Single-circuit cooling system with closed-loop water circulation
Single-circuit cooling system is used to cool down process and devices which have within a certain period of time a constant cooling water flow. Normally such cooling systems are used for production lines which function with ON/OFF regime as a whole.
Pumps, which ensure water flow, pump cold water from the cooling water pool and they supply it to the consumers. The water heated by the consumers is directed to cooling towers to be cooled again. Cooled water is then directed back to cooling water pools.
In open system of cooling towers a part of energy is transmitted by means of heat transfer from water into the air, one part is released through evaporation. That kind of cooling systems is used, when temperature of cooled cooling water is some degrees higher than the one of the wet bulb thermometer.
Due to a constant evaporation, when water is released from the system in the form of water vapour, the concentration of other substances is rising, which is also called condensed water. When the level of such concentration is too high, solubility of individual salts would be exceeded giving rise to formation of sediments and deposits, and the occurrence of corrosion on cooling system materials. For this reason a part of the system water is discharged and replaced by fresh water with a lower concentration of dissolved substances. This way we keep balanced quality of the system water in the system where eventual problems occurred by sediments, deposits or corrosion are manageable. Such water release is called TDS. In more modern systems this task is performed by automatic devices.
Dual-circuit water system treatment with closed colling system
In the event there are several users which needed to be cooled down and the needs for cooling water flow and cooling capacity can change through time, a dual-circuit cooling circulation system is the optimum solution.
The pumps which ensure water circulation in the circuit of users, also called primary circuit, pump cold water from the cold water pool and supply it to the users. Heated water from the users is directed to the heated water pool. The pumps which ensure water circulation in the circuit of cooling towers, also known as secondary circuit, pump cold water from the heated water pool and supply it to cooling towers where it cools down. Cooled water is directed to the cold water pool.
In open cooling system one part of energy is released by means of heat transfer from water into the air, another part of energy is released through evaporation. Such cooling systems are used when cooled water temperature is a few degrees higher than wet thermometer temperature. Theoretically, cooling water temperature is approaching to the wet thermometer temperature.
Due to a steady evaporation of water steam from the system, the concentration of other substances is rising, which is called thickening of the water system. When thickening becomes too strong or too high, this will overcome the solubility of some sorts of salt which could cause the appearance of lining or sediments, and corrosion on elements of cooling system. For this reason a part of the water system is released and replaced by fresh water with a lower concentration of dissolved substances. This is the way how we maintain a balanced quality of water system, where the problems of sediments, lining and corrosion can be manageable. Such water release is called total dissolved solids (TDS). In modernised systems this process is carried out automatically.
Open circuit cooling system
Cooling systems in circulation are the type of systems where cooling water is taken from immediate environment (from the lake, river or other type of watercourse). The water turns its path only once in the cooling process and then it comes back. In the past this was the cheapest way of cooling in locations which were rich in cooling water and this resulted in its relative frequency of usage. In terms of investment such process was quite simple, undemanding and there were no serious problems if the characteristics of water didn’t encourage 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.
We need to take care for the proper operation of the system by means of adequate preparation of cooling water.
Preparation of supplying water for closed cooling water systems
Losses due to evaporation and TDS need to be compensated by supplying water, which must be prepared in a way that do not cause formation of sediments, deposits or corrosion in the system.
To prevent formation of sediments and deposits we must remove substances from the supplying water, which could cause sediments. This can be achieved through ion exchange: softening and decarbonisation, or with chemical treatment.
To protect system material against corrosion and deposits, we need to add corrosion inhibitors into the supplying water. 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 inhibitors proportional to the flow of supplying water. In other cases we need to check regularly the level of inhibitors and add them if needed.
Chemicals for cooling water treatment normally include hardness stabilizers and dispersants. Hardness stabilizers are substances which prevent eventual rest of hardness to be released in form of sediments after the ion exchange process or any other water treatment process. Dispersants have a cleansing effect. They remove sediments, which are formed consequently locally or as a short-term excess of solubility of dissolved substances due to local temperature rise or due to short-term failures in the process of water preparation.
System water treatment for closed cooling water 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.
Preparation of cooling water for open cooling water systems
In open cooling water systems water is cooled with surface water or groundwater. Before entering into the cooling system, such water must be well-prepared so it does not cause formation of sediments, deposits or corrosion in the system.
To avoid entering any impurities and other particles into the system which would prohibit the cooling part of the process and increase resistance of water flow and heat transfer, we install, if necessary, filter devices. The choice of procedure and equipment depends on the type of impurities in the water source and the needs for water purity.
To prevent formation of sediments, deposits and corrosion we must keep chemical balance at increased temperatures by means of adding suitable compounds.
To protect material against corrosion we need do supply corrosion inhibitors into the supplying water. Only the right concentration of these substances in cooling water enables efficient prevention of corrosion. In the system of stable water quality, upon the supposition, that the supplying water quality is relatively stable, it is possible to supply inhibitors proportional to the supplying water flow. In the rest of the cases we need to check regularly the level of inhibitors and if needed to adjust their concentration.
Chemicals for cooling water treatment usually contain hardness stabilizers and dispersants. Hardness stabilizers are substances which prevent the water hardness to be released in the form of sediments. Dispersants have a cleansing effect. They remove sediments which occur as a consequence of local and short-term excess of solubility of dissolved matter due to local temperature increase or due to short-term failures in the process of water preparation.