Match the server spec to facility water

A rollout starts with two numbers: what the servers need and what the facility can give.

Know the relation

The CDU keeps the technology cooling system (TCS) apart from the facility water system (FWS). The CDU has an approach temperature.

TCS supply temperature = FWS supply temperature + CDU approach.

Worked example

This example uses made-up round numbers.

• The server specification needs a TCS supply of 30 degrees C or lower.
• The CDU approach is 3 K.
• The highest FWS supply is 30 - 3 = 27 degrees C.

W27 allows a facility water supply up to 27 degrees C. This design sits exactly on the limit and has no margin. Ask the vendor for the approach at full load and at part load. Ask the designer how much margin the plant keeps on a hot day.

For the same TCS supply, a colder FWS gives a larger approach. A larger approach can mean a smaller heat exchanger. A warmer FWS lets the plant use less mechanical cooling. Each choice has a cost, so write it down.

Check the class and the specification together

The W class names the facility water supply envelope. It does not set the temperature rise across the cold plates. The server specification sets that. Read the server specification first. Then find the class that the facility can meet.

One class for one hall?

Different rows can run different classes if separate CDUs serve them and the facility can supply each CDU at its own temperature. Document the class of each CDU in the commissioning and handover files.

Plan the heat budget

Add the heat load of every rack in the phase. State the share that goes to liquid and the share that stays in room air. Size the liquid side and the air side from those two numbers.

Sources

• ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments, 5th edition (2021)
• How to size a CDU and ASHRAE TC 9.9 liquid cooling classes, guides on this site