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Research of the sediment formation intensity at the run-around cooling systems equipment with water cooling towers.

UDC 621.182.11.001.57

Introduction. For circulating cooling systems, the solid mineral sediments formed as a result of crystallization of sparingly soluble inorganic salts in the transition of circulating water in the supersaturated state are the most dangerous. However, a visual inspection of water cooling towers and condensers in Ukrainian power plants shows significant sediments and drifts at the heat exchange surfaces of cooled condensers as well as at the evaporator surfaces of water cooling towers.

Many authors paid attention to the study of the formation sediment process at the heat exchange tubes. Most authors examined the sediments in once-through systems, exploring the water passing through the heat exchanger once [1 ... 5].

Briancon [3] and Andritsos [6] considered mainly the dependence of kinetics and mechanisms of sediments, depending on flow multiplicity of the heat exchange tubes surface, ribbing, and the recycling of the circulating water. Also, Andritsos shows that during formation of sediments on rough surfaces the turbulent transport of particles to the heat surface increases and thus decreases the induction period of the process.

Kishnevsky and Chichenin [7] studied the sediments on heat exchange surfaces by using scale inhibitors and without them.

Sediment formation is a complex process involving water supersaturating for salts with nucleation, their transport to the heat exchange surface with the possible sediment. To date, there is no reliable theory of thermodynamic prediction of carbonate salts sediments process on the heat exchange surface. Therefore, most of the known studies based on observational criteria. They allow calculate the probability of formation of sediment on heat exchange surfaces of condensers, but do not consider other types of heat exchangers [8, 9].

Practically in all previously published studies in the preparation of the material and salt balance of circulating cooling systems, it was assumed that most of the sediments are crystallized at the heat exchange tubes of condensers

In the works of Kishnevskiy [10] and Garrels [11] there is a studying method of water and chemical mode of circulating cooling systems at the scale model, as well as the possible criterion of sediment formation ([[DELTA].sub.Ca-Alk]) is offered, it is assumed that all the solid phase is deposited on the heat exchange surface condencer.

Chichenin [12] proposed a method of the study of the intensity sediments at the coupons without heating the coolant made from the materials typical for heat exchangers in which the heating of coolant is present. At the same time overlooked the processes occurring in the water cooling tower.

In the literature and in the open press author does not found researches aimed at understanding the mechanisms of calcium carbonate crystals sediments on the cooling surface of the water cooling towers and the pipeline surfaces of the run-around cooling systems.

Drifts and sediments at the evaporator surface reduce the operation efficiency of the cooling towers, and, moreover, with significant sediments threaten by major accidents related to the collapse of the irrigation surface.

Thus, study the intensity of sediments at the evaporation part of the water cooling tower and the distribution ratio of sediments mass between the condenser surface and cooling tower surfaces is actually.

The aim of this research is to study the intensity of sediments of hardly soluble salts crystals on the surfaces of the heating of evaporative cooling equipment and condensers from supersaturated circulating water in the circulating cooling systems of power plants with water cooling towers.

To achieve the aim is needed solve the following problems:

--Improve the methodology of research of intensity of sparingly soluble salts sediments on the surface of the cooling tower and evaporative heating surface of condenser.

--Improve the large scale stand of circulation cooling system.

--Carry out the experimental studies at the laboratory stand of large scale cooling system.

Materials and Methods. Formation of carbonate calcium CaC[O.sub.3] sediments in the cooling tower and the condenser takes place in different thermal and hydraulic conditions:

--in a condenser--at heating of circulating water with the release of CaC[O.sub.3] on the heated surface;

--in a water cooling tower--during cooling water by increasing the salt content on unheated surface.

Formation of crystallization centers in the circulation water during evaporative cooling in water tower occurs at the "liquid / water-steam mixture" interface.

Allocation of crystallization centers of sparingly soluble carbonate salts during heating of circulating water in the heat exchange tubes of the condenser is due to the thermal decomposition of readily soluble bicarbonate salts with the formation of dense and poorly soluble carbonate salts, and their subsequent sediments onto the heated surface of the tubes.

In the process of circulating water cooling in the cooling tower during its run-off along the cooling surface and aeration with counter vapor flow (saturation with C[O.sub.2] as result) flow salinity and mass of carbonate salts crystals are increasing. This eventually leads to the formation of loose sediments in the unheated watering surfaces of cooling towers.

The methodology of the research was to study the mass and size of sparingly soluble crystals in circulation water and sediments on heat exchange surfaces of the condenser and the evaporator surface of the water cooling tower. Experimental studies were carried out on a large-scale installation of the run-around cooling systems model (Fig. 1).

Detailed description of the stand is given in [7]. The design of water cooling tower and working cell is shown in [12].

The procedure of the experiment was as follows.

Previously prepared according the procedure described in [12], LAMsh brass tubes installed in the working cell 1 (Fig. 1).

Moreover, the flat elements (coupons) of 70x10x2 mm size which are made from LAMsh 77-2-0.05 brass material added to the water cooling tower. These elements are designed to compare the growth of sediments in the cooling tower and the condenser on the same materials. Coupons were mounted in the water cooling tower directly under the stream of circulating water dispersed by the nozzle, above the "crossed wheels" nozzle and parallel to air flow.

To control the sediments on the evaporator of the water cooling tower from the nozzle layer were selected and marked 12 elements, connected by a fishing-line.

In operation, the experiment was interrupted after 40 and 100 hours for extraction of nozzle control elements and coupons. Extracted from the installation control elements were placed in a drying oven until dry, then the check weighing held. Detailed sample processing method described in [12].

Control of deposits on the tubes, coupons and cross-disks conducted as weight gain. For elements made of LAMsh, it takes into account the effect of minor corrosion. Kinetics of growth of sediments was investigated in the condenser tubes, coupons and nozzles in a cooling tower.

Characteristics and composition of the source water taken from river Styr the following: pH = 7.4, evaporation rate [K.sub.e] = 1, water alkalinity [A.sub.w] = 1 mgEq/d[m.sup.3]; water hardness [H.sub.w] = 1.9 mgEq/d[m.sup.3]; [[Cl.sup.-]] = 0.7 mgEq/[dm.sup.3], [S[O.sub.4.sup.2-]] = 0.95 mgEq/[dm.sup.3].

In the pilot study the evaporation rate of the circulating water was maintained at 2, and pH = 8.

Every 12 hours there was sampled for the study of physical and chemical composition of water and the amount of sediments on the controlled areas of the experimental installation at given water velocity and air flow. Humidity, temperature of atmosphere air and air at the outlet of the cooling tower controlled using psychometric analysis.

To study the nature of the interaction processes between the particles of the solid phase in source and circulating water we used the analysis-of-variance method for coarse phase particles in optically scanned water layer, described in detail in [13].

Results. The obtained experimental data for determining the mass of sediment on the coupons, tubes and crossed disks are shown in Tables 1 and 2.

Visual inspection shows that the sediments in the water cooling towers are loose; moreover, the adhesion between the sediments and the heat exchange surfaces is flimsy.

Dependences of the intensity of sediments on the heating surfaces of the condenser, nozzles and control coupons considering the density of carbonate salts are shown in Fig. 2.

The intensity of sediments on the heated heat exchanger surface of the condenser tubes was 0.75 mg/([m.sup.2] x h) after 40 hours after the start of the experiment, and 0.30 mg/([m.sup.2] x h)--after 200 hours. The intensity of sediments on the unheated surface in the water cooling tower for LAMsh brass coupons was 0.30 and 0.10 mg/([m.sup.2] x h), respectively.

Thus, the intensity of sediment in the condenser is different from the intensity in the water cooling tower in more than two times, provided that they are made of the same material (LAMsh).

The intensity of sediments on the unheated surface in the water cooling tower on the polyethylene nozzles was 0.170 mg/([m.sup.2] x h) at the beginning of the experiment, and 0.07 mg/([m.sup.2] x h) at the end. This is a few less than for LAMsh brass coupons, but the difference is not so significant.

Conclusions. The conducted research lead to the conclusion that the processes of formation of carbonate salt sediments in the water cooling tower and in the condenser determined by the heat hydraulic conditions.

The obtained experimental data indicate that the intensity of sediments on the heat exchanger surface of the condenser tubes is more than two times higher than on the control coupons of irrigated surface of the water cooling towers made of LAMsh brass. Furthermore, it is shown that the intensity of sediments on unheated surface of LAMsh brass coupons is higher than on the polyethylene nozzles by 30 ... 40 %.

DOI 10.15276/opu.2.49.2016.09

References

[1.] Guliayenko, A.B., Kishnevsky, E.V., Maleenovsky, O.M., & Ochkov, V.F. (2010). Research of dispersion composition and characteristics of solid-phase particles in circulation water of the circulating systems of cooling. Odes'kyi Politechnichnyi Universytet. Pratsi, 1-2, 70-75.

[2.] Kishnevsky, V.A., Borovsky, B.N., & Shukaylo, B.N. (2005). Prevention of corrosion in steam air heaters and their condensate pipes. Odes'kyi Politechnichnyi Universytet. Pratsi, 2, 90-95.

[3.] Nevedrov, A.V., & Ushakov, G.V. (2003). A comparative analysis of physical methods for water treatment to reduce scale formation. Thermal Engineering, 50(11), 944-947.

[4.] Andritsos, N., Kontopoulou, M., Karabelas, A.J., & Koutsoukos, P.G. (1996). Calcium carbonate deposit formation under isothermal conditions. The Canadian Journal of Chemical Engineering, 74(6), 911-919. DOI:10.1002/cjce.5450740614

[5.] Kazi, S.N., Duffy, G.G., & Chen, X.D. (2012). Fouling and fouling mitigation on heated metal surfaces. Desalination, 288, 126-134. DOI:10.1016/j.desal.2011.12.022

[6.] Andritsos, N., Karabelas, A.J., & Koutsoukos, P.G. (1997). Morphology and structure of CaC[O.sub.3] scale layers formed under isothermal flow conditions. Langmuir, 13(10), 2873-2879. DOI:10.1021/la960960s

[7.] Briangon, S., Colson, D., & Klein, J.P. (1998). Modelling of crystalline layer growth using kinetic data obtained from suspension crystallization. Chemical Engineering Journal, 70(1), 55-64. DOI:10.1016/S1385-8947(98)00080-1

[8.] Kostoglou, M., & Karabelas, A.J. (1998). Comprehensive modeling of precipitation and fouling in turbulent pipe flow. Industrial & Engineering Chemistry Research, 37(4), 1536-1550. DOI: 10.1021/ie970559g

[9.] Karabelas, A.J. (2002). Scale formation in tubular heat exchangers--research priorities. International Journal of Thermal Sciences, 41(7), 682-692. DOI:10.1016/S1290-0729(02)01363-7

[10.] Kishnevsky, V., & Chichenin, V. (2014). Study of carbonate deposits on heat exchange surfaces of condensers. Eastern-European Journal of Enterprise Technologies, 3(8), 52-58.

[11.] Garrels, R.M., & Christ, C.L. (1990). Solutions, Minerals, and Equilibria. Boston: Jones and Bartlett.

[12.] Chichenin, V., Kishnevskiy, V., Hrytsaienko, A., Ahrameev, V., & Shuliak, I. (2015). Study of corrosion rate and accumulation of deposits under circulating water concentration in bench experiments. Eastern-European Journal of Enterprise Technologies, 5(8), 14-20. DOI:10.15587/17294061.2015.51205

[13.] Kishnevskiy, V., Chichenin, V., & Shulyak, I. (2013). Method of calculation of water chemistry of the integrated circulation cooling system with recirculation. Eastern-European Journal of Enterprise Technologies, 6(8), 10-14.

[TEXT NOT REPRODUCIBLE IN ASCII]

[1.] [TEXT NOT REPRODUCIBLE IN ASCII].--2010.--BNII. 1(33)--2(34).--C. 70-75.

[2.] [TEXT NOT REPRODUCIBLE IN ASCII].--2005.--BNII. 2(24).--?. 90-95.

[3.] [TEXT NOT REPRODUCIBLE IN ASCII].--2003.--No 11.--C. 6264.

[4.] Calcium carbonate deposit formation under isothermal conditions / N. Andritsos, M. Kontopoulou, A.J. Karabelas, P.G. Koutsoukos // The Canadian Journal of Chemical Engineering.--1996.--Vol. 74, Issue 6.--PP. 911-919.

[5.] Kazi, S.N. Fouling and fouling mitigation on heated metal surfaces / S.N. Kazi, G.G. Duffy, X.D. Chen // Desalination.--2012.--Vol. 288.--PP. 126-134.

[6.] Andritsos, N. Morphology and structure of CaC[O.sub.3] scale layers formed under isothermal flow conditions / N. Andritsos, A.J. Karabelas, P.G. Koutsoukos // Langmuir.--1997.--Vol. 13, Issue 10. -PP. 2873-2879.

[7.] Briangon, S. Modelling of crystalline layer growth using kinetic data obtained from suspension crystallization / S. Briangon, D. Colson, J.P. Klein // Chemical Engineering Journal.--1998.--Vol. 70, Issue 1.--PP. 55-64.

[8.] Kostoglou, M. Comprehensive modeling of precipitation and fouling in turbulent pipe flow / M. Kostoglou, A.J. Karabelas // Industrial & Engineering Chemistry Research.--1998.--Vol. 37, Issue 4.--PP. 1536-1550.

[9.] Karabelas, A.J. Scale formation in tubular heat exchangers--research priorities / A.J. Karabelas // International Journal of Thermal Sciences.--2002.--Vol. 41, Issue 7.--PP. 682-692.

[10.] [TEXT NOT REPRODUCIBLE IN ASCII].--2014.--C. 3/8 (69).--?. 52-58.

[11.] Garrels, R.M. Solutions, minerals, and equilibria / R.M. Garrels, C.L. Christ.--Boston: Jones and Bartlett, 1990.--450 p.

[12.] [TEXT NOT REPRODUCIBLE IN ASCII].--2015.--? 5/8 (77).--C. 14-20. DOI:10.15587/1729-4061.2015.51205

[13.] [TEXT NOT REPRODUCIBLE IN ASCII].--2013.--? 6/8 (66).--?. 10-14.

V.G. Ahrameev, PhD

Odessa National Polytechnic University, 1 Shevchenko Ave., 65044 Odessa, Ukraine; e-mail: ahvitalchik@mail.ru

Received June 10, 2016

Accepted July 22, 2016

Caption: Fig. 1. The model of run-around cooling system: 1--working cell (heat exchanger with external flow), 2--flowmeter, 3--ammeter, 4--voltmeter, 5--laboratory transformer (LATR), 6--indicator, 7--valve, 8--water cooling tower, 9--capacity, 10--pump, 11--"crossed wheels " nozzle, 12--coupons

Caption: Fig. 2. Dependence of the intensity of sediment on the heat exchange surfaces of water cooling towers and condenser: 1--LAMsh brass tube, 2--LAMsh brass coupons, 3--polyethylene nozzles
Table 1

Characteristics of the heat exchange surfaces

Data                 Nozzle     Tubes    Coupons

Quantity, pcs         350         8         3
Area S, [m.sup.2]    0.4613    0.02512    0.0039

Table 2
Mass of sediment on heat exchange surfaces

Time, hour         Sediments mass, g

             Nozzles   Tubes    Coupons

40           3.32136   0.7536   0.000288
100          4.1517    1.1304   0.00036
200          6.9195    1.5072   0.00048
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Author:Ahrameev, V.G.
Publication:Odes'kyi Politechnichnyi Universytet. Pratsi
Article Type:Report
Date:Jul 1, 2016
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