Abstract:
The invention is a power plant cooling system comprising a direct contact condenser ( 11 ), a cooling tower ( 12 ) with at least one heat dissipating unit ( 13 ), a pipeline ( 15 ) and a cooling water pump ( 16 ) suitable for circulating cooling water between the direct contact condenser ( 11 ) and the heat dissipating unit ( 13 ), as well as a de-aerating structural component ( 14 ) defining a de-aerating space adjoining to the top of a flow space of the heat dissipating unit ( 13 ). The inventive cooling system comprises a means suitable for maintaining a vacuum in the de-aerating space. The invention also relates to a method for operating the cooling system.

Description:
This application claims priority, under Section 371 and/or as a continuation under Section 120, to PCT Application No. PCT/HU2010/000135, filed on Dec. 2, 2010, which claims priority to Hungary Application No. P 0900749, filed on Dec. 3, 2009. 
     TECHNICAL FIELD 
     The invention relates to a power plant cooling system and a method for operating thereof. 
     BACKGROUND ART 
     The schematic diagram of a conventional Heller-type cooling system or in other words that of an indirect dry cooling system is shown in  FIG. 1 . The cooling system comprises a direct contact condenser  11 , which condenses the spent steam coming from a steam turbine  10  by means of cooling water re-cooled in an indirect dry cooling tower  12 . The cooling water warmed up in the direct contact condenser  11  is supplied to the cooling tower  12  in a pipeline  15  by means of a cooling water pump  16  driven by a motor  17 . 
     Heller cooling systems are known which comprise a so-called recuperative water turbine  18  built into the cooling water branch leading from the cooling tower  12  to the direct contact condenser  11 . The major task thereof is to absorb usefully the elevating height (drop) which is not needed for returning the cooling water to the direct contact condenser  11 . The power recovered on the water turbine  18  contributes to the operation of the motor  17  which drives the cooling water pump  16 , thereby reducing the energy need of the motor  17 . The motor  17  (electric motor) driving the cooling water pump  16  has two shaft ends. On one side it is coupled to the cooling water pump  16  and on the other side to the water turbine  18 , thereby creating a water machine group running with a common axis. Such an approach is disclosed by way of example in the Hungarian patent specification 152 217. 
     The air flow (draught) necessary for heat transfer is provided by the indirect dry cooling tower  12 . The draught can be a natural draught (chimney effect) and it can be an artificial draught (ventilator draught). Prior art cooling towers  12  have one or more heat dissipating units  13  which transfer the heat to be absorbed to the ambient air, and the cooling system also comprises a de-aerating structural component  14  which defines a de-aerating space coupled to the top of the flow space of the heat dissipating unit  13 . Generally, prior art heat dissipating units  13  are triangular cooling units (cooling deltas) arranged horizontally or standing vertically along the periphery of the cooling tower  12 , and are grouped into sectors, where triangular cooling units associated with a sector have a common cooling water inlet and common de-aerating structural component  14 . The common de-aerating structural component  14  generally comprises a de-aerating circular line connecting the top of the triangular cooling units of a sector, and an upright extending de-aerating rack pipe known per se coupled thereto. 
     In the course of the operation of the conventional Heller-type cooling system, the spent steam coming from the steam turbine  10  is condensed by chilled cooling water supplied to the direct contact condenser  11 . For the sake of improving the efficiency of steam recirculation, vacuum has to be ensured in the direct contact condenser  11 . It is the cooling tower  12  of an appropriate cooling capacity which ensures to reach this vacuum. As a consequence of the condensation of the exhaust steam, the cooling water is warmed up in the direct contact condenser  11 . The warmed up cooling water is removed from the vacuum space of the direct contact condenser  11  by the cooling water pump  16 , which then supplies it to the rack pipes located on the top of the triangular cooling units. 
     The de-aerating rack pipes may even reach 6 to 8 m above the top of the triangular cooling units, and the cooling water level may be 1 to 2 m above the top of the triangular cooling units during operation. The de-aerating rack pipes are opened on the top and hence atmospheric pressure prevails above the cooling water. 
     The elevating height of the cooling water pump  16  has to be determined in such a way that the cooling water is raised from the vacuum in the direct contact condenser  11  to the atmospheric pressure in the rack pipe, furthermore from the water level of the direct contact condenser  11  to the much higher water level of the rack pipe in such a way that it overcomes the hydraulic resistance of the forward-going branch as well. The driving force of the cooling water flow returning to the direct contact condenser  11  is the pressure difference which prevails between the atmospheric pressure and the vacuum (steam condenser shell pressure) of the direct contact condenser  11 , and furthermore the geodetic difference between the water level of the rack pipe and the water level of the direct contact condenser  11 . This driving force overcomes the hydraulic resistance of the returning branch and the direct contact condenser  11 . The available driving force is, however, much higher than that required for overcoming the hydraulic resistances. To absorb this extra driving power, generally a throttle valve or a much more cost efficient solution, the recuperative water turbine  18  mentioned above, is applied. 
     It is clear from the above disclosure of the conventional Heller-type cooling system that the cooling water pump  16  is not to be designed for overcoming the hydraulic resistance of the whole cooling water circuit, but for a higher load. Therefore, it is necessary to have the water turbine  18  so that the unnecessary elevating height (drop) can be utilised relatively cost efficiently (much more efficiently than by using throttle). However, the application of the water turbine  18  necessarily entails loss, too, resulting from the loss of the cooling water pump  16  and the water turbine  18 . 
     DESCRIPTION OF THE INVENTION 
     The object of the invention is to provide a power plant cooling system and a method of operation thereof, which reduce or eliminate the disadvantages of prior art solutions. The object of the invention is especially to create a power plant cooling system and a method of operation thereof which enable the reduction or elimination of the unnecessary elevating height (drop) in the return branch of the cooling water and eliminate the necessity of applying a recuperative water turbine. In such a way, the power necessary for circulating the cooling water can be reduced and the application of a cooling water pump with a lower elevating height is possible. 
     The invention is based on the recognition that if in the inner space of a de-aerating structural component—opening to atmospheric pressure according to the prior art—a lower than atmospheric pressure, i.e. a vacuum is maintained, the objects of the invention can be achieved. 
     Consequently, the invention is a power plant cooling system according to claim  1  or an operation method according to claim  8 . Preferred embodiments of the invention are defined in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary preferred embodiments of the invention will be described hereunder with reference to drawings, where 
         FIG. 1  is a schematic diagram of a prior art Heller-type power plant cooling system, 
         FIG. 2  is the schematic diagram of a power plant cooling system according to a first embodiment of the invention, 
         FIG. 3  is a magnified and supplemented schematic diagram of a detail of  FIG. 2 , 
         FIG. 4  is the schematic diagram of a power plant cooling system according to a second embodiment of the invention, and 
         FIG. 5  is the schematic diagram of a further preferred solution. 
     
    
    
     MODES OF CARRYING OUT THE INVENTION 
     One characteristic of the approach used by the invention is that a subatmospheric pressure, a vacuum is created in the heat dissipating units  13 , i.e. in the rack pipes at the top of the triangular cooling units. According to the invention, the definition of vacuum—as usually applied in this field of art—is a pressure generated in the steam condenser shell of the direct contact condenser  11 , which pressure is always lower than the atmospheric pressure, for example it is typically below 0.3 bar. Maintaining vacuum or any rate of subatmospheric pressure in the de-aerating space defined by the de-aerating structural component  14  entails the advantage that the cooling water pump  16  does not have to overcome the atmospheric pressure also in the forward-going branch, and accordingly the driving force of the cooling water in the return branch will also be lower. 
     The power plant cooling system according to the invention consequently comprises a means which is able to keep the pressure in the de-aerating space at a rate lower than the atmospheric pressure, which is preferably a vacuum maintaining means. 
     By way of example, the invention can be implemented in two especially preferred embodiments. The common characteristic of these embodiments is that the means suitable for maintaining the vacuum in the de-aerating space comprises a vacuum sealed valve designed to seal controllably the de-aerating space of the de-aerating structural component from the ambient air, and a vacuum line coupled to the de-aerating space. 
     According to the first embodiment shown in  FIG. 2 , the vacuum tight valve  19  is arranged close to the top of the triangular cooling units, hence the vacuum line  20  coupled below and only shown conventionally adjoins the de-aerating space below the water level which is created as a result of maintaining vacuum in the de-aerating space. Preferably, one vacuum sealed valve  19  is used in each sector, and they are preferably fixed on the rack pipes making the part of the de-aerating structural component  14 . 
     The vacuum tight valves  19  are closed by launching the operation of the cooling system, even before the triangular cooling units are filled up, and vacuum is generated in the triangular cooling units via the vacuum line  20 . Then the part of the de-aerating structural component  14  located below the vacuum tight valve  19  represents the space in which the lower than atmospheric pressure, vacuum is maintained. After filling up the triangular cooling units, in an operating state, the space below the vacuum tight valve  19  is filled up with cooling water. 
       FIG. 3  shows a magnified and further detailed section of  FIG. 2 . The vacuum line  20  is connected to the vacuum generating means  23 , preferably a so-called ejector, which also makes sure that the direct contact condenser  11  is under vacuum. The vacuum line  20  comprises a controllable exhaust valve  21 , which is opened during the creation of vacuum when the operation is started. As a de-aerating unit, a ball valve  22  on the top of the flow chamber of the heat dissipating unit  13  enabling a relatively smaller throughput is serving to transfer the air eventually accumulated during the operation. 
     The sectors of the heat dissipating units  13 , preferably triangular cooling units, are to be drained from time to time. This could be necessary, for example, at the time of maintenance and when a frost risk prevails. In such cases the controllable and motorised vacuum tight valves  19  are opened and the vacuum line  20  is separated by valve control from the de-aerating space, when providing its traditional function that the de-aerating circular line integrated in the de-aerating structural component  14  and the associated upright protruding de-aerating rack pipe enable the draining of cooling water from the triangular cooling units. 
     In the second preferred embodiment shown in  FIG. 4 , the vacuum line  20  is coupled to the de-aerating space, i.e. preferably to the rack pipe, above the water level that prevails in case of vacuum maintenance in the de-aerating space. Putting the system under vacuum/draining is implemented as described above, by the appropriate control of the vacuum tight valves  19  and the exhaust valve  21 . 
     The vacuum line  20  subjects suction effect to the de-aerating rack pipe, which raises the height of the water column in the rack pipe. The de-aerating structural component  14  as well as the rack pipe preferably integrated therein should be installed at such a height that the suction effect does not yet draw the cooling water into the steam condenser shell of the direct contact condenser  11 . 
     It is easy to see that the solution according to the invention may be combined also with an approach whereby the water level in the direct contact condenser  11  is raised; such an approach is shown in  FIG. 5  (where, for the sake of simplicity, the vacuum, i.e. the subatmospheric pressure generating unit is not shown). With the water level of the direct contact condenser  11  being hence raised, the extra elevating height (drop) evolving in the return branch of the cooling system can be reduced or even eliminated in the given case. 
     This approach can be applied especially in the case of the steam turbines  10  having a lateral, axial or upward outflow. The water level of the direct contact condenser  11  can be raised by locating the direct contact condenser  11  proper at a higher vertical position or by increasing the volume of water therein. 
     The higher the water level of the direct contact condenser  11 , the more the unnecessary extra elevating height (drop) can be reduced. The water level in the direct contact condenser  11  is preferably kept above the lower third of the vertical extension of the heat dissipating unit  13 , or more preferably above its halving level, and even more preferably above its topmost level. 
     The creation of vacuum at the top of the triangular cooling units and the raising of the water level in the direct contact condenser  11  provide broad combination options for the optimal use of local endowments. Both the approach according to  FIG. 2 , and the approach according to  FIG. 4  may be combined with the arrangement depicted in  FIG. 5 . 
     The invention, of course, is not limited to the above detailed embodiments, but further modifications and variations are possible within the scope defined by the claims. For example, instead of the de-aerating rack pipe, a de-aerating tank located in an appropriate vertical position can also be used.