US2252069A

US2252069A - Heat exchanger

Info

Publication number: US2252069A
Application number: US287702A
Authority: US
Inventors: Fletcher James
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1939-08-01
Filing date: 1939-08-01
Publication date: 1941-08-12
Anticipated expiration: 1958-08-12

Description

Aug. 12, 1941.

2 Sheets- Sheet 1 INVENTOR.

ATTORNEY.

Aug. 12, 1941. J FLETCHER 2,252,069

HEAT EXCHANGER Filed Aug. 1, 1939 2 Sheets-Sheet 2 IN VENTOR.

BY James Fletcher m Patented Aug. 12, 1941 UNHTED STATES PATENT QFFHQE HEAT EXCHANGER James Fletcher, Akron, Ohio, assignor to Babcock & Wilcox Company,

The Newark, N. J., a

6 Claims.

This invention relates to heat exchange apparatus of the vapor generating class and it is particularly concerned with tubular connections through which a liquid or fluid is introduced to pressure vessels employed in such apparatus. Certain types of vapor generators and steam boilers require the use of pressure vessels such as steel drums and headers arranged with metallic tubular conduits connected to them in a pressure-tight manner, and such arrangements usually involve thermal connections between these elements, and when the liquid or fluid flowing through such connections to the pressure vessel is at a temperature substantially different from the vapor or liquid contents of the drum, the metallic elements of the connections transfer heat between the fluid in the tubular connections and the drum walls. If the liquid in such connections is at a temperature substantially below the temperature of the contents of the drum, this transfer of heat because of the temperature differential, may cause the cooling of the drum metal in the zone of the connection to considerable extent below the temperature of the vapor Within the drum. When such a reduction in drum wall temperature occurs at the connection while the steel drum wall is maintained at the temperature of the drum contents at adjacent positions a condition occurs wherein high stresses are produced in the drum wall. are long continued cracks may appear in the drum metal, weakening that structure and necessitating extensive repairs, and such results are often aggravated when the fluid is such that it has a corrosive effect upon the metal.

In pressure vessels of the character referred to, for high pressure, for instance, above 600 lbs. per sq. in., the walls are of metal of high tensile strength and of substantial thickness, even when of moderate diameter, and of equal thickness for lower pressure and larger drum diameter. Tubes or connections-are required to supply fluid to, or withdraw it from, such pressure vessels in service, and the fluid flowing through such connections may have substantially lower or higher temperature than that contained in the pressure vessel.

The temperature of the metal of the pressure vessel is normally that of the fluid contained in it, especially when there is external insulation, and the temperature of the metal of the tube or pipe conveying fluid to, through, or from the vessel is the same as that of the fluid flowing in it, or very nearly so. The metal of the connecting tube and that of the vessel walls are in contact When such conditions 3 at a junction which, in high pressure work, is a joint that is rolled or welded, and the metal of the tube may extend wholly through the hole in the vessel wall. When the tube does not extend entirely through the hole, the flowing fluid will come into direct contact with the metal of the wall of the pressure Vessel and. tend to give it the fluid temperature at the contact surface, unless means are provided to prevent such contact.

When the flowing fluid has a different temperature than that of the fluid in the vessel steadily or momentarily, the tube metal in contact with the flowing fluid will have a different temperature than the metal of the vessel wall, and the metal of higher temperature will have expanded more than the other. This relative expansion will have an undesirable effect on the joint between the two metal parts. For example, if the tube is steadily conveying a fluid to the vessel and the flowing fluid is colder than the main body of fluid within the vessel, the tube metal will contract relative to that of the vessel and damage to the connection will result. A tube expanded into a hole in the vessel wall will act this way. In such a joint, and under these steady conditions, there will be a flow of heat from the hotter metal of the vessel wall to the colder metal of the tube in contact with it, so that the metal of the tube at this zone will not be as cold as the fluid flowing inside of it. However, if the fluid entering through the tube suddenly becomes colder than that in the Vessel, especially if, at the same time, its flow rate also increases, then the tube metal will suddenly cool and contract because of lack of time for heat flow. from the vessel wall to the tube metal to compensate, and leaky conditions will result. Such leaky conditions with high pressure fluid will increase and may become permanent because of the erosive efiect of such escaping fluid on the surrounding metal.

In addition. to the bad effect on joints between tube and vessel metal arising from difierent fluid temperatures, especially when there is a sudden change in that of the flowing fluid, there is another type of undesirable effect that may result and this is an excessive stress in the metal of the vessel that may cause-a crack or a rupture. An example of such a condition arises when the fluid entering the vessel is much hotter or much colder than the fluid in the vessel so that there is a temperature gradient and a heat flow in the metal of the vessel wall around the tube hole radially away from the hole, or toward it. When expand at the tube seat relative to the metal at a distance, and a crushing stress will be set up, which may exceed the metal resistance, and cause a failure of the joint between the tubular connection and the drum.

Such stresses beyond the elastic limit of the metal are serious, and may be more serious than the leaks due to relative expansion of the two metals at a joint. Some joints between the metal of the tube and that of the vessel wall are of the metallically integral type produced, for example, by fusion welding. Relative expansion of the two metals thus joined may result in an excessive stress in the junction metal and cause the joint to fail, independent of damage to the metal of the vessel wall or that of the tube. Such failures are doubly serious as to the operativeness and safety of such structures.

When such tubular connections supply feedwater, to the steam and water drum of a high pressure steam boiler and extend through the steam space of the drum, there exists a set of conditions particularly favorable to the production of the above indicated undesirable results. This is particularly true when the metal is highly stressed and simultaneously exposed to pure condensate.

When the operating characteristics of the vapor generator of which the illustrative tubular sections are parts involve wide changes in temperature of the liquid flowing through such connections, in short periods of time, and intermit-. tent heating and cooling of the metal in the Zone of the tubular connection, there will be resultant over-stressing of the metal, and fatigue failure due to intermittent stressing may occur more quickly than when operating conditions are steady.

This invention solves such problems by affording tubular connections which are particularly applicable to high pressure boilers wherein the feed from an economizer to the steam and water drum of the boiler varies widely in temperature due to operating conditions beyond the control of the designer.

The problem of supplying safer equipment for the foregoing conditions of service is solved in the present instance by a construction which minimizes the damaging temperature differentials and thus accomplishes a controlled relative expansion and contraction such as to materially reduce the liability of leakage or cracking due to stresses and in most cases to entirely prevent leakage and failure of the metal.

Other objects of the invention will appear as the accompanying description proceeds.

The invention will be described with reference to preferred embodiments which are indicated in the accompanying drawings.

In the drawings.

Fig. 1 is a vertical section of a vapor generator operating at high pressures and high temperatures;

Fig. 2 is a vertical section of a part of the high pressure steam and water drum included in the Fig. 1 installation, with an embodiment of the invention associated therewith;

Fig. 3 is a transverse section of the tubular drum connection indicated in Fig. 2, this section being taken on the line 33 of Fig. 2 and looking in the direction of the arrows. This view particularly shows means whereby the ferrule is held spaced internally from, and centered with respect to, the drum connected tube;

Fig. 4 is a vertical section through a drum and. tube connection adapted for operation with a steam generator operating at pressures in excess of 2000 lb. per sq. in.; and

Fig. 5 is a transverse section through the throttle ring of the Fig. 4 embodiment, this section being taken on the line 5-5 of Fig. 4 and looking in the direction of the arrows.

Fig. 1 of the drawings indicates a steam generating installation in which feedwater from the economizer i0 passes through the conduits II and 12 to a steam and water drum I4. This particular installation operates at steam pressures in excess of 2000 lb. per sq. in., and, consequently, the drum must have thick walls and be constructed of steel of high quality. The particular drum It has a diameter in excess of 5 ft. and has walls of a thickness in excess of 6 /2 in. The steam in the drum has a saturated temperature of about 650 F.

The conduits H and I2 conduct the discharge from the economizer ii) to the drum M, and they enter the steam space of the drum downwardly as indicated in Fig. 1. Each of these conduits may be connected with an internal feed pipe which conducts the entire flow of fluid through the tubes to a feedwater distributor located at another position in the drum, and the preferable arrangement is that such internal ferrules as those indicated at 24 and I08 in Figs. 2 and 4 shall be directly connected with such internal feed pipes.

A tubular drum connection by which one of the conduits H or 12 communicates with an internal feed pipe is particularly indicated in the drawings. As shown in Fig. 2 the conduit H is welded to an external sleeve or tubular connection 20. The latter forms a continuation of the conduit H and extends through an opening 22 in the drum wall 26. The major portion of the tube opening is slightly larger than the external diameter of the tubular member 20, and the positive connection between the drum metal and the metal of the member 20 is limited to the inner part of the drum wall. The inner end of the tubular member 20 when in assembled position may have tube metal extending into the grooves 28 and 39 formed within the tube seat.

The inner end of the tubular member 20, forming in effect the end of the conduit II, is tightly expanded against the tube seat. The extreme inner end of the tubular member 20 is then slightly belled out as indicated at 32 and is thereafter preferably seal welded to the inner face of the drum wall by the deposition of weld metal indicated at 34.

The ferrule 24 has an external diameter which is less than the internal diameter of the tubular member ZO and it is formed integrally with a circumferential collar 36 at its end disposed externally of the drum. Between this circumferential collar 36 and the inner end of the ferrule 24, the latter is formed with another integral annular collar, or throttle ring, 40.

The outside diameters of the circumferential collar 35 and the major part of the outside surface of the throttle ring 48 are such that the entire ferrule assembly may be moved within the tubular member 20 to the position indicated in Fig. 2 of the drawings, and the circumfercntially arranged spacers t2 near the lower end of the ferrule are also formed to permit this action. After the ferrule 26 is inserted within the tubular member 2!) and brought to the position indicated in Fig. 2 of the drawings, it is expanded against the inner surface of the member 30 at the position of the collar 36 and the throttle ring 49. There are thus formed two annular chambers 42 and M, within the member 26 and between it and the outside surface of the ferrule 28. These chambers are separated by the throttle ring to which has the same relation to the surfaces of the ferrule 25 and the member 29 as the throttle ring II2 has to the member Idll and the ferrule H38 in Fig. 5 of the drawings.

The upper part of the throttle ring so is provided with a steam inlet similar to the opening I20 shown in Fig. 5, and a drain opening of a greater circumferential extent is provided at the lower part of the throttle ring. This drain opening 55 corresponds with the opening I24. of the throttle ring construction indicated in Fig. 5, and the steam inlet opening d8 corresponds to the opening I26. As the tube 20 enters the drum in an upper quadrant above the normal water line of the drum which is at or slightly below the centerline, the annular chambers 42' and M are in communication with the steam space of the drum M. The two integral collars 3t and l which are expanded into the tube 2b in order to position the ferrule 2- 5 in proper position in the tube also effect a good thermal bond in the cylindrical zone of contact. This permits a limited transfer of heat from the ferrule 24 to the tube 20 at the position of the collars 35 and it.

The throttle rings separate the annular spaces between the outer tubes and their respective ferrules into two longitudinally adjacent chambers which, because of their different positions with respect to the vapor space of the drum (which is the source of heat when the invention is embodied in a steam boiler), will operate with different temperature conditions. The temperature of the tube 20 in the length between the collar 38 and the tube seat will be maintained at a value intermediate the drum temperature and the temperature of the fluid flowing through the drum with the temperature change gradual from end to end of the gradient.

In either embodiment of the invention the throttle ring positioned intermediate the ends of the ferrule inside of the tube (either 2G or Iilil) divides the annular space between the ferrule and the tube and is effective to prevent too steep a temperature gradient at the zone of junction between the ferrule and the tube around it. With the use of the throttle ring the temperature gradient is more gradual, and it extends from a position adjacent the throttle ring to a position adjacent the zone of junction between the ferrule and its external tube.

The arcuate ports I 20 and I'2 l, in the upper and lower quadrants of the collars, provide for the controlled movement of steam, or vapor, to the outside chambers 62 and I 22. They also provide for the controlled draining of condensate formed in these annular chambers, while still transferring some heat between the inner wall of the tube 20 or the tube Hit and the inner wall of the ferrule 24 or I08.

An increase inflow area of the arcuate openings I26 and I24 will permit greater freedomof flow of heating vapor into the outside annular chamber with a subsequent tendency to produce a steeper temperature gradient.

The extent of thermal contact of the expanded throttle ring with the interior wall of the outer tube effects the transfer of heat by conduction between the inner and outer tube of the ferrule and this transfer tends to counteract the temperature of the annular chamber.

When tubular connections such as those above described enter a drum radially above its horizontal centerline the cooled liquid in the annular space between the ferrule and the tube externally thereof can drain into the drum. This permits controlled circulation of the high temperature medium into the high annulus, replacing the cooled liquid drained from the annulus. When, as in the embodiments shown in the drawings, the tubular connections are in the steam space of a steam and water drum the circulation between the annular chamber and the steam space will be more rapid. This natural circulation maintains practically the full internal drum temperature over the entire length of the annulus when no throttle ring is employed and this results in a very steep temperature gradient across the ring section forming the junction between the ferrule and outer tubes at the end of the annular chamber remote from the drum.

With such tubular connections as those shown and described herein, the most satisfactory condition is to maintain the temperature of the medium in the annular chamber between the ferrule and its surrounding tube at a value approximately the same as the temperature of the internal medium in the drum for distances within the annular chamber equal to the length or width of the contact area between the outer tube and the drum metal. From this position in the annular chamber to its end remote from the drum, or to the position of the juncture between the inner or outer tubes, the temperature of the medium between the inner and outer tubes should follow a gradient from feedwater temperature at the end of the annular chamber remote from the drum to drum Wall temperature at the point of first contact between the outer tube and the tube seat.

As the port I25! restricts and controls the vapor into the chamber I22 and the port I 24 controls the drainage of condensate therefrom, the pressure of the vapor in the chamber I22 will be less than that of the annular chamber I28, and less than the pressure within the drum. This pressure differential will depend upon the frictional resistance of the port I20 to the flow of vapor introduced by condensation in the chamber I22, and the rate of condensation is determined by the temperature of the fluid flowing through the tube We or the end of the ferrule I08.

When the temperature differential between the fluid of the tube are and the fluid within the drum is greatest, the pressure in the chamber E22 will be at a minimum. The vapor temperature of the chamber I22 will be uniform throughout its length and will depend upon the pressure.

With the illustrative arrangement the ferrule H18 is subject to vapor heating on its outside surface, and the inside surface of the tube Ifill in its length corresponding to the length of the ferrule IIEB therein is also subject to such heating. The temperature of the vapor in the annular chamber I26 corresponds to that of the drum while that of the chamber I22 is lower. 1

The embodiment of the invention indicated in Figs. 4 and 5 of the drawings includes a feedwater tube I the inner end of which is expanded against the inner part of a tube seat formed in the drum wall I02. The tube I00 preferably enters the drum radially and at about an angle of 45 above the centerline of the drum. After the tube is expanded within the drum its inner end is preferably belied out as indicated at I04. It is also seal welded to the drum wall as indicated at I06.

Telescoped within the tube W0 and extending beyond the tube into the drum wall I02 there is a ferrule I08. This ferrule is provided at its outer end with an integral collar or boss I I0 and at a position intermediate its length, it is provided with another external collar or throttle ring I I2. Each of these collars fits tightly within the tube I00 and is secured to the tube by expanding operations with the ferrule. The throttle ring may have been previously fixed to the ferrule I00 by an expanding operation performed before the ferrule assembly is inserted within the tube I00.

The ferrule I00, at its inner end, is in free contact with the high temperature fluid within the drum and at positions spaced from the drum it is in good heat transfer relationship with the tube I00.

At the top of the collar II2 there is a small opening I20 providing a restricted passage for steam to the annular chamber 52. Any steam condensing in this chamber by reason of the cooling action of the low temperature fluid flowing through the ferrule I08, drains from this chamber through the arcuate drain opening I24, indicated in Fig. 5 of the drawings.

The ferrule I00, in conjunction with the tube I00 and the collars H0 and II2, forms the two annular chambers I22 and I20 the latter of which is in free communication with the fluid within the drum I02. The action of the high temperature fluid circulating through these annular chambers combines with the conduction of heat through the metal of the ferrule I08 and its collars H0 and H2 to transfer heat from the high temperature fluid in the drum to the metal of the tube I00 at such rates that the temperature curves indicated by the temperature diagram of Fig. 4 apply. The curves of this diagram indicate the metal temperatures along a section of the tube I00 within the influence of the heat transfer affected by the ferrule construction and the fluid action associated therewith. These curves represent the results of experimental tests. During the tests the temperature of the fluid within the tube I00 was approximately 75 F., and consequently the curves start at this temperature, at a point on the tube E00 about '7 removed from the drum wall I02, and the action of the various heat transfer influences above described, results in the gradual increase in the temperature of the tube I00 as the wall of the drum is approached. For example, an upper curve shows that the temperature of the tube I00 at a point 3" away from the drum wall is about 300 F., and from that point there is a rise in the temperature of the tube metal until at the drum wall the tube metal has reached the saturation temperature within the drum, approximately 412 F. Consequently, at a position adjacent the tube seat there is no drum metal temperature differential of sufficient extent to cause any damaging stresses across the wall of the drum at the tube seat. The lower set of curves of the thermal diagram of Fig. 4 indicates the original temperature of the metal of the tube I00 as being about 75 F. at a position '7 removed from the drum wall I02, and the temperature for both the bottom and the top of the tube gradually rises as the drum wall is approached. It rises to the saturation temperature of another set of operating conditions, that temperature being in this case slightly over 300 F.

With further reference to the embodiment of the invention indicated in Fig. 4 of the drawings, it is also to be noted that the tube I00 has a pressure-tight relation with the metal of the drum wall I02 over only a fraction of the thickness of the wall. Such relationship extends from the seal weld I00 to a position indicated by the reference character A, and from A to B the tube I00 may be considered as loosely fitting within the opening of the drum wall. This relationship prevents loosening of the tube seat due to creeping which might occur if the expanded tube seat connection between the tube and the drum extended over the entire thickness of the drum wall.

Referring again to Fig. 1 of the drawings, the economizer I0 is subject to contact with gases from the furnace I30. As shown, this furnace is fired by a plurality of pulverized fuel burners I32, E34, and I36, and the gases passfrom the furnace through an exit I38 to the bottom of the gas pass I40 in which the economizer I0 is located. The gases flow directly upwardly through this gas pass and through a flue MI in which one or more dampers 32 are located.

The furnace I30 is defined by water tubes connected into the boiler circulation. Some of these tubes I40 and I46 extend directly downwardly from the drum I I to the header I08. Roof tubes E50 extend from the drum I4 to the header I52 which is connected with the lower header I54 by the upright wall tubes I56. From the header I54 the floor tubes I58 extend at a downward inclination to the header I60. Suitable downcomer connections establish communication between the water space of the drum I4 and the headers I08 and I60, and in the present instance, some of these downcomers include the tubes I64 and I60 which extend across the path of the gases in the gas pass I40 and then downwardly alongside the wall I'I0 to the header I48. The side walls of the furnace may be defined by upright tubes directly connecting the headers I and I which have I appropriate circulatory connections with the drii m I4.

I claim:

1. In combination, a steam and water drum, a tube directly communicating with the steam space of the drum for the flow of a fluid at a temperature differing materially from the temperature of a fluid within the drum, means within the tube and extending past the junction of the tube and the drum to provide an annular chamber directly communicating with the interior of the drum, and an annular restrictor disposed transversely of said chamber to divide it into two shorter chambers and provided with a restricted opening to limit circulation of steam and water between the drum and said chamber.

2. In a steam boiler, a steam and water drum, economizer tubes entering the drum and expanded into the shell thereof, an auxiliary tube in each of said economizer tubes and extending from a point outside of said shell into said drum, said auxiliary tube being spaced from the interior of its economizer tube at the expanded portion thereof and arranged to conduct the water flowing through said economizer tube into said drum and maintain the water out of thermal contact with said expanded portion, and a throttle ring fixed around the auxiliary tube intermediate its ends so as to divide the annular space between each economizer tube and its auxiliary tube into a plurality of connected fluid chambers in communication with said drum.

3. In a fluid heat exchange system, a pressure Vessel for a fluid at one temperature, a tube fixed within an opening in the wall of the pressure vessel and functioning to conduct to the vessel a fluid at a temperature considerably dilferent from the temperature of the fluid within the vessel, and a tubular member fixed within the tube and having its outer surface spaced from the inner surface of the tube, said tubular member having a plurality of external annular collars in tight metal-to-metal contact with the member and the inner surface of said tube with one of the collars positioned at the end of the tubular member outside of the vessel and a second positioned intermediate the ends of the tubular member but within the tube and spaced from the end thereof, the second collar dividing the annular space between said tubular member and the tube into two component annular chambers in communication through an opening provided in said collar, the inner end of the tubular member extending to a position within the pressure vessel where its external surface is in free contact with the fluid within the vessel.

4. A thermal gradient tubular connection for fluid heat exchange apparatus comprising a thick walled pressure vessel, a tubular member communicating directly with the vessel, fluid flowing through said member to the vessel being at times at temperatures considerably lower than the temperature of the fluid within the vessel, and a thermal gradient ferrule secured within said member and spaced therefrom to afford an annular chamber in communication with the fluid within the vessel, and a second annular chamber in restricted communication with the first, the ferrule being provided with a throttle ring separating said chambers.

5. In a water tube steam boiler, a steam and water drum, an economizer, an economizer outlet tube in communication with the steam space of the drum and conducting fluid into the drum at temperatures materially less than the temperature of the fluid within the drum, and thermal gradient ferrule means including an inner tube with spaced collars fixed thereon and in metalto-metal contact with the inner wall of the economizer outlet tube in distinct and separated circular zones both of which are disposed externally of the drum wall, said means establishing such a thermal gradient in a section of the economizer outlet tube adjacent the drum that the metal of this section closely approaches the temperature of the fluid within the drum.

6. In a fluid heat exchange installation; a pressure vessel; and a thermal gradient tubular connection in communication with the vessel; the tubular connection including two concentric conduits with the conduit of smaller diameter extending through an opening in the wall of the vessel, said conduit being subject to the flow into the vessel of a fluid at a temperature materially differing from the temperature of fluid within the vessel, the outer conduit secured to the vessel in pressure tight relationship and extending externally thereof at least to a position wherein it is secured to the inner conduit, and ring means contacting said conduits and disposed transversely of the annular space between them, said ring means being located between said position and. said pressure tight connection and dividing the annular space between said conduits into two distinctive temperature zones.

JAMES FLETCHER.