WO2012114134A1

WO2012114134A1 - Arrangement for improving the cooling capacity and freeze protection of air-cooled heat exchangers subjected to the impact of wind

Info

Publication number: WO2012114134A1
Application number: PCT/HU2011/000017
Authority: WO
Inventors: Gábor Csaba
Original assignee: Gea Egi Energiagazdalkodasi Zrt
Priority date: 2011-02-24
Filing date: 2011-02-24
Publication date: 2012-08-30

Abstract

The invention relates to an arrangement for improving the cooling capacity and/or freeze protection of air-cooled heat exchangers subjected to the impact of wind, consisting either of guide elements arranged parallel with the axis of the cooling pipes (11) of the heat exchanger, or of a louvre structure (5) and guide elements (11) mounted on the louvres (9), and further consisting of support columns (2) arranged either independently or expediently such that they also strut the heat exchangers (5), where the support columns are adapted to fluid-dynamically cover the shortest outer extremities of the heat exchangers (9). The arrangement according to the invention is characterised by that the guide elements (9) are arranged towards the outer, upwind side of the heat exchanger (2) to divide the airstream flowing in between the support columns (10) to multiple but at least two portions, and the guide elements (9) have a geometric configuration adapted to reduce the parallel component of the air velocity vector of the air flowing beside the arrangement, where said parallel component is parallel with the plane of the support columns (10) arranged along the outer circumference of the heat exchangers (2), and further adapted to guide the flowing air towards a direction perpendicular to the plane of the support columns (10).

Description

Arrangement for improving the cooling capacity and freeze protection of air-cooled heat exchangers subjected to the impact of wind

Technical field of the invention

The invention relates to an arrangement for improving the cooling capacity and freeze protection of air-cooled heat exchangers subjected to the impact of wind.

Description of prior art

It is widely known that so-called dry cooling systems are often applied to cool power plant condensers or other industrial appliances. These dry cooling systems have the common characteristics that heat is transferred from the medium to be cooled (in most cases, water) to the cooling air applying closed surface heat exchangers separating the cooling air from the medium to be cooled. In case the medium to be cooled is water, the meteorological parameters of air flowing to the heat exchangers, that is, temperature and wind speed, affect the operation of the heat exchanger to a great extent. High wind speeds often cause undesirable drops in cooling capacity, while cold air having a temperature below 0°C may lead to freezing problems in the heat exchanger.

Especially serious problems may arise when these two adverse factors, strong wind and freezing temperatures occur at the same time. Natural-draft cooling towers may especially be put in danger by such meteorological conditions, as in their case either decreased heat loads, or low ambient air temperature may lead to overcooling of the water to be cooled and eventually to cooling pipe freeze. Under such circumstances the heat exchangers should be protected against freeze, which is achieved by applying means suitable for controlling the air flow, such as louvres.

Figs. 1a-c illustrate the above case, where cooling air flows through the air-water heat exchangers 2 of a natural draft cooling tower 1 in the direction of the arrows 3. Water entering the heat exchanger 2 through inlet 4 cools down and the cooled water leaves the heat exchanger 2 through outlet 6. The flow of air is controlled by a louvre 5. According to the solution illustrated in Fig. 1 the heat exchangers are disposed vertically along the outer circumference of the base of the tower 1 building. According to another known solution the heat exchangers are arranged horizontally, and are disposed inside the base portion of the tower, in a horizontal position above the legs. The louvres are arranged in a horizontal plane similarly to the previous arrangement. In order to allow the installation of as many heat exchangers as possible, the heat exchangers and the louvres are disposed in a so-called delta arrangement. Fig. 1c shows the magnified top plan view of such a delta arrangement. This consists of two heat exchangers that are termed heat exchanger columns 2, a support structure 10 and a louvre structure 5. The heat exchanger columns 2 are arranged at a specific angle with respect to each other, with the louvre 5 being disposed in the open space between the two columns. The louvres 5 are implemented as plates or profiles adapted to rotate about their axis like a flap to allow the air flow to be controlled between 0 and a maximum value corresponding to the completely closed and completely open positions. The adverse effect of wind, caused by the known flow pattern around a stationary cylinder, manifests itself such that at the heat exchangers located on the upwind side air pressure increases at the stagnation point, resulting in the overloading of these heat exchangers. At the same time, wind velocity increases, and air pressure drops, in the vicinity of heat exchangers located sideways and oriented at an angle with respect to the direction of wind, which causes decreased air flow there. In case of the former heat exchangers the disadvantageous effect of increased air flow that may cause the cooling water to freeze in winter periods can be alleviated by rotating the louvres toward the closed position. In the vicinity of the sideways located heat exchangers the pressure of air may drop by such an amount that the draft of the tower becomes insufficient for sustaining the flow of air through them, and consequently these heat exchangers cease to perform their cooling function. This cannot be mitigated by operating the louvres, because in this situation the louvres have very limited effect on the air flow. In case there is no pressure difference that could sustain the air flow, choking is also inefficient. The result of these two adverse conditions is that, since the tower has to remove a predetermined amount of heat from the medium to be cooled, the upwind heat exchangers get overloaded, and due to the decreased active heat exchanger surface the efficiency of the cooling tower decreases and water temperatures rise. In summer this rise of the temperature of water may, in case of a power plant cooling system, reduce the overall efficiency of the power plant and often leads to limiting power output. During winter periods, asymmetric operation may lead to pipe freeze. This may occur in the upwind located heat exchangers that, as it was explained above, are overloaded, but heat exchangers located sideways with respect to the direction of wind may also be in danger because totally a very limited amount of air flows through them.

The latter phenomenon may be explained as follows. On the water side the cooling tower usually consists of several parallelly connected heat exchanger groups. These groups, also termed as sectors or sections, comprising louvre arrays operable independently of one another, can be separated from the cooling water circulation system also on the water side, and can even be drained in case it is necessary. When the system is operating under full thermal load, all sectors are filled up and operating. The temperature and cooling capacity of the entire cooling system is controlled utilising the louvres. In case the temperature of cooling water drops below the setpoint, the louvres are rotated toward the closed position, and in the opposite direction in case the temperature rises above the setpoint. Because the system is divided into sections, and also because individual sections operate with different capacity in wind, control is also divided into sectors. The measured parameter is the water temperature at the outlet of each sector, and actuation is performed by the louvre drives at each section. Pipe freeze may occur in the sideways located heat exchangers in case the wind (and/or the depression occurring beside the tower) is so strong that the pressure difference between the inner and outer sides of the tower becomes too low, resulting in that the given sector cannot perform its cooling function. Under such circumstances the relationship between the outlet temperature and louvre position is not straightforward. In fact, it becomes exactly the opposite of what would follow from the control logic. The temperature of water at the outlet rises, and therefore the controller opens the louvres, which in this case does not result in a drop of the cooled water temperature. Air load of heat exchangers located at the outer corners of heat exchanger deltas 8 blown through the open louvres by the wind accelerating at the sides of the tower rises by such an amount that water freezes in the pipes. At the same time a vortex is formed beside the support column 10 located at the outer side of the other heat exchanger of the same delta, causing significant air pressure drop at this particular heat exchanger. Thereby the air flow through this heat exchanger also decreases almost to zero, and air may eventually flow backwards. The flow pattern is shown in dashed lines in the right hand portion of Fig. 1. The above described phenomenon may be eliminated with the application of different solutions, such as for instance by measuring the temperature of the corrugated tubes of the heat exchangers located at the most critical positions and closing the louvres if necessary, but as it involves the utilization of as many as a few hundred thermometers, long cables , and complex automation equipment, it is way too complex and costly. Closing the louvres in such a situation would result in eliminating some of those heat exchangers or sectors from the system that would still retain their cooling capacity. This would lead to overloading of the other heat exchangers and thereby increasing the danger of freeze damage. Besides, as it has been already mentioned, a rise of the temperature of the cooling water will result in a deterioration of efficiency.

Objective of the invention

The objective of the present invention is to eliminate the above mentioned adverse or disadvantageous effects. To provide such a solution it is worth contemplating the overall effect of wind on the cooling capacity of the cooling tower. First, it should be considered that in case a flat heat exchanger column consisting of corrugated tube arrays and utilized for instance for cooling water is subjected to wind, the cooling capacity of the heat exchanger will increase in straight proportion to the velocity of the wind blowing in a direction perpendicular to the heat exchanger. Now the question arises why this is not true for either a natural-draft or a forced draft cooling tower? The phenomenon can be traced back to the velocity and pressure distribution of air flowing around the building. Another circumstance, namely that wind causes depression at the top portion of an approximately cylindrical natural draft cooling tower, which depression should also increase cooling capacity, has not yet been mentioned. The explanation of the fact that cooling capacity decreases with increasing wind speed in spite of these circumstances to the contrary is that the negative effect of the pressure distribution generated by air flowing around the building at the level of the heat exchangers on cooling capacity surpasses the combined effects that contribute to rising it. Now, if we consider what was said above about the behaviour of a free standing flat heat exchanger subjected to wind, namely that wind blowing perpendicularly to the heat exchanger increases cooling capacity, we should provide a guiding arrangement that can partially transform the dynamic pressure of high-velocity air flowing around the tower building to static pressure, and can guide the air into heat exchanger deltas located sideways as seen from the direction of wind. The arrangement should also be capable of evenly distributing the air mass flow deflected in such a manner over the heat exchangers, instead of focusing it on the endangered outer corners, and of doing that without significantly raising air resistance. In order to reduce the danger of freezing damage the arrangement should provide that the component of air velocity that is parallel with the plane of the neighbouring two outer support columns 10 at the outer corners of the heat exchangers is reduced to the greatest possible extent. Another important consideration is that the creation of a continuous wind channel along the circumference of the tower 1 between the guide elements 9 and the outer support columns 10 should be prevented, in which wind channel horizontal air flows could build up in the plane of the outer support columns 10, increasing the danger of pipe freeze rather than reducing it. To achieve that, the guide elements 9 should expediently be placed directly on the triangle defined by the inner and outer support columns 10. The above objectives may be realised by differently configured guide elements 9.

The objective of the invention is achieved by providing an arrangement that consists either of guide elements arranged parallel with the axis of the pipes of the heat exchanger, or of a louvre structure and guide elements mounted on the louvres, and further consists of support columns arranged either independently or preferably such that they also strut the heat exchangers, where the support columns are adapted to fluid-dynamically cover the shortest outer extremities of the heat exchangers, and is characterised by that the guide elements are arranged towards the outer, upwind side of the heat exchanger to divide the airstream flowing in between the support columns to multiple but at least two portions, and the guide elements have a geometric configuration adapted to reduce the parallel component of the air velocity vector of the air flowing beside the arrangement, where said parallel component is parallel with the plane of the support columns arranged along the outer circumference of the heat exchangers, and further adapted to guide the flowing air towards a direction perpendicular to the plane of the support columns.

In a preferred embodiment of the arrangement according to the invention the guide elements are mounted on the louvre blades, with the guide elements having a longitudinally interrupted configuration to enable the rotatability of the louvre blades, and having a curved plane geometrical shape adapted to provide rotatability.

In another preferred embodiment of the arrangement according to the invention the louvre blades are divided into multiple portions along planes perpendicular to their axis, the louvre blade portions being disposed on a common shaft between neighbouring guide elements and having a width corresponding to the spacing between the guide elements, where the louvre blades are fixedly mounted on the louvre shafts that are rotatably received in through holes disposed on the guide elements.

In a further preferred embodiment of the arrangement according to the invention the louvre structure is disposed on the inner side of the heat exchangers, while the guide elements are arranged on the outer side thereof.

In another preferred embodiment of the arrangement according to the invention the guide elements are arranged to be rotatable about their shafts to provide that the guide direction of air is changeable, and also in a manner that they are able to shut the entire air flow cross section, the guide elements having a depth equal to or larger than their shaft sectioning distance.

Brief description of the drawings Figs. 1a, 1 b show the side elevation and top plan view of a known arrangement,

Fig. 1c shows a magnified view of the delta arrangement of the louvre of a known arrangement,

Fig. 2 is the top plan view of the heat exchanger delta of the arrangement according to the invention,

Fig. 3 shows an embodiment of the arrangement according to the invention having the guide elements mounted on the louvre blades,

Fig. 4 is a top plan view of the heat exchanger deltas of the arrangement according to the invention comprising a louvre for controlling air flow,

Fig. 5 shows the top plan view of another embodiment of the heat exchanger delta of the arrangement according to the invention,

and Fig. 6 shows a further embodiment of the arrangement shown in Fig. 3.

Fig. 2 shows the top plan view of a heat exchanger delta arrangement 8 in which guide elements 9 are disposed in front of the louvre 5 array, where the guide elements 9 are arranged parallel with one another and with the cooling pipes 11 of the heat exchanger 2. Alternatively to utilizing flat plates, the guide elements 9 may also have a wing-profile or curved geometrical configuration. The latter shapes have the advantage over flat plates that they can direct air into the interior of the heat exchanger delta 8 with lower profile depth and decreased air resistance. Guide elements configured in such a manner may also provide that cool air coming from the left in the figure can blow the pipes of the heat exchanger evenly along the entire length of the louvre 5 instead of primarily cooling the section next to the inner side of the support column 10. Thereby the air load of the cooling pipes 11 located next to the support column 10 and being subjected to a danger of frost decreases, while other cooling pipes 11 become under higher air load, resulting in an equalized cooling performance over the surface of the heat exchanger and decreased danger of pipe freeze, while the overall cooling capacity of the cooling tower increases. Thus, a single technical modification has had two different advantageous effects. While in windy periods of the summer the tower has increased cooling capacity, in periods posing a danger of frost the chance of pipe freeze has diminished. It is important that the axes of rotation of the louvre blades 5 (that in the embodiment shown in the figure are parallel with the plane of the drawing) should be set perpendicular to the axes of the pipes of the heat exchanger, and that the plane in which the longitudinal axes of the guide elements 9 lie should be parallel with the longitudinal axes of the cooling pipes 11 , independent of the orientation of the plane of the heat exchanger itself.

Fig. 3 shows an embodiment where the guide elements 9 are mounted on the louvre blades 5. The louvre blades 5 have a wing-profile like configuration, while the guide elements are implemented as flat plates. The louvre blades 5 may of course be made from plate material, and both components may have wing-profile like cross-sectional shape. In this embodiment the guide blades 9 are mounted on each louvre blade 5 in an interrupted configuration such that each guide blade 9 consists of multiple portions. Such a configuration does not bring about any decrease in the air guiding capability of the guide blades 9

According to the embodiment shown in Fig. 4 the louvre 5 is arranged towards the inside portion, proximate to the interior of the cooling tower, of the heat exchanger delta, while the guide elements 9 are disposed outside. In case the guide elements 9 are implemented to be rotatable about their axes set parallel to the cooling pipes 11 of the heat exchangers 2, the internal louvres 5 may be left out. According to such an arrangement the guide elements 9 perform the functions of both controlling and guiding the air flow. To be able to completely stop the air flow, the guide element should be configured such that in the direction of the flow it has a depth providing that after a 90-degree rotation the profiles of neighbouring elements touch each other.

Fig. 5 illustrates a solution that is more expensive but much more effective than the embodiments described above. In this embodiment guide elements 9 that have wing-profile cross sectional shape and are adapted to rotate under the effect of wind are disposed along the outer circumference defined by the heat exchangers, and the support columns 10 have a streamlined profile to provide that high-velocity air flowing in the direction of the arrows 3 is guided with the lowest possible loss to the surface of the louvre 5 and heat exchangers 2. The guide elements may have braking means to prevent the guide elements from suddenly swinging from one terminal position to the other. This solution may provide that the "wind factor" of the cooling tower is positive, that is, that the wind does not decrease but rather increase the cooling capacity of the tower. Alternatively to the arrangement shown in Fig. 5 the guide element 9 may also be disposed on the central line between the two support columns 10. In that case the guide elements 9 may be placed closer to the centre of the tower between the two streamlined support columns 10. Such an arrangement may further decrease the component of wind velocity that is parallel to the plane of the support columns 10.

Fig. 6 shows an embodiment where, similar to the embodiment illustrated in Fig. 3, the guide elements and louvre blades 5 are disposed in the same plane. However, in this embodiment the louvre blades 5 are divided into multiple portions, with the portions being disposed alternately with guide elements 9 along the louvre shafts. The louvre 5 portions are disposed on the louvre shaft in such a manner that they cannot be rotated about it. In addition, the guide elements 9 comprise bores adapted for receiving the louvre shafts and guiding the shafts to allow their free rotation. Thereby the rotatability of the entire louvre blade 5 through rotating the shafts is provided. The opened position of the louvre is shown in continuous lines in the drawing to the left, while the closed position is shown in dashed lines.

LIST OF REFERENCE NUMERALS

1 cooling tower

2 heat exchanger

3 opening

4 inlet

5 louvre, louvre blade

6 outlet

8 heat exchanger delta arrangement

9 guide element

10 support column

11 cooling pipes

12 shaft

Claims

1. Arrangement for improving the cooling capacity and/or freeze protection of air-cooled heat exchangers subjected to the impact of wind, consisting either of guide elements arranged parallel with the axis of the cooling pipes (11) of the heat exchanger, or of a louvre structure (5) and guide elements

(9) mounted on the louvres (5), and further consisting of support columns

(10) arranged either independently or expediently such that they also strut the heat exchangers (2), where the support columns are adapted to fluid- dynamically cover the shortest outer extremities of the heat exchangers (2), characterised by that the guide elements (9) are arranged towards the outer, upwind side of the heat exchanger (2) to divide the airstream flowing in between the support columns (10) to multiple but at least two portions, and the guide elements (9) have a geometric configuration adapted to reduce the parallel component of the air velocity vector of the air flowing beside the arrangement, where said parallel component is parallel with the plane of the support columns (10) arranged along the outer circumference of the heat exchangers (2), and further adapted to guide the flowing air towards a direction perpendicular to the plane of the support columns (10).

2. The arrangement according to Claim 1 , characterised by that the guide elements (9) are mounted on the louvre blades (5), with the guide elements (9) having a longitudinally interrupted configuration to enable the rotatability of the louvre blades (5), and having a curved plane geometrical shape adapted to provide rotatability.

3. The arrangement according to Claim 1 , characterised by that the louvre blades (5) are divided into multiple portions along planes perpendicular to their axis, the louvre blade (5) portions being disposed on a common shaft between neighbouring guide elements (9) and having a width corresponding to the spacing between the guide elements (9), where the louvre blades (5) are fixedly mounted on the louvre shafts that are rotatably received in through holes disposed on the guide elements (9).

4. The arrangement according to Claim 1 , characterised by that the louvre structure (5) is disposed on the inner side of the heat exchangers (2), and the guide elements (9) are disposed on the outer side of the heat exchanger (2).

5. The arrangement according to Claim 1 , characterised by that the guide elements (9) are arranged rotatably about their shafts (12) such that the guide direction of air is changeable.

6. The arrangement according to Claim 5, characterised by that the guide elements (9) are arranged such that they are able to shut the entire air flow cross section, the guide elements (9) having a depth equal to or larger than their shaft sectioning distance.