Cooling Tower Approach

Also known as approach to wet bulb

A = T_c - T_{wb}

Worked example: 29 degC basin with a 9 F approach → 24 degC wet bulb — press Try an example to run it live, then adjust anything.

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Cooling Tower Approach explained

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 formula

A = T_c - T_{wb}

Where

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

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

Approach — Glycol Loop Heat Transfer (Capacity Derate), Degrees of Superheat
Cold water temperature — Cooling Tower Range, Saturation pH (pHs) for Langelier's Index
Ambient wet-bulb temperature — Relative Humidity from a Sling Psychrometer, Wet-Bulb Temperature (Stull 2011)