CA1164637A - Method of fabricating a heat exchanger - Google Patents

Abstract

METHOD OF FABRICATING A HEAT EXCHANGER

ABSTRACT OF THE DISCLOSURE
Disclosed is a method of fabricating the heat exchanger of the type having first and second heat exchange compartments with a dividing partition therebetween and at least one heat pipe extended therethrough to effect heat transfer there-between. The method comprises the steps of, providing first and second heat pipe segments for each heat pipe, each of the first and second heat pipe segments including a mating surface end, providing an opening in the partition plate of each heat pipe, placing the mating-surface end of one of the heat pipe segments through a corresponding opening in the partition plate with a selected length of the mating-surface end extend-ing from the other side of the partition plate, securing the so-placed heat pipe segment to the partition plate, placing the mating-surface end of the other heat pipe segment in an abutting relationship with the mating-surface end of the first mentioned segment, and securing the abutting mating-surface ends together.

Description

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METHOD OF FABRICAI'ING A MEAT EXCHANGER
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BACKGROUMD OF '~HE INVENTION

This application is a division of Canadian Serial No. 374,117, filed March 27, 1981.

Field o the In~ention _ The present invention xelates to heat exchangers and, more particularly, to heat exchangers designed to preheat com-bustion air for a combustion furnace, using heat energy removed from the flue gas.
Prior Art The thermal efficiency of combustion furnaces and combustion systems has typically been increased by recovering heat energy ~rom the resulting flue gas and using this energy to preheat the combustion air. This preheating has been effected in a number of ways, including the use of recuperator type heat exchangers, by which thermal energy is transferred to the com-bustion air. These heat exchanger structures have ranged from comparatively simple devices, in which the flue gas and combus-tion air are carried in ad~acent ducts that are in heat exchange relationship with one another, to far more sophisticated devices that include tube-and-shell heat exchangers, thermal siphons, and heat pipe type heat exchangers.
Recent increases in the cost of hydrocarbon fuels have necessitated improvement in the overall thermal efficiency of combustion furnaces. ~he search fox these higher eficiencies has been complicated further by two factors: 1) the economic necessity of using fuels having a higher than preferred sulphur content and 2~ the need for fuels requiring greater than usual quantities of combustion air to realize the full heat value of the fuel. An example of one such high sulphur fuel, requiring large amounts of combustion air, is the coal typically available in the western United States.

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Prior tube-and-shell type heat exchangers, used as combustion air preheaters, have generally demonstrated adequate performanceO However, these types of heat exchanger~ require large sur~ace areas to effect efficient transfer. This large surface area requirement results in a cleaning and maintenance problem associated with the deposition of soot and other particles from the flue gas flows. In addition, these large ~urface heat exchangers are subject to corrosive attack when used in the lower temperature ranges because of acid vapor conden~ation. In a like manner, heat-pipe heat exchangers have also demonstrated good operating performance but their uppex temperature limit of operation is generally considered low when compared to the high temperature of 1ue gases pro-duced in the combustion process. The operation of heat pipes above their rated temperature limit results in performance degradation of the heat pipe and, occassionally, tube burn-out.
In addition, since heat pipes operate in the lower temperature ranges of the flue gas, they are also subject to corrosive attack by acidic components of the flue gas. While high-temperature heat pipes exist and can be fabricated to with-stand corrosive attack, these types of heat pipes generally require costly materials and heat transfer mediums, which are too expensive for conventional combustion air preheater applications.
SUMMARY OF THE INVENTION
In view of the above, it is an overall object of the present invention, among others, to fabricate a combustion air preheater that efficiently operates over a wide temperature range to transfer heat energy from flue gas to combustion air.

The present invention as disclosed also seeks to provide a combustion air preheater that provides effective heat transfer in a high temperature range, using a first-type 3~
of heat exchanyer, and effective heat -transfer in the low temperature range, using a second -type of heat exchanger to provide high overall efficiency over a wide temperature range.
The combustion air preheater in which the likelihood of acid attack is reduced by using a first large-surface heat exchanger in a higher temperature range and a second heat exchanger in a lower temperature range with the second heat exchanger confined to operation above the acid dew point to minimize acid vapor condensation.
Disclosed is a combustion air preheater for heating combustion air using heat energy transferred from the flue gas to the combustion air. The preheater includes a heat exchanger defined by a plurality of heat pipes extending between first and second heat transfer compartments for effecting heat transfer therebetween and another heat exchanger defined by a plurality of tubes supported in a shell by tube sheets for transferring heat energy from one side of the tubes to the other.
The invention to which this divisional application per-tains is a method of fabricating the heat exchanger of the typehaving first and second heat exchange compartments with a dividing partition plate therebetween and at least one heat pipe extended therethrough to effect heat transfer therebetween. The method comprises the steps of, providing first and second heat pipe seg-ments for each heat pipe, each of the first and second heat pipe segments including a mating-surface end, providing an associated opening in the partition plate of each heat pipe, placing the mating-surface end of one of the heat pipe segments through an associated opening in the partition plate with a selected length of the mating-surface end extending from the other side of the partition plate, securing the so-placed heat pipe seg-ment to the partition plate, placing the mating-surface end of 6 ~ ~
the other heat pipe segment in an abutting relationship with the mating-surface end of the first mentioned segment and se-curing the abutting mating-surace ends together.
By structuring a preheater in this manner, heat energy in a higher temperature range is efficiently transferred through the tube-and-shell hea~ exchanger and additional heat energy, in a lower temperature range/ is efficiently transferred to the combustion air to obtain the benefits of both type~ of heat exchangers.
Other features of the invention as disclosed include providing the heat pipes of the first-mentioned heat exchanger with extended heat transfer surfaces with the spacing of these surfaces arranged to maintain the temperature of the heat pipe mounting plate above the local acid dew point thus preventing or at least minimizing corrosive attack thereto, and fabricating the heat pipe in two parts to permit convenient assembly within the first-mentioned heat exchanger, reducing fabrication costs.
DESCRIPTION OF THE FIGURES
_ __ The above description, as well as the objects, features, and advantages of the present invention, will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodi-ment in accordance with the present invention when taken in conjunction with the accompanying drawings wherein:
FIG. 1 is an overall system view, in diagrammatic form, of a combustion furnace incorporating a combustion air pre-heater in accordance with the present invention;
FIG. 2 is a side elevational view, in cross-sectional schematlc form, of a combustion air preheater in accordance with the present invention;
FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2, showing an exemplary arrangement of heat transfer tubes;

~ 16~3~ 1 F~G. 4 is a side elevational view, in partial cross section, of an exemplary heat pipe;
FIG. 5 is an enlarged, detailed view of a portion of the heat pipe illustrated in FIG. 4, showing details of the fabrica~ion thereof; and FIG. 6 is a graphical representation of the temp erature (ordinate) of the flue gas ~solid line) and combustion aix (dashed line) with respect to transit time (abscissa) through the preheater with the tws curves displaced from one another for reasons of clarity.
DETAILED DESCRIPTION OF T~E P~EFERRED EMBODI~ENT
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A combustion ~urnace system incorporating a com-bustion air preheater in accordance with the present invention is shown in diagrammatic form in FIG. 1 and includes a com-bustion ~urnace 10 that receives a supply of hydrocarbon fuel such as petroleum or coal along an inlet line 12 and a source of combl~stion air through anothex line 14. q~he combustion furnace 10 produces heat energy as indicated for subsequent use in.the thermal cycle ~not.shown) and also produces a products~
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of-combustion flue gas directed through outlet line 16. As is conventional in the art, the flue gas is directed through the outlet line 16 to a combustion air preheater 18 and is passed therethrough to a preheater outlet 20 and i5 subsequently dis-charged through the system stack lnot shown). Incoming com-bustion air is provided to the preheater 18 through an inlet ~ine 22 and is passed through the pxeheater 18 in a heat exchange relationship with the hot ~lue gases to preheat ~he combustion air~ The so-preheated combustion air is then intro-duced into the combu.stion furnace 10 throu~h the aforementioned combustion air inlet line 14.

~ combustion air preheater 18 in accordance with the present invention is sho~m in FIG, 2 and is designed to 3 ~

efficiently transfer heat energy to the incoming combustion air from the outgoing flue gas in low and high temperature ranges to provide hi~h overall operating efficiency. As shown therein, the preheater 18 includes a primary heat exchanger, generally designated by the reference character 24 and a secondary heat pipe-type heat exchanger, generally designated by the reference character 26.
The primary heat exchanger 24 is a two stage tube-and-shell type exchanger in that it includes first and second bundles 28 and 30 of ~enerally horizontally disposed heat exchanger tubes 32 with the first bundle 28 disposed above the second bundle 30 and with the dividing line between the two bundles generally indica~ed a~ 34. The heat transfer tubes 32 may be distributed in their respecti~e bundles as exemplified in the cross-sectio~al view of FIG~ 3. The upper and lower tube bundles 28 and 30 each include tube sheets 36 at their opposite ends for suppoxting the tubes in the preferred dis~ribution with the bundles 28 and 30 and their ..
associated~tube sheets 36, enclosea hy an exterior shéll, -~
generally indicated at 38 (only partially shown in schematic form in FIG. 2) with tbe shell 38 enclosing the tube bundles as is conventional in the art. The shell 38 defines an upwaxdly facing 1ue gas inlet 40, a downwardly faci~g flue gas dis-charge outlet 42 that includes a soot and particulate matter trap 44, a combustion air inlet 46, and a combustion air out- ¦
let 48. A header or plenum chamber S0 is provided on the righ~
side o~ ~he prLmary heat èxchanger 24 to provide gas phase communication between the right ends of the upper and lower tube bundles 28 and 30. SOGt blowers or other devices designed to prevent or inhibit the accumulation of soot or other particula~e matter entrained in he flue gas on the exterior . ~
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surface of the tubes 32 may preferably be located as indicatedby the dotted-line cixcles 52.
The secondary heat exchanger 26 is defined by upper and lower heat transer compartments, 54 and 56, separated by an intermediate partition 58. A plurality of generally vertically aligned heat pipes 60 pass through appropriately sized openings in the parti~ion 58 and extend into the lower and upper compartments, 56 and 54, with thehea~pipes 60 being attached to the intermediate partitio~ 58 as described in more detail below. The hea~ pipes 60 are arranged in either parallel ox s~aggexed row formation~ as desired~ and are of conventional design, in that they are fabricated, as shown in FIG. 4, from straight, hollow tubes 62 which are sealed at both ends. Each tube 62 contains a select2d quantity of a heat transer liquid (e.g., ammonia) at a selected vapor pressuxe. The liquid L
collects in the lower portion o each tube 62, termed the evaporator section, and i5 adapted to vaporize in response to heat energy ~Qin) introduced into the evaporator section. The : -~-so-formed vapor rises upwar~ly-in the ~ube 62, as ind~cated by -~
the arrow 6~ in FIG. 4, and condenses in the upper, condenser portion of each tube, relinquishing the heat energy ~Qout) with the condensate falling under the influence of gravity to the evaporator section. The heat pipes 60 may be provided with various types of internal wicking materials (not shown) to assist in returning the condensate to the evaporator section when the heat pipes are used in a non-vertical alignment. As used herein, the ~erm "heat pipe" encompasses heat pipes with wicking material as well as without wiclcing material, the latter devices also referred to in the art as thermal siphons. Each pipe 60 is provided with a plurality of disc-like annular fins 66 that extend outwardly from the tube surface and are generally ~ ~i4~
equally axially spaced to prov.ide an extended heat transfer surface. Alth~ugh annular fins are shown in the figures, the fins may ~ake the form of any one of a number of surface configurations including spines, longitudinal fins, and spiral fins with certain of the fins or extended heat transfer surface configured as described in more detail below.
As shown in the detailed views of FIGS. 4 and 5, the fin 66 closest to ~he partition 58 on the evaporator side o~ the heat pipes (that is, the lower heat transfer compartment 56) is spaced at a distance d from the partition 58 which distance is greater than the inter-fin spacing d'. The spacing, as explained below, minimi~es the formation of corrosive materials on the partition 58 during operation of the preheater 18.
The heat pipes 60 of the secondaxy heat exchanger 26 can be facricated as shown in the detailed view of FIG. 5~ The heat pipe 50 can be initially manufactured in two separate parts~
an upper part and a lower part, with one of the parts, e.g., ... . , . I
` - the upper part, designed to be passed through an appropriately sized clearance opening in the partition 58 so that a stub portion 68 extends below the lower surface of the intermediate partition. Thereafter, the upper portion can be secured in place by an appropriate fillet weld, as indicated at 70 9 and the lower part of the two-part heat pipe 60 can be positioned and butt-welded to the upper part as indicated at 72 to complete the heat pipe fabrication. As can be appreciated, the above-mentioned fabrication tec~nique can be conducted with the stub portion o~ a lower part extending above the sur~ace of the partition 58 with the ~illet and butt-welding taking place above the surface of the partition 58.

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The primary and secondary heat exchangers, 24 and ~6, are connected together by ducting as shown in FIG. 2. A duct 74 extends between the upper compartment 54 of the secondary heat exchanger 26 to theinlet 46 of the lower tube bundle 30, and another duc~ 76 extends between the lower compartment 56 of the secondary heat exchanger 26 and the flue gas outlet 42 of the primary heat exchanger 24. Other ducting is provided to supply and remove flue gas and combustion air to and from the hea~ exchanger preheater 18. ~hese additional ducts (shown in dotted-line illustration) include a duct 78 for directin~ flue gas into the preheater 18, a duct B0 for directing ~lue gas away from the prehe~ter to the system stack, a duct 82 for directing combustion air into the preheater, and another duct 84 for directing preheated combustion air away from ~he preheater.
In operation, hish-temperature flue gases are directed through the duct 78 to the flue gas inlet 40 of theprimary heat exchang~r 24 as indicated by the arrow 86 in FIG. 2 downwardly ... ` over-the upper and then~ the lower tube.bundles, 28 and 3D, with a portion of.the therma.l energy in the f~ue gas being passed through the tubes 32. Thereafter, the flue gas exits ~he primary heat exchanger ~4 through the flue gas outlet 42 and passes through the duct 76 as indicated generally by the arrows 88 and 90. During the passage of the flue gas through the pr~mary heat exchanger 24, soot, including soot that is dis-lodged from the tubes32 by the soot blowers shown at the locations 52 and other particulate material collect in the trap ~4.
The heated flue gas then passes through the lower 1.
compartment 56 of the secondary heat exchanger 26 with additional heat energy being removed from the flue gas by ~he 4 ~

evaporator sections of the heat pipes 60 and transferred to the upper compartment 54. The flue gas, at a substantially lower temperature than its inlet temperature, is then passed through the outlet duct 80, as indicated generally by the arrow 92 to the system stack ~not shown). Incoming combustion air is directed through the duct 82 in the general direction of the arrow 94 through the upper compartment 54 of the secondary heat exchanger 26 and past the condenser sections of the heat pipes 60. The inc~ming combustion air is heated with heat energy supplied from the flue gas passing through the lower compartment 56. Thereafter the partially heated comhustion air is passed through the duct 74 in the general direction of the arrow 96 through the interior of the tubes 32 of the lower tube bundle 30 where the combustion air is again heated with thermal energy provided from the flue gas flowing on the exterior side of the tubes 32. The combustion air exits the tubes 32 of the lower bundle 30 and flows in the general direction of the arrows 98 to enter the tubes 32 of ..~he upper.bundle 2~ and pass therethrough haying i~ temp-._.... =
erature increased by receiviny additional heat energy from the flue ga~ flowing on the exterior side of the tubes 32 of the upper bundle 28. The preheated combustion air then exits the tubes 32 of the upper bundle 28 and is removed from the pre-heatex 18 through a duct 84 as indicated by the arrow 100.
As graphically illustr~ted in the graph of FIG. 6, the temperature of the flue gas (solid line) as it enters the preheater 18 is approximately 900 F. (460 C.) and is lowered to approxLmately 750 F. (400 C) as it passes over the tubes 32 of the upper and lower tube bundles, 28 and 30,by virtue of a portion of the heat energy th~reof being transferred through the walls of the tubes to the combustion air flowiny through the interior of the tubes. The flue gas then passes through the duct 76 and enters the lower compartment 56 of the secondary heat exchanger 26 at approximately 700 F. (340 C.) and is cooled further to its final exit temperature of 200 F.
t95O C.) by the transfer of additional heat energy from the flue gas to the evaporator section of the various heat pipes 60.
On the other hand, combustion air ~dotted line) enters ~he upper compartment 54 of the secondary heat exchanger 26 at approximately 100 F. (40 C.) and is heated to a temperature of approximately 500 F. (260 C.) with the heat supplied by the flue gas ~lowing in the lower compartment 56 of the secondary heat exchanger 26. The partially preheated combustion air then enters and flows through the tubes 32 of the lower bundle 30 and then flows through the tubes 32 of th upper bu~dle 28 where its tempexature is increased to approxi-mately 700 F. ~340~ C.).
As can ~e appreciated by consideration of the flue " gas=and combustion air ~lowing in`rë~ationship to the graphical ~
example of FIG. 6, it can be seen that a substantial portion of the heat energy in the flue gas is transferred to the incoming combustion air to effect preheating a~d an overall incr~ase in system efficien~y. By initially passing the high temperature ~lue gas through a tube-and-shell heat exchanger, efficient heat transfer can take place through the tubes without the need for extraordinarily large surface areas. By then passing the somewhat cooler flue gas khrough a heat pipe heat exchanger, efficient heat ~ransfer o~ the remai~ing heat in ~he ~lue ~as can take place at a lower t~rnperature without danger of 30 operatin~ the heat pipes at a temperature above their upper limits.

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According1y, a smaller tube-and-shell heat exchanger may be used than otherwise would be the case to effect a size reduction.in the overall preheater and to also limit problems associated wi~h acid dew formation. In addition, the ~in spacing arrangement described above in connection with FIG. 5 maintains the partition 58 in a warmerstate thus minimizing acid dew formation. Furthexmoxe, the ~abrication technique for the hea~ pipes described reduces assembly costs for the preheatex as a whole.
As will be apparent to those s~illed in the art, various changes and modifications may be made to the combustion air preheater of the present invention without departing from the spirit and scope of the present invention, as defined in the depending claims and their legal equivalent~ .

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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of fabricating a heat exchanger of the type having first and second heat exchange compartments with a dividing partition plate therebetween and at least one heat pipe extended therethrough to effect heat transfer there-between, the method comprising the steps of:
(1) providing first and second heat pipe segments for each heat pipe, each of said first and second heat pipe segments including a mating-surface end;
(2) providing an associated opening in the partition plate for each heat pipe;
(3) placing the mating-surface end of one of said heat pipe segments through the associated opening in the partition plate with a selected length of said mating-surface end extending from the other side of the partition plate.;
(4) securing the so-placed heat pipe segment to the partition plate;
(5) placing the mating-surface end of the other heat pipe segment in an abutting relationship with the mating-surface end of the first mentioned segment; and (6) securing said abutting mating-surface ends together.

2. The method claimed in Claim 1, wherein said securing steps include welding said segments.