Understanding Cycles of Concentration (CoC)
A very large portion of commercial and industrial businesses out there have either a cooling tower system, a boiler system, or both. Both of these systems take advantage of the large amount of heat that can be captured or released from the vaporization or condensation of water.
There’s one major catch to harnessing this large energy cache. When water evaporates (vaporizes, boils, etc.) any solids that were dissolved in the water are left behind. This means that we are left with less water, but the same amount of dissolved solids. The concentration of dissolved solids has increased through the process of vaporizing a portion of the water that we started with.
Chemical Treatment
High levels of total dissolved solids (TDS) can lead to scale buildup on surfaces reducing heat exchanger effectiveness. It can also lead to a lower pH and cause corrosion. The effects of these two issues can be treated with chemicals, but at some point the concentration will get too high to deal with and the amount of chemicals needed gets too costly.
System Blowdown
Another way to handle this issue is to get rid of the excess TDS by expelling the water with the higher concentration called “blowdown” and replacing it with water called “makeup”. We could send all the water that comes back through the system with a higher concentration of TDS to the drain and use 100% makeup water, but this would be very costly.
The middle ground of these two methods is to blowdown a specific amount of water that allows us to keep the TDS at a manageable level and also reduces the amount of makeup water that we have to add.
Cycles of Concentration (CoC)
The term that the industry uses to refer to the relative amount of TDS allowed to build up in the system is “Cycles of Concentration” (CoC). The value is the ratio of the amount of TDS in the system to the amount of TDS in the makeup.
Systems normally operate at various CoC’s between 2 and 10, but they’re usually around 4 or 5. Some other “rule of thumb” values are: evaporation losses are assumed to be about 1% of the system flow and drift losses are assumed to be about 0.04% of the system flow.
Both the makeup flow rate and the blowdown flow rate can be calculated from the evaporation rate and the CoC. The formula for the blowdown flow rate is the following:
b is the blowdown flow rate
e is the evaporation rate
c is the CoC
The formula for the makeup flow rate is the following:
m is the makeup flow rate
e is the evaporation rate
c is the CoC
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