As the performance levels and power densities of modern IT equipment racks increase, more and more companies are switching from air to liquid cooling because liquid provides a more efficient method of heat transfer. Despite some concerns about mixing liquid and electronics, liquid cooling technology has evolved to make those concerns more obsolete.
Under standard conditions, water conducts heat per unit volume much better than air, which means that liquid cooling increases both the cooling efficiency and the energy efficiency of data centers using it. are using. Plus, it’s easier to manage than high air volumes. ASHRAE Technical Committee 9.9 even added another liquid cooling classification to standardize the scope of liquid cooling applications.
Liquid Cooling Categories
Liquid cooling refers to any practice in which liquid enters a cabinet to remove heat, even when combined with air movement. There are two basic classifications of liquid cooling: direct, where liquid actually cools the server and/or components, and packaged, where air cools the server and/or components, but the heat is rejected in a liquid stream.
These two classifications can be broken down into several variants:
- Direct liquid cooling uses a conductive interface and is often called direct on-chip cooling. Coolant does not actually touch computer components; instead, the cooling fluid circulates through the plate on which the processor chips are mounted. This is a very efficient form of cooling, but requires plumbing in the cabinets, in addition to power and network cabling systems, so management can be difficult. High-performance research processors and supercomputers most often use direct on-chip cooling.
- Liquid immersion generally means that the servers are immersed in a vessel of non-conductive fluid that conducts heat away, but it also includes server-level immersion that seals each server in a container of liquid. The original coolant was mineral oil, although data centers may also use other fluids, such as 3M Novec. Enterprises can flood standard servers with relatively minor modifications, such as fan removal and sealed spinning hard drives. SSDs do not require modification. Full immersion provides liquid thermal density, which absorbs heat for several minutes after a power outage without the need for backup pumps. Tanks equivalent to 42U rack capacity can cool up to 100 kilowatts (kW) in most climates with only an outdoor heat exchanger or condenser water. Minimal mechanical refrigeration requirements make liquid immersion an excellent choice for natural cooling.
- Rear Door Heat Exchanger (RDHx) units are direct coupled indirect systems. They circulate liquid through coils built into the cabinet doors to draw heat away from the server before venting it into the room. Use them for total cooling applications or to bring high exhaust temperatures in cabinets within the cooling limits of air conditioners. For total cooling, an RDHx unit keeps the entire room at the temperature of the IT equipment inlet air. This makes hot and cold aisle cabinet configurations and air containment designs unnecessary since the exhaust air cools down to inlet temperature and can recirculate back to the servers. The most efficient RDHx units are passive, which means that the server’s fans circulate air through them. They are generally limited between 20 kW and 32 kW of heat removal. RDHx doors incorporating additional fans can cool higher thermal loads, up to 60 kW.
Other considerations for liquid cooling
High-density computer hardware that uses direct liquid cooling requires constant fluid circulation in the event of a power failure. High-performance devices enter self-protective thermal shutdown within seconds if cooling is interrupted. Such devices often require chilled water storage – which can sometimes use the residue from large header pipes – and auxiliary pumps on an uninterruptible backup power supply (UPS).
RDHx units may not always require backup pumps since the entire room remains at server inlet temperature. Depending on the size of the room, the air temperature may remain within ASHRAE limits until the generators start. Use fast-restart coolers to resume cooling quickly after power is restored.
Most liquid-cooled devices are both high-performance and mission-critical, so make sure power and cooling are redundant. Design double-ended piping by including isolation valves that isolate segments for maintenance without losing liquid flow to the rest of the cooling loop.
The electrical design is equally important. Many liquid-cooled devices shut down in seconds if cooling fails, so keep backup pumps and door fans on the inverter. Keep all mechanical parts on a separate, redundant UPS circuit.