Improving Cycles of Concentration

Cycles of Concentration, Cycles, and Concentration Ratio are all terms used to define how many times we can reuse the water in a system, before we have to remove some of the concentrated water (Blowdown) to prevent scaling or depositing from occurring. The more times we can reuse the water, the more efficient we are in the use of that water and improve the Sustainability of the process. To help understand how the term is used, how it is calculated and what the significance of it is, I’ll use a cooling tower to illustrate the principals involved.

A cooling tower is designed to evaporate heat from a process or HVAC system. As the higher temperature water evaporates, it leaves behind the minerals in the water. Think of a glass of distilled water in which we put a tablespoon of salt and stir it up. We can’t see the salt now; it is dissolved in the water. If we allow that glass to sit for a week or so, at least until all the water is evaporated, what do we have left in the glass? Salt – a tablespoon to be exact.

If we now add another tablespoon of salt to that glass and enough distilled water to fill it, and then stir it a bit, we now have 2 tablespoons of salt dissolved in the same volume of water. We can repeat this evaporation process and adding salt until we reach a point where no matter how much we stir the salt, we see it immediately start to settle on the bottom of the glass.

To measure what that limit is, we use a simple process of measuring a very soluble ion – Chlorides for example, in the city water (Make up Water), and compare it to the chlorides in the tower water (or concentrated water). The formula is:

Cycles = Chlorides in the Tower Water / Chlorides in the Make up Water

If our Make Up Water has Chlorides of 25 (ppm, but that’s another discussion), and the Tower Water has 100 Chlorides (ppm), then we have 100/25 = 4 cycles.

So what? Well, the more we can reuse the water before we have to send some to the drain to prevent scaling or settling, the less water we will need for makeup and the less chemical we will need for the new water being added. In fact at 3 cycles of concentration, 1/3rd of the water we are adding is due to blowdown. At 5 cycles, only 1/5th of the water being added is due to blowdown.

To help you increase your water, sewer and chemical dollar savings, ask your water treatment representative what you can do to increase your Cycles of Concentration in your systems.

Posted on behalf of Fred Lattin