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Air management metrics in data centers.

Article Excerpt
INTRODUCTION

The latest publications by ASHRAE (2005 and 2006) provide extensive guidance on the following:

* Liquid cooling of computer equipment: dielectric fluids, water and refrigerants

* Air distribution in equipment, cabinets and data center (hot cold aisles, etc)

* Energy efficiency of plant and systems.

There are also numerous valuable contributions on data-center air-management metrics. These metrics, primarily based on air recirculation (server exhaust to server inlet), show how the efficiency of the mechanical cooling system can be significantly improved.

Sharma et al introduce the concepts of supply and return heat indexes that measure the extent of air recirculation (2002). Schmidt et al define the Beta index that measures the extent of increase of the inlet temperature to the server due to air recirculation (2005). They correlate this to energy efficiency. Boucher represents the supply heat index in terms of temperatures (2006). Herrlin defines the rack cooling effectiveness to measure the extent of rack inlet over-temperature due to recirculation.

This paper proposes data center air management metrics that in addition to recirculation, also address negative pressure flow, bypass flow and balance of server and CRAC unit flows (Tozer 2006a). Defining three of these metrics e.g. recirculation, bypass and negative pressure ratios, will establish the overall air management performance of the data center. The metrics are practical and simple to apply and can provide a specific characteristic for each data center room (Tozer 2006b).

DATA CENTER AIR MANAGEMENT METRICS

This section explains the basics of the air management metrics which has been developed to establish how well the cooled air from the CRAC units reaches the server inlets. The main parameters are air mass flow rates and temperatures which are used to express the quantity and quality of energy supplied to the servers. These air management metrics are based solely on the consideration of sensible loads. For this conceptual model, the following flow rates are present. These are indicated in the following Figure 1.

[FIGURE 1 OMITTED]

CRAC flow (Mc): total air-flow rate produced by all operating CRAC units in the data center. This air flow rate is normally much more than what is needed by the servers, due to part load operation of IT equipment, and redundant CRAC units that are kept operational.

Negative pressure flow (Mn): air that is induced into the floor void due to Venturi effect. This is the effect of high velocity pressure (proportional to the square of air velocity), which reduces the static pressure (Bernoulli law of fluid dynamics). Once the velocity pressure is higher than the total pressure (made up of static and dynamic pressure), the static pressure at that point becomes negative. With negative static pressure under the floor, air will be induced into the floor void from the space above. In practice negative pressure flow is low and even negligible but is found near the CRAC discharge where air velocities are high, i.e. floor grills, floor openings, tile edges, and CRAC perimeter.

Bypass air flow (Mbp): this is air that leaves the floor grills and returns directly to the CRAC unit without cooling servers.

Recirculation airflow (Mr): air that is discharged from servers, which returns and mixes with air entering the servers to cool them.

For this conceptual model, average temperatures are used.

T = [SIGMA] m[c.sub.p] [T.sub.i]/[SIGMA] m[c.sub.p] (1)

Note: cp is specific heat

Typical temperatures for a legacy data center are provided as an example in Appendix A. The temperatures are:

* Tr = 21[degrees]C [69.8[degrees]F], return air temperature to CRAC (at CRAC), normally the CRAC return air set point

* Tc = 14[degrees]C [57.2[degrees]F], discharge air temperature from CRAC (at CRAC)

* Tf = 14.1[degrees]C [57.4[degrees]F], floor void temperature (after room air is drawn in), very close to Tc

* Ts = 21[degrees]C [69.8[degrees]F], server inlet air temperature (mixture of grill and recirculation air)

* Th = 28[degrees]C [82.4[degrees]F], server outlet air temperature (before cold air mixes with it)

Given these mass flow rates and temperatures, the following...

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