Fundamental Principles of Air Conditioners for Information Technology

Overview

The paper starts with the nature of heat: over 99% of the electricity IT equipment uses becomes heat, and roughly 50% of server heat originates in the microprocessor. The cycle consists of evaporation, compression, condensation and expansion; the evaporator coil runs at about 46°F (7.8°C), and the compressor lifts the refrigerant above 200 psi (1379 kPa) and above 125°F (52°C). This matters because the cooling load is essentially all of the electrical power, and the unit exists to carry that heat out of the room.

The most useful section is the energy rule of thumb: fans draw about 5-10% and compressors about 20-30% of rated cooling capacity, so about 350 W of electrical input removes 1000 W of heat at design, while poorly designed systems approach 1:1. A floor-mounted CRAC typically removes 35-150 kW. The example 50 kW unit moves over 8000 cfm (226.5 m3/min), draws about 30 kW three-phase, measures 178 x 89 x 193 cm and weighs 612 kg.

Service needs are given concretely: the 50 kW CRAC needs two ~25 mm refrigerant lines, a 22.2 mm condensate drain and a 6.4 mm potable water line for humidification. The matching air-cooled condenser is 304 x 122 x 122 cm, weighs 408 kg, moves over 20,000 cfm (566 m3/min) of outdoor air, draws about 5 kW three-phase and sits on a roof or concrete pad. The refrigerant discussion is dated by the R-22 phase-out era, so the paper is mainly a source of order-of-magnitude defaults and a connection checklist.

Key points

1. Over 99% of the electricity used by IT equipment becomes heat; roughly 50% of server heat comes from the microprocessor.
2. The evaporator coil runs at about 46°F (7.8°C); the compressor raises refrigerant above 200 psi (1379 kPa) and 125°F (52°C).
3. Fans draw about 5-10% and compressors 20-30% of capacity: about 350 W electrical per 1000 W of heat at design, approaching 1:1 in poor systems.
4. A floor-mounted CRAC typically removes 35-150 kW; the 50 kW unit moves over 8000 cfm (226.5 m3/min), draws about 30 kW three-phase, is 178 x 89 x 193 cm and 612 kg.
5. The 50 kW CRAC needs two ~25 mm refrigerant lines, a 22.2 mm condensate drain and a 6.4 mm humidifier water line.
6. Matching air-cooled condenser: 304 x 122 x 122 cm, 408 kg, over 20,000 cfm (566 m3/min), about 5 kW three-phase, roof or concrete pad.

Using it in design

• Give CRAC and condenser catalogue entries cooling kW, airflow (m3/min), electrical input kW, footprint, weight and service connections (refrigerant line sizes, condensate drain, humidifier water).
• Require every DX CRAC to be paired with an outdoor heat-rejection device and a drain point, and every humidifier-equipped CRAC with a water supply.
• Default the DX mechanical power estimate to about 0.35 kW per kW cooled, with a warning that legacy systems can be far worse.
• Flag refrigerant fields for phase-out regulations.

Who should read it

Mechanical designers sizing DX-based precision cooling units and planning their connections.

Related standards

• ASHRAE Standard 34

Publication details

Authors

Tony Evans

Publisher

Schneider Electric

Year

2014

Document number

WP 57

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