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Liquid cooling? Water cooling? Are these technical terms the same?

At the GTC China 2017 conference, which ended yesterday, Mr. Huang Renxun, co-founder and CEO of NVIDIA, said passionately: "We are all in the computer industry, and now we have a golden opportunity, because the foundation of the whole computer industry is being subverted. The way computers are manufactured has been radically changed, and software will no longer rely entirely on human coding ability.

At this annual gathering of AI developers, Baidu Apollo's sophisticated map, Huawei GPU accelerated intelligent analysis solutions, and Sino-Tech Dawning's artificial intelligence integrated application service platform all tell us that the era of artificial intelligence, whether hardware or software, has ushered in exciting opportunities.

We also saw the submerged Liquid cooling scheme in front of the booth of Dawning Company. It is introduced that this is a high-density high-performance computer specially designed to provide matching heat dissipation capability for in-depth learning, which can make the density of single cabinet reach 100 kW. In addition to Dawning's application of liquid cooling in AI field, the phased array radar, the first domestic carrier equipment launched in Dalian Shipyard in May, also applies full liquid cooling system in order to equip more transceiver units and expand the array area.

These seemingly too far away from our public life, liquid-cooled black technology is being introduced by major well-known enterprises and applied in some professional fields. Just recently, Dawning delivered China's first commercial immersed liquid-cooled server (see figure above) to Huazhong University of Science and Technology at a large-scale application conference of liquid-cooled servers, which is used for information storage system and parallel distributed computing system independently developed by Huazhong University of Science and Technology to build a healthy large data platform. Subsequently, Lenovo also released the first super-large-scale warm water cooling overrun cluster in China last week. It can be seen that the original intention of liquid Cooling Solutions is to achieve ultra-high density deployment and improve computing capacity.

Liquid cooling? Water cooling? Is that the same thing?

Friends who pay more attention to liquid cooling technology have probably captured some differences, such as liquid cooling servers and water cooling overrun clusters. What kind of existence are these? Today we will take this question to explore.

Liquid cooling refers to the replacement of air by liquid, which takes away the heat generated by CPU, memory strip, chipset, expansion card and other devices in operation. According to the current technological research process, liquid cooling is classified as water cooling and refrigerant cooling. The refrigerant can be used, including water, mineral oil, electronic fluoride, etc. According to the cooling principle, the liquid cooling is divided into two systems: cold plate liquid cooling (indirect cooling) and immersion liquid cooling (direct cooling).

According to Dawning and Lenovo's two products, they belong to indirect cooling, i.e. cold plate liquid-cooled servers. Baidu also showed X-Man 2.0 with cold plate radiator on GPU at Baidu AI Developer Conference in 2017. These refrigerants all use water.

We can see from the picture that all three servers inject water from the special fast interface, pass through the closed water supply pipeline, take away the heat of the CPU, and then flow out through the back water pipeline. The difference is that the pipelines used by Dawning and Baidu are hoses, while Lenovo uses copper tubes, which are easier to bend and more freely laid out than copper tubes. In addition, the second generation of Dawning cold plate liquid cooling server, the liquid cooling system includes memory modules.

Therefore, liquid cooling may be water cooling or other refrigerants, depending on the reliability level requirements of the application scenario; but water cooling must be within the scope of liquid cooling technology.

What GUI is warm water cooling?

In the field of liquid cooling technology, there are many subdivisions, such as submerged liquid cooling with phase change and non-phase change, and cold plate liquid cooling with liquid cooling and warm water cooling. Mingming uses water for cooling, so why use warm water? To solve this problem, we interviewed Cui Xintao, director of research and development of Dawning cold plate liquid-cooled server. Cui always thought about the moment as if to recall the original design ideas.

He said: "According to the second law of thermodynamics, heat can flow freely from high-temperature thermal energy to low-temperature thermal energy without external force. Server computing core CPU is the main source of heating, its normal operation generally requires that the core temperature is not higher than 80 C, while the outdoor temperature is far below 80 C. According to the second law of thermodynamics, the heat emitted by the CPU can be spontaneously transmitted to the outdoor without external power. So in theory, we can remove all the fans, air conditioners and compressors in the traditional air-cooling system, and use liquid instead of air with poor heat transfer coefficient as a new medium of heat conduction, which is the basic methodology of liquid-cooling technology.

So what is the concept of warm water? Cui always went on to explain: "Warm water, in fact, is what we call water in the temperature range of 30 - 45 degrees Celsius. When we developed the first cold plate liquid-cooled high performance server in 2013, and also the first one in China, in order to remove the compressor and realize natural cooling, we set the temperature of water between 35-45 degrees C and the temperature of water outlet between 45-55 degrees C through a large number of experimental data analysis, so that the water-cooled circulating system of the server will realize natural cooling throughout the year, thus greatly reducing the data. Heart PUE value. The liquid-cooled high-performance computing cluster we deployed in 2015 is the prototype system of the Earth System Numerical Simulation Device of the Institute of Atmosphere, Academy of Sciences. All of the systems use cold plate liquid-cooled servers with a total of 1375 computing nodes. Up to now, the PUE value of the system has been kept at about 1.17. Another case is China's largest grid simulation high-performance computing platform, which combines air-cooled servers and cold plate liquid-cooled servers, totaling 750 computing nodes, and the PUE value has been well maintained at about 1.27 for six months. It can be seen that using normal temperature water to cool the CPU, which is the core heating component of the computer, and abandoning the compressor as a solution can not only reduce the total cost of ownership, but also significantly increase the energy efficiency of the data center, which lays a foundation for realizing high-density deployment of computing capacity.

Is the higher the water temperature at the intake side the better?

Then you will have doubts, based on this methodology, whether the inlet temperature can be set higher than 45 C? Is the higher the water temperature, the better? In view of this problem, Cui's overall explanation is more rigorous: "In the case that natural cooling has been achieved throughout the year, the effect of heat dissipation and energy saving is achieved, it is not significant to increase the inlet temperature again. At present, the indoor side inlet temperature limit can reach 45 C. Even if the inlet temperature is raised again, the space for improving energy efficiency is not large, but the reliability of the liquid cooling system begins to decrease, and the heat dissipation effect of the chip will become worse, which will reduce the reliability of the computer system.

The technology of liquid cooling is gradually moving from behind the scenes to the front stage. The technology of cold plate liquid cooling is more mature and has begun the industrialization process. After the accumulation of high performance computing market, we hope that the dawn with rich experience in liquid cooling technology can continue to drive the development of liquid cooling industry. It is not enough to rely only on one company, which requires more manufacturers to invest more energy in liquid cooling technology and research and development of supporting raw materials, to form an industrial chain, and to improve the ecology in order to sustain and rapid development.

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