Rack Cooling: What is it?
All rack-mounted servers and data comms devices consume electrical power which ends up heating these devices. If they are not cooled, then their speed of operation and failure rate are badly affected. This is where rack cooling comes in. The phrase ‘rack cooling’ refers primarily to the process of cooling the rack in a cabinet. ‘Rack cooling’ can also be used to refer to the device that is engaged in the cooling. The amount of cooling necessary in a cabinet will vary enormously, depending on the equipment it houses. How server racks are cooled will depend on the location of the cabinet and the amount of cooling required.
Rack cooling in data centres uses complex cooling systems. These involve large cooling devices and underfloor ducting for the delivery of the cooling air to go to each cabinet, and the use of enclosed aisles so that the hot waste air the cabinets does not mix with the cooling air.
Server Room Cooling
When it comes to rack cooling for individual cabinets, there are a number of options. One way is to put cabinets in dedicated ‘server rooms’. The whole room is then cooled as much as possible with air conditioning, with the aim of keeping all the equipment inside the rack within the manufacturer’s recommended temperature range. This has the drawbacks of wasting space and consuming a lot of power for the cooling systems. But the main disadvantage is that it is of limited effect as the rack cooling air is not distributed evenly in the room or the rack, leading to ‘hot spots’ in the equipment.
Another method of rack cooling is by passive means, – that is to say by using uncooled air. This is done by removing all the panels from the server cabinet, using mesh doors and spacing out the kit inside to maximise the natural flow of air through the equipment to cool it. This has the drawbacks of limited cooling capacity and that the equipment is physically unsecured. Alternatively, special cabinets with built-in fans can be used to force uncooled air over the equipment. These are more effective but offer less cooling than active cooling systems.
Active cooling of a standalone cabinet uses a built-in air con unit to provide cooled air to the devices inside it. The system itself uses power to cool the air and provides more cooling capacity than passive methods. Active rack cooling methods offer the most cooling capacity using the smallest space possible. They offer a lot more flexibility on the location of cabinets compared to server rooms, but care needs to be taken when implementing them.
To get the system best performing, consider the airflow within the cabinet to ensure that there are no hotspots where cooling does not reach and where airflow through all of the servers is assured. Also, consider the fact that most air con systems will need to rest for up to 3 minutes from when they switch off to when they can switch on again, to protect their working parts. If the temperature in the cabinet rises too much in that time due to the high heat load inside, then the service life, speed and reliability of the equipment inside will be compromised. If this is the case, select a system that can cycle on and off rapidly.