First  what is drif:  Water ...

First  what is drif:  Water droplets suspended  in air and  discharged from the cooling tower is called drift. Drift is the byproduct  of the evaporating cooling process taking place in the C.T. Drift escapes  the system as mist or vapour  and that means loss of  cooling tower  and that means high make up water                                CALCULATION OF DRIFT LOSS:        With blow down blocked in , determine the con , of (CrO4-) at a measured intervals until the cycle of concentration returns to normal. So DRIFT LOSS (D) = (V/t)ln[Ci / Cf] where  V=VOL  capacity of the C.T. t = time in hour ,Cf = final con of CrO4- and Ci is that initial con.and using this formula at two definite time intervals  D1/D2 are calculated and then D = 1/2 (D1+D2)               source :Boiler Operation Engineering by P. Chottopadhyay .