Tube for boilers, heat exchangers, and the like

Nov. 6, 1934,

R. S. BROWN

1,979,859

TUBE FOR BOILERS, HEAT EXCHANGERS, AND THE LIKE

Filed Aug. 29, 1932 . 2 Sheets-Sheet 1

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Nov. 6, 1934. _{R s BROWN} 1,979,859

TUBE FOR BOILERS, HEAT EXCHANGERS, AND THE LIKE

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Patented Nov. 6, 1934

1,979,859

UNITED STATES PATENT OFFICE

1,979,859

TUBE FOR BOILERS, HEAT EXCHANGERS, AND THE LIKE

Roger Stuart Brown, Chicago, Hl.

Application August 29, 1932, Serial No. 630,847

Claims. (Cl. 257—262)

This invention relates to improvements in tubes used in air heaters, recuperators, low pressure water heaters, air coolers, or other heat exchangers, where both mediums are under rela5 tively low pressure. It will be realized that I do not wish to limit myself, except so far as set out specifically in the claims, to any particular use of my invention, it being adaptable to a wide variety of uses.

One object of the invention is to obtain a greater heat transfer per square foot of tube surface, and a greater number of square feet of surface per lineal foot of tube of a given width, thereby obtaining a greater capacity from apparatus 15 of a given size, under a given allowable pressure drop for the gases between which heat is exchanged. Another object is to lessen or minimize draft resistance, for example, by streamlining the exterior of the tubes, or by elimi20 nating tortuosities of the passage or path traveled by the gases. For example, by streamlining the exterior of tubes at a given velocity of the medium, for instace furnace gases, such medium is circulated across a bank of tubes at more uni25 form speed and with elimination of back eddies, and consequently less draft loss. A further object is lessening tb« tendency of soot to deposit in or on tubes by the elimination of dead spaces in the flow. Various other objects of my inven30 tion will appear from time to time in the course of the specification and claims.

I illustrate my invention more or less diagrammatically in the accompanying drawings, wherein—

Figure 1 is a transverse section through a bank of tubes;

Figure 2 is a similar section illustrating a variant form of tube;

Figure 3 is a side elevation and partial section ⁴⁰ of the tubes shown in Figure 1;

Figure 4 is the end elevation of one of the tubes of Figure 3;

Figure 5 is the side elevation and partial sec_4g tion of one of the tubes with transverse ribs ° shown in Figure 2;

Figure 6 is a section on the line 6—6 of Figure 5;

Figure 7 is a similar view to Figure 6 of the 50 variant form of my invention having transverse ribs with a circular periphery.

Figure 8 is a section of a further variant form of tube; and

Figure 9 is an elevation of a grating used for 55 end plates or center supports for the tubes,

Like parts are indicated by like symbols throughout the specifications and drawings.

Referring generally to the drawings, I illustrate a bank of tubes for example, for a boiler air heater, so arranged, and the exterior being 60 so shaped, as to provide passages of generally constant cross sectional area between the tubes, with passage turns limited to a minimum and with the elimination of projections, bends and sharp turns, which tend to cause eddies in the 65 circulation of the exterior medium. Staggering of tubes across the flow of the heating medium increases resistance to the flow, but such staggering is often helpful, in particular to bring the gases circulating outside of the tube in contact 70 with as great a percentage of its periphery as possible. It is to minimize the results of such staggering that I have developed the streamlining and uniformity of cross sections in the passages above referred to. 75

Referring for example to Figure 1, I illustrate tubes A A, each such tube having diametrically opposed points or elongations A¹ A¹, which are streamlined as shown in the figure. The tube is so shaped that the tubes may nearly touch at the 80 maximum diameter, or they , may be separated by a small space as shown. Note that the outside of the tube is so formed and the tubes are so spaced, that passages A² are formed around them which are substantially of equal width, and the 85 passages progress in a series of gently flowing arcs. Note also that the interior A⁵ of the tube substantially follows the outside outline, that is, the wall is of substantially uniform thickness. In Figure 3 is shown a side elevation and partial 90 axial section of two of the tubes as grouped in Figure 1, the end view being shown in Figure 4. By reversing the large and small ends of the tube as shown in Figure 3, it is possible to group them closer. The holes in the far end plate are shown 95 by the circles in Figure 1. The tubes in each row are placed with the large ends one way, and the tubes in the next row with the large ends the other way. Figure 3 shows one end of the tube forced back into a round shape at A⁹. The other ioo end is first forced back into the round shape as at A¹⁰, and then expanded into a larger round section A¹¹, whose outside diameter is slightly greater than the maximum diameter of a streamlined portion ALA¹. This permits withdrawing 105 the tube through the hole in the header into which A¹¹ is rolled or welded.

However where it is not desired to roll tubes into the header I may leave the tube section constant throughout, and weld them into holes of sim- HO

1,979,889 ilar shape in a header sheet. This permits of closer spacing with greater heat transfer.

While I have shown the large round end A¹¹on the tube of Figure 3 as made integral with 5 the tube, I may use a separate piece, as for instance is shown in Figure 3 of my co-pending application No. 466,755, filed July 9, 1930, which has since matured into Patent No. 1,870,760, dated August 9, 1932. This is particularly valuable 10 in connection with very thin-walled tubes which cannot stand the additional stretching to expand them out to a larger diameter. The ends A⁹ and A¹¹ may be rolled into grooved holes in headers or return fittings, or welded in, or otherwise suit15 ably fastened (see Fig. 3). I do not wish to be limited to any specific means for securing the tubes to the header, but I do find the means herein' shown practical and efficient.

Where the sides of the tube in section define ²⁰ the arc of a circle, I find it desirable to use the center of this circle to strike the concave arc defining the section of the concave sided streamline extension opposite. The length of the radius of the concave sides of the streamline extension ²⁵ will Toe the radius of the side of the tube plus the width of the passage.

Figure 2 shows a group of tubes of similar cross section of the body of tube but with a series of parallel fins A³ shrunk, pressed, or welded to ³⁰ the tube. They may also be applied by expanding the tube over a mandrel against the hole in the fins, or by brazing, or in any other way so that the fins are applied to the tubes in an effective heat-conducting relationship, or where a 35 cast tube is advantageous they may be integrally cast. Note that this transverse fin extends out from the tube approximately an equal distance at all points. That is, the periphery follows the general shape of the exterior of the tube. I find 40 it convenient to cut off the top and bottom straight across at A⁴. As shown in Figure 2, these fins extend out from the tubes from each side nearly across the space between. Clearance around the fins A³ may be left for the inter, 45 mediate and end supporting plate or grating. By keeping the height of the fin constant, I achieve the maximum heating effect without getting any one part of the fin too far away and, in certain cases, therefore, too much hotter than 50 the wall of the tube. These fins not only serve to greatly add to the heat-conducting effect on the outside of the tube, but they greatly help to keep the tube in shape, and in particular from bursting out of shape under even low pressures 55 of the medium inside.

In Figure 51 show a side view and section of the tubes grouped in Figure 2, showing how they rest on the fins in an end supporting plate or grate B, which is shown in front elevation in Figure 60 9. Figure 6 is a section on line 6—6, of Figure 5.

For high temperature work it is desirable that tubes be made seamless. With tubes of this design it is possible to start with a seamless round tube and deform it to shape shown by pressing, 65 rolling, or drawing over a mandrel. In Figure 6 note that the inside of the streamline extensions A¹ are not pressed tightly together, but . a small space A⁶ with a small radius at the bottom is left between the sides of each stream70 lining extension. This serves a twofold purpose.

It prevents cracking of the tube wall where it is bent sharply around this point, and it also in high temperature work has the great advantage of preventing the tip of this streamlined portion 75 from being overheated and causing burning or warping due to the unequal expansion. For example, if the cooler medium is inside of the tube, some of it will circulate through the space A® keeping the tip temperature down below the point of deterioration, which would not otherwise be f the case.

When the tubes are made with transverse fins as at A³ in Figure 5, they should be parallel to the flow of the gas flowing around the outside, and not necessarily perpendicular to the axis of j the tube as shown.

For very low pressures, as for instance several inches of water, I find the form of tube shown in Figure 8 to be economical. For such pressure a very thin wall is necessary, compared to the g diameter of the tube. It is difficult and expensive to make such a round tube, and I may, therefore, make the tubes by pressing or rolling two flat sheets into the characteristic form and fasten the edges A¹⁰ together, preferably by fusion, gas, 9 or arc welding, though other methods such as copper hydrogen brazing may be used for some purposes. With such design of tubes, the transverse ribs A³ of an approximately equal height around the tube are particularly valuable in m maintaining its shape.

It will be realized that whereas I have described and shown a practical and operative device, and have illustrated various modifications and applications thereof, that I do not wish to be limited iqi specifically to the forms herein described and shown, except so far as I limit myself by the language of my claims. I wish my description and drawings to be taken in a broad sense, illustrative and diagrammatic rather than as limiting ₁₁₍me precisely to the details indicated. The use and operation of my invention are as follows:

In current practice in the design of such low pressure heat exchangers as air heaters, round tubes are used arranged in a bank with rows 115 staggered across the direction of flow of the outer medium, usually by one-half the tube spacing across the direction of flow. In this arrangement, the gas streams passing through the bank find a minimum width of passage where adjacent 12Ο tubes come closest together. Once past this point, the passage widens out, tending to reduce the velocity of the stream, and causing back eddies in back of the tubes. This causes loss of velocity head. One phase of my invention is 125 the provision of a shape of tube whose surface confines the gases, as they pass between the tubes to passages of relatively constant cross-sectional area where the tubes are staggered. I obtain this result by. symmetrically streamlining the i₃₀tubes in cross-section and by arranging tubes so streamlined in such fashion that the distance between the opposed tubes is constant; the result is an easy passage for the gases outside of the tubes along a series or succession of flowing 135 curves without sharp points or angles, or restrictions in the passage or expansion chambers, or any means or method likely to delay or impede the flow of the exterior circulating medium.

Stated generally, by the various forms of tubes, 14η I illustrate, I obtain a uniform channel for the heated gases, a relatively smooth passageway of constant cross section between the tubes. I therefore obtain a minimum interference with the flow of the gases, and a minimum loss of 14«; velocity as the gas around the tubes passes each row of tubes. . At speeds below a few feet a secand, gases will travel in a stratified flow, and it is helpful to baffle or stir up the flow. But for most commercial air heaters and the like, 150

1,979,859 speeds greater than this are used and there is constant turbulence and flow within the medium and no necessity to agitate it since it provides its own turbulence. Instead of disturbing the 6 flow by changing the width of the channel, it is my idea to disturb it as little as possible, and so obtain the minimum draft loss for a given velocity.

When I speak of gases, I mean it to be taken in a broad sense, gaseous as distinctive from liquid. 10 Thus, it may be mixed gases of combustion or it may be air. Where one of the mediums to be heated or cooled is a liquid, I put it inside the tube. Much greater total heat transfer can be obtained by streamlining the gas passages to 15 form sinuous equal width passages, than by streamlining water passages. There is a very high film heat transfer on the water side. Increasing that by streamlining adds only a fraction to overall heat transfer compared to that 20 possible by streamlining and thereby increasing with a given draft the velocity in the gas passages.

In the use of my invention by reducing the loss of velocity head between rows of tubes it is pos25 sible to build a unit of the same heat transferring capacity with a lower draft loss for a given velocity. However, I prefer to take advantage of this fact by so reducing the free area between the tubes of a row that the velocity is increased 30 to produce the maximum draft which is allowable at the maximum duty. Then I obtain a greater velocity between the tubes than is at all possible with the designs embracing round tubes. An increased velocity results in a nearly pro35 portionate increase in the film heat transfer on that surface per square foot. If the increased velocity is obtained by placing the tubes closer together than would be the case in a similar design embracing round tubes, there is a further 40 increase of-heat transfer for the same velocity due to reducing the mean hydraulic depth of the passage. Furthermore, the area of my tube, even without transverse fins in effective contact with the exterior circulating medium as furnace gases, 45 can be made much greater and even twice the exterior area of the round tube of the same width, As a result of this an increased heat transfer is obtainable per lineal foot of tube, with an increased heat transfer per square foot, and a greater 50 square footage per lineal foot of tube, not so many rows of tubes are required to accomplish a given heat effect. This of itself will reduce the draft necessary at a given velocity.

By greatly increasing the amount of surface 55 and the heat transfer for a given volume of apparatus I achieve a great saving in space and in cost of headers and casing.

Another advantage of my symmetrical type of streamlined tube is that the gases or air may •K go through the bank in either direction with equally efficient draft loss and heat transfer in either direction. In many types of air heaters or other heat exchange apparatus it is necessary to lengthen the path of the gases circulating around K the tubes by dividing the bank with transverse baffles so that the gas will go across the bank at one end in one direction, and then be reversed and go back across the other end in the other direction; or there may be several passes back TO and forth. With non-symmetrical type of streamlining there is much higher resistance in one direction than in the other.