Description
Towersolve PM 400 – Range of Cooling tower chemicals to protect cooling water surfaces against the 3 common problems encountered
- Scaling
- Corrosion
- Microbiological growth
Open evaporative cooling systems can handle high heat loads with a minimum of water loss. This capability is also responsible for many of the problems associated with treating such systems. When a portion of cooling water is evaporated in the tower, the dissolved and suspended solids present in that water remain in the retained water. As evaporation proceeds, the concentration of total solids in the cooling water increases.
Since dissolved salt has a maximum solubility limit, deposition occurs when concentration of the salt in the cooling water is permitted to continue unchecked. In this situation, the salts of lowest solubility, such as calcium and magnesium carbonates and silicates, precipitate first. Any suspended matter in the tower will concentrate in a manner similar to that of dissolved slats. Concentration of airborne or waterborne debris can accelerate this sedimentation of suspended solids, eventually causing severe deposits problems.
The concentration effect described below can be prevented by removing a portion of the concentrated water from the cooling tower and replacing it with less-concentrated makeup water. This process called bleeding – is practiced on most cooling towers. In most instances, makeup water must be added to compensate for that lost by evaporation, windage and drift as well as bleed off.
Varying bleed off controls the degree of concentration in the tower. This is measured in terms of cycles of concentration which is the ratio between the total dissolved solids (TDS) in the cooling tower water and TDS in the system’s makeup supply. For example, cooling tower water containing four times as much TDS as its makeup supply would be operation at four cycles of concentration. An alternative to measuring total dissolved solids in the cooling water and makeup in order to calculate cycles of concentration is to measure a typical soluble ion, i.e. chloride.
Cycles of concentration are defined as:
Cycles of Concentration:
- Cooling Water Chlorides (ppm)
- Makeup Water Chlorides (ppm)
Determining optimum cycles of concentration for a cooling tower system is determined by its design, water characteristics, operating parameters and treatment program.
As mentioned earlier, makeup is added to compensate for other water losses in a system. Therefore, makeup also compensates for undesirable water losses such as leaks. Makeup, in litres per minute is determined by multiplying percentage of makeup by Q, the system circulation rate. The amount of makeup needed is governed by a cooling tower’s operating cycles of concentration.
A cooling tower with specific evaporation and windage losses can therefore, be regulated so that bleed off, or makeup, is actually a function of the tower’s operating cycles of concentration. In whish the curves show makeup requirements as a function of cycles of concentration for various range (evaporation) parameters. These data are based on assumed windage and drift of 0,2% of the circulation rate, a typical average for older mechanical draft cooling towers – newer units go as low as 0,005%.
An increase in cycles of concentration in the range of one to five can save significant amounts of makeup water, also results in significant treatment chemical savings.
Increasing cycles of concentration reduces this potential, however, promoting a scaling tendency instead. Control of calcium carbonate scale solely by cycles of concentration would dictate a maximum safe operating level of only 2 cycles for a cooling tower.
