Cooling Water Treatment A brief overview
Cooling Towers and Cooling Systems Treatment – Introduction
The following is designed to give a brief overview of water treatment for cooling systems. By its nature, the information is not fully comprehensive but we hope that it will give you an idea of the types of problems that may be encountered and common water treatment solutions to those problems. Further information is available from B & V Water Treatment’s technical specialists.
Cooling Towers and Cooling Systems Treatment – The basics
Water is widely used as a coolant with heat being transferred from hot process fluids into cooling water through a heat exchange surface. This cooling water then heats up but
In evaporative cooling towers, the evaporation of a small percentage of the water reduces the temperature of the rest allowing it to be used again. The evaporated water is replaced with make up water.
Evaporative condensers are often used to cool closed systems whereby pipework containing hot process fluids is sprayed with water to remove heat from the system.
If cooling systems are to operate effectively, their design and water treatment should ensure that they are safe and reliable and that they minimise the use of energy and water. Water treatment programmes aid these objectives by protecting the structural integrity of the system, through maintaining efficient heat transfer and by controlling microbiological contamination.
Water treatment programmes in cooling systems are aimed at controlling scale, corrosion, fouling and microbiological contamination.
The formation of insoluble calcium and magnesium salts results in scale, a rock like coating which may not only lead to a reduction in heat transfer but also act as a breeding ground for bacteria. One factor is the increase in concentration of dissolved salts in the system as water is lost through evaporation; if the hardness salts are kept in solution- scale will not form. In some cases, water softeners are used to remove the hardness from make up water; however this will lead to an increase in the corrosiveness of the water.
In many cases, scale inhibitor chemicals such as polyphosphates, phosphonates, acrylate polymers and copolymers are used. These either work by making the calcium / magnesium salts soluble or by altering the crystal structure so that it does not adhere to the heat exchange surfaces. The addition of acid (sulphuric) to lower the pH and alkalinity also reduces the potential for scale formation and is a means of scale control sometimes employed in larger cooling systems.
Corrosion in a cooling system results in the loss of metal from a surface, which may be seen as pitting and is often associated with the formation of deposits. It is accelerated by high levels of dissolved oxygen particularly in conjunction with low pH (low alkalinity) although excessive alkalinity can also be a factor as can temperature and the amount of dissolved solids. When dissimilar metals and alloys come into contact, an electrochemical reaction known as galvanic corrosion may also take place; this is affected by the pH and conductivity of water within the system.
Corrosion may also result from chloride attack on stainless steel and microbial activity (microbial corrosion).
Unfortunately the conditions which are optimum to reduce scale formation are those likely to favour corrosion. Chemical corrosion inhibitors which act by forming a film and thereby protecting the metal surface are often used. There are many types of corrosion inhibitors used in cooling systems including nitrites, phosphates, silicates, zinc, phosphonates, azoles and soluble oils.
The water within a cooling system provides optimal growth conditions for many species of bacteria, including Legionella spp. A build-up of biofilm within a system will not only inhibit the heat transfer efficiency of the system and block pipe work but will also encourage the growth of Legionella bacteria and may promote the formation of under deposit corrosion.
A correct biocidal treatment regime must therefore be implemented with regular checks to ensure that microbiological control is maintained.
There are two main types of biocide:
- Oxidising biocides simply oxidise microbial cells and will kill most micro-organisms including bacteria, algae, fungi and yeasts. Examples include sodium hypochlorite, chlorine dioxide and bromine.
- Non-oxidising biocides kill micro-organisms typically by disrupting the organism’s metabolism or damaging the cell wall. Examples include isothiazolines, 2-2-Dibromo-3-nitrilopropionamide, 2-bromo-2-nitropropane-1,3-diol, glutaraldehyde, tetrakis(hydroxymethyl)phosphonium sulphate and quaternary ammonium salts
Biodispersants are also used to break up any deposits of microorganisms allowing the biocides to attack the organisms more effectively. They inhibit the attachment of microorganisms to metal surfaces.
Sediment build up can lead to fouling particularly in low velocity areas. Fouling may be reduced by the use of products which clean the cooling tower packs and antifoam products.
Surfactants may also be used to penetrate deposits enabling them to be washed away from the deposition surface and anionic polymers may be used to prevent the agglomeration and subsequent deposition of suspended solids.
Testing should be conducted to ensure that any water treatment programme is working effectively.
B & V Water Treatment Products
B & V Water Treatment have many products which have been optimised for different systems. We employ all of the technologies described above (and many more). Our most popular cooling water products are viewable here but please speak to us to see which product we recommend for your needs. We are also able to offer products in a solid form.