M.Ing. / Deposition of scale on heating surfaces is a major problem in industry as well as households. The scale that forms on the heating surfaces acts as an insulator and results in decreased heat transfer effectiveness . These are two main approaches to prevent or reduce scaling. Although these approaches are claimed to be efficient, there is a need to evaluate or verify their efficacy. This calls for a method which should preferably enable quantitative and rapid evaluation of these techniques in the laboratory. A reliable, rapid and quantitative measurement method which was comprised of stripping the scale from heat exchanger pipes with a 10% acetic acid solution and measuring the Ca concentration in the acid after stripping, was developed. A total of 11 tests, 7 to test the reproducibility, 2 to test a physical water treatment device and 2 to test the effect of zinc, were conducted. The reproducibility amongst different pipes, and amongst different experiments could not be achieved. Attempts to explain the inconsistency through statistical analysis of the data showed that, the inconsistency in the results could partly attributed to chemical differences, particularly changes in calcium concentration and TDS. Metal contamination, particularly zinc could also be responsible for a part of the inconsistency.

Thesis (M. Eng.)(Metallurgical)--University of Pretoria, 2001. / Summaries in Afrikaans and English.

D.Phil. (Chemistry) / The formation ofcalcium carbonate scale is a major problem in industry as well as in households. The scale that forms acts as an insulator, decreasing the effectiveness ofheat exchangers, be it a water cooling systems or a water heating system This raises the energy costs ofthat company and the costs of cleaning are expensive. Physical water treatment has been used for at least 50 years in the treatment ofwater with scale forming potential to reduce the formation ofthis scale. In thisthesis, tests were made on a magnetic and constantelectric field antiscaling device on the effect they have, ifany, on CaC03 scale formation. An attempt will be made to give a mechanism for any effect found. It is hoped that a contribution is made by this research in clearing up the misconceptions that abound in this controversial field ofphysical water treatment. Water with a high calcium bicarbonate content was exposed to the antiscaling devices and then calcium carbonate was forced to precipitate out ofsolution. This precipitation was followed by meauring the pH and the crystal structure ofthe precipitated calcium carbonate was studied. The most important results were that by exposing the calcium bicarbonate solution to an antiscalingdevice, the start ofprecipitation ofthe calcium carbonate was significantly delayed and the crystal structure of the calcium carbonate that formed was predominantly aragonite as opposed to calcite that usually formed.

M.Ing. / Fouling and more specifically scaling is one of the most discussed and not yet well understood problems facing engineers. Scale formation which has enormous economic and environmental consequences does not only influence heat transfer characteristics and pressure drop through tubes, but influence the volumetric flow rate as well. To solve this problem Physical Water Treatment (PWT) devices have been developed which reportedly reduce scale formation. The efficiency of PWT devices is a controversial subject and many contradictions exits in the claimed effect. Researchers have been trying to prove the efficiency of PWT devices but in most cases the results are typically characterised by low reproducibility. An experimental measuring technique is thus required to measure scale formation and the influence PWT devices has on scaling. This technique must also produce reproducible results to clarify the questions surrounding PWT devices. This thesis is an experimental investigation in which an experimental setup and measuring technique is developed. It is used to evaluate a permanent magnet for the decrease of scale formation in tubes. The volumetric flow rate is used as the indicator of scale formation, which relies on the basic principle that the friction pressure drop increases if scaling takes place. The volumetric flow rate is monitored by measuring the time necessary to fill a container of known volume. The test section consists of three soft drawn copper tubes in which water with a velocity of ± 2 m/s flows. One of the tubes is used to evaluate the PWT device and the other two tubes are used as a control. The water temperature is kept at 53°C. The experiments are done in five phases in which the PWT device is placed onto one of the tubes in phase four and removed in phase five. It follows from the results obtained that a very sensitive experimental setup was designed and built which can be used to evaluate the efficiency of PWT devices. It follows from the variation in time necessary to fill the container in phase four and five that a clear conclusion can not be made. More experiments must be done in which reproducible results are obtained.

Oxide scale must be removed before cold drawing wire, otherwise it will cause bad surface quality, inferior die life and many wire ruptures. The nature of oxide scales and the methods of scale removal are reviewed, with particular emphasis on mechanical descaling. This is the major concern of the research, therefore a cantilever bending test has been developed to assess the ease of removal of the scale on commercial steel rod surface in the laboratory. A scanner method and a Finite Element model have been developed to evaluate the critical strain for scale cracking and removal after cantilever bending. Scanning electron microscopy (SEM), together with electron backscattered diffraction (EBSD), energy diffraction spectrum (EDS) and X-ray element mapping analysis, was used to characterize the scale before and/or after bending test. The effects of laying temperature, cooling conditions, ageing time, relative humidity and temperature, and coil positions on scale cracking and removal behaviour were studied. It was found that laying temperature has a larger effect on descalability than cooling conditions. The effect of relative humidity and temperature on descalability depended on a critical holding time. Beyond it, relative humidity and temperature had no further effect on descalability. The higher the environmental temperature, the less the critical holding time. Ageing time had an effect on descalability, but the effect was relatively small. Failure in tension started with first cracks formed at the places with high stress concentration. As tensile strain increased, new cracks formed midway between the existing cracks. Crack spacing stayed uniform but decreased until the scale segments spalled off the rod surface. The crack spacing increased with scale thickness and decreased with strain applied. Scale cracking and spallation mechanisms in compression depended on the relative shear strength of the oxide, the buckling stability of the layer and the relative shear strength of the interface. Spallation always required the propagation of a crack at the interface. The residual sub-layer left on the rod surface of EAF steel after the bending test was identified as magnetite. On the same sample, copper enrichment was found at the scale/metal interface, but within the metal side, and silicon enrichment was found at the scale/metal interface, but within the scale side.

Thesis (M.S.)--Ohio University, June, 2004. / Includes bibliographical references (p. 103-106).

Thesis (M.S.)--Ohio University, June, 2004. / Title from PDF t.p. Includes bibliographical references (p. 103-106)

M.Sc. / Dissolved salts, mainly carbonates and sulphates such as those of calcium and magnesium, form hard water, which when heated produces an insoluble precipitate of scale forming crystals. Scale formation affects performance in industry and ouseholds. Besides the problem of energy costs, there are all the additional problems and maintenance costs of scale removal. Traditional methods of combatting scale build-up based on chemical water treatment do exist. These include: the addition of strong acid to remove the bicarbonate ion from the water, the use of chemicals to prevent or retard the formation of hard scale and, other techniques such as ion exchange process. A cost-effective alternative to these methods is Physical Water Treatment (PWT). Many different types of equipment claiming to treat water physically, for scale reduction are commercially available. These include magnetic, electric (high frequency or radio frequency) or electrostatic, catalytic and ultrasonic devices. In this work attempts have been made to unravel the mechanisms involved in PWT. For this it was necessary to develop experimental methods to determine the effect of electromagnetic fields and selected magnetic and radio frequency (rf) water treatment devices on precipitation reactions. The most significant finding of this study involves the effect of Zn on scale formation. When supersaturated Ca(HCO3 )2 solutions were tested in a glass heat exchanger, a 77% reduction in the formation of scale was found when the solutions were spiked with Zn.

M.Sc. / Calcium carbonate scale formation is a major problem in industrial water and cooling systems as well as in household systems. The resulting reduction in heat transfer and the removal of the scale are very costly processes. One of the methods used in the past 50 years and investigated for nearly a century, is the use of physical fields to reduce, remove and / or inhibit scale formation. These physical fields are usually magnetic fields, but RF electric and electrostatic fields are also used. These fields are claimed to reduce the cost involved in the removal of scale to a minimum. To investigate these claimed effects on scale formation, a pure calcium bicarbonate solution was exposed to some of the commercial available units: The exposed solution was then forced to precipitate by elevating the temperature. The pH was monitored against time, to investigate the nucleation process of CaCO 3 and the resulting precipitate analysed for changes in crystal morphology. The principle conclusions made from the results were, firstly that the nucleation of CaCO 3 was significantly delayed in one of the units tested and that the preferred morphology was changed from the thermodynamically more stable calcite to the more unstable aragonite in that unit. Secondly, that a direct correlation existed between the release of trace amounts of metals from the unit and the observed effects.

M.Ing. / Scaling problems in a heating or cooling system can be recognised by one or more of the following symptoms. Reduction in heat transfer rate, in which the formation of an insulating deposit on a heat transfer surface significantly reduces the cooling or heating efficiency of the equipment. Reduced water flow, which results from a partial or complete blockage of pipelines, condenser tubes, or other openings. Even a small build-up of scale on a heat exchange surface reduces water flow. Scale may continue to build up in boilers until heat transfer is so low that the metal overheats, permitting the tubes to rapture under the operating pressure. Scale is usually found in water-handling equipment in which water is heated, i.e. hot-water storage tanks, boilers, etc. The magnitude of this problem may be appreciated by considering that scaling can cause degradation, or complete failure in thermal and hydraulic performance which increases initial and operating costs (Chan and Ghassemi, 1991). A fairly low-pressure boiler, with only 0.6 mm of calcium sulphate scale on the tubes results in a 180°C temperature drop. The cost involved due to heat transfer inefficiency and the removal of scale, in Britain alone, is estimated at £1 billion per annum (Darvill, 1993). Poor conductivity of a 25 mm thick CaC03 scale layer can decrease the heat transfer by 95% (Glater eta/., 1980), whereas a Si02 scale layer 0.5 mm thick results in a 90% decrease in heat transfer (Grutsch and McClintock, 1984).