US2604838A - Stack design - Google Patents

Description

ATTORNEY INVENTOR. d B. TTG I" I W B TRAVER STACK DESIGN Flled Dec 28 1949 July 29, 1952 Patented July 29, 1952 Ward B. Traver", ijHammond, Ind, assignorito .Standard Oil Company, Chicago, 111., a :corpo j ration of Indiana This invention relates to an improved stack design and it pertains moreparticularlyto an improvement in the design of tall, cylindrical stacks, such as steel stacks whichfin meat senceof said improvement, would have a tend encytoward lateral vibration at a particular-wind velocity. I

Application Dec ember 2 An object of theinvention is to providea cylindricaljstack which will avoid the-necessity of using uy wires or expensive stiffening structurehwhen said stack is of such height'and diameter that its natural frequency of lateral vi- ,bration is in the range of about 1 to 3 cycles per second. A further object is to provideamethod andgmeans for stabilizing existing stacks against lateral vibration. Qther objects wi1l;be apparent as the detailed description of the invention proceedshb V .7 Y .Stackswhich are of conical section from top .to bottom presenttno'lateral vibration problem and this invention is directed to cylindrical stacks, and to stacks having a tall, cylindrical portion"moun.ted on a conical base. The invention is particularly applicable to stacks of about 2 to 15 feet or more in diameter and'with a cylindrical section. height of the order, of 50 to 200 feet or more; Each such stackhas its particular natural frequency of lateral'vibration which can readily be calculated'by; those skilled in the art. "When such natural frequency of vibration-is in the range of about 1 to ,3 cycles per second, particularly in the case of stacks which are upwards of about 100 feet in height, it has long been known that there is a tendencyfor the stack to vibrate laterally at'a determinable wind velocity. Heretofore it ,has been the practice to employ guy wires to prevent such lateral vibration or to provide stiffening structure required for altering the natural frequency of the stack.

.. I have discovered that such stacks can be protarded layer-of the moving air stream near the stack surface, and the geometric arrangement must be such'as to break up this surface 'air stream: layer and create turbulence in order to avoid or minimize regular vortex formation. For

s, 1949; Serial No; 135,481

' '3 Claims. (01. 98-60) most stacks the protuberances should be from about one-fourth inch to' one' inch or more in height and they are-preferably in the form of staggered short {strips of light metal spaced circumferentiallyat about- 10 to 15'. degree intervals or at distances from one-half foot to two. feet apart. Close spacing of the, protuberances is more effective than the use of wider spacing but higher protuberances. The protuberances may be formed of vertical-angle bars of: light metal and such angle bars may provide increased stiffening and 're-enforcement, but for mypurpose such structural strength is immaterial and staggered protuberances are even more effective than angle bars. The protuberances maybe in the form'of woven'wireror expanded, metal around the upper portion of the stack providedcthat such material is of sufficientithickness to obtain" the required disturbances of air flow vat the surface of the stack. w

The invention will be more clearlyiunderstoo d from the followingfdescr'iption of azspecific exstack showing protuberances 'of'circular cross sectionarea; and. a l .7

Figure 5 is a detail;illustratingt thepuse of a fabric mesh protuberance arrangement. i ,3 g

In this particular example; thestack consists of a cylindrical portion llLrwhich-is 4 feet 3 inches in diameter and feetflhigh mounted on a frustro-conical steel base. I l,. 30 feethigh and 7 feet in diameter at its base; In this case the stackwall thicknesslis about e inch. The, stack base in this case is bolted to concrete founda- "tion and the natural frequency of la'teralivibration for this particular stack is shown bytcalculation to be about 1.6 cyclesper' second. :7 e a If such a stack were notre-enforced by" guy wires or stifiened by expensive are-enforcing means, it would be subject to suchintensive lateral vibration at a windvelocity of about 24 miles per hour as to be in danger ofrupture or collapse. At low wind velocities of the order of one mile per hour, the vortices 'on each side down-stream would be relatively equal and of no serious effect; At higherwind velocities there protuberances.

where F is the vortex frequency in cycles per second, V is the wind velocity in miles per hour and D is the stack diameter in feet.- y'I'his alter: H

nate vortex formation has no serious effect, toward causing stack vibration until said frequency substantially equals the natural frequency of the stack in lateral vibration. In this particular example, when the wind velocity reaches 24.. miles per hour, the frequency of vortex formation will be in resonance-with the stacksnatural frequency of lateral vibration andathe net result will be a gradual increase in amplitude of vibration. which may become sosevere as to cause failure ofthe stack. If the wind velocity exceeds 24 miles per hour, the frequency of vortex forma- 25 tion will nolonger be in resonance with the stacks natural frequency and at wind velocities sufficiently high to result in turbulent flow, there is no longer a vibration problem.

Referring again to the drawing, it will be noted that the upper half of the stack is provided with protuberances l2 which in this particular example are thin metal strips about 6 inches longand inch wide. These strips are spot welded, brazed or otherwise secured to the stack so that they are each in a vertical position but are spaced from each other by about. one foot andare staggered or geometrically arranged so as to give maximum disturbance to the surface layer of air which is flowing past the stack. With such protuberances, which in this case are firinch high, relatively closely spaced and geometrically arranged to disturb surface flow, the tendency toward vortex formation is markedly altered. I With such protuberances on the upper part of the stack, turbulent flow conditionsare obtained with a wind velocity so low that the frequency of vortex formation cannot attain-theflstacks natural frequency of vibration so that there is no longer any tendency for the stack to vibrate at the wind velocity of 24 miles per hour or in fact at any other wind velocity. In other words, by roughening the surface of at least the upper part of the stack in the defined manner, I lower I the maximum possible frequency of vortex formation to such extent that it is less than the stacks natural frequency of lateral vibration,

I have solved the vibration problem by'alterin fluid flow past the stack surface instead of by altering the stiffness of the stack itself or securing the stack against vibration by means-0f guy wires. 1 H

In the above specific example, I employ geometrically spaced metal strips. for distributing the surface flow ofv air past the stack but'it should 85 be understood that the disturbance of surface flow may be effected by various other types of Such protuberances may be along the entire cylindrical portion of the stack but usually it is only necessary that the upper half orupper one-third of the stack be provided with protuberances because of the higher wind velocities and the greater effectiveness of the force exerted on the, stacks upper end. The 1 vertical strips may extend from top to bottom in Fig.4. The protuberances may be in the form of a woven wire fencing or expanded metal lath fabric I6 as illustrated in Fig. '5. Such woven wire fencing or expanded metal lath is not as effective as the strip protuberances which extend at least about one-quarter inch to an inch or more from the stack surface unless the expanded metal iior woven wire extends outwardly far enouglitodisrupt the retarded surface-layer of the air stream flowing around the stack (in most 5 cases at "least about one-quarter inch).

The reason for having the protuberances at spaced intervals around the circumference of the stack is to protect-the stack against lateral vibration regardless of wind direction. If a stack were in a location where the wind was always from the same direction then it would only be necessary to have protuberances on each side of the stack about 70 to from the direction of the wind on the up-stream side. If strips or vanes were movably mounted and directed, for exampleby a wind vane so that the strips would always'be at about 70 to 80 from the direction of the wind on the up-stream side, the objects of my invention could be accomplished without employing the strips at spaced intervals around the entire stack. For practical purposes, however, it is usually desirable to employ fixed protuberances at suflzlciently close intervals so that the retarded surface or boundary layer flow isnecessarily disrupted regardless of wind direction. The boundary layer thickness of a flowing air stream is somewhat dependent on wind velocity, but at any particular velocity it increases in depth with increase in stack diameter. The minimum protuberance depth for a 2 foot diameter stack is approximately one-quarter inch, but protuberances of greater depth maybe employed. The minimum protuberance depth for a. ten foot diameter stack is about three-quarter inch... In all cases the protuberances may be two or. three inches, but the use-of higher protuberances may be objectionable because of increased drag;

I claim? 1 1. A :cylindrical stack. having a natural frequency'of lateral vibration in the range of about 1 to 3 cycles per second and having protuberances on at least the upper portion of its external surface, sa-id protuberances being at least about onequar'ter inch to one inch in depth and being spaced laterally around the stack at intervals of about six inches to twofeet whereby said protuberances disrupt the surface air stream flowing past the stack and interfere with regularly alternating formations of vortices on opposite sides of the stack.

2. The cylindrical stack of claim 1 wherein the protuberances consist of vertically aligned flat strips attached-tangentially to the surface of the stack of 'suiflcient width so that their free edges are-about one inch from the surface of the stack, spaced laterally around the stack at intervals of about ten to forty-five degrees.

3; The cylindrical stack of claim 1 wherein the protuberances consist of a sleeve of wire fabric around'the stack wherein the thickness of the fabric is not less than about one-quarter inch.

' WARD B. TRAVER.

N 0' references cited.

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