Controlling Cycles of Concentration
To a great extent the quality of the inflow water determine the extent depositions on heat exchange surfaces. The higher the levels of hardness and alkalinity in the inflow water, the greater will be the potential for scale accumulation.
Two key relationships - “cycles of concentration” and “bleed off water” are important.
Cycles of Concentration
The cycle of concentration (also referred to as concentration ratio) is defined as the ratio of the concentration of a specific dissolved constituent in the recirculated cooling water to the concentration of the same constituent in the makeup water. This figure establishes the minimum blowdown rate that must be achieved.
The concentration ratio (C) is determined by the following equation:
C = [E / (B + D)] + 1…………. (1)
Where:
E = Evaporation rate
B = Blowdown
D = Drift
Ignoring insignificant drift loss (D), the equation can be simply put as:
C = M / B…………….. (2)
Where:
M = Make up water equal to E + B.
The equation (2) tells us that as long as the amount of blowdown water is proportional to the amount of water entering the system, the concentration ratio will remain constant irrespective of variations in the inflow water chemistry.
Blowdown or Bleed-off
Evaporative loss from a cooling tower system leads to an increased concentration of dissolved or suspended solids within the system water as compared to the make up water. Over concentration of these impurities may lead to scale and corrosion formation, hence, fouling of the system. Therefore, concentration of impurities must be controlled by removing system water (bleed-off) and replacing with make up water. In order to control the total dissolved solids (TDS), bleed-off volume can be determined by the following formula:
B=E-[(C-1)*D] / (C-1)……………………… (3)
Where
B – Blowdown rate (L/s)
E – Design evaporative rate (L/s)
C – Cycle of concentration
D – Design drift loss rate (L/s)
The equation (3) tells us that it is practically impossible to ever achieve a concentration ratio of 1, because to do so would require an infinite amount of water. It also shows that as the concentration ratio increases the blowdown requirement decreases. As a rule of thumb, the minimum cycle of concentration shall be maintained at 5 to 6 for fresh water type cooling tower and 2 or less for seawater type cooling tower.
Increasing the blowdown is a simple way to reduce the levels of calcium and alkalinity in the water.
