DATA CENTRES OVERCOOLING

Air performance Metrics

Aflow = mc Aflow

Flow availability, ratio of CRACs to IT equipment air volumes:

mi = Tio = flow availability

There is one characteristic point for each data hall. For the example of weighted average temperatures provided above, the following air performance metrics result:

Flow performance (1-BP) = 0.50 Thermal performance (1-R) = 0.6 Availability of flow = 0.6 / 0.50 = 1.20

This unique point for each data hall can be represented on the following figure:

Tci – Tco =ηthermal – Tii

ηflow

Section of data hall air flows CRAH = computer room air handling unit; BP = bypass air; NP = negative pressure; IT = computer racks

The first diagram represents typical legacy data hall air flows shown in section, with temperature indicated on the colour scale and the relative air volumes represented by the thickness of the lines.

Air performance metrics of a data hall are based on the four characteristic weighted average temperatures which are:

Tci Temperature cooling unit (CRAH) in (example 22C)

Tco Temperature cooling unit (CRAH) out (example 17C)

Tii Temperature IT equipment in (example 21C) Tio Temperature IT equipment out (example 27C)

By considering the CRAH and IT loads to be equal, neglecting NP, and working with mass and heat equations, the following relationships are derived:

Flow performance, also equal to 1-BP, defines how much cooled air is actually being used by IT equipment:

ηflow = mf = Tci mc

ηflow mf

= flow performance

= air mass flow rate from cooling units supplied to IT equipment

mc = air mass flow rate through cooling units Tci = temperature entering cooling unit (in) Tco = temperature leaving cooling unit (out) Tii = temperature entering IT equipment (in) Tio = temperature leaving IT equipment (out)

Thermal performance, also equal to 1-R, defines how much of the air used by IT equipment actually comes from the cooling units (CRAHs):

ηthermal mi

ηthermal = mf = Tio = thermal performance

mi Tio

– Tii – Tco

= air mass flow rate through IT equipment Thermal and flow performance The results shown are typical for a legacy

facility: l Half of all cold air supplied by the cooling

units reaches the IT equipment, while the other half is wasted as bypass; and

l 60% of the air entering the IT equipment comes from the cooling units; the other 40% is recirculated warm air.

The ideal case is with flow performance and thermal performance equal to one, at the top right hand side of the diagram; it is possible to make improvements towards this ideal by implementing best practice.

Tio

– Tco – Tco

Efficient, flexible &#38; resilient cooling

As the leading UK manufacturer of IT cooling systems, we deliver cooling that works smarter not harder to reduce power draw and maximise uptime in critical, high density environments.

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T: +44 (0) 113 238 7735 E : f.farrelly@airedale.com W: www.airedale.com/ITcooling

44

CIBSE April 12 190mmx65mm v3.indd 1 CIBSE Journal April 2012

SmartCool OnRak INTEGRATED CONTROL SOLUTIONS 09/03/2012 11:21 www.cibsejournal.com

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