Calculating Total Cooling Requirements for Data Centers

Overview

The paper provides a quick worksheet to estimate the total heat load of a data centre or network room for air-conditioning sizing. Its central claim is that all power and cooling quantities should be expressed in watts, because IT equipment turns essentially all its input power into heat. Conversions: 1 BTU/h = 0.293 W and 1 ton of refrigeration = 3,530 W.

The method rests on simple linear rules. UPS heat with battery is 0.04 times UPS rated power plus 0.05 times the IT load (redundant capacity is excluded from the rating). Power distribution heat is 0.01 times the system rated power plus 0.02 times the IT load. Lighting is 14.32 W/m2 (1.33 W/ft2, from the ASHRAE 90.1-2016 computer-room value), and each person adds 100 W. In the worked example, 465 m2, 250 kW rated, 150 racks, 20 staff and 30 percent loading (75 kW IT) give 105 kW total heat, about 1.4 times the IT load. The breakdown is IT 71 percent, UPS 13, lighting 10, power distribution 4 and people 2.

Humidification and air mixing push CRAC capacity above the raw heat load: the oversize allowance ranges from 0 percent for a small room with ducted return to 30 percent for high mixing. For rooms under 372 m2 (4,000 ft2) the rule of thumb is CRAC rating = 1.3 x expected IT load plus redundancy. For large sites, envelope gains and recirculation need separate engineering analysis.

The paper's rules convert directly into a heat-load calculator, with unit conversions kept in a shared module. If the roof or walls are exposed to the outdoors, envelope load must be entered manually because the simple rules do not cover it. When a specific UPS model is selected from the catalogue, its efficiency curve should override the 4 + 5 percent rule, since newer units near 97 percent efficiency lose less heat.

Key points

1. 1 BTU/h = 0.293 W; 1 W = 3.41 BTU/h; 1 ton of refrigeration = 3,530 W
2. IT heat output (W) = IT electrical input (W); for VoIP/PoE devices up to about 30 percent of power can go to remote terminals
3. UPS heat = 0.04 x UPS rated power + 0.05 x IT load
4. Power distribution heat = 0.01 x system rated power + 0.02 x IT load
5. Lighting 14.32 W/m2 (1.33 W/ft2); 100 W per person
6. Example: 465 m2, 250 kW, 150 racks, 20 staff, 30 percent load (75 kW IT) gives 105 kW total heat (about 1.4x IT)
7. Example breakdown: IT 71 percent, UPS 13, lighting 10, power distribution 4, people 2
8. CRAC oversizing ranges from 0 percent (ducted return, small room) to 30 percent (high mixing)
9. For rooms under 372 m2, CRAC capacity = 1.3 x IT load + redundancy

Using it in design

• Implement as the default heat-load calculator: Q = IT + (0.04 x UPS + 0.05 x IT) + (0.01 x UPS + 0.02 x IT) + 14.32 x area + 100 x people
• Keep unit conversions in a shared units module
• Offer the 1.3 x IT + redundancy shortcut for rooms under 372 m2
• Require manual entry of envelope load when roof or walls face outdoors
• Let a catalogue UPS efficiency curve override the 4 + 5 percent rule (new units near 97 percent efficiency lose less)

Who should read it

Designers sizing air-conditioning capacity for small and medium data rooms.

Related standards

• ASHRAE 90.1
• ASHRAE TC9.9 Thermal Guidelines

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2017

Document number

WP 25

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