US3336974A

US3336974A - Serpentine tube boiler

Info

Publication number: US3336974A
Application number: US453381A
Authority: US
Inventors: Bernstein Ernest; David G Randall
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1965-05-05
Filing date: 1965-05-05
Publication date: 1967-08-22
Anticipated expiration: 1984-08-22

Description

Aug. 22, 967 E- BERNSTE'N ETAL 3,336,974

SERPENTINE TUBE'BOILER 2 Sheets-Sheet l Filed May 5, 1965 ug. 22, i967 E. BERNSTEIN ETAL 3,336,974

SERPENTINE TUBE BOILER Filed May 5, 1965 2 Sheets-Sheet 2 United States Patent flice 3,336,974 Patented Aug. 22, 1967 3,336,974 SERPENTINE TUBE BILER Ernest Bernstein, West Hartford, and David G. Randall,

United Aircraft Corpora- This invention relates in general to shell and tube heat transfer apparatus and more particularly to serpentine tube boilers which are particularly adapted to the eflicient generation of high temperature liquid metal vapor in a zero gravity environment.

In recent years considerable effort has been directed toward the development of a closed-circuit, Rankine-cycle, thermal power plant for the long term generation of electrical power in outer space. In a typical power plant of this type an eflicient heat transfer fluid, such as molten lithium, is circulated in a closed circuit, receiving thermal energy during its passage through a nuclear reactor and releasing this energy to a second heat transfer fluid in a liquid metal boiler. The second fluid, such as potassium, circulating in an independent second closed circuit extracts the heat from the lithium in the boiler wherein it is vaporized, the potassium vapor thus generated being utilized as the driving force in electrical turbo-generating apparatus.

In conventional boiler design, the force of gravity is utilized to effect a liquid-vapor separation in the boiler and to maintain the liquid being vaporized in intimate contact with the heated surfaces therein. In the absence of gravity, particularly in the environment of outer space, a substitute force or forces must be provided to perform the functions normally effected by gravitational forces. For this reason, in space power plant considerations, it has been necessary to depart somewhat from the traditional concepts of boiler design.

In addition to the zero gravity problem, it is quite obvious that a space system boiler must be compact and lightweight because of launching limitations, reliable because of its inaccessibility after launching, durable for maximum utility and safety, and relatively stable in operation. The problems inherent in such `a design are `further magnified because the system parameters usually specified require operation at elevated temperatures with working fluids of a very corrosive nature contained in vessels constructed ofthe so-called exotic alloys.

It is an object of this invention to provide a boiler of the shell-and-tube type which is particularly adapted to the efficient generation of liquid metal Vapor in a zero gravity environment.

A further object of this invention is to provide a boiler in which the centrifugal forces generated by a fluid flowing in an irregular path are utilized in lieu of gravity to effect liquid-vapor separation and maintain the liquid being heated in intimate contact with the heated surfaces internal of the boiler for efficient heat transfer.

A still further object is to provide a boiler in which vaporization is effected internal of a plurality of serpentine tubes which are adapted to accommodate the differential thermal expansion of the boiler over substantially the entire tube length.

Still another object is the provision of a heat exchanger in which each tube in the tube bundle is supported over substantially the entire tube length to minimize the deleterious effects of severe acceleration loading and induced vibration.

An additional feature is the compact boiler characterized and ease of construction.

provision of la lightweight, by high thermal efficiency Another feature is the provision of means whereby tube vibrations induced by two-phase fluid flow are effectively damped by the tube-locating and supporting spacers.

A further feature is the provision of a boiler which Will generate vapor of one hundred percent quality in a short developed length.

These and other objects and advantages will be evident or will be specifically pointed out in connection with the following detailed description of the embodiment of the invention shown in the accompanying drawings.

FIGURE l is a fragmentary longitudinal view of one embodiment of this invention shown partly in section and with part of the shell broken away.

FIGURE 2 is a sectional View taken on line 2 2 of FIGURE l.

FIGURE 3 is a fragmentary longitudinal view of a representative boiler tube shown partly in section, illustrating the serpentine configuration of the tube and the placement of the flashing orifice internal of the tube.

FIGURE 4 is a View taken on line 4-4 of FIGURE 3, illustrating the tube curvature in the plane of the serpentine bends.

FIGURE 5 is an enlarged fragmentary View of a portion of the serpentine tube shown in FIGURE 3, illustrating particularly the flashing orifice.

As is most clearly illustrated in the preferred embodiment of the invention shown in FIGURE l, the boiler may be seen to be generally of the shell--and-tube variety.

The fluid to be vaporized, typically a liquid metal such as potassium, enters the boiler through tube-side fluid inlet 10 and, after passing through the tubes 12, it is discharged from the boiler through tube-side fluid outlet 14. It will be noted that the diameter of outlet 14 is greater than the diameter of inlet 10 to accommodate the Volume increase associated with the conversion of the incoming liquid to vapor.

A shell-side fluid at high temperature, typically a liquid metal such as lithium heated by external means as in a nuclear reactor, is introduced to the boiler at shell inlet 16. The lithium passes through the tube bundle 18 around the outside of the individual tubes, surrendering heat in the process, and thence is discharged from the boiler through shell outlet 20 for recirculation back to the heat source. It is, of course, obvious that, depending on the desired heat transfer conditions, the shell-side fluid may be circulated counter to the tube-side fluid, in which case the positions of the shell inlet and outlet will be reversed.

Direct impingem-ent of the hot lithium on the tube bundle is avoided through the use of an impingement Ibaille 28 which is positioned in the inlet 16. Inlet tests conducted on a variety of heat exchangers demonstrated that, in the absence of such baffling means in high performance apparatus, severe vibrations could be promoted in the individual tubes leading t-o tube fatigue and consequent tube failure, usually at the point where the tube enters the tube sheet. Baflle 28 is shown in its most preferred embodiment as of a general mushroom shape, smoothly contoured to minimize pressure drop and supported in the inlet 16 by a plurality of radial legs 30. The fluid entering the shell through inlet 16 flows through the web formed by the legs 30, parallel to the stem 32, to the baffle crown 34 where it is distributed both axially and circumferentially with respect to the axis of the tube bundle. In this manner the impact for-ce of the incomf ing fluid stream is taken by the baille rather than by the several tubes adjacent the inlet, resulting in a more uniform temperature distribution, minimum tube vibration and reduced tube stress.

It will be understood that, at reduced power levels and lower lithium flow rates, the baille may be dispensed with entirely and that, depending particularly on the shape -of the inlet, alternate bafiie designs are contemplated. In high performance equipment, however, when the shell side fiuid is to be introduced transverse to the tube axes, means for preventing the direct impingement of the fiuid stream on the tubes will be found advantageous.

In order to reduce the shell-side pressure drop it has also been found `advantageous to provide plenum chambers 24 and 25 adjacent the shell inlet and outlet respectively. For this purpose a pair of generally spherical diffusers 22 and 26, forming extensions of the tubular shell 36, are provided at the shell inlet and outlet, 16 and 20, respectively, to enhance the fiow distribution of the lithium in the cross-ow areas of the boiler. The provision of these diffusers promotes radial penetration of the lithium into the interior of the tube bundle 18. Although it is not shown in the accompanying drawings, it is contemplated that, in some instances, an enlarged plenum chamber may be provided to permit fanning of the tubes in the cross-flow area to increase the tube-to-tube spacing and further reduce the shell-side pressure drop.

An inlet header or tube sheet 40 is connected to the inlet diffuser 22 and a corresponding outlet header 42 is connected to the outlet diffuser 26. As is illustrated in FIGURES 1 and 3, the individual tubes 12 in the tube bundle 18 extend through the respective headers at either end of the boiler and terminate at the outer surfaces of the headers. In the preferred construction an annular seal weld is effected between the end of the tube and the surface of the tube sheet at 52, and the individual tubes are expanded into the tube sheet to form a metallurgical bond between the outer surface of the tube and the inner surface 50 of the hole in the tube sheet. The other end of the tube is similarly connected to tube sheet 42.

Generally hernispherical closures 54 and 56 which contain and distribute the tube-side fluid to the headers are fixed to the headers 40 and 42, respectively.

Referring particularly to FIGURES l and 2, it may be seen that the individual tubes 12 are substantially identical and are nested in juxtaposition in circumferential layers about the axis of the tube bundle to provide a compact boiler configuration and provide the maximum heat transfer surface area per unit volume.

As is most clearly illustrated in FIGURE 3, the individual tubes 12 are formed with a serpentine configuration 58 over the greater portion of their length. The mixture of liquid and vapor generated within the tubes on boiling of the liquid is effectively separated within the tube by the centrifugal forces generated by the flowing fluid. Because of the tube undulations, the liquid is maintained in intimate scrubbing contact with the heated walls to permit additional vaporization of the liquid as a result of local boiling.

In a gravity field, the liquid is generally effectively separated from the vapor by the force of gravity, although tube-side boiling in a unit of the type described is -enhanced by the additional liquid-vapor separation produced by centrifugal force. In the absence of gravity, however, it is necessary to provide means for maintaining the liquid in -contact with the heated wall surface. Preferably, the liquid-Vapor mixture is continually subjected to a change in ow direction during its transit through the tubes and, to effect this continual change in direction, the tube is formed to present such a curved -path to the mixture.

Even in a gravity field, tests have established that vapor of 100 percent quality is more readily achieved in a unit of the type described than in conventional apparatus utilizing -straight tubes. This is true because at the higher vapor qualities, the effect of centrifugal force as a separation means is many times greater than that of gravity, particularly at the higher fiow rates.

As might be anticipated in a high performance heat transfer unit, means must be provided to accommodate the differential thermal expansion between the tube bundle and the shell as the unit is brought to its operating temperature. The serpentine section of the tube also performs this function. Moreover, the tube stress associated with bending as a result of the differential expansion is accommodated over the entire sinuous length rather than being concentrated at a single bend, and excessive localized stress in the tube wall is thereby avoided since the defiections at each of the serpentine bends is small.

In the fabrication of a boiler for space applications, it may be preferable to bend the tubes over their sinuous length from the plane of the sinuous bends to conform to the circular path of the tube layer in which the particular tube lies. This is illustrated in FIGURE 4. It is, of course, a means for increasing the compactness of the boiler with the anticipated weight-saving associated therewith. The radius of the bend thus imparted increases as the distance from the tube bundle axis increases and, for this reason, the transverse bend in the outer tube rows may be dispensed With entirely since its efficacy decreases With increasing diameter.

In a preferred tube configuration, as is shown in FIG- URE 3, the sinuous section of the tube is preceded by a straight portion of shorter length, constituting a fluid preheating area. Since the incoming fiuid, by proper design, remains a liquid in this area and the tube is filled, there is no need to provide a change in fiow direction in this area for maximum heat transfer efficiency.

Such a straight tube inlet portion serves an additional function. It facilitates the placement in the tube of a flashing orifice 70, shown most clearly in FIGURES 3 and 5. The preferred orifice construction comprises an annular insert 72, formed from a metallurgical bond promoting material, adjacent the inner wall of the tube and encircling a second annular insert 74 over a substantial portion of its length. The insert 74 terminates at its upstream end in an enlarged internally threaded end portion 75 which is adapted to receive a cooperating threaded orifice member 76. The insert 74 is fixedly held in position in the tube by frictional forces or more preferably by the formation of a metallurgical bond established by swaging the tube 12 and, consequently, reducing its diameter, with the pertinent inserts in their correct position. The insert 72 is utilized to promote formation of this metallurgical bond in a manner well known in the art.

In the embodiment shown the orifice diameter 78 of member 76 is of the same diameter as the passageway 80 in insert 74. However, member 76 is adapted to be insertable or removable after the tube bundle has been assembled to permit utilization of different orifice members 76 with varying orifice diameters to permit alteration of the tube-side pressure drop in varying installations. With different fluids, flow conditions and temperatures, preferred orifice diameters will change and, by this construction such changes may be effected after assembly of the tube bundle.

In the tube-side vaporization of liquid metals, the interposition of a flashing orifice has been demonstrated to be necessary for stability of operation. Its purpose is to effect a sudden pressure drop within the tube causing at least part of the liquid to flash into vapor and, thus, to initiate vaporization at a predetermined location along the tube axis. For this reason the individual tube orifices are located in a common plane perpendicular to the axis of the tube bundle.

It will, of course, be understood that in alternate boiler designs, the orifices may be located in different locations, as for example within the inlet header, and that the straight preheating tube section may then be dispensed with.

In high performance apparatus particularly, it is necessary to support the individual tubes within the tube bundle both from each other and from the shell. This is particularly true in apparatus designed for space applications since the internally and externally induced vibrations and acceleration loading could otherwise lead to premature tube failure. In the construction shown most clearly in FIGURE 2, the preferred tube spacing and tube positioning means comprises a plurality of wave Washers 90 and annular rings 92 positioned between each tube layer and further positioned at a plurality of locations axially of the tube bundle. Although for economic reasons the wave washers 90 and annular rings 92 are shown as separate elements, they may be preassembled into a unitary structure, or machined as a single element, and still perform their supporting and spacing function. In the apparatus shown, such tube support means is provided axially along the tube bundle at regularly spaced locations corresponding to one-half of the wave pitch of .the tubes.

The wave washer 90 may be seen to be generally annular and in the form of a sine wave when viewed along the axis of the tube bundle. The wave pitch of the washer 90 conforms to the tube spacing and the individual tubes are received in the troughs formed between the individual wave crests.

Annular rings 92 are provided between each tube layer, the spacing between adjacent layers corresponding to the combined thickness of the wave washer 90 and the annular ring 92. The rings 92 are adapted to conform closely to the outer periphery of the circumferential row of tubes which they encircle and further to conform closely at their outer periphery to the inner circumference of the abutting wave washer. The outer annular ring 96 is further adapted to closely conform to the inner surface of the tubular portion of the shell 36 to prevent bypass of the shell-side fluid between the shell and the tube bundle along surface 94.

The particular tube spacing means shown, comprising the combination of the annular rings 92 and the wave Washers 90, when positioned periodically along the axis of the tube Ibundle serve to damp any induced vibrations generated in the tubes as a result of two-phase uid flow or boiling instability, or from external sources.

Further, boiler fabrication and assembly is considerably simplified through this spacer construction, since the tube bundle may be completely assembled prior to incorporation into the shell and the headers. The tube spacing means and particularly the wave washers serve to position the remaining tubes within the tube bundle after a radial line of tubes has been established.

In the actual assembly of the boiler, it is the usual practice to form the straight portions of the sinuous tubes to a greater length than that desired in the finished product and, after installation and positioning of the inlet Y header 40 thereon, to cut the tubes off at the outer surface of the header. In this way, the concern with tube length tolerances is minimized since all tubes are cut flush with the header surface at assembly.

Since the tubes 12 are arranged in annular rows in the tube bundle, an open port is provided in the assembled tube bundle. To avoid a short-circuiting of the shell-side fluid through this port, a flow-defining insert 100 is provided along the axis of the tube bundle. For weight-saving considerations, insert 100 is preferably in the form of a tube, which is mounted at each end to the abutting header. To accommodate the differential thermal expansion between the tube `bundle and the shell, the insert is shown slidably mounted to the inlet header 40. For this purpose, a generally cylindrical pin 102 is ixedly mounted in the end of insert 40 so that a substantial portion projects beyond the end of the insert, the projecting portion of the pin riding in a blind hole 104 in the lower surface of header 40 after assembly of the boiler.

. In this construction, a boiler has been provided which will efliciently vaporize liquid metals, or other heat transfer liquids, in a zero gravity environment in a relatively simple and lightweight construction. Further, the boiler of this invention will be relatively immune to the deleterious effect of internallyv and externally produced vibrations and to acceleration loading. Further, through the interchange of orifices, substantially identical boiler apparatus may be easily adapted to accommodate changes in liow and temperature conditions and different fluids. Still further, vapor of percent quality may be readily generated in tubes of very short developed length.

While a preferred embodiment of the invention has been shown and described, it will be understood that various changes and modifications may "be made thereto without departing from the spirit of the invention as defined in the following claims.

We claim:

1. In a liquid metal boiler of the shell and tube type, a generally cylindrical tube bundle comprising:

a plurality of substantially identical tubes nested in juxtaposition in circumferential layers about the axis of the tube bundle, each of said tubes having a serpentine configuration over a major portion of its length,

means for supporting said tubes in radial and circumferential spaced relation about the axis of the tube bundle, and

a flashing orifice internal of each tube intermediate its length, each orifice being of equal size and positioned longitudinally of the tube bundle in substantial correspondence with each other orifice.

2. The tube bundle of claim 1 wherein:

each of said tubes is shaped over its serpentine length to conform to the circular path of the tube layer in which it is positioned.

3. The tube bundle of claim 2 wherein the tube supporting means comprises:

a plurality of circumferential wave washers positioned adjacent each tube layer, the wave pitch corresponding to the circumferential tube spacing, the washers being adapted to retain the tubes between ladjacent waves, and

a plurality of annular rings between each tube layer,

the thickness of the annular rings andthe wave washers defining the spacing between adjacent tube layers.

4. The tube bundle of claim 3 wherein:

the tubes are supported at spaced locations axially of the tube bundle.

5. The tube bundle of claim 3 wherein:

the tube supporting means are positioned axially of the tube bundle at regularly spaced locations over substantially the entire serpentine portion of the tubes, the spacing corresponding to one-half of the tube wave pitch.

6. A liquid metal boiler especially adapted to the genleration of vapor in a zero gravity environment comprismg:

a tubular shell having an inlet and an outlet,

a tube inlet header joined to said shell at one end,

a tube outlet header joined to said shell .at its other end forming a chamber therebetween,

a generally cylindrical tube bundle extending through said chamber and affixed to said headers, the tube bundle comprisinga plurality of substantially identical sinuous tubes nested in juxtaposition in circum- :ferential layers about the axis of the tube bundle,

a flashing orifice positioned in each tube adjacent the inlet header, each orifice substantially corresponding in internal diameter to each other orifice,

means for supporting said tubes in spaced relation about the axis of the tube bundle, and

means disposed in the shell inlet for preventing direct impingment of the shell-side uid on the tube bundle.

7. The `boiler of claim 6 in which:

each of said tubes comprises a straight inlet section of substantial length and a sinuous section of greater length, and

each orifice is positioned in said straight tube section.

eration of vapor in a zero gravity environment comprisa tubular shell,

a pair of diffusers attached to said shell, one at each end of said shell, each diffuser having a fluid connection thereto,

a header attached to each diffuser at its outer end forming a chamber therebetween,

a generally cylindrical tube bundle extending longitudinally through said chamber and affixed to said headers, the tube bundle comprising a plurality of tubes nested adjacent one another in annular rows about the axis of the tube bundle, each of said tubes having a straight portion of substantial length and a sinuous portion of greater length, each of said tubes being of constant cross-section throughout its length.

a ashing orice positioned in the flow path of each tube in the straight portion thereof, at least part of 30 said orifice being removable after assembly of the tube bundle,

a ow-dening insert positioned along the centerline of said tube bundle and affixed to each of said headers, said insert being slidably mounted to at least one of said headers,

a ba'le disposed in the liuid connection of one of said spherical diffusers for preventing direct impingement of the shell-side lluid on the tube bundle.

L1. The boiler of claim 10 wherein:

the individual orifices are substantially identical and are positioned in a common plane perpendicular to the axis of the tube bundle.

12. The boiler of claim 11 wherein:

the tube supporting means are positioned at regularly spaced locations axially of the tube bundle along the sinuous portion of the tubes.

References Cited UNITED STATES PATENTS 1,525,094 2/1925 Jones 165-161 1,748,121 2/1930 Gay 165--174 1,815,932 7/1931 Sieder 165-161 2,519,084 8/1950 Tull 165-163 3,118,497 1/1964 Olson 165-159- 3,134,432 5/1964 Means 165-161 ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Examiner.

Claims

1. IN A LIQUID METAL BOILER OF THE SHELL AND TUBE TYPE, A GENERALLY CYLINDRICAL TUBE BUNDLE COMPRISING: A PLURALITY OF SUBSTANTIALLY IDENTICAL TUBES NESTED IN JUXTAPOSITION IN CIRCUMFERENTIAL LAYERS ABOUT THE AXIS OF THE TUBE BUNDLE, EACH OF SAID TUBES HAVING A SERPENTINE CONFIGURATION OVER A MAJOR PORTION OF ITS LENGTH, MEANS FOR SUPPORTING SAID TUBES IN RADIAL AND CIRCUMFERENTIAL SPACED RELATION ABOUT THE AXIS OF THE TUBE BUNDLE, AND A FLASHING ORIFICE INTERNAL OF EACH TUBE INTERMEDIATE ITS LENGTH, EACH ORIFICE BEING OF EQUAL SIZE AND POSITIONED LONGITUDINALLY OF THE TUBE BUNDLE IN SUBSTANTIAL CORRESPONDENCE WITH EACH OTHER ORIFICE.