EcoWater CHC, Installation & Service Manual

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29
TECHNICAL ASSISTANCE
CYCLES OF CONCENTRATION
What are Cycles of Concentration ? Cycles are not the flow cycles of water in the system. It is a
comparison of the concentrations of dissolved salts in makeup water and basin water. Because of
evaporation, H
2
O left water to evaporate and leave the dissolved salts in cooling water. So the
concentration of the dissolved salts in the cooling tower water will increase. Cycles of 2 means the
concentration of certain species (like sodium or chlorides) in the cooling tower water is 2 times the
concentration in the makeup water.
How do you calculate cycles? The easy, and most popular method is to use TDS (Total Dissolved Solids).
Cycles are equal to the TDS (or conductivity) ratio of basin to makeup. This is based on the assumption
that all dissolved constituents in makeup will still be in basin water. The evaporation is the only source for
changes in concentration. This is true for systems with a successful chemical treatment. But for water
treated by CHC, parts of the dissolved calcium and bicarbonate are transformed into calcium carbonate
solids. TDS is only good for estimating the cycles, EcoWater CHC recommends the use of chloride
concentrations to calculate the actual cycles.
If the makeup and blow down rates are known, cycles can also be calculated by the ratio of makeup
volume to blow down volume. This method will give an average cycles over a certain time period, instead
of a snap shot value when using TDS or chloride.
TDS vs. cycles: When we tell customers we are going to reduce blow down significantly, the customers
may think there will be a huge bump up in cycles and TDS. Yes, cycles will go up, but not necessarily
TDS. Let's look at this in detail:
When using chemical treatment, acids or scale inhibitors are used to prevent the precipitation of calcium
carbonate, so all the dissolved solids from makeup will stay in the water. Therefore the TDS of the
cooling water is linearly proportional to the makeup TDS and cycles:
TDS (basin) = TDS (makeup) * cycles
Actually, the TDS (basin) may be even higher, because of the addition of treatment chemicals. This is
usually negligible; but for some water having very low TDS, such as water from the Pacific North West,
the contribution could be noticeable.
When water is treated by CHC, reactions in the CHC chamber will force some dissolved Calcium and
bicarbonate to form CaCO
3
colloids. These colloids are very small and are still suspended in water, but
they are not dissolved anymore. They are in a solid state, instead of a dissolved state. All other species,
such as chloride, sulfate and potassium will not be affected by CHC. Formed CaCO
3
may also contain a
small percentage of Mg, but most of Mg will be in water. So the TDS (basin) will not linearly increase
with cycles. The increase of TDS (basin) will be slower with cycles than in chemical treatment. Let's look
at an example:
References