Waterguide - Cooling water

What is cooling water?

Waste heat power from plants, industrial and commercial production sites must be dissipated, which means a cooling circuit is required. Coolants are used to remove the heat, with water being the most frequently used coolant.

Once‐through cooling is the simplest and earliest form of water cooling. In this case, water is extracted from a river or lake (sometimes groundwater or seawater is used too) and is pumped to the heat exchanger after treatment or conditioning (if required). The heated water is returned to the body of water it was extracted from.

Once‐through cooling results in a high water consumption and high cost (depending on the water source); moreover, it causes thermal pollution of the water body, which is increasingly being limited by law. In many instances, once‐through cooling is therefore replaced by other processes. Nevertheless, it is still the most cost‐efficient cooling option for large industrial plants (power plants, chemical industry) located near to water bodies. Nowadays, the heated water often passes a cooling tower before being returned to the water source in order to reduce the thermal pollution on the water body particularly in summer.

How does a cooling circuit work?

1. heat is transferred from machines to cold coolant
2. warm coolant dissipates heat to the environment
3. recirculation of the cooled coolant, if necessary

At many industrial sites, however, the water supply is limited or the costs for fresh water and wastewater continue to rise, making it necessary to recirculate the cooling water.  A recirculation system can be designed as an open or closed cooling circuit, hybrid solutions are also possible.

The open cooling circuit is the most common variant. In this case, water to be cooled is sprayed from the top of the cooling tower onto a fill medium and then trickles downwards through the fill. This creates a large surface that enables efficient heat exchange. Air is transported through the cooling tower (usually in counterflow) by a fan (induced or forced draft) or by buoyancy in a tall chimney (natural draft). Cooling occurs because of convection and evaporation, with evaporation accounting for the larger part of the cooling effect. Cooling water is then returned to the heat exchanger.

In the cooling water circuit, water is lost due to evaporation and drift as well as blowdown. These losses must be compensated with make‐up water. However, the water consumption is much lower than with once‐through cooling.

In a closed cooling circuit, cooling water does not trickle down inside the cooling tower, thereby coming in contact with the atmosphere. Instead, it flows through the tubes of a heat exchanger bundle that is mounted inside the cooling tower. Fans press or suck air through the cooling fins, thereby dissipating heat to the air. Cooling water does not evaporate like it does in an open cooling circuit, so the efficiency is lower than with open circuit cooling. On the other hand, no make‐up water is required in closed circuit cooling (unless water would be lost due to leaks).

A closed cooling circuit requires little or no maintenance of the softened or desalinated cooling water after an initial conditioning when filling the system (which, however, requires a particularly high‐quality cooling water treatment).

An advantage compared to an open cooling circuit is the lack of water demand, disadvantages are the lower cooling efficiency and the associated higher specific power consumption.

What are the different types of cooling?

• Wet cooling: water is trickled into the cooling tower, highest efficiency
• Dry cooling: air as cooling medium, no water consumption
• Hybrid cooling: Combination of wet and dry cooling tower, no steam vapour and small design

Of the cooling variants mentioned, wet cooling (i.e. open cooling circuit) is the most commonly used method as it has very good efficiency and low capital and operating costs. The type of construction (fan or natural draft cooling tower) often depends on the circulating water volume. Power plants are mainly equipped with natural draft cooling towers, industrial plants tend to have fan cooling towers, which can be better adapted to changing operating conditions. Fan cooling towers are often built-in cell design, row, or block arrangement.

At locations with little water, dry cooling (i.e. closed cooling circuit) offers advantages. No make-up water is needed, and no vapour clouds (mixture of air, water vapour and finest water droplets) are produced. However, the achievable cooling limit temperature is higher than with wet cooling, which can cause bottlenecks, especially in summer. Capital and operating costs are also significantly higher.

In hybrid cooling, the wet and dry sections are combined. The advantages are the compact design, the lower additional water requirement, and the reduced generation of steam vapours. In winter and when cooling requirements are low, water cooling can be partially dispensed with (dry operation). During this period, the cooling water is drained or stacked in a storage tank.

Why is it necessary to treat the cooling water?

When cooling water comes into intensive contact with ambient air and the constituent concentration increases due to cycles of concentration, operating problems can result from limescale build-up, biological growth and corrosion. Therefore, suitable water treatment is necessary to ensure safe and economical operation.

Limescale deposits become a problem in the cooling tower, especially in pipes, on fixtures and in droplet separators. Reduced heat transfer and higher energy consumption due to high water pressure are the result. Make-up water must therefore be softened or demineralised. In the circuit, deposits are avoided by dosing hardness stabilisers.

In the cooling circuit, the temperature, nutrient supply and incidence of light create very good conditions for the growth of algae, fungi and microorganisms. The resulting biofilms reduce the flow and cause blockages. Some microorganisms, such as legionella, also pose a health hazard. Biocide dosing can effectively reduce the biological activity.

Corrosion damage can be caused, for example, by incorrect pH values, by aggressive water, increased concentrations of chlorides or sulphates. The corrosion tendency is reduced by dosing corrosion inhibitor and, if necessary, adjusting the pH value.

Which Herco units can be used for industrial cooling purpose?

A water analysis is always necessary to select the right solutions, as the composition of the raw water differs depending on the region. Herco will work with you to develop the right concept for your customer's needs, while adhering to the specifications of the 42nd BImSchV. Ask us or watch our webinar "Cooltrol data".

Applied water treatment technologies:

• If necessary, filtration for the removal of suspended solids as well as iron and manganese by means of HEF filters.
• Softening systems to prevent limescale deposits, e.g. ECOTROL-D
• Reverse osmosis for desalination, in combination with antiscalant dosing for membrane protection and hardness stabilisation
• Desalination by means of Cooltrol data to control thickening as well as chemical dosing and to comply with legal documentation requirements

On our download page you will find a product overview.