Page 108 - Membranes for Industrial Wastewater Recovery and Re-Use

Table 3.3 Summary of terms used to define flows through recirculating cooling towers
Flow component Definition Flow rate, m' h-' Concentration of dissolved
constituent(s). mg IF'
Recirculating water Flow rate of water that is recirculating in tower QR CR
Blowdown Flow rate of water deliberately removed from system Qh Ch = CR
to limit salt build-up
Evaporation Water lost to atmosphere due to evaporation: -2% of Q, = QTAT/700 c, = 0
recirculating water flow per 10°C temperature drop
Drift Water lost due to entrainment in wind. When salt or Qd -0.2% for old units; Cd = CR
brackish water is used, drift may contain high Qd < 0.005% for efficient
concentrations of salts and/or minerals that can deposit designs
on crops, soils, or structures
Make-up water Flowrate ofwater added to flow stream to replace water by Qm = Qe + Qci + Qh + QL
evaporation, drift and blowdown
Other losses Losses ofwater due to leaks 01.
System capacity Total volume of water in system V
Temperature differential Difference between average water temperature following AT=T,"o,,-T,,,",,,,"C
evaporation and average water temperature returning to
tower
Cycle time Time required for water to travel around circulating loop t, = V/QR
Holding time index The half-life of chemicals added to the system tli2=(ln2)xVx(Rc-
1)/E