Cooling Tower Drift Problem

[ppeng01]

I've got a problem with a cooling tower. The cooling tower is cooling service water that is basically city water with the normal, small concentrations of biocide and dispersants which are added and monitored by the water chemical company. The tower is performing well thermally, but has failed the "drift" test - drift being the very small amount of mist carryover that results from the gas-liquid contact. The guarantee is 0.0005%, and the drift is being measured at 0.001%. The manufacturer is pointing to the water and is saying there must be some contaminant or chemical that is skewing the droplet distribution from the spray nozzles towards small droplet sizes. The small droplet sizes are then defeating the chevron style mist eliminators because they don't have enough mass for an inertial impaction type of mist eliminator and are following the air through. I am having a hard time visualizing how the small amount of water treatment chemicals could have a significant impact on droplet size distribution. I am used to thinking in terms of hydraulic pressure and design of single-fluid nozzles as the primary factors affecting droplet size. In any event, we are testing the water for chemical concentrations. The surface tension comes in close to water - which should be OK. What else should I be looking for as a cause of the excess drift? Overloading and flooding the mist eliminators? Anything else?

 

[IRstuff]

Seems to me that an obvious test is to turn off the chemicals, one by one, or all at once, and see if your drift problem changes.

I wouldn't be so quick to dismiss the notion that the chemicals are changing the behavior of the water; pure silicon is nearly an insulator, and just 1 ppm of boron, or arsenic, or phosphorus, will change it into a tolerable conductor.

Water additives like x-100 can be in concentrations down in the 0.1% range, so not much is required to affect water's properties. And it sounds like it's not a gigantic change, right? You're looking at what appears to be a factor of 20 change in drift, but the other side of the equation is that it's changing from 99.9995% to 99.9900% capture rate.

TTFN
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[racookpe1978]

Assuming you do the test (stopping the chemicals for short periods of time, be absolutely sure you get data at various wind speeds (direction I don't think will matter) - UNLESS you have directional issues now that you are not aware of.

For example, you get more droplets through when the wind is from the north because a row of south-side nozzles are a size J3 instead of a size or model J2. But if the wind is from the south, these extra large droplets are not "released" into the open air and measured, but are trapped by the structure because they are being blown back into the structure and walls.