US2564153A

US2564153A - Shielded heat exchanger

Info

Publication number: US2564153A
Application number: US690136A
Authority: US
Inventors: John E Comeau
Original assignee: Garrett Corp
Current assignee: Garrett Corp
Priority date: 1946-08-13
Filing date: 1946-08-13
Publication date: 1951-08-14
Anticipated expiration: 1968-08-14

Description

Aug. 14, 1951 J. E. COMEAU- v SHIELDED HEAT EXCHANGER 2 sheet -sheet 1 Ffld Aug. 15, 1946 INVENTOR.

JOHN E.

COMEAU ATORNEY A 14,1951 Y J. E. COME'AU 2,564,1 3

SHIELDED HEAT 'EXCHANGER I Filed Aug. 13, 1946 2 Sheets-Sheet 2 'I'IIII INVENTOR. JOHN E. COMEAU ATTORNEY Patented Aug. 14, 1951 SHIELDED HEAT EXCHANGER John E. Comeau, Culver City, Calii'., assignor to The Garrett Corporation,

Los Angeles, Calif.,

a corporation of California Application August 13, 1946, Serial No. 690,136

9 Claims. 1

My invention relates generally to heat exchangers, and more particularly to tubular crossfiow heat exchangers, and has as a principal object the provision of a heat exchanger in which the expansion of tubes, forming a bank or row between headers, is equalized to prevent the generation of unequal stresses upon the headers and the tubes.

With multiple-tube heat exchangers in which the tubes are arranged parallel to each other in rows and are secured at their ends to headers or manifolds, and in which heat is exchanged between a medium flowing through the tubes and a second medium flowing transversely across the tubes, it is a common experience to have some of the tubes buckle or tear loose from the headers, or for the headers to warp, because the tubes expand unequally. There is an operating temperature gradient from tube to tube across a layer or bank of tubes, generally attributed to a combination of the decrease in temperature differential between the hot and the cold medium, and an increase in the exterior film coemeient of heat transfer as the exterior medium progresses across the layer or bank of tubes, although other factors such as the type of inanifolding and the nature of the impingement of the cross-flowing medium upon the tubes may also afiect it. As a consequence of the temperature gradient, .the tubes expand longitudinally by difierent amounts. with the hotter tubes naturally expanding more than the colder ones, and the resultant unequal pressure on the headers causes the header plates to crack and the colder and less expanded tubes to pull away therefrom.

I have found that, contrary to general belief, the gradient is not always, or even generally,

'uniform across a bank of tubes. or even constant in direction. Circulation and incidence of the cross-flowing medium is an important factor, and the impingement of that medium upon the first tube in a bank, while strong upon the side of the tube impinged, may result in a dead space on the other side of the tube, causing that tube to be relatively inefi'ective as a heat exchanger. The streamlining of the flow is broken by the first tube, and the medium takes a labyrinthine, turbulent course through the remaining tubes, impinging on all sides of them and increasing their efliciency. It is not uncommon, therefore, to find the first tube hotter, when it contains the hotter medium, or colder, when it contains the colder medium, than the tubes immediately 1'01- lowing it.

It is an object of my invention to provide 66' means for deflecting the flow of fluid normally making contact with the tubes of a cross-flow heat exchanger so that exterior and interior tubes in a bank of tubes disposed successively in the direction of flow will be thermally affected by the fluid to substantially equal extents and thereby be caused to expand and contract substantially equally.

Still another object of my invention is to provide a cross-flow heat exchanger in which the aforesaid deflection of the flow aflects each of the tubes in proportion to its length as well as to the temperature normally to be expected therein.

A further object of my invention is to provide a cross-flow heat exchanger in which the portion of each tube exposed to the cross-flow of fluid is correlated in an expansion-equalizing manner to the total length of the tube and to the tube temperature produced by the fluids in internal and external contact with the tube, without, however, restricting or diverting the flow of air.

Yet another object of my invention is to provide a cross-flow heat exchanger in which the flow of fluid is deflected and guided in an expansion-equalizing manner by the tubes themselves.

Further objects of the invention will be apparent from, or will be specifically pointed out, in the following description of one, embodiment of the invention and important modifications thereof, illustrated in the accompanying drawings, in reference whereto:

Fig. 1 is an elevational view of one embodiment of my invention, the parts being shown for greater clarity as they would appear in the process of assembly;

Fig. 2 is a fragmentary elevation similar to Fig. 1 with the parts shown in their assembled positions;

Fig. 3 is a fragmentary plan view on the line 3-3 of Fig. 2;

Fig. 4 is a plan view on a reduced scale of a modified form of my invention with parts cut away to expose interior parts;

Fig. 5 is a fragmentary elevation of a. further modified form of my invention on the scale of Fig. 2 and from the same view-point; and

Fig. 6 is a sectional view on the line 66 of Fig. 5.

Having reference now to the details of the drawings and particularly to Figs. 1 to 3, I have indicated by H the walls of a passage I 2 suitable to conduct a flow of fluid, which for purpose of illustration may be air which it is desired to heat. Manifolds 13, of which only one is shown, enter opposite sides of the passage 12 and there support header plates M to which they conduct another fluid, which, by way of illustration, may be hot exhaust gases. The header plates l4 are connected across the passage l2 by a plurality of tubes I5, arranged in parallel rows disposed transversely to the flow of fluid in the passage, the rows being in turn arranged in parallel banks extending in the direction of flow, one of such rows l6 being shown in Figs. 1 and 2, and a bank I! being shown in Fig. 3. The ends of the tubes I5 are fixedly held in the header plates M in a manner well understood in the art, such as by spinning or welding and it will be seen that if some of the tubes are hotter. and therefore have elongated more than others, the tubes and the header plates will be subjected to strain.

To maintain the tubes at substantially equal mean operating temperatures and therefore at substantially equal lengths, I provide battles l8 for each of the banks H. The baffles I8 are U-shaped and are placed upon their respective banks I! with the closed end I9 of the U encompassing one of the tubes of the bank and the arms 20 of the U extending in the direction of fiow in the passage I2 and thereby Shielding the tubes which they embrace from direct impingeg,

ment by the air. One or more baflles may be used to shield a bank, the employment of the two banles of unequal size being illustrated in Fig. 3. Each bafile has a straight side 2| and a curved side 22, providing a the tubes l5 forming the bank may be shielded to different degrees. The bailles may be located intermediate the header plates, but in the embodiment of the invention shown in Figs. 2 and 3 the bafiles are shown adjacent one of the header plates 14 and have the straight sides 21 of the arms 20 provided with outwardly turned flanges 23. A multiple collar or frame 24, embracing all the tubes collectively engages the flanges 23 to hold them against one of the header plates 14 to which the collar is secured, as by Welding, and may serve the additional function of supporting a surrounding casing such as l3a. With the bafiles l8 thus held at the ends of the tubes against a header plate M, rather than at an intermediate portion of the tube, the header plate cooperates with the arms 20 of each baffle to aid in shielding the masked-oil? portion of each bank of tubes by preventing circulation of air around the sides 21 of the baffle.

In the embodiment illustrated in Figs. 1 to 3, a flow of air is conducted through the passage I2 in the direction of the arrow in Fig. 3. A flow of hot gases is conducted through the manifolds l3 and tubes IS. The air flow impinges on the first tube which it strikes in more or less stream-line flow and at relatively low velocity. It is then broken up to circulate around successive tubes in the bank and to impinge upon them at higher velocity, due to the restricted passages and thermal gaseous expansion, and with great turbulence, and the transfer of heat is more efficient- 1y performed by these later tubes than by the first one, and the first one remains relatively hot. As the air becomes hotter as it circulates along the bank of tubes, the heat differential decreases, and the rate of heat transfer again decreases, so that the mean operating temperature of the tubes in a layer or bank will first decrease toward the center of the exchanger in the direction of flow,

varying width for the arms 20, so that then increase toward the last tube, at which a maximum mean operating temperature is usually reached. The bafiles 18, however, protect portions of the tubes from the cooling effect of the air, maintaining those portions at higher temperatures so that the mean operating temperature of each tube is substantially equal to that of the other tubes. The mean operating temperature is defined as the mean temperature of the tube along its whole length under operating conditions. When the mean operating temperatures are substantially equal for all the tubes and the lengths and coefilcients of expansion of the tubes are equal, the tubes will expand and contract substantially equally.

It will be realized that numerous factors govern the width pattern and the variation of width patterns of the arms 20. Such factors are the length of the tubes l5, their coeflicient or thermal expansion, the initial temperature and nature of the fluid in the tubes [5, the nature and temperature gradient of the fluid in the passage l2, and the resultant mean operating temperature of the tubes and their temperature gradient. These factors will vary for each type of heat-exchanger installation, but are readily determinable therefor. The embodiments of the invention herein illustrated are not intended to indicate the proportions of the shielded and unshielded portions of the tubes, but to be illustrative of the principles of the invention.

In Fig. 4 I have illustrated a modified form of my invention in which the shielded portions of the tubes form extensions to the normally exposed length of the tubes. Manifolds l3 conduct hot gases to and from convexly shaped header plates 28 disposed on opposite sides of the air passage l2 defined by the walls I I. The crossflow of air is conducted through passage 12 in the direction of the arrow. The header plates 26 are curved after the manner of the curved sides 22 of the bafiles l8 in the hitherto described form of the invention so that the tubes 21, which are held in the header plates 26 in a manner well understood in the art, such as by spinning or welding, and which connect them in banks H are varied in length, the central tubes of each bank being longer than the end tubes. Subheaders 28 slideably embrace each of the tubes in each bank individually, and are disposed to form continuations of the walls ll. Only the central portions of the tubes 21, between the sub-headers 28, are exposed to the flow of air, the end portions 29 of the tubes between the sub-headers 28 and the header plates 26 being shielded therefrom by the sub-leaders. In this form of the invention the varying over-all length of the tubes is a factor to be considered, and the length of the shielded portion of each tube is correlated to the exposed length thereof and to the degree to which the tube is thermally affected by the air-flow by reason of its position in the bank [1, so that the expansion or contraction of all the tubes in a bank due to thermal changes is substantially equalized. The sub-headers 28 maintain the normal cross sectional area of the passage through the zone of cross-flow, so that in this form, the shielding means-that is, the sub-headers-do not restrict or divert the flow of air.

A further modification of my invention is illustrated in Figs. 5 and 6. This form of the invention is intended to be used with straight header plates I4, one of which is shown in Fig. 5, and obviates the need of U-shaped shields and means to retain such shields in position. Tubes 3| are plates disposed on opposite sides arranged in rows l6 and banks I! a hitherto described, and their ends are held by the header plates ll. The end portions 32 of corresponding tubes in each row are flattened so as to substantially close the space between ,them and thus create a baille extending from bank to bank. The

length of the portion of tube so flattened will vary with the position of the tube in its respective bank, and therefore with the degree to which it is thermally afiected by the air flow, so that a curve passed through the ends of the flattened portions will resemble the curved sides 22 of the baflles l8. Thus, successive tubes are shielded by the prior tube, and the extent to which each tube is shielded is proportioned to the degree to which it is afiected thermally by the air flow, and the resultant mean operating temperatures and the expansion or contraction of .the tubes are equalized. The flatteningof the tubes may be applied at or near their midpoints, but by flattening end portions, rather than central portions, the shielded area of the tubes is further protected from circulation of air by the adjacent header plate H.

It will be understood that the forms of apparatus herein delineated are illustrative, and that the scope of the invention is not limited thereto, but is to be considered as defined in the appended claims.

I claim as my invention:

1. In a heat exchanger, the combination of: means defining a fluid passage; a pair of header plates disposed on opposite sides of said passage; a plurality of tubes having their ends held by said header plates and being disposed in said passage in parallel rows transversely to the flow of fluid therein, said rows being in turn arranged in parallel banks and U-shaped baffles specific to each of said banks, having their closed ends disposed in counter-relation to said flow and their lateral elements disposed on opposite sides of said banks in overlying relationship to portions of the lengths of said tubes, said lateral elements varying in width so as to shield the lengths of individual tubes in each bank to extents proportionate to the degrees by which said individual tubes would be affected thermally by said flow were said baflles not present.

2. In a heat exchanger, the combination of: means defining a fluid passage; a pair of header of said passage; a plurality of tubes having their ends held by said header plates and being disposed in said passage in parallel rows transversely to the flow of fluid therein, said rows being in turn arranged in parallel banks; U-shaped baflies specific to each of said banks having their closed ends disposed in counter-relation to said flow and their lateral elements disposed on opposite sides of said banks in overlying relationship to portions of the lengths of the tubes, said lateral elements having one straight side and varying in width so as to shield the lengths of individual tubes in each bank to extents proportionate to the degrees by which said individual tubes would be afiected thermally by said flow were said baflies not present; outwardly turned flanges on said straight sides of said lateral elements; and a collar embracing all of the said tubes and engageable with said flanges and securable to one of said header plates to hold said bafiies thereagainst.

3. In a heat exchanger, the combination of: means defining a fluid passage; a pair of header plates disposed on opposite sides of said passage; and a plurality of tubes having their ends held by said header plates and being disposed in said posed successively in the passage in parallel rows transverse to the I or fluid therein, said rows'being in turn a in parallel banks wherein said tubes are dis-' direction 01 flow, said tubes being thereby affected thermally in difl'erent degree by said flow, said tubes having flattened end portions adapted to prevent the circulation of fluid between corresponding tubes of adjacent banks, the flattened portions of individual tubes varying in'length so as to shield successive tubes from said flow to extents proportionate to the degrees by which said successive tubes would be affected thermally by said flow were said portions not flattened.

4. In a heat exchanger wherein a plurality of tubes extends between header plates across a fluid passage and said tubes are arranged in banks parallel to and transversely of the major direction of the flow in said passage and parallel to each other, whereby successive tubes in each bank are thermally afiected by said flow in difierent degree, the improvement consisting of: a flattened portion along the length of each tube so as to prevent circulation of fluid between said tube and the corresponding tubes of adjacent banks, said flattened portions being aligned to form an area in said passage shielded from said flow and varied as to length with respect to individual tubes in each bank so as to effect temperatures of such extent in the shielded portions as to compensate for the different thermal eii'ects on said tubes and substantially equalize the overall expansion and contraction of all of said tubes.

5. A structure according to claim 4 in which said flattened portions of said tubes are end portions thereof, whereby the area of said passage shielded by said flattened portions is further protected from circulation of air by one of said header plates.

6. A heat exchanger having a conduit for mass flow of a first fluid, a plurality of paral el tubes extending across said conduit and through which a second fluid passes, said tubes having their ends held by header plates disposed on opposite sides of said conduit, there being a tendency for certain of said tubes to attain higher operating temperatures than certain other of said tubes because of diflerences in rates of heat transfer from one fluid to the other through the walls of said tubes, and means for shielding portions of the length of at least certain of said tubes from said mass now to cause all of said tubes to attain approximately the same mean operating temperature, said means shielding substantially greater proportions of the lengths of the tubes having a tendency to'opera'te at lower operating temperatures than of the lengths of the tubes having a tendency to operate at higher operating temperatures.

'7. A heat exchanger having a conduit for mass flow oi a first fluid to be heated, a plurality of parallel tubes extending across said conduit through which a second heating fluid passes, said tubes having their ends held by header plates disposed on opposite sides of said conduit and said tubes being arranged in banks parallel to said mass flow whereby there is a tendency for the tubes adjacent the edges of said banks to attain higher operating temperatures than the intermediate tubes of such banks, and means for shielding a portion of the length of at least certain 01' said tubes in each bank from said mass flow, said means shielding substantially greater proportions of the lengths of said intermediate tubes than or the lengths of the tubes adjacent 7 the edges of said banks to cause all of said tubes to attain approximately the same mean operating temperature.

8. A heat exchanger having a conduit for mass flow of a first fluid to be heated, a plurality of parallel tubes extending across said conduit through which a second heating fluid passes, said tubes having their ends held by header plates disposed on opposite sides of said conduit and said tubes being arranged in banks parallel to said mass fiow whereby there is a tendency for the tubes adjacent the edges of said banks to attain higher operating temperatures than the intermediate tubes of such banks, and means for shielding a portion of the length of at least certain of said tubes in each bank from said mass flow to raise the temperature of the shielded portion, the unshielded and shielded portions of the respective tubes having their lengths proportioned to give at their operating temperatures substantially equal expansion and contraction in all the tubes.

9. A heat exchanger having a passage for mass flow of a first fluid, a plurality of parallel tubes through which a second fluid passes, said tubes being arranged to extend across said passage in banks parallel to said mass flow whereby there is a tendency for certain of the tubes to attain I Number higher operating temperatures than others, header plates supporting the tubes at their opposite ends, sub-headers inwardly positioned from the header plates forming parallel baflles between unshielded tube portions of equal length in the mass flow, and shielded tube end portions of unequal length between each sub-header and its associated header plate, the end portions being of greater length for the tubes having a tendency to operate at lower temperatures than the end portions of the tubes having a tendency to operate at higher operating temperatures, so as to cause all the said tubes to attain approximately the same mean operating temperature.

JOHN E. COMEAU.

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

UNITED STATES PATENTS Name Date 1,550,332 Bancel Aug. 18, 1925 1,790,897 Brown Feb. 3, 1931 1,809,915 Smith June 16, 1931 2,269,619 Carpenter Jan. 13, 1942 2,288,598 Ramsaur July 7, 1942