EP0954735A1

EP0954735A1 - Natural-draught air condenser apparatus and method of operation thereof

Info

Publication number: EP0954735A1
Application number: EP97932956A
Authority: EP
Inventors: György P LFALVI; János GUBA
Original assignee: Energiagazdalkodasi Intezet; Energiagazdalkodasi Reszvenytarsasag
Current assignee: Energiagazdalkodasi Intezet; Energiagazdalkodasi Reszvenytarsasag
Priority date: 1996-07-17
Filing date: 1997-07-17
Publication date: 1999-11-10
Anticipated expiration: 2017-07-17
Also published as: WO1998002701A1; HU9601945D0; DE69709907D1; HUP9601945A1; AU3631497A; HU221152B1; EP0954735B1

Abstract

A natural-draught air condenser comprises air condensers (7, 7A, 11, 11A) arranged in sections (30, 30A) supplied with steam in parallel, each section (30, 30A) having two or more air condenser stages connected in series on the steam side, wherein subsequent stages are of a decreasing steam side cross section or cooling surface, and the air condensers (7, 7A, 11, 11A) are located at the bottom part of a cooling tower (5) in a way natural-draught air flow passes the air condensers in parallel. The air condensers (7, 7A, 11, 11A) of all stages are of such a circuitry that in them the steam and the generated condensate flow downwards in the same direction, and the air condensers (11, 11A) in either the last or in the last two stages are provided with auxiliary fans (12, 12A) to establish an artificial air flow in addition to the natural-draught air flow. The invention further relates to a method of operating a natural-draught air condenser wherein, upon start-up of the air condenser apparatus and in case of a stream flow disorder arising in the air condensers (7, 7A, 11, 11A), an artificial air flow is established in addition to the natural-draught air flow at the air condensers (11, 11A) in either the last or in the last two stages.

Description

Natural-draught air condenser apparatus and method of operation thereof

TECHNICAL FIELD

The invention relates to a natural-draught air condenser apparatus and a method of operation of such apparatus.

BACKGROUND ART

In thermal power stations, the exhaust steam of the turbine is usually condensed by cold cooling water. If providing the water is costly, air-cooled condensers, so-called air condensers are applied, where the steam flows in finned tubes so as to be condensed and the tubes are cooled from the outside by fan- supplied air. It is customary to install the air condensers in roof-like units, where the steam enters the air condensers from the top, while the cooling air is provided by supply fans located below the air condensers. In each air condenser apparatus, many units of identical size are located side by side. For example, the air condenser apparatus of a 200 MW output steam turbine may consist of thirty units.

The air condenser apparatus generally consists of several thousand tubes connected in parallel, in which the flow rate of steam decreases with progressing of the condensation. In principle, it is possible to build an air condenser, the tubes of which are exactly of a length necessary for the condensation of the steam, and in that case only the condensate is to be drained from the tubes.

In reality, however, this is not the case, due to many reasons. One is that the air condensers working under vacuum are not absolutely leak-proof and so in addition to the steam, a low concentration of non-condensing air also flows in the tubes, to be removed by vacuum pump. And, the other is that in the large number of parallel tubes it is practically impossible to ensure a uniform rate, i.e. the flow rate of steam is higher in certain tubes and lower in others. As a result, the end point of condensation changes and it happens that in certain tubes the condensate of the steam is already cooled by the air flowing outside. In a cold environment this could lead to freezing, but it is unfavourable anyway because at such points the air carried along with the steam is trapped and fills up the air condenser increasingly, and the "dead zone" without flow causes a loss in efficiency. An approach decreasing the disadvantages above has been described for example in patent specification EP-B1 0 390 990, where screens regulating the operating length of the cooling surface of fan type air condensers as well as air chambers and shutters enabling regulated recirculation of the cooling air for air condensers are applied.

In case of air condensers another known solution is when the steam is first guided to a primary air condenser stage, where it is only partly condensed and then the remaining steam is condensed in a second air condenser stage where the number of tubes is lower, consequently the steam can be adjusted again to the original flow rate. Such a solution has been described in patent specification DE-A1 3 010 816, where in the first air condenser stage the steam enters the air condensers at the top and then proceeds downwards along with the condensate generated, while in the second air condenser stage the steam enters the air condensers from the bottom, where a suction vacuum pump is linked to the upper collecting chambers, and so the steam, proceeding upwards, is exposed to the condensate flowing downwards, in a counterfiow manner. Such a counterflow air condenser is called dephlegmator in the literature. In this solution, pre-heated air is fed into the air condensers in order to provide protection against the risk of freezing which prevails at low ambient temperatures in spite of the application of dephlegmator stages.

The output of air condenser apparatus is generally regulated by fans with variable speed of rotation which can be operated independently. The output of an air condenser apparatus is basically determined by its first air condenser stage, because its surface is larger than, e.g. three to four times as large as, the surface of the second, dephlegmator stage.

Since natural-draught cooling towers are used widely and successfully in the power station industry, an air condenser apparatus with natural-draught has already been recommended, where without fans only the cooling tower ensures the flow of the cooling air. For regulating the air condensers, instead of fans, adjustable screen elements for example shutters have been built into the cooling tower.

However, this doubtlessly very simple solution does not always ensure steady operation. The first problem arises when starting up the steam turbine and the air condenser apparatus. According to experience, below a heat input of approx. 20%, a steady draught is not created in the cooling tower, and the uncertain air flow can be blocked by a small disturbance, for example a slight wind. Therefore, the condensation of steam may come to a halt any time, and this is not to be allowed for the sake of the steam turbine. Another problem arises during operation. Let us assume that the above mentioned dead zone has already developed - as a result of the local effect of the wind, for example - somewhere in the air condenser, and the increasing size of the air cushion prevents - like a plug - the steam flow. If the wind direction changes, the air cushion is displaced, and in a favourable case it reaches the vacuum pump, but this is not to be taken for granted. The air cushion causes a drop in output, and its displacement leads to a fluctuation of performance. Neither of these factors is favourable from the aspect of the steam turbine. In such a case, by partial closing of the shutters, the output of the cooling tower can be reduced, consequently the steam pressure will increase, and if subsequently the shutters are opened at the dephlegmator of the air condenser surface blocked by the air cushion, the intensive condensation developing here could be capable of "sucking out" the air cushion. In the meantime, however, the cooling tower runs at a decreased capacity, i.e. ultimately what happens is that the shutter control adjusts the air condensers to the worst performing condenser at any one time.

To resolve this problem, in patent specifications DE-A1 3 441 514 and EP- A2 0 553 435, such a cooling tower type air condenser apparatus has been recommended, where there is no fan at the condenser stages of the apparatus, but there are fans with variable output at the dephlegmator stages subsequent on the steam side.

As far as we know, however, no such equipment has been made so far, indicating that under the changing environmental conditions, attempts have not been successful to ensure continuous operation and the regulation of output.

DISCLOSURE OF INVENTION

According to the present invention, fans are also used for natural-draught air condenser apparatus, but only with an auxiliary character. It is also an important difference that no counterflow dephlegmators are applied, because in our experience operational troubles may arise in counterflow dephlegmators when the O 98/02701 ^{rL MU /.}uww

- 4 -

condensate flowing downwards and acting like a plug blocks the steam path. Therefore, it is believed to be important that for eliminating problems of a system we should not use a unit which in itself could be a source of disorders. According to the invention, the air condensers provided with auxiliary fans - moving away from the approaches recommended so far - also have a condenser circuitry, i.e. in these air condensers both the steam and the condensate keep flowing downwards in the same direction.

Thus, the invention is, on the one hand, a natural-draught air condenser apparatus, especially for condensing exhaust steam of a power station turbine, air condensers of which are arranged in sections supplied with steam in parallel, each section having two or more air condenser stages connected in series on the steam side, wherein subsequent stages are of a decreasing steam side cross section or cooling surface, and the air condensers are located at bottom part of a cooling tower in a way that as a result of the cooling tower natural-draught air flow passes the air condensers in parallel. According to the invention, the air condensers of all stages are of such a circuitry that in them the steam and the generated condensate flow downwards in the same direction, and the air condensers in the last stage or in the last two stages are provided with auxiliary fans to establish an artificial air flow in addition to the natural-draught air flow.

The advantage of the air condenser apparatus according to the invention is that due to the usage of air condensers of purely condenser type of operation, the cooling performance is higher in normal operation than that of the known solution having the same design but containing partly condenser type and partly dephlegmator type air condensers. A further benefit is the higher operational reliability stemming from the fact that we do not apply dephlegmators. Applying fans of auxiliary nature also represents energy savings, because in normal mode their operation is not necessary.

A preferred embodiment of the invention comprises a control equipment actuating the auxiliary fans only when the air condenser apparatus is started up and stopped, and when its operational status is disturbed. Advantageously, the control equipment is fitted with devices detecting the temperature or pressure of the entering steam and the temperature of the condensate exiting from air condensers of the first stage in each section. ln another preferred embodiment, the air condensers provided with the auxiliary fans are fitted with air chambers enabling a circulation of the artificial air flow passing through them and with adjustable flow control shutters.

In a third preferred embodiment, the air condensers provided with the auxiliary fans are fitted with spray nozzles directed to their outside surfaces, which nozzles being connected to a condensate conduit of the air condenser apparatus through a controllable valve.

In a further advantageous embodiment, the air condensers not provided with auxiliary fans are fitted with devices regulating the natural-draught air flow passing through them. Advisably, the devices regulating the natural-draught air flow consist of shutters.

A still another beneficial embodiment comprises valves for disconnecting one or more of the sections and devices for blocking the air flow in case of a low ambient temperature entailing a frost risk.

In practice, well applicable is the solution when each section has two air condenser stages and only the air condensers of the second stages are provided with said auxiliary fans, or each section has three air condenser stages and only the third stage's air condensers are provided with said auxiliary fans. In case of three air condenser stages, there could be auxiliary fans for the second and third stage's air condensers.

On the other hand, the invention relates to a method of operation of a natural-draught air condenser apparatus, which has air condensers for condensation of steam, the air condensers being arranged in sections supplied with steam in parallel, each section having two or more air condenser stages connected in series on the steam side, and wherein the air condensers are located at bottom part of a cooling tower in a way that due to the effect of the cooling tower natural- draught air flow passes the air condensers in parallel. According to the invention, upon start-up of the air condenser apparatus and in case of a steam flow disorder arising in the air condensers, an artificial air flow is established in addition to the natural-draught air flow at the air condensers in the last stage or in the last two stages.

Advantageously, the steam flow disorder is detected by sensing the temperature or pressure of the entering steam and the temperature of the condensate exiting from air condensers of the first stage in each section, and an occurrence of the disorder is established if the difference between the temperature of the entering steam and that of the exiting condensate exceeds a predetermined value.

In a preferred implementation of the method, in case of a steam flow disorder, at least one auxiliary fan for establishing said artificial air flow is started up in the section where the disorder occurred. Another way to proceed is starting up at least one auxiliary fan for establishing said artificial air flow in the section where the steam flow disorder occurred, and in sections not involved in the disorder, auxiliary fans are started up in an opposite sense of rotation. The efficiency of intervention can also be improved by - in addition to starting up an auxiliary fan or auxiliary fans in the section where the disorder occurred - spraying condensate on the air condenser or air condensers associated with the auxiliary fan or auxiliary fans, respectively.

Again, on the basis of the invention, the method may involve making a valve in an air suction conduit more open in the section where the steam flow disorder occurred, while making valves in air suction conduits more closed in sections not involved in the disorder.

In a further implementation of the method according to the invention, in case of low ambient temperature entailing a frost risk, the natural-draught air flow of the air condensers in the first stage is partly or fully suppressed. The method may also involve, however, disconnecting one or more of the sections in case of a frost risk.

When starting up the air condenser apparatus according to the invention, it is advantageous if the natural-draught air flow of the air condensers is fully suppressed, air is circulated at the air condensers in the last stage or in the last two stages, and when the temperature of these air condensers increases, the air circulation is stopped and the artificial air flow is directed into the cooling tower, then the suppression of the natural-draught air flow of the air condensers is stopped, and after that the artificial air flow is stopped at the air condensers in the last stage or in the last two stages.

BRIEF DESCRIPTION OF DRAWINGS

The invention will hereinafter be described on the basis of preferred embodiments depicted by the drawings, where Fig. 1 is a side view, partly a cross section of the air condenser apparatus,

Fig. 2 is the air condenser apparatus depicted in Fig. 1 where the block diagram of the control system is also shown,

Fig. 3 is the air condenser apparatus as shown in Fig. 1 , fitted with a system to provide protection against frost, and

Fig. 4 is the air condenser apparatus as shown in Fig. 1 , fitted with a protective system suitable in an environment of high risk of frost.

In the drawings, same reference numbers designate identical or similar parts.

MODES FOR CARRYING OUT THE INVENTION

Fig. 1 shows the design of a natural-draught air condenser apparatus according to the invention. Steam comes to the air condenser apparatus from a power station turbine 2 driving a generator 1 through a steam conduit 3, which air condenser apparatus is divided into independent branches, called sections, connected in parallel to the steam conduit 3. In the embodiment shown as an example, there are two sections 30 and 30A. The elements of section 30A are designated by the same reference numbers as those in section 3 but with an additional letter "A". Sections 30, 30A are connected on one side to the steam conduit 3 and on the other side to manifold 14 of a vacuum pump 15. Sections 30, 30A are located in a well separated way in a cooling tower 5 constructed on ground level 4. Hereinafter, only the design of section 30 will be described, because the two sections 30 and 30A are of identical structure in all respects.

Steam coming through the steam conduit 3 is supplied to steam distributor ducts 6 and from there to air condensers 7. From here, the condensate is supplied to condensate tank 18 through manifold 8 and conduit 17. The remaining steam is fed through conduit 9 connected to manifold 8 into steam distributor duct 10 and from there to air condensers 11 , and then the condensate generated in them flows also into condensate tank 18 through conduit 16. From the condensate tank 18 the condensate is returned to the boiler of the power station by condensate pump 19 through condensate conduit 20.

Air condensers 7 represent the first and air condensers 11 the second air condenser stage, which are connected in series on the steam side. Under the air condensers 11 there is an auxiliary fan 12 and, if it operates, an artificial air flow 23 is established through the air condensers 11 , while through air condensers 7 of the first stage a natural-draught air flow 22 is established, depending on the draught of the cooling tower 5. From the air the condensers 11 the air and, as the case may be, some remaining steam are supplied to the vacuum pump 15 through conduit 13 and manifold 14.

The whole cooling surface of the air condensers 7 and 11 is located within the cooling tower 5 in a roof shape arrangement as shown in the figure. However, the arrangement may also be different. The air condensers 7 and 11 have supporting structure not shown in the figure so that the air condensers 7 and 11 are located above air inlet openings 25 at the bottom part of the wall of the cooling tower 5. Between air condensers 7 and 11 and also between the outmost air condenser 11 and the wall of the cooling tower 5, plate wall 21 prevents any flow of false air. Within the cooling tower 5, air flows 22 and 23 are mixed. The extent and temperature of a resulting air flow 24 are determined by the air flows 22 and 23. The draught generated in the cooling tower 5 depends on the structural height of the cooling tower 5 and on the temperature of the air flow 24.

The air condenser apparatus according to the invention includes several subsequent stages on the steam side, with decreasing steam side cross section and cooling surface, respectively. The number of stages is arbitrary in principle, but because of the costs of connecting conduits and due to the pressure loss arising in them, for the sake of economics, three or two stages are generally used. For example, Fig. 1 shows two stages and the cooling surface of the air condensers 7 of the first stage is twice as large as that of the air condensers 11 in the second stage. In the case of three stages, the surface ratio can preferably be 3:2:1. All the stages consist of identical condenser type units, in which the steam and the condensate proceed in the same direction downwards. We do not apply any counterflow dephlegmator. The non-condensing air can be guided away through some conduits in a per se known way.

The operation of the air condenser apparatus is started in the following way. According to a customary method applied for fan-type air condensers, first the vacuum pump 15 is started up, thereby establishing a vacuum in air condensers 7 and 11. Then, through steam conduit 3, steam is supplied to air condensers 7 and through them to air condensers 11 , but since the draught has not yet developed, air flow 24 is limited. By starting up the auxiliary fans 12, air flow 23 starts condensation in air condensers 11. To make sure that the steam reaches that point, it must flow along steam conduit 3 and also through air condensers 7 of the first stage, and this steam flow flushes away any air eventually remaining in air condensers 7 and in the connecting conduits. When the loading of cooling tower 5 reaches approx. 20% of the nominal rate, a steady draught is established, auxiliary fan 12 is stopped, and from this time, natural air flow 22 is provided in all air condensers 7 and 11. The current rate of this air flow depends on the natural draught and thus on the output of turbine 2.

In the course of operation, e.g. under the impact of wind, the air flow may increase in an air condenser 7 and, therefore, a dead zone may develop there. This is detected by measuring the temperature of the steam in steam distributor duct 6 and also the temperature of the condensate flowing in manifold 8, and if this latter is lower than the steam temperature by at least a pre-determined value, this indicates that a dead zone has developed in the given air condensers 7 and so at that point the condensate is overcooled. If the temperature of the entering saturated steam is e.g. 30 °C, the temperature difference triggering the intervention could be e.g. 4 °C. In the case of overcooling, the appropriate fan 12 is started up, and so the output of air condensers 11 is increased. This starts a steam flow across the given air condensers 7, which flushes the air from the tubes and restores the original condition. If the overcooling is eliminated, fan 12 is stopped. If the overcooling continues partly, fan 12 may carry on its operation at a lower performance, but it is advisable to avoid this as it results in electric power consumption.

The intervention is only efficient if the operation of fan 12 makes an impact on the environment of the dead zone. To this end, as already mentioned, the air condenser apparatus is divided into several sections which are connected in parallel with and independent of each other. In the various sections, the air condensers are in the first, second and perhaps third stage, while the auxiliary fans are located at the air condensers of the second and/or third stage. In this way it is possible that if the wind blows and disorders occur in the section on the appropriate side, only the associated fan or fans must be started up. In normal operation, auxiliary fans 12 applied for the natural-draught air condensers 11 are not expected to eliminate fully the deteriorating effect of the wind, and the only objective is to ensure steady operation and prevent evolution of disturbing air cushions in air condensers 7.

Fig. 2 shows the control system of the air condenser apparatus depicted in Fig. 1. Control equipment 31 receives through line 33 a signal of a per se known detector measuring the temperature of steam coming through steam distributor duct 6. This signal is compared with a signal, coming through line 34, of another per se known detector measuring the temperature of condensate leaving the air condensers 7. Instead of measuring the temperature of saturated steam entering, its pressure can be measured, because the temperature can be calculated from the latter. If the control equipment 31 detects such a temperature difference which is higher than a predetermined value, i.e. a local overcooling occurs in section 30, fan 12 associated with this section 30 is started up. As a result, at this point, a larger air flow 23 than the earlier natural air flow develops, and since the larger air flow 23 is less prone to heating up, the total air flow 24 in the cooling tower 5 will be colder. The natural draught, including the cooling output, will deteriorate in the whole system, while the output of air condensers 11 at the operating fan 12 increases.

On the left hand side of Fig. 2 another control possibility is depicted, i.e. the operation - in a reversed sense of rotation - of the fan 12A associated with section 30A which is not involved in overcooling. At that time, flowing backwards on air condensers 11 A, the already heated air generates air flow 39, as a result of which the cooling capacity decreases there and at the same time less steam is delivered to air condensers 7A and 11A.

A third control possibility is provided by valves 38 and 38A fitted into the air suction conduits 13 and 13A. Once the already mentioned overcooling occurs, control equipment 31 makes valve 38 of section 30 according to the point of overcooling more open and valve 38A associated with section 30A not involved in the overcooling more closed. This measure also results in the fact that the cooling output increases in section 30 and decreases in section 30A.

A fourth possibility is provided by nozzle 37 connected through a controllable valve 35 and conduit 36 to condensate conduit 20 after condensate pump 19, as a result of which atomised condensate can be applied for wetting the surface of air condenser or air condensers 11 in section 30 corresponding to the point of overcooling, thereby increasing the cooling output. The effect is practically the same as provided by the actuation of fan 12, i.e. increasing the local cooling output, while the draught and cooling capacity of the whole natural-draught air condenser apparatus slightly deteriorate.

All of the possibilities presented above serve for eliminating the overcooling and flow disturbance arising in an air condenser section, and this is achieved by guiding extra steam to this section, at the expense of the other sections. The controlling of the control equipment 31 may also be carried out by supplying a set signal to its input 32, i. e. the devices described above (fans, atomisers and valves) may be controlled manually, too.

Fig. 3 depicts a system providing protection against frost, the use of which is justified in places where in the winter the air temperature could even drop to -15 °C. The difference between this equipment and the previous one is that air condensers 7 and 7A of the first stage are associated with controllable shutters 40 and 40A, respectively, which are in a closed status upon start-up. They are opened by control equipment 41, which measures by means of a signal coming through line 42 the temperature or pressure of the steam entering through steam conduit 3. In case this drops to a dangerously low rate or if this is required by an adjustment of a set signal supplied to input 43, shutters 40 and 40A close partially. In normal mode, all shutters 40 and 40A are fully open, and the apparatus works according to principles as described for apparatus shown in Figs. 1 and 2.

The protection system shown in Fig. 4 is suitable for use in an environment where a high risk of frost prevails, that is in places where the winter temperature could drop below -30 °C. The structure follows in principle the design shown in the previous figures, and so only the deviating details are described. Sectioning valves 54, 55, 56 and 57 of the apparatus enable in cold periods the total disconnection of one or more section of the air condenser apparatus, for example section 30 in Fig. 4. Shutters 40, 51 and 52 of the disconnected air condensers 7 and 11 are closed, the auxiliary fan 12 does not operate and so there is no steam flow. In the case of air condensers 11A cooled by auxiliary fan 12A in the operating section 30A, air chamber 50A is fitted with supply side shutters 51 A and recirculating shutters 52A. If the air condenser apparatus is started in very cold temperatures, fan 12A ensures air flow 53 through the open recirculating shutters 52A. In this case, shutters 51 A are closed on the supply side, and shutters 40A are also closed. The condensation of steam commences in air condensers 11 A, and therefore steam flows along steam conduit 3, steam distributor duct 6A and air condensers 7A in a way that in the meantime, air flow 24 does not exist in the cooling tower 5. When the temperature of condensate flowing in conduit 16A increases to a safe value, e.g. when it is warmer than +30 °C, the supply side shutters 51 A open, recirculating shutters 52A close, and then gradually shutters 40A open up and the operation of fan 12A stops. Subsequently, depending on the outside temperature and the loading, valves 54, 55, 56 and 57 as well as shutters 40 and 51 open, and so the disconnection of certain sections is discontinued, and the status shown in Fig. 3 evolves. Control equipment 58 looks after the process control, which control equipment 58 receives a signal of a per se known detector measuring the temperature of condensate flowing in conduit 16A through line 62 and a signal of a detector 60 measuring the outside ambient temperature through line 61 , and controls the mentioned units at its outputs. A set signal for the control equipment 58 can be adjusted on its input 59.

The air condenser apparatus shown in Fig. 4 provides a further possibility for performing another protection function. If, in the disconnected status of section 30, shutters 40 are opened, false air flows into the cooling tower 5, which reduces the temperature of the air current 24, and so the draught of the cooling tower 5 and along with it the output of the air condenser apparatus drop dramatically.

Of course, the air condenser apparatus according to the invention is not only suitable for condensing the exhaust steam of a turbine in a power station, but also for performing condensation tasks in other industrial facilities, e.g. in chemical plants.

It will be evident to those skilled in the art that the above disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention as defined by the following claims.

Claims

1. Natural-draught air condenser apparatus, especially for condensing exhaust steam of a power station turbine, air condensers of which are arranged in sections supplied with steam in parallel, each section having two or more air condenser stages connected in series on the steam side, wherein subsequent stages are of a decreasing steam side cross section or cooling surface, and the air condensers are located at bottom part of a cooling tower in a way that as a result of the cooling tower natural-draught air flow passes the air condensers in parallel, c h a r a c t e r i s e d in that the air condensers (7, 7A, 11 , 11 A) of all stages are of such a circuitry that in them the steam and the generated condensate flow downwards in the same direction, and the air condensers (11 , 11 A) in the last stage or in the last two stages are provided with auxiliary fans (12, 12A) to establish an artificial air flow in addition to the natural-draught air flow.

2. The air condenser apparatus according to claim 1 , characterised by comprising a control equipment (31) actuating the auxiliary fans (12, 12A) only when the air condenser apparatus is started up and stopped, and when its operational status is disturbed.

3. The air condenser apparatus according to claim 2, characterised in that the control equipment (31) is fitted with devices detecting the temperature or pressure of the entering steam and the temperature of the condensate exiting from air condensers (7, 7A) of the first stage in each section.

4. The air condenser apparatus according to claim 2 or claim 3, characterised in that the air condensers (11 , 11 A) provided with the auxiliary fans (12, 12A) are fitted with air chambers (50, 50A) enabling a circulation of the artificial air flow passing through them and with adjustable flow control shutters (51 , 52, 51 A, 52 A).

5. The air condenser apparatus according to claim 2 or claim 3, characterised in that the air condensers (11) provided with the auxiliary fans (12) are fitted with spray nozzles (37) directed to their outside surfaces, said nozzles (37) being connected to a condensate conduit (20) of the air condenser apparatus through a controllable valve (35).

6. The air condenser apparatus according to claim 1 or claim 2, characterised in that the air condensers (7, 7A) not provided with auxiliary fans are fitted with devices regulating the natural-draught air flow passing through them.

7. The air condenser apparatus according to claim 6, characterised in that the devices regulating the natural-draught air flow consist of shutters (40, 40A).

8. The air condenser apparatus according to claim 1 or claim 2, characterised by comprising valves (54, 55, 56, 57) for disconnecting one or more of the sections (30, 30A) and devices (40, 51 ) for blocking the air flow in case of a low ambient temperature entailing a frost risk.

9. The air condenser apparatus according to claim 1 or claim 2, characterised in that each section (30, 30A) has two air condenser stages and only the air condensers (11, 11 A) of the second stages are provided with said auxiliary fans (12, 12A).

10. The air condenser apparatus according to claim 1 or claim 2, characterised in that each section has three air condenser stages and only the air condensers of the third stages are provided with said auxiliary fans.

11. The air condenser apparatus according to any claim 1 or claim 2, characterised in that each section has three air condenser stages and only the air condensers of the second and third stages are provided with said auxiliary fans.

12. A method of operation of a natural-draught air condenser apparatus, which has air condensers for condensation of steam, the air condensers being arranged in sections supplied with steam in parallel, each section having two or more air condenser stages connected in series on the steam side, and wherein the air condensers are located at bottom part of a cooling tower in a way that due to the effect of the cooling tower natural-draught air flow passes the air condensers in parallel, c h a r a c t e r i s e d in that, upon start-up of the air condenser apparatus and in case of a steam flow disorder arising in the air condensers, an artificial air flow is established in addition to the natural-draught air flow at the air condensers in the last stage or in the last two stages.

13. The method according to claim 12, characterised in that the steam flow disorder is detected by sensing the temperature or pressure of the entering steam and the temperature of the condensate exiting from air condensers of the first stage in each section, and an occurrence of the disorder is established if the difference between the temperature of the entering steam and that of the exiting condensate exceeds a predetermined value.

14. The method according to claim 12 or claim 13, characterised in that, in case of a steam flow disorder, at least one auxiliary fan for establishing said artificial air flow is started up in the section where the disorder occurred.

15. The method according to claim 12 or claim 13, characterised in that, in case of a steam flow disorder, at least one auxiliary fan for establishing said artificial air flow is started up in the section where the disorder occurred, and in sections not involved in the disorder, auxiliary fans are started up in an opposite sense of rotation.

16. The method according to claim 12 or claim 13, characterised in that, in case of a steam flow disorder, at least one auxiliary fan for establishing said artificial air flow is started up in the section where the disorder occurred, and condensate is sprayed on the air condenser or air condensers associated with said at least one auxiliary fan.

17. The method according to claim 12 or claim 13, characterised in that, in case of a steam flow disorder, a valve in an air suction conduit is made more open in the section where the disorder occurred, while valves in air suction conduits are made more closed in sections not involved in the disorder.

18. The method according to claim 12 or claim 13, characterised in that, in case of a low ambient temperature entailing a frost risk, the natural-draught air flow of the air condensers in the first stage is partly or fully suppressed.

19. The method according to claim 12 or claim 13, characterised in that, in case of a low ambient temperature entailing a frost risk, one or more of the sections are disconnected.

20. The method according to claim 12, characterised in that, upon start-up of the air condenser apparatus, the natural-draught air flow of the air condensers is fully suppressed, air is circulated at the air condensers in the last stage or in the last two stages, and when the temperature of these air condensers increases, the air circulation is stopped and the artificial air flow is directed into the cooling tower, then the suppression of the natural-draught air flow of the air condensers is stopped, and after that the artificial air flow is stopped at the air condensers in the last stage or in the last two stages.