Analysis of Pumped Refrigerant Economizers: Supporting the California Energy Commission Approval of the Liebert DSE System for Use in California Data Centers

Overview

Title 24 requires computer rooms to use one of three prescriptive economization methods (air, water or pumped refrigerant) or to demonstrate equivalent efficiency via Exceptional Design Compliance. Prescriptive economizers must carry 100% of the cooling load; water economizers must do so at 40°F dry-bulb / 35°F wet-bulb and below, and containment is required above 175 kW per room. The note explains that the DX system switches off compressors and circulates refrigerant with a small pump when outdoor conditions allow, and that partial economization depends on load and return-air temperature rather than a fixed changeover point.

The comparison covers TDV energy and water use across all 16 California climate zones for a 1.2 MW reference data centre. The compliance model basis is 1,200 kW, N+2 redundancy, 80°F return air with containment, and the CBECC-Com/EnergyPlus TDV method. The pumped-refrigerant economizer was more efficient than water economizers in every climate zone except zones 10 and 15. In San Francisco (zone 3) at 90°F return air and 70% load, hours split 69% full economization, 25% partial and 6% compressor.

The water difference is decisive: for a 1.2 MW IT load a water economizer uses about 4.3 million gallons a year and an air economizer 2.7-3.2 million gallons, while the pumped-refrigerant system uses none. Refrigerant pump power is typically below 5% of compressor power for the same capacity. The note is a compliance dossier for California Title 24 and one manufacturer's system, so results do not transfer directly to other climates or codes.

Key points

1. Title 24 requires computer rooms to use one of three prescriptive economization methods (air, water, pumped refrigerant) or demonstrate equivalent efficiency via Exceptional Design Compliance.
2. Prescriptive economizers must carry 100% of the cooling load; water economizers must do so at 40°F dry-bulb / 35°F wet-bulb and below; containment is required above 175 kW per room.
3. The pumped-refrigerant economizer was more efficient than water economizers in every climate zone except zones 10 and 15.
4. For a 1.2 MW IT load, a water economizer uses about 4.3 million gallons/yr and an air economizer 2.7-3.2 million; the pumped-refrigerant system uses none.
5. Refrigerant pump power is typically below 5% of compressor power for the same capacity.
6. San Francisco (zone 3) at 90°F return air and 70% load: 69% full economizer, 25% partial, 6% compressor hours.
7. Compliance model basis: 1,200 kW data centre, N+2 redundancy, 80°F return air with containment, CBECC-Com/EnergyPlus TDV method.

Using it in design

• Add a pumped-refrigerant economizer DX type to the cooling-option library, with economizer hours modelled as a function of ambient, load fraction and return-air temperature.
• For California sites, check the Title 24 Section 140.9 economizer and containment (above 175 kW per room) requirements.
• Report water use alongside energy when comparing economizer types.

Who should read it

Mechanical designers working in California who assess DX economizer options against code.

Related standards

• California Title 24 Part 6 Sec. 140.9
• ASHRAE 90.4
• ASHRAE 90.1

Publication details

Authors

Vertiv

Publisher

Vertiv

Year

2016

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