Humidification Strategies for Data Centers and Network Rooms

Overview

The paper first shows why a humidity window is needed: too little moisture raises static-discharge risk, too much risks condensation and corrosion. Walking across an ungrounded raised-floor tile builds 250 V at 80% RH but 12,000 V at 20% RH; on synthetic carpet the figures are 1,500 V and 35,000 V. Room humidity shifts through infiltration, coil condensation and ventilation; cooling coils are typically held at 6.1-8.8°C, usually below the room dew point.

The control advice is concrete. The sensor belongs inside the rack front door in the top third; measuring at the exhaust has no effect on availability. Air at 90°F and 50% RH holds 96% more water by weight than air at 70°F and 50% RH, so RH-controlled CRACs with different return temperatures can demand-fight, which dew-point control avoids. The paper recommends running at the low end of the recommended range. The choice of sensing location has a practical consequence: the exhaust gives a different reading but does not affect availability, whereas the inlet side is the air the equipment actually sees.

In the humidifier comparison, steam-canister and infrared units are low capital, high operating cost; ultrasonic is high capital, low operating, high maintenance and needs RO or de-ionised water. The appendix works a 50 kW glycol CRAC at 72°F: at a 50% RH set point sensible capacity is 45.3 kW with 3.3 kW latent and a 3.2 kW humidifier load ($2,242.56 a year at $0.08/kWh); at 45% RH sensible capacity is 49.9 kW with no humidification. Some guideline values reflect the 2011 ASHRAE edition.

Key points

1. Static buildup: 250 V at 80% RH vs 12,000 V at 20% RH on a raised-floor tile; 1,500 V vs 35,000 V on synthetic carpet.
2. Summarised ASHRAE guidance: recommended 5.5°C dew point to 60% RH and 15°C dew point; allowable 20%-80% RH with a 17°C maximum dew point.
3. Cooling coils are typically held at 6.1-8.8°C, usually below room dew point; airflow through the coil exceeds 160 cfm (4.5 m3/min) per kW of IT load.
4. The minimum acceptable sensing point is inside the rack front door, top third; measuring at the exhaust does not affect availability.
5. Air at 90°F and 50% RH holds 96% more water by weight than at 70°F and 50% RH; RH-controlled CRACs can demand-fight, dew-point control avoids it.
6. Steam canister and infrared: low capital, high operating cost; ultrasonic: high capital, low operating, high maintenance, needs RO or de-ionised water.
7. 50 kW glycol CRAC at 72°F: at 50% RH sensible 45.3 kW, latent 3.3 kW, humidifier 3.2 kW ($2,242.56/yr at $0.08/kWh); at 45% RH sensible 49.9 kW and no humidification.

Using it in design

• Accept humidity limits as dew point plus RH in the environmental envelope settings, default to current ASHRAE recommended values rather than the 2011 ones quoted here, and recommend dew-point control mode for multi-CRAC rooms.
• Warn when the RH set point exceeds what is needed (for example above 40-45%) and show the humidifier kW and water penalty; require a vapour barrier on the white-space envelope.
• Place temperature and humidity sensors at the rack front, top third.
• Give CRAC catalogue entries sensible heat ratio, humidifier type and kW, and water-quality requirement (RO for ultrasonic).

Who should read it

Mechanical designers and operations engineers who set humidity control strategy and CRAC parameters.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines

Publication details

Authors

Tony Evans

Publisher

Schneider Electric

Year

2015

Document number

WP 58

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