Why does free cooling fail in tropical climates?

Evaporative cooling cannot chill water below the ambient wet-bulb temperature, and towers realistically bottom out about 5-8 K above it. Tropical design wet-bulbs of 27-30 °C put the achievable sink temperature at 33-36 °C or higher — at or above the coolant temperature most equipment needs, so mechanical refrigeration becomes mandatory on design days.

How is wet-bulb temperature used in data center cooling design?

Designers take the site’s 1% design wet-bulb (the value exceeded only 1% of hours), add the tower approach of roughly 6 K to get the coldest achievable water, and compare that against the coolant temperature the IT equipment requires (its ASHRAE W-class). Margin means free-cooling hours; no margin means compressors. Dry coolers follow the same logic against dry-bulb temperature with about 10 K approach.

Formula · 3 min read

Wet-bulb temperature & free cooling

Evaporative cooling can only chill water to near the wet-bulb temperature — plus a tower approach of ~5-8 K. Free cooling works when T_wet-bulb + 6 ≤ T_{coolant required}. At a tropical design wet-bulb of 28 °C, tower water bottoms out near 34 °C — even a 35 °C loop has no margin, which is the physics behind every tropical-derating warning on this site.

Industrial dry coolers with axial fans on a building roof — **PLATE 1** — Where facility heat lands: rooftop dry coolers. Their output tracks the weather — wet-bulb decides how cold "cold" gets. AI-generated illustration (Z-Image Turbo) — representative, not a specific site.

Formula

T_sink,min ≈ T_wet-bulb + 6 K (tower) · dry cooler: T_wb → T_dry-bulb + ~10 K

The 6 K is a typical cooling-tower approach; dry coolers run against dry-bulb with ~10 K approach. Whichever sink you use, if its achievable temperature sits above the coolant your equipment needs, mechanical refrigeration enters the chain — with its capital, energy and PUE cost. Higher W-classes raise T_coolant and buy back free-cooling hours.

Worked example

Singapore wb ≈ 28 °C → sink ≈ 34 °C → a 35 °C loop: no margin ❌ · Munich wb ≈ 20 °C → sink ≈ 26 °C → 35 °C loop: 9 K margin ✅

Typical 1% design wet-bulbs (verify per site): tropical coastal cities 27-30 °C, Gulf coast 30 °C+, Central Europe 20-22 °C, Northern Europe 17-19 °C.

Try it

Required coolant temp (°C)Site design wet-bulb (°C)Tower approach (K)

sink ≈ 34 °C · margin 1 K

⚠️ Only 1 K margin — free cooling on paper, mechanical assist on design days

Common mistakes

Averaging annual temperature. Free cooling fails on design days, not average days. Size against the 1% design wet-bulb, or accept mechanical backup.
Believing "chiller-free in any climate". At wet-bulbs near 30 °C even W45 loops have thin margin — and L2A cabinets derate on room air independently (approach temperature).
Forgetting water. Towers and adiabatic assists consume water and need treatment; in water-stressed regions dry coolers plus higher W-classes may beat evaporative despite fewer free hours.

ASHRAE W-classes · PUE math · Hot & tropical climates · Liquid vs air

Sources

Evaporative-cooling limit physics (wet-bulb as floor) with typical tower approaches of 5-8 K; design wet-bulb values are approximate 1% design points — always pull ASHRAE climate data or local meteo records for a real site.