Steam turbine with series connected direct-contact condensers

Field of the invention

The object of the invention is a condensing steam turbine comprising one or more turbine casings, the turbine casing or casings having at least two parallel exhausts separated at the steam side, with the exhausts being connected to condensers utilizing highly varying input-temperature coolant, the condensers being separated from one another at the steam side and being serially connected to one another at the coolant side. A further object of the invention is a method for extending the unrestricted operating range of such condensing steam turbines. Description of prior art

It is common knowledge that condensers are applied for condensing steam exhausted from steam turbines of condensing plants. Condensers are categorized into two main groups. Surface condensers, making up the first of these groups, usually consist of tube bundles. Cooling water flows through the interior of the tubes and steam condenses on the outer pipe surface. Latent heat released by the condensation of steam is absorbed by the water. The second group contains the so-called direct-contact condensers. As the name suggests, in these condensers cooling water is in direct contact with steam undergoing condensation, while the water itself gets heated. The present invention relates to condensing steam turbines equipped with serially connected condensers that have highly varying initial coolant temperature. Because in the vast majority of practical applications water is applied as coolant in the following we often refer to cooling water, but it has to be noted that other coolants capable of being circulated may also be utilized, such as antifreeze solution, and other liquids or gases. Under

condensers with highly varying initial coolant temperature we mean condensation apparatuses where the temperature of water fed into the condenser may move in a wide temperature range with respect to the operation of the steam turbine. Constant coolant temperature condensers utilize cooling water having a fairly low yearly temperature variation (such as fresh water cooled systems e.g. those using sea water or water from rivers as coolant), providing that the temperature of cooling water does not significantly rise in warmer periods during the year. In winter or during colder spells the temperature of the cooling water fed into the condensers may be adjusted e.g. by back-mixing to provide that the initial coolant temperature of the condenser stays in the 10 - 15 ⁰ C range. Contrary to that, the invention relates to steam turbine and condenser assemblies having highly varying cooling water temperature (applied primarily in air-cooled systems utilizing cooling water as an intermediate coolant between surrounding air and the condenser), which significantly affects the operating regime of the steam turbine. Such temperature variation is usually caused by meteorological factors (e.g. seasonal or daily changes, climate effects). For comparison, the temperature variation of the cooling water applied in a fresh water cooled plant operating under continental climate conditions is only 10 - 15 ⁰ C, posing no problems to turbine operation, whereas in case of an air cooled plant the temperature variation of ambient air is in the range of 60-65 °C, which may cause serious difficulties.

A high degree of cooling water temperature variation primarily occurs in indirect dry cooling systems, especially in those applying Heller-type condensation apparatus where closed-circuit surface heat exchangers (so- called air coolers) are applied for cooling warmed-up, clear cooling water. Cooling water is circulated by circulation pumps in a closed circuit with the direct-contact condenser disposed on one side of the circuit and water-air heat exchangers on the other. Air coolers are disposed either in natural draught cooling towers or in fan coolers. In both cases the temperature of

cooling water leaving the air coolers is determined by ambient air temperature, and thus the temperature of cooling water reintroduced into the direct-contact condenser is subject to uncontrollable change due to weather and seasonal effects. Because the design parameters of the steam turbine (dimensions, casing geometry, blade profile) are optimized for operating in a given temperature range, fluctuations of cooling water temperature may lead to a deterioration in steam turbine efficiency and/or to reduced power output. It is well known that modern steam turbines consist of three main portions. These are the high pressure (HP), intermediate or medium pressure (MP) and low pressure (LP) casings. It is also of common knowledge that the specific volume of saturated steam rapidly increases with decreasing pressure (and/or temperature). As condensers of condensing plants usually have an operating pressure of 40-250 mbar, due to the corresponding v = 5-

16 m /kg specific volume modern steam turbines in the 150-200 MW power range cannot be implemented with a single exhaust. Our invention relates to low pressure casings and direct-contact condensers or surface condensers applying highly varying initial temperature coolant (cooling water).

A steam turbine may have one or more exhausts. The low pressure turbine casings (or low pressure casings for short) of turbines having higher design power are usually implemented having a single inlet port branching into two opposite-direction inlet branches and two exhausts. The steam consumption of the low pressure casing is dependent upon the configuration of the inlet, more particularly the size and geometry of the casing, that is, the inlet cross section and the size and profile of blades arranged in the turbine casings.

Named after its shape, this casing configuration with two exhausts is usually called a "diabolo". Steam enters the casing through the inlet in the middle, then flows parallel with the axis of the diabolo in opposite directions and leaves the casing at the largest diameter blades rings. The exhaust is thus constituted by the blades of the last, highest diameter stage, two of

which are disposed in each diabolo. Such a two-exhaust low pressure casing is usually equipped with a single condenser. In such a configuration steam flows from the exhausts into a common interior space of the diabolo, which it leaves through a single exhaust hood (usually implemented as a downward- pointing duct). In this case the casing has a common steam space, with a single exhaust duct leading to the condenser and the two exhaust being connected in parallel. Configurations where a separating plate is disposed in the space between the two exhausts, or the two exhaust hoods (either the two separated exhaust hoods of a single diabolo or the exhaust hoods of two separate turbine casings) are connected to two, instead of one, condensers are designated as configurations with exhausts separated at the steam side. Serial connection of the condensers is possible only in case of such a configuration. Steam turbines comprising one, two or three diabolo-type low pressure casing or casings, respectively, therefore have two, four, or six exhausts (which number, as it has been explained above, does not necessarily equal the number of exhaust hoods or exhausts separated at the steam side). The low pressure casings are parallelly connected (at their inlets disposed in the middle of each casing) to the steam pipe connected to the exhaust or exhausts of the medium pressure casing.

It is also known that, in case a steam turbine has more than one exhausts that are separated by a wall or walls at the steam side and may therefore have different exhaust pressure, it may be expedient to serially connect two or more condensers separated at the cooling water side. Thereby the pressure of the first condenser (following the direction of cooling water flow from the input) will be lower than the pressure of the following one, and the pressure of only the last condenser will equal the pressure all condensers would have were they connected parallelly at the water side. Such a serial connection has already been in use with surface condensers having constant initial coolant temperature (or a coolant with such low temperature variation

that does not restrict operation), for instance condensers employing fresh water cooling.

In the 1970s and 1980s, when the Heller system was becoming more and more widespread, experiments were conducted in implementing serial condenser connection utilizing also direct-contact condensers. However, solutions provided so far have not proven satisfactory. Back in the 70s and 80s only steam turbines having at most two exhausts were fitted with direct- contact condensers, and even so the steam-side separation of the exhausts and balancing of axial force changes caused by condenser pressure variation were highly problematic. Nowadays the increasing requirements of plant unit power can only be met by installing steam turbines having even more exhausts. For instance, 600 MW-units with four exhausts and two low pressure casings, two direct-contact condensers connected in parallel are under construction. Units with a rated power of 1000-1200 MW, six exhausts, and three low-pressure casings are already in the design phase. In case of these units it may be worthwhile to consider possible solutions for the serial connection of direct-contact condensers at the cooling water side. Our invention relates to a steam turbine-condenser assembly with high seasonal variation of cooling water temperature that eliminates all the above mentioned disadvantages of serial connection. The inventive solution also removes a drawback that has not been touched upon so far but, as will soon become clear, is perhaps one of the most significant disadvantages. To understand this major disadvantage we should now turn to characteristics of low pressure turbine casings. As it is well known to those skilled in the art, low pressure turbines usually consist of five to seven stages. The steam consumption of such turbines primarily depends on the input steam pressure that is virtually unrelated to condenser back pressure. If such low pressure casings are serially connected at the input steam side, their steam throughput will be nearly the same irrespective of the slightly different back pressure values present in the turbine condensers. The operating regime of

low pressure turbines is affected by two constraints, both of which arise in connection with limiting factors related to blades of the exhaust stage. One of these limitations is the minimum pressure value corresponding to the maximum turbine output power (in case pressure drops below this value, the turbine's output will not increase further). In German and Hungarian literature this back pressure value is referred to as the "limit vacuum", while in English it is called the "choking point". Both terms shed some light on the physical nature of the phenomenon that is explained in detail in a number of textbooks and articles on steam turbine technology. Suffice it to state here that it is related to a critical pressure condition occurring at the last blade ring that results in choking (that is, a condition when no additional mechanical power is generated on the blades). In addition, potentially harmful blade oscillations may also occur. What is important for the purposes of the present invention is that a back pressure lower limit (and a corresponding temperature limit) exists.

The situation is even more complex when condenser pressure increases. Then, the volumetric steam flow passing through the last stages declines rapidly, resulting in harmful flow phenomena that above a certain pressure value cannot be tolerated. The phenomenon is explained in detail in literature. Without going into details suffice it to state that there exists an upper limit to condenser pressure (and condenser temperature), called turbine exhaust hood temperature limit, or for short exhaust hood temperature limit. The distance between the upper and lower limits (specified either as a pressure ratio or a temperature range) is a very important design and operating parameter. Experience shows that at full load the ratio of the upper and lower pressure limits is between 3.5 to 1 and 4 to 1 , which translates into a temperature range of 25-30 ⁰ C, meaning that in case for instance the temperature value corresponding to the choking point is 37,5 ⁰ C the upper limit of condenser temperature may lie around 67,5 ⁰ C. Both the range and

the absolute values are of course dependent on the actual design and sizing of the steam turbine. Although these are only exemplary values they provide a farily good approximation to the parameters for a 1000 MW system applying dry cooling towers designed for temperate climate. These parameter values will be used for the purposes of the present specification by way of example only, emphasising that inventive solution may also be implemented with different design parameters.

It is of common knowledge that different low pressure casings are utilized with different types of cooling systems. In a fresh water cooled plant for instance, due to the relatively low coolant temperature low pressure turbines with large exhaust area and low choking point are applied, with the corresponding condenser temperature being also low, usually 30 ⁰ C or even lower. In a system with wet cooling towers the above mentioned value is above 30 ⁰ C, and in a Heller type dry cooling system a value of 37.5 ⁰ C is acceptable. In a direct cooling system (ACC) the parameter value may lie around 40-45 ⁰ C, with the corresponding relatively small exhaust area values. These values are cited for easier comprehension of the present invention. The limits of optimal operation of varying input coolant (cooling water) temperature condensers are described with reference to Fig. 1 that illustrates the state of the art. Fig. 1 shows a schematic view of a condensing turbine and the corresponding direct-contact condenser. For easier comprehension the medium pressure (MP) casing of the turbine and details of the cooling system are not shown in the drawing. The steam flow G coming from the MP casing of the turbine is fed into parallelly connected diabolo type low pressure casings 1 , 2, 3, each casing having two exhausts. Exhaust branches of low pressure casings 1 , 2, 3 are connected to the inlet ports of respective condensers 1A, 2A, 3A, which condensers are connected serially. Cooling water is driven by gravity through condensers 1A, 2A, 3A. This is illustrated by showing condensers at different levels in the drawing. Fig. 1

shows a three-stage serial connection known per se. Serial connection may of course be implemented with two low pressure casings and two condensers, which could be illustrated by omitting low pressure casing 3 and the corresponding condenser 3A. Although in the present specification a three-stage variant is described, the invention may be implemented by serially connecting only two or more than three condenser stages. The low pressure casings 1 , 2, 3, having an identical configuration LPgwere selected for a temperature range 37,5-67,5 ⁰ C. This means that the low pressure casings 1 , 2, 3 with a configuration LP _β have a choking point of 37,5 ⁰ C and an exhaust hood temperature limit (upper limit of condenser temperature) of 67,5 ⁰ C.

The efficiency of a dry cooling system applied for removing heat at a given rate may be characterised with the difference of mean condenser temperature and ambient air temperature. This temperature parameter is designated with the abbreviation ITD (initial temperature difference). In our case an ITD = 30 ⁰ C has been selected (other values may also be possible). Depending on local conditions, a Heller-type system usually has an ITD between 25 ⁰ C and 35 ⁰ C. Let us now turn to temperature values shown in the graph below the schematic system drawing. In case of the ranges shown in the graph, the temperature values shown at the bottom are ambient air temperatures, while values shown at the top are condenser temperatures. Let us assume for simplicity that cooling water warms by 7.5 ⁰ C in each condenser stage, resulting in a total warming of 22.5 ⁰ C in condensers 1A, 2A, 3A. This implies that the same amount of heat power is removed from all three condensers 1 A, 2A, 3A, which is obviously not entirely true if the three low pressure casings 1 , 2, 3 have identical steam consumption, but the differences are minor and therefore can be neglected for the sake of easier comprehension. The total warming value of 22.5 ⁰ C was also selected for simplicity. This value may also be different in case of an actually implemented system. A further approximation was made assuming that in the

direct-contact condenser water reaches the saturation temperature corresponding to condenser pressure, or in other words the TTD (terminal temperature difference) of the condenser is zero. This is of course impossible, but since the TTD value is only 1-2 % of the ITD, the approximation can be allowed for easier comprehension of the invention. Let us assume that the lower condenser temperature limit (corresponding to the choking point) is 37.5 ⁰ C in case the three low pressure casings 1 , 2, 3 (having an identical configuration LP _β ) have a nominal steam consumption

G. This limit is first reached by condenser 1A receiving water with a temperature of t _wc . If the temperature of cooling water falls below t _wc (= 37.5

⁰ C according to our example), a choking point operational limit condition occurs, meaning that the efficiency of the turbine cannot be increased further (the power curve declines), and potentially damaging blade resonances may also occur. That implies that in case ambient air temperature falls below this limit cooling tower power has to be limited, and the water cannot be cooled further. The low pressure casing 1 thus reaches its peak power at a cooling water temperature t _wc = 37.5 ⁰ C. However, this is not the case with low pressure turbines 2 and 3. Because in each condenser stage the cooling water heats up by 7.5 ⁰ C, the condenser 2A connected to casing 2 has an input cooling water temperature 7.5 °C higher and the condenser 3A of casing 3 an input temperature 15 ⁰ C higher than the optimum value of 37.5 ⁰ C which (due to the above described limitation) can never be reached. On the whole the lowest mean condenser temperature (the parameter determining the power output of the turbine) will be 45 ⁰ C, which cannot be lowered further (since that would make casing 1 reach the choking point), even if casings having the configuration LP _β have a 37.5 ⁰ C choking point limit. The limit value of 37.5 ⁰ C is shown in a left slanted line pattern 4 in the diagram under casings 1 , 2, 3. The lowest mean condensation temperature limit of 45 ⁰ C is shown in a right slanted line pattern 6 under casing 2.

The upper condenser temperature limit creates a similar situation. Here the key role is played by the most downstream (with respect to the coolant flow) low pressure casing 3. As the input temperature t _wc of the cooling water reaches 52.5 ⁰ C, due to the temperature rise of 7.5 °C in each condenser the condenser 3A of low pressure casing 3 reaches the maximum allowed temperature of 67.5 ⁰ C (the exhaust hood temperature limit). Because this limitation is caused by the drop of volumetric steam flow, in case it is exceeded drastic and costly measures have to be taken. First, the steam consumption of the whole turbine must be lowered (at the main steam feed valve), to decrease the load on the cooling tower and thereby the temperature rise (and the corresponding specific volume drop) may be stopped. This however limits the power output of not only the low pressure casings 1 , 2, 3 but also that of the medium pressure casing MP and high pressure casing HP (not shown in the drawing). When the exhaust hood temperature limit is reached, condenser temperature of casing 2 is only at 60 ⁰ C and the condenser temperature of casing 1 is only at 52.5 ⁰ C. The 67.5 ⁰ C limit is shown also in a left slanted line pattern 5 under all three low pressure casings 1 , 2, 3. In this case the mean condenser temperature (the parameter determining the power output of the steam turbine) is 60 ⁰ C, approximately the same as the temperature of the condenser 2A of casing 2. The condenser temperature range 45-60 ⁰ C that is exploitable by casing 2 is illustrated in the diagram by an area with left slanted line pattern fill (above line 6). The ambient air temperature range where the turbine can be operated under optimal conditions is illustrated by a greyed area 7 in the diagram. In case of the exemplary cooling system having an ITD (difference between ambient air temperature and mean condenser temperature) of 30 ⁰ C, an ambient air temperture t _a = 15 ⁰ C will result in a mean condenser temperature of 45 ⁰ C.

This means that the cooled water temperature t _wc = 37.5 ⁰ C corresponding to the choking point limit of low pressure casing 1 will be reached at an

ambient air temperature t _a = 15 °C. If air temperature falls below this value, the cooling water temperature of condenser 1A will drop below the temperature t _wc = 37.5 ⁰ C corresponding to the choking point, and thus the turbine may be operated without choking point restriction above an ambient air temperature t _a = 15 ⁰ C.

The exhaust hood temperature limit of casing 3 (67.5 ⁰ C) is reached at a mean condenser temperature of 60 ⁰ C, which, assuming the ITD is 30 °C, corresponds to an ambient air temperature t _g = 30 ⁰ C.

Consequently, the system may be operated without restrictions caused by choking point and exhaust hood temperature limits in the ambient air temperature range t _a = 15-30 ⁰ C. Under an air temperature t _g = 15 ⁰ C power cannot be increased due to the choking point limit of casing 1 , although casings 2 and 3, respectively, have, heat drop reserves corresponding to 7.5 °C and 15 ⁰ C, which cannot be exploited. Above an air temperature t _g = 30 ⁰ C casing 3 reaches the highest condenser temperature of 67.5 ⁰ C corresponding to the exhaust hood temperature limit, resulting in that the steam throughput must be restricted, although casings 1 and 2 have, respectively, 7.5 ⁰ C and 15 ⁰ C reserves. If the temperature of ambient air exceeds this limit, the power output of the whole plant should be restricted, which has even more serious cost effects. The range of ambient air temperature where the turbine can be operated without restrictions is thus shown in the diagram by the greyed area 7 under casing 2. Objective of the invention

The objective of the present invention is to eliminate the above mentioned drawbacks by extending the unrestricted operating range of condensing steam turbines having serially connected condensers applying a coolant with high seasonal variation of initial coolant temperature. It has to be stressed once again that in case of condensers with highly varying coolant temperature the temperature of the coolant may be controlled. For instance,

in winter when the choking point limit is reached or when antifreeze action is required the cooling tower of a Heller-type system may be controlled with the application of shutters.

According to the invention the inventive objective is accomplished by providing a condensing steam turbine comprising one or more turbine casings, the turbine casing or casings having at least two exhausts separated at the steam side, with the exhausts being connected to condensers utilizing varying input-temperature coolant, said condensers being separated from one another at the steam side and being serially connected to one another at the coolant side. The major characterising feature of the inventive steam turbine is that the consecutive exhausts separated at the steam side are arranged such that the choking point and/or exhaust hood temperature limit of the exhausts increases in the direction of the coolant flow.

The turbine casings are preferably designed for different back pressure levels, which means that the casings are sized for the different output steam flow levels brought about by the serial connection of the condensers.

According to a preferred embodiment the exhausts are implemented such that they reach their choking point and/or exhaust hood temperature limit substantially simultaneously.

Further preferred embodiments of the inventive condensing steam turbine are specified in the dependent claims.

Another object of the invention is a method for extending the unrestricted operating range of a steam turbine, the condensing steam turbine comprising one or more turbine casings, where the turbine casings have at least two exhausts separated at the steam side, with the exhausts being connected to condensers utilizing varying input-temperature coolant, said condensers being separated from one another at the steam side and being serially connected to one another at the coolant side. The method is essentially characterised by:

applying exhausts that have different choking points and/or exhaust hood temperature limits, where the exhausts are connected to the condensers such that the choking point and/or exhaust hood temperature limit of consecutive exhausts increases in the direction of the coolant flow.

According to a preferred way of carrying out the inventive method the exhausts are arranged such that the temperature difference between the choking points and/or exhaust hood temperature limits of consecutive exhausts substantially equals the coolant temperature rise occurring in the corresponding condenser.

A further preferred way of carrying out the method is characterised by such an exhaust arrangement and connecting the exhausts with the condensers in such a fashion that the exhausts reach their choking point and/or exhaust hood temperature limit substantially simultaneously, at a given input temperature (t _wc ) of the coolant.

Brief description of the drawings

Further details of the invention are explained with reference to the accompanying drawings, where

Fig. 1 is a schematic block diagram of a prior art assembly consisting of a steam turbine and a direct-contact condenser, with coolant and air temperature values related to the unrestricted operating range being shown under the block diagram;

Fig. 2 is the schematic block diagram of the steam turbine and direct-contact condenser assembly according to the invention, with coolant and air temperature values related to the unrestricted operating range shown under the block diagram;

Fig. 3 is a schematic block diagram of a further steam turbine and direct-contact condenser assembly according to the invention; Fig. 4 is a schematic block diagram of a still further steam turbine and direct-

contact condenser assembly according to the invention; Fig. 5 is a schematic block diagram of yet another steam turbine and direct- contact condenser assembly according to the invention;

Fig. 6 is a diagram showing the yearly distribution of ambient air temperature; and

Fig. 7 is the exhaust loss diagram of a steam turbine.

Disclosure of the invention

The essential features of the invention will be described referring to Figs. 1 and 2 shown side by side. Temperature values shown on the right of Fig. 2 are the same as values indicated on the left of Fig. 1.

The inventive steam turbine illustrated in Fig. 2 differs from the steam turbine shown in Fig. 1 in that of the three low pressure casings 1 , 2, 3 connected in parallel only the middle casing 2 has a configuration LPg identical to the configuration of all three low pressure casings 1 , 2, 3 shown in Fig. 1. The leftmost low pressure casing 3 of Fig. 2 has a configuration LPQ and has the same inlet area (seen as a line section in the side view shown in the drawing) but a smaller exhaust area as the casing 2 that has a configuration LPg.

Similarly, the rightmost low pressure casing 1 , having a configuration LP^, has the same inlet area but a larger exhaust area than the casing having a configuration LPg. This is to illustrate that the three casings 1 , 2, 3 are configured such that an equal steam flow G is fed to each casing from the 3*G steam flow coming from the medium pressure casing MP, in a manner similar to the turbine where all three casings 1 , 2, 3 have a configuration LPg. Also the differently sized exhausts shown in the figure illustrate that the exhaust of casing having a configuration LPQJS capable of having a smaller volumetric steam flow, while the exhaust of casing 1 having a configuration LP^is capable of processing a larger volumetric steam flow with a nearly equal steam consumption G. The structural arrangement of turbine casings

will not be dealt with further in the present specification. In our case the different low pressure casings 1 , 2, 3 are arranged such that they have substantially equal steam consumption, and their terminal stage is implemented such that the casing 1 connected to the lower temperature condenser is capable of processing a larger volumetric steam flow than the casing 3 connected to the higher temperature condenser. The most important characteristic feature of the arrangement is that the choking point temperature of casing 1 , having a configuration LP^ is approximately 7.5 ⁰ C lower, while the choking point temperature of casing 3, having a configuration LPQ lies approximately 7.5 ⁰ C higher, than that of the middle casing 2, having a configuration LPg.

As it has already been touched upon, the temperature range between the lower and upper temperature limits for of low pressure turbine casings is 25- 30 °C (in our case, 30 ⁰ C) independent of the actual value of the lower limit, and therefore it can be maintained that the upper temperature limit of casings with configurations LP _β and LP^ will also follow the temperature steps of 7.5

°C of the exemplary system. It is now fairly obvious that due to their different configurations LP^, LP _β , LPQ the three low pressure casings reach their respective choking point limits of 30 ⁰ C, 37.5 ⁰ C and 45 ⁰ C simultaneously, and that the upper limits are also simultaneously reached at 60 ⁰ C, 67.5 ⁰ C, and 75 ⁰ C. In case the ambient air temperature t _a is 7.5 ⁰ C and the ITD is 30 ⁰ C the temperature of the cooling water in condenser 2A is 37.5 ⁰ C. As cooling water temperature rises by 7.5 °C in each condenser stage, the temperature of condenser 1 A is 30 ⁰ C, and the condenser 3A has a temperature of 45 ⁰ C. By the careful sizing of low pressure casings 1 , 2, 3 (having configurations LP^, LPg, LP _β ) we have obtained a design where the temperature values 30

⁰ C, 37.5 ⁰ C, and 45 ⁰ C correspond to the choking point limit.

Similarly, in case the ambient air temperature t _g = 37.5 ⁰ C, calculating with an

ITD of 30 ⁰ C the temperature of the cooling water in condenser 2A is 67.5 ⁰ C, while cooling water temperature in condenser 1A is 60 ⁰ C and in condenser 3A it is 75 ⁰ C. Due to the suitably chosen configurations LP^, LP _β , LPg of the respective casings 1 , 2, 3 these values correspond to the exhaust hood temperature limits.

It can thus be established that all three casings utilize the entire 30 °C temperature range available between the lower and upper temperature limits of unrestricted operation, and also that the corresponding ambient air temperature range where unrestricted operation is possible has increased from 15-30 ⁰ C to 7.5-37.5 ⁰ C, or in other words it has doubled with respect to conventional solutions. The lowest and highest utilizable ambient air temperature values are t _a = 7.5 ⁰ C and 37.5 ⁰ C.

The economic advantage provided by the inventive dry cooling system comprising a steam turbine-direct-contact condenser assembly utilizing different low pressure casings 1 , 2, 3 compared to the system applying identical low pressure casings is nearly as high as the savings provided by connecting direct-contact condensers of a dry cooling system are relative to connecting said condensers in a parallel fashion. This can be proven by the following simple calculations.

As it has already been mentioned, the efficiency of a cooling system of given cooling capacity is represented by its ITD, the difference between mean condenser temperature and ambient air temperature. In our case, ITD = 30 °C. The total cost of a cooling system with a given cooling capacity is inversely proportional to the final temperature difference of the cooling system, that is, the difference between the temperature of warmed cooling water leaving the last condenser stage (t _ww ) and ambient air temperature. In our case this difference is 37.5 ⁰ C. Without the serial connection of condensers the ITD would be 37.5 ⁰ C because in that case condensers 1 , 2

could not have lower temperatures than condenser 3. Therefore the cooling system involving serial connection of condensers is more efficient than the one without it by a ratio of 37.5/30, or in other words it is 20 % more efficient. Thus the savings are exactly 20 % of the total cost of the cooling system. However, these savings can only be realised if there are no restrictions placed on plant operation. As it is presented above, all prior art solutions suffer from such operational restrictions. The presented conventional system may be operated without restrictions in the ambient air temperature range of 15-30 ⁰ C, in contrast to the 7.5 °C-37.5 ⁰ C range achievable with conventional parallelly connected condensers or with the application of the present invention. In other words, a conventional parallelly coupled system with an ITD of 30 ⁰ C and the inventive system are equivalent from an operational point of view, since both provide the same power without restrictions in the range of t _a = 7.5 °C-37.5 °C. However, the inventive system is 20 % cheaper than the conventional parallel system. The prior art serial solution may provide the same power as the other two systems only in the air temperature range t _a = 15-30 ⁰ C. Outside this range the power output is restricted. Losses resulting from operating restrictions can be estimated according to the following calculation example. Let us assume a continental climate where t ajs above 15 ⁰ C in 1/3 of the year and it is below 7.5 ⁰ C also in 1/3 of the whole year. The share of periods with an air temperature above 30 ⁰ C is 1/11 , and the length of periods above 37.5 ⁰ C can be neglected. The temperature distribution of this exemplary climate is illustrated in a cumulative yearly temperature distribution diagram shown in Fig. 6. Ambient air temperature is shown on the vertical axis. On the horizontal axis the duration of occurrence of given air temperatures is shown in the percentage of the 8760-hour long year. In Table 1 below the condenser temperature differences between systems according to Fig. 1 and Fig. 2 are shown for different air temperature ranges (weighed by the durations of periods when air temperature is in the given range).

Table 1

Using data of Table 1 the mean temperature difference is calculated as [1/3*7,5 + 1/3*7,5/2 + 8/33*0 + 1/11*36 = 7,02 °C] , that is the prior art, restricted system has a temperature disadvantage of 7.02 °C compared to the system according to the invention. This almost equals the theoretical temperature advantage of 7.5 ⁰ C achievable by serial connection of the condensers. To calculate power output reduction due to steam consumption restriction a factor of 10 was applied, because to prevent condenser temperature from increasing further above the exhaust hood temperature limit as air temperature rises the ITD has to be decreased on average by (37,5-30)/2 = 3,75 ⁰ C, which can only be accomplished with a 12 %-steam consumption reduction. In case of a high-efficiency condensing power plant cycle a rule of thumb is that in case of constant steam consumption each 3 0C of condenser temperature rise brings about a power reduction of 1 %, and therefore a steam consumption reduction by 12 % would be equivalent to a condenser temperature rise of 36 ⁰ C (and the same amount of ITD reduction) if steam consumption could be kept constant. Thus, this value (36 ⁰ C) is included in the table under the period weight 1/11 , followed by an exclamation mark. As it is clearly shown by the example, the theoretically promising savings achievable by connecting the direct-contact condensers serially almost diminish to zero without the application of the solution according to the invention. It has to be noted that, because the necessary modifications do not increase

either the total exhaust area or the weight of the steam turbine, no turbine cost increases were taken into account.

A fully legitimate question arises at this point: In case it is possible to produce a larger low pressure casing with the configuration LP^, why should we apply the machine having three low pressure casings 1 , 2, 3 shown in Fig. 1 instead of utilizing a two-exhaust machine applying the larger low pressure casing with the configuration LP^ Such a turbine would be shorter and cheaper to produce than a turbine with three low pressure casings. The real- world situation is, however, that the number of low pressure casings 1 , 2, 3 rises because unit power is rising constantly, and the diameter of exhaust stages cannot be increased above a certain limit due to mechanical, flow mechanical and manufacturing limitations concerning turbine blades. It can thus be maintained that turbines comprising two or three low pressure casings 1 , 2, 3 usually have the largest feasible exhaust stages. This leads to another explanation of why the inventive solution is so advantageous. Let us assume that the casing having the configuration LPg has the largest feasible exhaust stage. In this case, Fig. 1 that if three such casings are connected serially the exhaust of the casing 1 connected to condenser 1A will operate at the choking point temperature of 37.5 ⁰ C. If an ITD = 30 ⁰ C is taken into account, then an ambient air temperature of 15 ⁰ C will correspond to a mean condenser temperature of 45 ⁰ C. Casing 3 connected to condenser 3A has a temperature of 52.5 ⁰ C, which is 15 °C lower than the upper temperature limit of 67.5 ⁰ C . The air temperature range the system can exploit without restrictions is the 15 ⁰ C range between 15 ⁰ C and 30 ⁰ C, which in most cases is insufficient to economically implement a Heller-type cooling system. To prevent such a situation, the low pressure casing 2 connected to condenser 2A of Fig. 1 has to be implemented with a smaller exhaust area, while casing 3 should be implemented with an exhaust area smaller than that of casing 2. In this case, the exploitable air temperature range widens to 30 ⁰ C.

The inventive solution has an additional advantage that becomes apparent contemplating the exhaust loss characteristics of the steam turbine (see Fig. 7). The diagram shows the exhaust loss (δ) characteristics of three low pressure casings 1 , 2, 3, where exhaust areas A1 , A2, A3 of the casings 1 , 2, 3 are in relation A1 > A2 > A3. In a warm period of the year the low pressure casing 3 (having an exhaust area A3 connected to condenser 3A releases a steam volume designated by V3. As it is seen in the diagram, it has a lower exhaust loss (δ i _β ) than it would have if it were implemented with the configuration LP _β having an exhaust area A1 (δ i^ ). The same line of thought may be applied in case of the low pressure casing 2 having an exhaust area A2 < A1 , which casing releases a volumetric steam flow V2 larger than V3 due to its lower temperature. In case this casing had the largest exhaust area A1 it would have an exhaust loss δ i f ^ , but since it has an exhaust area A2

(smaller than A1 ) its exhaust loss δ i f 2 ' ^{s a} ' ^so smaller. Because the exhaust losses δ i^, δ i _β θf both casings 2, 3 have become smaller (with respect to the largest exhaust diameter casing), the power and efficiency of the steam turbine have increased. This line of thought may of course be followed in case of a two-stage serial connection.

The invention may be implemented in a number of different ways. Fig. 3 shows an embodiment applying two-stage serial connection, involving two low pressure casings V, 2\ each of which have two differently sized exhausts

10, 20. In the drawing the reference numerals 10 and 20 designate, respectively, the exhausts of low pressure casings 1', 2' adapted for releasing a smaller and larger volumetric steam throughput, with condensers 1A, 2A being connected to said exhausts 10 and 20. The solution has the advantage that a single such low pressure casing V, 2' is sufficient for implementing a two-stage serial connection of condensers 1A, 2A. Utilizing two such casings

1 ¹ , 2' another two-stage serial connection (shown in the drawing) may be implemented; a three-stage serial connection being also possible with the

addition of a casing 3 comprising exhausts having the same area (not shown in the drawing).

Fig. 4 illustrates an embodiment where a low pressure casing 2 has a single exhaust and is built integral with the medium pressure casing MP. Cooling water first enters the direct-contact condenser 1A of a detached low pressure casing 1 that has two exhausts and is implemented such that it has a lower choking point temperature compared to the choking point temperature of the exhaust of low pressure casing 2 connected to a second condenser 2A. According to what has been stated above, the exhaust of casing 2 is implemented such that it reaches its choking point simultaneously with the exhaust of casing 1 at a temperature of condenser 2A that is higher than the temperature of the condenser connected to casing 1.

Fig. 5 illustrates a solution similar to the embodiment shown in Fig. 4 but comprising a three-stage serial connection of exhausts. Although all of the embodiments described above comprise Heller- type air cooled direct-contact condensers, the inventive solution may of course be successfully implemented utilizing any such type of condensers serially connected at the coolant side that have high seasonal variation, for instance due to changes in ambient air temperature, in the input temperature of the coolant.