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Tools · 02

What a few kelvin of approach costs you.

Start from the highest secondary supply temperature the IT equipment allows, subtract the CDU approach and the dry-cooler approach, and see the outdoor temperature below which dry free cooling can carry the load. Every kelvin of approach across the CDU, and every kelvin across the dry cooler, pushes the required outdoor temperature down and takes free-cooling hours with it.

°C

ASHRAE liquid-cooling classes W32, W40 and W45 set an upper facility-water-supply limit of 32, 40 and 45 °C for equipment declared to that class; they are operating envelopes, not set-points. Enter the supply your racks actually require.

4 K

Include the fouling allowance. Datasheet approach is usually clean-plate.

8 K

Illustrative dry-bulb distributions only, shaped from typical-meteorological-year climate normals for each city in 5 K bins and scaled to 8,760 h. Not station data and not a design basis. Replace with a named station's TMY or bin data for design.

Result
Facility supply required at the CDU— °C
Ambient dry-bulb at or below which dry rejection alone holds it— °C
Hours per year at or below that ambient— h
Share of the year on full free cooling— %
Hours needing trim (adiabatic assist, chiller or reduced load)— h
Sensitivity. Tightening the CDU approach by one kelvin is worth — hours a year in this climate at these settings. That is the number to weigh against the larger heat exchanger.
Method and limits
  • Facility supply required = secondary supply − CDU approach. Ambient threshold = facility supply − dry-cooler approach. Hours below threshold are read from the illustrative bin distribution with linear interpolation inside a bin.
  • Dry-bulb only. Adiabatic or evaporative assist, humidity, solar gain, wind, part load and redundancy staging are not modelled.
  • Approaches are single steady-state values. Real approaches vary with load, flow and fouling; use rated-load values with a fouling allowance.
  • Outputs are illustrative screening estimates, not equipment selections, guarantees or design hours. Verify with project-specific weather data, OEM limits and a rating from the equipment manufacturer.

Questions this page answers

How much does CDU approach temperature affect free-cooling hours?

Every kelvin of approach across the CDU, and every kelvin across the dry cooler, lowers the outdoor temperature at which the site can free-cool by about one kelvin. How many hours that costs depends on the local climate curve, which is why the tool shows the change against a stated climate profile rather than a single number. Where the CDU sits on the thermal path →

What CDU approach temperature is the industry benchmark?

Published targets cluster at 3–4 K. Google's Project Deschutes CDU specification lists 3 K at a 2,000 kW thermal load with 500 gpm on both loops; KAORI cites 4 K or less as a common design target for the heat exchanger and AVC states its CDUs are designed to 4 K, both manufacturer statements (OCP APAC Summit 2026 presentations). Compare a quoted approach only when the load fraction, both flows and the fluid are stated, then enter it in the tool to see what it costs in free-cooling hours. How we specify the CDU →

What are the ASHRAE W-classes?

ASHRAE liquid-cooling classes for IT equipment, W17, W27, W32, W40, W45 and W+, named for the maximum facility-water supply temperature in °C the equipment is declared for. A class is an operating envelope the equipment tolerates, not a set-point to run at.

Why does the tool call its climate data illustrative?

Because it uses a normalised 8,760-hour temperature distribution for each climate type, not a measured file for your site. Use it to see the direction and rough size of the effect, then rerun the calculation with a TMY file for the actual location before sizing equipment.