Cooling Tower Approach

Also known as approach to wet bulb

A = T_c - T_{wb}

A =

T_c =

T_wb =

Enter your known values, leave one input blank, and solves for the missing one. Try different units for next level excitement!

Learning zone

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.

Cooling Tower Approach

A = T_c - T_{wb}

Where

$A$ = Approach
$T_c$ = Cold water temperature
$T_{wb}$ = Ambient wet-bulb temperature

Missing one of these? Work it out first, then come back

Approach — Glycol Loop Heat Transfer (Capacity Derate), Cooling Tower Evaporation Rate
Cold water temperature — Cooling Tower Range, Saturation pH (pHs) for Langelier's Index
Ambient wet-bulb temperature — Cooling Tower Range, Stefan-Boltzmann Law