EP0687878A1 - Evaporative cooling tower - Google Patents

Abstract

The cooling tower has an OD (DA) of the tubes forming the tube coil of less than 25 mm. The straight line tube sections (3a,3b) of the immediately adjacent tubes (2a,2b) have a mean vertical tube distribution (S2) and are formed from tube centre to tube centre with between one and two times the OD. The vertical distance from tube centre to tube centre of the tube sections (3a,3b) corresponds to double the vertical tube distribution (S2). <IMAGE>

Description

The invention relates to an evaporative cooling tower for cooling or condensing fluids by means of cooling air with at least one additional coil coil heat exchanger, the coil of which consists of straight pipe sections and pipe sections connecting these pipe sections, with its start to a distributor and its end to a collector are connected that the straight, substantially horizontal pipe sections lying in the same plane of adjacent pipe coils are arranged with a division between one and three times their outer diameter, with straight pipe sections lying directly next to one another being offset from one another in the flow direction of the water acting on the pipe coils.

Evaporative cooling towers of this type are known, for example, from EP 0 007 829 B1 and EP 0 272 766 B1. In these evaporative cooling towers used for cooling or condensing fluids by means of cooling air, a coil coil heat exchanger replaces the cooling or trickle internals known from other designs in order to obtain a closed circuit for the fluid to be cooled or condensed. Such a type of cooling tower is also referred to as a fluid cooling tower, an evaporation condenser or a closed evaporation cooling tower.

In such an evaporative cooling tower, the respective fluid is cooled or condensed by means of air. In addition, the outer surface of the coil coil is sprayed or wetted with water to improve the heat transfer performance by using evaporative cooling to increase this heat exchanger significantly. Accordingly, three material flows are thermodynamically in contact in such an evaporation cooling tower, which, depending on the embodiment, are carried out in cocurrent and / or countercurrent and / or crossflow.

A decisive target variable in the design of such evaporative cooling towers is the lowest possible outlet temperature of the fluid, which, however, is physically limited by the temperature of the coolant used, that is to say the air. If the evaporative cooling system is also used, this limit value is determined by the moist air temperature, which is significantly lower than the dry air temperature. In particular at higher air temperatures in summer, the fluid outlet temperature can be reduced with relatively simple technical means, in that water is additionally injected into the cooling air flow. The finely divided drops of water evaporate for the most part and lead to a cooling of the air and thus to an improved cooling of the fluid in the heat exchanger.

In another embodiment, a larger amount of water is distributed as evenly as possible above the heat exchanger by means of spray nozzles or channels. The water trickles down through the tubes of the heat exchanger and is collected in the cooling tower trough. This spray or secondary water is then pumped upwards again for water distribution. A water film forms on the surface of the heat exchanger tubes and water droplets and tresses in the gaps, from whose surfaces a small amount of water evaporates into the cooling air flow. The loss of evaporation must be constantly supplemented with make-up water. The cooling air flowing into the evaporative cooling tower undergoes a change in temperature, is enriched with water vapor and finally discharged back into the environment. It extracts heat from the water film on the heat exchanger tubes, so that the fluid flowing inside the tubes is cooled or condensed. Evaporative cooling of this type in an evaporative cooling tower is therefore a combined heat and mass transfer process which, because of its very complex nature, permits reliable performance determination only after extensive investigations have been carried out.

The sprinkled heat exchanger preferably consists of smooth, non-finned tubes that are repellent to external pollution and have a high area-specific transmission capacity. The pipes are inexpensively processed into snakes and connected to distribution and collecting lines for the internal fluid. The directly adjacent, straight pipe sections of the coils are arranged in a thermodynamically advantageous manner vertically offset from one another, with larger ones alternating with smaller distances. These are called vertical divisions.

In order to achieve a higher surface-specific heat transfer rate in such evaporative cooling towers, adjacent coils of the evaporative cooling tower known from EP 0 007 829 B1 maintain a certain distance from one another in the horizontal direction. The pipe coils that do not touch each other have a preferred pipe geometry with an outer pipe diameter of 26.7 mm and an inner bending radius of 53.2 mm. This results in an average vertical division from the center of the pipe to the center of the pipe of the neighboring pipes of 66.5 mm (inner bending radius plus half the outer pipe diameter). This corresponds to approximately 2.5 times the outer tube diameter.

According to the prior art, tubes with a relatively large outside diameter of approximately 25 to 30 mm are preferably selected for the tube coil heat exchangers described. The manufacturing outlay can thereby be reduced since, in comparison with smaller pipe diameters with the same heat exchanger surface, a smaller number of pipes and coils is required per cooling tower cross section.

The tube coil heat exchanger described in the evaporative cooling tower of EP 0 272 766 B1, on the other hand, uses elliptical tubes for the straight tube sections, which likewise have a preferred spacing from one another in the horizontal direction.

From the prior art it can be seen that a high area-specific heat transfer rate of the heat exchanger in the form of a coil is achieved when the adjacent coils lying next to one another have a certain, but not too great distance from each other, regardless of which pipe shape or pipe diameter is used. Furthermore, the largest possible vertical division of the tubes is selected in the known prior art. On the one hand, this is done from a manufacturing point of view, since bending radii that are too small lead to buckling or other undesirable deformations of the pipe bends. On the other hand, the water drops between the pipes are involved to a not inconsiderable extent in the heat and mass transfer process already described and thus in the performance of the heat exchanger, it being obvious that larger spaces for the formation of drops are created with larger horizontal and vertical divisions, and thus the entire surface of all drops is enlarged. According to the common wisdom, an increased surface area of the trickle water to the air results in an increased heat transfer capacity of the evaporative cooling tower.

The invention has for its object to increase the area-specific transmission power in an evaporative cooling tower of the type described above or to achieve a reduction in the required surface of the heat exchanger and thus the material used, without having to accept a higher air-side pressure loss and thus an increased fan performance must become.

The solution to this problem by the invention is characterized in that with an outer diameter of the tubes forming the coils of less than 25 mm, the straight tube sections of the tube coils lying directly next to one another with an average, vertical tube division from tube center to tube center between one and two times the outer tube diameter are trained.

According to a further feature of the invention, the vertical distance from pipe center to pipe center of the straight pipe sections belonging to a pipe coil corresponds to twice the vertical pipe division. This results in a uniform pipe arrangement of all pipes in relation to the cross-sectional area perpendicular to the longitudinal direction of the straight pipe sections of the heat exchanger with the result of a compact design and increased performance.

The trickle water dripping at the lowest point of the pipe cross-section has to travel a considerably shorter distance in the pipe coil heat exchanger according to the invention until it hits the pipe directly below it in the same vertical plane. With the vertical division, the number of trickle water drops is reduced considerably, so that the entire heat and mass transfer surface of the trickle water also decreases. Of course, in accordance with the current teaching, reducing the total trickle water surface also results in a reduction in the heat transfer capacity of the evaporative cooling tower.

However, the performance contribution of the drops located between the rows of pipes has been greatly overestimated. A row of pipes combines the straight pipe sections that lie in the same horizontal plane. It has been found that reducing the drops in these tube-free spaces results in only a slight reduction in the overall heat transfer performance. However, this is more than compensated for by the rapidly increasing evaporation flow, which is released from the trickle film surfaces of the individual pipes into the air flowing around them. The pipes, which are closer together in the vertical direction, cause a more even and intensive flow around the pipes, which leads to an increase in the evaporation rate, which is defined as the evaporation flow per transmission surface. In this regard, it has also proven to be advantageous to design the vertical division as evenly as possible. The evaporation rate is increased by the choice of smaller pipe outer diameters in a previously unknown order of magnitude, so that the desired increase in the area-specific transmission capacity of the coil coil heat exchanger is achieved.

Furthermore, the reduced number of drops between the tubes reduces the air-side pressure loss, so that the air throughput is increased can. This leads to a further increase in the total transmission power.

In preferred embodiments of the invention, the vertical pipe division of the pipes corresponds to the simple one of the outer diameter. According to the invention, the tubes can also have an oval cross-sectional shape. Finally, the tubes can be finned.

Fig. 1

eine schematische, teilweise geschnittene Seitenansicht eines mit einem Rohrschlangen-Wärmetauscher ausgestatteten Verdunstungskühlturms;

Fig. 2

einen Schnitt durch den Verdunstungskühlturm gemäß der Schnittlinie II - II in Fig. 1;

Fig. 3

einige mit Luft und Rieselwasser beaufschlagte, gerade Rohrabschnitte gemäß einem teilweisen Querschnitt durch den Rohrschlangen-Wärmetauscher in Fig. 1;

Fig. 4

eine vergrößerte Darstellung des Rohrschlangen-Wärmetauschers gemäß Fig. 2;

Fig. 5

einen modifizierten Rohrschlangen-Wärmetauscher gemäß Fig. 4;

Fig. 6

eine weiter modifizierte Rohrschlangenanordnung des erfindungsgemäßen Rohrschlangen-Wärmetauschers gemäß Fig. 4;

Fig. 7

einen vergrößert dargestellten Teilschnitt durch einige Schlangen des Rohrschlangen-Wärmetauschers gemäß der Schnittlinie VII - VII in Fig. 4 bzw. Fig. 5;

Fig. 8

einen der Fig. 7 entsprechenden Teilschnitt einer Rohrgeometrie nach dem Stand der Technik und

Fig. 9

ein Diagramm mit dem Flächenbedarf des erfindungsgemäßen Rohrschlangen-Wärmetauschers als Funktion des Rohrdurchmessers und der vertikalen Rohrteilung.

An exemplary embodiment of the evaporative cooling tower according to the invention is shown in the drawing, namely:

Fig. 1

is a schematic, partially sectioned side view of an evaporative cooling tower equipped with a coil coil;

Fig. 2

a section through the evaporative cooling tower along the section line II - II in Fig. 1;

Fig. 3

some straight pipe sections to which air and trickle water have been applied according to a partial cross section through the coil heat exchanger in FIG. 1;

Fig. 4

an enlarged view of the coil heat exchanger according to FIG. 2;

Fig. 5

a modified coil heat exchanger according to FIG. 4;

Fig. 6

a further modified pipe coil arrangement of the pipe coil heat exchanger according to the invention according to FIG. 4;

Fig. 7

an enlarged partial section through some coils of the coil heat exchanger according to the section line VII - VII in Fig. 4 and Fig. 5;

Fig. 8

7 corresponding partial section of a tube geometry according to the prior art and

Fig. 9

a diagram with the area requirement of the pipe coil heat exchanger according to the invention as a function of the pipe diameter and the vertical pipe division.

The evaporative cooling tower shown schematically in FIGS. 1 and 2 serves to cool or condense fluids and is equipped with a coil coil heat exchanger 1 for this purpose. This consists essentially of a plurality of coils 2a and 2b, which are arranged close to one another in vertical planes without spacing. The coils 2b are offset in an alternating sequence to the coils 2a in the vertical direction. In the exemplary embodiment, each tube coil 2 a and 2 b is formed in each case by six straight tube sections 3 a and 3 b lying parallel to one another and at the same distance above one another and tube bends 4 a and 4 b connecting them to one another, cf. Fig. 2. This results in twelve rows of pipes arranged vertically one above the other, one row of pipes in each case combining the straight pipe sections 3a and 3b lying next to one another in a horizontal plane.

All coils 2a and 2b are connected to a box-shaped distributor 5 and a collector 6. The fluid to be cooled or to be coded is fed to the distributor 5 through an inlet connection 7 and is distributed through this to the individual pipe coils 2a and 2b. The individual partial flows are then brought together again by the collector 6. An outlet connection 8 is connected to this, through which the cooled or condensed fluid flows.

The cooling of the fluid in the individual coils 2a and 2b of the coil heat exchanger 1 is carried out by air. For this purpose, a motor 9 drives a radial fan 11 via a V-belt 10, which sucks in the cooling air from the surroundings according to the arrows shown in FIGS. 1 and 2. This is pressed through the outlet opening 12 of the radial fan 11 into the distribution space 13 in order to separate the individual coils 2a and 2b from feed below. For this purpose, the evaporative cooling tower has housing walls 14 which close off to the outside.

The coils 2a and 2b of the coil heat exchanger 1 are additionally acted upon with water in order to increase the cooling effect of the air by using evaporative cooling. The water is drawn in from the reservoir of a cooling tower trough 15 with a pump 16 and fed to the nozzle water distribution 18 via a riser 17. This consists of several pipes, not shown, and attached nozzles to distribute the water over the coil heat exchanger 1.

The water is sprayed into a multiplicity of drops which, after striking the outer surfaces of the straight, horizontally running pipe sections 3a and 3b as shown in FIG. 3, form thin downward-flowing trickle films 19. At the lowest point 20 of the respective pipe cross-section, the trickle water flows out in the form of drops and falls through a horizontal plane 21 formed between the pipes, the drops being further divided by the opposing air and distributed in the intermediate spaces between the pipes. The drops then hit the tubes immediately below, where further trickle films 19 are formed. This continues from tube to tube.

The drops from the lowest tubes of the coil heat exchanger 1 fall into the distribution space 13 according to FIGS. 1 and 2 and are finally collected in the cooling tower trough 15. On the way from distribution to collection, water evaporates from the surfaces of the trickle drops and films into the counter-flowing air flow. The air, which is strongly enriched with water vapor, finally flows through a droplet separator 22, which retains entrained water drops, and then leaves the evaporative cooling tower. The evaporation loss that occurs is continuously supplemented according to FIG. 2 by additional water via line 23.

In Fig. 4 the coil coil heat exchanger 1 according to FIGS. 1 and 2 is shown enlarged in order to better recognize essential structural details can. The straight pipe sections 3a and 3b are connected to prefabricated pipe bends 4a and 4b, for example by welding, in order to obtain pipe coils 2a and 2b which have a uniform vertical division s 2 between the straight pipe sections 3a and 3b (see FIG. 7). In the exemplary embodiment, this value corresponds to 1.5 times the outer pipe diameter d _A and is equal to the radius of the pipe bends 4a and 4b up to the pipe center. The coils 2a and 2b are supported several times by horizontal, strip-shaped support elements 24, which in turn are attached to an outer, solid support frame 25, and are positioned in their spatial position.

5 shows a modified pipe coil design of the pipe coil heat exchanger 1 according to FIGS. 1, 2 and 4. Here, the straight pipe sections 3a and 3b are not connected with pipe bends, but with short, straight pipe sections 26a and 26b. For this purpose, the pipe ends are cut at a 45 ° angle. The straight pipe sections 26a and 26b are particularly simple to manufacture and are therefore more economical than the prefabricated pipe bends 4a and 4b according to FIG. 4. They also allow the mean vertical division of the straight pipe sections 3a and 3b to one another to be even more favorable at 1.0 times the value to reduce the outer pipe diameter.

The goal of maximum heat transfer performance of the evaporative cooling tower requires the selection of an outer tube diameter that is less than 25 mm. This can lead to the fluid speed in the pipes becoming too high. A further exemplary embodiment according to FIG. 6 shows an arrangement of pipe coils which are nested in one another in the vertical direction for these cases. The pipe coils, each with four rows of pipes, are connected to the box-shaped distributor 28 and to the similar collector 29. This corresponds to a doubling compared to the previous exemplary embodiments according to FIGS. 2, 4 and 5, so that the internal fluid velocity is halved.

In Fig. 7 are based on an enlargement of the coil coil heat exchanger 1 according to FIGS. 4 and 5, some pipe coil sections with the preferred geometrical relationships shown in cross section. In this embodiment, the outer tube diameter d _{A is} 18 mm; the inner tube diameter d _I 15 mm; the mean horizontal pitch s₁ 37 mm and the mean vertical pitch s₂, which here is equal to the bending radius r _B , 27 mm. The vertical division s₂ is uniform and here corresponds to 1.5 times the outer tube diameter d _A.

This representation in Fig. 7 is compared with Fig. 8 a prior art arrangement having an outer tube diameter d _A of 26 mm and a vertical division s₂ as the average of the two vertical offsets s _2a and s _2b of 65 mm . This corresponds to 2.5 times the outer pipe diameter d _A.

The diagram in FIG. 9 shows the curve profiles for the required surface area of a coil coil heat exchanger 1 according to FIGS. 1 to 6, which is exposed to air and additionally sprinkled with water, as a function of its geometric data. The starting point of the consideration is a geometric arrangement according to FIG. 8 corresponding to the known prior art, which is characterized by an outer tube diameter of 26 mm and a vertical division ratio of 2.5. With this pipe geometry, a defined surface area of the coil heat exchanger of 100% is required to cope with the cooling capacity.

If one follows the upper curve profiles, which represent the performance with the known vertical division ratios of 2.0 to 3.0, a reduction in the heat exchanger surface required can only be ascertained with relatively small pipe diameters. However, the material savings that can be achieved in this way are not sufficient in view of the larger number of pipes required, which mean an increased production outlay.

It can also be clearly seen in this diagram that, with the tube diameters of more than 25 mm previously used, the vertical tube division ratio according to the invention only brings about a marginal reduction in the heat exchanger surface required. A significant effect is only achieved with pipe diameters smaller than 25 mm. The required surface falls with a vertical pitch smaller than 2.0 times the pipe diameter, especially with decreasing pipe diameter. However, the curves are flatter again for pipe diameters smaller than 16 mm. Furthermore, it was found that an even smaller vertical pitch than 1.0 times the outer pipe diameter again leads to a reduction in the performance of the heat exchanger, since the pipes are then moved very close together and there is a steep increase in the air-side pressure loss.

These basic results also apply to finned tubes and tube shapes that deviate from the circular shape, such as the oval tube. In these cases, the hydraulic diameter should be used as the assessment standard.

Reference symbol list:

1

Pipe coil heat exchanger

2a

Pipe coil

2 B

Pipe coil

3a

Pipe section

3b

Pipe section

4a

Elbows

4b

Elbows

5

Distributor

6

Collector

7

Inlet connection

8th

Outlet connection

9

engine

10th

V-belts

11

Centrifugal fan

12th

Outlet opening

13

Distribution room

14

Housing wall

15

Cooling tower trough

16

pump

17th

Riser

18th

Nozzle water distribution

19th

Trickle film

20th

lowest point

21

level

22

Droplet separator

23

management

24th

Support element

25th

Support frame

26a

Pipe piece

26b

Pipe piece

28

Distributor

29

Collector

d _A

outer tube diameter

d _I

inner tube diameter

r _B

Bending radius

s₁

horizontal division

s₂

vertical division

s _2a

vertical offset

s _2b

vertical offset

Claims (5)

Evaporative cooling tower for the cooling or condensation of fluids by means of cooling air with at least one additional coil coil heat exchanger, the coil of which consists of straight pipe sections and pipe sections connecting these pipe sections are connected with their beginning to a distributor and with their end to a collector that the straight, essentially horizontal pipe sections of adjacent pipe coils lying in the same plane are arranged with a division between one and three times their outer diameter, straight pipe sections lying directly next to one another being offset from one another in the flow direction of the water acting on the pipe coils,
characterized,
that with an outer diameter (d _A ) of the coils (2a, 2b) forming tubes smaller than 25 mm, the straight tube sections (3a, 3b) of the directly adjacent coils (2a, 2b) with an average vertical tube pitch (s₂) from the center of the tube Pipe center between one and two times the outer diameter (d _A ) are formed. Evaporative cooling tower according to claim 1, characterized in that the vertical distance from pipe center to pipe center of the straight pipe sections (3a or 3b) belonging to a pipe coil (2a or 2b) corresponds to twice the vertical pipe division (s₂). Evaporative cooling tower according to claim 1 or claim 2, characterized in that the vertical pipe division (s₂) of the pipes corresponds to the simple one of the outer diameter (d _A ). Evaporative cooling tower according to at least one of claims 1 to 3, characterized in that the tubes have an oval cross-sectional shape. Evaporative cooling tower according to at least one of claims 1 to 4, characterized in that the tubes are finned.

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