Water-Cooled Servers: Common Designs, Components, and Processes

Overview

The architecture has three layers: the facility water system (FWS), the secondary technology cooling system (TCS) loop, and the CDU separating them. Cold plates, wetted materials and controls are described too. The paper warns explicitly that the water-quality tables for the two loops are often mixed up. The condensation rule is clear: keep TCS supply at least 2 degC (3.6 degF) above the room dew point, using proportional, binary or bypass valves.

Filter size is dictated by the smallest flow path in the cold plate, and a 35-40 mesh (about 500-400 micron) strainer guideline is given for the FWS. Water-quality limits are split by loop. pH is 7-9 for FWS and 8.0-9.5 for TCS. Total hardness is below 200 ppm for FWS and below 20 ppm for TCS (as CaCO3). TCS conductivity is 0.2-20 micromho/cm, total suspended solids below 3 ppm and turbidity below 20 NTU.

The tenfold hardness gap shows why running both loops to one specification is an error. In the product survey, maximum facility-water temperatures ranged 10-24 degC for most products, and eight products accepted 40 degC or more. The paper also covers water treatment, monitoring and maintenance planning, but it is a white paper compiling common practice, not a design code. The tables show the TCS loop has to be kept far cleaner than the FWS loop.

Key points

1. Keep TCS supply at least 2 degC (3.6 degF) above room dew point.
2. pH: FWS 7-9; TCS 8.0-9.5.
3. Total hardness: FWS under 200 ppm; TCS under 20 ppm (as CaCO3).
4. TCS conductivity 0.2-20 micromho/cm; total suspended solids under 3 ppm; turbidity under 20 NTU.
5. FWS strainer guideline 35-40 mesh (about 500-400 micron).
6. Filter size is dictated by the smallest flow path in the cold plate.
7. Surveyed products: max facility-water temperature 10-24 degC for most; 8 products at 40 degC or above.

Using it in design

• Enforce TCS supplyTemp >= roomDewPoint + 2 degC.
• Keep separate FWS and TCS water-quality specs (pH, hardness, conductivity, TSS) in commissioning checklists.
• Default FWS strainer to 500 micron and TCS filter to 50 micron (with the Vertiv guide).
• Label loops clearly as FWS or TCS in the UI to avoid the common spec mix-up.

Who should read it

Mechanical engineers who design, commission and operate water-cooled server loops.

Related standards

• ASHRAE Liquid Cooling Guidelines for Datacom Equipment Centers
• ASHRAE TC9.9 Thermal Guidelines (W classes)
• OCP cold plate requirements

Publication details

Authors

ASHRAE TC 9.9

Publisher

ASHRAE TC 9.9

Year

2019

Document number

TC 9.9 White Paper

Document language

English

Source type

Standard

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation