Expansion accommodating means and method

Sept. 8, 1970

R. J. NEARY ET AL

3,527,291

EXPANSION ACCOMMODATING MEANS AND METHOD

Filed May 16, 1968

Sheets-Sheet 1

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Sept. 8, 1970 r. j. nearv et al 3,527,291

EXPANSION ACCOMMODATING MEANS AND METHOD Filed May 16, 1968 2 Sheets-Sheet 2

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United States Patent Office ”

3,527,291

EXPANSION ACCOMMODATING MEANS AND METHOD

Robert J. Neary, Westfield, and Walter J. Markowski, Cranford, N.J., assignors to Aero-Flow Dynamics, Inc. (The Wing Co. Division), Linden, N.J., a corporation of New York

Filed May 16,1968, Ser. No. 729,660

Int. CI. F28f 7/00; F16113/04

U.S. CI. 165—83 5 Claims

ABSTRACT OF THE DISCLOSURE

Banks of heater tubes of high slenderness ratio extending between fixed upper and lower headers in an air heater unit, each tube having independently functioning expansion joint formed by compressible bellows within its length at upper end thereof, and preloaded compression spring exerting bias on bellows in direction of tube expansion; cup element between spring and bellows transmits spring force to latter; fixed but vertically adjustable common plate support for lower ends of all said springs, plate being suspended by inverted U-bolts from upper header; method provides prevention of tube buckling while accommodating tube expansion by establishing compression spring force substantially equal in amount and opposite to the sum of certain forces exerted by said bellows on its associated tube during operation of heater unit.

This invention relates to the accommodation and control of thermal expansion as occurs in steam and other hot fluid conduits, especially those conduits which extend' between substantially fixed points such as a pair of fixed steam headers to which the conduits are attached at their respective ends. More particularly, the invention relates to bellows-type expansion devices which are incorporated in such conduits for the purpose.

Although the invention has other uses, it was made and will therefore be described in connection with improving air heating units of the so-called “face and bypass” type, which are situated in air ducts and which have pivotable dampers for proportion-control of the total flow of air as between the “face” passages of the heater which contain steam heated tubes, and the “bypass” passages of the heater which are merely open passages. Such heaters are generally illustrated in Horn et al. U.S. Pat. 3,107,724.

Unless accommodated, linear thermal expansion in a tube having fixed ends will cause it to deflect or buckle due to internally developed stresses in the tube. This is particularly true in tubes having high slenderness ratio, such as those which would preferably be provided in the face passages of such face and by-pass heater units. However, although such lateral deflection of a tube might be tolerated under other circumstances, the relative closeness of the dampers to the heating tubes as is sometimes desired in such heater units makes any significant tube deflection intolerable since the tubes may then touch and interfere with the operation of the dampers. Accordingly, heating tubes in such heaters are sometimes made loopshaped, or are otherwise bent to particular configuration to accommodate thermal expansion. Alternatively, where straight tubes are used, one of the tube headers is mounted for “floating” movement away from the opposite fixed header, thus permitting thermally induced increase in tube length to occur between the headers. Such bent tube and floating header arrangements are disadvantageous for several reasons. For example, bent tubes involve construction difficulties, and are unstable in operation insofar as control of buckling is concerned.

Floating header arrangements are satisfactory only where operating pressures are relatively low, and where there is only a relatively narrow range of expected pressure and/or temperature differential between the respective tubes in the bank.

The use of straight length individual tubes in the tube banks of such heaters is preferred, at least for the reason that tube replacements are facilitated. Thus, the provision of fairly low cost but in any event effective and long-lasting means to accommodate tube expansion is a desirable objective. In addition, it is desirable that such expansion-accommodating means be associated with each individual tube in a tube bank such that abnormal expansion of one will have no effect upon the other tubes in the bank.

Where the tube bank extends between two fixed headers, the provision of a compressible bellows within the length of each tube, such that the length of compression of the bellows would equal the length of expansion of the tube, was thought to be a convenient solution to the problem. However, although the bellows accommodates the linear expansion it is found that, especially where the tube has high slenderness ratio, tube buckling still occurs due to reactive forces on the tube which are developed in the bellows by reason of the bellows tending to resist its own compression, and by reason of the additional direct force generated by fluid pressure within the bellows which tends to expand it. These forces act in direction opposite to the direction of tube expansion, and therefore are exerted directly against the cross-sectional area of the tube. Even where the spring rate of the bellows is low, such forces may exceed the sustainable buckling load of the tube especially in instances where the tube is relatively long and thin.

The present invention provides a spring and bellows arrangement which, when installed within the length of each such heater tube, reduces the aforementioned buckling forces on the tube to a level which is at least below that which would cause its deflection or buckling. Further, the arrangement is such that these forces are not only reduced to such level under anticipated conditions of maximum tube expansion, but also are maintained at such low level throughout the entire range of expanded or unexpanded conditions of the tube. The possibility for tube buckling is therefore eliminated. Moreover, where such spring and bellows arrangement is provided in each of a plurality of adjacent heater tubes extending between the same fixed headers, the expansion in each tube will be accommodated without affecting any other, so that any otherwise possible buckling of the entire tube bank as a result of the buckling of fewer than all of the tubes, is also eliminated.

Briefly and generally describing the invention in its preferred embodiment, each long and thin copper tube of a vertical bank of such tubes has within its length an expansible bellows portion, the bellows being of similarly thin and appropriately configured metal and situated near the upper end of the tube adjacent one of the two fixedposition steam headers between which all of the tubes are attached. Actually, the bellows is a short length, prefabricated stainless steel element which is brazed on to the end of its associated heater tube, thereby completing the overall tube length. At a level below its associated bellows, each tube passes through a horizontally disposed spring support plate, the latter being suspended from the relatively heavy upper steam header pipe by a pair of inverted U-bolts whose closed ends pass over the header pipe and whose respective legs are adjustably connected to the spring support plate. All of the tubes are slidable through the common spring support plate, which also incorporates appropriate tube guides or sleeves for the purpose.

3,527,291

With reference to each tube and its bellows portion, a steel coil spring, which is loosely sleeved around the heater tube, extends and is pre-loaded in compression between the spring support plate on which it rests and the bellows. In the preferred embodiment, the spring connection to the bellows is via a loose-fitting cup-like element which surrounds and seats against the underside of the bellows and has an outwardly projecting flange at its upper end, adjacent the upper end of the bellows, which engages and constrains the upper end of the coil spring. Conventional rubber or plastic O-rings are respectively interposed between the top of the spring and the projecting top flange of the cup, and between the lower end of the bellows and the inwardly projecting bottom flange of the cup, for the purpose of evenly distributing the forces of engagement between these respective elements as will be apparent.

The desired uniform preloading of all of the springs in the tube tank is easily accomplished and determined by take-up of the aforementioned adjustable connections between the common spring support plate and the legs of the referred to inverted U-bolt suspension.

Preloading of the springs is done when the tubes are initially installed, i.e., before the tubes are heated. The compressed force in each spring is uniform in amount, and is substantially equal to the maximum sum of the oppositely directed tube buckling forces that will be exerted on the tube with which the spring is associated during heater operation, these forces being developed by steam pressure within the bellows and by the resistance of the bellows to its compression during thermally induced tube expansion, as aforesaid. That is, the initial compression in each spring is such that any result of such combined forces, in direction tending to compress and buckle its associated tube, will be ideally zero, but in any event less than the critical buckling load which the tube can sustain during operation. Careful determination of the spring rate (i.e., the spring constant) of each coil spring will ensure its same effectiveness under all conditions of heater operation.

Although perhaps not technically a joint, for convenience in description the spring and bellows arrangement provided by the invention will be characterized and referred to herein as an expansion joint.

These and other objects, features and advantages will become apparent from the following detailed description of the invention, when taken together with the accompanying drawings in which:

FIG. 1 is a front sectional elevation of a “face and by-pass” type air heating unit which includes heating tubes having expansion joints in accordance with the invention;

FIG. 2 is a side sectional view from lines 2—2 of FIG. 1;

FIG. 3 is an enlarged and somewhat diagrammatic sectional elevation of an expansion joint in accordance with the invention, to illustrate its principles;

FIG. 4 is an enlarged and sectionalized front elevation of a typical bank of tubes of the heater of FIG. 1, the tubes having expansion joints in accordance with the preferred embodiment of the invention; and

FIG. 5 is a side sectional view from the lines 5—5 of FIG. 4.

Referrng to FIGS. 1 and 2, an “integral face and bypass” type unit heater 10 comprises a rectangular steel frame 11, which mounts a horizontally disposed upper steam or hot water header pipe 12 and a similar lower steam or hot water header pipe 13 is fixed, vertically spaced relation to each other. In the particular arrangement shown, respective vertically extending pivotable damper pairs 14α, 14δ divide the frontal area of the heater 10 into respective and alternately adjacent “face” passages 15 and “by-pass” passages 16 through which air to be heated will flow when the heater 10 is mounted within an air duct (not shown). In the illustrated posi4 tion of the damper pairs 14α, 14b the by-pass passages 16 are closed by the dampers themselves and their locations are therefore only generally indicated by reference numeral 16, whereas the face passages 15 are fully opened and therefore can be seen in FIG. 1. The particular damper configuration and arrangement, and the manner of pivoting of the dampers to concurrently open and close the alternate face and by-pass passages to proportion the total airflow therebetween, do not form a part of the present invention, and therefore need not be further described.

A vertically extending tube bank, comprising four heater tubes 18, is disposed within each face passage 15 as seen in FIG. 1. The parallel tubes 18 of each bank are aligned within the centerline plane of the heater frame as seen in FIG. 2, and are horizontally spaced apart to permit airflow therebetween. Numerous outwardly projecting fins 18α are mounted along the lengths of each tube 18 to facilitate heat transfer as will be apparent, and the upper end of each tube includes a bellows which forms an integral portion of its length as will subsequently be described. Each tube, as it includes such bellows, is attached at its upper end to the header 12 and at its lower end to the header 13, as by brazing. The tubes pass through suitably enlarged apertures (unnumbered) of the upper and lower frame elements 11α, lib so as to be free-standing between the headers 12 and 13.

In operation, steam or hot water, which is continuously admitted to either the upper header 12 or lower header 13 via either the upper conduit 19 or lower conduit 20, as the case may be, flows through all of the tubes 18 to the opposite header from which it is discharged via the other of the conduits 19 or 20. If steam is used as the heating fluid, it may be admitted at pressure anywhere from about 2 p.s.i.g. (pounds per square inch, gauge) to about 200 p.s.i.g., and at corresponding temperature of from 212° F. to 388° F. If hot water is used, the water may be at temperature anywhere from about 150° F. to about 400° F., the heater 10 being capable of so-called “high temperature” hot water performance under corresponding water pressure. Thus, the heater tubes 18 may be subjected to high fluid temperatures which will cause their thermal expansion, and to correspondingly high internal pressures.

Depending upon the overall size of the heater 10, th· length of each heater tube as it extends between the fixed headers 12, &lt;13 may be from about 36 to about 108. In the preferred embodiment the tubes are copper, each having nominally % diameter, and wall thickness of about 0.035. Thus, each tube has relatively high slenderness ratio, and will sustain only limited buckling load. Accordingly, and because a tube having length for example of 96 will expand linearly about 36 under certain normal conditions of heater operation, a means for accommodating tube expansion must be provided. In addition, and because tube buckling could interfere with the pivotable operation of the dampers 14α, 14Z&gt;, these means must be such as to accommodate the expansion without permitting or causing the tube to buckle. The present invention provides such thermal expansion accommodating means in each of the banks of tubes 18 of the heater, the preferred form of expansion accommodating means in any tube bank actually comprising a separately acting expansion joint 21 in each tube 18 as illustrated in detail in FIGS. 4 and 5. However, for a better understanding thereof, the underlying features and principles of operation of each expansion joint will first be described with reference to a modified form of expansion joint, generally designated by reference numeral 22, as illustrated somewhat diagrammatically in FIG. 3.

Because it could be substituted for each expansion joint 21, the expansion joint 22 will be described with reference to a typical heater tube 18 and the upper header 12 of the heater unit 10. The expansion joint 22 includes a compressible bellows portion 23 within the length of the tube in which expansion is to be accommodated; a compression spring 24; and fixed spring support which is generally

3,527,291 ⁵indicated by numeral 25. The bellows 23 is made of stainless steel, actually as a part of a separately fabricated length of stainless steel tubing 26 whose lower end is attached as by brazing 27 to the upper end of the tube 18, and whose upper end is attached as by brazing 28 to the upper header 12 as shown. The accordion-like alternately inward and outward annular bends 23α of the bellows are only diagrammatically illustrated, but the number and arrangement of such bends and the spring rate of the thus formed bellows is such that the bellows will compress easily under manual pressure a distance as great as the anticipated distance of thermal expansion of the tube 18 (e.g. 16 under the described conditions) during heater operation. Without more, it will be understood that when steam or hot water flows through the tube 18 and tubing 26, the linear distance of expansion of the total tube length will be accommodated by compression of its bellows portion 23.

However, upon such compression of the bellows 23 in the direction of arrow A in FIG. 3, the reactive force in opposite direction as indicated by the arrow F_B is established in the bellows by its own resiliency, or resistance to expansion. In addition, the pressure of the fluid flowing within the bellows 23 tends to expand the bellows, the pressure acting within the outwardly projecting bend areas 23α of the bellows. This additional force due to fluid pressure is also exerted on the tube 18, and is indicated by the arrow F_P in FIG. 3. As will be apparent, the combined forces F_b and F_P acting downwardly on the upper end of the tube 18, whose opposite end is fixed, will tend to buckle the tube. In high slenderness ratio tubes as are being described, even these relatively small forces F_Band F_P are sufficient to cause actual buckling.

To counterbalance the sum of the anticipated forces F_Band F_P so that any resultant force tending to buckle the tube 18 is substantially zero, or in any event is alwaysless than the sustainable buckling load, the compression spring 24 is preloaded in compression between the bellows 23 and a horizontal plate 29 of the spring support 25 as shown in FIG. 3. Because it is suspended by a pair of inverted U-bolts 30 which straddle the fixed upper header 12, the support plate 29 provides a fixed support against which the coil spring compression forces will act. In this embodiment, the coil spring 24 has diameter only slightly larger than the tubing 26 to be freely sleeved therearound, the upper end of the spring being in direct engagement with the underside of the lowermost laterally projecting portion of the bellows 23 as shown. As will be apparent, the spring bias is exerted on the bellows in the direction of tube expansion, i.e., direction opposite to forces F_Pand F_b.

The preloaded compression force, in amount equal to the sum of the maximum forces F_B and F_P as may be anticipated, is effected by screw take-up on the respective nuts 31 by which the support plate 29 is mounted on the legs 30α of the pair of inverted U-bolts 30. That is, during assembly the plate 29 and coil spring 24 are loosely slidable on the tube 18 until the latter, including its attached bellows tubing 26, is attached between the headers 12 and 13. The U-bolts 30 are then positioned on header 12, and the plate 29 is then slidably mounted on their legs 30α via the oversized plate apertures 32, the nuts 31 engaging suitable threads at the free ends of the U-bolt legs 30α as shown. Either as guided by a previously computed distance of linear compression, or by using a torque wrench, the coil spring 24 is compressed to the desired preloading by a suitable number of equal turns being taken on all of the nuts 31. The expansion joint 22 is thus preconditioned both to accommodate thermally induced linear expansion of tube 18 by compression of its bellows portion 23, and to absorb the forces F_P and F_B as would otherwise be absorbed by the tube 18 as the same are developed during heater operation. That is, the tube 18 has been preloaded in tension substantially equal and opposite in

G amount to the anticipated forces F_P and F_B. As steam or hot water is introduced under pressure, the force F_P develops but its action on tube 18 is counteracted by the spring 24. As tube 18 expands, the bellows 23 is compressed an equal distance in the direction of expansion, but the developing reactive force F_B is also counteracted by spring 24.

Referring now to FIGS. 4 and 5 which show the preferred embodiment of the invention as actually incorporated in the heater 10 of FIGS. 1 and 2, the mode of operation of each expansion joint 21 is essentially that as described in connection with FIG. 3. Each expansion joint 21 includes the same compressible bellows portion 23, which is actually a part of a preformed length of stainless steel tubing 26 attached as by brazing 27, 28 between the tube 18 and upper header 12 to become an integral extension of the tube 18, as in the FIG. 3 embodiment, and therefore these like elements are identically numbered. However, the connection between each compression spring 24α and the underside of its associated bellows 23 is effected via a cup-like stainless steel cylindrical element 24 which surrounds the bellows 23. The cup 35 has length or height substantially equal to that of the bellows and (referring to FIG. 5) has a peripherally extending, outwardly projecting upper flange 24α and a peripherally extending, inwardly projecting lower flange 35b as shown. The inside diameter of the central portion 35c of the cylindrical element or cup 35 is slightly larger than the largest diameter of the bellows 23 to provide an insulating air space therebetween. It is slidably received from the lower end of the bellows, the aperture formed by its inwardly projecting lower flange 35b being only slightly greater than the outside diameter of the tubing 26. A flexible O-ring 36, of rubber or suitable plastic material, is loosely interposed between the lower flange 35b and the underside of the bellows 23 to equalize the distribution of the forces which will be developed therebetween. The diameter of the compression spring 24j is greater than that of the central portion 35c so that the spring is loosely received therearound. A similar rubber or plastic O-ring 37, of diameter equal to that of spring 24α is loosely interposed between the upper end of the spring 24α and the underside of the outwardly projecting upper flange 35α of the cup 24 against which it acts, similarly for equalizing the distribution of forces developed therebetween. The lower end of the spring 24α is seated on a support plate 38 which is common to all of the expansion joints 21 in the tube bank, the plate 38 being part of a common fixed support generally indicated by reference numeral 39, as shown.

As in the FIG. 3 embodiment, the fixed support 39 includes a pair of inverted U-bolts 40 which straddle the upper header 12 of the heater 10. However, the spring support plate 38 rests on a lower plate 41 of the support 39, the ends of the plate 41 being slidably received on the U-bolt legs 40α and adjustably connected thereto by nuts 42 at its underside which engage suitable threads (unnumbered) of the legs 40α. The lower plate 41 includes integrally formed, downwardly extending cylindrical tube guides 43 which surround the respective upper ends of the heater tubes 18. Of course, each tube 18 is freely slidable through its associated tube guide 43 and through its associated aperture 38α of the support plate Y38. The tube guides 43 guide the tubes 18 in their vertical expansive movement during operation, and incidentally effect a reduction in the slenderness ratio of each tube as will be apparent.

The cup 35, spring 24α, and plates 38 and 41 are slidably mounted on the tubes 18 and tubing 26 before the latter are brazed in place between the upper and lower headers 12, 13 of the heatei 10. Thereafter, the plate 41 is mounted on the legs 40α of the U-bolts 40, and the nuts 42 threaded in place. The nuts 42 are then tightened to an equal extent such that all of the

3,527,291 springs 24α are initially placed under equal compression. As in the previously described embodiment, the preloaded compressive force in each spring 24α is predetermined to be substantially equal in amount to the the sum of the maximum forces F_P and F_B (see FIG. 3) as are anticipated to be developed with respect to be developed with respect to each tube 18 during heater operation. The spring force is transmitted to the lower end of its associated bellows 23 via the cup 35, as shown.

Considering the four tubes 18 and their respectively associated expansion joints 21, it will be noted that expansion of any tube 18, and the referred to developed forces therein, are accommodated independently of the perhaps unequal expansion and development of similar forces in any of the remaining tubes 18. That is, the operation of each expansion joint 21 in the tube bank is the same as that described in connection with FIG. 3, although a common fixed support 39 is utilized in conjunction with all of the tubes of the bank. Because each tube may expand to a greater or lesser extent independently of the others, the tube bank as a whole can operate under significantly higher pressure and/or temperature conditions, and increased ranges of expected pressure and/or temperature differentials than have been permissible in such heaters in the past.

Although operation of each expansion joint as shown in FIGS. 4 and 5 is essentialy the same as that of the FIG. 3 embodiment, the inclusion of the cup 35 in the manner as has been described lends stability to the arrangement; permits the compression spring to have larger diameter for greater spacing from its associated heater tube 18 so that heat will not materially affect its spring rate; and permits the compression spring 24α to be as long as desirable, yet conveniently accommodated within a shorter overall height of each joint 21.

Because the respective U-bolts 30 and 40 in the described embodiments are not subjected to significant heat during operation, any linear expansion thereof during operation is negligible. Accordingly, in the foregoing explanations with regard to the balancing of forces, the effects of any such expansion of the U-bolts 30 or 40 has been disregarded. However, under other conditions in which such hanger supports might be subjected to considerable heat and therefore appreciable expansion during operation, it will be apparent that such expansion may result in some downward displacement of the spring support which is suspended therefrom, and such displacement must be taken into consideration when determining the amount of preloading of the respective springs 24 or 24α.

Thus has been described a method and means for accommodating linear expansion in heater tubes having high slenderness ratio, which achieves all of the objects of the invention.