COOLING TOWERS – APPROACH – CONSTANT MYTH

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.

WBT	Approach	WCT
28	4	32
27	4.5	32.5
26	5	31
25	5.5	30.5
24	6	30
23	6.5	29.5
22	7	29

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.