© Strojni{ki vestnik 46(2000)8,509-516 ISSN 0039-2480 UDK 536.24:546.212:66.06:628.16.04/ © Journal of Mechanical Engineering 46(2000)8,509-516 ISSN 0039-2480 UDK 536.24:546.212:66.06:628.16.04/ .09:537.84 .09:537.84 Pregledni znanstveni ~lanek (1.02) Review scientific paper (1.02) Prepre~evanje izlo~anja vodnega kamna na povr{inah prenosnikov toplote z uporabo naprave za magnetno obdelavo vode The Preventation of Surface Precipitation on Heat Exchangers Using a Magnetic Water-Treatment Device Andrej Pristovnik - Lucija ^repin{ek Lipu{ - Jurij Krope V nalogi predstavljamo metodo za nadzor vodnega kamna na temelju magnetne obdelave vode (MOV) v prenosnikih toplote. Podali smo teoretičen pregled tvorbe kotlovca pri industrijskih prenosnikih toplote s poudarkom na obarjanju kalcijevega karbonata (CaCO3) in kalcijevega sulfata (CaSO4) ter osnovne izračune za uspešno uporabo naprav MOV pri preprečevanju nastajanja vodnega kamna. © 2000 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: prenosniki toplote, zaščita proti kotlovcu, magnetna obdelava vode, magnetohidrodinamika) Magnetic water treatment (MWT), a water-conditioning method for scale control in heat exchangers (HEs), is discussed. The theoretical possibilities of scale formation in industrial processes with the emphasis on the precipitation of CaCO3 and CaSO4 as the main scale components, are reviewed. Some preliminary calculations for a theoretical understanding of the scale problem in HEs and its prevention using MWTs are contributed. © 2000 Journal of Mechanical Engineering. All rights reserved. (Keywords: heat exchangers, scale control, magnetic water treatment, magnetohydrodynamic) 0 UVOD Problem izločanja vodnega kamna se pojavlja pri vseh tehnoloških procesih, ki uporabljajo naravno vodo. To pa še posebej velja v primeru uporabe prenosnika toplote, pri katerem pride do povišanja temperature in posledično do prenasičenja soli, ki tvorijo vodni kamen (predvsem CaCO3 in CaSO4). Obstaja več dobro znanih in uporabnih metod za preprečevanje nastajanja vodnega kamna. Uporaba nekaterih pomeni velik finančni strošek druge pa onesnažujejo okolje. V zadnjih letih se kot alternativa kemičnim metodam vedno bolj uveljavlja t.i. magnetna obdelava vode (MOV). Čeprav je metoda znana že petdeset let in z ekonomskega in okoljevarstvenega vidika zelo sprejemljiva, prav procesna industrija še naprej dvomi o njeni učinkovitosti in uporabi ([1] do [4]). 1 NASTANEK VODNEGA KAMNA Naravna voda je dejansko bogata raztopina/ disperzija mnogih ionov: Ca2+, Mg2+, Na+, K+, HCO3, SO42 in Cl-. Ioni Na+, K+ in Cl - so inertni, preostali pa so vključeni v t.i. medfazno ravnotežje. Zaradi 0 INTRODUCTION The build-up of scale deposits is a com-mon and costly problem in many industrial pro-cesses which use natural water supplies, especially in heat-exchange processes, where a high oversaturation of scale-forming components (i.e. CaCO3 and CaSO4) is established. There are many well-known scale-prevention methods, but they are costly and environmentally unfriendly. MWT is be-ing used more and more as an alternative method for scale control. The process industry remains skeptical about this non-chemical method despite its long history and examples of favorable economic ben-efits ([1] to [4]). 1 SCALE FORMATION Natural waters are rich solution/dispersion systems which contain the ions: Ca2+, Mg2+, Na+, K+, -HCO3-, SO42- and Cl-. The Na+, K+ and Cl ions are inert, while the others are incorporated into an inter- gfin^OtJJlMlSCSD 00-8 stran 509 |^BSSITIMIGC A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation sprememb obratovalnih razmer (sprememba tlaka, temperature, vrednosti pH) pride do prenasičenja in soli se v obliki vodnega kamna izločajo na stene cevi, prenosnikov toplote in drugih naprav, ki so v stiku z vodo. Najpomembnejši parameter za nadzor vodnega kamna je delež kalcijevih ionov Ca2+. Določimo ga s pomočjo t.i. karbonatnega ravnotežja ((1) do (4)). Parametra (c) in (K) pomenita koncentracijo in konstanto ravnotežja. CO2(g)+H2OoH2CO3, H2CO3<->H++HCO3, phase equilibrium. Due to the natural supersaturation of the supplied water or supersaturating due to changed operating conditions (such as a pressure drop, temperature or pH increase) hard scale precipi-tates in pipelines and on the walls of equipment. The most important parameter in scale con-trol is the concentration of Ca2+ ions, determined by carbonate equilibrium ((1) to (4)), where the parameter c is the concentration and parameters K is the equilibrium constant. Kg = p c H2CO3 CO2 HCO3 ^H++CO3- CaCO3(s)^Ca2++CO2-, K1 K c H2CO3 c -c H+ CO32 - K =c (1) (2) (3) (4). Iz pogoja o električni nevtralnosti (5) in z upoštevanjem ionskega produkta vode (6) lahko izpeljemo odvisnost koncentracije kalcijevih (Ca2+) ionov kot funkcije vrednosti pH in temperature. From the condition of the solution’s electric neutrality (5) and the water dissociation equilibrium (6), the concentration of Ca2+ ions can be derived as a function of pH and temperature. 2cCa2+ + cH+ = 2cCO23 - + cHCO-3 + cOH- HO<->H++OH- Kw=cH+cOH- KW -cH2+ + ( cH2+-KW ) +8cH2+-K\2 + H K 1/ 2 4c (5) (6) (7). Konstante ravnotežja so odvisne od temperature (7). V naravnih vodah (pH < 7) vodi zvišanje temperature in vrednosti pH do znižanja ravnotežne koncentracije Ca2+ ionov (7). Pri znižanju tlaka pride do znižanja koncentracije H CO3 (1) in posledično s povečevanjem vrednosti pH pospešeno obarjanje CaCO3 ((2) do (4) in (7)). S temperaturo (do 40 oC) se zvečuje topnost CaSO4, pri višjih temperaturah (okoli 100 oC) pa naglo zmanjšuje. Iz opisanega je razvidno, da se bo v nizkotemperaturnih sistemih v glavnem izločal kalcijev karbonat (CaCO3) in v visokotemperaturnih sistemih (toplovodi, uparjalniki, prenosniki toplote) pa kalcijev sulfat (CaSO4). 2 ZMANJŠANJE UČINKOVITOSTI PRENOSA TOPLOTE Obloge vodnega kamna, ki nastanejo na površinah prenosov toplote, zmanjšujejo pretočne zmogljivosti in predvsem učinkovitost prenosnikov toplote ter s tem zvišujejo investicijske, obratovalne in vzdrževalne stroške. Brez primerne obdelave napajalne The equilibrium constants in equation (7) are temperature dependent. In natural waters (with a pH less than 7), a rise in temperature and pH leads to a reduction of the Ca2+ equilibrium concentration ac-cording to equation (7). The pressure drop leads to a lower concentration of H2CO3 according to equation (1) and causes CaCO3 precipitation with a pH increase according to eqs. ((2) to (4) and (7)). The solubility of CaSO4 increases as the temperature increases to approximately 40oC and then rapidly decreases at higher temperatures around 100oC. As a result, CaCO3 is the main scale compo-nent in low-temperature water systems, while in high-temperature water systems (especially in high-pres-sure heat exchangers and boilers) CaSO4 prevails. 2 HEAT EXCHANGE REDUCTION The scale formed on heated surfaces re-duces the flow capacity and heat exchange efficiency which leads to higher investment, operation and main-tenance costs. Hard scale can be a severe industrial problem without properly supplied water condition- VBgfFMK stran 510 A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation vode so tako nastale trdovratne obloge težak industrijski problem; terjajo periodično čiščenje z mehanskimi postopki in jedkanjem s solno kislino. Naslednja ocena bo pokazala, kako vodni kamen izrazito znižuje prenos toplote. Moč toplotnega toka P skozi kovinsko steno površine S pri temperaturni razliki DT je za nov prenosnik st (sl. 1.a) določena z enačbo (8). Prestopnostni koeficient a1 je tu praktično enak konvekcijskemu koeficientu plasti vode na obeh straneh stene. Konvekcijski koeficient kovine je namreč bistveno višji kakor za vodo ing. It demands periodic cleaning using mechanical methods and HCl etching. The following preliminary calculations show how scale drastically reduces the exchange of heat. With a new wall of a HE (Fig. 1/a), the heat-flow intensity (P1) through the metallic wall of area S at a temperature difference DT is determined by equa-tion (8), where the heat transition coefficient (a1) is practically equal to the convection coefficient of the water layer on both sides of the wall due to the much higher value of the heat-conduction coefficient of the metal P1 =a^S-DT (8). Obloge vodnega kamna (sl. 1.b) znižujejo The formation of scale (Fig. 1/b) reduces moč toplotnega toka P’ in je ta določen z enačbo (9). the heat flow intensity (P’) according to equation (9), Tukaj se lahko celokupni prestopnostni koeficient a’ where the total heat-transition coefficient (a’) can be izračuna iz a1 nove stene in a2 nastalih oblog po calculated using a1 of the new wall and a2 of the enačbi (10). Velja za postavko iste temperaturne razlike formed scale according to equation (10) at the same med ogrevano in hladilno vodo DT = DT + DT . temperature difference DT = DT + DT. The coeffi- Koeficient a2 je odvisen od celotne debeline oblog cient a2 depends on the total scale lining thickness Dy2 po zvezi (11), kjer je l2 toplotna prevodnost (Dy2) according to equation (11), where l2 is the heat vodnega kamna. conductivity of the scale. P' = a'-S-DT = a1-S-DT1=a2-S-DT2 a 1 1/a1 + 1/a2 Dy2 (9) (10) (11). (a) kovinska stena metal wall (b) | kovinska stena metal wall ci- Sl. 1. Temperaturni krivulji skozi: (a) novo kovinsko steno in (b) skozi kovinsko steno z oblogo vodnega kamna Fig. 1. The temperature curve through a new metallic wall (a) and through a metallic wall covered with scale (b) Preglednica 1 prikazuje nekaj vrednotenj Table 1 represents some estimations for the relativnega zmanjšanja učinkovitosti prenosa toplote relative drop of the heat-exchange efficiency (z) de- x, ki je definirana z enačbo: fined by equation: z = P1 -P, = 1- a, 1 1 + a1Dy2/ l2 (12) gfin^OtJJlMlSCSD 00-8 stran 511 |^BSSITIMIGC A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation Preglednica 1. Relativna zmanjšanja učinkovitosti prenosa toplote (pri izbrani praktični vrednosti a1 = 500 W/m2K za kovinsko steno) zaradi oblog CaCO (l2 = 1,75 W/mK) oziroma CaSO (l2 = 0,50 W/mK) Table 1. Relative drops of heat-exchange efficiency at chosen practical values a1 = 500 W/m2K due to CaCO lining (l2 = 1.75 W/mK) and CaSO lining (l2 = 0.50 W/mK), respectively Dy2 z(CaCO3) z(CaSO4) 1,5 mm 5,5 mm ) 30% 60% 85% ) 60% 85% 95% 20 mm Rezultati potrjujejo praktične izkušnje, da zaradi nizke toplotne prevodnosti CaCO3 in CaSO4, celo tanke obloge vodnega kamna izrazito zmanjšujejo učinkovitost prenosa toplote. V visokotlačnih grelnih napravah je ta problem še posebej močno izražen, saj se v večinskem deležu izloča kalcijev sulfat, ki ima manjšo toplotno prevodnost od kalcijevega karbonata. V mnogih primerih se je izkazalo, da omogočajo naprave za magnetno obdelavo vode razmeroma učinkovit sistem za nadzor vodnega kamna. Eden od uspešnih preskusov naprav MOV domačega proizvajalca Panorama Ptuj [6] pomeni vgradnja le-teh v prenosnik toplote Toplotne oskrbe Maribor (TOM) [5]. Naprave so bile instalirane na ceveh s hladno napajalno vodo in so učinkovito preprečile nastanek vodnega kamna. V preglednici 2 sta predstavljena rezultata vgradnje naprav za magnetno obdelavo v prenosnika toplote. Results prove that even thin scale linings drastically reduce the heat-exchange efficiency be-cause of the low heat of conductivity of the scale components CaCO3 and CaSO4. In high-pressure boilers the problem will be even greater due to the main scale component, CaSO4, which has a lower heat of conductivity than CaCO3. These theoretical predictions are in accor-dance with many practical results, where scale formation on HE surfaces demanded a preliminary treat-ment of the supplied water. In many cases MWT turned out to be a very efficient method for scale control. The installation of MWT devices to prevent hard scale in the HEs in the TOM town heating station [5] was one of the successful domestic tests of the Panorama Ptuj magnetic device [6]. These devices were installed on the cold water pipeline entrance of the HE and efficiently solved any problems with hard scale. Table 2 represents some observations on the scale in the two HEs which were supplied with magnetic ally treated water. Preglednica 2. Rezultati naprave za magnetno obdelavo proizvajalca Panorama Ptuj v toplotni postaji TOM-a Table 2. Results of Panorama Ptuj devices in the TOM station Prenosnik toplote HE star cevni register U old U-pipe register nov spiralni register new spiral register obloge ob vgradnji scale at MWT installation da yes ne none prvi pregled time of the first control 8 mesecev po vgradnji 8 months after installation 11 mesecev po vgradnji 11 months after installation stanje po prvem pregledu state after the first control obloge, odstranitev z vodnim visokotlačnim curkom present scale was removable with high-pressure water jet tanke plastne obloge, odstranitev z vodnim visokotlačnim curkom thin powder scale was removed with jet drugi pregled time of the second control 16 mesecev po vgradnji 16 months after installation 17 mesecev po vgradnji 17 months after installation stanje ob drugem pregledu state after the second control oblog ni bilo,površina je bila veliko bolj čista kakor pred samo vgradnjo without a new scale, surfaces were cleaner than before the installation time enako kakor pri prvem pregledu the same as at the first control 3 NADZOR VODNEGA KAMNA V PRENOSNIKIH TOPLOTE V naravni vodi, bogati z raztopljenimi/ dispergiranimi snovmi, delujejo naprave za magnetno obdelavo vode neposredno na samo stabilnost in 3 THEORETICAL PRINCIPLES OF MWT SCALE PREVENTION ON HE SURFACES The nature of MWT devices acting on supplied water as a rich solution/dispersion system is to alter its crystallization habits and dispersion stability to form VBgfFMK stran 512 A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation kristalizacijo dispergiranih delcev. Kristali, ki se izločajo po obdelavi, so večji in modificirani. Prav na teh kristalih se neposredno iz vode izloči večji del soli, tako da se na stenah naprav nabere neprimerno manj vodnega kamna. Ob pretakanju vode skozi napravo za magnetno obdelavo prihaja do sprememb, ki pa se izražajo (najverjetneje) v spremenjeni ionski hidrataciji prek magnetohidrodinamičnega premika ionov in koncentracijskega vpliva na dispergirane delce v sami napravi MOV [7]. Izračuni kažejo, da se med magnetno obdelavo vode agregatne tvorbe, sestavljene iz CaCO3 in CaSO4 trdno sprimejo. Iz samega načela staranja kristalov namreč kosmiči, v katerih so delci med seboj šibko povezani, niso tako zaželeni kakor goste agregatne tvorbe [8]. Po teoriji DLVO (Deryagin, Landau, Verwey, Overbeck) ([9] in [10]) smo opravili numerično analizo koagulacije in kosmičenja nemagnetnih delcev vodnega kamna in prišli do sklepa, da v naravnih vodah prevladuje koagulacija, ki je odvisna od same naprave MWT, medtem ko je zaradi nizke vrednosti Hamakerjeve konstante in nizke magnetne susceptibilnosti pri večjih delcih (a > 0,1 mm) mogoča le kosmičenje. Po drugi strani pa se bodo magnetohidro-dinamično nastali kosmiči pod vplivom turbulentne pulzacije razbile. Do pulzacije prihaja v večini naprav MOV, kjer je priporočena pretočna hitrost od 0,5 do 2 m/s. Ob preseženi vrednost Reynoldsovega števila (104) imamo opraviti s turbulentnim tokom Re = Parametra h in r pomenita viskoznost in gostoto vode. Pri pretočni hitrosti 0,5 m/s je kritična velikost delovnega preseka znotraj naprave MOV 2 cm in pri 2 m/s pa 0,5 cm. Iz pulzacijske teorije [11] smo za izračun pulzacijske dolžine (b) in pulzacije delcev(v) s polmerom (a) izpeljali sistem enačb: r- bigger modified crystals, which in suspended form offer surfaces for scale precipitation and in that way hard scale formation indirectly prevails on equipment walls. The change in the water’s behaviour when the water flows through the magnetic field is most probably a result of altered ion hydration, by magne-tohydrodynamic shifts of ions and concentration effects on the dispersed particles in the working channel of the MWT device [7]. Some calculations have been made showing that all aggregates, formed from scale components (CaCO3 and CaSO4) during MWT, are compact-strongly adhered. In other words, the flocks in which constituent particles are weakly bonded are not as favorable for scale prevention as the compact aggregates according to the principles of crystal aging [8]. A numerical analysis of the coagulation and flocculation of the nonmagnetic scale components, based on the Deryagin, Landau, Verwey, Overbeck theory ([9] and [10]), has been made. It offered an estimation that in natural waters only flocculation from big particles (with radius a > 0,1 mm) is possible due to the low Hamaker constant and low magnetic susceptibility of these components, while a coagulation prevails and depends on the MWT working conditions. On the other hand, the magnetohydrody-namically formed big flocks will be shattered by turbulent pulsations which appear in the majority of practical MWT devices, where the recommended values of water flow velocity are in range from 0.5 to 2 m/s for efficient anti-scale treatment. The Reynolds number Re, defined by equation (13), characterizes turbulent flow, if it is greater than 104 (13). h The parameter h is the viscosity and r is the mass density of water. For a water flow of velocity 0.5 m/s, the critical thickness of the working channel (d) is 2 cm, and for 2 m/s, the critical thickness is 0.5 cm. From the turbulent pulsations theory [11], the equation system was obtained for the evaluation of the pulsation length (b) and the pulsation for a particle with radius a (vb). b = 207 d 0,17v logRe/ 7 Re7/4 Stabilni kosmič z 10kT vezno energijo med delci (k je Boltzmannova konstanta) lahko razbijemo s turbulentno pulzacijo samo, če je gostota kinetične energije rv2/2 večja od gostote vezne energije 10k T/ (4pa3/3). Določimo lahko ti. kritični polmer kristalnega delca (a*): 1/3 Re 1/ 4 (14) (15) A stable flock with a 10k T bonding energy between constituent particles (k is the Boltzmann constant) would be shattered by turbulent pulsation, if the kinetic energy density rv2/2 were greater than the bonding energy density 10kBT/(4pa3/3). A crystal particle radius is therefore: A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation 3 15kBT / prvb (16). Tako je pri v = 0,5 m/s kritični polmer 0,25 mm in 0,13 mm pri v = 2 m/s. Povzamemo lahko, da bo za priporočene pretočne hitrosti proizvajalcev naprav MOV turbulenca razbila CaCO3 in CaSO4 kosmiče. V suspendirani obliki bodo ostali le najbolj močno vezani agregati. V primerjavi s kemijskimi metodami priprave vode za nadzor vodnega kamna je magnetna obdelava še najbolj podobna suspendiranju kristalnega prahu. Naslednji izračuni določajo potrebno količino prahu za preprečevanje izločanja CaCO na stenah prenosnikov toplote z relativno površino SHE = S storn Vvode, kjer sta Sstene površina sten in Vvode Da bi se zagotovil hiter prenos toplote, so v skladu z enačbo (17) [12] priporočane visoke vrednosti SHE, in sicer med 100 in 1000 /m. dT = a dt cpr V enačbi (18) je iz kvocienta x oborjene mase v jedru vode (dm ) in mase na stenah (dm ) razvidno, da se bo vodni kamen nalagal v tanjših oblogah pri nižjih vrednostih SHE. The critical radius a is 0.25 mm for a water flow of velocity v = 0.5 m/s and a is 0.13 mm for v = 2 m/s. A theoretical conclusion could be made for all recommended ranges of water flow velocity that turbulence will deaggregate CaCO3 and CaSO4 flocks. Only highly adhered aggregates will remain in a suspended form. In a comparison with chemical scale-prevention methods, the suspending of crystal powder is the most similar to the MWT method. The following calculation estimates that the necessary amount of powder for the prevention of CaCO3 precipitation on the walls of a HE with a relative surface: SHE = S l/V , where S ll is the area and V is the water volume. wae To ensure a quick heat exchange, high values of SSE from 100 to 1000 /m are recommended according to equation (17) [12]. DTS (17). x dm rV dms In this relationship for the heating rate dT/dt, the parameter cp is the heat capacity of water. A thin-ner scale lining will be formed at lower values SHE , as can be predicted from the x-quotient (of precipitated mass in the bulk of water-dmv and precipitated mass on the walls - dms) in equation (18). So, the optimal value SHE in HE designing should be found. (18). rS rS s wall s HE V modificirani obliki sta enačbe za hitrost kristalne rasti (r) določila Nancollas in Reddy [13], in sicer na podlagi obarjanja iz jedra raztopine s temperaturo T na površino naprav s temperaturo T ((19) in (20)). Pri tem velja, da je parameter k določen empirično, MCaCO3 je relativna molska masa kalcijevega karbonata in R splošna plinska konstanta. The relationship of crystal growth rate r has been determined by Nancollas and Reddy [13] and is represented by equations (19) and (20) in a modified form for precipitation in the bulk of a solution with temperature T1 and on the equipment walls with temperature T2, where k is an empirical parameter, MCaCO3 is the relative molecular mass of CaCO3 and R is the universal gas constant. r = kMCaCO exp| RT1 | powder b1 DG -DG SHE=kMC exp RT2 Sb wall 2 (19) (20) (21) Kristalna rast je odvisna od sestave raztopine in trdnine: - stopnje prenasičenja b, ki je ob stenah prenosnikov toplote (b2) višje kakor v jedru raztopine (b1), in od - aktivacijske energije DG, ki je odvisna od kristalne faze. V primeru obarjanja CaCO sta kristalni fazi kalcit in aragonit. V suspendiranem prahu, nastalem z magnetno obdelavo, je opažen povečan delež The crystal growth rate depends on solution and solid phase composition by: - b supersaturation degree (defined by 21), which is higher at the HE walls (b2) than in the bulk of solution (b1); - DG activation energy depending on crystal phase. In the case of CaCO precipitation, crystal phases aragonite and calcite are formed. A powder, formed from magnetically treated water, has an increased VH^tTPsDDIK stran 514 A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation aragonita. Za hipotetični primer vzamemo vrednost DG1 za aragonit in vrednost DG2 za kalcit. Z zamenjavo r in r SHE v enačbi (18) z izrazoma (19) in (20) dobimo zvezo (22) za količino prahu, ki je potrebna za učinkovit nadzor CaCO oblog: fraction of aragonite. In an ideal case a DG1 value could be taken for aragonite and a DG2 for calcite. With the substitution rv and rsSHE in (18) by (19) and (20), equation (22) is obtained and a neces-sary powder surface is estimated: powder b1 x b2 exp DG1 RT DG2 RT (22) Da bi se učinkovito preprečile obloge trdega vodnega kamna za temperaturno območje 40 do 100oC in za zahtevano učinkovitost, je potrebna količina prahu S ah istega reda, kakor je površina sten prenosnikov toplote Sstene 4 SKLEP Za nadzor vodnega kamna na stenah prenosnikov toplote je potrebna optimizacija velikosti površine za prenos toplote glede na obratovalne razmere. Zraven kemičnih postopkov za zmanjšanje koncentracije Ca2+ ionov se priporoča uporaba naprav MOV. Dobro načrtovana naprava MOV, ki zagotavlja zadostno količino suspendiranih delcev v obliki praška, lahko učinkovito prepreči nastanek kotlovca. Problem učinkovitega načrtovanja naprav MOV je nezadostno poznavanje samega mehanizma delovanja teh naprav. Mehanizem je zapleten in je neposredno odvisen tudi od obratovalnih razmer in sestave napajalne vode. Na srečo so na temelju empiričnih izkušenj izdelali lepo število učinkovitih naprav MOV. S tem zadovoljujejo veliko povpraševanje po tej preprosti in cenovno ugodni rešitvi za preprečevanje nastanka vodnega kamna. For efficiency request ^b1 b2 and operational temperatures between 40oC and 100oC, the necessary powder surface S d should be of the same order as the surfaces of the heat exchanger walls Swall to effectively prevent hard scale. 4 CONCLUSION For scale control in HEs an optimization of the heat-exchange surface area is recommended for simultaneous high heat transition and scale prevention. In addition, besides chemical methods for the reduction of the Ca2+ concentration, the alternative method of MWT is recommended. A well-designed MWT device which assures the formation of a suspended scale powder with a surface area comparable to the exchange surface area can effectively prevent hard-scale formation. The prob-lem with designing MWT devices is an insufficient theoretical understanding of the MWT mechanism. The mechanism is complex and depends directly on operational conditions and the composition of the supplied water as a solution/dispersion system. Fortunately, numerous MWT devices of different constructions have been designed on an empirical basis resulting from several decades of testing and are available to satisfy a large demand for such easy and cheap solutions to industrial scale problems. 5 OZNAKE 5 SYMBOLS premer delca dolžina pulza koncentracija specifična toplota premer cevi aktivacijska energija prva konstanta ravnotežja pri disociaciji HCO druga konstanta ravnotežja pri disociaciji HCO plinska konstanta ravnotežja topnostni produkt ionski produkt vode empirična konstanta hitrosti kristalne rasti Boltzmannova konstanta molska masa masa moč toplotnega toka tlak a b c c p d DG K1 K2 Kg Ks Kw k kB M m P p m particle radius m pulsation length mol/L concentration J/kgK heat capacity m thickness of working channel J/mol activation energy ml/L equilibrium constant of the first step of H2CO3 dissociation ml/L equilibrium constant of the second step of H2CO3 dissociation mol m2/NL gas equilibrium constant mol2/L2 soluble product mol2/L2 dissociation product of water 1/mol m3s empirical constant of crystal growth rate J/K Boltzman constant kg/mol molar mass kg mass J/s heat flow intensity N/m2 gas pressure A. Pristovnik - L.^. Lipu{ - J. Krope: Prepre~evanje izlo~anja - The Preventation of Precipitation splošna plinska konstanta Reynoldsovo število hitrost kristalne rasti s raztopini hitrost kristalne rasti na kovinskih stenah površina absolutna temperatura čas hitrost pretoka hitrost turbulentne pulzacije debelina sloja vodnega kamna koeficient toplotne prehodnosti stopnja prenasičenja viskoznost koeficient toplotne prevodnosti gostota snovi učinkovitost nadzora vodnega kamna učinkovitost prenosa toplote R J/molK Re - r kg/m3s r kg/m2s S m2 T K t s v m/s v m/s Dy m a J/m2sK b mol2/L2 h Ns/m2 l J/msK r kg/m3 x - z - 6 LITERATURA 6 REFERENCES universal gas constant Reynolds number crystal growth rate in bulk of water crystal growth rate on walls surface area temperature time flow velocity turbulent pulsation velocity scale thickness heat transition coefficient supersaturation degree water viscosity heat conductivity mass density scale control efficiency heat exchange efficiency [I] Kittner, H.(1970) Wassertechnik 20(4), 136. [2] Tebenihin, E. F., B.T. Gusev (1970) Obrabotka vody magnitnym polem v teploenergetike. p.145, Izdatel‘stvo Energija Moskva, Moskva. [3] Grutsch, J. F. (1977) USA/USSR Symposium on physical-mechanical treatment of wastewaters; p. 44, EPA-Cincinati. [4] Grutsch, J. F., J.W. McClintock (1984) Corrosion and deposit control in alkaline cooling water using magnetic water treatment at Amoco’s largest refinery. CORROSION/84, No.330, Texas. [5] Krope, J., L. Crepinsek (1994) Magnetic water treatment for process systems. Research Project B2-6504-0795-94, Ministry for Science and Technology, Slovenia. [6] OPz Panorama Ptuj (prodajni prospekt), Osojnikova 1, 2250 Ptuj, Slovenia. [7] Krope, J., L. Crepinsek (1998) Magnetohydrodynamics of colloid systems. Research Project L2-06990-0795-98, Ministry for Science and Technology, Slovenia. [8] Khamskii, E.V.(1969) Crystallization from solutions. Consultants Bureau, New York-London. [9] Voyutski, S. (1978) Colloid chemistry. MIR publisher Moscow. [10] Hunter, R.J. (1996) Introduction to modern colloid science. Oxford Science Publications, New York. [II] Kulskii, L.A., V.Z. Kochmarskii, V. V. Krivtsov (1983) Intensifying and destabilizing factors of magnetic antiscale treatment of water. Himiya i tehnologija vody, Vol. 5, No. 4, 296-301. [12] Krope, J., E. Kiker (1996/98) Planing and dimensioning of heat recuperaters in water / steam systems. Research Project Maribor. [13] Nancollas, G. H., M.M. Reddy (1974) Crystal growth kinetics of minerals encountered in water treatment processes. Aqueous-Environmental Chemistry of Metals, New York. Naslova avtorjev: mag. Andrej Pristovnik dr. Lucija Črepinšek Lipuš Fakulteta za strojništvo Univerze v Mariboru Smetanova 17 2000 Maribor prof. dr. Jurij Krope Fakulteta za kemijo in kemijsko tehnologijo Univerze v Mariboru Smetanova 17 2000 Maribor Authors’ Addresses: Mag. Andrej Pristovnik Dr. Lucija Črepinšek Lipuš Faculty of Mechanical Eng. University of Maribor Smetanova 17 2000 Maribor, Slovenia ProfDr. Jurij Krope Faculty of Chemistry and Chemical Technology University of Maribor Smetanova 17 2000 Maribor, Slovenia Prejeto: Received: 15.8.2000 Sprejeto: Accepted: 10.11.2000 VBgfFMK stran 516