Calcium carbonate scaling in a plate heat exchanger in the presence of particles N. Andritsos * , A.J. Karabelas Department of Chemical Engineering, Chemical Process Engineering Research Institute/CERTH, Aristotle University of Thessaloniki, P.O. Box 361, GR 57001, Thermi-Thessaloniki, Greece Received 27 August 2002; received in revised form 3 June 2003 Abstract Scale formation of CaCO 3 in a plate heat exchanger is investigated in the presence of various types of added particles under isothermal conditions. The parameters examined include the degree of supersaturation, the type of particles, the flow velocity, the particle concentration and the direction of flow inside the heat exchanger. The key result of this work is the strong synergistic effect of fine aragonite particles on the deposition rate and the morphology of the deposits. On the contrary, the presence of fine titanium oxide and of relatively large calcite particles does not seem to affect the main scaling characteristics of CaCO 3 . Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction Fouling, the accumulation of undesirable deposits on industrial equipment surfaces, is a complex phenomenon with severe economic consequences, affecting a wide range of industrial processes. Research on fouling has made significant progress over the past 30 years in un- derstanding the governing mechanisms of the fouling process, although several aspects of this phenomenon have not been elucidated yet, and predictive tools are inadequate. The deposits may consist of crystalline, particulate and biological matter or can be products of chemical reactions or corrosion. In most cases more than one type of matter is present in the deposits. It is generally accepted that five main fouling categories exist, depending on the prevailing process [1,2]; i.e., crystalli- zation (or precipitation), particulate, chemical reaction, corrosion and biological fouling. Precipitation fouling usually refers to the formation of a solid layer on equipment surfaces arising from the direct crystallization on the wall of dissolved inorganic salts. Fluid supersaturation is brought about through temperature or pH changes, or through mixing of in- compatible water streams. The term ‘‘scaling’’ often re- fers to the formation of deposits of inverse-solubility salts (CaCO 3 , CaSO 4 , Ca 3 (PO 4 ) 2 etc.), although it gen- erally denotes the hard, and adherent deposits due to inorganic constituents of water [3]. Suspended solids are almost always present in in- dustrial processes, such as cooling water systems. They can accumulate on equipment surfaces forming in gen- eral, ‘‘loose’’ deposits. Moreover, their presence can af- fect the formation of crystalline deposits in various ways. There is a common belief that the presence of particulates renders the scale deposits ‘‘softer’’ and less coherent. On the other hand, several investigators report that the presence of particles tends to increase significantly the deposition rate of CaCO 3 [4,5]. Calcium carbonate is by far the most common scale compound, forming tena- cious layers in several industrial systems, including cooling water circuits, potable water supply lines, desa- lination units, and petroleum and gas production systems [3]. The main causes of calcium carbonate deposition are CO 2 loss from the solution (resulting in pH increase) and the temperature or salt concentration increase. The cal- cium carbonate solubility decreases with increasing pH and temperature and increases with increasing partial * Corresponding author. Present address: Department of Mechanical and Industrial Engineering, University of Thessaly, 383 34 Volos, Greece. Fax: +30-231-049-8189. E-mail address: andritso@cperi.certh.gr (N. Andritsos). 0017-9310/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0017-9310(03)00308-9 International Journal of Heat and Mass Transfer 46 (2003) 4613–4627 www.elsevier.com/locate/ijhmt