US2596642A - Heat exchanger - Google Patents

Description

y 13, 1952 e. K. w. BOESTAD 2,596,642

HEAT EXCHANGER Filed Ma 15, 1946 s Sheets-Sheet 1 4 m1 xp y 13, 1952 G. K. w. BOESTAD 2,596,642

HEAT EXCHANGER Filed May 15, 1946 3 Sheets-Sheet 3 fig ATTORNEY Patented May 13, 1952 HEAT EXCHANGER Gustav Karl William Boestad, Lidingo, Sweden, assignor, by mesne assignments, to Jarvis C. 'Marble, New York, N. Y., Leslie M. Merrill, Westfield, N.J., and PercyH. Batten, Racine,

. Wis., as trustees Application May 15, 1946, Serial No. 669,754 In Sweden May 28, 1945 6 Claims.

The present invention relates to heat exchange and more particularly to plate heat exchange structure for transfer of heat between two gaseous media, either by the so-called regenerative method, in which the plate structure is heated by contact with a hot gas and thereafter gives up its heat to a cooler gas subsequently flowing through the same channels in the structure, or by the so-called recuperative method in which heat is transferred from a hotter to a cooler medium by conduction through plates separating adjacent channels in which the hot and cold media flow respectively at the same time.

The general types of apparatus to which the invention relates are well known, an example of the regenerative type being disclosed'in U. S. Patent No. 2,023,965 granted to Ali Lysholm December 10, 1935, and an example of the recuperative type being disclosed in U; S. Patent No. 2,064,931 granted to Alf Lysholm December 22, 1936. In all such apparatus the heat transfer capacity of a device of given size is a function of the rate of heat transfer between a gaseous medium and the plate structure, but the commercial utility or efficiency of the device is determined not alone by the coefficient of heat transfer obtained, but also by the factor of resistance to flow of the gaseous media through the device, as well as the factors of cost and weight of the required plate structure, ease of cleaning the gas passages, etc.

In earlier devices of the kind under consideration, satisfactory coefficients of heat transfer were obtained by constructions resulting in highly turbulent gas flow, but these were obtained at the 7 cost of high resistance to flow and consequently with high pressure drop through thedevice, coupled usually with difficulty in cleaning the plate surfaces. To improve this, plate structures of which the above mentioned Lysholm patents are representative were developed, in which substantially straight and unobstructed gas passages or channels are provided, having one ormore walls formed by a plate or plates provided with obliquely disposed and relatively shallow corrugations or furrows for inducing a lateral component of flow in the boundary layer of the gas column which through molecular friction with the core of the column operates to prevent stratification of the gas and to progressively bring all of the gas into best exchange relation with the wall of the channel, thereby maintaining a satisfactorily high rate of heat transfer to the wall of the channel while at the same time maintaining pared with; previous types. Y

relatively =v'er'ylow resistance to gas flow as comr It is the general object of the present inven tion to improve upon the open channel type of apparatus just described by the provision of novel forms and arrangements of plate structures which will improve the rate of heat transfer, which will even further reduce resistance to gas flow, which will provide channels having proportions making'the walls more readily cleanable, which will enable the devices to be constructed from plates of like configuration so that costs of producing the plates are reduced, and which for a given heat transfer capacity will require less plate material than heretofore, thus further reducing cost and also reducing weight.

For a better understanding of the more detailed nature and objects of the invention and the advantages to be derived from its use, reference may best 'be had to the ensuing portion of this specification, taken in conjunction with the accompanying drawings forming a part hereof, in which:

Fig. 1 is a plan view of a portion of a plate adapted to be used in carrying out the invention;

Fig. 2 is an end section of the plate shown in Fig. 1;

Fig. 3 is a perspective view showing plates of the kind illustrated in Fig. 1, assembled in accordance with the invention;

Figs. 4 to 6 inclusive are views similar to Fig. 3 and showing difierent specific plate forms and arrangements for carrying out the principles of the invention;

Fig. 7 is a view similar to Fig. 3 showing a variation in the rib arrangement of the plate structure, and Figs. 8 and 9 are plan views illustrating still other variations in plate rib arrange-: ments.

In the plate structures as heretofore developed the lateral flow of the boundary layer has been produced in such manner that the friction between the boundary layer and the core of the gas column tends to produce a unidirectional rotative component of flow of the whole gas mass in the channel, resulting in a tendency for the gas to move forward through the channel as a single column in a generally helical path of flow. Such apparatus has gone into extensive commercial use and experience has shown that best results with it are obtained with gas channels many times as wide as they are deep. This results in a channel having a relatively low value of hydraulic radius, that is, the ratio of cross sectional area over perimeter, and it is obvious that from the standpoint of flow resistance, the higher the value of the hydraulic radius the-better will be the results.

On the'other hand, if the previous teaching is followed, alteration of the channels to a narrower and deeper form to increase the hydraulic radius, results in less emcient heat transfer and is therefore impractical.

I have discovered, however, that the highly desirable increase in hydraulic radius is obtainable, not only without decrease in heat transfer efiiciency but with materially increased efilciency, by so locating and arranging the oblique furrows in the wall or walls of the channel that the lateral components of flow induced in different portions of the boundary layer at the same section tend to subdivide the gas column as a whole into a plurality of sub-columns flowing parallel to each other in the same general direction through the same channel, but with adjacent sub-columns within the same channel having lateral components of flow of opposed rotative sense with reference to the longitudinal axis of the channel, thus giving continuous helical paths of flow of opposed rotative sense to adjacent sub-columns.

This novel characteristic of flow may be obtained with a variety of specifically different plate structures, but for the, purpose of explaining the invention so that its advantages may be made available it is sufiicient to describe a few embodiments which not only produce the desired flow but which also result in ancillary advantages which will hereinafter be pointed out.

Referring now to the embodiment shown in Figs. 1 to 3, the plate structure comprises a plurality of similar plates indicated generally at H). These are usually of thin sheet metal capable of being rolled or stamped to desired configuration, but the invention is not necessarily limited to the use of metallic plates. The plates ID are formed to provide parallel spacing ridges l2 and 14, respectively, the former projecting from one side of the plate and the latter from the other. Between these ridges the plates are corrugated to provide a series of oblique furrows l6, which as will be observed from the drawings are relatively shallow as compared with the height of the ridges l2 and I4 and which are relatively narrow as compared with the distance between the ridges. As shown, the furrows are inclined at an angle of about 30 (which, however, may be varied) to the longitudinally extending ridges and insofar as the invention is concerned the furrows may be formed by corrugations of wavelike form as shown or with more sharply defined plane surfaces.

As seen from Fig. 3, the plates are assembled with the ridges of adjacent plates in laterally staggered relation to form a series of parallel channels l8 the confronting areas of the opposite side walls of which present furrows inclined obliquely and similarly toward the same lateral wall of the channel. With this arrangement, the boundary layers of gas at the opposite side walls of the channel are each guided toward the same lateral wall of the channel and as a result these portions of the boundary layer at different places on the perimeter of the section, have lateral flow components of opposite rotative sense with reference to the longitudinal axis 20 of the channel, as indicated by arrows 22, the general direction of flow of the gases through the channel being indicated by arrows 24. When the boundary layer gas streams on the opposite sides of the channel reach the lateral wall they are deflected thereby and since they are moving in opposite rotative sense they meet and mutually deflect each other to induce a return lateral component of flow across the central or core region of the channel, as indicated by arrows 26.

Thus the column of gas in the channel is subdivided into sub-columns having generally helical paths of flow of opposite rotative sense and as willreadily be observed from Fig. 3, the depth of the channel in relation to its width may be made considerably greater than if only a single helically flowing column were produced, because of the more positive lateral movement produced in thecentral core portion of the channel which operates to bring all of the molecules of gas rapidly very near or into actual contact with the wall surface of the channel. It will be understood that lengthwise of the channel the furrows of any one row incline in the same direction from end to end of the channel, so that each subcolumn of gas maintains rotative movement in the same sense throughout the length of the channel.

While it might at first appear that the above described structure would result in such turbulent flow that increased resistance to flow would result, actual test results with commercial forms of plate structures show that to be not the case and that in fact-due, I believe, to the use of channels of greater hydraulic radius than heretofore employed in such open channel construction, the flow resistance or pressure drop is decreased in comparison with previous constructions under like conditions. By way of example, I have tested ferrous metal plate structure of the kind above described in which the channels were of 37.5 mm. width and 4.6 mm. depth (average distance between plates) with furrows 2.3 mm. deep arranged as shown in Fig. 3, under like conditions and with the same test equipment, in comparison with plate structure of the most efiicient kind known to me and now generally employed and which produces a single helix type of flow in a channel having a width of 50 mm., a depth of 3.8 mm. and furrows 2.7 mm. deep, and have found that within the range of gas velocities commercially employed in such apparatus the unit pressure drop in the new apparatus is approximately 30% less than in the old, the variation in improvement changing generally inversely with the rate of gas flow.

Such tests have also demonstrated that the new structure has a higherrate of heat transfer than the old, showing an improvement in heat transfer coeflicient from gas to metal of approximately 18% for the range of gas velocities tested, the-improvement being substantially constant.

Depending upon the specific conditions of use, such as nature and temperature of the gas, velocityof flow desired through the apparatus and; other like factors, the relation between width and; depth of channel giving the optimum result may vary considerably, particularly when it is taken into account that in some instances a very high rate of heat transfer is more important than the amount of pressure drop while in other instances ;the reverse may be true and in still other cases the criterion may be dependent on a combination' of both factors. Consequently, dimensions and proportions such as those given above are ,to'be considered illustrative only and in no sense limitin as to the scope of the invention.

Insofar as the physical construction of the plates isconcerned, this may be varied considerably within the scope of the invention. For ease inimanufacture, the plates may be ridged and corrugated as shown in Fig. 4, with the ridges 28' extending only on one side of the general plane of each-plate, or, as shown in Fig. 5, the spacing of tbpiplates may be effected by longitudinally extending ridges formed by separate ribs 30 spot welded, brazed or otherwise secured to the plates.

In all cases, the ridges, whether formed by por tions of the plates or by separate ribs, constitute spacing means which also provide the lateral walls of the channels.

Within the scope of the invention, the arrangement of the oblique furrows is not limited to the arrangement shown in the above described examples. As shown in Fig. 6, the furrows between adjacent spacing ridges may have a herringbone or chevron form, with the furrows of confronting areas of adjacent plates bein inclined generally parallel to each other, thus tending to create in each channel four sub-columns as indicated by the arrows 32.

As will be evident from the foregoing, plate structures capable of carrying the invention into effect can in most instances be made of packs or groups of plates of like configuration and as will be seen particularly from Figs. 3 and 4, the desired spacing between plates can readily be secured by means of spacing ridges which are in the form of open notches, this construction being particularly advantageous from a cost standpoint, since no separate distance pieces need be produced and attached to the plates. It is evident, however, that with plates such as shown in Fig. 4, lateral movement of one plate relative to another would permit the plates to "nest together since their configuration is the same. This cannot occur in the usual recuperative heat exchanger construction, which requires the plates to be fastened at their ends to headers or the like for distribution of the gases to the alternate sets of channels through which they flow. In the usual form of regenerative heat exchanger, however, this does not necessarily hold true. In such construction the plate structure is ordinarily carried in sector-like compartments formed in a cylindrical rotor, and for reasonsof cost, ease of replacement in case of corrosion or other damage and other considerations, the individual plates usually are not rigidly fixed with respect to one another. Thus under the influence of vibration or due to slight fiexure or other cause, one or more plates of a pack of plates of like form might shift position sufiiciently to nest with an adjacent plate, thus not only closing a row of channels but also operating to loosen the remaining plates of a confined pack of plates not rigidly connected to one another. Heretofore nesting difficulties in regenerative heat exchange packs have not ordinarily been metwith for the reason that adjacent plates have not been of like configuration in the most satisfactory and widely used of the prior forms of construction.

In accordance with a further aspect of the present invention, the advantages to be derived from use of plates of like configuration may be retained without risk of nesting, by the manner in which the parallel spacing ridges are distributed with reference to each other and/or to the marginal edges of the plate. Thus, for example, for plates of the form shown in Fig. 4, the plates can be made alike but with the distance between adjacent spacing ridges different across the width of the plate. When such plates are assembled into a pack, alternate plates are reversed to produce an assembly such as shown in Fig. 7. In the figure the difference in distance between the spacing ridges has been exaggerated for purposes of illustration, but it will be evident that with relatively small differences in distances of spacing, risk of nesting because of lateral movement is obviated, and the difference in the widths of the channels resulting from the variable spacing may be kept suinciently small to have no substantial effect on the overall performance of the plate pack.

For some configurations of plates, different distances between spacing ridges, coupled with reversal of alternate plates, would not be practical because of the requirement that confronting furrows on adjacent plates bear a symmetrical relation to each other in each channel.

For such cases the desired object can be economically achieved, while retaining uniformity of distance between spacing ridges, by forming the sheet material in strip form longer than the required plates, with uniformly spaced ridges interrupted at intervals along their length. One such arrangement is shown in Fig. 8, in which the spacing ridges 38 are formed in longitudinall-y coextensive rows and the plates are sheared from the strip so that the spacing ridges are unequally spaced with respect to the end marginal edges of the plate, the distance a being less than distance b. With such plates, reversal of alternate plates results in a longitudinal offset between the spacing ridges as indicated by the dotted line positions of spacing ridges 38a on such a reversed plate lying below the plate shown in full lines.

Fig. 9 shows still another arrangement in which the interruptions in the spacing ridges all are staggered and the plates cut so that when alternate plates are reversed their spacing ridges are located as indicated by the dotted outlines 48a.

Obviously different specific contours of surface between the spacing ridges may be employed with different ridge arrangements above described by way of example, and it will further be apparent that many variations and changes in the several structures herein shown can be made Without departing from the invention, the scope of which is defined by the appended claims.

Iclaim:

1. A heat exchange plate structure comprising a pack of plates with parallel spacing ridges providing a plurality of parallel straight channels of substantially constant cross sectional area for flow of gaseous heat exchanging fluid between adjacent plates, each of said channels having spaced side walls formed by confronting portions of adjacent plates and lateral Walls formed by adjacent spacing ridges and the width of said side walls being several times that of said lateral walls, and a plurality of furrows in each of the side walls of each channel, said furrows being relatively narrow and shallow as compared with the width and depth of said channels and the furrows in confronting areas of the opposite side walls of each channel extending obliquely and similarly toward the same lateral wall of the channel to thereby induce a lateral component of flow toward the same lateral wall in the boundary layers of gaseous fluid flowing through the channel adjacent to 'said confronting areas of said opposite walls of the channel.

2. A structure as defined in claim 1 in which the pack is formed of like plates each provided with spacing ridges, the distances between adjacent ridges on the same plate being different, alternate plates being disposed substantially degrees from the position occupied by said ad- 3. A structure as defined in claim 1 in which the pack is formed of like plates each provided with spacing ridges having ends differently located with respect to the end marginal edges of the plates, alternate plates 'being disposed substantially 180 degrees from the position occupied by said adjacent plates to prevent adjacent plates from resting.

4. A structure as defined in claim 3, in which said ridges are interrupted at intervals along their length.

5. A heat exchange plate structure comprising a pack of plates with parallel spacing ridges providing a plurality of parallel straight channels of substantially constant cross sectional area for flow of gaseous heat exchanging fluids between adjacent plates, each of said channels having spaced side walls formed by confronting portions of adjacent plates and lateral walls formed by adjacent spacing ridges and the width of said side walls being several times that of said lateral walls, and a plurality of furrows in each of the side walls of each channel, said furrows being relatively narrow and shallow as compared with the width and depth of said channels and the furrows in confronting areas of the opposite side walls of each channel extending obliquely and substantially parallel with each other toward the same lateral wall of the channel to thereby induce a lateral component of flow toward the same lateral wall in the boundary layers of gaseous fluid flowing through the channel adjacent to said confronting areas of said opposite side walls of the channel.

6. A heat exchange plate structure comprising a pack of plates with parallel spacing ridges providing a plurality of parallel straight channels of substantially constant cross sectional area for flow of gaseous heat exchanging fluid between adjacent plates, each of said channels having spaced side walls formed by confronting portions of adjacent plates and lateral Walls formed by adjacent spacing ridges and the width of said side walls being several times that of said lateral walls, and a plurality of furrows in each of the side walls of each channel, said furrows being relatively narrow and shallow as compared with the width and depth of said channels and being of chevron form, the portions of the furrows in confronting areas of the opposite side walls of each channel extending I obliquely and substantially parallel with each other toward one of the lateral walls of the channel.

GUSTAV KARL WILLIAM BOES'IAD.

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

UNITED STATES PATENTS

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