COC or the cycle of concentration is a abbreviation or term used when talking about cooling water. This value specifies how often the fresh water is used, before it is blown down or bleed off from the cooling tower.
In detail this means, in a open cooling water system there are usually cooling towers and consumers, such as heat exchangers or similar. The cooling water is circulated all the time.
The cooling water leaves the cooling tower at a certain temperture and comes back from the consumers in a higher temperature. This higher temperature will be cooled down in the cooling tower when part of the water is evaporated. Using the effect of evaporation chill.
As only water can evaporate, the minerals solved or suspended in the cooling water can not. So the amount of minerals in the cooling water are increasing over time. These minerals will lead to scaling or fouling in the system. To minimize this, the water has to be replaced regular. This is done by the so called blow down or bleed off. Part of the higher concentrated cooling water is drained from the system and replaced by feed water. The feed water holds far less minerals. This fresh water is used to replace the water evaporated and bleed off. The new water is diluting the left cooling water and so brings down the overall concentration in the cooling water.
The cycle of concentration specifies the relation in between the concentration of minerals in the feed water and the cooling water.
So assumed the feed water has 100 TDS and the cooling water in the loop has 400 TDS, the COC will be 4. As higher the COC as less water is needed for replacement.