U.S. patent application number 12/599930 was filed with the patent office on 2011-01-06 for indirect heat exchange device and method of exchanging heat.
Invention is credited to Dominicus Fredericus Mulder.
Application Number | 20110000650 12/599930 |
Document ID | / |
Family ID | 38626912 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110000650 |
Kind Code |
A1 |
Mulder; Dominicus
Fredericus |
January 6, 2011 |
INDIRECT HEAT EXCHANGE DEVICE AND METHOD OF EXCHANGING HEAT
Abstract
Indirect heat exchange device comprising heat exchanger tubes
arranged in at least 2 layers each of which layers comprises at
least 2 heat exchanger tubes wherein the heat exchanger tubes are
eccentrically finned (53) heat exchanger tubes having a ratio of
surface area of the fins to surface area of the tube of at least 5,
and in which device the heat exchanger tubes have similar
eccentricity; and method of exchanging heat with the help of such
device.
Inventors: |
Mulder; Dominicus Fredericus;
(Amsterdam, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
38626912 |
Appl. No.: |
12/599930 |
Filed: |
May 14, 2008 |
PCT Filed: |
May 14, 2008 |
PCT NO: |
PCT/EP2008/056036 |
371 Date: |
August 30, 2010 |
Current U.S.
Class: |
165/104.34 ;
165/181 |
Current CPC
Class: |
F28B 1/06 20130101; F28F
1/24 20130101; F28F 1/36 20130101 |
Class at
Publication: |
165/104.34 ;
165/181 |
International
Class: |
F28D 15/00 20060101
F28D015/00; F28F 1/10 20060101 F28F001/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 2007 |
EP |
07108131.9 |
Claims
1. Indirect heat exchange device comprising heat exchanger tubes
arranged in at least 2 layers each of which layers comprises at
least 2 heat exchanger tubes wherein the heat exchanger tubes are
eccentrically finned heat exchanger tubes having a ratio of surface
area of the fins to surface area of the tube of at least 5, and in
which device the heat exchanger tubes have similar
eccentricity.
2. Indirect heat exchange device according to claim 1, wherein the
ratio of surface area of the fins to surface area of the tube is at
least 7.
3. Indirect heat exchange device according to claim 2, wherein the
direction of the eccentricity is parallel to the direction in which
fluid outside the tube normally flows.
4. Indirect heat exchange device according to claim 3 further
comprising a fan having a blow or suck direction across the heat
exchanger tubes and defining an upstream side of the heat exchanger
tubes, wherein the geometric centroid of the cross-section of the
envelope defined by the fins of the heat exchanger tubes is
arranged upstream from the axis of the tubes.
5. The indirect heat exchange device according to claim 3 arranged
in a heater having flow direction of heating fluid across the heat
exchanger tubes and defining a downstream side of the heat
exchanger tubes, wherein the geometric centroid of the
cross-section of the envelope defined by the fins is arranged
downstream from the axis of the tubes.
6. A method of exchanging heat between a first fluid and a second
fluid, the method comprising: providing a indirect heat exchange
device according to any one of claim 1; passing first fluid through
the heat exchanger tubes of the device; and passing second fluid
along a flow direction across the indirect heat exchange device,
wherein the eccentricity of the heat exchanger tubes is parallel to
the flow direction of second fluid.
7. The method according to claim 6, wherein the first fluid is a
cooling fluid, in particular air, and wherein the geometric
centroid of the fins of each of the heat exchanger tubes is
arranged upstream from the axis of the tubes.
8. The method according to claim 6, wherein the first fluid is a
heating fluid, in particular comprising combustion products, and
wherein the geometric centroid of the fins of each of the heat
exchanger tubes is arranged downstream from the axis of the
tubes.
9. The method according to claim 8, wherein the upstream and
downstream differential temperatures at the tip of the fins of the
heat exchanger tubes are substantially equal.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an indirect heat exchange
device comprising finned heat exchanger tubes and to a method of
exchanging heat between a first fluid and a second fluid.
BACKGROUND OF THE INVENTION
[0002] Finned heat exchanger tubes can be used in indirect heat
exchange devices wherein a first fluid, which is passed through the
interior of the finned tubes, can exchange heat with a second fluid
outside the tubes.
[0003] For a variety of reasons, the geometric centroid of the
cross-section of the envelope defined by the fins of a heat
exchanger tube sometimes does not coincide with the axis of the
tube. DE-A-1451143 describes an indirect heat exchange device of
which the fins of the outer heat exchanger tubes contain additions
to shade these outer fins from the sun or other sources of heat.
GB-A-281,289 describes finned heat exchanger tubes of which the
fins are arranged eccentric relatively to the centre of the tube,
which tubes are arranged in layers having opposite eccentricity in
order to force gases to take a sinuous path in order to increase
the efficiency of the apparatus. U.S. Pat. No. 4,002,198 describes
a desublimator for isolating sublimation products comprising finned
tubes intended to be alternately subjected from the inside to a
heating medium and a coolant, the transverse fins of which tubes
are arranged in rows staggered laterally in opposite directions by
an amount corresponding to the whole spacing between adjacent fin
edges to provide additional turbulence surfaces causing greater
pressure drops. U.S. Pat. No. 4,440,216 teaches to foreshorten the
fins at the top of a heat exchanger tube in order for liquid to be
more uniformly distributed over the tubes. The fins of liquid
treated heat exchanger tubes have a relatively small surface area,
i.e. the ratio of surface area of fins to surface area of the tube
will be substantially less than 5.
[0004] Finned heat exchanger tubes may in particular be used in
air-cooled heat exchanger devices, wherein the fluid outside the
tubes is air.
[0005] Air-cooled heat exchangers can also be referred to as air
coolers. Air coolers are described in Perry's Chemical Engineers'
handbook, 7th edition, 1997, pages 11-47 to 11-52. Air coolers are
for example used in refinery, petrochemical and chemical processes
to cool or condense process fluids inside the tube with air outside
the tube. Air coolers typically include a bundle of finned tubes,
and a fan, which fan moves air across the tubes.
[0006] In air-cooled heat exchangers, the heat transfer from the
fluid inside the tube to the tube itself is typically much more
efficient than the heat transfer between the fluid outside the tube
(air) and the tube itself. The efficiency of heat transfer can for
example be expressed by a so-called film coefficient as defined in
Perry's, pages 5-12 to 5-19. In order to compensate for a
difference in film coefficients, the external surface area of the
heat exchanger tube is increased by means of fins, so that the
product of film coefficient and surface area inside and outside of
the heat exchanger tube is of the same order of magnitude.
[0007] Heat transfer due to free convection can be described by the
following equation:
Q t = hA ( T W - T .infin. ) ##EQU00001##
The rate dQ/dt of heat exchanged Q (also referred to as duty) with
the surrounding fluid is proportional to the object's exposed area
A, and the difference between the object temperature T.sub.w and
the fluid free-stream temperature T.sub..infin.. The constant of
proportionality h is termed the convection heat-transfer
coefficient, also referred to as film coefficient [units
W/(m.sup.2.K)].
[0008] The flow of fluid outside the tube is typically induced by a
fan. The higher the air velocity, the higher the heat transfer
coefficient and the higher the duty. However, the air velocity is
often limited, such as by the maximum noise level of a fan, e.g. 80
dBa. For a given fan rotating at a certain speed, the air velocity
across a bundle of finned tubes is determined by the static
pressure drop (resistance) of the bundle. A higher air velocity
will be achieved if the pressure drop (resistance) is lower.
[0009] Finned tubes are also employed in heaters or furnaces, such
as fired heaters, for improving the heat transfer from the heating
fluid surrounding the tubes to fluid that is flowing inside the
tubes. It has been observed that coking of fluid inside heat
exchanger tubes occurs preferentially at the upstream (upwind) side
of the flow of fluid outside the tubes, for example in heaters for
crude oil entering a crude distillation unit. Typically the heating
fluid is combustion gas from the combustion of a fuel, rising
upwardly in a heater.
[0010] It is desired to increase the efficiency of heat transfer in
heat exchange devices comprising finned heat exchanger tubes.
SUMMARY OF THE INVENTION
[0011] To this end there is provided an indirect heat exchange
device comprising heat exchanger tubes arranged in at least 2
layers each of which layers comprises at least 2 heat exchanger
tubes wherein the heat exchanger tubes are eccentrically finned
heat exchanger tubes having a ratio of surface area of the fins to
surface area of the tube of at least 5, and in which device the
heat exchanger tubes have similar eccentricity.
[0012] Finned heat exchanger tubes have an axis and are provided
with fins, the fins defining an envelope having a cross-section,
wherein the cross-section of the envelope has a geometric centroid.
In eccentrically finned heat exchangers, this geometric centroid is
spaced apart from the axis of the tube.
[0013] The geometric centroid of an area, such as of the
cross-section of the envelope of the fins, is similar to the center
of mass of a body. Calculating the centroid is based on the
geometrical shape of the area. Cartesian co-ordinates C.sub.x,
C.sub.y of the geometric centroid can for example be determined by
integration over the area A, C.sub.x=.intg.x dA/A, C.sub.y=.intg.y
dA/A, A=.intg. dA.
[0014] The axis of the tube is the longitudinal axis of the
interior of the tube.
[0015] Eccentricity is defined as the spacing, both in magnitude
and direction, between the axis of the tube and the geometrical
centroid of the envelope of the tube as positioned in the device.
Finned heat exchanger tubes of similar eccentricity in the heat
exchange device are finned tubes having an eccentricity which is
the same both in magnitude and in direction for their position in
the heat exchange device. The influence of the position in the
device on the eccentricity of a tube is clear from FIGS. 1, 3 and 4
of GB-A-281,289 where the eccentriciy of tubes in adjacent layers
is opposite in direction due to the different position of tubes in
adjacent layers.
[0016] In the indirect heat exchange device according to the
present invention, most, preferably all, finned heat exchanger
tubes of the device have a similar eccentricity. Preferably, the
finned heat exchanger tubes of the device according to the present
invention have the same eccentricity both in magnitude and
direction.
[0017] It is preferred that the direction of the eccentricity of
the heat exchanger tubes of the device is parallel, i.e. either the
same or opposite in direction, to the direction in which fluid
outside the tubes normally flows.
[0018] A substantial part of the static pressure drop due to a
finned heat exchanger tube is caused by the fins. It has now been
found that the effectiveness of finning with respect to heat
transfer is higher at the upstream side of the tube than at the
downstream side. In the description and in the claims, the upstream
(also referred to as upwind) side is the side at which the fluid
flow direction outside the tubes is towards the finned tubes, and
at the downstream (downwind) side the fluid flow outside the tubes
is away from the tubes.
[0019] The different effectiveness can be observed for conventional
concentric circular fins in that the temperature of the tips of
such fins is lower on the upstream side than on the downstream
side. The difference in temperature between the fin tip and the
fluid surrounding the fin tip, hereinafter referred to as the
differential temperature, is also higher for the fins at the
upstream side of such conventional heat exchanger tubes. For this
reason it is advantageous to arrange the finning eccentrically on
the tubes, or in other words, to use non-concentric fins. It will
be clear that the difference in effectiveness is more pronounced
for heat exchanger tubes having a relatively high surface area,
i.e. having a ratio of surface area of the fins to surface area of
the tube of more than 5, more specifically at least 6, more
specifically at least 7, more specifically at least 8, more
specifically at least 9, and most specifically at least 10. It is
especially preferred for the indirect heat exchange devices of the
present invention to contain such high surface area heat exchanger
tubes. The ratio of surface area of the fins to surface area of the
tube preferably is at most 25. The surface area of the fins is the
surface area of the fins to be in contact with the fluid outside
the tube while the surface area of the tube is the surface area of
the tube in contact with the fluid inside the tube.
[0020] A particular phenomenon in heat transfer by finned tubes is
recirculation, i.e. eddies formed in the fluid at the downstream
side, which hamper efficient heat transfer. This effect is also
minimized by having the larger part of the fin surface at the
upstream side. By proper design for a particular application it can
be achieved that the upstream and downstream differential
temperatures at the tips of the fins of the heat exchanger tubes
are substantially equal.
[0021] The fin can have any suitable shape such as circular,
elliptical, oval, polygonal, or egg-shaped (i.e. roughly oval with
somewhat different radii at the tips; the larger radius can
suitably be arranged at the downstream side). An elliptical shape
has shown good results.
[0022] Because the heat transfer is optimised, less finning is
required to achieve the same duty. Moreover, if less finning is
used, the static pressure drop over a bundle will reduce so that
the maximum air velocity for a given fan capacity will increase, so
that the overall duty can be increased.
[0023] The indirect heat exchange device according to the present
invention comprises at least 2 layers, preferably at least 3
layers, more preferably at least 4 layers of heat exchanger tubes.
Preferably, the number of layers is at most 10, more preferably at
most 9. Further, each layer comprises at least 2, more preferably
at least 3, more preferably at least 4 heat exchanger tubes. The
number of layers and the number of tubes is the number of times the
tube is present independent from whether the tubes are connected to
each other such as via a tube bend.
[0024] The heat exchanger tubes in adjacent layers are preferably
arranged staggered with respect to each other while the tubes in
the device still have similar eccentricity.
[0025] The heat exchange device according to the present invention
can further comprise a fan having a blow or suck direction across
the heat exchanger tubes and defining an upstream side of the heat
exchanger tubes, and wherein the geometric centroid of the
cross-section of the envelope defined by the fins is arranged
upstream from the axis of the tube.
[0026] The problem of preferential coking in a heater can also be
solved with the help of the heat exchange device according to the
present invention. According to the present invention, the
geometric centroid of the cross-section of the envelope defined by
the fins preferably is arranged downstream from the axis of the
tubes with respect to the direction of heating fluid flow across
the heat exchange device (typically the upper side). Accordingly,
in a particular aspect the invention provides an indirect heat
exchange device arranged in a heater having flow direction of
heating fluid across the heat exchange device and defining a
downstream side of the heat exchanger tubes of the device, and
wherein the geometric centroid of cross-section of the envelope
defined by the fins is arranged downstream from the axis of the
tubes. In this way a more equal heat transfer around the
circumference of the tube is achieved, so that temperature
differences at the inner wall between the upstream and downstream
sides are minimized. This will suppress preferential coking at the
upstream side within the tubes.
[0027] In the description and in the claims the expression "the
geometric centroid of the cross-section of the envelope defined by
the fins is arranged upstream (or downstream) from the axis of the
tube" refers to a position of the geometric centroid in a plane
parallel to a plane through the tube axis and perpendicular to the
direction of the flow of fluid outside the tubes, and which plane
is more upstream (or more downstream) than the plane through the
tube axis, respectively. In advantageous embodiments the geometric
centroid is in an upstream (or downstream) position along the
direction of fluid flow outside the tubes with respect to the axis
of the tube.
[0028] The invention also provides the use of the indirect heat
exchange device according to the invention for exchanging heat
between a first fluid inside the tubes and a second fluid outside
the tubes. Accordingly, the invention provides a method of
exchanging heat between a first fluid and a second fluid, the
method comprising
[0029] providing an indirect heat exchange device according to the
invention;
[0030] passing first fluid through the heat exchanger tubes of the
device;
[0031] passing second fluid along a flow direction across the heat
exchanger tube, wherein the direction of the eccentricity of the
heat exchanger tubes is parallel to the flow of direction of the
second fluid.
[0032] Preferably, the upstream and downstream differential
temperatures, as defined above and with respect to the flow of
fluid outside the tubes, at the tip of the fins of the heat
exchanger tubes are substantially equal during use in the method
according to the invention.
[0033] The second fluid can be gas optionally in combination with a
limited amount of liquid. Preferably, the second fluid is gas
only.
[0034] When the second fluid is a cooling fluid, in particular air,
the geometric centroid is preferably arranged upstream from the
axis of the tube. When the second fluid is a heating fluid, in
particular comprising combustion products, the geometric centroid
is preferably arranged downstream from the axis of the tube.
[0035] Heat exchanger tubes for use in the device according to the
present invention can be manufactured in many different ways. A
suitable method of manufacturing comprises
[0036] providing a tube having an outer surface and a
circumference;
[0037] providing an elongated strip of fin material having a length
direction, the strip having a straight side along its length
direction, and a side opposite the straight side, wherein the width
of the strip varies along the length direction defining maxima and
minima, wherein the maxima are spaced apart in length direction
substantially by the circumference of the tube;
[0038] spirally winding the strip around the tube so that the
straight side is attached to the outer surface of the tube.
[0039] Using this method a finned heat exchanger tube can be
obtained, which has an eccentric envelope with respect to the axis
of the tube, wherein the geometric centroid of cross-sections of
the envelope extends a line parallel to the longitudinal axis of
the tube. The elongated strip can be efficiently manufactured by
cutting from an elongated strip with parallel straight sides, so
that two elongated strips are obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] The invention will now be described in more detail and with
reference to the accompanying drawings, wherein
[0041] FIG. 1 shows schematically a conventional finned heat
exchanger tube in perspective view;
[0042] FIG. 2 shows schematically the conventional finned heat
exchanger tube of FIG. 1 in transverse cross-section;
[0043] FIG. 3 shows schematically a first embodiment of a finned
heat exchanger tube for use in a device according to the invention
in transverse cross-section;
[0044] FIG. 4 shows schematically a second embodiment of a finned
heat exchanger tube for use in a device according to the invention
in transverse cross-section;
[0045] FIG. 5 shows schematically a indirect heat exchange device
and a fan according to the invention.
[0046] Where the same reference numerals are used in different
Figures, they refer to the same or similar objects.
DETAILED DESCRIPTION OF THE INVENTION
[0047] Reference is made to FIG. 1, showing schematically a
conventional finned heat exchanger tube 1. The tube is provided
with fins 3 of circular cross-section. The fins are obtained by
helically winding a strip of metal around the inner tube 5. The
fins define an envelope 7 having a circular cross-section 8. The
geometric centroid of the circle 8 is in the centre 9, which
coincides in this case with the longitudinal axis 10 of the tube 1.
The conventional finned heat exchanger tube 1 is shown in
transverse cross-section in FIG. 2.
[0048] Reference is now made to FIG. 3, showing schematically a
finned heat exchanger tube 21 for use in a device according to the
invention. The tube is provided with fins 23 defining an envelope
24 of elliptical cross-section 25, eccentrically with respect to
the longitudinal axis 30 of the tube 21. I.e., the geometric
centroid 31, which is at the cross section of the major and minor
axes 32,33 of the ellipse, is spaced apart from the axis 30.
[0049] FIG. 4 shows schematically another embodiment of a finned
heat exchanger tube 41 for use in a device according to the
invention. Here the fins 43 define an envelope 44 of circular cross
section 45. The centre 46 of the circle 45 is spaced apart from the
longitudinal axis 50 of the tube 41.
[0050] Reference is now made to FIG. 5 showing schematically a
device 51 according to the invention comprising eccentrically
finned heat exchanger tubes 53, in an assembly 54 with a fan 55,
for example to form an air-cooled heat exchanger. The device in
this example comprises 4 layers of tubes when viewed along the blow
direction 58 of the fan 55, each of which layer comprises 3 or 4
heat exchanger tubes. Each tube has an upstream side 60 and a
downstream side 61, wherein the upstream side is closer to the fan
55 than the downstream side in the case of a fan that blows. The
finned tubes 53 are eccentric elliptical as discussed with
reference to FIG. 3.
[0051] During operation of the assembly 54, a first fluid is passed
through the interior 62 of the tubes 53, and the fan blows second
fluid (e.g. air) across the tubes along the blow direction 58, so
as to exchange heat between the first and second fluids, e.g. to
cool the first fluid against air.
[0052] The elliptical fins are non-concentrically arranged such
that the geometric centroid of their envelope is below the axis of
the tubes in FIG. 5, at the side of the blowing fan.
[0053] Computational Fluid Dynamics calculations have been
performed, in order to compare the heat transfer duty and pressure
drop of a four layer bank of finned heat exchanger tubes according
to the invention with an analogous arrangement of conventional
circular finned tubes. The calculations were performed using a
so-called EFD. Lab software package.
[0054] The model assumes copper tube cores with aluminium fins. The
tube core has a fixed temperature of 100.degree. C. The ambient
temperature of the air is 30.degree. C. The tubes are in cross
flow, with an ambient air velocity of 4 m/s. The following
parameters were used in the calculations.
Finned tube dimensions (all examples): Bare inner tube outer
diameter: 25.4 mm Fin thickness: 0.4 mm Fin pitch (10 fins/inch):
2.54 mm Fin spacing: 2.14 mm Ratio of surface area of fins to
surface area of tube: 20 Bank dimensions: Tube pitch: 63 mm Stagger
angle: 60 degrees 4 layers each comprising several tubes
Example 1
[0055] The device according to the invention comprised ellipsoid
and eccentrically finned tubes.
Major diameter: 74.4 mm Minor diameter: 42.98 mm Minimum fin
height: 10 mm Maximum fin height: 39 mm Magnitude of eccentricity:
15 mm
Comparative Example 2
[0056] The device not according to the invention comprised
conventional concentric circular finned tubes.
Fin height: 15.88 mm Outer diameter of fin envelope: 57.15 mm
[0057] In Example 1, a duty of 1366.9 W per meter length of the
finned tube was obtained, at a pressure drop of 101.5 Pa. In the
Comparative Example 2, the duty was somewhat higher, 1505.5 W/m,
but at a much higher pressure drop namely 132.0 Pa. The ratio of
duty to pressure drop was 18% higher in the Example 1 according to
the invention.
[0058] The embodiment of a heater wherein preferential coking is to
be suppressed would be similar to FIG. 5, but instead of the fan a
burner would be arranged, and the elliptical fins would be arranged
with the geometric centroid of their envelope above the axis of the
tubes in FIG. 5, away from the burner.
* * * * *