Cooling tower mass balance gives an indication about make-up water requirement in system. Cooling Tower Makeup is required due to water losses resulting from Drift, Evaporation & Blowdown.
Water make-up (M ) = Total water losses = Drift Losses ( D) + Evaporation Losses (E ) + Blow down Losses (B)
M = D + E + B
M = Make up water Requirement in m3/hr
D = Drift Loss in m3/hr
E = Evaporation Loss in m3/hr
B = Blow Down in m3/hr
Drift Losses (D) or Windage:
Drift losses may be assumed to be:
D = 0.3 to 1.0 percent of Circulating water (C ) for a natural draft cooling tower
D = 0.1 to 0.3 percent of Circulating water (C ) for an induced draft cooling tower
D = about 0.01 percent or less of Circulating water (C ) if the cooling tower has windage drift eliminators
Evaporation Losses (E ):
It is calculated on the basis of heat balance around the cooling tower
C = Circulating water in m3/hr
= Latent heat of vaporization of water = 540 kcal/kg (or) 2260 kJ / kg or
Ti – To = water temperature difference from tower top to tower bottom in °C ( cooling tower inlet hot water and outlet cold water temperature difference)
Cp = specific heat of water = 1 kcal/kg / °C (or) 4.184 kJ / kg / °C
Alternatively to find the evaporation loss by the following formula.
Evaporation loss in m3/hr = 0.00085 x 1.8 x circulation rate in m3/hr x (Ti-To) in 0C
BLOW DOWN OR DRAW OFF:
The process of cooling water circulates the cooling tower part of water evaporates thereby increasing solids per unit volume of solution also the amount of material dissolved in a unit volume of solution.
To control the Cycle of Concentration blow down or Draw off is given. Blowdown can be calculated from the formula
B = E/ (COC-1)
B = Blow Down (m3/hr)
E = Evaporation Loss (m3/hr)
COC = Cycle of Concentration.
CYCLE OF CONCENTRATION:
The cycle of concentration is a dimensionless number. It is a ratio between parameter in Cooling Water to the parameter in Makeup water. It can be calculated from any the following formulae.
Cycles of concentration (COC) = Silica in Cooling Water / Silica in Makeup Water
Cycles of concentration (COC) = Ca Hardness in Cooling Water/ Ca Hardness in Makeup water
Cycles of concentration (COC) = Conductivity of Cooling Water / Conductivity of Makeup water
Cycles of concentration (COC) = Concentration of chlorides in circulating water /Concentration of chlorides in make-up water
The cycle of concentration (COC) normally varies from 3.0 to 7.0 depending on the Process Design and Manufactures Guidelines. In some large power plants, concentration cycles of the cooling tower may be much higher.