What does "drift rate" mean?

For cooling tower products and applications, the industry defines the effectiveness of a drift eliminator product in terms of its drift rate. This rating is usually presented in the form of a percentage. You may ask, “Percentage of what?”… Percentage of circulating water flow rate!

Let’s use this tower design criteria as an example so that we can work through the calculation together:

Entering Water Temp: 95°F
Exiting Water Temp: 85°F
Site Wetbulb Temp: 78°F
Circulating Flow Rate: 1,000 gpm

If this is a counterflow cooling tower where we plan to utilize the CF80MAx drift eliminator, we can look at the published drift rate for this product and see it is 0.0005%. Since we now know that this percentage is in relation to the circulating flow rate, we can calculate the volume of water exiting out of the tower in the form of drift droplets, or “drift loss,” as such:

Drift Loss (gal/m) = 1000 gal/min x (0.0005/100) = 0.005 gal/min

Expressed over an hour period: 0.005 gal/min x 60 min/hr = 0.3 gal/hr

If we substitute another product’s drift rating of 0.002%, we would get a drift loss of 1.2 gal/hr, or 4 TIMES GREATER drift loss.

Hopefully this provides a clear example and/or refresher on a drift eliminator’s drift rate. The next question is, “Where does a manufacturer get that drift rate number, and is that rate consistent as my fan rpm changes?” We’ll tackle that topic in a separate article!