As Datacentres increase in size and racks become more densely populated, controlled cooling of the equipment becomes crucial.
As energy efficiency in the datacentre becomes more critical, the method of providing efficient cooling to their high-density racks needs detailed consideration. Aisle containment is a solution that can benefit both new build and existing datacentres by helping deliver consistent air temperatures to the server racks.
Aisle Containment
A cooling system that delivers cooling air throughout the room is uncontained. It can lead to unpredictable air flows within the whole room, resulting in inefficient cooling and higher energy usage. Although this may be acceptable in small rooms with minimal equipment, the larger densely populated datacentres will benefit from the containment of this airflow by increased cooling efficiency and corresponding reductions of energy usage. Containment separates the cool and hot air to avoid mixing, meaning that warmer & drier air is sent to the air conditioners for cooling. Datacentre racks are arranged into hot and cold-aisles, cool air is drawn into the front of the Cold-Aisle racks and exhausted through the rear of the cabinets into the Hot-Aisle. Unless these aisles are isolated by enclosing one or the other aisle, we have an uncontained system. An uncontained system is unpredictable and may require complex CFD (Computational Fluid Dynamics) modelling.
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There are two types of aisle containment to consider adopting: Cold-Aisle or Hot-Aisle containment.
Cold-Aisle Containment System (CACS)
A CACS only has the cold-aisle enclosed. The hot air is expelled to the hot-aisle, i.e. the rest of the datacentre and ideally drawn into the Computer Room Air Conditioner (CRAC). A consistent hot air/cold air row-orientated layout of the racks will minimise the mixture of hot and cold air. However, the air temperature within the datacentre CACS will increase, so monitoring the temperatures of both the hot and cold air flows is crucial to avoid excessive heat return to the rack equipment.
Cold-Aisle Containment Example
Hot Aisle Containment System (HACS)
A HACS can give up to 40% better energy savings and is the preferred option for new build datacentres. HACS’s only enclose the hot-aisle to contain the heated airflow output from the rack equipment. The rest of the datacentre or the underfloor area becomes a cold-air return plenum. Consistent row layout again is required to minimise the mixing of hot and cold air. This type of row-oriented HAC is perfect for new build datacentres, smaller datacentres and high-density server racks. It is also possible to have the hot air ducted away to the CRAC where the heat is exchanged and tied into a combined heat & power (COHAP) system.
This course is best suited to new data centres as it will require bespoke air plenums and ducting to handle the airflows effectively.
Hot-Aisle Containment Example
CACS is more straightforward to implement as being less costly to install; it only requires doors at each end of the row and a roof. It is also easy to install in an existing datacentre, as there no requirement to contain exhaust air. Cold air can leak and mix with warm air decreasing overall efficiency.
HACS minimises air leakages' impact into the datacentre room area, making it cooler, providing a suitable working staff environment. HACS is more costly as it will require bespoke ducting to remove warm air for recirculating. It is vital for both CACS and HACS that there are environment and temperature monitors, and consideration is given to fire monitoring and suppression systems, placed away from obstructions.
Working Environment Temperature
With CACS, the room becomes the hot-aisle with HACS the reverse is true. Overall ambient temperatures mustn't vary to uncomfortable levels for datacentre staff, with a CACS system air outside can rise above 26˚C and as high as 38˚C, where a high density of servers are deployed. This heat level can be very uncomfortable for staff and visitors, so it is imperative to monitor the ambient temperatures. ASHRAE Standard TC9.9 (2008) recommends IT inlet temperatures of 18-27˚C. Find out more.
Benefits of Containment
- Energy Savings: Can achieve 10 to 40% savings over an uncontained datacentre facility.
- Cooling System Temperatures: Careful setting of a higher input air temperature will raise the return temperature, increasing the air conditioner's efficiency (AC), resulting in further energy savings.
- Hot-Spot Elimination: Containment minimises hot and cold air mixing, ensuring the incoming air temperature is uniform with air leaving the AC. Incoming air temperatures can be raised without the risk of rack hot-spots, assisting with AC efficiency by reducing running hours.
- Reduced Humidification: Increasing the supply air temperature, reduces the chance of the system operating at or below dew point temperature. Therefore it minimises the need to reduce the air humidity once again saving energy.
- Cooling System Sizing: The need for oversizing of cooling with fan sizes and numbers is unnecessary as containment enables the delivery of predictable cooling leading to greater efficiency
