Concetration Factor & Bleed-off Control
An evaporative cooling system works by transferring heat from the process being cooled to the recirculating cooling water at a heat exchanger and then rejecting that heat at the cooling tower by evaporating a small proportion of the water. Evaporating just 1.8% of the recirculating flow achieves a temperature reduction of 100C. Since water is evaporated as pure H2O the dissolved minerals (TDS) in the remaining cooling water become more concentrated. Some increase in the concentration factor is often beneficial since it tends to makes the cooling water less corrosive but if it this allowed to go unchecked it can lead to scale formation and fouling. It is therefore often necessary to control the concentration factor by bleeding-off or blowing down a proportion of the cooling water and replacing it with less concentrated make-up water. This is best done using a conductivity controlled bleed-off system which automatically controls the concentration factor at the desired conductivity.
Cooling Tower Scale Control
Scale or lime-scale is a hard, rock-like deposit of calcium or magnesium salts that forms in heat exchangers and cooling tower packing as a result of heat and increased concentration factor. Scale formation impairs heat transfer, interferes with flow and cooling, and can be a breeding ground for legionella bacteria. The scaling tendency of a water supply will depend on the hardness of the water but if not adequately treated even relatively soft waters can become highly scaling when concentrated by evaporation. Cooling water scale control is achieved by either:
- Removing the hardness from the make-up water using a water softener
- Adding a scale inhibitor chemical which extends the solubility of the hardness salts and thus preventing scale formation
- Dosing sulphuric acid to lower the pH and alkalinity and reduce the scaling potential (this technique is generally restricted to large industrial systems)
- Limiting the system concentration factor to a range within which the hardness salts can remain soluble
Cooling Water Corrosion Control
Corrosion may be the result from a number of different mechanisms such as general oxygen driven attack, galvanic corrosion cells caused by dissimilar metals, chloride attack of stainless steel and microbially induced or influenced corrosion (MIC). As a general rule the more scale forming the less corrosive water tends to be to steel and the LSI and RSI indices can be used to predict this. This means that provided scale formation can be effectively inhibited it is beneficial to operate a cooling system at an elevated concentration factor therefore close control of the cycles of concentration is critical to cooling water corrosion control. In effect you are making the water chemistry work for you rather than having to fight against it. It is also important to control the microbial activity and physical fouling since both can be cause of severe localised corrosion. Once these basics have been dealt with you need a good corrosion inhibitor which is matched to the cooling water chemistry and system metallurgy, and to control its dosage effectively. Contact us to protect your cooling system from corrosion problems.
Cooling Tower Microbiological Control
An evaporative cooling system is often an ideal growth environment for a whole range of microbes such as bacteria, algae, fungi and protozoa which can lead to problems such as:
- Microbial slimes which interfere with heat transfer
- Highly localised microbial induced corrosion
- Reduced effectiveness of corrosion inhibitors
- Fouling caused by suspended solids being trapped in the microbial slimes
- Reduced flow and poor water distribution within the cooling tower
Effective cooling water microbiological control is not only essential to the efficient operation of a system but also necessary to control the growth of legionella bacteria and the risk of Legionnaires disease. In order to maintain microbial control effective biocides need to be added on a regular basis and the results monitored using dip-slides and periodic laboratory analysis.
Controlling scaling, corrosion and microbial growth will reduce problems of fouling but a cooling system may still become fouled with sediment in the incoming water supply (particularly if it isn’t mains water) and airborne dust and debris washed out of the atmosphere by the cooling tower. In some instances fouling may also be the direct result of process contamination. Deposits tend to settle in low-velocity areas and can cause reduced flow, blockages, impaired heat transfer, under deposit corrosion and encourage microbial growth including legionella. If the incoming water contains a lot of suspended solids then pre-filtration or clarification may be advisable. To keep the suspended solids mobile in the recirculating water the chemical programme should always incorporate a polymer dispersant and if the solids level is high then the incorporation of a side stream filtration system in a of the cooling circuit will help to keep the system clean. Since an evaporative cooling system is in effect an air scrubber so some build-up of deposits with time is inevitable so in addition to these on-going control measures the system should be taken off-line periodically for disinfection and the physical cleaning. Contact us to help you keep your cooling system free from fouling.
Dosing & Control Equipment
The best cooling water treatment programme in the world will be rendered ineffective if the concentration factor and the concentration of inhibitors and biocides cannot be accurately controlled. A dosing and control system should therefore incorporate:
- An automatic bleed-off control system (preferably conductivity controlled)
- Automatic inhibitor dosing (preferably water meter controlled based on the volume of make-up water)
- Aautomatic biocide dosing (either timer controlled for non-oxidising biocides or preferably redox controlled for oxidising biocides like bromine)
Cooling System Legionella Control
In light of the recent Edinburgh Legionella outbreak the HSE is drawing attention the risks and good practice concerning cooling towers and evaporative condensers in cooling water treatment. The whole safety bulletin can be found here: HSE Cooling Tower Safety Bulletin – This will open in a new page. The essential elements of good treatment programme are the same as those required for cooling tower legionella control – scale and corrosion control, effective microbial control and maintaining system cleanliness. In the India, because Legionnaires ’ disease is potentially fatal, owners and operators of cooling systems are required to notify them to the local authority and should adhere to the HSE Approved Code of Practice for cooling tower legionella control, ACoP L8. This means that you should:
- Appoint a responsible person to manage the system
- Carry out a formal legionella risk assessment
- Prepare a scheme of precautions and control measures to control the risk
- Implement and manage these control measures and monitor to ensure that they are working
- Keep comprehensive records because you may need to prove that you are meeting these requirements.