Heat exchanger construction

A plate-fin heat exchanger construction including peripheral channels each receiving a mass for protecting the plates against stress failure.

BACKGROUND OF THE INVENTION 
This invention relates to a heat exchanger construction. More specifically, 
this invention relates to a plate-fin heat exchanger core including means 
for strengthening the core and for protecting the peripheral edges thereof 
against thermal stress failure. 
Heat exchangers in general are well known in the prior art, and typically 
comprise a heat exchanger core having dual fluid flow paths for passage of 
two fluids in heat exchange relation with each other without intermixing. 
The fluid flow paths commonly comprise a plurality of relatively small 
and/or intricately shaped passages formed within a heat exchanger core so 
as to maximize the available core surface area for absorbing and 
transferring heat from one fluid to another. 
In the prior art, plate-fin heat exchangers have become popular largely 
because of their simplicity of fabrication and ease of assembly. Such 
plate-fin heat exchangers comprise a core formed by a stacked series of 
thin plates connected together in a spaced relationship so as to provide 
fluid flow regions between the plates. Extended surface fin elements are 
interposed between the plates to form a multiplicity of relatively small 
fluid flow paths within the flow regions, and to increase the available 
surface area for absorbing and transferring heat. Suitable manifolds 
supply the two fluids to the heat exchanger for flow through the flow 
paths in the core without intermixing. 
A common problem with plate-fin heat exchangers comprises stress failure of 
the thin plates, particularly at their outer, peripheral edges. More 
specifically, the heat exchanger experiences substantial thermal gradients 
and stresses upon start-up and/or shut down, and these thermal gradients 
are particularly pronounced at the peripheral edges of the core. The 
thermal gradients result in substantial expansion or contraction of the 
thin core-forming plates which all too frequently causes the plates to 
crack or separate. Such cracking or separation of the plates allows 
undesirable leaking and intermixing of the fluids, and thereby shortens 
the useful life of the heat exchanger. 
Another common problem with plate-fin heat exchangers comprises so-called 
creep failure of the relatively thin core-forming plates. That is, during 
sustained thermal loading at operating temperatures, the thin plates 
experience a relatively slight and random stretching and contracting known 
as creep. This slight creeping of the plates with respect to each other 
contributes to eventual cracking or separating of the plates, particularly 
at the peripheral plate edges. 
Some prior art heat exchangers have included devices for protecting the 
peripheral edges of the plates in a plate-fin heat exchanger. In one 
arrangement, these protective devices have comprised outwardly projecting 
fins which are primarily intended to protect the plates against damage 
from erosion or contact with foreign objects. See, for example, British 
Pat. No. 585,192. Other protective devices have included plate-like 
shields for shielding the plates against high temperature radiant heat 
energy. See, for example, U.S. Pat. Nos. 370,865; 2,093,686; 3,150,714. 
Still other prior art techniques have involved the attachment of fin-like 
elements to designated areas exposed to high heat. See, for example, 
German Pat. No. 1,122,080. However, none of these prior art techniques 
satisfactorily resolve the problems of stress failures resulting primarily 
from the substantial thermal gradients experienced at the peripheral edges 
of a plate-fin heat exchanger. 
The present invention overcomes the problems and disadvantages of the prior 
art by providing an improved heat exchanger construction including 
appropriately sized masses mounted at the peripheral edges of the 
core-forming plates in a plate-fin heat exchanger for controlling 
expansion and contraction of the plates so as to reduce stress failures. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a plate-fin heat exchanger construction 
comprises a plurality of relatively thin plates having trough-shaped edges 
and connected together in inverted pairs to form central flow regions 
between the connected plates. A plurality of generally corrugated first 
fin elements are received within the central flow regions between the 
connected pairs of plates to form a first series of relatively small fluid 
flow passages, and the plate pairs are arranged in an alternating stack 
with a plurality of generally corrugated second fin elements forming a 
second series of relatively small fluid flow passages. Suitable 
manifolding is provided for directing flow of a first fluid through the 
first flow passages in heat exchange relation with a second fluid 
manifolded for flow through the second flow passages. 
The trough-shaped edges of each connected pair of plates form a generally 
outwardly presented channel in which is embedded an elongated 
heat-absorbing strip of predetermined size and shape. In a preferred 
embodiment, the elongated strip comprises a thermal mass formed from a 
suitable heat absorbing and retaining material such as a metallic or 
ceramic wire, and is wrapped peripherally about the connected pair of 
plates. The strip absorbs and retains heat upon start-up and/or shutdown 
of the heat exchanger to smooth out and minimize thermal gradients at the 
peripheral plate edges. 
In another embodiment of the invention, the strip is pretensioned prior to 
operation of the heat exchanger. In this manner, the core-forming plates 
are maintained under compression during operation of the heat exchanger to 
controllably limit thermal expansion of the plates, and thereby help to 
avoid stress failures. Alternately, the strip may be formed from a 
suitable material which is resistant to thermal creeping relative to the 
plates at normal heat exchanger operating temperatures, whereby the 
creep-resistant strip serves to controllably limit relative movement of 
the plates to protect the peripheral edges of the plates against creep 
failure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A heat exchanger 10 is shown generally in FIG. 1, and comprises a heat 
exchanger core 12 carried within a housing 14, with the housing and 
associated conduits being illustrated in dotted lines. As shown, the heat 
exchanger core 12 comprises a laminated stack of plate-fin elements formed 
from suitable metallic, ceramic, or similar materials, and forming two 
fluid flow paths for passage of two fluids in close heat exchange relation 
with each other. More specifically, one end of the core 12 is suitably 
formed to provide a cylindrical inlet manifold 16 communicating with a 
conduit 20 or the like for supplying flow of a first fluid such as air to 
one of the fluid flow paths. As shown, this first fluid flow path is 
formed by a plurality of relatively small flow passages 18 which direct 
the air through the core 12 of the heat exchanger to a cylindrical outlet 
manifold 22. The outlet manifold 22 is disposed at the opposite end of the 
core 12 from the inlet manifold 20, and communicates with a conduit 24 or 
the like for conducting the air collected in the outlet manifold 22 away 
from the heat exchanger. 
A second fluid such as a heated gas is supplied to the interior of the heat 
exchanger housing 14 as by an inlet conduit member 26. The hot gas flows 
through a second fluid flow path comprising a plurality of relatively 
small flow passages 28 in the heat exchanger core 12. These second flow 
passages 28 cause the hot gas to flow in close heat exchange relation with 
the air circulating through the first passages 18 whereby the gas 
transfers a substantial amount of heat energy to the air prior to exiting 
the housing 14 as by an outlet conduit member 30. Thus, the hot gas is 
substantially cooled within the core of the heat exchanger 12. 
An enlarged fragmented portion of the heat exchanger core 12 is shown in 
FIG. 2. As shown, the core 12 comprises a plurality of relatively thin 
plates 32 each having a generally planar central portion 33 bounded by a 
generally troughed or U-shaped peripheral edge 34. The trough-shaped edge 
34 forms a recess or channel 35, and terminates in an outwardly extending 
lip or fin 36. To form the core 12, the plates 32 are connected in 
inverted pairs 37 with the trough-shaped edges 34 of each plate pair in 
abutting contact with each other and with the channels 35 in an aligned, 
back-to-back relation. In this configuration, the central portions 33 of 
each connected plate pair 37 are spaced substantially in parallel from 
each other to define an extended flow region 38 between the plates 32 
comprising the connected pair 37. Moreover, as will be described in more 
detail, the peripheral edges 34 of the connected plates 37 including the 
outwardly extending fins 36 combine to form an outwardly presented 
peripheral recess 44. 
Extended surface fin elements 40 are positioned within the flow regions 38 
of each pair 37 of connected plates 32. These fin elements 40 have a 
generally corrugated and/or offset path configuration to form the 
plurality of relatively small fluid flow passages 18, and thereby comprise 
the fluid flow path for the circulating air. These flow passages 18 are 
adapted to communicate between the inlet air manifold 16 and the outlet 
air manifold 22, (FIG. 1) with the fin elements 40 providing substantial 
heat transfer surface area along the lengths of these passages 18. 
The plates 32 connected in pairs 37 and including the fin elements 40 are 
arranged in an alternating stack with a second plurality of extended 
surface fin elements 42 to form the assembled heat exchanger core 12. More 
specifically, as shown in FIG. 2, the core is formed by interposing one of 
the second fin elements 42 between connected pairs 37 of the plates 32 so 
as to provide an extended flow region 45 generally parallel with and 
between adjacent flow regions 38. The second fin elements 42 are 
substantially similar to the first fin elements 40, including a generally 
corrugated configuration. The second fin elements 42 thus form the 
plurality of relatively small fluid flow passages 28 comprising the flow 
path for the heated gas. Accordingly, the two fluids flowing through the 
core 12 pass in close heat exchange relation with each other, with the fin 
elements 40 and 42 providing substantial heat transfer surface area. 
The heat exchanger core 12 is assembled by connecting together the plates 
32 in pairs 37, and the fin elements 40 and 42 into the stacked or 
laminated arrangement described above and shown in FIG. 2. In practice, 
the components may be connected together by a variety of techniques, such 
as welding, brazing, or the like. However, in a preferred embodiment, the 
components are connected together by brazing, with a sealed braze joint 
being provided between the aligned channels 35 of the peripheral 
trough-shaped edges 34 of each connected pair 37 of plates 32. In this 
manner, the flow regions 38 formed by the connected pairs 37 of plates 32 
form air flow paths which are isolated from the gas flow paths defined by 
the flow regions 45, whereby the two fluids pass in close heat exchange 
relation without intermixing. 
An elongated strip 46 of predetermined size and shape is received within 
the outwardly presented recess 44 of each pair 37 of connected plates 32. 
The strips 46 each comprise an elongated element of suitable thermal mass 
properties for absorbing and retaining heat energy to protect the 
peripheral edges 34 of the plates 32 against crackage due to stress 
failure. That is, upon start-up and/or shut down of the heat exchanger, 
the thermal strips 46 comprise heat sinks serving to absorb heat energy to 
smooth out and minimize expansion effects due to thermal gradients. 
Each strip 46 is formed from a suitable metal or ceramic strip, wire, or 
the like, and is embedded within the associated recess 44. Each strip 46 
is wrapped completely about its associated pair 37 of plates 32 to 
completely occupy the recess 44, and thereby also physically strengthen 
the plates 32. The strips 46 may be secured in position as by brazing upon 
formation of the heat exchanger, or they may be secured in position after 
the heat exchanger is assembled. The precise physical characteristics of 
the strips 46 such as size mass, etc., will vary according to the 
composition and operating environment of the plates 32 of the heat 
exchanger core 12. The selection of suitable strips 46 for a given heat 
exchanger is believed to be within the skill of the art, and accordingly 
is not described in detail. 
Each strip 46 may be adapted for placing the associated pair 37 of plates 
32 under compression to further strengthen the heat exchanger core. 
Specifically, each strip 46 may be tensioned so as to place the associated 
plates 32 under continuous peripherally inward compression. Such 
peripheral compression serves to limit expansion of the plates upon 
start-up, and thereby also helps to reduce stress failures. Placing the 
plates under compression also allows the plates to withstand relatively 
greater fluid pressures so as to prolong core life, or to allow the use of 
relatively lightweight or low strength plates in high pressure fluid 
applications. 
The strips 46 may be chosen from a material having suitable thermal 
properties so as to yield the desired combined thermal mass and/or 
stressing effect on the plates 32. For example, the strips 46 may be 
chosen to have a coefficient of thermal expansion which is less than the 
coefficient of thermal expansion of the plates 46. Thus, as the heat 
exchanger temperature increases upon start-up, the strips 46 expand less 
than the plates 32 so as to place the plates 32 under peripheral 
compression during operation. Alternately, the strips 46 may be chosen 
from a material having a higher coefficient of thermal expansion than that 
of the plates 32. In this example, the strips 46 may be mounted on the 
plates 32 during brazing at elevated temperatures of the heat exchanger 
core during assembly. During cool-down, the strips 46 will attempt to 
shrink more rapidly than the plates 32, but will be prevented from normal 
shrinkage as they become secured in solidifying braze alloy material 
whereby the strips 46 will effectively become pretensioned to place the 
plates 32 under peripheral compression. Conversely, the use in the 
preceding example of strips 46 having a lower coefficient of thermal 
expansion than the plates will function to place the plates under 
peripheral tension which may be required in some heat exchanger 
applications. Still further, if desired, the strips 46 may be pretensioned 
to place the plates 32 under compression as by mechanical means such as 
stakes, turnbuckles, crimps, and the like. 
The strips 46 may also be chosen from a suitable material which is 
relatively resistant to creeping when compared with the plates under 
sustained thermal loads. That is, during prolonged operation of a heat 
exchanger at elevated operating temperatures, the materials tend to 
experience a relatively random expansion and contraction phenomena known 
as creep. Selecting the strips 46 from a material which is more 
creep-resistant than the plates 32 tends to strengthen the plates against 
stress failures due to creep, and thereby prolong heat exchanger operating 
life. 
While the embodiment of FIG. 2 illustrates the invention in a counter flow 
heat exchanger application, the invention may be adapted for use in a 
cross flow heat exchanger as shown in FIG. 3. That is, plates 32 including 
the trough-shaped peripheral edges 34 may be connected in pairs 37 to form 
the flow regions 38 receiving the first fin elements 40. The connected 
pairs 37 of plates 32 are arranged in an alternating stack with second fin 
elements 142 forming the fluid flow regions 45. As shown, the fin elements 
142 define a plurality of relatively small flow passages 128 for passage 
of a fluid at a right angle to the passage of fluid through the flow 
regions 38. Against, as in the previous embodiment, the trough-shaped 
edges 34 include elongated strips 46 of predetermined size and shape 
received within the peripheral recesses 44 for protecting the plate edges 
34 against stress failure. These masses 46 may be suitably adapted as 
described above with respect to the previous embodiment to further 
strengthen the heat exchanger core by placing the plates 32 under 
compression, or to reduce the occurrence of creep failures, etc. 
An enlarged fragmented portion of the connected peripheral edges 34 of 
another alternate plate-fin heat exchanger construction is shown in FIG. 
4, and illustrates a further modification of the invention. As shown, the 
connected plates 32 include the trough-shaped peripheral edges 34 forming 
back-to-back channels 35 and the outwardly presented recess 44. An 
elongated strip 46 comprising a suitable thermal mass is received within 
the recess 44, and may be adapted as described above to place the plates 
32 under compression, etc. The peripheral edges 34 are further 
strengthened by additional elongated strips 50 received within the 
back-to-back channels 35. These strips 50 are also formed from a 
preselected metallic or ceramic material having suitable thermal mass 
and/or creep resistant properties for controlling thermal gradients and 
relative movements at the edges 34, and may be used further to place the 
plates 32 under compression. Alternately, as illustrated in FIG. 5, these 
thermal mass strips 50 may be used separately from the thermal mass strips 
46 of FIGS. 1-4, if desired, for protecting the plate edges 34 against 
stress failure. 
Still another arrangement of the invention is shown in FIG. 6. As shown, a 
pair 37 of plates 32 for the heat exchanger core includes the 
trough-shaped peripheral edges 34 forming the outwardly presented 
peripheral recess 44. A plurality of elongated strips 146 are received 
within the recess 44 and these strips 146 comprise a plurality of wraps of 
a suitable wire-like material. The plurality of strips 146 form a thermal 
mass of predetermined size, shape and thermal properties to absorb and 
retain heat energy to protect the plates 32 against stress failure. 
Importantly, these strips 146 are circumferentially wrapped about the 
plates 32 and may be pretensioned to place the plates under continuous 
compression. Further, if desired, a suitable resinous, ceramic, or other 
bonding material 147 such as braze alloy or the like may be provided for 
anchoring the strips 146 in position. 
A wide variety of further modifications and improvements of the invention 
are believed possible without varying from the scope of the invention. 
Accordingly, the embodiments presented herein are not intended to limit 
the invention, except by way of the appended claims.