Data Center Cooling: CRAC/CRAH redundancy, capacity, and selection metrics (Hussein Shehata)

Overview

Hussein Shehata's 2014 article in the Uptime Institute Journal uses layout modelling of down-flow CRAC/CRAH units with a raised floor to show the practical density limit of perimeter cooling and the space cost of redundancy. The article is freely readable and is an expert community piece. The example CRAC delivers about 90 kW net sensible in a 3 x 1 m footprint; layouts were modelled with 10, 14 and 18 m air throw and with units on one or two sides.

The author argues that passive hot-air-removal cabinets cannot remove more heat than the cold air supplied, debunking claims of 20 kW or more per cabinet. The model states its assumed clearances: 500 mm side and 1,000 mm long-side maintenance space, at least 2 m from the nearest cabinet to the CRAC, a 1,000 mm hot aisle and a 1,200 mm cold aisle. These assumptions show under which conditions the results hold.

A comparison in the same 186 m2 hall with four CRACs shows how extra redundancy reduces usable capacity and cabinet count. N+2 gives 180 kW for 40 cabinets at 4.5 kW (72% space use), while N+1 gives 270 kW for 52 cabinets at 5.2 kW (95%). The author also derives a cooling-per-perimeter-metre formula, P = K/L + M - (6.4 x R/S), where P is kW per perimeter metre, K the CRAC net capacity, L the length, M the side clearance, R the redundancy and S the one- or two-sided layout.

The conclusion is that for efficient space use with CRAC-only cooling the average load per cabinet should not exceed about 6 kW; a 20 kW average cabinet in N+2 needs a CRAC area about twice the IT cabinet area. Higher densities require row- or cabinet-based active cooling, and passive chimney cabinets are viable only for a few cabinets per row. The article's limit is that it rests on one CRAC example and the assumed clearances.

Key points

1. Example CRAC: about 90 kW net sensible in a 3 x 1 m footprint; layouts modelled with 10, 14 and 18 m air throw and units on one or two sides.
2. Clearances assumed: 500 mm side and 1,000 mm long-side maintenance space, at least 2 m from nearest cabinet to CRAC, 1,000 mm hot aisle and 1,200 mm cold aisle.
3. Same 186 m2 hall with four CRACs: N+2 gives 180 kW for 40 cabinets at 4.5 kW (72% space use) while N+1 gives 270 kW for 52 cabinets at 5.2 kW (95%).
4. Formula: P = K/L + M - (6.4 x R/S), where P = kW per perimeter metre, K = CRAC net capacity, L = length, M = side clearance, R = redundancy, S = one/two-sided layout.
5. For efficient space use with CRAC-only cooling, average load per cabinet should not exceed about 6 kW; a 20 kW average cabinet in N+2 needs CRAC area about twice the IT cabinet area.
6. Passive hot-air-removal cabinets cannot remove more heat than the cold air supplied, debunking 20 kW+ per cabinet claims.
7. Higher densities require row- or cabinet-based active cooling; passive chimney cabinets are only viable for a few cabinets per row.

Using it in design

• When only perimeter CRAC/CRAH cooling is selected, warn above about 6 kW average per cabinet and require row, rack or liquid cooling for 20 kW and above.
• Use the per-perimeter-metre formula and throw distance to auto-check CRAC count and placement.
• Show the space lost to each extra redundant unit.
• Compare N+1 and N+2 in the same hall in terms of cabinet count and capacity.

Who should read it

Mechanical designers who size raised-floor CRAC/CRAH cooling and assess redundancy and space use.

Related standards

• ASHRAE TC9.9 Thermal Guidelines

Publication details

Authors

Hussein Shehata

Publisher

Uptime Institute Journal

Year

2014

Document language

English

Source type

Industry body

Access

Free to read

Licence

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

Citation