What is approach temperature in liquid cooling?

Approach temperature is the difference between the coolant supply temperature and the room air temperature across the CDU’s heat exchanger: approach = coolant supply °C − room air °C. Capacity scales roughly in proportion to it, so warmer rooms or cooler coolant directly reduce how much heat a liquid-to-air cabinet can reject.

Why does liquid cooling capacity drop in hot climates?

Because the heat exchanger’s driving temperature difference shrinks. A cabinet rated 86.5 kW with 45 °C coolant in 25 °C air (20 °C approach, Vertiv XDU070) has only 10 °C of approach in a 35 °C room — roughly half the capacity. Below about 5 °C of approach, capacity effectively collapses. Always derate against the vendor’s capacity-vs-ambient curve.

Formula · 3 min read

Approach temperature & derating

Approach temperature is the gap between coolant supply and room air temperature: approach = T_coolant − T_air. Liquid-to-air capacity scales roughly linearly with it — which is why a cabinet rated 86.5 kW at 45 °C coolant / 25 °C air (Vertiv XDU070) delivers about half at 35 °C air.

Formula

capacity ≈ rated × (approach / approach_rated) · approach ≤ 5 °C → ≈ 0

A heat exchanger transfers heat in proportion to the temperature difference across it. When room air warms from 25 °C to 35 °C, the approach on a 45 °C coolant loop falls from 20 °C to 10 °C and capacity halves. Below roughly 5 °C of approach, the exchanger can no longer move meaningful heat — capacity collapses. This is the first-principles reason hot climates derate everything.

Worked example

50 kW-class @ 45 °C / 25 °C: 25 °C → 50 kW · 30 °C → 37.5 kW · 35 °C → 25 kW · 40 °C → ≈ 0

Try it

Rated capacity (kW)Room air (°C) 25Coolant supply (°C) 45

50.0 kW at approach 20 °C · 100% of rated

✅ 100% of rated capacity available at 25 °C air

Common mistakes

Comparing nameplate to a tropical room. Nameplate capacity is quoted at 25 °C air / 45 °C coolant. At 35 °C ambient you get roughly half; buy the derated number, not the brochure number.
Chasing capacity by lowering coolant temperature. Below the room's dew point, supply lines and cold plates condense water onto live electronics. This is exactly what ASHRAE W-classes exist to prevent — coolant temperature is a floor, not a free lever.
Forgetting the model is linear. Real capacity curves bow (fan power, flow limits); always get the vendor's capacity-vs-ambient table. The linear model here is the honest first pass — Vertiv is one of the few publishers of real curves (XDU070).

Derating simulator (full curve) · Rack builder applies this model live · Hot & tropical climates · Coolant flow rate

Sources

Model anchored to Vertiv CoolChip CDU 70 (XDU070): 86.5 kW at 45 °C coolant / 25 °C air — datasheet SL-71252 and application guide SL-71111 (published capacity-vs-ambient curves).