Contents:
👉Mass balance of cooling tower
👉Types of Water Losses in cooling tower:
👉Evaporation loss
👉Drift or windage loss
👉Blow down loss
👉COC
●There are three types of water losses in cooling tower.
I.e evaporation loss, drift or windage loss and blow down loss.
Mass Balance Of Cooling Tower:
●Cooling tower mass balance gives an idea about make-up water
requirement.
●Cooling tower makeup has to substitute the water losses
resulting from evaporation, drift or windage and blow down .
● M = E + D + B
Where M=Make-up water requirement in M³/Hr
E=Evaporation loss in M³/Hr
D=Drift /Windage loss in M³/Hr
B=Blow down loss in M³/Hr
Types of Water Losses in cooling tower:
1.Evaporation Loss:
●Evaporation loss usually represents the efficiency of cooling tower and the heat of equipment.
●There is a formula to calculate evaporation loss volume:
● Evaporation loss (m3 /hr) = C x (T1-T2) x Cp
λ
Where
C= Circulation rate (m3 /hr)
λ= Latent heat of vaporization of water=540kcal/kg/ °C
T1= Cooling Tower Inlet water temperature in °C
T2= Cooling Tower Outlet water temperature °C
Cp=Specific heat of water=1 kcal/kg/ °C
Example: If circulation rate is 1750 m3/hr.T1=33 °C,T2=30 °C ,
Then Evaporation loss= 1750 x (33-30) x1
540
= 9.72m3/hr
There is an alternative formula to calculate evaporation loss volume:
● Evaporation loss (m3 /hr)
=0.0008 x 1.8 x C x (T1-T2)
C= Circulation rate (m3 /hr) ,T1= Cooling Tower Inlet water temperature
T2= Cooling Tower Outlet water temperature
(Reference: Perry’s Chemical engineers Hand book)
Example: If circulation rate is 2000 m3/hr.T1=33 °C,T2=30 °C ,Then Evaporation loss=0.0008 x 1.8 x 2000 x (33-30) = 8.64 m3/hr
2.Drift Loss or Windage Loss:
●Drift loss or windage loss of cooling tower is normally provided by the cooling tower manufacturer based on the process design.
● There is a formula to calculate evaporation loss volume:
● For natural draft cooling tower,
Drift or Windage loss (m3 /hr) = 0.3 to 1.0 x C
100
[Where C= Circulation rate (m3 /hr)]
●For induced draft cooling tower , D= 0.1 to 0.3 x C
100
●For cooling tower with drift eliminator, D= 0.01 x C
100
3.Blow Down Loss:
●.As the cooling water circulates the cooling tower part of water evaporates, there by increasing the total dissolved solids in the remaining water.
●To control the COC (Cycle of concentration) blow down is given.
● There is a formula to calculate blow down loss :
● Blow Down loss (m3 /hr) = E
COC-1
COC= Cycle of concentration , E= Evaporation loss
Example: If Evaporation loss is 8 m3/hr, COC=3 ,
Then Blow Down loss= 8 = 4 m3/hr
(3-1)
What is COC ?
●Cycles of concentration (COC) is a ratio between parameter in cooling water to the
parameter in makeup water.
●The COC is a dimensionless number.
● It can be calculated from any following formulae.
●Cycles Of Concentration(COC)=
Ca Hardness in Cooling water
Ca Hardness in make up water
Or COC= Conductivity of cooling water
Conductivity of make up water
Or COC= Concentration of chlorides in cooling water
Concentration of chlorides in make up water
NB: COC normally varies from 3 to 7 depending on process design.
