Drift Emissions

Cooling tower emit large volumes of liquid emissions containing the same chemical constituents as the circulating water.  These liquid emissions, called drift, become a regulated pollutant when teh liquid droplets evaporate to form crystals or particulate matter less than 10 microns in diameter (PM10). 

CleanAir's staff has a long history of cooling tower drift emissions testing and consulting.  CleanAir is the only company with the capacity to perform both the Heated Glass Bead Isokinetic (HBIK) and the Sensitive Paper (SP) test methods that determine total mass emission rates, drfit droplet size distributions and PM10 emissions.  In addition to drift testing, CleanAir routinely writes drift emissions test protocols for submission for our clients for regulatory approval.

Heated Glass Bead Isokinetic (HBIK) Test Method
The HBIK test method (CTI ATC-140) is specifically designed for determining the mass emission rate (or drift) from a cooling tower. The test method is based on collecting tracer elements entrained in the exit air stream of the tower.   Air samples are drawn over a heated glass bead collection apparatus that evaporates moisture and deposits the trace elements on the beads.  

Air samples are drawn at the same angle and speed as the exit air stream, thus the test is called "isokinetic".   Data is acquired along each of two perpendicular traverse diameters of a fan stack to obtain a representative composite sample. Tracer element concentrations in the circulating water are compared with recoevered train tracer mass to calculate the drift flux of the sampling area.   Drift emissions can then be calculated using the area of the stack and circulating water flow rate.

Sensitive Paper (SP) Test Method
The Sensitive Paper (SP) Test Method permits the calculation of cooling tower drift droplet size distribution and can be used to determine cooling tower drift rates in some applications. Drift droplet size distributions are required for drift deposition modeling and can also be used for evaluation of cooling tower PM10 (particulate matter) emission rates.  (Particulate Matter less than 10 microns is aerodynamic diameter.  PM10 is a nationally regulated criteria pollutant regulated under the National Ambient Air Quality Standards (NAAQS).

Each sensitive paper consists of a chemically treated filter that generates a well-defined stain distribution when impacted by water droplets. In a typical mechanical draft cooling tower test, 24 filters are sequentially exposed at the exit plane of each tested fan stack. The air velocity is also measured at each of the discrete sampling stations. The size of the stain is a function of the momentum of the droplet that inertially impacts upon the filter.

After the stain sizes are counted and grouped, the droplet size distribution associated with each sampling station is calculated through the application of the empirically generated calibration equations.

PM10 Formation
Cooling tower drift is defined as the emitted percentage of the circulating water from the cooling tower that is entrained in the exhaust air stream and emitted from the cooling tower. Drift is differentiated from evaporation, in that drift contains the same chemical constituency as the circulating water whereas evaporation/condensation consists of pure water vapor. PM10 forms when cooling tower drift evaporates to a salt crystal. The size of the crystal is a function of the TDS and the size of the emitted water droplet. The attached charts illustrate the distribution of emitted mass for a fictional cooling tower.

The diameter of the airborne particle that is produced by the evaporation of the liquid water from a drift droplet can be calculated by:

and

where

msalt    =  mass of salt particle, g
rsalt     =   density of particle, g/cm3
rw       =   density of drift droplet, g/cm3
CTS     =   concentration of solids in circulating water, ppm
dp      =    diameter of solid particle, microns
dd      =    diameter of drift droplet, microns