cools the liquid in the tube. The
design of the fins on the tube outer surface increase heat transfer
by several different mechanisms. The fins increasing surface area
of the tube, and the fins form boiling nucleation sites which serve
to promote boiling. The more surface area on a condensing tube, the
more rapid the flow of heat. When fins are formed on a tube it
increases the surface area of the tube, which serves to increase
the rate of heat transfer across the tube. Other deformations in
the tube outer surface which increase surface area will also tend
to increase the rate of heat transfer.
[0029] When heat is transferred from a liquid inside a tube to a
vapor formed by boiling a liquid on the tube outside, the heat
transfer is considered in several distinct steps. The same basic
steps apply when heat is transferred through a barrier, such as a
tube wall, between any two mediums with different temperatures. The
first step is transfer of heat from the liquid inside the tube to
the tube inner surface. Liquid flowing through the tube tends to
form an essentially stagnant layer on the tube inner surface. In
laminar flow, there is a second liquid layer next to the stagnant
layer that is moving slowly, or just sliding by the stagnant layer.
Then there is a third layer next to the second, which is moving a
little bit faster, and so on such that the fastest flowing liquid
is furthest from the tube wall. These layers tend to insulate the
tube wall and hinder the flow of heat.
[0030] In turbulent flow, there is still a stagnant layer next to
the inside tube wall, but the rest of the liquid is flowing and
mixing together as one large mass. The stagnant layer still tends
to insulate the tube from the main body of the liquid, but the
mixing promotes heat transfer by keeping a larger temperature
differential between the stagnant layer and the next liquid layer.
In either case, anything which disturbs the static layer or
promotes more mixing helps to reduce the insulating effect within
the tube, and therefore increases the rate of heat transfer.
[0031] After heat is transferred from the main body of liquid to
the stagnant layer, it has to flow across the stagnant layer. Then,
heat has to flow across the interface between the stagnant layer
and the tube inner surface. Any interface provides some resistance
to heat flow. After heat flows to the tube inner surface, it has to
flow through the tube to the tube outer surface. To facilitate this
heat flow, heat transfer tubes are usually made out of a material
which readily conducts heat, or a heat conductor. Copper is one
material which is considered to be a good conductor of heat. Then,
the heat flows across the interface between the tube outer surface
to any liquid contacting the outer surface There can be heat flow
across a stagnant liquid layer on the tube outer surface, and then
the liquid absorbs the heat and boils. The boiling liquid absorbs a
specific amount of heat, called the heat of vaporization, to change
from the liquid state to the vapor state.
[0032] An interface between the fins and the tube exists if the
fins are constructed separately from the tube, and then attached.
This is true if the fin and tube are constructed of the same
material, such as copper, or from different materials. Any
interface causes some resistance to heat flow. If the fins are
formed from the tube wall, there is no interface and heat flow is
improved. In this discussion, fins formed from the tube wall are
referred to as being monolithic with the tube, and it is preferred
that fins be monolithic with a tube to minimize resistance to heat
flow.
[0033] The tube should be made from a malleable substance so the
fins can be formed from the tube without cracks or breaks forming
in the tube wall. Cracks or breaks limit the structural integrity
and strength of a tube, and can also provide resistance to heat
flow. Generally these tubes are used in shell and tube heat
exchangers, and the ends of the tubes are affixed in tube sheets of
the heat exchanger. A malleable tube can be easier to install in a
heat exchanger tube sheet. The tube should also be constructed from
a material which readily conducts heat. Copper is often used in
tube construction because of its malleability and heat
conducting
Special Evaporation Principles
[0034] Evaporation tubes have specific design features which are
different than those features preferred for a condensation tube.
Evaporation tubes are typically immersed in the liquid to be
evaporated, so the tube outer surface is constantly wet. Factors
which can enhance evaporation include providing a nucleation site
for the initial formation of bubbles, providing enclosed areas
where liquid can be superheated, and providing holes or access
ports to the enclose areas where vapor can escape and more liquid
can be introduced.
[0035] Nucleation sites for boiling are often very small
imperfections or sharp points on the boiling surface. An enclosed
area on a tube provides for a relatively small quantity of liquid
to be essentially surrounded by heat transferring surfaces from the
finned tube, so the amount of heat transfer surface area per volume
of liquid is large. This allows for the liquid to be rapidly heated
to facilitate boiling or vaporization. This can result in the
liquid being temporarily superheated, which is when the temperature
of the liquid is greater than the liquid's boiling temperature.
Vapors are less dense than liquids, so when a liquid vaporizes it
expands. If the vaporizing liquid is enclosed, it produces pressure
as it vaporizes. Vapors also expand as they are heated, so heating
of a vapor in an enclosed area also increases pressure.
[0036] Small holes in the enclosed area allow for the small
quantity of liquid to escape after is has vaporized, and the
pressure from vaporization tends to push the vapor out of the hole.
Normally, surface tension would reduce liquid flow through small
holes, unless there is a large enough pressure difference to force
or push the liquid through the hole. The escaping vapor leaves a
reduced pressure in the enclosed area, which draws liquid in
through the small holes after the vapor has escaped, and the
process repeats. This serves as a sort of pumping action, where
liquids are drawn into enclosed area, vaporized, and pushed out of
the enclosed areas.
[0037] The small hole in the enclosed area has to be small enough
to prevent a liquid from freely flowing through the hole. The small
holes can be one continuous hole, as long as it serves to prevent
the liquid from freely flowing into the enclosed area. To prevent a
liquid from flowing through a continuous hole or a series of small
holes, there must be a hole gap small enough that the liquid
surface tension prevents the liquid from passing. Reference in this
description to several small holes is intended to include one long
hole with a gap small enough to prevent liquid from flowing
through, such that the long hole serves the same function as
several small holes. The long hole serves essentially as several
small holes which are connected together.
Finned Tube Main Body
[0038] One embodiment of the finned tube 10 of the current
invention is shown in different perspectives in FIGS. 1, 2 and 3.
This discussion focuses on the embodiment shown, but this
discussion is not intended to be limiting. Other embodiments are
possible, and will be apparent to one skilled in the art.
[0039] The tube 10 includes a main body 12 which has an outer
surface 14 and an inner surface 16. The main body 12 is the base
for any shapes or structures on the outer or inner surface 14, 16.
This main body 12 should be made of a material which conducts heat
readily. Metals are generally good conductors and are frequently
used for the construction of tubes of the current invention. Copper
is a particularly common metal used for tube 10 construction, but
aluminum, other metals, various alloys and even non-metallic
materials are also possible. The material should also be malleable
or formable such that the various structures on the inner and outer
surface 14, 16 can be formed without damaging the integrity of the
tube body 12. This allows for the structures to be formed from the
tube body 12, which results in the structures being monolithic with
the tube body 12.
Tube Fins
[0040] The tube 10 has at least one fin 20 formed on its outer
surface 14. The fin 20 generally protrudes or extends
circumferentially from the tube body outer surface 14, and is
usually helical. The tube 10 often has a first end 22 and a second
end 24 without any fins 20 which facilitates forming a seal between
a tube end 22, 24 and a heat exchanger tube sheet. These ends 22,
24 are generally smooth. There is typically some transition area
between the smooth ends 22, 24 and the finned portion of the tube
10.
[0041] It is possible that one single fin 20 is helically wound
around the entire length of the finned portion of the tube 10. It
is also possible that there will be a plurality of fins 20
helically winding around the tube 10. In either case, when looking
at a section of the tube body outer surface 14, it will appear as
though there are several adjacent circumferential fins 20
protruding from the tube body outer surface 14. When viewed along
the axial direction of the tube 10, fin 20 sections next to each
other are referred to as adjacent fins 20, despite the fact that
they might be the same fin 20 helically wrapping around the tube
body outer surface 14. The fin 20 is formed from the material of
the tube body 12, so the fin 20 is monolithic with the tube body
12.
[0042] Each fin 20 has several parts including a fin base 26, a fin
top 28, and a fin side wall 30. The fin base 26 is at the point
where the fin 20 connects to the tube body outer surface 14. The
fin top 28 is opposite the fin base 22 and is the highest point of
the fin 20 relative to an axis of the tube 62. The fin side wall 30
includes a left side wall 32 and a right side wall 34 opposite the
left side wall 28. A channel 36 is defined between two adjacent
fins 20 over the tube body 12 such that the channel 36 is between a
right side wall 34 of one fin 20 and a left side wall 32 of an
adjacent fin 20. The fin 20 can be approximately perpendicular to
the tube body 12 such that the fin 20 extends essentially straight
out from the tube body outer surface 14. In such a case, the fin 20
would extend radially from the tube 10. It is also possible for the
fin 20 to be positioned at other angles to the tube body outer
surface 14.
[0043] The fin 20 also has a fin upper portion 38, which is
deformed or molded from the fin 20 to form a roof 40 over the
channel 36. The fin upper portion 38 can be split to extend both
left and right of the fin 20, as in FIG. 3, or the fin upper
portion 38 can be deformed in just one direction from the fin 20,
as shown in FIGS. 4 and 5. The roof 40 does not have to completely
cover the channel 36, and there should be holes 42 or pores 42
defined by and penetrating the roof 40 to into the channel 36.
Notches 43 can be formed in the fin top 28 to define the holes 40,
but the holes 40 can be formed in other ways as well. The roof 40,
the left and right fin side walls 32, 34, and the tube body outer
surface 14 define a boiling cavity 44, which is basically an
enclosed channel 36.
[0044] The boiling cavity 44 is very effective at promoting boiling
or evaporation on the tube exterior. Liquid in the boiling cavity
44 is surrounded on four sides by tube surfaces which transfer heat
to the liquid. The tube surfaces facing the liquid in the boiling
cavity 44 are the tube outer surface 14, the left and right fin
side walls 30, 32, and the roof 40. A liquid droplet in the boiling
cavity 44 has a relatively low volume with a relatively high
surface area in contact with the boiling cavity surfaces 14, 30,
32, 40, which results in the liquid being heated rapidly. As the
liquid boils and turns into a gas, it expands and increases the
pressure inside the boiling cavity 44. This forces the boiled gas
out through a roof hole 42, and the exiting gas leaves a partial
vacuum or low pressure inside the boiling cavity 44.
[0045] The partial vacuum inside the boiling cavity 44 facilitates
motion. The partial vacuum pulls and moves drops of liquid inside
the boiling cavity 44 which were next to the exiting gas into the
location where the exiting gas was, which serves to agitate the
moving liquid drop and thereby promote heat transfer. Also, when
the vaporized gas exits the boiling cavity 44, the low pressure
produced pulls more liquid from somewhere along the outside of the
cavity 44 through a roof hole 42 into the cavity 44. Without the
low pressure in the boiling cavity 44, surface tension would tend
to prevent liquids from readily passing through the roof holes 42
into the boiling cavity 44, so the pressure has to be enough to
overcome the liquid surface tension. This action of moving and
mixing liquids inside the boiling cavity 44 combined with pushing
out vaporized gas and pulling in additional outside liquids is
referred to as a pumping action, and it greatly increases the rate
of heat transfer and vaporization of liquids.
[0046] The fin 20 can be deformed or shaped in a wide variety of
ways, such as forming wings or side fins (not shown) extending from
the fin side wall 30 below the roof 40 to form an upper and lower
boiling cavity 44. Many different boiling or evaporation fin 20
configurations are possible within the current invention. The size
of the boiling cavity 44 and the roof holes 42 can be varied, and
specific sizes are more efficient for certain compounds. For
example, if the tube 10 were to used for the refrigerant R22,
different sized holes 42 and boiling cavity 44 dimensions would be
employed than if the tube 10 were to be used for the refrigerant
R123.
Inner Surface Ridges
[0047] Heat transfer across the tube 10 can be improved by
providing better transfer of heat between the tube body inner
surface 16 and a liquid within the tube 10. A ridge 50 or a
plurality of ridges 50 can be defined on the tube body inner
surface 16 to help facilitate more rapid heat transfer, and these
ridges 50 can be monolithic with the tube body 12. The ridges 50 on
the inner surface 16 are generally helical and have a depth 52 and
a frequency or pitch. The frequency is the number of ridges 50
within a set distance. The ridges 50 are also set at different cut
angles relative to the tube axis 62. There can be several ridges 50
formed within the tube 10, and the number of ridges 50 allows for a
predetermined cut angle, ridge depth 52, and frequency. The number
of ridges 50 is referred to as the number ridge heads or the number
of ridge starts.
[0048] The depth 52 and the frequency of the ridges 50 can vary,
and the cut angle can be set to cause the cooling liquid to swirl
within the tube 10. A swirling liquid tends to increase heat
transfer by increasing the amount of agitation within the cooling
liquid. Agitation tends to minimize or eliminate the layers of
fluid in laminar flow, and agitation also tends to minimize the
thickness of the stagnant layer of fluid next to the tube inner
surface 16. Additional measures which can induce vortexes and local
agitation at or very near the tube inner surface 16 further reduce
the stagnant layer of fluid next to the tube inner surface 16, and
thereby increase heat transfer. However, variations and
3-dimensional contours or texture also tend to increase the
resistance to flow within a tube 10, so more pressure is required
to push a given amount of fluid through a pipe in the same amount
of time. A larger ridge depth 52 and a smaller ridge frequency tend
to increase the rate of heat transfer, but they also increase the
resistance to flow inside the tube 10.
Tube Indentations
[0049] Referring now to FIG. 6, the tube 10 includes indentations
60 defined and depressed into the outer surface 14 such that the
inner surface 16 protrudes or extends towards the tube axis 62
directly opposite the indentation 60 in the outer surface 14. This
basically means the indentation 60 goes through the tube body 12
and pushes the inner surface 16 inward. This inner surface
protrusion is referred to as a rib 64, which is the inner surface
16 counterpart to the outer surface 14 indentation 60. The
indentation 60 loops around the tube 10, and the loops can be in a
variety of forms. For example, the loops can be helical as shown in
FIG. 7, or they can be a plurality of radial indentations 60 which
form successive rings around the tube 10 as shown in FIG. 8, or
they can be a plurality of indentations 60 helically looping around
the tube 10 in opposite directions as shown in FIG. 9. Other
indentation 60 patterns are possible, and are within the scope of
the current invention.
[0050] Referring now to FIGS. 3 and 6, the ribs 64 on the inner
surface 16 affect fluid flow patterns inside the tube 10. The rib
64 projects into the tube 10, and fluid flowing over the rib 64
tends to form vortexes and eddies downstream from the rib 64. These
vortexes decrease the stagnant fluid layer next to the inner
surface 16, and therefore increase the heat transfer rate inside
the tube 10. The rib 64 should not form a barrier which
significantly impedes flow through the tube 10, so the rib 64
cannot have a rib height 66 greater than a tube inner radius 68.
The rib height 66 is the height from the top of the rib 60 to the
tube inner surface 16. Preferably, the ratio of the rib height 66
to a nominal tube outside diameter 70 is between 0.02 and 0.2,
where the nominal tube outside diameter 70 is measured from the fin
tops 28.
[0051] A helical rib 64 and the corresponding indentation 60 can
wrap around the tube 10 either in the same direction as the
internal ridges 50, or in the opposite direction of the internal
ridges 50. If the rib 64 wraps in the same direction as the ridges
50, the rate of heat transfer and the resistance to flow is not
increased as much as if the rib 64 wraps the opposite direction as
the ridges 50. The ridges 50 induce a swirling flow direction, and
the rib 64 also induces a swirling flow direction, as long as both
are helical. When the rib 64 spirals counter to the ridge 50, the
change in induced flow direction between the rib 64 and the ridges
50 accounts for the greater heat exchange rate and resistance to
flow. The double helical indentation 60, and thus the double
helical rib 64, is particularly effective when the inside liquid
flow rate is relatively low, and discontinuous rib 64 belts are
particularly effective when the inside liquid flow is laminar. The
rib direction and height 66 can be set to keep the tube resistance
to flow within a 1.5 fold increase of the resistance to flow
without the rib 64.
Tube Use in a Heat Exchanger
[0052] The tube 10 is often used in a heat exchanger 72, as shown
in FIGS. 1 and 10. The tube first and second ends 22, 24 are fixed
to two tube sheets 74 with a tube side inlet 76 and a tube side
outlet 78 for fluid flow through the tube 10 interior. The tubes 10
are contained inside a shell 80. In the example shown, the heat
exchanger 72 is a dual pass heat exchanger 72 with fluid entering
and exiting from the same side of the heat exchanger 72 for flow
through the tubes 10. There is also a shell side inlet 82 and a
shell side outlet 84 for flow past the outside of the tube 10. The
heat exchanger 72 shown is a drip exchanger, with shell side fluid
being collected at the bottom of the shell 80 and recirculated to a
spray device 86 positioned above the tube 10 by a pump 88.
[0053] A drip exchanger 72 or drip evaporator 72 uses less
refrigerant than a flooded evaporator. The flooded evaporator has
the shell 80 filled with liquid refrigerant such that the tubes 10
are immersed in liquid, but the drip evaporator 72 is mostly filled
with gas or vapors. The drip evaporators 72 tend to be more energy
efficient than the flooded type, and they use less refrigerant.
Often the refrigerants used are chlorofluorocarbons (CFCs) or hydra
chlorofluorocarbons (HCFCs), which have many regulations
controlling their use, so means of using less refrigerants are
desirable.
[0054] For an evaporator tube 10 to function most efficiently, the
outer surface 14 (or the evaporating surface, which can be the
inner surface) should be kept wet with liquid to be evaporated.
With a flooded heat exchanger, the tube outer surface 14 is
immersed in liquid, so keeping the surface wet is not a
consideration. With a drip evaporator, there can be portions of a
tube outer surface 14 which are not wet, and which therefore cannot
evaporate any liquids. The way a liquid flows over a tube 10, the
type of spray device 86 used to distribute the liquid, and other
factors can affect the rate of evaporation. It has been noted that
tubes 10 with indentations 60 tend to keep more of the tube outer
surface 14 wet than tubes 10 without indentations 60. This may be
because the indentations 60 serve to collect and re-distribute the
liquid along the tube 10, or because the indentations 60 allow for
easier liquid access to the tube boiling cavities, or it may be due
to other reasons. Whatever the reason, measurements of overall heat
transfer rates with tube indentations 60 have shown increases of up
to 20% over heat transfer rates for tubes 10 without indentations
60.
[0055] A heat exchanger 72 with tubes 10 that are 20% more
efficient can utilize tubes 10 that are 20% shorter, or fewer tubes
10 with larger diameters can be used. By changing the heat
exchanger 72 design, pressure drop issues from tubes 10 with a
higher flow resistance can be addressed. It should be noted the
tubes 10 of the current invention also can be used in flooded type
evaporators. The internal rib increases the tube internal heat
exchange rate, which can benefit the tube overall heat exchange
rate.
Tube Forming Process
[0056] Finned tubes 10 are generally formed from relatively smooth
tubes 10 with a tube finning machine, which is well known in the
industry. The tube finning machine includes an arbor 90 as seen in
FIGS. 11 and 12, with further reference to FIG. 3. Frequently, a
tube finning machine will include three or more arbors 90
positioned around the tube 10, so the tube 10 is held in place by
the arbors 90. The arbors 90 are positioned and angled such that
each complements the others. A tube 10 is provided and fed through
the finning machine such that a tube wall 92 is positioned between
the arbor 90 and an inner support 94. The arbor 90 deforms the tube
outer surface 14, and the inner support 94 can deform the tube
inner surface 16. Actually, the arbors 90 hold various tools or
discs, and the tools contact and shape the tube outer surface 14,
so the arbors 90 serve as a form of tool holder. The tube wall 92
is generally rotated relative to the arbor 90 and moves axially
with the inner support 94 as it rotates.
[0057] The arbor 90 generally includes several fin forming discs 96
which successively deform the tube wall 92 to form one or more
helical fins 20 on the tube outer surface 14. Successive filming
discs 96 tend to project deeper into the tube wall 92 such that
fins 20 are formed and pushed upwards by the finning discs 96. The
inner support 94 can include recesses 98 such that helical ridges
50 are formed on the tube inner surface 16 as fins 20 are formed on
the tube outer surface 14.
[0058] After the fin forming discs 96 have formed the fins 20,
various other discs can be included on the arbor 90 to further
deform and define aspects of the final tube 10. There are a wide
variety of discs which can be included to produce many different
shapes, include a boiling cavity 44. One of many examples is shown.
After the fin forming discs 96, a notching disc 100 notches the fin
top 28. The notching disc 100 has teeth which press into the fin 20
to form the notch 43. In FIG. 11, a portion of the fin 20 is shown
in front of the notching disc tooth, and in FIG. 12, the tube wall
92 is shown sectioned such that there is no material shown in front
of the notching disc tooth forming the notch 43. The notch 43
becomes a hole 42 in the roof 40 as the fin 20 is further deformed.
The fin splitting disc 102 splits the fin top 28. Then, a
flattening disc 104 flattens the fin tops 28 to form the roof 40.
The roof 40 can be formed with a small gap between adjacent fin
tops 28 to produce the hole 42, or other methods can be used to
produce the hole 42.
[0059] After the fins 20 and the inner ridges 50 are formed, the
indentation 60 is produced in a subsequent step, as shown in FIG.
6. An indentation disc 106 is rolled over the tube 10. The
indentation disc 106 cuts through the fin 20 and the boiling cavity
44, and presses inward on the tube inner surface 16 to form the rib
64. The indentation disc 106 cuts into and interrupts the boiling
cavity 44, and can provide relatively easy access by a fluid to the
boiling cavity 44 at discrete locations.
[0060] This is one example of how the various deformations of the
original relatively smooth tube 10 are produced. There are other
possible orders and designs of discs and tools which can be used as
well.
EXAMPLE DIMENSIONS
[0061] The dimensions of the current invention can vary, but
example dimensions are provided below which will give the reader an
idea as to at least one embodiment of the current invention.
[0062] The inter-fin distance is the distance between a center
point of two adjacent fins 20 and this distance can be between 0.3
and 0.7 millimeters.
[0063] The fin 20 has a thickness between the left and right side
wall 32, 34, and this thickness can be between 0.1 and 0.5
millimeters.
[0064] The fin 50 has a height measured from the fin base 22 to the
fin top 24, and the fin height can be between 0.3 and 1.5
millimeters.
[0065] The ridge 74 formed on the tube body inner surface 16 has a
depth 52, and this depth can be between 0.1 and 0.5 millimeters.
The internal ridge angle with the axis 62 can be set at 46.degree.,
and the ridge starts can vary between 8 and 50.
[0066] The hole 64 defined in the barrier 58 can have an area
between 0.01 and 0.2 square millimeters.
[0067] The tube wall 92 thickness can vary between 0.75 and 3
mm.
[0068] The indentations 60 have an indentation depth and a pitch,
wherein the pitch is the distance between two adjacent indentations
measured axially along the tube 10. The indentation depth can be a
ratio of the nominal tube outside diameter 70, and the ratio can be
between 0.02 and 0.2, with the indentation depth ranging between
0.25 to 7 mm. The indentation pitch can a ratio with the nominal
tube outside diameter 70, and this ratio can be 0.25 to 2, with the
pitch ranging between 3 and 75 mm.
[0069] The nominal tube outside diameter 70 can range between 12
and 39 mm.
[0070] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed here. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *