Water Cooling System Coolant Distribution Unit (CDU) For Data Center & IT Liquid Cooling Systems

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With the vigorous development of cloud computing and big data technology, the performance of IT equipment has gradually improved, which directly leads to the continuous increase in server power consumption, especially the CPU as a key component of the server. As performance improves, power consumption increases significantly. Common computer room cooling mainly relies on air cooling to cool servers, which can no longer meet the requirements of high power density equipment. Data center cooling technology solutions are gradually getting closer and closer to electronic equipment. From the room to the cabinet row, from the cabinet row to the rack and cabinet door for heat exchange, to now cooling chips and other equipment inside the server. Close to the heat source, nearby cooling has become a technological development trend. With the development and improvement of chip cold plate and coolant enhanced heat exchange technology, liquid cooling has become a new hot spot in the industry.

The principle and three forms of liquid cooling

According to the current research on the technological process and based on the cooling principle, liquid cooling technology is mainly divided into three main forms: cold plate, immersion and spray.

1. Cold plate liquid cooling

The original intention of developing cold plate liquid cooling technology is to avoid direct contact between coolant and servers. This technology precisely cools components such as CPU and memory, which are the main heat sources. The liquid cooling channel consists of a water-cooled heat pipe radiator, a liquid cooling distribution unit, a liquid cooling maintenance unit, a liquid cooling temperature control unit, a natural cooling unit, and primary/secondary cooling loop pipelines. In the cold plate liquid cooling system, high-power components such as the CPU use liquid-cooled cold plates for heat dissipation, while other small amounts of heat-generating components such as hard drives and interface cards still use air-cooling heat dissipation systems.

Cold plate liquid cooling technology is based on conventional air-cooled servers. The CPU and memory sides are close to a plate heat exchanger. The heat of the chip is transferred to the fluid in the plate through thermal conduction. The fluid is an insulating medium, which can be deionized water or ethylene glycol. Solution, fluorinated liquid, etc., or phase change heat pipe (the heat pipe conducts heat to the water system outside the computer room through the heat exchanger). The cooling plate takes away the main heat of the server through direct contact with the server’s PU/GPU (high heat flow density components) (there are two forms of heat pipes and liquid heat dissipation in the cold plate), and the heat of the remaining components (low heat flow density components) can be By taking away the higher temperature wind, we call this heat dissipation technology that combines liquid cooling and air cooling as liquid/gas dual-channel heat dissipation technology.

Compared with traditional rack-mounted air-cooled servers, this technology significantly improves resource utilization, significantly increases the energy efficiency of the data center while reducing the total cost of ownership. Moreover, the installation and maintenance of servers are basically the same as those of conventional air-cooled servers, so the operation and maintenance difficulty of this liquid cooling technology is basically the same as that of traditional air conditioners.

2. Immersion liquid cooling

Immersion liquid cooling can be divided into phase change immersion liquid cooling and non-phase change immersion liquid cooling technology according to whether the cooling medium changes phase during the heat exchange process.

The principle of non-phase change immersion liquid cooling technology is to immerse IT equipment directly in insulating coolant. After the coolant absorbs the heat generated by the IT equipment, it transfers the heat to the water in the heat exchanger through circulation, and then transfers the heat through the water circulation. Go to an outdoor radiator. Due to the fanless design of the server, this technology has lower noise and the noise value can be controlled below 45dB. It also eliminates fan power consumption and reduces the overall power consumption of the server by more than 10%.

The principle of phase change immersion liquid cooling technology is to immerse IT equipment in a cooling fluid with a boiling point lower than the working temperature of the IT equipment. When the operating temperature of the IT equipment reaches the boiling point of the cooling fluid, it will cause local boiling of the cooling fluid, causing cooling. During the boiling process of the working fluid, the heat generated during the operation of the IT equipment is taken away.

Immersion liquid cooling has clear advantages. First, in immersion liquid cooling, the coolant is in direct contact with the heating device, which has low convection thermal resistance and high heat transfer coefficient; secondly, the coolant has high thermal conductivity and specific heat capacity, and the operating temperature change rate is small ; Third, this method does not require a fan, reducing energy consumption and noise, and has high cooling efficiency; finally, the coolant has excellent insulation properties, a high flash point, is non-flammable, and is non-toxic, harmless and non-corrosive. Therefore, this liquid cooling technology is suitable for large data centers, supercomputing, industrial and other computing fields and scientific research institutions that have high demands for heat flow density and green energy saving, especially for areas located in severe cold, high altitude, or with special terrain and limited space. Data centers that have high environmental noise requirements, are close to people’s offices and residences, and need to be quiet have obvious advantages.

3. Spray liquid cooling

The spray liquid cooling system uses a certain type of coolant and directly or indirectly absorbs heat through the coolant to take away the waste heat released by the device to the external environment of the IDC for centralized heat dissipation. The main feature of spray liquid cooling is that the coolant with insulating and non-corrosive properties is sprayed directly onto the surface of the heating device or the extended surface in contact with the heating device, absorbs heat and is drained away. The drained hot fluid is directly or indirectly connected to the external environment. Large cold source for heat exchange.

Spray liquid cooling refers to the modification and deployment of corresponding spray devices to IT equipment. A liquid cooling method that specifically cools devices that generate excessive heat when the equipment is running. The characteristic of this method is that it does not require major changes to the computer room infrastructure, and only requires a small amount of modifications to the server to achieve better cooling performance. The spray liquid cooling cabinet system includes three parts: the spray liquid cooling cabinet system (including pipelines, liquid distribution system, liquid return system, PDU and other components), liquid cooling server, and coolant. The spray liquid cooling cabinet is connected to the indoor heat exchanger through pipelines, that is, the waste heat of the chip in the cabinet is absorbed by the coolant and then transferred to the indoor heat exchanger and exchanges heat with the outdoor heat exchanger.

The spray liquid cooling system has the characteristics of high device integration, strong heat dissipation efficiency, high efficiency, energy saving and silence. It is one of the effective means to solve the problem of deploying high power consumption cabinets in IDC computer rooms, reducing IT system cooling costs, improving energy efficiency and reducing TCO.

Application of cold plate liquid cooling system

Of the above three forms of liquid cooling, cold plate liquid cooling is the most widely used. The design can be divided into two categories, one is to deploy the coolant distribution unit (CDU) outside the cabinet, and the other is to have no coolant distribution unit (CDU).

The two different implementations and some design considerations are described in detail below.

1. Design implementation method including CDU

The most common form of liquid cooling implementation within a data center is the use of CDUs to separate the facility cooling system (the cooling side outside the computer room) from the rack cooling system (the cooling side inside the computer room). No need to expose sensitive IT cooling components to facility cooling systems. The CDU can be located within an IT rack to distribute liquids to equipment within a single rack, or it can be installed as a floor-standing external unit that distributes liquids to multiple racks. Figure 6 shows an example of a CDU-based liquid cooling unit.

In addition to circulating coolant, the CDU transfers heat from the cabinet cooling system to the facility cooling system. Prevent condensation within the equipment by regulating the coolant to a temperature above room temperature dew point. And it can provide flexible and diverse coolant temperatures to meet the requirements of different IT equipment.

2. Design points including CDU

In the cold plate liquid cooling system, the CDU plays a vital role. If the CDU fails, the entire cooling system will be paralyzed, so the CDU setting must consider redundancy or fault tolerance. The coolant pump in the CDU is most likely to fail, so redundant pumps should be set up. It is also equipped with a cut-off valve to ensure that water pumps that are not working properly can be cut off and online maintenance can be achieved. The power supply of the CDU also requires a UPS power supply to enable uninterrupted operation.

Regulating temperature is one of the main functions of the CDU. The temperature of the water supply provided by the CDU can be controlled through the use of bypass loops, proportional control valves, variable speed pumps, etc., in response to changes in equipment water temperature and heat load. In addition to setting an upper limit (the highest water supply temperature that can be safely provided to IT equipment), the water supply temperature must also set a lower limit (the lowest water supply temperature that can be safely provided to IT equipment). This limit can be dynamic in nature. , and depends on the dew point in the cooling space. The CDU monitors the ambient dew point and raises the supply temperature of the auxiliary water circuit to a level of at least 2°C above the room dew point temperature to prevent condensation from occurring.

3. CDU-free design implementation

Today, most data center liquid cooling equipment is liquid-cooled through CDUs. However, the liquid cooling form without CDU also has certain advantages in some aspects. First, the space occupied in the main engine room is reduced; secondly, there is no intermediate heat exchange link and the energy consumption of the transmission and distribution system is reduced, making the system more energy efficient; finally, the heat source and the final heat dissipation medium are tightly coupled, improving the coolant efficiency. temperature, while reducing the working pressure of the equipment and avoiding condensation.

4. Key points of design without CDU

Corrosion and scaling issues with non-CDU liquid-cooled server materials can be addressed by using materials that are chemically incompatible with various water-based components.

In non-CDU designs, facility design teams must work closely with IT equipment manufacturers to ensure appropriate materials are selected for the entire liquid cooling system.

Non-CDU cooling systems also require filtration on the upstream side of the water supply. Available as rack-level filtration units or equipment-level filtration units, they are designed to provide terminal filtration of equipment and pipes in liquid cooling systems. Today, the most common liquid heat transfer device used in electronics cooling is the microchannel cooling plate. These devices move liquid coolant through efficient channels to maximize the heat density (specific heat) of the liquid coolant. Reducing the size of the fin spacing in cold plate designs often results in superior heat transfer performance, but can also be a challenge for data center water system cleanliness. Therefore, sufficient communication must be carried out between data center operators and liquid-cooled server manufacturers.

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