Several things must be considered when selecting fill for a cooling tower, including: thermal performance, retention of suspended solids, attachment of biofoulers, resistance to chemicals and UV, price, delivery, assembly, durability, and even ultimate disposal. This is a complex decision. You will not likely be making your own fill. There are several reputable manufacturers, who are more than anxious to suggest which of their products are best suited to your particular application. Some will even assist you with what questions to ask. Unless this is simply an academic exercise, you should not attempt to design a cooling tower with no experience. An effective design requires many years of experience and the learning curve is steep. Cooling towers cost millions of dollars. Design flaws can be extremely difficult and expensive to correct. If you are studying this topic, try to get on with a manufacturer and work a year or two. The lessons you learn will be invaluable. There's nothing quite like crawling in, out, and all over a cooling tower while it's running, especially one that has problems because you get to see what's wrong and how to fix it. There's no shortage of cooling towers with problems. Most plants forget about them until they plug up with junk or the LP steam turbine trips offline. People who change the oil in their car on strict schedule will neglect a cooling tower for years and wonder why it doesn't work.