Cooling Tower Approach

Wet bulb is the coldest temperature evaporation can ever reach, so it is the floor a cooling tower is pushing against, and approach is how far above that floor the tower actually delivers. Unlike range, approach genuinely measures the tower: same load, same flow, and a fouled, scaled or air-starved tower will show a wider approach on the same day. The 1950s design convention of 7 °F approach persists; 5 °F is aggressive and expensive in fill and fan power, while nothing on earth reaches 0 °F.

An example a service tech runs weekly: on a 78 °F wet-bulb afternoon a tower delivering 85 °F basin water is at a 7 °F approach and is performing to design. If that same tower delivers 92 °F on the same wet bulb, the approach has doubled and something is wrong: plugged fill, a fan belt slipping, a bird screen matted with cottonwood, or scale on the fill sheets from a program running too many cycles. The trap is comparing basin temperatures between days; without the wet bulb the number means nothing, which is why a sling psychrometer or a hygrometer belongs in the truck.