Intro
Water is the most important resource on the planet, and it is a task of humanity to preserve its quality for future generations. Therefore, modern industry strives to enhance environmental sustainability along with the development and expansion of production volumes.
The use of no-contact water supply systems is a promising solution that helps to minimize consumption of natural water resources, reduce the load on treatment facilities and decrease environmental pollution.
What does the closed-loop water supply system include?
In traditional industrial water supply systems water is used for technological processes and then treated before being discharged into the environment.
In closed-loop systems, water is subjected to a purification process after its initial use and then returned to the technological process or a consumption system.
Closed- loop circulating water systems can be divided into two types:
- “Clean” cycles (no-contact systems),
- “Dirty” cycles (contact systems).
In this article we will consider conditionally “clean” cycles.
Conditionally “clean” cycles.
The so-called “clean” cycles differ from “dirty” systems in which incoming water does not require deep purification as its contamination is minimal. The example of it is equipment cooling, where the water does not come into direct contact with contaminants.
Schematic diagram of a so-called “clean” circulating water supply system.
1. Water intake from a source.
Water comes from a natural water source (river, lake, well) or a central water supply system. At this stage mechanical filters may be used to remove large contaminants (suspended particles, sand, debris).
2. Primary water treatment.
Before coming into the system water is subjected to:
- Clarification and filtration – removal of suspended particles.
- Disinfection (if necessary) – treatment with chlorine, ozonation or UV irradiation.
- Adjustment of chemical composition – correction of hardness and pH to prevent deposits.
3. Using in cooling systems / production processes.
Purified water is supplied to process consumers – heat exchangers, condensers, cooling units, etc. This water is heated up and evaporates partially.
4. Collection and return of heated water.
After use the water is collected and returned to the circulation system. Here it is important to consider two flows:
- Clean water – suitable for reuse with minimal purification.
- Contaminated water – requires additional treatment.
5. Cooling of circulating water
To reduce water temperature the following facilities are used:
- Cooling towers – evaporative cooling (a part of the water is lost as steam).
- Dry-type coolers are air heat exchangers.
- A cooling pond is an artificial reservoir in which water is cooled naturally.
6. Additional purification
Circulating water passes through filters, coagulation, biological or chemical treatment for removing mechanical impurities, oils, hardness salts and biological contaminants.
7. Source water make-up
Water losses (evaporation, leaks, sludge discharge) are compensated by adding fresh water to the system. The amount of make-up water is minimal, since the water is repeatedly reused.
8. Reuse in a technological cycle
Purified and cooled water returns back into production, closing the recycling processes.
Schemes of circulating systems.
There are several types of circulating systems:
- open loop,
- closed loop,
- double loop.
Let us take a closer look at these schemes using the example of heat removal from heated equipment.
Open-type circulation system.
It consists of an effective supply of water in the quantity, required for cooling equipment that was heated by some heat source during physical and chemical processes and further removal of heated water for cooling in cooling towers, irrigation pools or cooling ponds.
The essence of the open-type circulation system is to cool down heated water by contact with the environment, where the water volume passes through a mass of air with a lower temperature.
This scheme is used for all cooling circuits at thermal and electric power plants, metallurgical plants, and chemical industry facilities.
Open cycles are used in cases, where there are no strict requirements for the presence of dissolved oxygen, and the amount of biological impurities from the air can be controlled by adding a biocide. Dust and small solid particles that have entered the water from the environment can easily be captured in filters before being fed to the cooling equipment.
They can also be used for cooling turbine steam at power plants, blast furnaces, continuous casting machines, electric furnaces, converters in chemical industry and air conditioning systems.
Closed-loop clean recirculating system.
A closed-loop recirculation system is a system in which water also constantly circulating without discharge. In a closed system heat is removed through heat exchangers while in an open recirculation system cooling occurs through evaporation in cooling towers,.
The main differences from the open-loop system:
🔹 No water losses. In a closed loop water does not evaporate, it means, no constant make-up is required.
🔹 Minimal contamination. Water does not come into contact with air, subsequently, there are no sedimentations, pollutants and microorganisms.
🔹 Temperature stability. In closed-loop systems it is easier to control the coolant temperature.
🔹 Energy efficiency. Due to lower water and heat losses, less energy is required for the system operation.
🔹 Reduced power consumption due to the absence of “stream disruptions” and control of cooling water pressure at the pump suction side.
To avoid capital expenditures for system make-up, clean closed loops are applied. Water-to-water heat exchangers (plate-type or tubular) or water-to-air heat exchangers (air-cooled devices) are used to cool the heated water.
M HEAVY TECHNOLOGY has successfully implemented this method in projects for cooling BF No. 3 and BF No. 4 at PJSC “MK AZOVSTAL”, BF No. 2 at PJSC “Zaporizhstal” etc.
Double-loop circulation water supply system.
A double-loop circulation water supply system is a combination of two independent loops: a closed and an open one.
🔹 The first loop – cooling water circulates inside a closed system, cooling the equipment by means of heat exchangers. It does not contact with the external environment. Therefore, pollution and water losses are minimal.
🔹 The second loop – here the first loop water is used for process needs and is cooled in cooling towers, cooling ponds or irrigation basins.
The closed loop can contain not only water, but also a heat transfer medium of any chemical composition, such as propylene glycol, alcohol, etc., which is cooled down by means of plate-type or tubular heat exchangers with water from the open loop. After that the heated open loop water is coming to cooling towers, cooling ponds or irrigation basins for cooling.
M HEAVY TECHNOLOGY specialists successfully applied this approach at PJSC “Zaporizhstal”, Blast Furnaces No. 2, No. 3, No. 4 and 5 and at PJSC “Ilyich Iron and Steel Works”, Blast Furnace No. 4, where chemically treated water, heated by a blast furnace, was cooled down by means of heat exchangers and used in the secondary loop for the blast furnace shop needs.
Main difference from other systems.
✅ Difference from an open-loop system. In a double-loop system the water, directly contacting with the equipment, does not evaporate or become contaminated. All evaporative losses occur in the second loop.
✅ Difference from a closed- loop system. In a closed loop heat is removed through heat exchangers without external cooling, and in a double-loop system the cooled water of the first loop transfers heat to the second loop, where it is dissipated.
The main advantage of such a system is reduction of contamination of the main heat-transfer medium and water savings due to more efficient heat removal.
Advantages of conditionally clean systems:
● Water conservation. Water reuse reduces freshwater consumption.
● Reduction of costs for water purification and treatment due to reduction of volume and pollution of wastewater.
● Reduced environmental impact. Less contaminated wastewater emissions.
● Process stability: Using a stable water source reduces dependence on external conditions.
Industrial use
Conditionally pure closed-loop systems are used in steelmaking industry, power engineering, chemical and oil-refining industry.
For example:
- In metal industry such systems are used for cooling units, rolling mills, furnaces etc.
- In power industry – for cooling turbines and heat exchangers.
- In chemical industry they are used for technological support of synthesis and processing.
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Water treatment for clean recirculating systems.
Technical solutions and innovations.
To maintain water quality in closed systems, the following modern technologies are used:
- Filtration and ultrafiltration – for removing mechanical impurities.
- Reverse osmosis – for removing dissolved salts.
- Use of corrosion inhibitors – to prevent equipment wear.
- Biological treatment – for decomposition of organic impurities.
Examples of successful projects with clean water supply cycles.
Our company has extensive experience in creating clean open-, closed- or double-loop systems. The examples are blast furnace cooling systems at PJSC “Azovstal”, PJSC “Zaporizhstal”, PJSC “Kamet-Stal”, PJSC “Ilyich Iron & Steel Works” etc.
Implementation of our solutions at these enterprises made it possible to:
- significantly reduce water consumption,
- reduce power demand,
- increase reliability of metallurgical units
- reduce to a minimum or completely eliminate contaminated water discharge.
Conclusion
Conditionally “clean” closed-loop water supply systems are an important step towards sustainable industrial development. Their implementation makes it possible both to save resources and to reduce the impact on the environment. That is especially important in the context of tightening environmental standards. Investing in such technologies is not only a contribution to environment protection, but also a real economic benefit for enterprises.
