Close Coupled Cooling and Reliability (Matt Mescall)

Overview

The article was written by Matt Mescall and published in the Uptime Institute Journal in 2014. It explains how in-row, above-row and rear-door heat exchanger (RDHx) cooling can meet Concurrently Maintainable (Tier III) and Fault Tolerant (Tier IV) requirements.

The starting point is the difference between room CRACs and close coupled cooling (CCC) units. Below about 2-4 kW per rack, room CRACs can share one or two redundant units for the whole room, whereas close coupled units only serve their own aisle and cannot cool other aisles. The redundant unit must therefore sit in the aisle that needs it, and redundancy must be provided per aisle and within the unit's throw distance.

The article covers piping and electrical design. Concurrent Maintainability needs dry pipe isolation: draining a pipe while disassembled is allowed, but hot tapping and pipe freezing are not. Fault Tolerant cooling must respond autonomously to failures, provide Continuous Cooling and compartmentalize chilled-water or refrigerant piping outside the room of use.

On the electrical side, any planned electrical outage may take out no more than the redundant number of cooling units in any single aisle; 2N CCC with an A/B split simplifies this. N+R CCC needs dual feeds via an internal transfer switch or an external manual or automatic transfer switch; for Fault Tolerance all transfers must be automatic and fans, pumps and CDUs need uninterruptible power. For RDHx, a front-to-back row layout fails Tier III/IV because a door out of service blows hot exhaust into the next rack.

Key points

1. Room CRACs below about 2-4 kW/rack can share one or two redundant units; close coupled units cannot cool other aisles, so the redundant unit must be in the aisle that needs it.
2. Concurrent Maintainability needs dry pipe isolation: draining a disassembled pipe is allowed, hot tapping and pipe freezing are not.
3. Fault Tolerant cooling must respond autonomously, provide Continuous Cooling and compartmentalize piping outside the room of use.
4. A planned electrical outage may take out no more than the redundant number of cooling units in any single aisle; 2N CCC with A/B split simplifies this.
5. N+R CCC needs dual feeds; for Fault Tolerance all transfers must be automatic and fans, pumps and CDUs need uninterruptible power.
6. RDHx in front-to-back rows fails Tier III/IV; hot/cold aisle layouts can still recirculate over the rack top and at row ends.

Using it in design

• For in-row and RDHx designs, evaluate cooling redundancy per aisle or row, not per room, and check unit throw distance to the farthest rack under N-1.
• Catalogue close coupled units with a serves-scope (aisle/rack) and power-feed type (single, dual-ATS).
• Warn when a front-to-back RDHx layout is labelled Tier III.
• Build dry-isolation valves and compartmentalisation into the piping design from the start.

Who should read it

Mechanical and electrical engineers designing Tier-targeted, high-density cooling.

Related standards

• Uptime Institute Tier Standard: Topology

Publication details

Authors

Matt Mescall

Publisher

Uptime Institute Journal

Year

2014

Document language

English

Source type

Industry body

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation