cooling tower water treatment: boosting your cooling tower efficiency

cooling tower water treatment: reduce water consumption, corrosion and scaling

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Cooling tower water treatment is an essential industrial mechanism: in heat-transfer equipment, corrosion and deposits will interfere the normal transfer efficiency. In the cooling tower side it translates into a smaller heat rejection and a lower cooling power capacity.

A cooling tower is, typically, an open water-recirculating system that takes water from the tower basin, passes the water through any equipment that requires cooling (chiller or industrial process) and then returns the water back to the tower where, in the tower packing, part of the water is evaporated to cool down the water that remains. 

This recirculating system repeats this process, taking in sufficient freshwater makeup to balance the water evaporated and water blown-down. 

With the water availability issues that we face today, reducing the amount of water used as makeup and re-using the blow-down water is a major concern, and the focal point of Watercore solutions for cooling towers.

Watercore engineers collaborate with facility managers to offer the best specialised water treatment solutions for their cooling towers. Water quality is a critical factor in cooling equipment and can have a damaging impact on cooling towers performance

Common problems linked to poor water management procedures are: low concentration cycles, high water consumption, corrosion, biological growth and scaling of Calcium and Magnesium carbonates and sulphates, heat-transfer parts and equipment.

Most Common Cooling Tower Water Treatment Options

1. cooling tower water treatment goal : maximum water recycling with minimum corrosion and scaling

Normally, in cooling tower water recirculation, water does not suffer a noticeable deterioration other than the increase in the concentration of inorganic salts and some organic compounds. Most organic compounds are related to the chemical dosing used as part of the corrosion and scaling prevention programs.

The main goal of any cooling tower water treatment protocol will try to increase the recirculation cycles while controlling corrosion and scaling risk. This requires the least soluble salts and corrosion-related ions to be removed in advance from the make-up water and and the presence of a controlled water blowdown.

2. Low concentration cycles and high water consumption in cooling towers:

There is a simple relationship between the makeup, blowdown and evaporation water in cooling tower systems. Drift losses are usually assumed to be insignificant:

Makeup = Blowdown + Evaporation (+ Drift)

The easiest way to measure makeup and blowdown water is to install meters in those locations. The amount of water lost by evaporation can be calculated as the difference between makeup and blowdown. 

When water evaporates from the cooling tower, all dissolved and suspended solids stay. If there were no water loss other than evaporation this situation would continue without limit and eventually the solubility of the dissolved solids would exceed their solubility limit causing massive scale and corrosion.

A controlled flow of concentrated cooling water is sent to drain to prevent an over-concentration of the dissolved and suspended solids. This is called blowdown or bleed. 

Cycles of concentration (COC) refers to the concentration ratio between the makeup and the blowdown and most cooling towers operate within a COC range of 3 to 10, where three is generally considered as minimum efficiency and 10 cycles is considered an appropriate efficiency.

Increasing cycles of concentration saves water, as essentially means that water is recirculated longer through the system before being blown down, and reduces the amount of treatment chemicals required.

Optimization programs based on chemical dosing (corrosion inhibitors, dispersants and scale inhibitors) combined with softening to reduce hardness (calcium and magnesium) from the make-up water and blowdown water reuse by means of water desalination techniques can bring the cooling tower system to peak efficiency both in terms of water use and energy use.

3. corrosion in cooling towers:

Corrosion is a complex chemical redox process that can be easily understood as follows:

Corrosion starts with the oxidation of processed metals such as steel or copper, which have a natural tendency to return to the native ore oxide states that exist in nature in the rocks from which they are extracted.

For example, Iron in its natural state is an oxide (Fe3O4, FeO, and Fe2O3). When processed into iron and steel, the oxygen is removed to form pure iron (Fe°) with the use of energy to transform the oxide into the metal. In the presence of water and oxygen, Iron (Fe°) naturally returns to an oxidised state.

If the oxidation products form an insoluble protective oxide film on the metal surface that prevent further oxidation, the process is called passivation and corrosion doesn’t proceed in a destructive form.

If the oxidation products from soluble oxides, these tend to dissolve in the circulating cooling water and the oxidation continues endlessly in what is considered an accelerated corrosion process.

Whether or not the protective passivating film is generated as part of the oxidation process depends on different factors such as the water PH, presence of biological contaminants, the water alkalinity and the dosing of corrosion inhibitors.

4. biological growth and legionella in cooling towers:

Cooling towers are an ideal environment for bacterial growth due to the warm water temperature.

Microorganisms in cooling systems are problematic for three main reasons: 

  • Form biofilms that stick to surfaces in the cooling system and having a strong insulating properties, block an effective heat transfer in heat exchangers. 
  • Increase fouling and corrosion.
  • In the case of Legionella, can represent a risk to human health
 
Monitoring and adequate control of biocide dosing and routine testing of biological activity are critical to ensure biological growth is controlled and limited throughout the cooling system.

5. scaling problems in cooling water:

When any salt is first added to water, the ions dissolve more or less rapidly. As more salt is added, the concentration of dissolved ions increases but the salt keeps on dissolving until the solubility limit is reached.  

When the solubility limit is reached, the system reaches equilibrium and the solution is called a saturated solution. This means that the liquid contains the maximum concentration of ions that can exist in equilibrium with the solid salt and any excess of those ions tend to precipitate.

The solubility limit is temperature and PH dependant. Many common scale-forming (Carbonates and Phosphates) minerals have inverse temperature solubility, what means that their solubility decreases with an increase in temperature. This solubility drop leads to localized scaling on the surfaces of heat exchangers, reduces heat transfer efficiency and represent a major problem in cooling tower systems.

The most common scale forming minerals in cooling tower systems are:

  • Calcium Carbonate (CaCO3)
  • Calcium Phosphate ( Ca3(PO4)2 )
  • Silica
  • Calcium Sulphate (CaSO4)
 
It must be noticed that Calcium brings a high scaling risk to the equation. Softening the makeup water to remove Calcium and Magnesium hardness allows to operate the cooling system at much higher COC than otherwise possible when using raw, unsoftened source water.

6. typical limits for circulating water in cooling systems:

ParameterPHHardness
(mg/l CaCO3)
Conductivity
(µS/cm)
Chlorides
(mg/l)
Sulphates
(mg/l)
TDS
(mg/l)
Threshold/
limit
6 – 930 – 500< 1800< 150< 150< 1300

7. watercore : helping with cooling tower water treatment

Controlling corrosion, microbial growth, scaling and water use in cooling towers is essential for optimizing the system performance and efficiency.

When deciding the right water treatment for your cooling tower, Watercore can help with a range of solutions that will improve the water chemistry and tower operation based on your particular needs.

 

8. learn more about cooling tower water treatment: parameters that have an impact in your facility

Mild or carbon steel is one of the most commonly used materials for construction of heat exchangers and boiler and cooling water systems. However, this metal is easily corroded in water when Carbon dioxide (CO2) and oxygen (O2) are present.

Carbon steel is sensitive to pH and suffers significant corrosion at low pH. The corrosion rate drops as the pH increases over 8.5, but corrosion increases at very high pH (over pH 12). As with other chemical reactions, corrosion increases with temperature. Consequently, high-heat-flux or high-temperature exchangers will suffer greater metal loss than transfer piping.

The amount of make-up water required by a cooling tower depends on many factors such as the air hygrometry and design parameters like the ΔT across the cooling tower but for quick assessments the following value is usually accepted:

  • Make-up water needed in open recirculation cooling towers :  3 m3/h per MW

 

In normal operation and due to the evaporation of part of the circulation water, the salt content in circulating water is continuously increasing in cooling water circuits. If no special measures are taken, the continuous accumulation of salts will lead to scale formation or incrustations after reaching the solubility limit. In order to avoid inadmissible salt content in the recirculation water circuit a certain level of water purging or blowdown is required.

The primary reason why scaling forms is that solubility of Calcium and Magnesium salts in water decreases as the temperature and concentrations increase. When feed-water salinity is elevated or blow-down is not enough the concentration of the scale-forming salts exceeds solubility and precipitation, hence scale, starts. Our water treatment solutions designed for cooling towers can help reducing the scaling risk in cooling towers.

The blowdown amount will depend on concentration cycles or recirculated water salinity. The limits of performance for each cooling tower will depend on the design materials, fill packing used, and the process for which it works but in general 1800 μS/cm or  1300 ppm TDS are considered industry limits for the circulating water salinity in cooling tower systems. 

Since cooling tower water losses due to blowdown, evaporation and entrained droplets are replaced with make-up water, make-up water salinity will define the starting salinity level of the circulation water. 

The higher the salinity of the make-up water, the lower the number of concentration cycles, the higher the blowdown amount and total water consumption.

Removing salinity (TDS or Total Dissolved Solids) in the cooling tower make-up water can increase the number of recirculation cycles, reduce the cooling tower blow down (waste water), and therefore reduce the overall water consumption up to 50%. 

Desalination as make-up water treatment for cooling towers also reduces the amount of chemicals (scale inhibitors) required otherwise for water conditioning.

Nanofiltration and reverse osmosis are two common methods used in water treatment for cooling towers as both reduce the amount of TDS in make-up water .

Scale typically occurs inside the cooling tower water circuit in the fill packing, where the ions in the water concentrate by evaporation, and in heat-transfer elements (i.e. at the chiller’s condenser) reducing water throughput and heat exchange efficiency. 

However it is also common to find scale incrustations in the basin and drift eliminator.

Fouling is different from scaling in the way that fouling deposits are formed from insoluble material suspended in water. Suspended solids, organic contaminants like oil, corrosion products, and microbial growth are typically responsible for boiler and cooling water fouling.

FactorSolution
Make-up water salinity RO Desalination
Make-up water hardnessWater Softeners
Corrosion preventionDosing of corrosion inhibitors
Corrosion MonitoringLPR corrosion monitoring system
Blodown water reuse RO Desalination
PH AdjustmentChemical dosing

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