Why liquid: heat, flow and temperature rise
Liquid cooling foundations for buyers. Lesson 1, 12 min.
Almost all the power a server uses becomes heat. A cooling system must move that heat out of the room. Liquid cooling uses a liquid, usually water or a water mixture, to carry the heat away from the chips.
Why a liquid
Water holds much more heat per unit of volume than air. A small pipe of water can carry the heat that needs a large flow of air.
The one equation to know
Heat moved = mass flow x specific heat x temperature rise.
For water, the specific heat is about 4.18 kJ per kg per K. One litre of water has a mass of about 1 kg. Use these values for a first estimate.
Worked example
A rack makes 100 kW of heat. The coolant gains 10 K as it passes through the cold plates.
- Mass flow = 100 / (4.18 x 10) = about 2.39 kg/s.
- That is about 143 litres per minute.
The same rack with a 5 K rise needs about twice the flow: about 287 litres per minute. A smaller temperature rise needs more flow. A larger heat load needs more flow.
Limits of the estimate
A water and glycol mixture holds less heat per kg than water, so it needs more flow for the same load. Use the figures from the coolant maker and the server maker for a real design.
This site gives education and records, not engineering advice. Do not size real equipment from this lesson.
Sources
- How to size a coolant distribution unit (CDU), guide on this site
- Rack heat load and coolant flow calculator, /tools/rack-heat-coolant-flow on this site