US2462421A

US2462421A - Crossflow heat exchanger

Info

Publication number: US2462421A
Application number: US560369A
Authority: US
Inventors: Paul A Pitt
Original assignee: Solar Aircraft Co
Current assignee: Solar Aircraft Co
Priority date: 1944-10-26
Filing date: 1944-10-26
Publication date: 1949-02-22
Anticipated expiration: 1966-02-22

Description

Feb. 22, 1949. P. A. PITT CROSS-FLOW HEAT EXCHANGER Filed 001;. 26, 1944 2 Sheets-Sheet 1 JQ JUL 1 /77;

INVENTOR.

ATTORNEY.

22, p PITT CROSS-FLOW HEAT EXCHANGER 2 Sheets-Sheet 2 Filed Oct. 26, 1944 INVENTOR.

PAUL A. PITT ATTORNEY Patented Feb. 22, 1949 CROSSFLOW HEAT EXCHANGER Paul A. Pitt, San Diego, Calif., assignor to Solar Aircraft Company, San Diego, Calif., a corporation of California I Application October 26, 1944, Serial No. 560,369

Claims. (Cl. 257-245) This invention relates to heat exchangers of the cross-flowv type, in which two fluids between which heat transfer is to take place flow in directions substantially at right angles to each other. More particularly, it relates to that type of cross-flow heat exchanger in which a series of generally fiat thin metal sheets are so secured together at their edges as to define the two sets of passages.

The invention is particularly useful on aircraft for heating air by the exhaust gas from the engine where the temperature differential between the exhaust gas and the air to be heated is frequently very great. Thus the temperature of the air at high altitudes is frequently 60 F. or more below zero whereas the exhaust gas may be 1800" F. or more above zero. As a result, different parts of the heat exchanger, only a few inches apart, may acquire temperatures differing by as much as 1200 F. Because of the resultant different expansions and contractions oi relatively closely spaced parts of the heat exchange structure, severe stresses are developed that eventually result in the break-down of a structure that would be adequate when subject to less severe temperature differentials.

A general object of the present invention is to increase the life of heat exchangers of the type mentioned.

Another object is to reduce the temperature difierentials between inside and outside portions of a heat exchanger of the type described and thereby reduce differential expansions between the parts.

Another object is to prevent vibration of the plates or sheets of a heat exchanger.

Still another object is to provide a structure that permits the sheets to bend in response to difierential expansion caused by unavoidable temperature differentials.

Ordinarily there is a marked difference in the temperatures of the outside sheets of a heat exchanger and the interior sheets because the interior sheets are exposed only to the cooling efiect of the cooling fluid that is flowing through the heat exchanger whereas the outside plates are exposed not only to the cooling medium flowing therethrough but also lose heat by radiation. In accordance with the present invention, I reduce the difierential temperature between the inner and outer sheets of the exchanger by throttling the flow of cooling fluid through the outer passages so that less heat is absorbed from the outer plates by the cooling fluid, to compensate for. the heat lost from those plates by radiation.

I reduce vibration of the plates, in accordance with the present invention, by providing corrugations or beads extending in both directions across the plates and either in contact with or welded to the corrugations of the adjoining plates so as to reduce the free areas of the plates to stiffen them and reduce opportunity for drumming. I also so form the corrugations that they facilitate gradual bending of the sheets in response to differential expansion of diflerent portions of the sheets so as to reduce the overall stresses due to unequal expansion and which ordinarily produce failure of the structure within a relatively short time.

Various more specific objects and features of the invention will become apparent from the following detailed description of a specific em-' bodiment of the invention, as illustrated in the drawing:

-. In the drawing:

Fig. 1 is a side elevation view of the core of a heat exchanger in accordance with the invention, portions of the structure being broken away;

Fig. 2 is a cross section of the core shown in Fig. 1, with the outer casing shown in section and a portion of the core broken away to show how it appears in a diiferent plane;

Fig. 3 is a perspective view, with parts broken away, showing the corner construction of the heat exchanger; and

Fig. 4 is a plan view of one of the sheet units that make up the core.

Referring to Fig. 1, there is disclosed a crossflow heat exchanger having a core of special construction in accordance with the invention, which core is adapted to be contained in a casing. The particular exchanger illustrated is of approximately square cross section and is of greater length than width. It is intended for use for heating air from the exhaust gas of an airplane engine and, as usually employed, the exhaust gas is passed longitudinally through the exchanger and the air is passed through it transversely.

The core of the exchanger consists of a plural; ity of similar units I3, each of which constitutes a conduit extending longitudinally of the exchanger and, in this instance, conveying hot exhaust gas. Each unit l3 consists of two formed sheets l5 and IS in spaced relation over the major portion of their areas but brought together and welded at their lateral edges I I, and having opposite corrugations II spaced inter- 3 mediate the edges I4, the corrugations on the two sheets I5 and I6 being also welded together so that three distinct longitudinal passages I8, I8 and I8 are formed by each unit I3.

The lateral edges I4 of eacl. unit I3 are flattened at the corners, as shown in Fig. 3, to form vertical corner edges 22 on each unit.

The lower sheet I5 of each intermediate unit I3 is connected to the upper sheet I6 of the next unit therebelow at each end by a clip as indicated in Figs. 1 and 3, and each intermediate cross-flow passage 2| is defined by the sheet I5 ofone unit I3 and the sheet I6 of the next adjacent unit I3.

The joinder of the horizontal end edges of the units by the clips 20, as described, maintains the corner edges 22 of the various units in alignment and they are welded together to maintain a separation of the cross-flow passages 2| from the longitudinal passages I8.

To conduct fluid to and from the longitudinal passages I8, transition members 24 and 25 are provided at the opposite ends of the core. The transition member 24 is rectangular in crosssection at its inner end and fits in and is welded to arectangular collar 2 which in turn is welded to the left end edges of the top wall I6 of the top unit I3 and of the bottom wall I5 of the bottom unit I3. The other two edges of the collar 24I are welded to the vertical edges of the core, which are constituted by the aligned corner edges 22 of the various units I3. The outer end of the square collar 2 is turned out to form a flange 242. The transition member 25 is joined to a square collar 25I having an outwardly extending flange 252, and the collar 25I is joined to the right end of the core of the exchanger in the same manner that the collar 24I is joined to the left end thereof.

Upper and

lower casing walls

26 and 21, respectively, are provided above and below the top and bottom units I3, to provide a pair of outer transverse passages 2Ia for conveying fluid past the outer sides of the upper and lower units I3. These

casing walls

26 and 21 are secured at their ends to the transition members 24 and 25 and are provided with groove-defining attachment flanges 28 for receiving conduits which convey fluid to and from the cross-flow passages. As previously stated, ordinarily the fluid flowing through the cross passages 2| and 2Ia will be the cooler fluid, such as air, whereas the fluid flowing through the longitudinal passages I8 would be exhaust gas at a high temperature.

Although the outer cross-flow passages 2-Ia are highly desirable for the reason that they provide additional capacity for the exchanger, I have found that as a result of the heat loss to the air flowing through passages 2Ia, in addition to the heat loss by radiation to and through the outer casing wall 26, which is exposed to the surrounding air, the outer sheet I6 of the top unit I3 may run at a much lower temperature than the inner sheet I5. Likewise the lower sheet I5 of the bottom unit I3 may run at a lower temperature than the top sheet of that unit. These differences in temperature produce differential expansions in the two walls of the outer units I3 which materially shorten their lives.- I have found that this inequality in temperature may be largely corrected by providing baffles 29 at one end of each passage 2Ia. These baffles 29 are preferably provided at the inlet end although this is not essential. The bailies are so proportioned by test as to produce substantially .equal temperatures in the two sheets I5 and I6 forming the outer units I3.

As already described, the sheets I5 and I6 of each unit I3 are welded together at their edges I4 and at the beads or corrugations II so that the lateral dimensions of the sheets between points of joinder are relatively small, thereby reducing the opportunity for free vibration of the plates.

.To further stiffen and support the plates, they are provided with externally projecting transverse corrugations 30, and the corrugation 30 of each plate I5 of one unit I3 contacts the corrugation 30 of the adjacent plate I6 in the next adjacent unit I3. This construction is most clearly shown in Fig. 1. It has the advantage of providing mutual support between the plates at relatively closely spaced points longitudinally so as to reduce the unsupported length of any section of each sheet.

It is found in practice that substantial temperature difierences may obtain between the edge portions I4 and the corrugated portions H of each unit I3, relative to the unsupported areas intermediate the joined portions I4 and I1. These differences in temperature result in unequal expansion, which in older designs have within the undulate portion. It will be observed that both sheets I5 and I5 of each unit I3, and of the different units I3, are all curved similarly, so that the passages are of substantially the same cross sectional area at all points. An exception is that the cross-flow passages 2I are larger adjacent the corrugations I'I. However, these enlarged portions are beneficial to promote turbulence of the air flowing through the passages 2 I.

The heat exchanger described is preferably constructed by first forming the units I3 individually by seam-welding the edges I4, and the corrugations II together. ments I3 are stacked together and flame or are welded together at the ends 20. Following assembly of the core, along the vertical corner edges 22, along the end edges of the top wall ft of the top unit I3, and along the end edges of the bottom wall I5 of the bottom unit I3, the transition members 24 and 25 may be welded to the ends of the core, after which the upper and

lower casing walls

26 and 21 can be welded or joined in other manner to the transition members 24 and 25.

Although for the purpose of explaining the invention a particular embodiment thereof has been described in detail, various departures from the exact construction shown can be made without departing from the invention which is to be limited only to the extent set forth in the appended claims.

I claim:

1. In a heat exchanger of the cross-flow type having a first set of generally flat parallel passages extending in one direction for conducting a first fluid that is above ambient temperature and having a second set of generally flat parallel passages interleaved with the first set and extending in a direction substantially at right angles to Thereafter, the elesaid one direction for conducting a second, cooler, fluid, the construction comprising: a plurality of generally fiat sheets in generally parallel spaced relation with successive pairs of sheets joined directly together along one pair of opposite edges and defining fiat conduit units constituting the sole essential structure forming said first set of passages, and the adjacent sheets of each adjacent pair of said conduit units being joined directly together along their other pairs of opposite edges for defining all of the interior passages of said second set of passages; auxiliary casing walls parallel and in outwardly spaced relation to the outermost of said sheets for defining therewith the outermost passages of said second set of passages; and baiile means extending across said outermost passages at one end thereof for restricting the velocity of flow of said second cooler fluid through said outermost passages more than through the interior passages of said second set to reduce the temperature difi erential of said outer sheets relative to the remaining sheets.

2. In a heat exchanger of the cross-flow type having a first set of generally flat parallel passages extending in one direction for conducting a first fluid that is above ambient tem erature and having a second set of generally flat parallel passages inter-leaved with the first set and extending in a direction substantially at right angles to said one direction for conducting a second, cooler, fluid, the construction comprising: a plurality of generally fiat sheets in generally parallel spaced relation with successive pairs of sheets joined directly together along one pair of opposite edges and defining flat conduit units constituting the sole essential structure forming said first set of passages, and the adjacent sheets of each adjacent pair of said conduit units being joined directly together along their other pairs of opposite edges for defining all of the interior passages of said second set of passages, said sheets having straight corrugations extending longitudinally of and projecting into said first passages and the corrugations the full length of each pair of sheets that define one of said first passages merging and being welded together, whereby said sheets are reinforced.

3. In a heat exchanger of the cross-flow type having a first set of generally flat parallel passages extending in one direction for conducting a first fluid that is above ambient temperature and having a second set of generally flat parallel passages inter-leaved with the first set and extending in a direction substantially at right angles to said one direction for conducting a second, cooler, fluid, the construction comprising: a plurality' of generally fiat sheets in generally parallel spaced relation with successive pairs of sheets joined directly together along one pair of opposite edges and defining flat conduit units constituting the sole essential structure forming said first set of passages, and the adjacent sheets of each adjacent pair of said conduit units being joined directly together along theirv other pairs of opposite edges for defining all of the interior passages of said second set of passages, each pair of adjacent sheets that defines a passage of said first set being longitudinally straight at their joined edges and being longitudinally undulate intermediate their joined edges, whereby the intermediate portions can yield" to compensate for relative expansion and contraction of the edge portions with respect to the intermediate portions.

4. A heat exchanger as defined in claim 3, in which the two sheets of each pair defining one of said second passages have juxtaposed corrugations extending longitudinally of and projecting into the second passages into contact with each other.

5. In a heatexchanger of the cross-flow type having a first set of generally flat parallel passages extending in one direction for conducting a first fluid that is above ambient temperature and having a second set of generally flat parallel passages inter-leaved with the first set and extending in a direction substantially at right angles to said one direction for conducting a second, cooler, fluid, the construction comprising: a pluralityof generally flat sheets in generally parallel spaced relation so joined together at their edges that they constitute the sole essential structure defining said first set of passages and all the interior passages of said second set, each pair of adjacent sheets that defines a passage of said first set being longitudinally straight at their joined edges and having straight longitudinal corrugations welded together and spaced from each other and from said joined edges, the two sheets of each pair defining one of said second passages having juxtaposed straight corrugations extending longitudinally of and projecting into the second passage into contact with each other, said sheets being longitudinally undulate intermediate their joined edges and said joined corrugations and intermediate the corrugations projecting into said second passages, whereby the unsupported portions of said sheets can yield to compensate for relative expansion and contraction of said different portions of the sheets.

PAUL A. PITT.

nernnanons crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 849,878 Zivi Apr. 9, 1907 1,472,954 Behringer Nov. 6, 1923 1,734,962 Clarke Nov. 12, 1929 1,751,725 Cross Mar. 25, 1930 1,775,103 Hume Sept. 9, 1930