Evaluating the Limits of Door Heat Exchanger Solutions Optimized for Open Rack V3

Overview

The study compares an off-the-shelf (OTS) door heat exchanger with a design sized specifically for ORv3. Three applications are examined: traditional (facility water makes the rack thermally neutral to the room), air-assisted liquid cooling (AALC; closed-loop in-rack cold plates reject heat to aisle air through the rear door) and hybrid (cold plates plus a door HX). The metrics are approach temperature, water-side delta T, liquid-captured load fraction and server fan power. Assumptions are 30 C ambient, sea level, 12-15 C air delta T and a W27 facility water system.

The introduction stresses the limits of air cooling. According to the Uptime Institute's 2022 survey rack power can approach 40-50 kW but typical racks remain under 20 kW. Server fan power has reached 10-20% in dense servers, and because fans share the UPS source with the server, fan power rising from 2% to 10% of server power cuts UPS capacity by about 8%. Conventional room CRAH and aisle containment approaches are described as limited to under 20 kW per rack. ORv3 differs from ORv2 with 44 OU, movable PSU/BBU shelves and liquid cooling support, and can carry 30 kW of IT with two power shelves.

Results vary by application. In AALC the custom design can supply 44-46 C water to cold plates for 30 kW at 2500 cfm, where the OTS design needs 3500 cfm, 1000 cfm more. Total fan power is 990 W for the custom design and 2180 W for OTS. In the traditional case, for 29.5 kW and 3500 cfm the OTS design needs 19 C facility water while the custom design can run at 26 C, a 7 C gain. In the hybrid case a 30 kW rack is cooled with 1500 cfm and 30 C water, when at least 50% of the load is captured by cold plates, at 32 C exhaust air.

The paper also describes an 80 kW hybrid rack: a 30 kW door HX with a 4U, 50 kW in-rack CDU (5 C approach), 27 C facility water and 30 C air, with total fan power of 1.97 kW (about 2.5% of capacity) and a density of about 9 kW per square foot. The authors call this feasible; the results rest on modelling and assumptions rather than measured field data. The suggested next step is a Door HX base specification for ORv3.

Key points

1. Raising server fan power from 2% to 10% of server power consumes about 8% of UPS capacity because fans share the UPS-backed supply.
2. Conventional air-side approaches such as room CRAH and aisle containment are limited to under about 20 kW per rack.
3. Assumptions: 30 C ambient (cold-aisle upper limit), sea level, 12-15 C air delta T, W27 facility water.
4. Traditional door HX: purposeful design allows 30 C room air with 27 C facility water (upper end of W27).
5. AALC: 30 kW of IT is supported at 2500-3500 cfm with supply air of 30 C or less; a tailored door HX cuts fan power by up to 55% and rack airflow by 1000 cfm (990 W versus 2180 W).
6. Traditional example: for 29.5 kW and 3500 cfm the OTS design needs 19 C facility water, the custom design 26 C.
7. Hybrid: a 30 kW rack with 1500 cfm and 30 C water (CDU upstream, at least 50% of load on cold plates) holds 32 C outlet air; total fan power about 1.5%; cold-plate water may reach 40 C.
8. Example hybrid rack: 30 kW door HX plus 4U 50 kW CDU cools 80 kW with 27 C facility water, about 9 kW per square foot and 2 kW (2.5%) fan power; rack outlet water 43 C.
9. Placing the CDU upstream of the door HX saves in-rack space and power but gives the coil warmer water; placing it in the rack downstream lets the coil use facility supply temperature.
10. Door HX is a mature technology and, among OCP ACS technologies, has the least impact on IT hardware design.

Using it in design

• Build a rear-door HX sizing helper that takes rack kW, airflow (cfm), facility water temperature and liquid-capture fraction and returns approach, leaving air temperature and fan power, with 27 C facility water and 30 C air as default reference points.
• Warn when air-only options are selected above about 20 kW per rack and suggest a door HX or hybrid solution.
• Include server fan power in UPS sizing; as the fan fraction rises, usable UPS capacity falls.
• When selecting a door HX, compare how an ORv3-specific coil raises the permissible facility water temperature (26 C instead of 19 C in the example) and water-side delta T, in terms of PUE, WUE and pump power.
• In hybrid designs decide whether the CDU sits upstream or downstream of the door, where the approach penalty is paid, and the in-rack space and power impact.
• For AALC compare the achievable cold-plate water temperature (44-46 C) with component case temperature limits.

Who should read it

Mechanical engineers planning a move toward liquid cooling in air-cooled facilities, ORv3 rack designers and door heat exchanger manufacturers.

Document contents

1. License
2. Acknowledgements
3. Terminology
4. Management Summary
5. Introduction
6. Goals of the document
7. Door HX Technologies & Facility Applications
8. Boundary conditions and Parameters of Interest
9. Comparison of ORV2 and ORV3
10. Two Different Door HX Solutions
11. Off-the-shelf (OTS)
12. Specific to ORV3 (Custom)
13. Performance Under Specific Applications
14. Air-assisted (liquid cooling in an air-cooled facility)
15. Traditional
16. Hybrid
17. Door HX Cooling Management
18. Summary of Benefits and Limitations
19. Next Steps
20. References

Related standards

• OCP ACS Door Heat Exchanger Requirements for Open Rack
• ASHRAE TC 9.9 Liquid Cooling Guidelines (W-classes)
• OCP Open Rack V3 Base Specification

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Publication details

Authors

George, J.; Fernandes, J. (Meta); Cacho Alonso, J.C. (Rittal); Kiernan, K. (Amphenol); Thompson, M. (nVent Schroff); Boisvert, P. (Boyd)

Publisher

Open Compute Project Foundation

Year

2023

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

CC BY-SA 4.0 — share with attribution under the same licence

Licence evidence

www.opencompute.org

Citation