1、Research on Thermal and Resistance Performances of Sea Water Cooling TowerTang Yong (唐勇), Liao Neiping (廖內(nèi)平), Ren Jiachi (任家馳)(CPECC Southwest Electric Power Design Institute, Chengdu, 610021) Abstract: This paper describes research on influence mechanism and extent of concentrated sea water on ther

2、mal and resistance performances of natural draft cooling tower by combining practices in sea water circulating supply system for a 2×1000MW project.Key words: thermal and resistance performances of sea water cooling tower1GeneralIn recent years, large-capacity generator units are used for more

3、and more coastal power plant projects, however, under limited conditions for plant construction, especially limit of the sea water environment where power plants locate, it has increasingly become a practical need to adopt circulating water supply system with sea water cooling tower for coastal proj

4、ects having large-capacity generator units. By adopting sea water circulating supply system, adverse effects on water environment caused by large amount of warm water discharge from DC water supply system can be avoided, meeting requirements of environmental protection. Sea water cooling tower has b

5、een researched for over 50 years and applied for over 40 years internationally, especially in the United States, Britain, France, Germany, South Africa, etc. frequently. According to preliminary statistics, sea water cooling tower has been adopted in more than 60 power plants in the world. Research

6、has been conducted on sea water cooling tower since late 1980s domestically and experiments have been performed on application of small industrial tower, while there is no actual project operation performance for large generator units adopting sea water circulating supply system yet. However, with d

7、evelopment and requirements of power construction in our country, there have been large power plants adopting sea water circulating supply system in design and construction, among which, a 2×1000MW project designed by our institute is the first large generator unit adopting sea water circulatin

8、g supply system domestically, for which 13000m2 natural draft sea water cooling towers, the largest in our country, are designed as cooling facility for the power plant. Currently, the two 13000m2 natural draft sea water cooling towers have been completed in the power plant, which has been put into

9、trial production and operation. This paper conducts preliminary discussion on influence mechanism and extent of concentrated sea water on thermal and resistance performances of sea water cooling tower by combining design and relevant experimental research achievements of sea water cooling towers for

10、 the 2×1000MW project.2Theoretical Analysis on Influence of Concentrated Sea Water on Thermal Performance of Cooling TowerSea water, of which the main components include ions such as Na, Mg2+, Ca2+, K+, HCO32-, SO42- , CI , Br-, etc., is a kind of diluted electrolyte solution. That contains the

11、 8 components described above and with content of 3.45, in which content of NaCI take up 2.7%, is called sea water of standard concentration. The research shows that, at the same temperature and pressure, differences in main physical properties of sea water and fresh water are as below:lSea water ha

12、s higher concentration than fresh water; lSea water has lower saturated steam pressure than fresh water; lSea water has lower specific heat than fresh water; lSea water has heat of vaporization approximately equal to fresh water, with little difference. For cooling tower, except physicochemical prop

13、erties of the media, all the cooling processes and cooling mechanisms are basically the same, either in sea water cooling or in fresh water cooling. Researches and practices conducted home and abroad indicate that difference in thermal performance resulted from different media in cooling tower can a

14、lso be analyzed by enthalpy potential method based on Merkel theory. When conducting thermal calculation for the cooling tower by enthalpy potential method, it is deemed that heat absorbed by air in the cooling tower is approximately equal to that radiated from evaporation cooling and contact radiat

15、ion in saturated air layer at water surface of the spray unit in the tower. The contact radiation is in direction proportion to difference between water temperature and air temperature; considering comparison under the same conditions, the temperature difference is the same for fresh water and sea w

16、ater, therefore, capacity of contact radiation is the same for cooling towers with sea water or fresh water as the cooling medium, accordingly, the difference lies in evaporation radiation only. Theoretically, evaporation radiation capacity within unit area and in unit time is in direct proportion t

17、o the steam pressure difference between saturated steam at water surface and in the air. Evaporation radiation capacity dH in unit time is:dH=p(PSat-PAir) dF （2-1）Where: p is the coefficient of radiation medium subject to partial pressure difference. is heat of vaporization of the medium.Qualitative

18、 analysis performed through formula above indicates that sea water has a smaller PSat than fresh water in the cooling tower, thus, within the same cooling area, thermal performance of the cooling tower taking sea water as medium is lower than that taking fresh water as medium. However, as salt conte

19、nt increases, density of sea water increases and saturated steam pressure of the sea water will decrease, thus thermal performance of the sea water cooling tower will reduce further.3Correction for Thermal Performance of Sea Water Cooling TowerTheoretical analysis on thermal calculation method of co

20、oling towerAccording to theoretical analysis in the previous section, we can see that thermal performance of sea water cooling tower decreases gradually as salt content in water increases, and the specific performance decrease shall be researched by combining thermal calculation method of the coolin

21、g tower. At present, enthalpy potential method is used in thermal calculation for most of the cooling towers home and abroad. Based on principle of thermal calculation by enthalpy potential method, total heat radiation includes contact radiation and evaporation radiation in the cooling tower, and th

22、e radiation mechanism and calculation method are the same no matter fresh water or sea water is used as medium. When conducting thermal calculation by enthalpy potential method in case of fresh water as cooling medium, the following expression shall be used: (3-1)Where: enthalpy difference is the co

23、efficient of radiation medium3).K is the coefficient of flow considering heat radiation of evaporable water.Q is the quantity of cooling water (m3/h). is specific heat of water (KJ/kg.). is density of water (kg/ m3). and are enthalpy of air and saturated air layer at water surface respectively (KJ/k

24、g). (3-2) (3-3)Where: -Enthalpy of dry air;-Enthalpy of water vapor;-Specific heat of dry air;-Specific heat of water vapor;-Specific heat of wet air;-Air temperature;P-Atmospheric pressure;Pa-Partial pressure of water vapor in air;Pa-Partial pressure of water vapor in saturated air (the value is th

25、e same as steam pressure of the water at the same temperature with air).When conducting thermal calculation by enthalpy potential method in case of sea water as cooling medium, since the sea water above has different physical properties from fresh water, formula (3-1) can be corrected as below when

26、calculating heat exchange capacity of the sea water cooling tower: （3-4） （3-5）Where: is enthalpy of saturated air layer at sea water surface (KJ/kg); is enthalpy difference between saturated air layer at fresh water surface and saturated air layer at sea water surface at the same temperature (KJ/kg)

27、;CS is specific heat of sea water (KJ/kg.). is density of sea water (kg/ m3).The others are the same as above.If enthalpy of saturated air layer at fresh water surface at the same temperature is applied in formula 3-4, then: （3-6） (3-7)Formula below can be obtained by integrating formula (3-6): (3-8

28、)Where, is a value between and ; sea water flow in kg/h is considered by combining both specific heat and density. In formula (3-8), Ns is the cooling coefficient representing thermal performance of sea water cooling tower; compared with cooling coefficient N of fresh water cooling tower, that of se

29、a water cooling tower has an additional correction coefficient C(t'), then correlation between cooling coefficients of fresh water cooling tower and sea water cooling tower is as below: （3-9）For thermal calculation of cooling tower, correction coefficient of thermal performance at tm, average te

30、mperature of the water going into and coming out of the tower can be used for substitution approximately. Formula (3-9) is as below: （3-10）3.2Theoretical calculation method for correction of thermal performance of sea water cooling towerAnalyzing according to formula (3-7) and theories above, if dif

31、ference Di" between saturated enthalpy of sea water and that of fresh water is known, then difference in thermal performance of sea water and fresh water cooling towers can be calculated. Di" can be calculated after is " and i" are calculated according to formula (3-3), and parti

32、al pressure of saturated steam of sea water in the formula can be obtained according to Raoult law. According to Raoult law, “at a certain temperature, steam pressure drop of non-volatile non-electrolyte diluted solution is in direct proportion to mole fraction of the solute, or steam pressure of th

33、e diluted solution is equal to product of steam pressure of pure solvent and mole fraction of the solvent.” Saturated steam pressure of sea water can be expressed as below:ps " = pn " · Xn (3-11) Xn + Xs = 1(3-12) Where: p " s -Steam pressure of sea water (saturated steam pressur

34、e); pn " -Saturated steam pressure of fresh water; Xn Mole fraction of water in sea water; Xn Mole fraction of salt in sea water.Different salt contents and saturated steam pressures of sea water at different temperatures can be calculated based on Raoult law. Taking sea water with salt content

35、 of 875087500ppm as an example, see Table 3-1 for difference in saturated steam pressure at water surface and in fresh water.Table 3-1Comparison in Steam Pressures of Sea Water with Different Salt Contents (at 35, standard atmosphere pressure) Salt Content (mg/kg) Mole Number of WaterMole Number of

36、SaltMole Fraction of WaterMole Fraction of SaltSteam Pressure of Fresh Water (Pa) Steam Pressure of Sea Water (Pa) Difference (Pa) 8750 55.07 0.28 99.49 0.51 5623 5594 29 17500 54.58 0.56 98.98 1.02 5623 5565 58 35000 53.61 1.13 97.94 2.06 5623 5507 116 52500 52.64 1.69 96.88 3.12 5623 5448 175 7000

37、0 51.67 2.26 95.81 4.19 5623 5388 235 87500 50.69 2.82 94.73 5.27 5624 5327 297 When partial pressures of saturated sea water steam and of saturated fresh water steam are obtained according to formula (3-11), is", i" and Di" can be calculated; see Fig. 3-1 for decrease in saturated en

38、thalpy of sea water (relative to fresh water) with different salt contents at different temperatures.Salt water (salt content 105000ppm)Salt water (salt content 87500ppm)Salt water (salt content 70000ppm)Salt water (salt content 52500ppm)Salt water (salt content 35000ppm)Temperature (C)Enthalpy decr

39、ease (%)Fig. 3-1Decrease in Saturated Enthalpy of Sea Water (Relative to Fresh Water) with Different Salt Contents at Different TemperaturesWith saturated enthalpies of sea water and fresh water, correction coefficient of thermal performance of sea water cooling tower relative to fresh water cooling

40、 tower can be calculated.Estimation of thermal performance of sea water cooling tower for the 2×1000MW projectTotal salt content (including sodium, calcium and magnesium salt) in makeup sea water for sea water cooling tower in the project is 20079ppm, in which content of sodium chloride is 1716

41、7ppm and pH=7.3. For sea water cooling tower in the project, sea water density, specific heat and conductivity coefficient at different concentration ratios at temperature between 3040 are listed in Table 3-2.Table 3-2Properties of Circulating Water at Different Concentration Ratios of the 2×10

42、00MW ProjectTemperature20 30 40 Water QualityDensity (kg/m3) 998.00 996.00 993.00 Fresh waterSpecific heat (J/g.) 4.18 4.18 4.18 Conductivity coefficient0.36 0.37 0.38 Density (kg/m3) 1013.30 1010.79 1007.56 Sea waterSpecific heat (J/g.) 4.12 4.12 4.12 Conductivity coefficient0.36 0.37 0.37 Density

43、(kg/m3) 1020.95 1018.28 1014.95 Specific heat (J/g.) 4.10 4.10 4.10 Conductivity coefficient0.36 0.36 0.37 Density (kg/m3) 1055.58 1052.62 1048.74 Specific heat (J/g.) 3.83 3.83 3.83 Conductivity coefficient0.35 0.36 0.36 Density (kg/m3) 1063.31 1060.29 1056.28 Specific heat (J/g.) 3.79 3.79 3.79 Co

44、nductivity coefficient0.35 0.35 0.36 in summer and relative humidity is 71%. According to calculation, correction coefficients of thermal performance decrease of sea water cooling tower at different concentration ratios are listed in Table 3-3.Table 3-3Estimation of Thermal Performance Decrease of S

45、ea Water Cooling Tower for the 2×1000MW ProjectConcentration ratio of sea water1.0 2.0 3.0 Correction coefficient0.969 0.937 0.905 The table above provides decrease range of thermal performance of sea water cooling tower for the project relative to fresh water cooling tower, based on which prel

46、iminary thermal calculation can be conducted for sea water cooling tower. Thermal performance of cooling tower is a complicated parameter which involves influences in several aspects; therefore, correct thermal performance of sea water cooling tower shall be determined by combining experiments on in

47、door simulative tower packing.Indoor simulative tower experimentSince it is the first time in our country to adopt large generator units with sea water circulating supply system in this project, thermal performance of the sea water cooling tower has direct effects on safe and reliable operation of t

48、he power plants, for this reason, indoor simulative tower experiments have been conducted for this project to verify theoretical calculation results. In the experiments, bi-direction wave and bi-oblique wave packing are selected according to quality of sea water makeup (prepared manually) in the pro

49、ject, and experiments with combination of 1.0m and 1.5 high packing with 5 different concentration ratios of salt water are performed. The experiment results indicate that coefficient in thermal performance expression of packing decreases as concentration ratio of sea water increases, and ratio of a

50、ir to water is independent of sea water density. Taking cooling coefficient of fresh water packing as reference, by fitting the experiment results, relation curve (curve in Fig. 3-2) of correction coefficient of thermal performance of sea water cooling tower at different concentration ratios with co

51、ncentration ratio can be obtained; and see data points in Fig. 3-2 for thermal correction coefficient at different concentration ratios obtained by theoretical calculation according to formula (3-10). The figure shows that the theoretical calculation results are well fitted with the experiment resul

52、ts.Concentration ratio of sea waterCorrection coefficient of thermal performanceFig. 3-2Comparison between Theoretical Calculation and Experiment Results4Influence of Concentrated Sea Water on Resistance Performance of Cooling Tower PackingBy research on influence of sea water with different salt co

53、ntents on resistance performance of the packing through indoor simulative tower, the experiment results indicate that, resistance of packing changes little as salinity increases, indicating that salinity of sea water has no obvious influence on resistance of the packing, which is related to type and

54、 height of the packing only. See Fig. 4-1 for resistance change of packing with salinity change of sea water at the same spray density (10m3/m2.h) and the same wind speed (/s).BBSalinity ()BBResistanceFig. 4-1Influence of Sea Water Salinity on Packing Resistance5ConclusionDomestic application resear

55、ch on large sea water cooling tower has been conducted from basic research and experimental research stages to industrial application and engineering practice stages. Existing research achievements indicate that: (1)At the same temperature and pressure, main differences in physical properties between sea water and fresh water lie in: sea water has higher density than fresh water; sea water has lower saturated steam pressure than fresh water; sea water has lower specific heat than fresh water; and sea water