The two most common polymers used in cooling tower fills and drift eliminators are polyvinyl chloride (PVC) and polypropylene (PP). Both materials offer the benefits of durability, compatibility with the water environments of most cooling towers and processes, and cost-effective manufacturing techniques to provide many different engineered profiles suitable for cooling tower uses. However, one of the impediments that both polymers present is their hydrophobic nature. The surface energy of PVC and PP are high enough to want to repel water and make it bead up on the surface. Unfortunately, this action is detrimental to the performance of fills and drift eliminators, which achieve their full potential when the water fully wets the polymer surface.
The resistance to wetting out is related to the surface energy of the polymer. Here is a table listing the available Surface Free Energy (SFE) of several different materials:
PP has much lower surface free energy than PVC, therefore its draw to water is less. Lower SFE yields greater “beading action.” For example, Teflon (PTFE) has very low SFE, and we all know how water beads up on Teflon-coated cookware and runs right off. That is the exact opposite of what we want water to do on fills and drift eliminators.
Brentwood’s previous testing conducted on fill materials shows the benefits of conditioning (also called, “seasoning” or “aging”) on fill packs. Both PVC and PP packs of the same model were tested at various stages of conditioning and at different water loadings. The fills’ performance was directly related to the amount of time spent being conditioned up until the time that the pack fully wetted out. For PVC, this occurred in about half the time that PP required. For some low water loading applications, the asymptotic curve suggests that PP might never be fully wetted, and therefore, can never meet 100% capability.
Drift eliminators are affected the same way as fills when it comes to their performance in relation to surface conditioning. The efficiency of a drift eliminator relies on the captured drift droplets forming a thin film of water on the drift eliminator surface so that the water drains back into the wet section of the cooling tower properly. Any beads of water on the surface are exposed to the air stream and are susceptible to being stripped off the drift eliminator and exiting the tower.
Brentwood has had several reports over the years, where new PVC drift eliminators were installed, stating that there was a problem with the product since there were signs of drift issues. The problems disappeared after the eliminators had sufficient time to condition via normal operation of the tower with full heat load, which is typically at least 1,000 hours of service.
What is important to note is that as drift emission restrictions become more stringent, the potential for any perturbation in the air/water interface to cause a failed drift test increases dramatically. If you look at the comparative rates of conditioning of the film fills above, you will note that the water loading on the fill plays a part in the efficiency. Consider that even after three weeks in the accelerated aging tank – at 3.5 gpm/ft2 (8.6m3/hr·m2) – the fill was only at just over 90% capability. If you were to extrapolate the capability out by month, it does not seem like it would ever reach full potential. That means that the water might never fully wet out the surface.
Now take into account that those are the results of a fill product that has water intentionally sprayed onto the plastic itself. What might be the difference for a drift eliminator that only gets a tiny fraction of the water spray that a fill sees? If the water is not able to ever form a complete film on the surface of the drift eliminator, then the full potential of the drift eliminator efficiency will never be realized. This shows just how important the material of construction can be with respect to a drift eliminator.