The feed water used for cooling the hot circulating fluid in the system generally has dissolved mineral impurities and suspended matter, also known as Total Dissolved solids (TDS) and Total Suspended Solids (TSS), respectively. As a result of heat absorption from the circulating hot fluid, the cold feed water evaporates in the atmosphere as pure vapor leaving behind the impurities in the system. Due to the loss of solvent, the TDS and TSS in the feed water increases and reaches a saturation level beyond which the water fails to dissolves more solids. The mineral salts precipitate out of the water as scale and form a sludge at the bottom of the cooling tower water tank. This can cause problems like clogging of the nozzles, increased microbial growth, lower heat transfer and corrosion; reducing the overall efficiency of the plant.
Blow downs are constantly done in the cooling tower to remove the depositing scale and sludge in the water tank. The blow down at normal operating conditions or at elevated pressure effectively throws out the impurities from the bottom of the tank in a very short time. This reduces the TDS and TSS level in the feed water. The time for which the blow-down is carried out is very critical as the efficiency of the plant depends on the amount of water available in the plant at any given time. Thus a blow down requires supervision and some technical know-how.
A blow down is usually done by a specific blow down valve provided in the tank bottom. The valve is opened and water is allowed to drain out at a fast rate, carrying with itself large amount of scale and sludge deposited in the tank bottom. The amount of water to be blown down is kept at below 25 percent of the total level. The time period involved depends upon the pressure and flow-rate of the blow down. Accordingly, make-up water is added to the tank to maintain effective heat transfer and overall efficiency of the system.
Proper and timely blow down can provide for major cost cuts in maintenance and repair by minimized carryover deposits in the plant and result in longer life of the equipments. It prevents undesired corrosion and microbial growth in the tank by removing the scale and sludge. Lower TDS and TSS levels mean higher heat transfer rate and lower operating cost, less feed-water consumption, less chemical treatment and higher heating efficiency.