Cooling Audit for Identifying Potential Cooling Problems in Data Centers

Overview

The audit starts by comparing CRAC and heat-rejection sensible capacity with a heat-load worksheet. Its coefficients are: UPS with battery = 0.04 x UPS rating + 0.06 x IT load; power distribution = 0.02 x rating + 0.02 x IT load; lighting at 21.53 W/m2 (2.0 W/ft2); 100 W per person; redundant UPS capacity is excluded. It then covers CRAC coordination and set points, supply-loop temperatures, aisle and rack inlet measurement, perforated-tile airflow, blanking panels, underfloor obstructions and hot/cold aisle layout.

Numeric tolerances are given. CRAC set points should be 20-25 C and 40-55% RH, with filters changed quarterly. Chilled water is 7.2 C (+/- 1.7 C); condenser water is at most 32.2 C for water-cooled and 43.3 C for glycol-cooled systems. CRACs in a defined area (4-5 adjacent units) should run in the same mode, since demand fighting can cut efficiency by 20-30%. In a 50 kW example, lowering the RH set point from 50% to 45% at 22 C raised sensible capacity from 45.3 to 49.9 kW and removed 3.2 kW of humidifier load ($2,242.56 per year).

Measurement positions are fixed so results can be repeated: aisle temperatures at 1.5 m above the floor at about every fourth rack, rack inlets at bottom, middle and top 50 mm from the equipment face and logged for 48 hours, with 20-25 C as the default acceptable inlet. The airflow formula is CFM = 3,412 x Q / (1.085 x delta T in F); at an 11 C rise, about 75.5 L/s (160 CFM) is needed per kW.

Perforated-tile limits are tight: a typical tile about 25% open supports only 1.25 to 2.5 kW per cabinet at one tile per cabinet. Measurements show 50-80% of available cold air escapes prematurely through unsealed raised-floor cable openings. An appendix gives the assumptions of the humidification cost example and a fill-in audit form.

Key points

1. Heat-load coefficients: UPS 0.04 x rating + 0.06 x IT load; distribution 0.02 x rating + 0.02 x IT load; lighting 21.53 W/m2; 100 W per person.
2. 4-5 adjacent CRACs in an area should run in the same mode; demand fighting can cut efficiency 20-30%.
3. In the 50 kW example, cutting RH set point from 50% to 45% raised sensible capacity from 45.3 to 49.9 kW and removed 3.2 kW of humidifier load ($2,242.56 per year).
4. CRAC set points are 20-25 C and 40-55% RH; filters are changed quarterly.
5. Chilled water 7.2 C (+/- 1.7 C); condenser water at most 32.2 C (water-cooled) or 43.3 C (glycol-cooled).
6. Aisle temperature at 1.5 m at about every fourth rack; rack inlets bottom, middle, top at 50 mm over 48 hours; default acceptable 20-25 C.
7. CFM = 3,412 x Q / (1.085 x delta T in F); about 75.5 L/s per kW at an 11 C rise.
8. A tile about 25% open supports 1.25-2.5 kW per cabinet; 50-80% of cold air escapes through unsealed cable openings.

Using it in design

• Use these coefficients as the default room heat-load method and compare the result against the lower of CRAC sensible capacity and heat-rejection capacity.
• Compute required airflow as Q x 3412 / (1.085 x delta T) with delta T defaulting to 20 F, and warn when rack kW per standard 25%-open tile exceeds 2.5 kW.
• Flag mixed CRAC set points within a zone (demand fighting) and validate set points against 20-25 C and 40-55% RH.
• Export a measurement-point plan (aisle points every fourth rack at 1.5 m, rack inlets bottom/middle/top) with water tolerance checks, and warn on unsealed cable cut-outs in raised-floor designs.

Who should read it

Facility and cooling engineers auditing existing air-cooled data rooms.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines

Publication details

Authors

Kevin Dunlap

Publisher

Schneider Electric

Year

2014

Document number

WP 40

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

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

Citation