Heating and Ventilating

 

Close Control: Keeping cool in computer rooms

Derek Siveter, managing director of Stulz UK, examines the latest technology for precision AC applications
Close Control: Keeping cool in computer rooms
THE increased cooling requirements in computer rooms, particularly the installation of high density servers, has put increasing pressure on manufacturers to take a fresh look at the design of computer room air conditioning units (CRAC units). The demand from the IT market has been: air volumes balanced to server cooling requirements; low energy costs; increased cooling capacity; smaller footprint.

Stulz has developed CyberAir, a range of CRAC units with electronically commutated fans from 15kW to 100kW capacity in direct expansion, chilled water and combinations of dual fluid and free cooling to meet additional redundancy and energy saving requirements.

The new generation of IT equipment, typically 7U Blade Servers and IU Pizza (U denotes the server height that fits into standard sized racks), are designed with air intake at the front and discharge at the back of the server. To cool these high density servers the racks are positioned front to front and back to back, creating a cold aisle/hot aisle configuration.

It is important to supply the correct quantity of conditioned air to the cold aisle. Too little air and the servers at the top of the rack will short cycle. Too much air and the cold supply will short cycle to the return.

The variable volume EC fans in the CyberAir unit ensure the correct air volume is supplied by automatically adjusting fan speed as determined by return air temperature or static pressure in the floor void as the servers internally regulate the air flow from cold aisle to hot aisle.

The rising cost of energy makes reduction of energy usage essential when selecting CRAC units. EC fans are more efficient than ac motors, typically 90% efficient against 70%. As the fan laws state, fan power is a cube function of air volume. Therefore, a reduction in the CyberAir’s EC fan speed, which will supply the correct volume of air for the cold aisle, will have an impact on consumption.

Most data centres are designated business critical facilities with minimal or no single points of failure in the electrical or air conditioning supply. Therefore, data centres are built with N+1 redundant air conditioning capacity. It is thought that, by turning off and sequencing the +1 standby unit, energy will be saved as it will with traditional fixed speed fans. CyberAir with variable speed EC fans will reduce the fan power consumption further by running the +1 unit at a pre-determined lower air volume.

An example, taking a 400kW cooling requirement for a data centre with five 100kW CRAC units, is shown below. Traditionally, based on US equipment, the largest CRAC unit had a capacity of 70kW of sensible cooling and an airflow of about 5.5m3/s. The compact design and availability of larger scroll compressors means the CyberAir unit has a capacity of 100kW and the EC fans will supply 8.0m3/s, an increase of 40% on what was previously the largest manageable unit.

The compact design of the CyberAir unit means all the components for a full function 100kW cooling capacity CRAC unit can be accommodated within the 255cm length with maintenance access from the front. To meet the high density cooling capacity requirements of modern data centres, CyberAir units can be located side by side around the perimeter and provide cooling capacities that were not possible previously.
1 October 2006

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