WO2011159595A2

WO2011159595A2 - Counterflow heat exchanger

Info

Publication number: WO2011159595A2
Application number: PCT/US2011/040144
Authority: WO
Inventors: David A. Scearce
Original assignee: Pb Heat, Llc
Priority date: 2010-06-15
Filing date: 2011-06-13
Publication date: 2011-12-22
Also published as: US20110303400A1; WO2011159595A3; WO2011159595A4; EP2583044A2

Abstract

A counterflow heat exchanger having a plurality of heat exchanger elements in a stacked, spaced-apart arrangement forming a plurality of inter-element passages. Each element has fluid passages formed between facing sections of a first plate and a second plate. A first-fluid passage has an outer passage circumscribing an inner passage in fluid communication with the outer passage. An inlet-port traversing passage in fluid communication with the outer passage, an outlet-port traversing passage in fluid communication with the inner passage and a second- fluid traversing passage circumscribed by the inner passage traverse the first and second plates. A first-fluid inlet header includes the inlet-port traversing passage of each element. A first-fluid outlet header includes the outlet-port traversing passage of each element. A second-fluid inlet passage in fluid communication with the plurality of inter-element passages includes the second-fluid traversing passage of each element.

Description

TITLE OF THE INVENTION

[0001] Counterflow Heat Exchanger

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is related to U.S. Provisional Patent Application No. 61/354,943, filed June 15, 2010, and incorporated herein by reference and claims the earlier filing date of the provisional application.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to a device and method for transferring heat from flue (or combustion) gas through an interposing wall (or plate) to a fluid without allowing the flue gas and fluid to mix. More particularly, the present invention relates to a counterflow heat exchanger and method for cooling flue gas below a condensing temperature.

[0004] Most fossil fuels are combusted with ambient air in a chamber, such as a boiler. The combustion product gas is exhausted from the chamber through a flue. Typical flue gas from the combustion of fossil fuels contains substantial amounts of uncombusted nitrogen, and to a lesser degree carbon dioxide and water vapor respectively formed by the combustion of carbon and hydrogen with atmospheric oxygen. In volume, the water vapor can be as much as seven to eleven percent of the flue gas. The water vapor contains energy in the form of latent heat.

[0005] Absent a cost effective device and method for cooling the hot flue gas below the condensing temperature of the water vapor to recover the latent heat and to transfer the energy in the hot flue gas to a working fluid, the flue gas energy would be exhausted through the flue and wasted. Accordingly, a devise able to cost effectively recover latent heat from hot flue gases is desirable.

BRIEF SUMMARY OF THE INVENTION

[0006] Briefly stated, one embodiment of the present invention is directed to a counterflow heat exchanger comprising a plurality of heat exchanger elements in a stacked, spaced-apart arrangement forming a plurality of inter-element passages. Each element comprises a first plate adjacent a second plate. A first- fluid passage is formed between facing sections of the first plate and the second plate. The first-fluid passage comprises an outer passage circumscribing an inner passage in fluid communication with the outer passage. An inlet-port traversing passage traverses the first and second plates. The inlet-port traversing passage is in fluid communication with the outer passage. An outlet-port traversing passage traverses the first and second plates. The outlet-port traversing passage is in fluid communication with the inner passage. A second- fluid traversing passage traverses the first and second plates. The second-fluid traversing passage is circumscribed by the inner passage. A first-fluid inlet header comprises the inlet-port traversing passage of each element. A first-fluid outlet header comprises the outlet-port traversing passage of each element. A second-fluid inlet passage comprises the second-fluid traversing passage of each element. The second-fluid inlet passage is in fluid communication with the plurality of inter-element passages.

[0007] Another embodiment of the present invention is a counterflow heat exchanger element comprising a first plate adjacent a second plate. A first-fluid passage is formed between facing sections of the first plate and the second plate. The first-fluid passage comprises an outer passage circumscribing an inner passage in fluid communication with the outer passage. An inlet-port traversing passage traverses the first and second plates. The inlet-port traversing passage is in fluid communication with the outer passage. An outlet-port traversing passage traverses the first and second plates. The outlet-port traversing passage is in fluid communication with the inner passage. A second-fluid traversing passage traverses the first and second plates. The second-fluid traversing passage is circumscribed by the inner passage.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0008] The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0009] In the drawings:

[0010] Fig. 1 is a perspective view of the front, top and right side of a preferred

embodiment of a counterflow heat exchanger in accordance with the present invention;

[0011] Fig. 2 is a partial exploded perspective view of the counterflow heat exchanger of Fig. 1;

[0012] Fig. 3 is an assembly diagram of the counterflow heat exchanger of Fig. 1 ;

[0013] Fig. 4 is a front perspective view of a heat exchanger element of the counterflow heat exchanger of Fig. 1; [0014] Fig. 5 is a side perspective view of a transfer seal of the counterflow heat exchanger of Fig. 1 ;

[0015] Fig. 6 is an enlarged cross sectional view of the counterflow heat exchanger of Fig. l ;

[0016] Fig. 7 is an enlarged partial view of the cross sectional view of the counterflow heat exchanger of Fig. 6;

[0017] Fig. 8 is a schematic diagram of a cross sectional view of the counterflow heat exchanger of Fig. 1 ;

[0018] Fig. 9 is a schematic diagram of a heat exchanger element of the counterflow heat exchanger of Fig. 1 ; and

[0019] Fig. 10 is a front perspective view of another embodiment of a counterflow heat exchanger element in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0021] As used in the description of the invention and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The words "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. The words

"comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0022] The words "right," "left," "lower" and "upper" designate directions in the drawings to which reference is made. The words "inwardly" and "outwardly" refer to directions toward and away from, respectively, the geometric center of the structure to which reference is made, and designated parts thereof. The terminology includes the words noted above, derivatives thereof and words of similar import.

[0023] Although the words first, second, etc., are used herein to describe various elements, these elements should not be limited by these words. These words are only used to distinguish one element from another. For example, a first passage could be termed a second passage, and, similarly, a second passage could be termed a first passage, without departing from the scope of the present invention.

[0024] The following description is directed towards various embodiments of a counterflow heat exchanger in accordance with the present invention.

[0025] Referring to the drawings in detail, where like numerals indicate like elements throughout, there is shown in Figs. 1-9 a preferred embodiment of the counterflow heat exchanger generally designated 10, and hereinafter referred to as the "heat exchanger" 10 in accordance with the present invention. In a preferred application, the heat exchanger 10 is for cooling flue (or combustion) gas below the condensing temperature of water vapor contained in the flue gas. In other applications, the heat exchanger 10 may used for cooling the hot fluid without producing a condensate.

[0026] Referring to Figs. 1-3 and 6, the heat exchanger 10 comprises an outer gas-tight wrapper 12 housing a plurality of heat exchanger elements 100 in a stacked, spaced-apart arrangement 14 secured between a first support plate 16 and a second support plate 18 connected by a plurality of fasteners 20 such as tie rods with threaded ends and corresponding nuts.

[0027] In the illustrated embodiment, the outer wrapper 12 is a separable two piece shell having a generally rectangular cross-sectional shape for conformance with the corresponding rectangular shape of the support plates 16, 18. In another embodiment (not shown), the outer wrapper 12 may have a cylindrical cross-sectional shape for conformance with circular support plates (not shown). In general, the outer wrapper 12 may have any desired shape allowing the outer wrapper 12 to form a gas-tight enclosure housing the arrangement 14 of heat exchanger elements 100.

[0028] In the illustrated embodiment, the outer wrapper has an upper section 22 with an exhaust port 24 and a lower section 26 with a condensate drain 28 extending from a fluid collector (or trap) 30 formed in the bottom of the lower section 26. In an alternate embodiment, the trap 30 may be a component separate from the outer wrapper 12 and part of a drainage system to which the outer wrapper is plumbed.

[0029] In another embodiment (not shown), the outer wrapper 12 may be more than two sections or may be separable into front and rear sections instead of upper and lower sections depending on the form factor desired for a particular installation. [0030] The edges of the upper and lower sections 22, 26 forming the outer wrapper 12 are configured to join each other and the first (or front) support plate 16 and the second (or rear) support plate 18 in a manner able to form a gas tight enclosure for the arrangement 14 of heat exchanger elements 100 enclosed therein. For example, the outer edges of the upper and lower sections 22, 26 of the outer wrapper 12 may be outwardly flanged to join the sections to each other. Other outer edges may be inwardly flanged to join the first and second support plates 16, 18 to the upper and lower sections 22, 26 of the outer wrapper 12. The flanges may be channeled to receive or support a gasket or elastomeric material between adjoining surfaces and to receive a fastener releasably joining the component parts.

[0031] The outer wrapper 12 may be fabricated from a wide variety of heat resistant materials including, but not limited to, polyphenylene sulfide, glass-filled polypropylene or similar high-performance thermoplastics. The outer wrapper 12 may also be fabricated from coated steel or high grade stainless steel.

[0032] Referring to Figs. 2-4, in one embodiment, the heat exchanger elements 100 comprising the arrangement 14 are formed by circular plate pairs. Each element 100 comprises a first plate 102 adjacent a second plate 104. A first-fluid passage 106 is formed between facing sections of the first plate 102 and the second plate 104 by opposing channels in the facing sections. The first-fluid passage 106 comprises an outer passage 108 circumscribing an inner passage 110 in fluid communication with the outer passage through one or more interconnecting passages 112. In some embodiments, the outer passage 108 may be spaced from the inner passage 110 by a web 109 through which the interconnecting passages 112 pass. An inlet-port traversing passage 114 in fluid communication with the outer passage 108 traverses the first and second plates 102, 104. In some embodiments, each element 100 may have additional port- traversing passages, such as a second (or alternate) port traversing passage 114_a, to provide the ability to accommodate alternate uses and plumbing configurations. For example, the alternate port traversing passage 1 14_a may be used in some embodiments as an additional inlet-port traversing passage or in other embodiments as a drain port. An outlet-port traversing passage 116 in fluid communication with the inner passage 110 also traverses the first and second plates 102, 104. In addition to the inlet-port and outlet-port traversing passages 114, 116, the first and second plates 102, 104 of each element 100 are traversed by a centrally-positioned, second-fluid traversing passage 118 circumscribed by the inner passage 110.

[0033] In some embodiments, the plates forming each element 100 may have a generally circular shape and may have concentric outer and inner passages as shown in Fig. 4. In other embodiments, the plates may have a square or rectangular shape such as the heat exchanger element 100' shown in Fig. 10. In general, the plates may have any desired shape allowing the outer wrapper 12 to form a gas-tight enclosure housing the arrangement 14 of elements 100.

[0034] Referring to Fig. 10, in some embodiments, a first- fluid passage 106' of each heat exchanger element 100' may have one or more generally concentric fluid passages 106_n' between the outer and inner passages 108', 1 10'. Interconnecting passages 1 12' provide a fluid path between the one or more concentric fluid passages 106_n', the outer passage 108' and the inner passage 110'. The interconnecting passages 1 12' may be radially oriented or may be angled with respect to a radius. Other embodiments may have more than three concentric passages or less than three concentric channels with one or more interconnect passages connecting adjacent concentric passages. In still other embodiments, the passages need not be concentric channels. Although circumscribing each other, the passages may have a generally meandering shape. Each heat exchanger element 100' may also have an inlet-port traversing passage 114', outlet-port traversing passage 1 16' and a centrally-positioned, second-fluid traversing passage 1 18' circumscribed by the inner passage 1 10'.

[0035] In some embodiments, the outwardly facing surfaces of each element may have raised, radially-oriented dimples 120 arranged and may be configured to allow generally uniform radial flow of a fluid over the surfaces (See, e.g., Fig. 4). In other embodiments, the outwardly facing surfaces of each element may be smooth. (See, e.g., Fig. 10.)

[0036] The material from which the heat exchanger elements are fabricated is preferably a metal such as 316 stainless steel. Alternatively, 304 or 400 series stainless may be used. Other thermally conductive materials, titanium for example, compatible with the combustion gas products and the cooling (or working) fluid may also be used. The channels in the first and second plates 102, 104 forming the passages in each element 100, 100' are preferably created by stamping or hydroforming. Alternatively, the plates and the channels therein may be formed by casting.

[0037] Referring to Figs. 2, 3 and 6, the stacked, spaced-apart arrangement 14 into which the plurality of heat exchanger elements 100 are assembled has a first-fluid inlet header 32 comprising the inlet-port traversing passage 114 of each element 100 and a first-fluid outlet header 34 comprising the outlet-port traversing passage 1 16 of each element 100. In some embodiments, a plurality of transfer seals 36 described below couple the inlet-port traversing passages 1 14 comprising the first-fluid inlet header 32 and the outlet-port traversing passages 116 comprising the first-fluid outlet header 34. The plurality of heat exchanger elements 100 are spaced-apart by the plurality of transfer seals 36 forming a plurality of inter-element passages 38 in the assembly 14.

[0038] Referring to Figs. 3 and 5-7, in some preferred embodiments, each transfer seal 36 may be a generally cylindrical component with a central passage 40 extending axially therethrough. The transfer seal 36 may have a generally cylindrical central portion 42 configured to make sealing contact with one of the inlet-port traversing passages or outlet-port traversing passages when the plurality of heat exchanger elements are assembled into the spaced-apart arrangement. A nipple 44 extends axially from each side of the generally cylindrical central portion 42. Each nipple 44 has a nipple outer diameter sized for insertion in one of the inlet-port traversing passages 114 or outlet-port traversing passages 116. In some embodiments, the nipple outer diameter may be less than the inner diameter of the inlet-port or outlet-port traversing passages 114, 116 allowing each nipple to be inserted in the traversing passages with a clearance fit when the heat exchanger elements 100 are assembled to form the spaced-apart arrangement 14. Alternatively, in other embodiments, the nipple outer diameter may be greater than the inner diameter of the inlet-port or outlet-port traversing passages 114, 116 requiring each nipple to be inserted in the traversing passages with an interference (or press) fit. In some embodiments, the outer diameter of the central portion 42 of each transfer seal 36 may be sized to be greater than the outer diameter of the nipples 44. In other embodiments (not shown), the outer diameter of the central portion 42 of each transfer seal 36 may be sized to be less than or equal to the outer diameter of the nipples 44. In some embodiments, a

circumferential O-ring channel 46 in the outer surface of each nipple 44 retains an O-ring 48 sized to make sealing contact between the nipples 44 and one of the inlet-port traversing passages 114 or outlet-port traversing passages 116 when the elements 100 are assembled into the arrangement 14. The generally cylindrical central portion 42 of the transfer seal 36 may have an axial extent sufficient to provide a desired predetermined spacing for the inter-element passages 38 formed between the plurality or heat exchanger elements 100 of the spaced-apart arrangement 14.

[0039] The transfer seals 36 may be fabricated from the same material identified above for the plates 102, 104 forming the each element 100. The O-ring 48 may be made from any elastomer compatible with flue gas products and is preferably made for ethylene propylene diene monomer (M-class) rubber or Viton™, a brand of synthetic rubber and fluoropolymer elastomer made by DuPont Performance Elastomers L.L.C.

[0040] When the plurality of heat exchanger elements 100 is assembled together with the transfer seals 36 into the arrangement 14 in accordance with Fig. 3, a second-fluid inlet passage 50 comprising the second- fluid traversing passage 118 of each element 100 is formed. The second-fluid inlet passage 50 is in fluid communication with the plurality of inter-element passages 38.

[0041] The second-fluid inlet passage 50 has a first end 50a coupled to a second-fluid inlet port 52 in the first support plate 16. In some embodiments, the second end 50b of the second- fluid inlet passage 50 may be coupled to a removable closure 54 configured to seal the second- fluid traversing passage 1 18 of a last element of the plurality of heat exchanger elements 100 in the stacked arrangement 14.

[0042] In some embodiments, the stacked arrangement 14 of heat exchanger elements 100 is secured between the first support plate 16 and the second support plate 18 by a plurality of fasteners 20 connecting the two plates 16, 18. For ease of assembly and disassembly, the fasteners 20 preferably are tie rods that terminate in a threaded portion for receiving a threaded nut and extend between and through penetrators 58 in the first and second support plates 16, 18.

[0043] In some embodiments (not shown), the first and last elements of the plurality of elements 100 may replace the first and second support plates 16, 18. In such embodiments, each element, including the first and last elements, may have penetrators (see, e.g., Fig. 10) through which the tie rods pass. Further, the last element may not have a second-fluid traversing passage. Instead, the central area circumscribed by the inner passage 110 remains a continuous web. The internal surface of the last element may or may not be provided with a thermally insulating insert.

[0044] In some embodiments, the second-fluid inlet port 52 may traverse the first support plate 16 and the second support plate 18 may have a first-fluid inlet port 60 and a first-fluid outlet port 62. When the plurality of heat exchanger elements 100 are housed in an enclosure formed by the outer wrapper 12, the first-fluid inlet port 60 is in fluid communication with the first-fluid inlet header 32 and the first-fluid outlet port 62 is in fluid communication with the first-fluid outlet header 34. The second-fluid inlet port 52 is in fluid communication with the second-fluid inlet passage 50. The exhaust port 24 is in fluid communication with the inter- element passages 38. The condensate drain 28 also is in fluid communication with the inter- element passages 38. [0045] During operation, the internal pressure in the inlet and outlet fluid headers 32, 34 acts circumferentially on the internal surface of the nipples 44. Since the internal pressure serves to reinforce the radial sealing surface, the transfer seals 36 do not depend on the tie rods 20 to compress the O-rings 48. The stress in the tie rods 20 is predominantly induced by the internal pressure of the working (or first) fluid.

[0046] In one embodiment, the heat exchanger 10 may be designed to be used in

conjunction with a gas or liquid fuel combustion system, such as a gas or oil burner. In another embodiment, the heat exchanger 10 may be designed to be used with a solid fuel burner or other source of hot gases. The method for manufacturing the heat exchanger 10 and the materials from which the heat exchanger elements are fabricated may be application specific. For example, the elements could be stamped from aluminum or stainless steel for condensing boilers, or from steel for non-condensing boilers. Alternatively, the heat exchanger elements could be die-cast aluminum for condensing boilers and die-cast steel for non-condensing boilers or they could be cast aluminum for condensing boilers or cast iron for non-condensing boilers.

[0047] Figs. 8 and 9 are representative flow diagrams of the heat exchanger 10 in use.

Combustion gases enter the center cylindrical opening or second fluid inlet passage 50 through a gas flue (not shown) attached to the second- fluid inlet port 52 in the first support plate 16. The gases flow axially through the second- fluid inlet passage 50 and radially outwardly through the inter-element passages 38 between the elements 100 of the arrangement 14. As the gases flow radially outwardly, the gases successively contact the concentric fluid passages (e.g., the inner passage 1 10 and then the outer passage 108) comprising the first- fluid passage 106, and flow over and in contact with the web 1 9 which acts as an extended heating surface between the concentric fluid passages. When the gases flow beyond the extent of the arrangement 14, the gasses are captured by the outer, gas-tight wrapper 12 and exit through the exhaust port 24.

[0048] In one embodiment a first fluid (e.g., a coolant or working fluid) from a heating system enters the inlet header 32 of the heat exchanger 10 through the first- fluid inlet port 60 of the second support plate 18. The working fluid travels radially inwardly in a counter flow direction to the flow of the combustion gasses from the outermost concentric fluid passage 108 through the interconnecting passage 1 12 (and intervening concentric fluid passage ways, if any,) to the innermost concentric passage way 1 10 and into the outlet fluid header 34. The working fluid exits the heat exchanger 10 through the first-fluid outlet port 62 in the second support plate 18 and returns to the heating system. [0049] As the combustion gas and the working fluid cross-flow through the heat assembly 14, water vapor in the combustion products condenses. The latent heat released during condensation transfers to the plurality of heat exchanger elements 100 increasing the

temperature of the working fluid. The condensate drops to the bottom of the outer wrapper 12 and exits the heat exchanger 10 through the condensate drain 28 in the outer wrapper 12.

[0050] The transfer of energy to the working fluid and the rise in the temperature of the working fluid is diagrammatically represented by the change in the stippling of the working fluid from lightly stippled (a relatively cool inlet temperature) to more heavily stippled (a relatively hot outlet temperature) as the working fluid flows radially inwardly from the outer passage 108 to the inner passage 110.

[0051] In typical combustion system applications, the following operation conditions may occur. The maximum internal pressure may be 160 psig and the maximum temperature may be 210-250°F. The liquid flow rates may vary with inlet rate to develop a 10-100°F temperature difference across the heat exchanger. The gas flow rates are sufficient to provide reasonable combustion at rated inlet (Gas CFH x 9.5 x 1.5). Combustion side pressure is usually less than 14 in of water but could be more.

[0052] The above disclosed embodiments may be modified to accommodate a wide range of operating conditions more or less demanding than the conditions set forth above. For example, in some embodiments, the number of heat exchanger elements may be increased or decreased to accommodate greater or lesser thermal loads. Further, the elements may be designed to have more or less passages (having a concentric or other circumscribing shape) to allow scaling of the design to larger or smaller sizes. Still further, although the embodiments disclosed above were directed to combustion systems, the heat exchanger 10 is not limited to combustion application. Rather, the heat exchanger 10 may be implemented in various embodiments suitable for use in instances where there is a desire to capture the latent energy associated with the condensation of one or more vapors in a gas stream or, stated more generally for non-condensing applications to capture some of the thermal energy in a gas stream. Therefore, the invention disclosed above is not limited to the particular embodiments or applications disclosed. Rather, the disclosure is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims

CLAIMS We claim:

1. A counterflow heat exchanger comprising:

a plurality of heat exchanger elements in a stacked, spaced-apart arrangement forming a plurality of inter-element passages, each element comprising:

a first plate adjacent a second plate;

a first- fluid passage formed between facing sections of the first plate and the second plate, the first- fluid passage comprising an outer passage circumscribing an inner passage in fluid communication with the outer passage;

an inlet-port traversing passage traversing the first and second plates, the inlet- port traversing passage in fluid communication with the outer passage;

an outlet-port traversing passage traversing the first and second plates, the outlet- port traversing passage in fluid communication with the inner passage; and

a second- fluid traversing passage traversing the first and second plates, the second-fluid traversing passage circumscribed by the inner passage;

a first-fluid inlet header comprising the inlet-port traversing passage of each element; a first-fluid outlet header comprising the outlet-port traversing passage of each element; and

a second- fluid inlet passage comprising the second- fluid traversing passage of each element, the second-fluid inlet passage in fluid communication with the plurality of inter- element passages.

2. The counterflow heat exchanger according to claim 1 , wherein the outer passage, the inner passage and the second fluid passage are concentric.

3. The counterflow heat exchanger according to claim 1 , wherein each element has a generally circular shape and the outwardly facing surfaces of each element have raised, radially- oriented dimples arranged and configured to allow generally uniform radial flow of a fluid over the surfaces.

4. The counterflow heat exchanger according to claim 1 , further comprising a plurality of transfer seals having a central passage therethrough, the plurality of transfer seals coupling the inlet-port traversing passages comprising the first-fluid inlet header and the outlet-port traversing passages comprising the first-fluid outlet header.

5. The counterflow heat exchanger according to claim 4, wherein each transfer seal comprises:

a generally cylindrical central portion configured to make sealing contact with one of the inlet-port traversing passages or the outlet-port traversing passages when the plurality of heat exchanger elements are assembled into the spaced-apart arrangement.

6. The counterflow heat exchanger according to claim 5, wherein each transfer seal further comprises: a nipple extending axially from each side of the generally cylindrical central portion; each nipple having a nipple outer diameter sized for insertion in one of the inlet-port traversing passages or the outlet-port traversing passages with a clearance fit; the central portion of each transfer seal having a transfer seal outer diameter sized to be greater than the nipple outer diameter; a circumferential O-ring channel in an outer surface of each nipple; an O-ring retained in the O-ring channel and sized to make sealing contact between the nipple and one of the inlet-port traversing passages or the outlet-port traversing passages when the plurality of heat exchanger elements are assembled into the spaced-apart arrangement.

7. The counterflow heat exchanger according to claim 5, wherein the generally cylindrical central portion of each transfer seal has an axial extent sufficient to provide a predetermined spacing for the inter-element passages formed between the plurality of heat exchanger elements of the spaced-apart arrangement.

7. The counterflow heat exchanger according to claim 1, wherein the plurality of heat exchanger elements are spaced-apart by a plurality of transfer seals coupled to the inlet-port traversing passages and the outlet-port traversing passages.

8. The counterflow heat exchanger according to claim 1 wherein the plurality of heat exchanger elements are housed in an enclosure having a first-fluid inlet port in fluid

communication with the first- fluid inlet header, a first-fluid outlet port in fluid communication with the first-fluid outlet heard, a second- fluid inlet port in fluid communication with the second-fluid inlet passage, an exhaust port in fluid communication with the inter-element passages and a condensate drain in fluid communication with the inter-element passages.

9. The counterflow heat exchanger according to claim 1 wherein the stacked arrangement of heat exchanger elements is secured between a first support plate and a second support plate connected by a plurality of tie rods.

10. The counterflow heat exchanger according to claim 1 wherein the second- fluid inlet passage has a first end coupled to a second- fluid inlet port and a second end coupled to a closure configured to seal the second- fluid transfer passage of a last element of the plurality of heat exchanger elements in the stacked arrangement.

11. A heat exchanger element comprising:

a first plate adjacent a second plate;

a first- fluid passage formed between facing sections of the first plate and the second plate, the first-fluid passage comprising an outer passage circumscribing an inner passage in fluid communication with the outer passage;

an inlet port traversing the first and second plates, the inlet port in fluid communication with the outer passage;

an outlet port traversing the first and second plates, the outlet port in fluid

communication with the inner passage; and

a second-fluid passage traversing the first and second plates, the second-fluid passage circumscribed by the inner passage.

12. The counterflow heat exchanger element according to claim 11 , wherein the outer passage, the inner passage and the second-fluid passage are generally concentric.

13. The counterflow heat exchanger element according to claim 11 , wherein the element has a generally circular shape and the outwardly facing surfaces of each element have raised, radially-oriented dimples arranged and configured to allow generally uniform radial flow of a fluid over the surfaces.