In a cooling tower, heat is removed from cooling water by a combination of loss of sensible heat and by means of evaporation. As was discussed earlier, we can estimate the evaporation in a cooling tower to be equal to 17% of the recirculation rate for every 10°F T (the temperature difference between the hot water in and the cold water out in a cooling tower).

In order to better understand the operation of a cooling tower there are some other terms with which you should be familiar. One of these is the approach of the tower. The approach is defined as the difference between the cold water temperature of the tower and the wet bulb temperature of the atmospheric air near the tower. 

On the surface it would appear that evaporation in a cooling tower would not have a negative affect on the cooling water chemistry. However, we must remember that most of the water we encounter is not "pure". Specifically, when a raindrop falls through the atmosphere it collects a variety of dissolved gases. Once that drop contacts the earth’s surface, it picks up their contaminants such as dissolved and suspended solids. 

In essence, by the time the water is used for cooling tower makeup, it contains a variety of contaminants. These contaminants, coupled with process leaks or other surface contaminants, intensify the potential water problems in a cooling system.

Specifically, as water evaporates, it leaves behind almost all of the contaminants it contained. The net effect is that the contaminants are "concentrated" in the cooling water.
As an example, let's say you fill a beaker with 100 ml of water that has a chloride concentration of 50 ppm. You then put the beaker on a table and allow water to evaporate to the 50 ml mark. If you then analyzed the water, the chloride concentration would be 100 ppm. The reason is simple: one half of the water evaporated and the dissolved chlorides it contained were left behind. Therefore, the chlorides were concentrated in the beaker 2 times. In a cooling tower we refer to this as "2 cycles of concentration". The term cycles of concentration or simply cycles, is used to define the number of times the tower water has been concentrated over that of the makeup water. If we had allowed the water in the beaker to evaporate to 25 ml, the chloride concentration would have been 200 ppm and the cycles of concentration 4 (200 ppm divided by 50 ppm). ultimately exceed their solubility limit. When this happens, scale can form on heat exchange surfaces or precipitated solids can drop out in low flow areas.

We can control this concentration mechanism by removing a portion of the concentrated water from the cooling tower and replacing it with makeup water. This process called "bleedoff", is practiced in most cooling towers. In most instances, enough makeup water must be added to compensate for that lost by evaporation, windage and drift as well as bleedoff.

As the quantity of bleedoff in a cooling tower is decreased, the cycles of concentration are increased, the amount of makeup water needed is decreased, and the quantity of chemical treatment is decreased. Therefore, in order to prepare a competitive water treatment program, we must determine the maximum number of cycles at which the tower can be safely operated.

Choice of proper cycles of concentration for a cooling tower system is determined by its design, water characteristics, operating parameters and treatment program; these factors are discussed later in this manual.

The nomenclature employed in the following discussion is shown below.

E = Evaporative water loss, as a percent of the recirculation rate

W = Windage and drift water losses, as a percent of the recirculation rate

B = Water lost by bleedoff (and other losses to waste), as a percent of the recirculation rate

Q = Circulation rate, in gpm

V = Volume of the system, in gallons

M = Makeup water, needed, as a percent of recirculation rate

C = Cycles of concentration

T = Overall retention time, in days

P - Dosage of treatment chemical required, in parts per million (ppm)

Cycles of concentration can be calculated based on a number of different variables such as chlorides, total dissolved solids (TDS) or calcium. The variable used will depend on the operating conditions of the tower. For example, the use of chlorides for cycles determination in a tower using chlorine may not be reliable because the chlorine will contribute chlorides to the water. However, as an example, if cycles are based on chlorides, the formula for calculating the cycles of concentration is as follows:

As mentioned earlier, makeup is added to compensate for other water
losses in a system. Therefore,

M = E + H + B

Makeup also compensates for such undesirable water losses as leaks. Makeup, in gallons per minute, is obtained by multiplying percentage of makeup by Q, the system circulation rate. 

The amount of makeup needed is governed by a cooling tower's operating cycles of concentration. A material balance around the cooling tower yields the following relationship: