EP3097377B1 - Improved tube for a heat exchanger - Google Patents

Description

The invention relates to an element for an industrial type heat exchanger, in particular a condenser, of the type comprising a generally tubular body.

Condensers comprising such elements, also called "tube condensers", are widely used industrially, in particular for the production of electricity. A first fluid, typically water in the liquid state, is circulated inside a plurality of tubes, while a second fluid in the gaseous state, generally steam, is brought close to the tubes, externally thereto. There is then a heat exchange between the first and second fluids, through the wall of the tubes, which exchange causes the condensation of the fluid in the gaseous state.

Industrial type condensers must be able to condense large quantities of steam in a minimum of time. The volume of vapor that they are able to condense per unit time at least partly characterizes their performance. To do this, industrial condensers are generally equipped with hundreds of tubes, or even thousands, of great length, typically up to about twenty meters.

Originally, industrial condensers were equipped with smooth tubes. To improve their performance, particularly with regard to the flow of condensed steam, a new type of tube has been used, the body of which retains its generally tubular shape, but the wall of which has a twisted shape extending on at least one segment of said body. This twisted shape of the wall results in an outer surface having a domed relief extending helically along the segment in question, and a correspondingly shaped groove on the inner surface of the body. US2011/174469 , which represents the document of the state of the art closest to the subject of claim 1, discloses a heat exchanger element of the type comprising a tubular body whose wall is at least partly delimited by an internal surface and an outer surface, the wall having a twisted shape over at least a segment of said body, and the inner surface having at least one groove in shape correspondence with said wall and extending helically over said segment.

This particular configuration substantially improves the heat exchanges at the level of the tubes: on the one hand, the twisted shape gives the wall a larger contact surface between the fluids, inside and outside the tubes; on the other hand, it causes turbulence in the fluid flowing inside the tubes, which is generally beneficial for the heat exchanges at the level of the tubes. The twisted shape also improves the evacuation of drops that form on the outer surface of the tubes.

In the art, tubes having a configuration of this type are said to be "corrugated", or, more precisely, "provided with corrugations". The pitch of the twist, also called pitch of corrugation, is generally greater than 20 millimeters.

The Applicant has found that, in general, the actual performance of corrugated tubes, in particular with regard to their ability to condense a fluid on their outer surface, is quite markedly lower than the expected performance.

The invention aims to improve the existing.

The proposed heat exchanger element includes the features of claim 1.

• la figure 1 montre un schéma d'un échangeur thermique générique ;
• la figure 2 est une vue en plan d'un élément de tube pour l'échangeur de la figure 1 ;
• la figure 3 est une vue en coupe longitudinale d'une portion de paroi d'un l'élément de tube pour l'échangeur de la figure 1 ;
• la figure 4 représente une portion torsadée d'un élément de tube pour un échangeur thermique, vue en perspective et partiellement coupée.

The invention will be better understood on reading the detailed description below, given in relation to the appended drawings, in which:

• the figure 1 shows a diagram of a generic heat exchanger;
• the figure 2 is a plan view of a tube element for the exchanger of the figure 1 ;
• the picture 3 is a longitudinal sectional view of a wall portion of a tube element for the exchanger of the figure 1 ;
• the figure 4 shows a twisted portion of a tube element for a heat exchanger, seen in perspective and partially cut away.

The accompanying drawings contain elements of certain character. They may therefore not only be used to complete the description of the invention, but also contribute to its definition, where applicable.

We refer to the figure 1 . It shows, generically, an industrial type heat exchanger in the form of a condenser 1.

The condenser 1 comprises a plurality of elementary tubes 3 held relative to each other in one or more bundles 5 by plates 7 distributed along the tubes 3. Each plate 7 is thus crossed by each of the tubes 3 of the bundle 5.

Condenser 1 further comprises a pair of header boxes 9 into which the opposite ends of each of tubes 3 respectively open.

One of the boxes 9 is in fluid communication with a fluid inlet 11, while the other of these boxes 9 is in fluid communication with a fluid outlet 13.

Inlet 11 and outlet 13 can be connected to the rest of a circuit in which a first fluid circulates. Typically, the first fluid enters condenser 1 through inlet 11 in liquid form. It circulates from the corresponding collector box 9 to the other collector box inside the tubes 3, in one or more passes. From there, the first fluid leaves condenser 1 for the rest of the circuit through outlet 13.

The bundle 5 of tubes 3 is housed in an enclosure 15 formed inside what is called a calender 17 in the art. The shell 17 is equipped with a fluid inlet 19 and a fluid outlet 21 which open into the enclosure 15.

The inlet 19 and the outlet 21 make it possible to connect the condenser 1 to a circuit in which a second fluid circulates.

The second fluid enters enclosure 15 through inlet 19 in gaseous form. In contact with the tubes 3, the second fluid exchanges heat with the first fluid circulating inside these tubes. The first fluid being generally introduced at a temperature lower than that of the second fluid, the latter condenses on the outer surface of the tubes 3. The second fluid in liquid form leaves the enclosure 15 through the outlet 21.

Condenser 1 type condensers are widely used in industrial power generation. This involves in particular condensing water vapor using cold water circulating inside the tubes. To do this, very long tubes are used, up to twenty meters each.

We refer to the figure 2 . It shows a TE tube element that can be used in a condenser like condenser 1.

The tube element TE comprises a body BDY in the general shape of an elongated hollow cylinder, or tubular, of length TL. The body BDY has two longitudinal end sections ES1 and ES2 connected to each other by a central section CS of length CL. The length TL corresponds to the total length of the tubular element TE, including the central section CS and the end sections ES1 and ES2.

The ES1 and ES2 end sections are generally cylindrical, with an outside diameter TOD. The diameter TOD corresponds to the nominal outside diameter of the TE element, as it is commonly referred to in the art. End sections ES1 and ES2 each have a smooth outer surface and inner surface.

The central section CS has a wall which extends along the body in a twist, or in a helical manner, or even in a helicoid, forming turns LP around the longitudinal axis LA of the tube element TE. Here, the LP turns are contiguous.

This twisted conformation of the wall of the element TE results in an outer surface which, over the length of the central section CS, has a helical relief, made of hollows and bumps. This relief is likely to improve the heat exchange capacities of the TE element, because the outer surface of the latter is then larger than that of a smooth tube of the same outer diameter. In addition, it improves the evacuation of drops that form on the outer surface of the TE element. The central section CS retains the general appearance of a hollow cylinder, having an outside diameter COD.

Reference is made to figures 3 and 4 . They show, in a generic way, a twisted portion CW of the wall and a parameterization of this twisted shape. The result of the twisted shape of the wall CW is an internal surface IS having a relief made of ridges and hollows, in shape correspondence with the hollows and bumps, respectively of the external surface OS. Seen otherwise, the internal surface IS is provided with a groove which extends according to a helix with contiguous turns along the section CS. Seen still otherwise, the inner surface IS has a helicoid shape.

This relief is likely to improve the heat exchange capacities of the TE element, due to the generation of vortices in the fluid which flows inside the TE element.

The twisted wall CW has a thickness TT. The thickness TT corresponds to the nominal thickness of the TE element, that is to say the thickness of the wall of the smooth tube at the origin of the TE element. The central section CS has an inside diameter CID. The CID diameter corresponds to the diameter of a gauge just capable of passing through the TE element internally.

The twisted section CS has an outside diameter COD which corresponds to the nominal outside diameter of the element TE on the twisted section CS, that is to say the diameter of a cylindrical envelope surface of this section.

The twist shape has a pitch CP, if necessary considered inside the tabular element. The depth CD of the internal groove resulting from the twisted form is considered with respect to an internal envelope surface of the tubular element, or, seen otherwise, as the radial distance between the bottom of the hollows of the internal surface IS and the top of the ridges.

Although not represented in the figures, we can note TID the dimension which corresponds to the nominal internal diameter of the tube, as it is usually designated in the art, that is to say here the nominal internal diameter of the smooth terminal sections ES1 and ES2.

$$\mathrm{FR}<24$$

$$\mathrm{1,5}<\mathrm{R}<\mathrm{2,5},\mathrm{et}\mathrm{en}\mathrm{particulier}\mathrm{R}=\mathrm{1,7}$$

According to a general aspect of the invention, the tube element TE has, on the twisted section CS, a nominal outside diameter COD of between 18 and 30 millimeters. The pitch CP of the twist is less than 3.5 millimeters. The depth CD is such that the ratio of the pitch CP raised to a real power R of between 1.5 and 2.5 over the depth CD, which is called aspect ratio FR, remains lower than a ceiling value TV. In particular, the ceiling value TV is close to 24. In particular, the power R is close to 1.7. In other words, the depth CD verifies the conditions COND1, COND2 and COND3 stated below: $$\mathrm{FR}={\mathrm{CP}}^{\wedge }\mathrm{R}/\mathrm{CD}$$

$$\mathrm{FR}<24$$

$$1.5<\mathrm{R}<2.5,\mathrm{and}\mathrm{in}\mathrm{particular}\mathrm{R}=1.7$$

Tables 1A and 1B below bring together characteristic data of the twisted section CS for TE tube elements according to three variant embodiments of the invention (table 1A), and two example embodiments (table 1B). The dimensions are expressed in millimeters. Each time, the tube elements conforming to these variant embodiments are such that they satisfy the conditions COND1, COND2 and COND3, in particular with R=1.7. <u>Table 1A</u>: Variant 1 Variant 2 Variant 3 TOD 19.05 22.22 25.40 CODE 18.90 22.07 25.25 TT 0.5 0.5 0.5 CP between 2 and 3.5 between 2 and 3.5 between 2 and 3.5 CD between 0.15 and 0.3 between 0.15 and 0.3 between 0.2 and 0.3 Example 1 Example 2 TOD 19.05 22.22 CODE 18.90 22.07 TT 0.5 0.5 CP 2 2.5 CD 0.16 0.25

Tables 1A and 1B show that the twisted part of the tubes according to the invention has a very small pitch, less than 3.5 millimeters, and preferably less than 3 millimeters, compared to the pitch values conventionally used in tubes with corrugations, typically greater than 20 millimeters. Consequently, the tubes according to the invention differ from conventional tubes in that the twisted section has a shape resembling a spiral.

Table 2 below brings together the dimensional characteristics relating to a set of tube elements (referenced I, ..., XII) each having a twisted central section. The tube elements are distinguished from each other by the profile of their respective twisted section, characterized by values of pitch CP and depth CD different from each other. Dimensions missing from Table 2 are common to tubes I to XIV. In particular, the tube elements all have an outside diameter of 22.22 millimeters and a wall thickness of 0.5 millimeters. The tubes are made of grade 2 titanium.

The CP pitch and CD depth values are expressed in millimeters.

Table 2 also shows the corresponding values of the aspect ratio FR, calculated for a power value R of 1.7. <u>Table 2</u>: CP CD FR I 2.0 0.14 23.21 II 2.3 0.19 21.92 III 2.5 0.25 18.84 IV 3.1 0.17 39.44 V 3.2 0.39 18.06 VII 3.3 0.24 31.83 VII 3.3 0.32 24.16 VIII 3.3 0.44 17.57 IX 4.2 0.47 24.42 X 4.3 0.49 23.96 XI 4.8 0.40 36.47 XII 4.9 0.64 23.24 XIII 1.3 0.04 39 XIV 1.5 0.06 33

In this table 2, the tube elements II, III, are in accordance with the embodiment variant 2. The tubular element III is moreover in conformity with the example embodiment 2. In addition, the elements I, II, III, V, and VIII are in accordance with the invention in that they have CP pitch values of less than 3.5 millimeters and additionally verify the conditions COND1, COND2 and COND3.

Tube elements IV and VI have dimensions in accordance with embodiment variant 2, except that they do not satisfy conditions COND1 to COND3.

The tube element X satisfies the conditions COND1, COND2 and COND3 with R = 1.7.

Table 3 below collates the results of heat exchange capacity measurements carried out on the elements in table 2.

In table 3, the coefficient K represents a heat exchange capacity measured for the tube element considered. The K coefficient is expressed in Watt per square meter and per Kelvin (Wm ^-2 .K ^-1 ). The HER value, expressed as a percentage, corresponds to the improvement in the value of K for the element considered compared to a smooth element having otherwise similar dimensions. <u>Table 3</u>: K HER I 7901 50% II 7646 45% III 8098 54% IV 6669 26% V 8599 63% VII 6771 28% VII 6778 29% VIII 7912 50% IX 6659 26% X 8081 53% XI 6553 24% XII 7698 46% XIII 6473 23% XIV 6007 13%

Table 3 shows that compliance with COND1, COND2 and COND3 conditions is generally associated with a significant increase in heat exchange performance. The rows corresponding to tube elements I, II, III, V, VIII, X and XII show coefficient K values at least 45% higher than the reference value for a smooth tube (5272 Wm ^-2 .K ^{- 1} ). The comparison in Table 2 of lines VII and X on the one hand, and lines I and XIII and XIV on the other hand, also shows a slight increase in heat exchange performance as soon as the FR ratio exceeds the ceiling value of 24. When the FR ratio is higher than the ceiling value, the increase in heat exchange performance compared to a smooth tube is generally less than 30%.

Table 3 proves that the tubular elements in accordance with the invention have greatly improved heat transfer capacities in comparison with smooth elements on the one hand, and elements whose twisted section deviates from the profile provided by the invention.

Table 4 below collates the results of measurements carried out on the tube elements of table 2.

Table 4 also collates the values of the so-called Darcy coefficient, or Darcy-Weisbach, for the tubes considered, as well as the DCR increase in the value of this coefficient relative to a smooth reference tube. The Darcy coefficient corresponds to a pressure drop coefficient. This dimensionless quantity represents the influence of the type of flow (laminar or turbulent) and the appearance of a pipe (smooth or rough) on the pressure drop. Here, the DARCY coefficient is calculated for a flow rate of 2.5 cubic meters per hour.

An increase in the DARCY value is generally unfavorable to the performance of a tube element within a condenser. In particular, an increase in the DARCY value implies an increase in the energy consumption necessary for the circulation of the fluid inside the tubes. In other words, an increase in the DARCY value harms the condensation of water vapor on the exterior of the tubular element with constant energy consumption. <u>Table 4</u>: DARCY DCR I 0.032487593 48% II 0.032098063 46% III 0.032823286 49% IV 0.052203857 129% V 0.054736888 140% VII 0.043978584 95% VII 0.038960463 74% VIII 0.050991452 124% IX 0.071932623 211% X 0.068924461 199% XI 0.052814985 132% XII 0.073328679 217% XIII 0.029523637 41% XIV 0.03016296 44%

Table 4 generally shows that the tubes in accordance with the invention exhibit a substantial increase in the DARCY value. However, this increase remains limited (less than 140 and less than for certain tubes not in accordance with the invention, as indicated by a comparison with lines X and XII). On the other hand, the relative increase in the DARCY coefficient is very low (near, or even less, to 100%) for the elements conforming to the second variant embodiment (tubes II and III) and for tube I, in comparison with the other tubes tested. The tubes of the second embodiment variant and tube I show an increase in the DARCY value which is very clearly lower than the others.

It follows from the foregoing that the tubes having a twisted section in accordance with the invention are capable of greatly improved performance as regards their ability to condense a gas circulating externally. This improved performance is the result of a twist shape which greatly improves the heat exchange capacities and significantly limits the effects of pressure drop.

The comparison of examples I to II with examples XIII and XIV highlights that the reduction in the pitch makes it possible to reduce the DARCY value but that compliance with the conditions COND1, COND2, COND3 makes it possible to obtain an improved heat exchange performance.

In addition, the tubes in accordance with the variant embodiments 1 to 3, and to Examples 1 and 2, are likely to have even greater condensation performance due to heat exchange capacities comparable to the other tubes according to the invention and losses. significantly reduced loads compared to these tubes.

From the general embodiment of the invention, and on the basis of tests whose results are at least partly mentioned in Tables 2 to 3, it is considered that the characteristics below, which are optional, complementary or substitution, are likely to further improve the condensation performance of a tube:
The outer diameter of the TOD tube is between 19 and 26 millimeters, preferably between 20 and 26 millimeters, and even more preferably between 20 and 23 millimeters. In particular, the outer diameter TOD is close to 19.05 millimeters, 22.22 millimeters, or 25.4 millimeters.

The outer diameter COD of the twisted part is between 18 and 26 millimeters, preferably between 20 and 26 millimeters, more preferably still between 20 and 23 millimeters. In particular, the diameter COD is close to 18.90 millimeters, 22.07 or 25.25 millimeters.

The pitch CP is strictly greater than 2 millimeters. It is less than 3 millimeters.

The thickness TT of the wall CW of the tube is between 0.4 and 1 millimeter, for example of the order of 0.5 millimeter.

The invention is not limited to the embodiments described above, but encompasses all the variants that a person skilled in the art may consider.

Claims (8)

1. Industrial type condenser element (TE) comprising a tubular body (BDY) made from grade 2 titanium, designed for liquid water circulation inside, the wall (CW) of which is at least partly delimited by an inner surface (IS) and an outer surface (OS), the wall (CW) having a twisted shape on at least one segment (CS) of said body (BDY), and the inner surface (IS) having at least one groove with a shape corresponding to said wall (CW) and extending helically over said segment (CS), such that on said segment (CS), the outer surface (OS) has a diameter (COD) comprised between 18 and 30 millimetres, while the groove has a pitch (CP) of less than 3.5 millimetres and a depth (CD) such that the ratio of the pitch (CP) at a real power comprised between 1.5 and 2.5 to the depth (CD) is less than a threshold value close to 24, said depth (CD) being comprised between 0.05 millimetre and 0.6 millimetre, in particular greater than 0.15 millimetre.
2. Element according to claim 1, in which the real power is close to 1.7.
3. Element according to one of claims 1 and 2, in which the body (BDY) has, on at least said segment (CS), an outside diameter (COD) comprised between 18 and 26 millimetres, preferably between 20 and 26 millimetres, and even more preferentially between 20 and 23 millimetres.
4. Element according to claim 3, in which the body (BDY) has, on at least said segment (CS), an outside diameter (COD) close to 18.90, 22.07 or 25.25 millimetres.
5. Element according to one of the preceding claims, in which said pitch (CP) is strictly greater than 2 millimetres.
6. Element according to one of the preceding claims, in which said pitch (CP) is less than 3 millimetres.
7. Element according to one of the preceding claims, in which the wall (CW) has a thickness comprised between 0.4 and 1 millimetre, preferably close to 0.5 millimetre, on at least part of said segment (CS).
8. Heat exchanger comprising at least one element according to one of the preceding claims

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