US2595457A - Pin fin heat exchanger - Google Patents

Description

y 1952 s. HOLM ETAL 2,595,457

PIN FIN HEAT EXCHANGER Filed June 3, 1947 4 Sheets-Sheet l xi K ,6

INVENTORS refl Ho/m Ar/h or Jen 5e ATIURNEY May 6, 1952 s. HOLM ET AL PIN FIN HEAT EXCHANGER' 4 Sheets-Sheet 2 Filed June 3, 194'? INV EN TO S re 6 0 01 I" Jen sen A 7709MB Y May 6, 1952 PIN FIN Filed June 5, 1947 S. HOLM ET AL HEAT EXCHANGER 4 Sheets-Sheet 3 Patented May 6, 1952 PIN FIN HEAT EXCHANGER Sven Holm and Arthur Jensen, Wellsville, N. Y., assignors to 'The Air Preheater Corporation,

New York, N. Y.

Application June 3, 1947, Serial l lo. 752,008

9 Claims.

This invention relates particularly to improvements in extended surface for heat exchange apparatus in the form of pin-like fins projecting from plate surfaces into the path of the heatin gases.

A heat exchanger capable of transferring large quantities of heat between a gas and air or the like, as in a gas turbine plant, becomes a rather bulky piece of equipment if designed as a shell and tube type heat exchanger. A plate type exchanger might occupy less space but even when provided with conventional finned surfaces of types heretofore employed it would still be undesirably large.

To attain a really compact design it is desirable to have heating surface in contact with each fluid of at least 100 sq. ft. per cubic foot of volume by utilizing extended surface. The present invention contemplates an ideal form of extended surface wherein the passage bounding plates are provided with a myriad of small diameter fins in the form of studs or pins attached to the plates. To attain the high performance required in a gas turbine plant the diameter of the pinfins We utilize is in the order of .02 to .125 inches and the passage forming plates have thicknesses of around a quarter to one half the fin diameter; of course the plates could be of heavier gauge without materially lowering the heat transmission, but then the apparatus would be bulkier and heavier for the same amount of heating surface.

An object of the invention is to provide a heat exchanger having the high performance characteristics afforded by pin-fin extended surface.

Other objects and advantages of the invention will become apparent upon consideration of the following detailed description of a number of illustrative embodiments of the invention when read in conjunction with the accompanying drawings in which:

Figure 1 is a longitudinal sectional view diagrammatically illustrating a high capacity heat exchanger embodying the present invention.

Figure 2 is a fragmentary cross-sectional view on the line 2-2 in Figure 1 and illustrates a pin-fin type of extended surface consisting of continuous lengths of wire sinusoidally formed with zig-zag or U-like bends disposed so as to lie between and in good heat conducting relaticnship with plates bounding fluid passages of a heat exchanger in accordance with the present invention.

Figure 3 is a framentary plan view or Partial section along the line 3-'-3 in Figure 2, of the heat exchange passages of the apparatus.

Figure 4 is a sectional view on the line 44 in Figure 2, viewing the heating gas passages of the apparatus in the direction of gas flow.

Figures 5, 6, 7, and 8 are fragmentary perspective views showing various ways of forming continuous lengths of wire to create pin-like fins as extended surface for the walls of heat exchange passages;

Figure 9 is a graph showing the optimum plate spacings for fins of very small diameter (.02") for the mass velocities of 10,000, 6,000 and 3,000 lbs. per square foot per hour.

Figure 10 is a graph illustrating the relative effectiveness for a gas mass velocity of 10,000 pounds per square foot per hour of heat exchange coil for pin-fins of the diameters of .03, .04 and .0625 inches when disposed at various spacings;

Figures 11 and 12 are graphs similar to Fig. 10 but for mass velocities of 6000 and 3000 lbs. per square foot per hour.

Figure 13 is a graph illustrating the optimum spacings for pin-fins of a diameter of .125 inch.

Figure 14 is a graph plotting the optimum values shown in Figs. 10 to 12 as expressed in terms of plate spacing and plotted against mass velocity.

Figure 15 is a graph derived from Figs. 9 to 14 showing the loci of optimum plate spacings for pin fins between .02 to .125 inches at velocities of 1,000, 10,000 and 30,000 pounds per square foot of core per hour, the enclosed area delineating the range of plate spacings for various fin diameters.

Figure 16 is a graph showing the heat exchange efliciency of an exchanger embodying th invention as compared with tube and plate type exchangers of conventional design.

A heat exchanger embodying the present invention particularly designed for use in a gas turbine cycle is shown in Fig. 1 wherein hot gases at low pressure enter the inlet 10 of a casing ii at high velocity and flow centrally along the longitudinal axis of the latter towards a bafile plate i2 located midway between the ends of the casing. Baille l2 defines the inner end Wall of a central space i 3 bounded on either side by a plate type heat exchange mass designated as a whole by the numeral It. The gases flow from space It transversely through the heat exchange masses l4 and the partially cooled gases enter a passage [5 provided between these masses and the outer casing II to flow thence through similar heat exchange masses IS. The gases exit into a central space H and are Withdrawn from the apparatus through the gas outlet l8. Air or other fiuid to which heat from the gases is imparted is admitted to the apparatus through the inlet connections 53 and flows longitudinally of the heat exchange masses l6, M in passages intermediate their gas passages and alternating with the latter. The air flows serially through the air passages of the masses l6 and I4 and having absorbed heat from the gases is discharged from the apparatus through the air outlets IS. The individual exchange masses [4, I6 may be in the form or" rectangular shaped plate piles forming parallel fluid passages; alternatively the plates may be disposed radially to create masses that are annular in end view, in which case the pairs of masses i i and iii respectively, at either side of the central axis combine to form a single annular mass around the central spaces l3 or l1. The heat exchanger described above is in its general arrangement similar to that disclosed in the copending application of Hilmer Karlsson and Sven Holm, filed on April 3, 1946, under Serial No. 659,272.

Referring now to Figures 2 to 5, the heat exchange masses Hi and [6 of the exchanger consist of a series of metallic plates 20, 2|, spaced to form gas and air passages 22 and 23 disposed in alternating arrangement. As customary in plate type heat exchangers the sides of the gas and air passages are suitably closed, as by providing fillers 24 (Figure 2) or by joining the edge portions of adjacent plates in alternation along their side and end edges as indicated at 25 in Figure l. The surfaces 26 of the walls 20, 2| that face air passages 23 may be provided with fins in the form of metallic channel members 21 extending longitudinally of the passages 23 and having their base or bight portions 28 (Figure welded to the surface 26 with the legs or uprights 29 extending normally so as to constitute fins. The axes of the channels may be parallel with the direction of air flow or alternatively the fins 2? may be divided into sections 21A, 21B, and 21C, (Figure 3) that are reversely inclined on the plates 28 to impart a swirling movement to the air stream.

In accordance with one embodiment of the present invention, there are located in the gas passages 22 and secured to the gas side 30 of the plates 23, 2|, a plurality of heat exchange elements designated as a whole by the numeral 3|. Each element 3| consists of a continuouslength of sinusoidally bent wire formed into a plurality of zig-zig bends or loops 32 of roughly U-shape with the loops alined on the longitudinal axis of each wire. The elements 3| as shown extend in the direction of gas flow and parallel to other similar elements, although they may be inclined on the plates so as to slant across the stream. As shown in Figure 5 the stretchers or yokes 33 of alternately upright and inverted loops are brazed or welded after assembly between the plates to the plate surfaces 3|) on opposite walls of the pasages 22 with the legs 34 of the Us extending preferably perpendicular to the passage walls to form pin-like fin extensions thereof. If desired the elements 3| may be slanted so that the U-legs 3d are inclined in planes transverse.

to the gas stream. The parts of the wire element iii in contact at the yokes 33 of their bends with the walls 38 of the passage are preferably flattened out as illustrated at 35 in Figures 3, 5 and 6, and the area of each of these portions in contact with the passage walls is preferably twice the cross-sectional area of the element wire so as to afford an unrestricted area for heat flow from the gas through the pin-like heat exchange fins 34 and the walls 30 of which they form extended surfaces. The gauge of the wire is also such that when bent, the deformed portions 33 at the bend are of sufiicient area to avoid bottlenecks and thus preclude choking the flow of heat through the fins to the plates 20, 2|.

In the arrangement shown in Figure 6 alternate elements 3| in the gas passages have the yoke portions 33 of their upright U-shaped loops attached only to the plate 2| forming the bottom wall of the gas passage whereas the adjacent elements have the yoke portions 33A of their inverted U-shape loops abutting the other wall fastened thereto. This construction provides a plurality of heat exchange elements 3| attached to the surfaces of plates at opposite sides of the gas passages with the elements projecting from one side intercalated with those on the other side. One advantage of this construction is that while providing extended pin-fin surfaces for both walls of a passage the apparatus may readily be taken apart, as for cleaning, because the elements in the gas passages are not attached to the two walls of the passage in which they are located. At the same time efiicient heat transfer is afforded through the legs 34 of the U-bends to the particular plate to which the alternate elements 3| are attached.

The heat exchange elements illustrated in Figure 6 are formed similarly to those shown in Figures 2 to 5 except that the legs of the U-shaped portions are slanted or inclined outwardly from the yokes 33. This increases the length of the pin-like fin parts 34.

The construction disclosed in Figure 6 herein differs further from the other illustrated forms of the invention in that the air passages as well as the gas passages are provided with bent wire elements 3|.

In the construction illustrated in Figure 8 a further increase in the amount of heat exchange surface is attained first by corrugating the wall plates 40, 4| which obviously increases their area and secondly, by fastening the wire elements 3| in the bases of the grooves 42 of the corrugated plates so that a closer spacing of elements may be provided. The wire elements 3| are shown mounted both in the gas and air passages 22, 23. Since the form shown in Figure 8 utilizes plates with corrugations of the same radius as the wire which is brazed or seam welded thereto it therefore does not require a flattened-out portion on the wire because a sufiicient area of contact is obtained between the surface of the wire and the groove of like radius in the plate. This type of construction is more suitable for use with a counterfiow type of exchanger than a cross-flow type.

In Figure 7 the leg portions 34 of the bends in the wire element are inclined toward each other like sides of a trapezoid with the result that the gaps or spaces 31 between adjacent bends is narrowed. This also lengthens the leg portions 34 of the bends thereby providing a greater amount of heat exchange surface for the pin-fins. The cellular structure of plates and wires is further strengthened since only small gaps 31 between the loops are not in contact with and secured to the yoke portions 33 of the fins.

Whether the flow of gas is parallel to the plane of the wire elements Figures 2 to 8 or transversely relatively close spacing of the elements or coil convolutions at the far end of the axial inlet space [3 may be adopted to increase flow resistance. In Figure 3 it may be observed that the group A of elements 3| at the lower end of the figure which corresponds to the right hand end of the heat exchange masses Id in Figure 1 are more closely spaced than either those at the central part B or at the upper end C of the figure corresponding to the outer or left end of the masses I4.

The specific constructions so far described are exemplifications of the broader principle that underlies the invention of providing a myriad of pin-like fins to form extended surfaces for passages through which fluids flow in heat exchange relationship. The upright fin portions 3|, 34, disposed between the plates as distinguished from the yoke parts interconnecting the latter and lying on the plates will be seen to be of the same fundamental form as simple fin pins attached at their ends to the plates. We have discovered that to obtain optimum heat transfer there are certain critical relationships that should be maintained between the diametrical size (D) of the pin fins and the spacing (t) of the walls forming the passages, which last is equivalent of the fin length. The mutual spacings of the pin fins laterally and in the direction of gas fiow as related to their diameters are also important factors but have more influence with respect to maintaining acceptable pressure drops than in governing the rate of heat transfer except insofar as their aggregate mass and surface affect the overall hydraulic diameters of the fluid passages. However, such spacings in the range between two and four times the fin diameter are preferred.

In Figures 9, 10, 11 and 12 the plate spacings are plotted on the abscissa in major graduations of tenths of an inch while the heat transmission appears on the ordinate scale, in terms of B. t. u. 'per hour degree Fahrenheit for unit of weight in pounds. In all cases the ratio of spacing of the fins transversely to the fin diameter (ST/D) or in the direction of fiow (Sr/D) is assumed to be 3.125. These curves show, for example, that for pin-fins of .02, .03, .04 and .0625 diameter optimum effectiveness is attained with pin lengths or plate spacings of .18 and .225 and .26 and .275 inches, respectively, for a velocity of 10,000 and spacings of .20 and .25 and .275 and .30 inches at I a velocity of 6000 while at a velocity of 3000 spacings of .25 and .280 and .35 and inches give proportionately lesser results.

The optimum spacings for different velocities are shown in Figure 14 for the several diameters of pin fins in the range between .02 and .45 inches. From this diagram the correlation of fin diameter to plate spacing or fin length may be determined to show the range of plate spacings for different fin diameters between given limits of mass velocity. The formula for the lower limit of plate spacing at the 30,000 foot velocity curve (Fig. 15) being derived as and that for the upper limit of plate spacing lies parallel to the 1000 foot velocity curve being where t equals plate spacing and D fin diameter the range of or area outlined by these curves is In other words for mass velocities between 1000 and 30,000 the proper spacings to attain optimum heat transfer with various diameters of fins, or vice versa, will be found to lie in the range of the area defined by the curve. Thus, being given a determined mass velocity and pin diameter that is considered suitable from a performance point of view the proper range of plate spacing for the given pin diameter lies between the values t2 and h which are functions of the pin diameter and may be read from the graph or determind by interpolating the given values in the formula. As may be seen from the graphs there is small loss in effectiveness for the heat exchanger fins over a wide range of loads.

Analysis of the various optimum spacings that may be determined from Figs. 9 to 15 will show that expressed mathematically in terms of the relation between plate spacing and fin diameter (t/D) optimum results are attained when the plate spacings are in the range of from three to twelve times the fin diameter with fins of the order of .02 to .125 inches.

Heat transmission and draft loss also being affected by the spacing of the fins with respect to each other it is to be understood of course that the usual precepts governing the selection of plate gauges and fin spacings are followed. However, we have discovered that optimum results are attained when the plate thickness is between one quarter and one half, the fin diameter and the latter are spaced in the direction' of gas flow (Sr) and transversely (Sr) thereof at intervals of no less than two nor no more than four times the fin diameter. Figs. 9 to 15 all relate to fins spaced at intervals 3.125 times their diameter.

Pin fins according to the invention can be made very small in diameter giving not only a large amount of surface but also a relatively large free flow area in relation to the frontal area, which lessens the pressur drop. Making the pins from wires also allows very close spacing, resulting in turbulence which increases the heat transfer rate especially for low velocities. Welding by seam or flash welders or brazing will attach the wires in metal to metal contact with both plates of a passage resulting in a very strong structure.

Thus by selecting a fin diameter and a plate spacing that is properly correlated therewith the maximum amount of heat transfer surface per unit of core volume may be attained while at the same time providing for a minimum of friction work value or pressure drop. The heat exchanger also will have a minimum overall dimension thus readily accommodating it to space requirements.

A heat exchanger embodying the invention has a maximum heat transfer per unit of weight for a given hydraulic diameter and its cost is low compared to conventional tube or plate ex" changers. Figure 16 shows that for a given friction work a pin fin heat exchanger has a much higher heat transfer effectiveness than other known types of heat transfer surfaces. The curves I, 2 and 3 represent tubular exchangers, the curve 4 a heat exchanger with interrupted strip-fin surface and the curve 5 test results for a comparable pin-fin exchanger embodying the present invention. In addition the exchanger has great structural strength due to the myriad of fins connecting opposite walls of its passages. It consequently is capable of withstanding high pressures and may be used for heat transfer where the pressure difference between the fluids is great.

The constructions described are of particular utility in gas turbine cycles wherein heat is to be abstracted from the gases at low pressure and having a low density and low mass velocity. Because of such conditions it is important that the flow be well broken up on the gas side and the pin-like fins formed by the wire strips or the coils constitute a most eflicient heating surface because the gas film is constantly broken up by them.

Heat exchangers embodying pin-fin extended surface in accordance with the invention may readily be adapted for transferring heat between various fluids because the ratio of the heat transfer surface contacted by a fluid flowing on one side of an intervening passage wall to the amount of surface on the other side of the wall and in contact with the other fluid may readily vary to suit conditions imposed by the particular fluids.

Although specific embodiments of the invention have been described in detail herein many changes and variations may be made without departing from the essentials of the invention and, therefore, it is desired and intended that all such changes and variations be included within the scope of the following claims.

What is claimed is:

1. In a heat exchanger having metallic plates spaced apart as walls of a fluid passage; heat exchange elements mounted in spaced parallel relation in said passage each consisting of an individual continuous metallic wire of uniform cross-section bent to substantially sinusoidal form to provide a plurality of U-shaped loops therein with leg portions extending back and forth in upright relation between said walls with intervening stretcher or yoke portions that connect leg portions of said U-loops that are contiguous along the axis of each wire approximating in length half the height of said legs and contacting both of said plates over their length and being disposed in alinement in substantially the general direction of gas flow through said passage; and good heat transfer bonds between the yoke portions of the loops in each wire element and said plates whereby said pin-like portions constitute extended surface projecting therefrom into the path of fluid flowing in said passage.

2. In a heat exchanger having extended surface in the form of bent wire elements as defined in claim 1 wherein the bends or loops formed in said wire are of roughly trapezoidal form so that the aforesaid leg or pin-fin forming parts thereof are inclined with respect to the surfaces of said walls and act to deflect the fluid streams towards said surfaces.

3. In a heat exchanger as defined in claim 1 having parallel walls of heat transfer material spaced apart in the range between .125 and 1.00 inches the myriad of pin-fins constituted of heat transfer wires with diameters in the order of .02 to .125 inch attached at their ends to both walls of said passage and positioned in closely spaced relation.

4. In a heat exchanger as defined in claim 1 having heat transmitting parallel walls spaced no less than .125 inch and no more than 1.00 inch apart, the myriad of pin-fins being constituted of heat transfer wires with diameters in the order of .02 to .125 inch attached at their ends to both walls of said passage and positioned in spaced relation at distances no less than two and not exceeding four diameters.

5. A heat exchanger as defined in claim 1 having a. fluid passage defined by parallel heat transmitting plates spaced no closer than .125 and no further apart than 1.00 inch, the myriad of pin fins of heat transfer material being attached at their ends to both plates and positioned in transverse and laterally spaced relation at intervals no less than two diameters and no greater than four diameters, the spacing between said plates being in the range where t represents the plate spacing and D the pin diameter with .02 and .125 inch as the lower and upper limits of pin diameters.

6. A heat exchanger as defined in claim 1 having a fluid passage defined by parallel closely spaced plates of heat transmitting material, the myriad of pin fins of heat transfer material being attached at their ends to both plates, the spacing between said plates being no less than three times the fin diameter and no greater than twelve times said diameter with .02 and .125 inch as the lower and upper limits of pin diameters.

'7. A heat exchanger as defined in claim 1 having a fluid passage defined by parallel closely spaced plates of heat transmitting material, the myriad of pin fins of heat transfer material being attached at their ends to both plates and positioned in transverse and laterally spaced relation at intervals no less than two and no greater than four diameters, the spacing between said plates being no less than three times the fin diameter and no greater than twelve times said diameter where the plates have a thickness of between one quarter and one-half the fin diameter with .02 and .125 inch as the lower and upper limits of pin diameters.

8. In a heat exchanger having a plurality of envelopes each comprising a pair of metallic plates spaced apart as opposite walls of a gas passage; a plurality of individual heat exchange elements mounted in spaced parallel relation between said plates each consisting of an individual continuous metallic wire of uniform cross-section bent to substantially sinusoidal form to provide a plurality of U-shaped loops therein with leg portions extending in upright relation from said plates to constitute a plurality of pin-like fins disposed in a plane parallel to the direction of flow of the stream of heating gas and with the intervening stretcher or yoke portions that connect leg portions of said U-loops contiguous along the axis of each wire contacting said plates over substantially their length and being disposed in alinement in the general direction of gas flow between said plates; good heat transfer bonds between the yoke portions of the loops in each wire element and one or the other of said plates whereby said pin-like portions constitute extended surface projecting therefrom into the path of fluid flowing thereover; metallic channel members bonded to the other sides of said plates with their longitudinal axes extending in the general direction of flow of the fluid to be heated to form extended surface on the outer walls of said envelopes; means closing the spaces between said plates along a pair of opposite marginal edges thereof to define the sides of said gas passage; means maintaining the superimposed stacked envelopes in spaced relation and closing the spaces between contiguous stacked envelopes along a pair of opposite marginal edges of said contiguous envelopes to define the sides of the passages for the fluid to be heated; and inlet and outlet manifolds for the heating gas and for the fluid to be heated in communication with the gas passages between said plates and with the passages for fluid to be heated between said stacked envelopes, respectively.

9. In a heat exchanger having a metallic plate forming a wall of a fluid passage; heat exchange elements mounted in spaced parallel relation in said passage each consisting of an individual continuous metallic wire of uniform cross-section bent to substantially sinusoidal form to provide a plurality of U-shaped loops therein with leg portions extending in upright relation from said wall with intervening stretcher or yoke portions that connect leg portions of said U-loops that are contiguous along the axis of each wire approximating in length half the height of said legs and contacting said plate over their length and being disposed in alinement in substantially the general direction of gas flow through said passage; and good heat transfer bonds between the yoke portions of the loops in each wire element and said plate whereby said pin-like portions constitute extended surface projecting therefrom into the path of fluid flowing in said passage.

SVEN HOLM. ARTHUR JENSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 777,853 Hanscom et a1 Dec. 20, 1904 1,782,171 Lattner Nov. 18, 1930 2,093,256 Still Sept. 14, 1937 2,172,667 Nelson Sept. 12, 1939 2,247,199 Kritzer June 24, 1941 2,268,680 Von Linde Jan. 6, 1942 2,505,619 Holm Apr. 25, 1950 FOREIGN PATENTS Number Country Date 91,697 Sweden Mar. 10, 1938 452,231 Great Britain Aug. 19, 1936 559,107 Great Britain Feb. 3, 1944 788,410 France Oct. 10, 1935 889,916 France Jan. 24, 1944

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