feasibility study and optimization of cooling towers

International Conference

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Feasibility Study and Optimization of Cooling Towers for New Nuclear Power Plant at the Krko Site

Ale Buri, Robert Bergant GEN energija, d.o.o.

Cesta 4. julija 42 SI-8270 Krko, Slovenija

[email protected], [email protected]

ABSTRACT

Since the cooling capability of Sava river is already exploited due to the once through cooling of existing nuclear power plant, cooling towers as a closed third cycle are predicted for the new nuclear power plant which will be located next to the existing one. Therefore, feasibility of different cooling tower (CT) designs and their adequacy to meet the plant and environmental parameters were examined. At two generator output power levels 1200 MWe and 1700 MWe, three cooling tower designs representing three basic cooling air philosophies forced-, induced- and natural draught were applied. The feasibility evaluation for cooling towers analyzed two separate nuclear power plant systems: Circulating Water system (CWS) and Service Water system (SWS).

1 INTRODUCTION

When selecting a wet cooling tower, there are many factors to consider. The weighting for the selection criteria for a cooling tower application are always dependant on the local conditions and preferences of the end user. The considered factors at selecting appropriate wet cooling tower might include:

Technical factors: cooling tower performance over a range of ambient conditions, flexibility of operation and maintenance of desired water temperatures, space requirement including provision for air inlets and shape;

Economic factors: capital cost, operating costs, lifetime cost and reliability; Environmental factors: noise, plume generation, visual impact.

In accordance with NPP Krko II pre-preparation phase studies, the use of the following cooling tower designs for two generator output power levels, 1200 MWe and 1700 MWe, were examined:

Natural draught, Cell-type (Multi-cell) and Round forced draught hybrid.

There are of course other designs such as Fan Assisted Natural Draught (FAND) and Hybrid Cell-type. These three were chosen because each is indicative of one of the three basic wet cooling tower air philosophies (forced-, induced- and natural draught). For example the layout of a FAND tower is similar to the Round Forced Draught Hybrid. The natural draught cooling towers have been selected as a single unit where possible, with an alternative where appropriate for two or more smaller units being limited in height to 150 m.

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Material of construction for all types is expected to be reinforced concrete; however, it is also possible to offer the multi-cell induced draught models in FRP (fiber-reinforced plastic) material.

The heat loads for the SWS applications are too low for the use of natural draught or round forced draught hybrid cooling towers. For these applications only cell-type is considered.

2 COOLING TOWER APPLICATIONS AND EFFECT OF TEMPERATURE

At cooling tower applications there are some terms frequently used and therefore some explanation is needed. Wet bulb air temperature is the minimum temperature which may be achieved by purely evaporative cooling of a water-wetted, ventilated surface. Dry bulb air temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture; Range indicates the difference between the hot and cold circulating water temperatures, while approach indicates the difference between the wet bulb and cold water temperatures. Heat load is the amount of the heat that the cooling tower is required to dissipate.

The ambient conditions at the design point for cooling tower were defined in order to meet the required thermal performance. For this study following main ambient conditions with respect to local psychrometric measurements and experience with recent installed cooling cells at existing power plant NEK were selected: dry bulb air temperature 26.0 C and wet bulb air temperature 23.6 C at 76.9 % relative humidity (r.H.) [1]. At other ambient conditions the cooling tower performance will vary in a predictable manner. These variations are presented with cooling performance characteristic diagrams. Each of cooling tower types and applications have their own cooling performance characteristic with respect to ambient wet bulb temperature. Cooling performance characteristic diagrams were calculated by SPX Cooling Technologies GmbH with dedicated software compliant with Acceptance Test Codes CTI ATC-105 and ATC-105S.

Tables 1, 2 and 3 are representing respective cooling solutions for two generator output power levels, i.e. 1200 MWe and 1700 MWe. Footprint differences at the same power level emerging mainly from selected different process parameters in CWS by different power plant suppliers: condenser back pressure, circulating water flow and respective inlet/outlet and condenser saturated temperatures. Differences at these parameters are resulting different power efficiency and accordingly different dimensions of cooling installation. For instance; lower condenser saturated temperature means larger useful work on the one hand (thus increasing power generation), but gives lower approach temperature moving cooling tower outlet temperature or condenser inlet temperature closer to design air wet bulb temperature and thus requires larger cooling tower installation on the other hand. The optimization of the design is not straightforward; therefore some sort of compromise between two diametrical parameters should be done. Based on the results and dimensions of CTs from Tables 1, 2 and 3, it is notable, in some cases additional preliminary cold-end optimization has to be carried out by plant designers in order to reduce disproportion of CTs between larger and lower generator output power levels.

2.1 Natural Draught (CWS)

Natural draught cooling towers (NDCT) shown in Fig. 1 rely on the difference in air density between ambient air and the warm air leaving the cooling tower to induce a flow of air through the cooling fill and thereby cool the circulating water flow. The buoyancy of air

Proceedings of the International Conference Nuclear Energy for New Europe, Bled, Slovenia, Sept. 14-17, 2009

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leaving the cooling tower is enhanced during times of low ambient temperature and this result in an acceleration of cooling efficiency as the ambient temperature falls. The flow of air through the cooling tower varies according to the prevailing ambient conditions. Water is either delivered to the cooling tower through one or more external riser pipes, or in some cases through an internal central concrete riser duct, before being distributed over a network of plastic pipes with spray nozzles. These nozzles ensure an even distribution of water over the entire cooling fill surface.

Figure 1: Natural Draught Cooling Tower

Table 1: Natural Draught Cooling Tower dimensions NDCT

solutions per NPP

Power range [Mwe]

N of cooling towers

Total Height,

rHges [m]

Eff. Height, Hw [m]

Air Inlet Height, Hle [m]

Process Diameter,

Dr [m]

Outlet Diameter,

Da [m] GEN01 1 170 159,3 10,7 118 73 GEN02 2 130 122,5 7,5 89,3 55 GEN05 2 190 179 11 125,7 78 GEN06

1700

3 150 139 11 106,9 67 GEN10 1 175 164 11 119,5 75 GEN11 2 134 126 8 90,3 56 GEN14 2 188 177 11 124,3 78 GEN15

1200

3 155 145 10 105,6 66

Figure 2: Performance characteristic curve for Natural Draught Cooling Tower at

temperature range 14 K with water flow rate 181170 t/h


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Cooling performance characteristic diagrams show the hot water temperature versus wet bulb temperature for a selection of relative humidities (40%, 60%, 80% as different design points and 90% as an extra one). Nine cooling performance characteristic diagrams are generated for each NDCT cooling application. For each of three different cooling ranges, three circulating water flow rates are considered, similar as shown in Fig. 2 as an representative example.

2.2 Multi-cell Induced Draught (CWS)

In this type of cooling tower, shown in Fig. 3, fan machinery located on the roof deck of the tower structure is utilized to deliver a constant volume of air through the cooling fill as presented in Fig. 4. The cooling performance, shown in Fig. 5, still improves in response to lower wet bulb temperature; however as result of the air flow being constant during summer and winter, the cooling characteristic tends to be more resistant to temperature change than in natural draught cooling. Although the volume of air delivered by the induced draught fans remains constant throughout the year, the mass air flow increases as a function of exit air density. This means that during colder ambient conditions the absorbed fan power rises, and conversely during warm ambient conditions the fan power falls, within a range of 10% of the nominal design point. Water is again delivered to the cooling towers through external riser pipes (1 per cell), or in the case of back to back blocks (as in the CW layouts of this study) 1 per 2 cells.

Table 2: Multi-cell dimensions Multi-cell Induced Draught

Power range [Mwe]

N of

cells

Total cell length, l x

w [m] GEN03 38 313 x 33 GEN07

1700 62 510 x 33

GEN12 36 297 x 33 GEN16

1200 56 461 x 33

Base cell dimensions: 16,5m x16,5m x20m Figure 3: Multi-cell Induced Draught CT

Figure 4: Cell-type Operation Principle


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Figure 5: Performance characteristic curve for Multi-cell Induced Draught Cooling

Tower at 100% Fan Power with water flow rate 212731 t/h

2.3 Circular Fan Assisted Hybrid (CWS)

Fig. 6 shows the principle behind hybrid cooling towers in a Mollier diagram. The principle of operation is based on maintaining (up to the plume-free point) the exiting air (M) above the area of oversaturation. Warm water arriving from the process is first delivered to the dry section of the cooling tower, arranged vertically around the periphery of the upper floor. After passing through these finned tube heat exchangers, water is then delivered to the wet section distribution system mounted below, where it is then sprayed over the entire cooling fill area.

Figure 6: Hybrid Operation Principle


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Figure 7: Circular Fan Assisted Hybrid

Table 3: Circular Fan Assisted Hybrid dimensions Round Forced

Draught Hybrid

Power range [Mwe]

N of cooling towers

Total Height,

Hges [m]

Outside Diameter, Dges [m]

Process Diameter,

Dr [m]

Number of Fans Wet /

Dry

GEN04 1 55 161 121 44 / 44 GEN08 1 55 188 144 56 / 56 GEN09

1700 2 55 143 103 32 / 32

GEN13 1 55 159 119 44 / 44 GEN17

1200 1 55 178 138 52 / 52

Figure 8: Performance characteristic curve for Circular Fan Assisted Hybrid at 100%

Fan Power and 100% water flow rate

Such plume abated cooling towers have two separate streams of process air, one for the dry section and one for the wet section, as shown in Fig. 7. In the case of circular hybrid tower designs, each section is equipped with forced draught axial flow fans arranged in two


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levels around the periphery of the tower. The dry section fans deliver a warm stream of dry air that mixes with saturated air leaving the wet section.

A series of mixer ducts then ensures the wet and dry air streams are thoroughly mixed such that the air leaving the cooling tower does not produce a visible plume under pre-agreed ambient conditions and plant load. For the hybrid cooling tower as an input a typical plume-free point 5 C at 90% relative humidity was selected. This is the ambient condition up to which the hybrid cooling tower has practically no visible plume. Below this point the plume is abated (i.e. reduced) but clearly visible.

Since the air flows are delivered mechanically, the performance characteristic of this cooling tower type, shown in Fig. 8, is very similar to the induced draught Multi-cell type described above.

2.4 Cooling performance characteristic curves - conclusions

Cooling performance characteristic curves in general show dependence of cooling performance according to ambient conditions. Wet Bulb Temperature changes result in cooling performance changes; with temperature rise, cooled water in CWS exiting cooling tower becomes warmer, consequentially decreasing cooling performance in condenser and thus lowering plant efficiency.

Natural draught cooling towers are most dependent upon ambient conditions. This is because at lower ambient temperatures the air density difference between the ambient air and the warm air in the cooling tower is larger so the natural draught effect is increased. Due to this effect NDCTs become more efficient at lower temperatures (and less efficient at higher temperatures). The negative aspect of this is that these towers are more prone to icing than the other types.

For the forced draught towers (Circular hybrid) and the induced draught towers (Multicell-type) the difference is smaller because the air volumes entering the towers remain constant. There is a slight difference because the fans of the induced draught towers pull warmed air through tower, whereas the forced draught hybrid towers force cold ambient air into tower. It causes the Multicell-type towers to be the least effected by cold ambient air because the cold air is denser and hence the hybrid tower fans force a greater mass of air into the tower at cold ambient conditions.

3 ECONOMIC FACTORS

Figure 9: Capital costs of different CT designs


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The economic factors determining the choice of cooling tower depends on the interplay of capital and running costs. Fig. 9 represents capital costs of different CT designs at different applications in two specified power ranges. Specific application power ranges and CT designs are described under appropriate designations (e.g. GEN01) in Tables 1, 2 and 3.

The cost of maintenance, as one of important contributors to running costs, is however difficult to assess in this phase. For all types the cooling fill is equally subjected to wear and tear. The additional mechanical components of the fan assisted towers (particularly the hybrids) are a source of significant maintenance expense. During operation, power consumption, as a sum of pump power and fan power, has a significant impact on running costs and those are connected with CT design. Disposal costs reflect the quantity of concrete used at construction and the difficulty of demolishing the tower after operation lifetime. In Table 4 economic factors are evaluated over selected CT designs.

Table 4: Economic factors of cooling towers Natural Draught Circular Hybrid Multi-cell Capital costs ++ + ++ Maintenance +++ + ++ Power consumed +++ + ++ Disposal costs + ++ +++ +++ Best ++ Mid + Worst

For the Circular hybrid CT the proportion of the dry section electrical power required to maintain plume-free operation to 5C, 90% r.H. was estimated, followed by plume-abated operation if temperature sinks and/or relative humidity rises. Investigation indicates a year round average power consumption of 41% of total installed dry section fan power for 1996 and 33% for 2000.

Figure 10: Representative data of Fan Power variations during the day (based on average

hourly ambient data, 1996)

Figure 11: Representative data of Fan Power variations during the day (based on average

hourly ambient data, 2000)


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The year 1996 was taken as the average coldest year and 2000 as hottest year in last 20 year observation period. The figures of average power consumption presented in Fig. 10 and 11, by their nature, are independent of the application, being based on the environmental data supplied for these years [3] and a modelling of the fan power requirement to counteract air saturation. Figures 10 and 11 are representing calculations for two months period, but average data were calculated for all 12 months of plume abated operation. It can be seen that warm weather conditions require less power for plume abated operation compared to cold weather conditions.

Comparison between combining the installed power for dry fan section and wet fan section at Circular Fan Assisted Hybrid and installed power for multi-cell cooling towers is shown on Figure 12. Energy consumption at hybrid CT is varying according to ambiental conditions. Thus a year round average power consumption for plume-abated operation (Figures 10 and 11) could be compared to multi-cell power consumption, especially at GEN04 to GEN03 (1700 Mwe) and GEN13 to GEN12 (1200 Mwe) variants.

Figure 12: Installed Fan power for Circular Fan Assisted Hybrid and

multi-cell cooling towers

4 ENVIRONMENTAL IMPACTS

The environmental impacts of each cooling tower types are summarized in the Table 5. When assessing the environmental impacts of footprint the round CT is more space efficient due to the recirculation effects suffered by the Multi-cell layout. Visual Impact of height and plume at the NDCT makes it the most exposed one. The Multi-cell has smallest visual impact, because it is low and its plume does not reach high elevations. Impact of noise at NDCT is most acceptable, because it does not require fans for operation, since Multi-cell CT has one fan for each cell whereas Circular Hybrid is equipped with two sections of fans. For any given degree of sound attenuation there, noise level will correspond to the number of fans installed at each design. Assessing Plume impact on environment leads to conclusion that the Multi-cells have the biggest impact, because the plume is emitted at a low height. The Circular hybrid is plume abated, therefore its plume impact on environment is negligible. Drift Losses due to water droplets escape in the cooling tower discharge are the smallest at Circular hybrid CT. Circular hybrid has high performance drift eliminators in order to improve the plume abatement performance and at the same time the best drift eliminating performance.


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Table 5: Environmental Impacts of cooling towers Natural Draught Circular Hybrid Multi-cell Footprint/Size +++ +++ + Visual impact + ++ +++ Noise level +++ + ++ Plume ++ +++ + Drift losses ++ +++ ++ +++ Best ++ Mid + Worst

5 CONCLUSIONS

In a comparison, represented in this paper, where the design ambient data are the same for the applications, the main influencers are:

Range For a given heat load the range is inversely proportional to the water flow rate, i.e. the design can be a large range and a low flow rate or vice-versa. A larger range reduces the size of cooling tower required (whichever type is chosen) but increases the size of the condenser, assuming the steam pressure is constant. The optimal split between the cooling tower and condenser sizing requires a cold-end optimisation to be carried out.

Approach As the required approach decreases, the size of cooling tower increases intensively. The approach of 3 K is technically the lowest acceptable value.

Heat load changes in power, e.g. load follow operation of plant, results in changes of heat that the cooling tower is required to dissipate.

Effect of temperature variation on different cooling tower types at ambient conditions away from the design point the different types of cooling towers produce different circulating water temperatures. This effect can be seen on the Performance Curves in Section 2.

Above presented effects influence the average annual power produced. They also determine the sizing required for a specific type of cooling tower. All determining factors for the type of cooling tower must reflect local considerations, including Space available, Height restrictions and Acceptability of plume.

At the economic criteria capital cost versus running costs should be considered in terms of a project time frame. With all costs amortized over this period a comparison can be made. However, economically quantifying the value of plume-free operation is not objectively possible here the value is in terms of the project acceptability.

For the three types of cooling towers considered in this paper the following conclusions can be drawn: Natural Draught Cooling Towers use the least parasitic energy. They require the least maintenance because they have no major moving parts, use space efficiently and emit plume at a high height; Multi-cell Induced Draught Cooling Towers can be installed quickly, they suffer from recirculation, require a lot of space and use a significant amount of energy; Circular Hybrid Cooling Towers produce no plume up to certain ambient conditions and thereafter abated plume, require extensive noise abatement measures, are most expensive among considered cooling options, use an enormous amount of energy while their maintenance is intensive.

This study clearly showed all cooling approaches are feasible. Decision in selecting appropriate cooling option among presented ones and further balancing other process parameters (cold end optimization) is indeed in hands of the owner and investor, normally trying to drive optimization to the optimum, gaining most efficiency with respect to investment cost and return rates leaving least impact on environment and ensuring best local community acceptance.


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REFERENCES

[1] SPX GmbH, S. Phillips et al, Feasibility Study and optimization Of Cooling Towers For The New Nuclear Power Plant at the Krko Site (NPP Krko II), Ratingen, Germany, 2009

[2] DMP-477-CT-L: CT cooling tower system extension, 2008

[3] ARSO, Meteorological data from AMP NEK for years 1996 and 2000, Ljubljana, Oct. 2008

[4] Black & Veatch, Drbal, Lawrence F., POWER plant engineering, Chapman & Hall; Springer Science+Business Media, New York, 1996

[5] Robert C. Rosaler, Standard Handbook of Plant Engineering; Ch 3.4, McGraw-Hill, New York, 2004


1 INTRODUCTION 2 COOLING TOWER APPLICATIONS AND EFFECT OF TEMPERATURE 2.1 Natural Draught (CWS) 2.2 Multi-cell Induced Draught (CWS) 2.3 Circular Fan Assisted Hybrid (CWS) 2.4 Cooling performance characteristic curves - conclusions 3 ECONOMIC FACTORS 4 ENVIRONMENTAL IMPACTS 5 CONCLUSIONS

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