In cooling towers, the heat is rejected by the partial evaporation of water as it flows through the unit. The evaporation will bring along a rapid concentration of the chemical species naturally dissolved in the water.
The principal scale found in cooling towers is calcium carbonate which comes from the decomposition of calcium bicarbonate. This process of decomposition may also form derivative salts with magnesium, sodium or silica among others. Whichever may be the case, their solubility would depend on conductivity, hardness, pH and temperature.
Given no control, the increase of salts concentration can compromise the integrity of the system and reduce its efficiency. In a more extended scenario, scales formed in the cooling tower can travel all the way to the condenser, create hard deposition and affect the steam condensation process. If the cooling system fails to drop down the temperature in the condenser, then the vacuum will decrease and interfere in the overall efficiency of the plant.The problem can be tackled by applying an adequate chemical treatment program, but also it is required the design of an effective blowdown system. In fact, the operation of a cooling tower is measured by the “cycles of concentration”. This ratio is determined by comparing the concentration of dissolved solids between the blowdown and the makeup water. The optimization of this ratio will reduce the scale buildup potential, decrease the water consumption and limit the blowdown disposal.The monitoring and control of the blowdown quality and flow is essential to ensure the correct operation of the cooling tower. The logic control loop can be implemented by mean of a control valve linked to a conductivity meter. The conductivity controller will automatically trigger the blowdown valve to keep the level of concentration as close as possible to the setpoint. The setpoint would depend on the material specifications of the cooling tower and the chemical treatment program used for operation.
In most cases, low pH is also required to keep solubility below the saturation point. Acid treatments change the equilibrium of carbonates toward the formation of more soluble compounds. This way the setpoint of conductivity can be increased to operate at higher cycles of concentration and prevent the deposition of salts.
The below image shows the effect of lack of balance in the blowdown, and also it was aggravated by deficiencies in the chemistry control of the system. The problem was specifically due to high levels of chloride and conductivity.
