U.S. patent application number 13/674162 was filed with the patent office on 2014-05-15 for condensing air preheater with heat pipes.
This patent application is currently assigned to EXXONMOBIL RESEARCH AND ENGINEERING COMPANY. The applicant listed for this patent is EXXONMOBIL RESEARCH AND ENGINEERING COMPANY. Invention is credited to Scott C. CHAPMAN, Hyungsik LEE, Brandon J. PENCE.
Application Number | 20140131010 13/674162 |
Document ID | / |
Family ID | 50680546 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140131010 |
Kind Code |
A1 |
LEE; Hyungsik ; et
al. |
May 15, 2014 |
CONDENSING AIR PREHEATER WITH HEAT PIPES
Abstract
An air preheater comprises flow conduits for heat transfer
between a heating gas and combustion air which is to be pre-heated
with heat pipes extending between the flow conduits for the two gas
streams to provide heat transfer The fins on the heat pipes in at
least the condensation zone of the flow conduit of the heating gas
are provided with one or more serrations on the sides of the pipes
facing away from the oncoming gas flow. The serrations assist in
removing any liquid film which may accumulate on the fins during
operation in the condensing mode. A corrosion resistant coating
material such as glass may be provided on the condensing side of
the finned heat pipes.
Inventors: |
LEE; Hyungsik; (Fairfax,
VA) ; PENCE; Brandon J.; (Fairfax, VA) ;
CHAPMAN; Scott C.; (Fairfax, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EXXONMOBIL RESEARCH AND ENGINEERING COMPANY |
Annandale |
NJ |
US |
|
|
Assignee: |
EXXONMOBIL RESEARCH AND ENGINEERING
COMPANY
Annandale
NJ
|
Family ID: |
50680546 |
Appl. No.: |
13/674162 |
Filed: |
November 12, 2012 |
Current U.S.
Class: |
165/104.15 ;
165/104.21; 431/215 |
Current CPC
Class: |
Y02E 20/34 20130101;
F23L 15/045 20130101; F28D 15/0275 20130101; F28D 15/02 20130101;
Y02E 20/348 20130101; F28F 1/36 20130101 |
Class at
Publication: |
165/104.15 ;
431/215; 165/104.21 |
International
Class: |
F23L 15/04 20060101
F23L015/04; F28D 15/02 20060101 F28D015/02 |
Claims
1. An air preheater comprising: a first flow conduit for a heating
gas, a second flow conduit in heat transfer communication with the
first flow conduit for a gas to be heated, heat pipes extending
from the first flow conduit into the second flow conduit, the heat
pipes having: an evaporation zone located in the first flow
conduit, a condensation zone located in the second conduit, and
fins on the heat pipes which extend out from the heat pipes into
the respective flow conduits, the fins on the heat pipes in at
least the first flow conduit having at least one serration in the
fins on the sides of the pipes facing away from the oncoming gas
flow.
2. An air preheater according to claim 1 in which the fins on the
heat pipes in at least the first flow conduit have a plurality of
serrations on the sides of the pipes facing away from the oncoming
gas flow.
3. An air preheater according to claim 1 in which the fins on the
heat pipes in at least the first flow conduit have two or three
serrations on the sides of the pipes facing away from the oncoming
gas flow.
4. An air preheater according to claim 1 in which the fins on the
heat pipes in at least the first flow conduit have a plurality of
serrations extending around the entire periphery of the pipes.
5. An air preheater according to claim 1 in which serrated fins
extend out from the heat pipes into both the first and second flow
conduits, with at least one serration in the fins on the sides of
the pipes facing away from the oncoming gas flow.
6. An air preheater comprising: a first flow conduit for a heating
gas comprising a vapor component which is susceptible to
condensation upon cooling in the preheater, the first flow conduit
in the preheater having a condensation region in which condensation
of the vapor component of the heating gas is likely to occur; a
second flow conduit in heat transfer communication with the first
flow conduit for a gas to be heated, heat pipes extending from the
first flow conduit into the second flow conduit, the heat pipes
having: an evaporation zone located in the first flow conduit, a
condensation zone located in the second conduit, and fins on the
heat pipes which extend out from the heat pipes into the respective
flow conduits, the fins on the heat pipes in at least the first
flow conduit having at least one serration in the fins on the sides
of the pipes facing away from the oncoming gas flow.
7. An air preheater according to claim 6 in which the fins on the
heat pipes in the condensation region of the first flow conduit in
have a plurality of serrations on the sides of the pipes facing
away from the oncoming gas flow.
8. An air preheater according to claim 7 in which the fins on the
heat pipes in the condensation region of the first flow conduit
have two or three serrations on the sides of the pipes facing away
from the oncoming gas flow.
9. An air preheater according to claim 6 in which only the fins on
the heat pipes in the condensation region of the first flow conduit
in have a plurality of serrations on the sides of the pipes facing
away from the oncoming gas flow.
10. An air preheater according to claim 9 in which only the fins on
the heat pipes in the condensation region of the first flow conduit
have two or three serrations on the sides of the pipes facing away
from the oncoming gas flow.
11. An air preheater according to claim 6 in which the fins on the
heat pipes in the second flow conduit have a plurality of
serrations extending around the entire periphery of the pipes.
12. An air preheater according to claim 6 in which serrated fins
extend out from the heat pipes into both the first and second flow
conduits, with at least one serration in the fins on the sides of
the pipes facing away from the oncoming gas flow.
13. An air preheater according to claim 6 in which the fins are
helical fins on the heat pipes.
14. An air preheater according to claim 11 in which the fins are
helical fins on the heat pipes.
15. An air preheater according to claim 12 in which the fins are
helical fins on the heat pipes.
Description
[0001] This invention concerns a condensing air preheater system
that utilizes finned heat pipes for effecting the heat
transfer.
BACKGROUND OF THE INVENTION
[0002] Waste heat recovery units are used to capture additional
heat from the flue gas as it leaves boilers and fired heaters. Air
preheaters and economizers comprise one type of heat recovery unit
and have the capability to increase the capture of the latent heat
in the flue gas but their increased heat recovery potential is
limited by working conditions which restrict condensation of the
flue gas.
[0003] Preheaters such as these may operate in condensing or
non-condensing mode. In the condensing mode, the heating fluid is
initially in the vapor state and during its passage through the
exchanger but when it gives up heat to the cooler combustion air,
condensation from the vapor state takes place forming a liquid
phase in the stream. A typical instance of this is with heat
exchangers using flue gases from a combustion process in a furnace
or a boiler as the heat source; as these exhaust streams will
contain not only nitrogen from the original combustion air and
carbon dioxide from the combustion process but also water from the
combustion of hydrocarbons, the liquid phase which forms on cooling
will be water which will form on and around the heat exchange
elements in the exchanger. Not only does this liquid phase impede
good heat transfer but since acid components including carbon
dioxide and, often, sulfur oxides, it is also frequently corrosive
towards the metal components of the exchanger.
[0004] Condensing air preheaters and economizers using a heat
transfer surface to condense water vapor from the flue gases are
inherently more efficient in their potential to realize an enhanced
recovery of the waste heat. While there are inherent advantages
with these heat exchangers, they come at a price as the condensate
is usually laden with corrosive constituents such as sulfuric acid.
The design and materials of construction of these types of
equipment is therefore of the utmost importance for optimal
performance and reliability.
[0005] Tubular and heat pipe heat exchangers are examples of this
sort of equipment that operate in a condensing environment. From an
operational perspective, their design is favored by an increased
surface area as this increases the heat transfer potential, i.e.
greater surface area translates into a larger contact surface both
between the flue gas and the transfer components and between the
transfer components and the cool fluid on the other side of the
exchanger; the heat transfer area can be optimally increased with
minimal contributions to the overall size of the heat exchanger by
the use of finned tubes.
[0006] The transfer of heat between the two streams is also
susceptible to improvement by the use of heat pipes extending
between the two sides of the exchanger. Heat pipe air preheaters
consist essentially of a bundle of self-contained heat pipes. Each
heat pipe is partially filled with a working fluid, most commonly
water or hydrocarbon, and sealed. In a typical design, the heat
pipes are arranged in an array of parallel rows, attached at their
respective midpoints to a divider plate which both supports the
pipes and provides a barrier between the flue gas and combustion
air which typically pass in countercurrent flow to maximize heat
transfer between them. Heat from flue gas, for example, evaporates
the working fluid collected in the lower or evaporator end of the
pipe and the vapor flows to the upper or condenser end of the pipe
located in the other half of the exchanger where it gives up heat
to the incoming combustion air. Condensed fluid returns by gravity
to the evaporator end. The process continues indefinitely as long
as there is a temperature difference between the flue gas and the
combustion air.
[0007] Combustion air preheaters using heat pipes to effect the
heat transfer between the hot exhaust gases to the cooler
combustion air are described, for example, in U.S. Pat. No.
5,085,270 (Counterman). This patent describes a preheater which has
a number of finned heat pipes arranged in parallel, superposed rows
with a divider plate providing a barrier between a flue gas stream
and a counterflowing combustion air stream.
[0008] Currently, air preheaters with heat pipes are designed for
non-condensing operation based on two significant considerations.
First, no cost-effective way is available to protect the heat
transfer surface from corrosion from acidic condensate without a
reduction in the heat transfer. The current options for corrosion
protection are to fabricate the heat pipes with expensive corrosion
resistant materials or to cover them with the corrosion resistant
film such as Teflon.TM. or enamel coatings. Employing expensive
corrosion resistant materials is cost-prohibitive. Covering the
surface with a corrosion-resistant coating such as a
perfluorocarbon e.g. Teflon.TM. film makes it impossible to attach
fins outside the heat pipes, which is the most common technique to
enhance heat transfer for the current non-condensing units. Enamel
coatings are being used to protect a part of air preheaters from
cold end corrosion, but are applied to bare tubes, not finned
tubes. Second, eliminating condensate is crucial in improving the
performance of the condensing air preheater but it is difficult to
remove condensate from the finned heat pipe surfaces of current air
preheaters. The liquid condensate layer creates a resistance to
heat transfer due to its low thermal conductivity. With horizontal
heat pipes, a thick condensate layer forms on the lower portion of
tubes and with vertical finned heat pipes, the back side of the
pipe can become flooded by condensate. Consequently, these two
issues need to be resolved if finned heat pipes are to be applied
to condensing air preheaters.
SUMMARY OF THE INVENTION
[0009] We have now developed an improved condensing mode air
preheater with heat pipes in which the heat recovery rate from the
heating gas is substantially improved. Operation in the condensing
mode implies, of course, that the heating gas comprises partly or
exclusively a component which is susceptible to condensation under
the conditions encountered by it in the preheater, i.e. when heat
is transferred out of the gas by the heat pipes into the combustion
air which is being heated. Flue gas is a particular example of such
a heating gas as it contains water vapor from the combustion
process; waste steam may be another. Condensation is most likely to
occur in the condensation region or section at the end of the heat
pipe array where the heating gas has been cooled to its greatest
extent. According to the invention, serrated fins and, optionally,
coatings are used on the heat pipes in the section where
condensation occurs to improve heat transfer and to prevent
corrosion; they may suitably be used on all the pipes in the entire
heat pipe array if desired. The serrations may be imposed
selectively on certain portions of the heat pipe circumference or,
alternatively, around the entire periphery of each pipe. Another
possibility is that fins with selectively located serrations may be
used in certain areas and completely serrated fins in other
locations.
[0010] According to the present invention, the air preheater
comprises: a first flow conduit for a heating gas, a second flow
conduit in heat transfer communication with the first flow conduit
for a gas to be heated, heat pipes extending from the first flow
conduit into the second flow conduit to provide heat transfer
communication, the heat pipes having: an evaporation zone located
in the first flow conduit, a condensation zone located in the
second conduit, and fins extending out from the heat pipes into the
respective flow conduits, the fins on the heat pipes in at least
the first flow conduit having at least one serration on the sides
of the pipes facing away from the oncoming gas flow.
[0011] The heating gas flowing in the first conduit is, as pointed
out above, particularly susceptible to condensation with consequent
formation of liquid films in the region of the conduit where the
heating gas reaches its lowest temperatures, namely, the
condensation section of the conduit. The serrated fins are
primarily located in this region as this is where the problems
described above arise. The serrated fins may be provided
exclusively on the heat pipes located in the condensation section
or, optionally, on all the pipes in the first conduit. The
preheater can therefore be regarded in more detail as comprising: a
first flow conduit for a heating gas comprising a vapor component
which is susceptible to condensation upon cooling in the preheater,
the first flow conduit in the preheater having a condensation
region in which condensation of the vapor component of the heating
gas occurs during operation in the condensing mode; a second flow
conduit in heat transfer communication with the first flow conduit
for a gas to be heated, heat pipes extending from the first flow
conduit into the second flow conduit to provide heat transfer
communication, the heat pipes having: an evaporation zone located
in the first flow conduit, a condensation zone located in the
second conduit, and fins on the heat pipes which extend out from
the heat pipes into the respective flow conduits, the fins on the
heat pipes in at least the first flow conduit having at least one
serration in the fins on the sides of the pipes facing away from
the oncoming gas flow.
[0012] Further features and constructional options are described
below.
DRAWINGS
[0013] FIG. 1 is a vertical section of an air preheater with finned
heat pipes;
[0014] FIG. 2A is a cross section of a heat pipe with a circular
fin with one serration on its downstream side;
[0015] FIG. 2B is a cross section of a heat pipe with a circular
fin with three serrations on its downstream side; and
[0016] FIG. 3 is an isometric drawing of a sectioned heat pipe with
fins serrated around the entire circumference of the pipe.
DETAILED DESCRIPTION
[0017] Heat pipe air preheaters essentially consist of a bundle of
self-contained heat pipes which act to transfer heat from a fluid
providing the heat source to the air which is to be heated as a
heat sink. In a typical design, heat pipes are arranged in an array
with rows of pipes which extend transversely to the directions of
fluid flow in each conduit. A number of transverse rows are
positioned at successive locations along the gas flow paths of the
two respective conduits. The heat pipes are attached at their
respective midpoints to a divider plate which both supports the
pipes and provides a barrier between the flue gas and combustion
air which usually flow in countercurrent to one another to optimize
heat transfer. Each heat pipe is partly filled with a working
fluid, most commonly water or hydrocarbon, and sealed. The warm
flue gas transfers heat to the evaporator ends of the heat pipes to
evaporate the working fluid and the heated vapor flows to the
other, condenser end, where it gives up heat to the incoming
combustion air flowing over the condenser ends of the pipes.
[0018] The basic components of a heat pipe are the container and
the working fluid, optionally with a wick or capillary structure.
The function of the container is to isolate the working fluid from
the outside environment. It has to therefore be capable of
maintaining a pressure differential across its walls and a high
thermal conductivity to enable transfer of heat to take place from
and into the working fluid with minimum temperature drop across the
walls of the container. Typical container materials are copper,
nickel, aluminum and aluminum alloys.
[0019] The working fluid within the container is selected to have a
vapor temperature range appropriate to the intended operations. The
vapor pressure over the operating temperature range must be
sufficiently great to avoid high vapor velocities, which tend to
setup large temperature gradients and cause flow instabilities. The
fluid should exhibit good thermal stability, a vapor pressure not
too high or low over the operating temperature range a high latent
heat, high thermal conductivity, low liquid and vapor viscosities
and acceptable freezing or pour point. A high value of surface
tension is desirable in order to enable the heat pipe to operate
against gravity and to generate a high capillary driving force with
a small contact angle so as to wet the wick (if present) and the
container material. The selection should also be based on
thermodynamic considerations which are concerned with the various
limitations to heat flow occurring within the heat pipe such as
viscous, sonic, capillary, entrainment and nucleate boiling
levels.
[0020] Typical working fluids which may be suitable in the present
air preheater application include, for example, acetone and other
ethers, alcohols such as methanol, ethanol, propanol, butanol,
hydrocarbons such as toluene, perhalocarbons, water, Mercury is
normally excluded for environmental reasons although possibly
otherwise suitable. Liquid metals such as sodium, lithium and
sodium/potassium alloy offer the possibility of high temperature
application but are not usually required in the present
service.
[0021] While a wicking material is not required with the vertical
pipe positioning, it is not excluded although it may create a
complexity in design. The prime purpose of the wick is to generate
capillary pressure to transport the working fluid from the
condenser to the evaporator. It must also be able to distribute the
liquid around the evaporator section to any area where heat is
likely to be received by the heat pipe. These two functions may
require wicks of different forms with the selection of wick
depending on various factors, several of which are linked to the
properties of the working fluid. The maximum capillary head
generated by a wick, for example, increases with decreasing pore
size while wick permeability increases with increasing pore size,
requiring a balance to be struck between these two factors with the
pore sizing appropriate to the selected values. Another feature of
the wick to be optimized, is its thickness with the heat transport
capability of the heat pipe being raised by increasing the wick
thickness.
[0022] Wick material has a porous structure and is typically made
of materials like steel, aluminum, nickel or copper in various
ranges of pore sizes. They are fabricated using metal foams, felts
and sintered powders. Sintered powders provide high thermal
transfer capacity, low temperature gradients and high capillary
forces for anti-gravity applications. Screen mesh is used in many
the products and provides readily variable characteristics in terms
of heat transport and orientation sensitivity, according to the
number of layers and mesh counts used Fibrous ceramics have also
been used although they have little stiffness and usually require a
support by a metal mesh. More recently, interest has turned to
carbon fibers as a wick material. Carbon fiber filaments have many
fine longitudinal grooves on their surface, have high capillary
pressures and are chemically stable. The small capillary driving
force generated by the axial grooves is adequate for low power heat
pipes when operated horizontally, or with gravity assistance. The
tube can be readily bent and when used in conjunction with screen
mesh the performance can be considerably enhanced.
[0023] The heat pipes are preferably arranged vertically with the
evaporator end at the lower location and the condenser at the upper
location so that the fluid which condenses at the cooler end will
return by gravity to the warmer evaporator end. The process
continues indefinitely as long as there is a temperature difference
between the flue gas and the combustion air. Horizontal or inclined
pipes may however be used although their use is less favored as a
wicking material needs to be provided to promote movement of the
condensed material back from the condenser end to the evaporator
end. An inclination of generally 5 to 15 degrees from the
horizontal will offer advantages in efficiency over a completely
horizontal position. The capacity of the individual heat pipe
depends upon several factors, including its inclination angle and
the temperature differential between its ends, increasing both as
the inclination angle and the temperature differential
increase.
[0024] As shown in FIG. 1, the preferred design utilizes vertical
heat pipes with external fins. The preheater 10 has a conduit 11
its lower section through which the high temperature fluid, which
is the flue gas, flows from inlet end 14 to outlet end 16. A
dividing wall 12 separates the lower conduit 11 from the conduit 13
in the upper section of the preheater in which the low temperature
fluid, which is combustion air, flows from inlet end 19 to outlet
end 17 in countercurrent fashion to the flow of the flue gas in the
lower section. Heat pipes 15 are arranged in an array of plural
parallel rows extending across the gas flow paths with successive
rows arranged along the gas flow paths in the conduits. The heat
pipes may be fixed in either the triangular or square
configurations characteristic of heat exchangers in order to
maximize contact between the two gases and the heat pipes. The
pipes are fixed at approximately their midpoint in dividing wall 12
so as to extend from the lower conduit 11 into the upper conduit 13
to effect heat transfer from the warm flue gas to the cool
combustion air. The heat pipes have the construction described
generally above with a working fluid of selected appropriate
characteristics within the container and optionally, a wicking
material. The vertical disposition of the heat pipes may eliminate
the need for the wick material because the condensed working fluid
formed in the upper low temperature condensation zone will fall
gravitationally to the high temperature (lower) zone of the pipe.
The design is then simplified with a reduction in manufacturing
cost. The flue gas passes from the outlet 16 of the lower conduit
11 to stack or to treatment, e.g. scrubbing, before discharge to
stack. The combustion air, heated by its passage through the
preheater, passes from outlet 17 of the upper conduit to the burner
of the furnace, boiler or other fired equipment.
[0025] In its passage through the conduit and past the heat pipes,
the flue gas is cooled as heat is transferred to the heat pipes and
from them, into the combustion air on the other side of the
dividing wall. As the flue gas is progressively cooled by its
passage over the heat pipes, the potential for condensation of the
water in it increases as its temperature decreases with
condensation taking place in the condensation section 18 towards
the outlet end of the lower conduit. It is in this condensation
section of the flue gas conduit that the serrated fin heat pipes
are used to reduce the problems which occur with operation in the
condensing mode. The serrated fins may be used on an exclusive
basis in this section of the conduit or, alternatively, on all the
heat pipes in the array as a whole.
[0026] Circular fins are attached on both the upper 20 and lower
sections 21 of heat pipes 15 and extend out from the body of the
pipe into the gas flow path in the conduit to assist heat transfer
between the two respective fluids and the heat pipes. As shown in
FIG. 2A, a single serration is provided on the downstream (back)
side 25 of fins in the lower section; FIG. 2B shows a section of a
heat pipe with three serrations 26a, 26b, 26c on the back or
downstream side (downstream is determined by reference to the
direction of flow of the gas over the pipe with the downstream
facing away from the direction of the oncoming gas flow as
indicated by arrows 27, 28, respectively). The purpose of the
serrations on the fins is to enable condensate to be removed
efficiently by the gas flow in the lower conduit so enhancing heat
transfer by the removal of the insulating liquid layer on the
finds. These serrations can be fabricated with a milling machine
after attaching the fins to the containers of the heat pipes. Since
the purpose of the serrations is to remove condensation, the
serrated fins may be provided at least or exclusively on the heat
pipes in the area where condensation occurs in the lower conduit
when operating in the condensation mode. There is, however, no
prejudice, apart from cost considerations, to the use of serrations
on all the heat pipes in the lower conduit. Since condensation is
not a problem in the upper conduit, it is not necessary to provide
serrations on the fins in the upper evaporation zone although they
may be provided in that zone to improve heat transfer to the
combustion air by increasing access by the air to the hotter
portion of the fins in the region immediately next to the
containers of the heat pipes. The fins on the tubes typically have
a height (root-to-tip) of about 1.3 cm with an overall thickness of
about 1 mm with 2 fins/cm. These dimensions may be varied according
to individual requirements and manufacturing convenience and cost
although with possible changes to heat transfer efficiency.
[0027] To provide protection against corrosion in the condensation
zone, a corrosion-resisting coating material is applied to the heat
pipe and finned surface in this zone. The coating material may, for
example, by enamel, a perfluorocarbon e.g. Teflon, or a
polyphenylene sulfide (PPS)-based material, e.g. Ryton.TM. (Chevron
Phillips) or Fortron.TM. (Ticona). With the use of the
corrosion-resisting coating it is possible to fabricate the heat
pipe with cost-effective materials, including carbon steel, copper,
or aluminum, with reduced risk of corrosion. The coating materials
can be mixed with other materials such as carbon nano tube or
carbon fiber, to further improve the thermal and mechanical
performance of the heat pipes.
[0028] Corrosion resistant glass coating is a preferred coating
material, being favored over other coating/lining materials for its
ability to withstand strong acids at elevated temperatures (i.e.
dew point temperature of the flue gas). Glass is also known to be
inert, thereby eliminating any potential of cross-contaminating the
condensate and increasing any potential adverse environmental
effects. The glass coating will also provide a smooth surface which
will increase the rate of condensate coming off the surface of each
tube.
[0029] The glass coating is a surface treatment specifically
selected for its ability to adhere onto the metallurgy of the heat
exchanger and to withstand the operating environment of the heat
exchanger. For the glass to perform as expected, it needs to be
applied in a controlled environment following carefully prescribed
operating procedures and quality control practices. A critical
element in the application of the glass coating is to round off any
sharp edges on the surface of the finned tubes. This assists in the
overall application of the coating as the glass needs to be 100%
pinhole-free before being put into service.
[0030] One variation is to make use of serrated fins as an
alternative to circular fins as shown in FIG. 3 where the fins 30
(only one indicated) are arranged helically around the central heat
pipe. Serrated fins more effectively promote condensate removal,
and at the same time provide additional enhancement to heat
transfer. If additional milling is required to form the serrations
around the entire periphery of the fins, this configuration may be
more expensive to fabricate, when compared to the simpler variant
with a limited number of serrations on the back side of the pipes
as shown in FIGS. 2A and 2B although pipes with continuously
serrated helical fins as shown in FIG. 3 may be fabricated by
winding a serrated strip onto the exterior of the pipe and
fastening the strip onto the pipe by soldering or welding or,
alternatively, by mechanically embedding the strip in a groove cut
into the outer diameter of the tube and locking the strip in place
by rolling to force the groove to close tightly around the base of
the fin. A coating material is desirable in this case also to
protect the heat transfer surfaces from corrosion.
* * * * *