Power Play

Decrease temps and cooling costs

Using the equipment cabinet as an architectural feature can cool heat loads in excess of 20 kW per cabinet.

by Ian Seaton

Case studies have shown that by employing passive cooling technology to overcome thermal challenges, network managers can decrease data center cooling costs up to 40 percent. The basic principle of passive cooling technology involves using the equipment cabinet as an architectural feature in the data center that secures the isolation between chilled supply air and heated return air. When using a ducted exhaust cabinet, this isolation can effectively cool heat loads in excess of 20 kW per cabinet, while maximizing cooling unit efficiency and allowing the air temperature in the data center to be increased.

A case study of a casino's 600 kW data center demonstrates the savings that can be achieved by using passive cooling technology. This data center contains two rows of low-density deployments with 2 kW-2.5 kW per cabinet, two rows of 8 kW cabinets, two rows of 16 kW cabinets, four short rows of connectivity and three rows with 1 kW-1.5 kW per cabinet.

The original design was based on standard hot aisle and cold aisle separation with free-space return air. There were severe hot spots in the 8 kW and 16 kW cabinets.

The ducted exhaust cabinets and improved tile locations in the containment scenario eliminated seven CRAC units from the plan.

The containment strategy included adding vertical exhaust ducts to the higher-density cabinets to remove the return air into the suspended ceiling space, and plenum extensions were added to the cooling units to capture the return air. Suspended ceiling tiles over the hot aisles of the low-density cabinets were replaced with ceiling grates to bring the return air into the suspended ceiling space and minimize its impact on the overall room.

A computational fluid-dynamics model confirmed the site audit that the hot air containment changes had eliminated the hot spots. More importantly, the initial scenario required 14 computer room air conditioning (CRAC) units with a cooling capacity of 385 tons and still experienced hot spots, compared to the containment scenario, which only required seven CRAC units for a cooling capacity of 220 tons and resulted in no hot spots.

The ducted exhaust cabinets and improved tile locations in the containment scenario eliminated seven CRAC units from the plan, totaling 165 tons of excess cooling capacity (40 percent of the original capacity). The reduced capacity resulted from the improved efficiency of the water-cooled CRAC units at higher return air temperatures and the elimination of the need to over-provision airflow to compensate for the wide variation in static pressure typically found under the access floors of most data centers.

Following this plan, this data center could see more than $80,000 in annual savings, plus the reduced capital outlay for the unneeded CRAC units.

This example demonstrates the challenge of effective cooling in a free-space return air environment and the benefits of contained return air using ducted exhaust cabinets. The free-space return air environment requires additional cooling capacity to overcome the mixing of hot and cold air in the room.

Immediate savings were realized using partial isolation with ducted exhaust cabinets for the highest heat load and by carefully locating supply and return vents throughout the room. Complete isolation with ducted exhaust cabinets and the use of economizers can further reduce cooling cost.

Ian Seaton, technology marketing manager for Chatsworth Products, New Bern, N.C., serves as editor of the rack and cabinet chapter and functions as a thermal consultant on the mechanical working group for the new BICSI data center design standard.

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