It is yet another myth with some of the users that when cooling tower is designed at 280C WBT with 40C Approach, it means it is suppose to give some approach thru out the year. This is the Myth. Approach is never constant. Approach varies with WBT. Lower the WBT, higher is the approach. A typical variation is shown in the following table & graph.
If you observe closely the graph, we design Cooling tower at worst conditions of WBT hence absolute value of CWT will be highest, which can be designed as best cold water as further input to next utility equipment.
Another interpretation is even when WBT can reach at 280C, and if we design at WBT of 250C with 30C Approach, where design CWT will be 280C. in this case when WBT goes to 280C, approach may be 20C however CWT will be 300C, which will be higher than design CWT of 280C.
When you design Cooling Tower with lower approach, tower size goes up. The need for lower L/G gets generated. Hence the initial investment goes up. Also as you increase airflow, power consumption goes up. This will add to operational costs. A proper balance need to be achieved by optimizing plant efficiency. This means an integrated or holistic approach. For example when we work on better approach, in power plant systems Condenser vacuum improves, this will reduce steam consumption or improve power output.