Cycle Monitoring:  Finding The "Just Right" Solution To Water Reuse In Cooling Towers

Why do we want to reuse water in a recirculating cooling water system to the greatest extent possible?

• Water can be expensive and is available in limited quantities in many locations.
• Treatment chemicals added to the water cost money.
• When it's finally no longer usable, the effluent costs money to treat.

What keeps us from using the same water over and over?

• A goodly percentage of system water is lost to evaporation.
• The dissolved and suspended solids that are left behind when the pure water evaporates build up to destructive levels unless they are removed from the system by a process called blowdown (some call it bleed-off); blowdown results in water being lost to the system.
• Additional recirculating water, containing both water and its impurities and additives, leaves the system from drift, leaks, and intentional siphoning (draw-off).

If you didn't ever bleed water from the system and only added water as necessary to maintain the system's volume, what would be the consequences of recycling endlessly?

• With each cycle, the impurities in the system water become more concentrated. As saturation levels are reached for the scale-forming salts in the system water they will precipitate out to form harmful crystalline deposits. The potential for organic and inorganic fouling as well as underdeposit corrosion may also increase as the tower cycles up.
• In this environment some treatment chemicals might also lose their effectiveness—biocides and dispersants for instance.
• To stave off scaling and other problems as long as possible, you would probably apply more treatment chemicals, thereby eating up some of your savings on water.
• In a nutshell, there would be no "endlessly" because the system would fail!

Why is cycle monitoring the answer?

• Cycle monitoring enables you to find the happy medium between the expensive no-reuse option and system failure (which is not an option at all). It involves careful analysis of the chemistry of the makeup water and recirculating water followed by a determination of the cycles of concentration for all key parameters (calcium hardness, chloride, silica, total alkalinity, etc.). Once these are known, generally there will be one parameter whose concentration limit will dictate when blowdown must be initiated.

How are cycles of concentration calculated?

• Each time an amount of solids equal to the amount of solids in the makeup water is added to the system, this is counted as one cycle. For instance, if an analysis of the makeup for sulfate content indicates 200 ppm and 2,000 ppm is found in the recirculating water, the tower has cycled 10 times (2,000 ÷ 200).
• Suppose the industry maximum for calcium hardness is 800 ppm and the makeup introduces 150 ppm; this would indicate only 5 cycles could be run through before scaling becomes problematic (800 ÷ 150 = 5.33).
• In the scenario above, it wouldn't matter that your calculations show silica could be safely cycled up to 10 times because the water will be scale forming after 5 cycles.
• The number of cycles in a cooling tower is frequently determined by doing a chloride test (such as our K-1549) on the make-up and system water, as chlorides are very soluble, remain in solution, and are easy to measure even at low levels. If you're applying a chlorine-based biocide, such as bleach or hydantoin, to avoid a false-high reading on your cycles of concentration with the chloride test, test for calcium hardness (a good indicator unless the system water exceeds 800 ppm; use K-1567) or test for silica (appropriate up to 150 ppm in the system; use K-1273).
• Another method is to determine your cycles based on magnesium. To determine magnesium concentrations, subtract your calcium hardness test result from the total hardness result. Magnesium is very soluble in the operating conditions of most systems. Use the K-1514.
• Remember to filter out the suspended solids in cloudy sample water before testing. Filtering will not affect the levels of chloride, calcium, total hardness, or silica.