Water treatment

For open-circuit cooling systems

Cooling systems in circulations are the type where cooling water is taken from the immediate environment (from the water stream, lake or any other kind of watercourse). The water turns its path only once in the cooling system and then it comes back. In the past this was the cheapest way of cooling in locations which were rich in cooling waters and thus resulted in its relative frequent use. In terms of investment such process was quite simple, undemanding and no problems occurred if the water characteristics were not such as to cause formation of deposits or sediments in the system.

Today, environmental regulations and taxes are directing us to use close water cooling systems, thus the cooling systems in circulation are rarely built. They can be accepted conditionally if ecological criteria and technical requirements allow performance of such way of water cooling. Although the problem there is the need to prepare the entire circulation of water. When ecological taxes were introduced the economic viability was questionable for such systems, since the operating costs are relatively high especially if pollution of cooling water occurs during the cooling process.

In the cooling cycle some problems occur mainly because of some particles which deposit in installations and devices. And there can be problems with microbiology, deposits, sediments and corrosion as well.

Bigger particles can be removed by means of mechanical filters. In the market today we can find various methods of mechanical filtration, f. e. self rinsing filters with cartridges.

Depositing and accumulating of smaller particles, of which removal by means of filtration would be too expensive, and formation of bio films is best prevented by constructional solutions. This can be done by eliminating of areas with too low velocity of water current. Dosing of biocides into the cooling water is absolutely unacceptable. Occasional sanitation is possible, but it requires suspending the part of the system which is being cooled. Problems with corrosion and deposits can be solved by adding phosphate preparations. However we need to pay a careful attention to the level of dosing if we don’t want to cause negative impacts on the environment.

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For closed-circuit cooling systems

System water is dense and full of impurities which enter the water circulation through cooling towers, where water is in direct with ambient air. The water temperature is often between 25 and 50°C. Such conditions are ideal for formation of algae, fungi and bacteria. Bio films and accumulated algae reduce heat exchange and prevent resistance in installations.

To avoid such effects, we need to dose biocides. It needs to be emphasised that we do not achieve sterility of cooling water with the use of biocides, but we control biological growth and keep it within acceptable limits. There are several methods of the use of biocides available.

With the occasional so-called shock-dosing we take measures when biological growth exceeds the desired limit. Preventively, we can dose chemicals several times a week, or we opt for biocides based on regular biological tests.

Through constant dosing at a considerably lower concentration as the one using shock treatment we can prevent biological development in the system water. It depends on economic calculation and technological requirements which method will be selected.

With the use of system biocides resistance of microorganisms often occurs what demands replacement of the dosing recipe.

The second but not less important or smaller problem are particles, which enter the system water through cooling towers. Dust, pollen, leaves and other forms of impurities can be disposed on devices and installations and present a source of problems. Besides, this organic matter is food for microorganisms living in cooling water. This is the reason why it is advisable to place a filter into the circulation, which can remove such impurities from the water. The ratio between the circulation of the filtered water and the basic water flow depends on the quantity of impurities entering the system and on special technological requirements. With the use of circulation filtering cooling water can never be absolutely clean. However, its quality is stable. Filtration is performed on sand filters and self-rinsing filters with cartridges. Sand filters, compared to the ones using cartridges, have more advantages: they rinse well, they remove a wide range of sizes of particles from water, have the ability to bind large quantities of impurities. Their weak points are they take a lot of space in case of large flow devices or plants, and they are heavy and more expensive than filters with cartridges.

If absolute water quality is required, which is directed to the cooling process, the whole process must be filtered. In such a case filters with cartridges or replacement of cold cooling towers systems with the open ones are solution to the problem.

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For warm- and hot water systems

Prepartion of supplying water

Losses due to breaks or leaks or repair intervention must be compensated by supplying water. This must be prepared in a way it does not cause formation of deposits, sediments or corrosion.

To prevent formation of sediments and deposits we must remove substances from supplying water, which could form sediments. This can be achieved through ion exchange (softening) or in soluble form by chemical treatment.

Water softening is one of the most frequent methods for preparation of supplying water in medium size and large heating systems.

To protect system material against corrosion and deposits, we need to add into the supplying water corrosion inhibitors, hardness stabilizers and dispersants or the so called multifunctional chemicals which contain all required components. Only the right concentration of these substances in the system water enables efficient system protection. In the system with stable water quality, upon the supposition that the quality of supplying water is stable as well, it is possible to add chemicals proportional to the supplying water flow. In other cases we need to check regularly the level of inhibitors and add them if needed.

The corrosion dynamics can be ascertained and estimated with weight loss measurements on corrosion coupons. These must be installed in the circulation in compliance with the proper standards.

System water treatment

The first and the biggest problem in the system is corrosion. To avoid it we need to add chemical oxygen binders. Dosing of such chemicals into the system is normally performed if needed or based on the analysis of heating water.

The second and not less important problem are particles which appear in heating water along with corrosion. Such impurities can be removed through partial or circular filtration.

The ratio between the filtered water flow and the basic flow depends on the quantity of impurities and special technological requirements. Through circulation filtration heating water can never be absolutely clean, however the quality is stable.

Filtration is performed on bag filters or on self-rinsing filters with cartridges.

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For steam boilers

Preparation of feeder water

For steam boilers the difference between the produced steam and the return condensate must be replaced with feeder water. Feeder water must be prepared in such a way that does not cause formation of sediments, deposits or corrosion. The method of feeder water preparation depends on the type of boiler, working pressure, the quantity and quality of return condensate.

To prevent formation of sediments and deposits we must remove some substances from the feeder water, which could form deposits. This can be done by means of ion exchange (softening or demineralisation) or with membrane separation (reverse osmosis).

To protect system material against corrosion we supply corrosion inhibitors into the feeder water and we regulate pH. Only the right concentration of such substances in the system water enables efficient corrosion prevention. In the system with stable water quality, under supposition that the quality of raw water is relatively stable, inhibitors can be supplied proportional to the feeder water flow. In other cases we must control the level of inhibitors regularly and adjust (add) their quantity if needed.

Corrosion dynamics can be ascertained and estimated with measurement of weight loss on corrosion coupons. These must be embedded into the circulation subject to the appropriate standards.

Chemicals for boiler water treatment normally contain hardness stabilizers and dispersants. Hardness stabilizers are substances, which prevent that eventual rest of hardness after ion exchange or in any other process of water treatment would be released in the form of deposits. Dispersants have a cleansing effect. They remove deposits which occur as a consequence of local and short-term excess of solubility of dissolved substances due to a local temperature increase or due to short-term failures in the process of water preparation.

In case of smaller boiler systems we normally dose a multifunctional chemical which contains all needed components to protect the system.

Feeder water treatment

The first and the biggest problem in the system is corrosion.

At the inlet into the water supply tank water is normally thermally degasified. Additionally we need to add chemical binders of oxygen. In a larger systems dosing of such chemicals is normally carried out if or when needed, based on the prior feeder water analysis. When selecting a binder of oxygen we need to pay attention to the steam purpose.

The second and not minor problem is the quality of reverse condensate. In order to cut costs of feeder water, we pay attention to the proportion of reverse condensate to be as high as possible, to be of adequate quality and clean.

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For air humidifiers and air chambers

In humidifiers and air chambers water vapour is needed for the process of air treatment. Water vapour can be provided by steam generators, by spraying or evaporation on a surface which is in contact with the air needed to be moistened.

Water treatment for humidifiers and air chambers is similar to the water treatment for low pressure steam boilers. It additionally needs dosing of biocides and TDS to prevent any problems caused by excessive thickening and exceeding solubility of individual types of salt.

Preparation of supplying water

Losses due to evaporation or TDS need to be replaced by supplying water. Such water must be prepared in the way that does not cause formation of sediments, lining or corrosion in the system.

To prevent formation of sediments and linings we need to remove from the supplying water some substances which could form such sediments. This can be achieved by means of ion exchange; softening, decarbonisation and demineralisation, or by means of membrane separations (reverse osmosis).

To protect system materials against corrosion and linings, we need to inject into the supplying water corrosion inhibitors, hardness stabilisers and dispersants or multifunctional chemicals which contain all the needed components. Only the right concentration of such chemicals in the system water can enable efficient system protection. In the system with stable water quality, upon the supposition that also supplying water is of good quality and stable, it is possible to inject chemicals proportional to the flow of supplying water. In other cases we need to regularly control the level of inhibitors and to supply them additionally if needed.

The corrosion dynamics can be ascertained and estimated with measurements of weight loss on corrosion coupons. These must be embedded in the circulation in accordance with the appropriate standards.

System water treatment

System water is thickened and contains impurities which enter the system through their contact with the air. The water temperature is usually around 20°C. Such circumstances are favourable for formation and growth of microorganisms (algae, fungi and bacteria). Bio-films and accumulation of algae represent a constant threat for infections.

To prevent microbiological growth in the system water we need to inject biocides. It must be emphasised that with the use of biocides we do not achieve water sterility, but we control biological growth and keep it within acceptable limits. There are two methods of dosing biocides available shock dosing and constant dosing.

With the occasional shock dosing we take measures when biological growth exceeds the desired level. Preventively we can dose chemicals several times a week, or we decided for biocide on the basis of regular biological tests.

With constant dosing using much lower concentration as the one used in shock dosing we want to prevent biological growth in the system water.

When using system biocides the so called resistance of microorganisms can appear which demands replacement of the dosing recipe.

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