Hi friend, in this blog we are going to discuss about one of the most common cooling tower problem is scale. I will update blog about cooling tower in detail and its problems & solution in upcoming blog.
What is scale?
Scale is a hard deposit of predominantly inorganic material on heating transfer surfaces caused by the precipitation of mineral particles in water. As water evaporates in a cooling tower or an evaporator condenser, pure vapor is lost and the dissolved solids concentrate in the remaining water. If this concentration cycle is allowed to continue, the solubility of various solids will eventually be exceeded. The solids will then settle in pipelines or on heat exchange surfaces, where it frequently solidifies into a relatively soft, amorphous scale.
Factors:
The principle factors responsible for scale formation are:
1. As alkalinity increases, calcium carbonate- the most common scale constituent in cooling systems - decreases in solubility and deposits.
2. The second—more significant—mechanism for scale formation is the in-site crystallization of sparingly soluble salts as the result of elevated temperatures and/or low flow velocity. Most salts become more soluble as temperature increases, however, some salts, such as calcium carbonate, become less soluble as temperature increases. Therefore they often cause deposits at higher temperatures.
3. High TDS water will have greater potential for scale formation.
Types:
Typical scales that occur in cooling water systems are:
1. Calcium carbonate scale - Results primarily from localized heating of water containing calcium bicarbonate. Calcium carbonate scale formation can be controlled by pH adjustment and is frequently coupled with the judicious use of scale inhibiting chemicals.
2. Calcium sulfate scale - Usually forms as gypsum is more than 100 times as soluble as calcium carbonate at normal cooling water temperatures. It can usually be avoided by appropriate blowdown rates or chemical treatment.
3. Calcium and magnesium silicate scale - Both can form in cooling water systems. This scale formation can normally be avoided by limiting calcium, magnesium, and silica concentrations through chemical treatment or blowdown.
4. Calcium phosphate scale - Results from a reaction between calcium salts and orthophosphate, which may be introduced into the system via inadequately treated wastewater or inadvertent reversion of polyphosphate inhibitors present in recycled water.
The most common type of scaling is formed by carbonates and bicarbonates of calcium and magnesium, as well as iron salts in water. Calcium dominates in fresh water while magnesium dominates in seawater.
Control:
Scale can be controlled or eliminated by application of one or more proven techniques:
1. Water softening equipment – Water softener, dealkalizer, ion exchange to remove scale forming minerals from make up water.
2. Adjusting pH to lower values - Scale forming potential is minimized in acidic environment i.e. lower pH.
3. Controlling cycles of concentration - Limit the concentration of scale forming minerals by controlling cycles of concentration. This is achieved by intermittent or continuous blowdown process, where a part of water is purposely drained off to prevent minerals built up.
4. Chemical dosage - Apply scale inhibitors and conditioners in circulating water.
5. Physical water treatment methods – Filtration, magnetic and de-scaling devices
