Reservoir and Pumping Unit (RPU) Specification, Ver 1.0

Overview

The RPU is designed for in-rack or rack-level closed-loop liquid cooling in air-cooled data centres. Designs should comply with Open Rack v2, Open Rack v3 and Olympus rack architectures, and standard 19-inch (EIA-310) racks should also be natively supported. Rack manifolds, quick-connect fittings and cold plates are out of scope. The document is a reference for vendors to build interchangeable, competitive products and does not aim to favour any vendor.

The AALC concept rests on the premise that rising component power and density are pushing the limits of air cooling: closed-loop liquid cooling with an effective liquid-to-air heat exchanger (LTA HX) can extend existing air-cooled facilities and ease the move to facility water. The RPU is the hub: it powers the LTA HX section (a door heat exchanger or side car), monitors fans and sensors, and passes status, alarms and sensor data to the rack management device or top-of-rack switch.

Physical limits: fit within 4RU (at most 175.3 mm from the top of the rack rails), maximum chassis width 445.25 mm, recommended maximum length 820 mm and, in ORV3, at most 787.8 mm from the equipment catch edge. Weight primed with coolant, rail kit included, must not exceed 75 kg (60 kg preferred; ORV3 rails cap system weight at 80 kg). Power input is 12 VDC (11-13 V) from an ORV2 busbar, 48 VDC (46-52 V) from an ORV3 busbar, or 230 VAC via C14 on EIA racks. At 50 LPM the RPU may draw at most 2% of rack power and the whole liquid-cooling solution at most 5%.

Performance targets are a 20-50 LPM flow range based on 25% propylene glycol (extension to 70 LPM to be evaluated), mandatory N+1 pump redundancy, and AALC total power at or below 5% of rated cooling capacity, of which the RPU should take at most 40%. Supply and return use 20 mm dry-break quick connects; fill, drain and vent use 3 mm dry-break connectors. Control uses two setpoints, differential pressure (pump speed) and supply temperature (fan speed), over Redfish. Required sensors include temperature (0-100 C, +/-1 C), flow (10-100 L/min, +/-1 L/min) and pressure (0-145 psi, +/-1.5 psi).

Safety and reliability sections detail leak behaviour. A leak in the RPU or the IT rack stops pumps and fans, raises a Cooling Loss Siren (CLS) to system management and signals the IT rack to stop pumping; the IT rack's own response (emergency or controlled power-down, throttling) is implementation dependent. Pumps target at least a 10-year L5 life at 45 C inlet, fuses need at least 50% margin, accessible surfaces stay at or below 65 C and at least a 100-micron filter is used. Production requires gas leak testing, burn-in and test-to-failure on 3 units.

Key points

1. The RPU fits in 4RU (at most 175.3 mm from rail top), maximum chassis width 445.25 mm, recommended length 820 mm, at most 787.8 mm in ORV3.
2. Primed weight with rail kit must not exceed 75 kg (60 kg preferred); with ORV3 rails, system weight must stay below 80 kg.
3. Power input: ORV2 12 VDC (11-13 V), ORV3 48 VDC (46-52 V), or 230 VAC via C14 for EIA racks; A/B feed recommended for AC.
4. At 50 LPM the RPU draws at most 2% of rack power and the whole liquid cooling solution (heat exchanger and fans included) at most 5%.
5. Flow is 20-50 LPM on 25% propylene glycol (70 LPM extension to be evaluated); N+1 pump redundancy is mandatory.
6. AALC total power is at most 5% of rated cooling capacity, of which the RPU takes at most 40%.
7. Supply and return use 20 mm dry-break quick connects; fill/drain/vent use 3 mm dry-break connectors; leak alerts go to the IT rack and switch via RJ45 GPIO.
8. Control uses two setpoints, differential pressure (pump speed) and supply temperature (fan speed); monitoring and control use Redfish.
9. Pumps target at least a 10-year L5 life at 45 C inlet; fuses need at least 50% margin; accessible surfaces at most 65 C; filter of at least 100 microns.
10. On a leak the RPU stops pumps and fans, sends a Cooling Loss Siren and asserts a pump-stop signal to the IT rack.

Using it in design

• Model AALC (RPU plus door or rack liquid-to-air heat exchanger) as a cooling option for air-cooled halls, counting the RPU's 4RU space, 75 kg mass and the 2%/5% parasitic power caps against rack U-space, weight and power budgets.
• Enforce N+1 pumps and the 20-50 LPM window (25% PG) in the liquid-loop calculator.
• Require leak-detection signal wiring from the RPU to rack management in the bill of materials.
• Include all AALC heat in the room heat load, since it is ultimately rejected to air.
• In service planning, note that the pump is hot-swappable but any other RPU failure means replacing the whole unit, which requires powering down the rack and entering the hot aisle to disconnect large dry-break connectors.
• Specify at least a 100-micron filter in the loop; cold plates and valves may demand finer filtration.

Who should read it

Mechanical and rack designers adding liquid-cooled racks to air-cooled halls, and RPU and AALC product developers.

Document contents

1. Scope
2. License
3. Acknowledgements
4. Terminology
5. Overview
6. RPU Requirements
7. Physical
8. Interfaces
9. Performance
10. Monitoring and Control
11. Operational
12. Sensor Requirements
13. Control Scheme
14. Alarms and Field Safety
15. Environmental Conditions
16. Serviceability and Operational Impact
17. Compliance and Safety
18. Production Test Requirements
19. Long Term Reliability

Related standards

• OCP ACS Door Heat Exchanger Requirements for Open Rack
• OCP Open Rack V3 Base Specification
• ASHRAE TC 9.9 water-cooled server white paper
• EIA-310

Read the document

Publication details

Authors

Li, Y.; Mithani, N.; Chong, D.; Xu, J.; Fernandes, J. (Meta); Turner, C.; Varela, S.; Shadbolt, V. (CoolIT Systems)

Publisher

Open Compute Project Foundation

Year

2022

Document number

Ver 1.0

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

OWFa 1.0 — Open Compute open specification licence; redistribution permitted

Licence evidence

www.opencompute.org

Citation