US3387592A - Tubular heat-transfer elements - Google Patents

Description

June 11, 1968 L. F. H. M. FOUCHE'. ETAL 3,387,592

TUBULAR HEAT-TRANSFER ELEMENTS m/z/E/v TUES a'e/ckw/mem Faye/ ATI'OZN'YJ Mia Filed Oct. 26, 1966 V arc/aw United States Patent 7 Claims. (31. 122-406) The present invention has essentially for its object a tubular heat-transfer element or unit intended notably for forced circulation heat-transfer steam-generator of the type comprising a tube of substantially rectilinear configuration about which a heat-transfer fluid is circulated, and in which the fluid to be heated is circulated, this fluid being successively in the liquid state in the economizer, in the state of a vapor liquid mixture in the vaporizer, this vaporizer comprising in turn a first zone in which the heat-transfer coefficient is relatively increased and another zone in which the heat-transfer coefficient is relatively low, and finally in the state of vapor in the superheater, said fluid being caused, during at least part of its travel to follow a helical path due to the provision of fins or like deflection means of helical configuration which are disposed inside said tube.

As a rule forced-circulation heat-transfer steam-generators comprise essentially clusters of straight vertical tubes mounted in a casing, the circulation of the heattransfer or heating fluid (consisting for example of carbon dioxide) taking place externally of the tubes and parallel to their axes in the downward direction, the fluid to be heated, consisting for example of ordinary water, circulating upwards in these tubes.

It is known that the reheating and vaporization of water, and eventually the superheating thereof, require as a rule a relatively long circuit within the tubular elements.

It has already been proposed to direct the water to be heated along a helical path by providing helical fins or the like within said tube, in order to elongate the path followed by the water while limiting the necessary tube length.

However, in none of the various arrangements proposed up to now for imparting this helical path to the water stream the least account is taken of the successive physical states of this water as it flows along the heattransfer tube.

The successive states of the fluid to be heated, for example water, circulating in heat-exchange tubes, led the applicants to consider the following elements of a steam generator:

(1) The economizer in which water is present only in liquid phase, this economizer corresponding, in the heattransfer steam-generator of the type set forth hereinabove, to the lower portion of said heat-transfer tube.

(2) The vaporizer in which a mixture of liquid phase and vapor is circulated, this vaporizer corresponding, in the above example, to the intermediate portion of the tube, that is, the zone located just above the economizer.

This vaporizer comprises in turn two separate zones differing from each other by the composition of the mixtures of the two liquid and vapor phases:

A first zone located immediately after the economizer, in which water in the liquid state has reached its boiling point, this zone being characterizer, as a consequence of the considerable convections due to this ebullition, by a high heat-transfer coefficient. In this first zone the inner wall of the tube is coated with a liquid film.

A second zone following the first one, in which the water-and-vapor mixture occurs in the form of water droplets in a steam medium, the convection currents in "ice this mixture being however less considerable so that this mixture has a relatively low heat transfer coeflicient. In this zone, the inner wall of the tube is coated with vapor.

It should be noted that the limit between the first and second zones defined hereinabove is dependent on the value of the heat flux and corresponds to different vapor contents according as the critical flux corresponding to the fluid pressure and output, and to the geometrical characteristics of its flow, is attained or not.

(3) The superheater following the second zone aforesaid and corresponding therefore to the upper portion of the tube; only vapor-phase fluid circulates in this third zone.

The hitherto proposed solutions aiming at improving the heat-transfer co-eflicient did not, from the practical point of view, provide the advantageous results one might feel entitled to expect therefrom, due to the lack of consideration for the various states in which the fluid, notably water, occurs.

The present invention has for its object a tubular heattransfer element of the type broadly set forth hereinabove, which is designed by taking due consideration for the successive states of the circulating fluid, for example, this element being remarkable notably in that it comprises a core or like member extending within and coaxially to the tube on nearly the whole of its length in order to form in the economizer, the vaporizer and the superheater an annular space of which the cross-sectional area is substantially the same along these three sections and in which the fluid to be heated is circulated, the aforesaid helical fins being provided in the economizer, in said second zone of the vaporizer and in said superheater.

It will be seen that the heat-transfer'tube according to this invention provides for the water to be heated an an nular passage having the same shape and dimensions throughout the path followed by this water while constraining the water, during the initial portion of its travel, that is, when it is in liquid phase and at a relatively low temperature, a helical path which, by increasing the length of its travel and the convections tends to increase the rate of heat transfer; then, as the water attains its boiling point and therefore its heat transfer coeflicient has attained a high value, a substantially rectilinear path in said annular space; finally, when the vapor prevails and all the more when the vapor phase alone exists, so that the heat-transfer coeflicient has again resumed a relatively low value, another helical path.

The device according to this invention will thus permit of adapting the water circulation conditions to the variations in the heat-transfer coefficient of the water which are due to the changes in its physical state.

According to another feature characterizing this invention, the aforesaid core consists of a tubular or like hollow body com-prising at least two flattened portions disposed the one adjacent to the limit between said economizer and said vaporizer, and the other substantially at the limit between said first and second zones of the vaporizer.

This specific configuration of the core-forming central tube will facilitate the mounting and centering of this tube in the heat-transfer tubular body proper while avoiding any breaks in the continuity of the cross sectional dimensions of said annular space except in zones corresponding to changes in the physical states of the circulating water.

According to another feature characterizing this invention, the aforesaid hollow core or tubular body is provided with orifices whereby its inner space can communicate with the aforesaid annular space in order to create a pressure equalization between said inner space and said annular space.

This pressure equalization permits of avoiding the distortion of the inner core and the creation of strains in this core as a consequence of the substantial pressure values attained in said annular space.

According to another feature characterizing this invention, the aforesaid orifices are disposed in the vicinity of said flattened portions and also in the vicinity of the core end by which the liquid to be heated is admitted into the device. The orifice provided on the water inlet side, that is, at the base or bottom of said core, may be used for draining the core.

According to a further feature of this invention, the aforesaid core consists preferably of at least three tubes flattened at either ends and butt-welded to each other at said flattened ends.

The manufacture of this core by welding separate tubes end to end permits of eliminating the difiiculties usually encountered in the production of tubular pieces of relatively great length; consequently, their construction is simplified and their cost reduced accordingly.

Other features and advantages of this invention will appear as the following description proceeds with reference to the attached drawings showing diagrammatically by way of example a typical form of embodiment of the tubular heat-transfer element of this invention. In the drawings:

FIGURE 1 is an axial section of the element, and

FIGURE 2 is a fragmentary section taken along the line IIII of FIGURE 1, to show the flattened portions of the core.

In the form of embodiment illustrated the tubular heattransfer element according to this invention comprises a straight tube 1 of cylindrical cross-sectional contour, provided with rectilinear external fins or ribs 2 and communicating on the one hand with a neck 3a for introducing the fluid to be heated, for example water in the liquid phase, this neck being connected to the main body of the tube 1 through a tapered portion 1a, and on the other hand with a neck 31) for discharging the heated fluid, for example superheated water steam, this neck being connected to the main body 1 through another tapered portion 1b.

This tube 1 further comprises, in succession, in the direction of flow of the water as shown by the arrows F, an economizer E in which water circulates in the liquid phase, the vaporizer V in which water circulates in the form of a mixture of liquid water and water steam, this vaporizer comprising in turn a first zone Z1 in which the inner wall of the tube 1 is coated with a liquid film, and a second zone Z2 in which the inner wall of the tube 1 is coated with steam, and finally the superheater S in which only water steam circulates.

Housed in this tube 1 is a core of generally tubular configuration, designated by the reference numeral 4, disposed coaxially to the outer or main tube 1 and corresponding substantially to the length of this main tube between the tapered portions 1:: and 1b. The diameter of this core 4 is substantially smaller than that of tube 1 whereby an annular space 5 is formed between the tube 1 and the core 4 to permit the circulation of the fluid to be heated.

The core 4 consists of three elementary tube sections 6, 7 and 8; each elementary tube section is flattened at its ends and formed therefore with flattened portions 6a, 6b; 7a, 7b; 8a, 8b. Tubes 6 and 7 on the one hand and 7 and 8 on the other hand are butt-welded at their adjacent ends.

Tube 6 is disposed in the economizer section E of the main tube 1 and projects slightly into the next portion V where the vaporization begins. It is known that in a steam generator of the conventional type comprising a single passage the point at which the vaporization commences is dependent on the load, so that the amount of projection of this tube 6 into the vaporizer-forming portion is a function essentially of the rated load, due allowance being made, however, for overload operating conditions and also for low-load operating conditions of the tubular heattransfer element.

To ensure a proper balance between the pressure values prevailing inside the tubular core 6 and the tubular inner space 5 respectively, a communication orifice 9 is provided in the vicinity of the end 6a where the liquid-phase water is introduced, in order to permit the draining of the element, at least when the heat-transfer tube is operated in a vertical position, as generally assumed.

The inner or core-forming tube 6 is provided on its outer surface with helical fins or ribs 10 consisting for example of one or more crimped or welded steel strips forming in the annular space 5 a helical path for the liquidphase water. This helix has a constant pitch of a value selected to impart an optimum velocity to the water, this velocity corresponding to the optimum heat-transfer and pressure-loss conditions. This pressure loss may be calculated to ensure a satisfactory flow stability.

The upper portion of the tube 6 constituting the core of the element is filled with water at a slightly higher pressure, this overpressure being dependent on the leakages at its upper end, that is, on the thickness 2 (see FIG- URE 2) of the passage left in its flattened end portion 6b connected to the flattened end portion 7a of the next tube 7.

This intermediate tube 7 is a plain tube extending in the zone Z1 of the vaporizer in which water has attained its boiling point and therefore a high heat transfer coeflicient, even in the absence of an appreciable forcedconvection. Due to the high value of this heat-transfer coefficient a moderate water circulation rate may be accepted; in other words, any helical fins or ribs may be dispensed with. Under low load conditions nearly complete vaporization may be attained, but the reduction in the heat-transfer coefficient is then immaterial for an excess of heat-transfer surface area is available.

In the vicinity of its two flattened ends the tube 7 is provided with orifices such as 11a and 11b providing a communication between this tube and the annular space 5. It should be noted that the tube ends are flattened in such a manner as to leave a minimum cross-sectional passage area between adjacent tubes.

Under these conditions a very moderate pressure loss is produced in the boiling water circulation. In the inner portions of tube 7 the temperature cannot be as high as in the aforesaid annular portion 5, so that the water level in this tube 7 is of course subordinate to the pressure loss and in certain cases a moderate downward circulation may take place, this circulation being fed through the orifices 11b.

The last tube 8 extends in the zone Z2 of vaporizer V and in the superheater S, and comprises helical fins 12 like the first tube 6.

In the zone Z2 this tube 8 is adapted to increase to a substantial degree the heat-transfer coefficient. This increment is obtained whenever the vapor richness of the mixture is such that the inner wall of the main tube 1 has dried up. Moreover, when the heat flux is equal to or higher than the critical flux, the presence of helical fins permits of continuing the vaporization with a very high heat-transfer coeflicient due to the centrifugal action exerted on the water which causes the water film deposited on the inner wall of tube 1 to attain high vapor contents values. Therefore, the device operates as if the critical flux necessary for breaking the water film formed on the inner wall of tube 1 were increased considerably.

In the superheater S the tube 8 improves the heattransfer coefficient as a consequence of the increment in vapor flow velocity.

The pitch of the helical fins 12 may have a different value in zone Z2 and in the superheator. In the case considered herein, the fins have a pitch increasing gradually from the portion in which vaporization takes place with a high heat-transfer coeificient, and the portion corresponding to high vapor richness values. Moreover, the pitch may be increased near the end of the superheater with a view to reduce pressure losses.

Orifices such as 13 are also provided in the vicinity of the inner end 8a of tube 8 which is connected to tube 7.

The inner space of tube 8 contains only steam, perhaps with a slight moisture content at its lower portion. The internal overpressure is dependent on the pressure loss sustained by the vapor and also on the low leakage output at the flattened end 8b which, like the preceding flattened end 8a provides a restricted passage therein. This overpressure is still limited so that any risk of bursting the tube and delivering appreciable quantities of non-vaporized water is safely precluded.

The flattened portions 6a and 8b are adapted properly to merge or fit into the tapered portions 1a and 1b of the main tube 1 and the major dimension of the other flattened portions 6b, 7a, 7b and So has a length very slightly inferior to the inner diameter of tube 1, whereby the assembly consisting of the core-forming inner tubes 6, 7 and 8 butt-welded to one another can be introduced without diificulty into the tube 1.

Of course, suitable distance pieces 14 may be provided between the plain tube 7 and the main tube 1.

Similarly, the fins 10 and 12 secured to the outer surface of tubes 6 and 8 respectively have an outer diameter very slightly inferior to the inner diameter of tube 1 to permit the introduction of the core into this tube.

As a result, between the fins and the inner wall of the tube 1 a passage is provided so that the circulation of fluid along these helical fins is attended by a certain degree of leakage. This leakage is not detrimental under normal tolerance conditions, for the fluid flows at a very high speed along the wall of tube 1 and therefore with a very satisfactory heat-transfer coefficient.

Advantageously, a local expansion may be contemplated at one or several points of tubes 6 and 8 so that the fins 10 and 12 will engage the inner wall of tube 1 without any play. Similarly, the aforesaid distance-pieces 14 provided bet-ween the tubes 7 and 1 may advantageously cOnsist of resilient member. This particular mounting prevents possible vibration of the core in the main tube 1.

The tubular heat-transfer element thus constructed is particularly advantageous in that its pressure loss is reduced to a very low value although it consists of a singlepassa-ge steam generating tube. In fact, in zone Z1, that is, in the major portion of the vaporizer, the pressure loss is extremely low due to the absence of fins. The total pressure loss is only slightly higher than that measured in a natural-circulation steam generator. This pressure loss is a factor promoting a good distribution among the various tubular elements of the steam generator.

On the other hand, the inner volume of the tubular element according to this invention is relatively high and due to the communications provided in the inner core 4 the inner space of the core-forming tubes is thus filled with water along a substantial portion of its length. Thus, a thermal inertia is obtained which is a stability factor and the water in tube 6 has a function very similar to that contained in the reservoir of a natural-circulation steam generator.

The helical fins which, in the case illustrated, consist of steel strips inserts welded to the outer surface of the end tubes 6 and 8, may also be formed by machining or knurling the walls of these tubes, or through any other suitable and known process.

Besides, the flattened portions provided at the connections of tubes 6, 7 and 8 for separating the core portions from one another without ensuring a complete fluid tightness may have a shape other than that illustrated, for example a cross-like configuration. On the other hand, these flattened portions may also be replaced by partitions or 6 plugs carried by the ends of these tubes and provided with small passage orifices, the core having in this case the appearance of a tubular member of constant cross-section.

Of course, this invention should not be construed as being limited by the specific form of embodiment illustrated and described herein which is given by way of example only and to which many modifications and variations may be brought without departing from the scope of the invention as set forth in the appended claims.

What we claim is:

1 A tubular heat-transfer element, for heat-transfer and steam-generator of the forced-circulation type, comprising a tube of substantially rectilinear general configuration about which a heating fluid is circulated and within which a fluid under pressure to be heated is circulated, said tube comprising an economizer section, a vaporizer section, said vaporizer section having in turn a first zone in which the heat-transfer coemcient is relatively high and a second zone in which said heat-transfer coefficient is relatively low and a superheater section, a core member fitted internally and coaxially in said tube and extending substantially throughout the tube length, an annular space between said tube and said core member the cross-sectional are-a of said annular space being substantially the same along the tube, and helical fins housed in said annular space and provided in said economizer section, in said second vaporizer zone and said superheater section, whereby the fluid to be heated is caused to follow a helical path in said economizer section, a substantially rectilinear path in said first vaporizer zone and again a helical path in said second zone and in said superheater section.

2. Element according to claim 1, wherein said core consists of a tubular body, having an inner wall defining an inner space and comprising at least two flattened portions disposed the one in the vicinity of the limit between said economizer portion and said vaporizer portion, and the other substantially at the limit between the first and second zones of said vaporizer portion.

3. Element according to claim 2, wherein said tubular body is provided with orifices permitting the communication between said inner space and the aforesaid annular space in order to balance the pressures between said inner space and said annular space.

4. Element according to claim 3, wherein said orifices are disposed in the vicinity of said flattened portion and also in the vicinity of the tubular body and where the fluid to be heated is introduced.

5. Element according to claim 2, wherein the width of said flattened portions, in the direction of their major dimension, is very slightly inferior to the inner diameter of said tube, whereby they engage said inner wall of the tube while permitting the sliding movement of said tubular body in said tube.

6. Element according to claim 2, wherein said tubular body comprises at least three tube sections flattened at both ends and butt-welded to each other at said flattened ends.

7. Element according to claim 1, wherein the pitch of said helical fins in said second zone of the vaporizer portion and in the superheater section increases in the direction away from the limit between said first and second zones of the vaporizer section.

References Cited UNITED STATES PATENTS 3,294,070 12/1966 Bell 122-32 3,249,154 5/1966 Legrand -109 FOREIGN PATENTS 625,059 8/ 1961 Canada. 980,412 1/ 1965 Great Britain.

KENNETH W. S-PRAGUE, Primary Examiner.

Claims (1)

1. A TUBULAR HEAT-TRANSFER ELEMENT, FOR HEAT-TRANSFER AND STEAM-GENERATOR OF THE FORCED-CIRCULATION TYPE, COMPRISING A TUBE OF SUBSTANTIALLY RECTILINEAR GENERAL CONFIGURATION ABOUT WHICH A HEATING FLUID IS CIRCULATED AND WITHIN WHICH A FLUID UNDER PRESSURE TO BE HEATED IS CIRCULATED, SAID TUBE COMPRISING AN ECONOMIZER SECTION, A VAPORIZER SECTION, SAID VAPORIZER SECTION HAVING IN TURN A FIRST ZONE IN WHICH THE HEAT-TRANSFER COEFFICIENT IS RELATIVELY HIGH AND A SECOND ZONE IN WHICH SAID HEAT-TRANSFER COEFFICIENT IS RELATIVELY LOW AND A SUPERHEATER SECTION, A CORE MEMBER FITTED INTERNALLY AND COAXIALLY IN SAID TUBE AND EXTENDING SUBSTANTIALLY THROUGHOUT THE TUBE LENGTH, AN ANNULAR SPACE BETWEEN SAID TUBE AND SAID CORE MEMBER THE CROSS-SECTION AREA OF SAID ANNULAR SPACE BEING SUBSTANTIALLY THE SAME ALONG THE TUBE, AND HELICAL FINS HOUSED IN SAID ANNULAR SPACE AND PROVIDED IN SAID ECONOMIZER SECTION, IN SAID SECOND VAPORIZER ZONE AND SAID SUPERHEATER SECTION, WHEREBY THE FLUID TO BE HEATED IS CAUSED TO FOLLOW A HELICAL PATH IN SAID ECONOMIZER SECTION, A SUBSTANTIALLY RECTILINEAR PATH IN SAID FIRST VAPORIZER ZONE AND AGAIN A HELICAL PATH IN SAID SECOND ZONE AND IN SAID SUPERHEATER SECTION.

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