The first commercial blade server was launched by Compaq less than a decade ago and the high density data centre revolution was sparked. It already feels like a long time since data centre designers first started pointing out the inadequacy of perimeter cooling solutions with a raised floor plenum for reliably cooling anything much beyond the 1.75kW/ rack norm. Although with care, densities of 4kW/ rack can be achieved, perimeter mounted CRAC units have been demonstrated to be able to deliver reliable cooling redundancy in the event of the loss of a unit from the system. That is, unless units are drastically oversized.
Physical characteristics of raised floor cooling designs which can contribute to inefficiency in today’s data centres include bypass leakage, poor air coupling to the IT load, uneven distribution of cooled air, hot spots that require a lowered supply temperature, the need to over-provision computer room air handling (CRAH) units, under floor obstructions and blockages, inability to adjust to dynamic power variation, and other problems.
Oversizing of infrastructure, over compensation with low air inlet temperatures and ‘stranded capacity’ all escalate capital equipment cost as well as data centre operating expenses. Worse still, they do not necessarily ensure a reliable and adequate cooling solution.
A lot of these problems are overcome with newer approaches that are effective even without a raised floor and traditional CRAH units. Such approaches result in improved PUE along with improved compatibility with high-density and dynamic loads. Without doubt, traditional cooling approaches will continue to be used, but there is an increasingly good business case for aisle containment. Containment prevents the mixing of hot and cold air streams to offer more efficient data centre cooling infrastructure. It can also be fitted into new designs or added retrospectively to existing data centres.
Keeping the Cold aisle contained
Containment of the cold aisle is typically retro fitted in traditional perimeter-based data centre environments. Traditional cooling environments use the entire room as a hot air return plenum and use cold air via the raised floor plenum to the cold aisles. This allows the rest of the data hall to become a large hot air return plenum while separating the hot and cold air streams.
Whilst a considerable step forward from legacy cooling solutions, cold aisle containment does have drawbacks such as the inefficiencies which result from distances and pressures required for adequate air distribution. Inefficiencies are caused in a room-based approach because there is a requirement to move cold air from the perimeter CRACs to the load. A row-based approach brings the source of the cooling in close proximity to the IT equipment and therefore less energy is required to deliver cold air to its destination.
There are also density limitations when using cold air containment with distribution through a raised floor. On average the limit is around 6 kW per rack and higher densities can only be achieved through the use of customised designs. Additional fan assisted devices can be used to improve airflow but this further reduces the efficiency of the overall cooling system by increasing overall power consumption and adding heat to the cold air supply. Density limitations can be avoided if cold aisle containment is used together with row based cooling.
However, the solution can still have limitations when deployed in a row-based approach. For example, since cold aisle containment minimises the overall pool of cold air available to the server, any loss of power and/or cooling supply will result in more rapid temperature increases in the server load. This will shorten the amount of time available to the data centre manager before overheating and thermal shutdown starts to occur.
To realise maximum benefits, all cold aisles in the entire data hall should be contained, otherwise mixing of hot and cold air in uncontained areas will occur, diminishing expected energy and cost savings.
An open and shut case for hot aisle containment
The Hot Aisle Containment System (HACS) encloses the hot aisle, collecting exhaust air from IT equipment, cooling it and making it available to the air intakes of racked IT equipment.
HACS ensure efficient air distribution by completely separating supply and return of air paths and eliminating the mixing of hot and cold air streams. HACS hold many of the advantages of cold aisle containment whilst avoiding many pitfalls. The efficiency of the HACS will be higher because the hot aisle is capable of maintaining higher temperatures. The net effect of elevated return air to the cooling unit enables better heat exchange across the cooling coil, better utilisation of the cooling equipment and overall higher efficiency.
Hot aisles typically operate +38?C with high density servers. Consider the practicalities of maintaining the entire data hall at this temperature with cold aisle containment in order to return the same level of efficiency. And whilst it is true that cold aisle containment would enable higher temperatures overall (up to +39?C), it’s unlikely that the average data centre manager would consider this a sensible solution.
Another advantage of hot aisle containment is improved flexibility. HACS create a room neutral cooling zone and does not impact the temperature of the surrounding room. Since HACS do not deliver any hot air to the outside room, they eliminate the mix of hot and cold air that leads to inefficiency. Furthermore, as a row-based cooling approach, a HACS can be easily installed in an existing environment - there is no need for specialised duct work and no adjustment required to the existing HVAC systems to manage elevated temperatures. There is, additionally, no requirement for a raised floor with HACS, making the solution suitable for a wide range of installations from small to mega data centres.
The added bonus is higher availability. Since HACS only ‘contain’ hot air, the rest of the data hall provides a much larger reservoir of cool air which the servers can draw upon to extend runtime.
Conclusion
Data centre professionals recognise that legacy solutions have limitations for cooling today’s high-density environments: They can be inefficient, costly and wasteful from a carbon footprint perspective. Row-based cooling and segregation of hot and cold streams can be used to improve upon this situation.
Containment offers the benefit of cooling systems that can be set to a higher temperature (saving energy) while still supplying the load with safe operating temperatures and reduction of humidification/ dehumidification costs. In addition, better physical infrastructure utilisation enables right sizing to fit power consumption and cooling needs, it means equipment is running at higher efficiencies and it leads to better PUE.
Both hot aisle and cold aisle containment offer considerable advantages over legacy cooling systems in terms of system efficiency and ability to handle high density equipment including blade servers. However, HACS, with its row-based cooling architecture is more efficient, more flexible, provide better ride through capability, and offer a way to better address the need to manage higher density requirements without making the entire data centre hotter than necessary.
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Tags: Power & Cooling, Design & Facilities Management |
