Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A HEAT TRANSFER TUBE
AND METHOD OF MANUFACTURING SAME
BACKGROL1ND OF THE INVENTION
The present invention relates generally to heat transfer tubes. In particular,
the invention relates
to a heat transfer tube having a refrigerant surface configuration that is
suitable for use in air
conditioning and refrigeration system heat exchangers in both evaporating and
condensing
applications, as well as a method of manufacturing same.
A shell and tube type heat exchanger has a plurality of tubes contained within
a shell. The
tubes are usually arranged to provide multiple parallel flow paths for one of
two fluids
between which it is desired to exchange heat. In a flooded evaporator, the
tubes are im-
mersed in a second fluid that flows through the heat exchanger shell. Heat
passes from the
one fluid to the other fluid through the walls of the tube. Many air
conditioning systems
contain shell and tube type heat exchangers. In air conditioning applications,
a fluid, commonly
water, flows through the tubes and refrigerant flows through the heat
exchanger shell. In an
evaporator application, the refrigerant cools the fluid by heat transfer from
the fluid through the
walls of the tubes. The transferred heat vaporizes the refrigerant in contact
with the exterior
surface of the; tubes. In a condenser application, refrigerant is cooled and
condenses through
heat transfer to the flui3 through the walls of the tubes. The heat transfer
capability of such a
heat exchanger is largely determined by the heat transfer characteristics of
the individual tubes.
The external configuration of an individual tube is important in establishing
its overall heat
transfer characteristics.
There are a number of generally known methods of improving the efficiency of
heat transfer
in a heat transfer tube. One of these is to increase the heat transfer area of
the tube. One of
the most corr>rrlon methods employed to increase the heat transfer area of a
heat exchanger
tube is by placing fins on the outer surface of the tube. Fins can be made
separately and
attached to the outer surface of the tube or the wall of the tube can be
worked by some
process to form fins on the outer tube surface.
In a refrigerant condensing application, in addition to the increased heat
transfer area, a finned
tube offers unproved condensing heat transfer performance over a tube having a
smooth outer
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surface for another reason. The condensing refrigerant forms a continuous film
of liquid
refrigerant on the outer surface of a smooth tube. The presence of the film
reduces the heat
transfer rate across the tube wall. Resistance to heat transfer across the
film increases with
film thickness. The film thickness on the fins is generally less than on the
main portion of the
tube surface due to surface tension effects, thus lowering the heat transfer
resistance through
the fins.
In a refi-igerarrt evaporating application, increasing the heat transfer area
of the tube surface also
improves the heat transfer performance of a heat transfer tube. In addition, a
surface configura-
tion that promotes nucleate boiling on the surface of the tube that is in
contact with the boiling
fluid improve performance. In the nucleate boiling process, heat transferred
from the heated
surface vaporizes liquid in contact with the surface and the vapor forms into
bubbles. Heat
from the surface superheats the vapor in a bubble and the bubble grows in
size. When the
bubble size is sufficient, surface tension is overcome and the bubble breaks
free of the surface.
As the bubble; leaves the surface, liquid enters the volume vacated by the
bubble and vapor
remaining in the volume has a source of additional liquid to vaporize to form
another bubble.
The continual forming of bubbles at the surface, the release of the bubbles
from the surface and
the rewetting of the surface together with the convective effect of the vapor
bubbles rising
through and mixing the liquid result in an improved heat transfer rate for the
heat transfer
surface.
The nucleate boiling process can be enhanced by configuring the heat transfer
surface so that it
has nucleation sites that provide locations for the entrapment of vapor and
promote the
formation of vapor bubbles. Simply roughening a heat transfer surface, for
example, will
provide nucleation sites that can improve the heat transfer characteristics of
the surface over a
similar smooth surface. Nucleation sites of the re-entrant type produce stable
bubble columns
and good surface heat transfer characteristics. A re-entrant type nucleation
site is a surface
cavity in which the opening of the cavity is smaller than the subsurface
volume of the cavity.
An excessive influx of the surrounding liquid can flood a re-entrant type
nucleation site and
deactivate it. By configuring the heat transfer surface so that it has
relatively larger communi-
cating subsurface channels with relatively smaller openings to the surface,
flooding of the vapor
entrapment or nucleation sites can be reduced or prevented and the heat
transfer performance of
the surface innproved.
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In a falling film type evaporator, spreading of liquid filin on the heat
transfer surface and
promotion of a thin filin are important to improve the ability to transfer
heat.
It is desirable from a logistics and manufacturing point of view to have a
heat transfer tube with
an external heat transfer surface that has good heat transfer performance in
both refrigerant
condensing and evaporating applications in the flooded and falling film
evaporator modes so
that a single tube configuration may be used in both condensers and flooded
evaporators.
SUMMARY OF THE INVENTION
The present invention is a heat transfer tube, and a method of manufacturing a
heat transfer
tube, having m external surface configured to provide improved heat transfer
performance in
both refrigerant condensing, flooded evaporation and film evaporation
applications.
The tube has one or more fin convolutions formed on its external surface.
Notches extend at
an oblique angle across the fin convolutions at intervals about the
circumference of the tube.
The portion of a fin convolution between adjacent notches in the fin
convolution forms a
spike. The diistal tip of the spike is split into two tip portions. Each tip
portion extends
outward froir~ the proximal base of the fin toward the split fin tips in the
adjacent fin
convolution.
The notches and split spike tips further increase the outer surface area of
the tube as compared
to a conventional finned tube. The grooves between adjacent fin convolutions,
over which
the split fin tips extend form reentrant cavities that promote refrigerant
pool boiling in a
flooded evaporator.
In a condensing and falling film evaporation applications, the relatively
sharp spike tips
promote drainage and spreading of refrigerant from the fin. In most
installations, the tubes in
a shell and tube type air conditioning heat exchanger run horizontally or
nearly so. With
horizontal tubes, the notched and split fin configuration promotes drainage of
condensing
refi-igerant from the fins into the grooves between fins on the upper portion
of the tube
surface and also promotes drainage of condensed refrigerant off the tube on
the lower portion
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4
of the tube surface. In film evaporation mode, the sharp tips and notches, and
low surface
tension of refrigerant aid in liquid spreading on the tube surface and along
the tube axis. This
promotes good wettability in a horizontal shell and tube falling film
evaporator.
Manufacture of a notched split tip finned tube can be easily and economically
accomplished
by adding a notching disk or disks and a splitter disk or disks to the tool
gang of a finning
machine of the type that forms fins on the outer surface of a tube~by rolling
the tube wall
between an internal mandrel and external finning disks. The notching tool is
configured to
impart a twist to the sound spikes in order to facilitate splitting of the
spike tips.
BRIEF DESCRIPTION OF THE-DRAWINGS
The accompanying drawings form a part of the specification. Throughout the
drawings, like
reference numbers identify like elements.
FIG.1 is a pictorial view of the tube of the present invention.
FIG. 2 is a view illustrating how the tube of the present invention is
manufactured.
FIG. 3 is a plan view of a portion of the external surface of the tube of the
present invention.
FIG. 4 is a plan view of a portion a single fin convolution of the tube of the
present inven-
tion.
FIG. 5 is a generic sectioned elevation view of two adjacent fin convolutions
of the tube of
the present invention. ..
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG.1 is a pictorial view of heat transfer tube 10. Tube 10 comprises tube
wall 11, tube
inner surface 12 and tube outer surface I3. Extending from the outer surface
of tube wall 11
are external fin spikes 22. Tube 10 has outer diameter Do as measured from
tube outer
surface 13 including the height (H~ of fm spikes 22 as shown in FTGS.1 and 2.
The tube of the present invention may be readily manufactured by a rolling
process. FIG. 2
illustrates such a process. In FIG. 2, finning machine 60 is operating on tube
10, which is
made of a malleable metal such as copper, to produce both interior ribs and
exterior fins on
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the tube. Fimung machine 60 has one or more tool arbors 61, each containing a
tool gang 62,
comprised of a number of finning disks 63, notching disk 66 and splitting disk
67. Extending
into the tube i.s mandrel shaft 65 to which is attached mandrel 64.
Wall I1 is pressed between mandrel 64 and finning disks 63 as tube 10 rotates.
Under
pressure, metal flows into the grooves between the finning disks and forms a
ridge or fin on
the exterior surface of the tube. As it rotates, tube 10 advances between
mandrel 64 and tool
gang 62 (from left to right in FIG. 2) resulting in a number of helical fin
convolutions being
formed on the; tube. The number of convolutions is a function of the number of
finning disks
63 in tool gang 62 and the number of tool arbors 61 in use on finning machine
60. In the
same pass and just after tool gang 62 forms fin convolutions on tube 10,
notching wheel 66
impresses oblique notches in to the metal of the fin convolutions. Following
formation of the
oblique notches, splitting disk 67 splits the tip of each fin convolution into
two portions.
Mandrel 64 may be configured in such a way, as shown in FIG. 2, that it will
impress some
type of pattern into the internal surface 12 of the wall of the tube passing
over it. A typical
pattern is of one or more helical rib convolutions. Such a pattern can improve
the rate of heat
transfer between the fluid flowing through the tube and the tube wall.
FIG. 3 shows, in plan view, a portion of the external surface of the tube.
Extending from
outer surface 13 of tube 10 are a number of fin convolutions 20. Extending
obliquely across
each fin convolution at intervals are a pattern of notches 30. Between each
pair of adjacent
notches in a given fin convolution is a fin spike 22 having two distal tips
23.
FIG. 4 is a plan view of a portion of a single fin convolution of the tube of
the present
invention. The angle of inclination of notch base 31 from tube longitudinal
axis AT is angle
ac. The angle of inclination of the distal tip 23 of fin 22 from longitudinal
axis of the tube AT
is angle ~3. During manufacture of the tube (see FIG. 2), the interaction
between rotating and
advancing tube 10 and notching wheel 66, may result in the axis of fin spike
22, indicated in
FIG. 4, is turned slightly from the angle between the teeth of the notching
wheel and the fin
convolution ::o that tip axis angle (3 is oblique with respect to angle a,
i.e., (3 ~ oc. However, it
is possible to have ~i = oc as a specific case. It is this turning of the
spike that allows the
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splitting disk 67 to reliably split the spike because the notched spike
presents a wider face for
splitting than would the unnotched fin convolution.
It has been found that if tie angle of the notching wheel is greater than
4U° and the spacing
between adjacent teeth on the notching wheel is less than 0.0125 each, the
spikes will be
caused to twist. The twisting of the spikes enables the splitting of the
spikes to be done more
efficiently. Specifically, without the twisting, the fin tip thickness would
be too small to
reliably split the spikes. With the twist the shape of the spikes after
notching and just before
splitting is essentially a parallelogram. After splitting the parallelogram is
split along its
diagonal to create two triangles.
FIG. 5 is a-pseudo sectioned elevation view of two adjacent fin convolutions
of the tube of
the present invention. The term pseudo.is used because it is~unlikely that a
section taken
through any part of the fin convolutions would look exactly as the section
depicted in FIG. 5.
The figure, however, serves to illustrate many of the features of the tube.
Fin convolu-
tions 20A and 20B extend outward from tube wall IZ. Fin convolutions 20A and
20B have
proximal portions 21 and spike portions 22. Extending through fin convolution
20A is a
.notch having notch base 31. The overall height of fin convolutions 20A and
20B is Ht. The
width of proximal portion 21 is Wr and the width of spike portion 22 at its
widest dimension
is Wt. The outer extremity of spike 22 has two distal tips 23. The notch
penetrates into the
fin convolution to height H" above outer wall surface 13.
It should be understood that notching wheel 66 (FIG. Z) does not cut notches
out of the fin
convolutions during the manufacturing process but rather impresses notches
into the fin
convolutions by displacing material from the notched area. The excess material
from the
notched portion of the fin convolution moves both into the region between
adjacent notches
and outwardly from the sides of the fin convolution as well as toward tube
wall 11 on the .
sides of the fin convolution . As a result, W~ is greater than Wr: The
distance between
similarpoints_on.adjacentfin_convolutions, nr~pitchis.pf. The
anglebet<x~the.t~co. _ _
distal tips 23 on a spike portion 22, or split angle, is angle b. A distal tip
extending from one
side of a fin convolution extends toward the adjacent fin convolution on that
side leaving gap
g between tips.
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7
The relatively large number of sharp distal tips promote condensation on the
surface of the
tube when the tube is used in a condensing application. Because the distal
tips overlie the
volume between adjacent fin convolutions, a reentrant cavity is formed and
thus forms a tube
surface that promotes evaporation.
We have tested two families of prototype tubes made according to the teaching
of the present
invention using refrigerant R-134a. The pertinent parameters of the two
prototypes are:
Protot~rpe Family A --
nominal outer diameter (Do) -- 1.9 cm (3/4 inch),
fin pitch (Pf) -- 0.6 mm (0.024 inch) or 16.5 fins per cm (42 fins per inch),
fin height (Hf)-- 0.79 mm (0.031 inch),
notch base height (H") -- 0.58 mm (0.023 inch),
notch angle (a) -- 50 degrees, 30 degrees, 45 degrees
split angle (b) -- 70 degrees, 90 degrees, 110 degrees
notch density, or number of notches in a fin convolution per tube
circumference --
80, 140.
Protoh,~pe Family B --
nominal outer diameter (Do) -- 1.9 cm (3/4 inch),
fin pitch (P f) -- 0.45 mm (0.018 inch) or 22 fins per cm (56 fins per inch),
fin height (Hf)-- 0.58 mm (0.024 inctl),
notch base height (Hn) -- 0.35 mm (0.014 inch),
notch angle (a) -- 50 degrees
split angle (8) -- 90 degrees, and
notch density, or number of notches in a fin convolution per tube
circumference --
14~0.
We compared the performance of the two prototypes to the performance of a tube
having a
smooth external surface over a range of heat flux conditions. In an
evaporation application,
the performance of Prototype Family A is an average of about 2.5 times that of
the smooth
tube and the performance of Prototype Family B is about 3 times the smooth
tube perform-
ance. In a condensing application, the performance of Prototype Family A is an
average of
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8
about 19 timers that of the smooth tube and the performance of Prototype
Family B is about
23 times the .smooth tube performance.
Extrapolations from test data indicate that comparable performance will be
obtained in tubes
having nominal 12.5 millimeter ( 1/2 inch) to 25 millimeter ( 1 inch) outer
diameters where:
a) the fin pitch is 0.038 to 0.76 millimeter (0.015 to 0.030 inch), or
0.038 mm <_ Pf <_ 0.76 mm (0.015 inch <_ Pf <_ 0.030 inch);
b) the ratio of fin height to tube outer diameter is between 0.026 and 0.067,
or
0.026 <_ Hf / Do <_ 0.067;
c) the notch density is 60 to 190;
d) the angle between the notch axis and the tube longitudinal axis is between
20 and 65 degrees, or
20° <_ a. <_ 65°
e) the height of the notch base is between 0.50 and 0.8 of the fin height or
0.50<_:Hn/Hf_<0.8and
f) the angle between the two distal tips on a spike is between 70 and 130 de-
grees, or
70° _< b _< 130°.
The tested prototypes have three convolutions or "starts." The optimum number
of fin
convolutions or start depends more on considerations of ease of manufacture
than upon the
effect of the number on heat transfer performance. A higher number of starts
increases the
rate at which the fin convolutions can be formed on the tube surface.
