WO2008089777A1

WO2008089777A1 - Heat exchanger with dual concentric tubes and calender

Info

Publication number: WO2008089777A1
Application number: PCT/DZ2008/000002
Authority: WO
Inventors: Chérif BOUGRIOU; Khireddine Baadache
Original assignee: Bougriou Cherif; Khireddine Baadache
Priority date: 2007-01-24
Filing date: 2008-01-21
Publication date: 2008-07-31

Abstract

The invention relates to a heat exchanger with dual tubes and a calender, and to a superimposition of heat exchangers with triple concentric tubes that can operate according to a single or dual phase mode (condenser and evaporator). It is similar to a traditional tube and calender exchanger, the difference being that the tubes are replaced with dual coaxial tubes, which results in an increased heat-exchange surface without increasing neither the volume nor the geometrical shape on the calender side. The heat exchanger with dual tubes and a calender can be built in different manners depending on the state of the fluids. The simple tubes or notches can be provided with vanes having various shapes. Accordingly, the heat exchanger operates with three fluids (2 hot fluids and a cold fluid H-C-H or the opposite C-H-C) instead of two. Instead of straight tubes, the calender may include concentric tubes having a helical, coiled or spiralled shapes. If one of the fluids is a gas, the heat exchanger can be of the crossed co-current type, or most often crossed with counter-flow and dual tubes. The heat-exchangers with dual tubes and a calender can be used in the following fields: industry, pharmacy, thermal power plants, nuclear power plants, transport, distillation, agri-food industry, heating, cooling, air conditioning, heat pumps, cryogenics.

Description

HEAT EXCHANGERS WITH DOUBLE CONCENTRIC TUBES AND CALANDER

Technical field to which the invention relates ^r

The invention relates to heat exchangers with double tubes and calender, used in the fields: industrial, pharmaceutical, thermoelectric power station, nuclear power plant, transport, distillation, food industry, heating, cold, air conditioning, heat pump, cryogenics, etc. They are more particularly used in the field of heat exchangers.

_* State of the prior art

There are tube and shell heat exchangers and simple or co-current and countercurrent cross-heat exchangers for several decades. The single tubes or grooves are of different geometrical types that can have fins of various shapes, see references [1-11]. These heat exchangers can operate in single or double phase (condenser and evaporator).

They have lower thermal performance and compactness than our two new types of heat exchangers (coaxial double tube heat exchanger and shell and coaxial double tube heat exchanger). The performance of these two new exchangers is estimated to be 70% better than conventional heat exchangers.

The invented heat exchangers are similar to the conventional exchangers mentioned above, the difference being that single or corrugated tubes with or without fins are now replaced by double coaxial tubes (double jacketed tubes). The outer diameter of the envelope of the double concentric tubes is of the same order as the tubes used in conventional tube and shell heat exchangers. Likewise, two new tubular plates are added to maintain and distribute a fluid in the inner tubes of the concentric double tubes. The two old tubular plates are still used for the maintenance and distribution of the fluid passing through the annular passage formed by the two concentric tubes. So instead of the exchanger working with two fluids (a hot fluid and a cold fluid), now the heat exchanger works with three fluids (2 hot fluids and a cold CFC fluid or the reverse FCF). The fluids can be of the same nature or not.

The documents and studies that allowed us to determine this invention are summarized in the various studies on coaxial tube heat exchangers, crossed (single, co-current and countercurrent.) In the end, this allowed us to The invention is the recent work on heat exchangers with triple concentric tubes.

Purpose of the Invention

The present invention aims to intensify the heat exchange and increase compactness by increasing the exchange surface volume occupied by these devices. That is, the decrease in the cost and weight of this type of heat exchanger. For the same operating requirement, these new heat exchangers are smaller (tube length and volume reduced). This is made possible by the addition of a second heat exchange surface and a third fluid, without increasing the volume of the heat exchanger or the geometrical shape on the side of the shell (outer casings). Apart from the addition of a distributor and a collector and an exchange surface for the third fluid, the accessories: pumps (or fans), tube plates, calender, baffles, etc., remain unchanged. The second heat exchange surface is a tube of nominal diameter less than the diameter of the tube of the first exchange surface. Both tubes are mounted concentrically.

Statement of Figures

Our invention is illustrated and explained by way of indication and not limitation, by appended technical drawings, several sections of the exchanger concentric double tubes and calender, and crossed heat exchanger, are represented:

Fig. 1 is a perspective view of the double concentric tube and shell heat exchanger;

Fig. 2 is a perspective longitudinal section of this heat exchanger;

Fig. 3 illustrates the longitudinal section of this heat exchanger in the direction B-B;

Fig. 4 relates to the perspective of the cross heat exchanger with concentric double tubes;

Fig. 5a and 5b details the distribution of the fluids in the double concentric tubes and the collectors (or the distributors) of the crossed heat exchanger;

Fig. 6a and b represent the collectors (or distributors) of concentric form.

Presentation of the essence of the invention

In the concentric double-tube heat exchanger and calender shown in FIG. 1, we see the grille, the 3 distributors, the 3 collectors, the distribution boxes and the outside of the four tubular plates. The two fluids of the same temperature level enter through the first and the third distributor and exit through the third and the first collector respectively. The fluid of different temperature level with the other two fluids enters and leaves always by the collector (distributor) adjoining. The first fluid (same temperature level or the same type as the third fluid) enters through the first distributor and passes through the first tube plate and out through the fourth tube plate and the last collector as shown in Figs. 2 and 3. While the second fluid enters the heat exchanger through the second distributor and passes through the annular passages formed by the inner tubes and the second tube plate and leaves the exchanger through the third tube plate and the penultimate collector (see Figures 2 and 3). The third fluid enters the exchanger through the third distributor and passes through the exchanger on the outer side of the double envelopes (shell side) and leaves the heat exchanger by the first manifold in the same manner as the conventional tube and shell heat exchangers (Figures 2 and 3). These exchangers concentric double tubes and calender are constructed differently, depending on the state of the fluids present. The tubes can be finned tubes and corrugated. Generally, the fluid flowing on the side of the shell can circulate to multipass due to the presence of baffles. This allows to irrigate better all the tubes. Baffles are perforated plates of different shapes: segments, disks, circular orifices, circular trunks, etc. The tubes can be arranged in the beam in a staggered or aligned arrangement.

It is illustrated in FIG. 4, the case where the circulation of the fluid outside the outer casing (calender side) is perpendicular to the double jacket (double concentric tube), which is the case of crossed heat exchangers (gas-liquid or gas -gas). The circulation of the fluids will have to be cross-type against the current because; thermally, it is the most efficient for this type of heat exchanger. Thus, the manifold and the distributor of the fluids passing in the double envelopes are adjacent. The two fluids of the same nature or the same level of temperature enter the same side of the heat exchanger. The fluid different from the other two fluids flowing in the annular passage of the double jacket generally passes in the opposite direction of the global circulation of the other two fluids. The external collector (or external distributor) corresponds to the fluid circulating inside the inner tubes of the double envelopes. The other two (inner) correspond to the fluid circulating in the annular passages formed by the concentric tubes, see fig. 5a and 5b.

Embodiment of the invention

The embodiment of this invention is similar to that of cross or tube and shell heat exchangers and straight or helical concentric dual tube heat exchangers. The only novelty lies in the mounting of double concentric tubes on two tubesheets for each end of the heat exchanger. The first plate has holes of diameter equal to the inside diameter of the inner tube of the jacket while the second tube plate has holes of diameter equivalent to the inner diameter of the double concentric tube casing. The first plate and the second plate (or third and fourth plate) are separated by a cylindrical trunk on which is welded the collector (or distributor) of the second fluid.

As regards the cross-heat exchanger with concentric double tubes, its embodiment is identical to cross-heat exchangers against the current. The only difference is the presence of another collector and a second distributor on both sides of the heat exchanger.

The collectors (or distributors) have different diameter holes. The first has holes of diameter equal to the inner diameter of the inner tube of concentric double tube, the holes the other collector have a diameter equivalent to the inside diameter of the double concentric tube casing, see fig. 4, 5a and 5b. In the case where the collectors (distributors) are concentric, the central manifold (distributor) is connected to the inner tubes and the external collector (distributor) is connected to the envelopes, see fig. βa and βb.

The invention will now be illustrated in more detail by the following example of comparison between the concentric double-shell and shell heat exchanger and the tube and shell heat exchanger. This example is not intended to limit the invention in its frame and spirit.

Example

As an example of calculation, we take again the example of the dimensioning of a heat exchanger with tubes and calender, given in the Techniques of the engineer, treated Energy Engineering [27]. This heat exchanger serves to cool a flow rate Qi = 15 m ³ / h of dodecane of T _e i = 120 ^° C. at T _sl = 60 ^° C. with industrial water circulating in the tubes whose temperature varies from T _e2 = 20 ⁰ C to T ₃₂ = 30 ⁰ C. The physical properties of dodecane at an average temperature of 90 ^° C. are as follows: P ₁ = 750 kg / m ³ , c _pl ≈ 2260 J / (kg K), X ₁ ≈ 0.151 W / (K m) and μi = 7.5 x 10 ^-4 Pa s.

The physical properties of industrial water at an average temperature of 25 ⁰ C are as follows: p ₂ = 1000 kg / m ^3, c _p2 = 4180 J / (kg.K), X ₁ = 0.607 W / (m. K) and μx = 8.9 x 10 ^"4 Pa. s.

The exchanger consists of a bundle of N _t = 66 mild steel tubes of thermal conductivity λ _p = 50 W / (m K), inner / outer diameters (D ₂ ZD ₁ ) of 20/24 mm, in normal triangular pitch p = 30 mm. The heat exchanger has two tube side passes. The calender has a diameter D _c = 337 mm and has baffles of thickness δ = 5 mm spaced a distance b = 100 mm. The free section left by the baffles is 25%.

In order to determine the length of the tubes to be installed, the following steps must be calculated:

The exchanged heat power is: Φ = IU ₁ Cpi (T _e i - T _s i) = • 423750 W

The flow rate m ₂ of industrial water is:

36.5 m _* ³ / h

The ratio of the heat capacities of the two fluids and the efficiency of the exchanger are respectively:

According to Kern [1], the corrective coefficient of the logarithmic temperature difference corresponding to the values of Z and η calculated is F = 0.97.

The heat exchange coefficient h ₂ of the industrial water in the tubes is:

_{V2 =} - m - _{= 0j978} ^

P ₂ NtpS _P 2

with N _tp = N _t / 2 = 33 tubes per pass

Sp ₂ - section of internal passage of the tubes The calculation of the Reynolds number and the number of Prandtl gives

_Re2 _{= 21974} ₌ PH2D2 _{and pr2} μ2 ₌ £ p2 ₌ μ ₂ _6jl3 λ2 using the Colburn correlation, the Nusselt number is:

Nu ₂ = 0.023 Re§> ⁸ Pr ^{^1/3} = 125.2

The heat exchange coefficient is:

h _{2 =} ^λzNu _{2 =} 3801 W / (m ² .K) D ²

On the side of the shell, the cross section between baffles and the speed of the dodecane are respectively: Spi = (p - di) (b - δ) = 5.7 x 10 ^-4 m ² and

_V1 = _ * L_ = _ *! * _ = 0.65 m / s PlNcSpI PiDcSpi

The calculation of the Reynolds number, the Prandtl number and the Nusselt number gives:

Nui = 0.36 Re? ⁵⁵ Pr ^{/ 3} = 163.3

The heat exchange coefficient on the shell side is:

₌ λiNui _{= 102? w / (m} 2 _K) Di

Taking into account the conductive resistance of the wall, the overall heat exchange coefficient is:

K = = 900 W / (m ² .K)

DiIn 2λ _p D2 h2

The average logarithmic temperature difference and the heat exchange surface of the exchanger are respectively:

Hence the length of the tubes in the grille is:

L = = 1.90m π D2N _t Now, we will calculate the length of the tubes to be installed in the case of a heat exchanger with double concentric tubes and calender. The data of the tube and shell heat exchanger above are kept and each tube of inner / outer diameters (D ₂ / Dχ) of 20/24 mm is placed concentrically in a tube of inside / outside diameters (d _2). / dχ) of 8/4 mm mild steel.

In this example, the dodecane flow circulating on the shell side and inside the inner tubes of inner / outer diameters (da / di) are equal to half the total flow rate of cooled dodecane in the tube and shell heat exchanger above.

The cooling water passes countercurrently in the annular passages formed by the envelope of diameters (D ₂ / Di) and the inner tubes.

The heat exchange coefficient h ₃ of dodecane in the inner tubes is:

_{V3 =} ≡a = 5.024m / s

PlNtpS _p 3 with S _P3 = inner passage section of the inner tubes m ₃ = mi flow of dodecane circulating inside the inner tubes

The calculation of Reynolds number and Prandtl number gives:

using the Colburn correlation, the Nusselt number is:

Nu ₃ = 0.023 R _e > ⁸ Pr ^ ³ = 142.6

The heat exchange coefficient is: _h3 = MW ₌ 5385 W / (m ² .K)

The heat exchange coefficient h ₂ of industrial water in the annular passages is:

Y ₂ = - = 1,164 m / sp ₂ NtpSp2

The passage section S _P2 is determined by:

The calculation of Reynolds number and Prandtl number gives:

Re ₂ ^{= P> 2V 2 h} = 15696 and d _h = D ₂ - di = 0, 012 m ₂

Using the Colburn correlation, the Nusselt number is:

Nu ₂ = 0.023ReJ ⁸ Pr ₂ ⁷³ = 95.7

The heat exchange coefficient is:

h ₂ = ^λ2Nu2 = 4840 W / (m ² .K) dh

On the side of the grille, the speed of the dodecane is: mi mip _n .

VJ = ⁱ - = = 0.325 m / s

P _l NcSpi PiDcSpi

The calculation of the Reynolds number and the Nusselt number gives:

_Rei = PLII ^ ^l ₌ 7 and ₈ o9 Nui = O, 36R _ei ^o. ⁵⁵ p _r ; ^{/ 3} = 111.5

hl

The heat exchange coefficient on the shell side is:

h _{l =} A ^l N.il ₌ 702 W / (m ² .K) Di

Taking into account the conductive resistance of the wall, the overall heat exchange coefficient Ki, ₂ (shell-side dodecane and cooling water) and the overall heat exchange coefficient K ₂ , ₃ (cooling water and dodecane inner tube side) are calculated respectively by:

In this case the heat flow is:

φ ⁼ [Ki, ₂ S _{1 2} F + K ₂ , ₃ S ₂ JΔTML

^with Si, ₂ ⁼ Nt ^{π D} 2L

S ₂ , 3 ⁼ Nt ^{π d} 2L

Hence the length of the double concentric tubes in the shell is:

Φ

L = r 1 = 1,27m NtlKi π ₂ D ₂ + K ₂ F, 3d ₂ jΔTML

For this example, it is deduced that the length (or volume) of the tube and shell exchanger is approximately 50% greater than the length (or volume) of the concentric double-shell and shell heat exchanger.

In the case where the conductive resistance is very small (copper tubes), the difference in volume between the two exchangers is approximately 53% (L = 1.21 m).

The most interesting result is that one can cool the double the dodecane flow rate, ie 30 m ³ / h with a length (or volume) of the heat exchanger with double concentric tubes and calender smaller by 23% compared to at the length of the tube and shell heat exchanger operating with the old dodecane flow rate of 15 m ³ / h.

In any case the length of the exchanger with concentric double tubes and calender is further reduced, by increasing the dodecane flow. This length is 62% smaller in the case where the two exchangers operate with a dodecane flow rate equal to 30 m ³ / h.

Claims

1. Heat exchanger with concentric double tubes and calender, characterized by the heat transfer between three fluids and its compactness.

2. Thermal apparatus according to claim 1, can be extended to heat exchangers; crossed, crossed in co-current and crossed against the current.

3. Heat exchanger according to claim 1, can be mounted with concentric tubes in form; helical, serpentine or spiral in a shell.

4. Heat exchanger according to claim 1 to 3 can be single tubes, corrugated or fin shapes of different.

5. Apparatus according to claims 1 to 4, can be used in all areas of applications of heat exchangers. It can operate in single phase (heater or cooler) or with phase change of fluids, such as condensers and evaporators