US3088494A - Ribbed vapor generating tubes - Google Patents

Description

y 7, 1963 P. H. KOCH EI'AL 3,088,494

RIBBED VAPOR GENERATING TUBES Filed Dec. 28, 1959 3 Sheets-Sheet 1 IO a 16 1 3: c d

K j INVENTORS Paul H. Koch Edward A. Pirsh Howard, S. Swanson ATTO RNEY May 7, 1963 P. H. KOCH ETAL RIBBED VAPOR GENERATING TUBES 3 Sheets-Sheet 2 Filed Dec. 28, 1959 I so STEAM QUALITY X./OBY WEIGHT 2 n H O a 5 H m 1 U I G E m... g I ll l1 m |ll |l| .lnlln'l'l m 0 w OM n .F DP n O 0 O w O O 0'0, B ND LE O O OE FT 1 Maia (\MQB 4. m m v EXO F A wn WW5 luwl llll EiWllJ illltnWIl E A m E U 9 B wmM 5 r L U TA GT D -i W N F. 0 W H B I LE BMO w G IB 5 N B OB IER 9H 0 U l RTP m T as R Y E r: R R mH U H w E IT TT m A GB mo A F. OH N O HRE L n b T IT BM TEL C 8N L l. O S X O F U I E OT M 0M0 NG B 1 R MER E 0 H5 0 sT R m FF P L I AM .l I l. I I ll 1 0!! S E K 8 L C FI! iti I I 8 1 i l it I Illl UO p F N 5, O 5 O O O O 7 0 0 O O m H 0 9 8 7 INVENTORS May 7, 1963 P. H. KOCH ETAL RIBBED VAPOR GENERATING TUBES Filed Dec. 28, 1959 3 Sheets-Sheet 3 FIGS 40 30 e 2 7%? '7 25/370 W h I 4 f ficfr 707' /7""/77'7' 77;- /7 /4 4m 4%; X2

. y h ZONE C I 2 yz/g/j/j/ ////4/ a! 0.02 f, 010

/7 774x2 4 @fi 0 so so STEAM QUA Y x/ BY WEIGHT LJ ATTORNEY United States Patent Office Patented May 7, 1963 3,088,494 RIBBED VAPOR GENERATING TUBES Paul H. Koch and Edward A. Pirsh, Akron, and Howard S. Swenson, Alliance, Ohio, assignors to The Babcock &

Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Dec. 28, 1959, Ser. No. 862,232 3 Claims. (Cl. 138-37) This invention relates in general to the construction and operation of tubular vapor generating elements and more particularly to internally ribbed tubular vapor generating elements of circular cross-section adapted in operation to contain a sub-critical pressure vaporizable fluid stream increasing in quality as it flows therethrough. The tubular vapor generating elements of the invention are especially suited for use in the furnace of a vapor generator wherein the elements absorb heat at relatively high rates.

In the operation of a steam boiler, for example, at subcritical pressures steam is formed in the water inside of the tube, and at successively greater distances along its length there will be an increasing fraction of the flowing fluid in the form of steam and a decreasing fraction in the form of water, depending upon the rate of heat absorption. The change from liquid to vapor occurs both at a solid-liquid interface, as at the inside surface of a tube, and at a liquid-vapor interface, as with water surrounding steam bubbles. It has been observed that two distinct types of boiling, known as nucleate boiling and film boiling, can occur at a solid-liquid interface. Nucleate boiling is characterized by the formation and release of steam bubbles on the inside of the heat absorbing surface with the water still wetting the surface, while in film boiling the inside of the heat absorbing surface is covered by a film of steam. To transfer heat from the surface to the fluid a temperature gradient is necessary. The magnitude of this gradient depends mainly on whether nucleate or film boiling is taking place. In nucleate boiling the steam bubbles generated at nucleation points on the heat transfer surface are rapidly detached therefrom and move into the bulk liquid, and the resulting agitation of the mixture produces an excellent heat transfer coefiicient. It is well known that the wall metal temperature of a steam generating tube will not rise above the temperature of the contained fluid enough to weaken or otherwise damage the tube so long as the tube is wet with water on the inner wall surface opposite the heat receiving outer surfaces, i.e. as long as nucleate boiling is taking place, even with high heat transfer rates through the metal of the tube wall due to the contact of hot gases and/or radiation from a furnace. in film boiling a film of steam forms over the heat transfer surface so that steam generation does not occur at the heat transfer surface but at the liquid-vapor interface. The [steam film prevents the liquid from wetting the surface and the resulting heat transfer coefficients are poor. The steam film in film boiling acts as a layer of insulation which retards the beat being transferred from the heat absorbing surface to the water, and therefore the temperature of the heat absorbing surface rises to a higher level than that resulting from nucleate boiling under the same heat flux and mass flow conditions.

The actual process involved in the formation of the vapor nuclei on the solid-liquid interface surface of a tube is the subject of several interesting but unsubstantiated theories. However, observation shows that the nuclei originate at selective points on the surface. As the heat flux across this surface is increased the number of nucleation points increases until the entire surface is covered, thereby replacing the liquid-solid interface with a vapor film. Boiling then proceeds from the liquid-vapor interface. The heat flux at which this vapor film forms is calied the break-down point" of nucleate boiling, sometimes known as the burn-out point, at which the boiling heat transfer coefficient is not the controlling resistance to heat flux and surface metal oxidation temperatures may be exceeded. While it is weil known that internally ribbed tubes will improve heat transfer, the application of a tube of this type has been limited because it was not known that the improvement would be as great as it is in regard to extending the nucleate boiling range with certain proportions and arrangements of the lands and grooves of the tube. Moreover, there has been no recognition in the art that the proportions and arrangement of the lands and grooves of an internally ribbed tube perform a vitally important part in the successful operation of such a tube.

The main object of the invention is the provision of an improved construction of a tubular vapor generating clement particularly adapted for sub-critical pressure operation, and which is characterized by simple but effective means for promoting the maintenance of nucleate boiling of the liquid in a iiquid-vapor stream flowing therethrough over the inner wail surface of the vapor generating element irrespective of the position of the tube and without requiring any distortion or substantial weakening of the tube wall. A further and more specific object is an improved wail construction of an internally ribbed steam generating tube of circular cross-section of the character described which is adapted for use in parts of the steam generator wherein the tubular element is subject in use to a high heat flux from a high temperature heat source while conducting a sub-critical pressure vaporizable fluid stream increasing in quality as it passes through the tubular element. The tubular element has its internal wall formed with at least one helical groove and turbulence promoting helical lands or ribs intermediate the convolutions of the groove. According to the invention, the lands and grooves of the tubular element are specially proportioned and arranged so as to promote the maintenance of nucleate boiling of the fluid passing therethrough and so as to extend the steam quality limit at which nucleate boiling of the fluid deteriorates into film boiling to a point far beyond that provided by smooth tubes or known types of ribbed tubes under the same conditions of pressure, heat flux, mass flow and fluid supply temperature.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a longitudinal section of an internally ribbed tube made in accordance with the invention and formed with a single continuous helical groove;

FIG. 2 is a longitudinal section of a modification of the ribbed tube of FIG. 1;

FIG. 3 is a sectional elevation of a forced flow steam generator in which tube lengths embodying the invention are adapted for use;

FIG. 4 is a graph plotting the inside tube metal temperature profile of smooth and internally ribbed tube lengths against fluid enthalpy and quality; and

FIG. 5 is a summary of data in graph for establishing the zones within Whch nucleate boiling of a vaporizable fluid will be maintained in ribbed tubes having their lands and grooves proportioned and arranged in accordance with the invention.

The ribbed tubes of the invention are particularly designed for use in the lower part of the enclosure walls of the furnace of a forced flow high pressure steam generator having small liquid and heat storage capacity, such as illustrated in FIG. 3 and disclosed in greater detail in a copending application of Paul H. Koch, Serial No 781,576, filed December 19, 1958; and are specially constructed and proportioned to assure nucleate boiling of the liquid flowing therethrough substantially throughout the ribbed tube length under the worst foreseeable conditions of operation. It will be understood, however, that the ribbed tubes illustratcd and hereinafter described can also be advantageously used in natural circulation steam generators and other forms of vapor generators, where the vapor generating tube conditions render the use of ribbed tubes desirable in a portion of the vapor generating section thereof.

As shown in FIGS. 1 and 2, each of the tube lengths is formed of suitable carbon or alloy steel tubing of circular cross-section. The inner wall surface 12 of the tube of FIG. 1 is formed throughout its length with a single continuous helical groove 14 to provide fluid turbulence promoting helical lands or ribs 16 intermediate the convolutions of the groove having a slope angle of about with the sides of the lands defining the adjoining groove convolutions. The slope angle of the lands is the angle between a line tangent to the land and a line normal to the longitudinal axis of the tube. While the tube length of FIG. I of the invention is formed with a single continuous helical groove, it will be understood that the tube lengths may be formed with a plurality of parallel continuous helical grooves, as shown in FIG. 2. Relative to the terms of dimensions employed and as illustrated in FIGS. 1 and 2, pitch, 1), means the distance between corresponding points of consecutive lands measured parallel to the tube axis. The lead or spiral lead, I, of a land is defined as the distance, parallel to the tube axis, that the land advances in one complete revolution. For a tube having only one continuous groove, as shown in FIG. 1, the pitch and lead are identical. In the case of a tube having a pair of parallel continuous helical grooves, as shown in FIG. 2, the pitch is equal to the lead divided by the number of grooves. In addition, minor inside diameter, d, is measured between opposing land face portions; "111 represents the Width of the lands at their inside face; "it" is the height of the lands; and s is the distance, measured parallel to the tube axis, between the top and bottom of each side of the lands.

The purpose of the ribbed inside tube surface is to change film boiling, which would be present in a smooth tube under certain conditions of operation, to nucleate boiling, thereby reducing the temperature difference between the liquid and the tube wall. Reduction of this temperature difference maintains the temperature of the tube more nearly equal to that of the liquid, thus holding the tube temperature within tolerable limits.

We have found that the proportions and arrangement of the lands and grooves of a ribbed tube of the character described play a critically important part in the operating characteristics of a tube of this type, particularly in respect of the maintenance of nucleate boiling of the liquid passing through the tube length. The primary requirement is that the inside surface of the tube should be wetted at all times by water in the liquid phase, that is, by a Water-steam mixture of suitable quality for given conditions. Having met this requirement, the water film resistance is negligibly small and the overall conductance depends upon gas film and tube wall resistance to heat flow. Any departure from this sound practice means the introduction of a steam film resistance.

The proportioning of the tubes of the invention to insure nucleate boiling of the contained fluid requires the evaluation of a number of operating conditions. In establishing the limits of these conditions use was made of accumulated data on limiting values and design criteria derived largely .from experience. The ribbed tubes of the invention have no application to a vapor generator operating at or above the critical pressure of 3206 p.s.i.a. since the film boiling problem encountered with sub-critical pressure operation is absent. With the operating pressure at or above the critical pressure, the addition of heat to a vaporizable fluid causes it to pass directly from the liquid phase to the vapor phase without any bubble formation or boiling, without any latent heat of vaporization, and with substantially only a single phase existing at any one time. The present invention is, accordingly, directed to internally ribbed tubular elements wherein the vaporizable fluid passing through the elements is at a pressure less than the critical pressure of 3206 p.s.i.a., and preferably no greater than 3000 p.s.i.a., the heat flux to the tube length is preferably no less than 500,000 lbs./hr.-ft. and the mass flow of the fluid passing through the tube lentgh is preferably no less than 500,000 lbs./hr.-ft. These represent the severest operating conditions to be expected in the appliaction of the tubular elements of the invention. Heat transfer tests on smooth tubes with heat applied about their full circumference show that they cannot successfully be operated at these limits, the reason being that the tube metal temperature would rise to levels which would result in the rupture of the tube. This result can be plotted as shown in FIG. 4, which shows local values of inside tube metal temperature at various positions along the length of a tube having a smooth inside surface plotted against the corresponding values of enthalpy and steam quality at these positions, with the pressure, heat flux and mass flow set at the limits specified above and with water supplied to the tube length at a temperature a few degrees below the saturation temperature corresponding to the pressure. While the water is still in a sub-cooled condition the tube temperature suddenly rises to a level far above the allowable limit, with the result that the tube becomes overheated and fails. The sudden increase of temperature occurs at the location where sub-cooled boiling, which is, in effect, a form of nucleate boiling, terminates and film boiling begins.

From extensive experimental work we have found that a ribbed tube having its lands specially proportioned and arranged in accordance with the invention will inhibit the breakdown from nucleate to film boiling. Because nucleate boiling is maintained, the tube temperatures remain at a low level through a wide range of steam qualities and the tube operates safely and successfully within this range. This result is also shown in FIG. 4 in comparision with the smooth tube operating under the same conditions.

Under nucleate boiling conditions heating the tube causes steam bubbles to form at so-called nucleation points on the inner surface of the tube. The rapid formation and subsequent removal of the bubbles into the main stream of flow creates turbulence at the tube surface which, in turn, promotes high heat transfer rates. Under conditions of nucleate boiling the inside tube surface is maintained at a few degrees above the saturation temperature of the fiuid. It can be seen in the smooth tube curve of FIG. 4 that, in the region marked nucleate boiling, the inner surface temperature of the tube is only slightly higher than that of the fluid temperature. In this region, when the fluid is still in a sub-cooled condition and the inside metal temperature is about 20 F. higher than the fluid temperature, small steam bubbles are formed. These bubbles are removed by the stream and they collapse as soon as they penetrate into the fluid. As the fluid passes through the heated tube a point is reached Where the bubble removal is no longer in equilibrium with the bubble formation and the bubbles unite and form a film of superheated steam along the tube wall. Bubble coalescence signifies incipient film boiling which is characterized by a sharp increase in the metal temperature of the tube due to increased resistance of heat ilow by the steam film.

It was found that the onset of film boiling and the resulting sharp increase in tube metal temperature occurs in a matter of only a few inches along the length of the tube.

As shown in FIG. 4, a ribbed tube operates safely far past the smooth tube limits. So far, in fact, that nucleate boiling in a ribbed tube could be retained up to a steam quality of 70% under the same conditions of pressure, heat flux, mass flow and fluid supply temperature as the smooth tube. The lands and grooves on the inner tube surface, in comparison to a smooth tube, increase the eddy formations adjacent the tube surface, create a considerably higher level of secondary flows adjacent the inner surface, and increase the rate at which water flows towards and away from the inner surface, thereby making more water available to replace the steam generated. The effect of disturbing the solid-liquid interface of the tube by discontinuties in the form of lands and grooves is to increase the convention heat transfer coeificients. Turbulent mixing adjacent to the heating sunface, excited by boundarylayer boiling and promoted by the lands and grooves, makes possible the high heat transfer coeificients.

Test work on ribbed tubes showed that the proportions and arrangement of the lands and grooves thereof are critically important to the successful operation of a tube of this type. In FIG. 5 is a graphical summary of the results of a series of tests on ribbed tube lengths having various land and groove proportions and arrangements with varying mass flows, heat fluxes and fluid pressures and water supplied to the tube lengths in a slightly subcooled condition, that is, at a temperature a few degrees below the saturation temperature corresponding to the pressure. The quality of the steam at the point at which nucleate boiling of the fluid deteriorated into film boiling was determined for each tube length by plotting the inside metal temperature profile of the tube length against the enthalpy and steam quality, the resulting curves being similar in form to that of the ribbed tube curve of FIG. 4. Then, as shown in FIG. 5, the locus of these transition points was plotted against the ratios of: land pitch to land height; land lead to minor inside diameter; land height to minor inside diameter; and land width to land pitch, of the corresponding tube lengths to thus define zones A, B, C and D.

Each of the zones A, B, C and D is of rectangular form, and has horizontal upper and lower boundaries which establish the critical limits of each of the above mentioned ratios within which nucleate boiling of fluid will be maintained and beyond which film boiling will occur, with its attendant difficulties, in the ribbed tubes under the circumstances hereinafter described. The upper and lower boundaries of each zone provide an envelope for a family of curves each of which extends between the upper and lower boundaries and fixes the maximum allowable steam quality leaving the ribbed tubes for the maintenance of nucleate boiling of the vaporizable fluid passing therethrough for any given constant condition of pressure, heat flux and mass flow. With the lands and grooves of a ribbed tube proportioned and arranged in accordance with the ratio limits shown in FIG. 5, the steam quality at which nucleate boiling deteriorates into film boiling will be extended to the most favorable degree and far beyond the steam quality at which breakdown of nucleate boiling occurs in a smooth tube or prior types of ribbed tubes under the same conditions of pressure, mass flow, heat flux and fluid supply temperature. All four of the plotted ratios must fall within their corresponding zones in order to maintain nucleate boiling to the optimum steam quality limit, otherwise film boiling will exist at the optimum quality limit which would prevail for a ribbed tube formed in accordance with the invention. If one or more of these ratios falls outside their corresponding zones, the breakdown of nucleate boiling will occur at a steam quality considerably lower than the optimum for a ribbed tube having the proper ratios. For example, one of the ribbed tube lengths tested had ratios of p111, l/d, h/ d and w/p corresponding to the points e" in the respective zones, as shown in FIG. 5. With a mass flow of 500,000 lbs./hr.-ft. a heat flux of 220,000 B.t.u./hr.-ft. and a pressure of 3,000 p.s.i.a., nucleate boiling was maintained in this tube length up to a steam quality of 78 percent. Another tube length had ratios of p/h, l/ d, Md and w/p corresponding to the points f" on FIG. 5, with the ratio of Ur! falling within zone B, the ratio of p/h falling on the edge of zone A and the ratios of w/p and h/ d falling outside zones D and C, respectively. The breakdown of nucleate boiling in this tube length occurred at a steam quality of 15 percent under the same conditions of mass flow, heat flux and pressure as the first tube length.

The upper and lower limits of the zones A, B, C and D, and thereby the ratio limits corresponding to the zones, remain the same with varying heat flux, mass flow .and pressure. For the same pressure and heat flux, the quality at nucleate boiling breakdown varies directly with the mass flow; the higher the mass flow the higher the quality. With the same pressure and mass flow the quality at nucleate boiling breakdown varies inversely with the heat flux; the higher the heat flux the lower the quality. For the same heat flux and mass how the quality at nucleate boiling breakdown varies inversely with the pressure.

Thus it will be seen from FIG. 5 that the ribbed tube of the invention should have its lands and grooves proportioned and arranged as follows to maintain nucleate boiling through an optimum range of steam qualities for any given constant condition of pressure, heat flux and mass The preferred ratio of s/h is 0 to 1.5.

By way of example and not of limitation, the tube of FIG. 1 has a single continuous helical groove to provide lands intermediate the convoiutions of the grooves, and is manufactured according to the following specification:

Outside diameter .875"-* -.008"

.004 Minor inside diameter (d) .445"i.006" Minimum thickness 0.179 Land height (h) .018i.003" Land pitch 2) ,{32 i /i2 Land width (w) ,"i%2" Rib side contour within s/h 0 to 2.5

Again by way of example, the tube shown in FIG. 2 has two continuous helical grooves to provide lands intermediate the convolutions of the grooves, and is manufactured according to the following specifications:

Outside diameter 1.75":.008" Minor inside diameter (d) 1.25":.008" Minimum thickness 0.180 Land height (h) 0.050" :.003" Land pitch (p) "i Land width (w) d fil Rib side contour within s/h Oto 2.5

While in accordance with the provisions of the statutes we have illustrated and described herein the best form and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a sub-critical pressure vapor generator having a tube length subject to heat from a heat source and normally conducting a vaporizable fluid increasing in quality as it flows therethrough, the improvement comprising said tube length having its internal wall formed with a plurality of helical grooves to provide turbulence promoting helical lands intermediate the convolutions of the grooves with the lands and grooves being proportioned and arranged so that the ratio of p/h is from 10 to 25, the ratio of w/p is from .2 to .55, the ratio of Nd is from .1 to 2.5, and the ratio of h/d is from .03 to .08, where p is the pitch or distance between corresponding points of consecutive lands measured parallel to the tube axis, 11 is the height of the lands, w is the width of the lands, 1 is the lead or distance that the land advances in one complete revolution measured parallel to the tube axis, and d is the minor inside diameter of the tube.

2. In a sub-critical pressure vapor generator having a tube length subject to heat from a heat source and normally conducting a vaporizable fluid increasing in quality as it flows therethrough, the improvement comprising said tube length having its internal wall formed with at least one continuous helical groove to provide lands intermediate the convolutions of the groove, with the lands and groove being proportioned and arranged so that the ratio of p/lz is from 12 to 20, the ratio of w/p is from .3 to .4, the ratio of l/d is from .1 to 1.5 and the ratio of h/ d is from .04 to .06, where p is the pitch or distance between corresponding points of consecutive lands measured parallel to the tube axis, It is the height of the lands, w is the width of the lands, 1 is the lead or distance that the land advances in one complete revolution measured parallel to the tube axis, and d is the minor inside diameter of the tube.

3. A vapor generating tube having its internal wall formed with at least one helical groove and helical lands intermediate the convolutions of the groove, with the lands and groove being proportioned and arranged so that the ratio of p/h is from 10 to 25, the ratio of w/p is from .2 to .55, the ratio of l/d is from .1 to 2.5, and the ratio of h/a' is from .03 to .08, where p is the pitch or distance between corresponding points of consecutive lands measured parallel to the tube axis, h is the height of the lands, w is the width of the lands, 1 is the lead or distance that the land advances in one complete revolution measured parallel to the tube axis, and d is the minor inside diameter of the tube.

References Cited in the file of this patent UNITED STATES PATENTS Bailey Apr. 14, 1942 OTHER REFERENCES

Claims (1)

1. IN A SUB-CRITICAL PRESSURE VAPOR GENERATOR HAVING A TUBE LENGTH SUBJECT TO HEAT FROM A HEAT SOURCE AND NORMALLY CONDUCTING A VAPORIZABLE FLUID INCREASING IN QUALITY AS IT FLOWS THERETHROUGH, THE IMPROVEMENT COMPRISING SAID TUBE LENGTH HAVING ITS INTERNAL WALL FORMED WITH A PLURALITY OF HELICAL GROOVES TO PROVIDE TURBULENCE PROMOTING HELICAL LANDS INTERMEDIATE THE CONVOLUTIONS OF THE GROOVES WITH THE LANDS AND GROOVES BEING PROPORTIONED AND ARRANGED SO THAT THE RATIO OF P/H IS FROM 10 TO 25, THE RATIO OF W/P IS FROM .2 TO .55, THE RATIO OF L/D IS FROM .1 TO 2.5, AND THE RATIO OF H/D IS FROM .03 TO.08, WHERE P IS THE PITCH OR DISTANCE BETWEEN CORRESPONDING POINTS OF CONSECTIVE LANDS MEASURED PARALLEL TO THE TUBE AXIS, H IS THE HEIGHT OF THE LANDS, W IS THE WIDTH OF THE LANDS, L IS THE LEAD OR DISTANCE THAT THE LAND ADVANCES IN ONE COMPLETE REVOLUTION MEANSURED PARALLEL TO THE TUBE AXIS, AND D IS THE MINOR INSIDE DIAMETER OF THE TUBE.

Images