Cooling Systems for Data Center

There are many cooling methods for data centers, which can be divided into mechanical refrigeration and natural cooling according to the cooling method. Among them, mechanical refrigeration can be divided into air-cooled direct expansion air conditioning system, air-cooled chilled water system, water-cooled chilled water system and centralized cooling water system, etc. , Natural cooling includes technologies such as fresh air, air plate exchange, rotary heat exchange, evaporative cooling and liquid cooling.

First, the air-cooled direct expansion system

This is the most traditional cooling method. The air conditioner is composed of an indoor unit and an outdoor unit connected by fluorine pipelines. To dissipate heat, the structure of the air-cooled air conditioner is very simple. Usually, one air conditioner internal unit corresponds to one or two external units.

1. The principle of air-cooled direct expansion system

The air conditioner is composed of a compressor, a condenser, an expansion valve, and an evaporator. The refrigerant circulates in the system under the action of the compressor, and undergoes a two-phase phase transition from liquid to gas to liquid to complete a refrigeration cycle. The refrigerant boils and evaporates into steam after absorbing the heat of the machine room in the evaporator, the corresponding pressure is called the evaporation pressure, and the temperature is called the evaporation temperature; the compressor continuously sucks the steam generated in the evaporator and compresses it to condensing Pressure, and then sent to the outdoor condenser, the refrigerant condenses into a liquid under the condensing pressure, and transfers the released heat to the outdoor air.

2. System Features

The air-cooled direct expansion system has a simple structure and flexible and convenient layout. Since each air conditioner is independent of each other, there is no single point of failure and high reliability. It only needs to adopt the N+X method for design and layout, and it can meet the cooling requirements of a class A machine room. Require.

When the air-cooled direct expansion air conditioner unit is used, the horizontal distance and vertical distance between the indoor unit and the outdoor unit of the air conditioner should be as short as possible to prevent the cooling capacity of the air conditioner from attenuating; the arrangement of the external unit should be conducive to heat dissipation, and the condenser can be appropriately enlarged according to the requirements of outdoor site installation Heat dissipation, improve heat dissipation and reduce air conditioning energy consumption.

3. Fluorine pump energy saving

Fluorine pump energy saving is mainly composed of liquid receiver, fluorine pump, pipeline valves, etc. When the outdoor ambient temperature is higher than the indoor ambient temperature, the compressor system runs normally, and the fluorine pump system stops working; when the outdoor ambient temperature is below 5°C, When the temperature is much lower than the indoor ambient temperature and reaches the set point of the system control, the compressor will stop working and the fluorine pump will start. After exchanging heat with indoor air in the evaporator, the refrigerant directly enters the air-cooled condenser to exchange heat with the outdoor cold source, and the refrigerant cooled to a liquid state overcomes the tube resistance under the action of the fluorine pump and returns to the evaporator to continue heat exchange , to achieve the effect of energy saving, the normal working modes include compressor mode, fluorine pump mode and combined cooling mode.

Second, centralized water-cooled chilled water air conditioners

1. Principle of water-cooled chilled water system

The water-cooled chilled water system is mainly composed of water-cooled host, plate replacement, chilled water circulation system, terminal equipment, cooling water circulation system, cooling tower fan system, etc.

The water-cooled main engine compresses the refrigerant into a liquid state through the compressor and sends it to the evaporator for evaporation. The refrigerated circulating water system sends the normal temperature water into the evaporator coil through the chilled water pump for heat exchange with the refrigerant. The normal temperature water cools down and becomes low temperature frozen water. Chilled water is sent to the end cooling coil to absorb the heat of the machine room, thereby reducing the temperature of the machine room.

2. Features of water-cooled chiller

Water cooling and centrifugal units are adopted, with high efficiency and large capacity. Water cooling avoids the disadvantage of wind cooling in summer, which is easy to be poorly cooled. It is conducive to the safe operation of the system and better energy efficiency. It is suitable for scenarios where the heat load of the data center is large.

3. Natural cooling

The water-cooled chilled water system adopts water cooling, which is not restricted by the outdoor dry bulb temperature, and the efficiency is not affected when working in summer. In winter, plate replacement can be used to obtain natural cooling, and the energy efficiency ratio is high throughout the year. Water-cooled refrigeration units generally use high-power centrifugal units, and the energy efficiency is above 6.0, which is significantly higher than the compressor energy efficiency of air-cooled machine room air conditioners. The disadvantage is that the composition of the water-cooled refrigeration unit is complex and requires many facilities, including refrigeration room, cooling tower, cooling water pump, chilled water pump, water collector, water separator and other facilities. The water system is complex and requires professional maintenance and on-duty. In addition, the cooling water system needs to consume a large amount of cooling water resources. During the operation of the water system, dirt will be generated, and water treatment is required.

4. System Features

Advantages: Cooling tower is used for heat dissipation, small footprint and good heat dissipation effect; high energy efficiency of centrifugal compression; free cooling can be realized by using plate replacement in winter.

Disadvantages: complex system structure, large investment, two water systems for cooling and freezing, complex construction and operation and maintenance; low reliability of the water system, there is a possibility of flooding the machine room; there is a single point of failure, and equipment backup, ring network or Dual systems to solve reliability issues.

Common layout: The cooling machine has high energy efficiency, and the final load is equipped with plate replacement and natural cooling to achieve better energy efficiency. It is currently a commonly used solution in large data center centers.

Third, centralized air-cooled chilled water air conditioning system

The composition of the chilled water side is relatively complicated. For example, the design of the water pump can use a single-stage pump or a two-stage pump; there are also various forms of correspondence between the water pump and the chiller, and it can be multi-machine to multi-pump design, or one machine and one pump. ; The pipeline to the end can be single-pipeline, double-pipeline and ring network structure, and the cold storage method can be considered in series or parallel.

1. Principle of air-cooled chilled water

The air-cooled chilled water system uses an air-cooled water machine. The air-cooled water machine uses a shell-and-tube evaporator to exchange heat between water and refrigerant. The refrigerant system absorbs the heat load in the water to produce cold water. The air-cooled water machine uses air-cooled fins. The heat is exchanged by the condenser, and the heat is discharged to the outside atmosphere by the cooling fan. Compared with the water-cooled chilled water system, the air-cooled chilled water system saves the cooling water system, has a compact structure, does not require a special refrigeration room, and is simple and convenient to operate and maintain. The disadvantage is that the capacity of the air-cooled chilled water system is small, and the air-cooled chilled water system is prone to high pressure in summer, so it is suitable for scenarios where the outdoor ambient temperature is not too high and the heat load of the computer room is moderate.

The compressor of the air-cooled water machine is designed with a screw machine or a magnetic levitation centrifuge. The system has a compact structure and simple operation and maintenance, but the system capacity is not as good as that of a water-cooled water machine. When designing an air-cooled water machine, a magnetic levitation compressor should be selected, and a screw compressor can also be selected.

2. Natural cooling

The air-cooled chilled water system can use natural cooling. When the outdoor temperature is low, the cold air can be used to directly cool the chilled water, so as not to open or reduce the cooling time of the compressor. This method is called natural cooling. The unit is called a natural cooling unit, and it can also be equipped with an independent natural cooling module externally. The working mode of the natural cooling module realizes cooling without a compressor in winter and saves power consumption of the compressor.

3. Features of air-cooled chilled water system

The auxiliary facilities required by the air-cooled chilled water system are relatively simple, generally chilled water pumps, water collectors, water distributors and pipe networks, etc. Compared with water-cooled chilled water systems, they are simpler and less investment.

Advantages:

①The air-cooled unit can be directly arranged outdoors, without occupying a dedicated refrigeration room, and without installing cooling towers and pump rooms, easy maintenance, convenient operation, and no professional maintenance;

② There is no cooling water system, so there is no power consumption of the cooling water system and evaporation loss of cooling water.

Disadvantages:

①The air-cooled chilled water unit is large in size and occupies a large area. At the same time, the outdoor unit has high noise. The air cooling and outdoor units are arranged in a centralized manner, and there is a heat island effect, which deteriorates the environmental conditions of the external local space;

②The operation of air-cooled chilled water is easily restricted by the outdoor environment. When encountering high temperature in summer, the efficiency is greatly reduced, and the cooling capacity decreases with the increase of outdoor temperature, which is exactly the opposite trend of the total heat load demand of the data center.

In addition, in order to solve the problem of water freezing in the pipeline in winter, the chilled water system can be added with ethylene glycol solution for anti-freezing, and the system with ethylene glycol solution can better solve the problem of freezing in winter.

Fourth, water-cooled fluorine system (centralized cooling water air-conditioning system)

1. Principle of centralized cooling water system

Centralized cooling water is mainly to solve the problem that the air-cooled condenser cannot be arranged and the high pressure in summer. The principle of the system is that the computer room air conditioner is composed of a compressor, a condenser, an expansion valve, and an evaporator, which are connected by pipes to form a closed system. The refrigerant in the system The circulating flow undergoes a two-phase phase transition from liquid to gas to liquid, completing a refrigeration cycle. The difference is that the high-pressure and high-temperature gas discharged from the compressor enters the shell-and-tube condenser to exchange heat with cold water to complete the condensation effect. The cold water from the cooling tower enters the shell-and-tube condenser under the action of the water pump and becomes hot water, returning to the cooling tower. , re-cool down.

2. Composition and structure of centralized cooling water

The composition of the centralized cooling water system is relatively simple. It is mainly composed of a closed cooling tower, a water pump, a shell-and-tube condenser and a constant pressure system. The typical architecture diagram, for the sake of system safety, also needs to consider system redundancy and backup.

3. Features of centralized cooling water system

Advantages:

①The centralized cooling water system only has the cooling water system, the system is relatively simple, and the construction investment is relatively low;

②A closed cooling tower is used to solve the complicated problem of cooling water maintenance;

③A shell-and-tube condenser is used to solve the problem of water entering the machine room.

Disadvantages:

①The characteristics of the water system determine that there is a single point of failure, which needs to be resolved through system backup;

② The efficiency of the centralized cooling water system is between that of air-cooled air conditioners and water-cooled chilled water.

Scenario: Suitable for centralized cooling water system of large and medium-sized data centers.

Fifth, dual cold source air conditioner

For safety, some data centers sometimes use dual-cooling source air conditioners. Dual-cooling-source air-conditioning means that the air-conditioning system has two different types of cooling systems. Since the system has two different types of cooling sources, there is no single point of failure, and the system runs High reliability. The dual cold source can adopt various methods such as chilled water/chilled water, chilled water/fluorine refrigeration, etc. For the fluorine system and chilled water dual cold source air conditioner, since the unit can either use direct evaporative refrigeration or chilled water for cooling, The system has high reliability and no single point of failure.

Sixth, liquid cooling technology

With the rapid development of high-density equipment in data centers, server-level cooling has begun to appear and be used, the most typical of which is liquid cooling technology. Liquid cooling technology is to directly cool the equipment through the liquid, and the liquid will directly take away the heat generated by the heating element of the equipment; the use of liquid cooling can realize the natural heat dissipation of servers and other equipment, which is more efficient and energy-saving than the traditional refrigeration system.

Commonly used coolants for liquid cooling include water, mineral oil, and fluorinated fluids. Among them, water is an excellent heat dissipation medium, which is cheap and pollution-free, and has a good specific heat capacity. However, since water is not an insulator, it can only be used in non-direct contact liquid cooling technology. Mineral oil is a relatively cheap insulating coolant. Single-phase mineral oil is odorless, non-toxic and non-volatile, and is an environmentally friendly medium. However, mineral oil has a high viscosity and is easy to remain. There is a risk of burning under certain conditions. Fluorinated liquid, because of its insulating and non-combustible inert characteristics, will not cause any impact on equipment. It is the most ideal and most widely used immersion cooling liquid in current applications, but the price is relatively expensive.

According to the contact method between the liquid and the heating device, it can be roughly divided into cold plate type (indirect contact), spray type and full immersion type (direct contact).

1. Cold plate

It is to fix the liquid-cooled cold plate on the main heat-generating device of the server, and rely on the liquid flowing through the cold plate to take away the heat to achieve the purpose of heat dissipation; the liquid-cooled cold plate solves the heat dissipation of the components with large heat generation in the server, and other heat-generating components Small devices have to rely on air cooling, so servers that use cold plate liquid cooling are also called air-liquid dual-channel servers; the liquid on the cold plate does not touch the cooled device, and a heat conduction plate is used in the middle to transfer heat, which has high safety; the disadvantage is that After adopting cold plate liquid cooling, the computer room still needs to install room-level air conditioners to ensure the temperature of the computer room.

2. Spray liquid cooling

It is to store liquid and open holes on the top of the chassis. According to the position of the heating element and the amount of heat generated, the coolant is sprayed on the heating element to achieve the purpose of cooling the equipment; the sprayed liquid is in direct contact with the cooled device, and the cooling efficiency is high. ; The disadvantage is that when the liquid encounters a high-temperature object during the spraying process, there will be a small amount of drifting and evaporation, and the mist and gas will be scattered outside the chassis along the holes and gaps of the chassis, resulting in a decrease in the cleanliness of the computer room environment or affecting other equipment.

3. Immersion liquid cooling

The heating element is directly immersed in the cooling liquid, and the heat generated by the operation of the server and other equipment is taken away by the flow and circulation of the liquid. The immersion liquid cooling has higher heat dissipation efficiency due to the direct contact between the heating element and the cooling liquid. Compared with the cold plate liquid cooling, it has lower noise and can solve higher heat density. It is a typical direct contact liquid cooling. The immersion type is divided into two-phase liquid cooling and single-phase liquid cooling. Single-phase liquid cooling is commonly used, and the heat dissipation method can be in the form of dry cooler and cooling tower; . The heat transfer efficiency of two-phase liquid cooling is higher, but the control is relatively complicated, the pressure will change during the phase change process, the requirements for the container are very high, the cooling liquid is easily polluted during use, and the practical application is less.
Single-phase liquid cooling, the cooling liquid always maintains a liquid state in the process of circulating heat dissipation, and no phase change occurs. Therefore, single-phase liquid cooling requires a higher boiling point of the cooling liquid, so that the control of cooling liquid volatilization and loss is relatively simple, and it is compatible with IT equipment components According to the application scenario specifications, fluorinated liquid can be used with dry cooler or with cooling tower and plate heat exchanger.

Seventh, common energy-saving technologies

Data centers need cooling throughout the year. When the outdoor temperature is lower than the indoor temperature in winter and transitional seasons, there are abundant cold sources in nature. Rational development and utilization of natural cooling sources is a key measure to reduce energy consumption in data centers and PUE in computer rooms. When selecting the location of the data center and determining the technical solution, the natural cooling source should be utilized and developed according to local conditions. According to the carrier of the natural cooling source, the natural cooling source technology scheme is roughly divided into cold air natural cooling, evaporative cooling and cold water natural cooling. According to the heat transfer path, it can be divided into direct natural cooling and indirect natural cooling.

1. Fresh air system

Directly use the cooling capacity of the outdoor cold air, filter the outdoor cold air through the fan and directly introduce it into the machine room, avoid turning on the compressor and save energy consumption. This kind of cooling efficiency is the highest, and it is suitable for use when the air quality is good.

When using the direct fresh air system, it is necessary to focus on evaluating the annual air quality in the area, because the outdoor air contains harmful components such as dust, sulfur dioxide, hydrogen sulfide, and ozone. When using direct fresh air, there is a risk of polluting the computer room and corroding the cabinet. It is necessary to consider certain air treatment methods such as physical filtration, chemical filtration, and humidity control to meet the environmental requirements of the computer room.

2. Board replacement system

The plate is used to isolate the outdoor air for heat exchange, and the cooled air is sent into the machine room. Since the outdoor air cannot directly enter the computer room, the air panel is suitable for use in air-polluted environments, and the cooling efficiency is lower than that of fresh air systems. It is suitable for environments with air pollution and is not suitable for installing fresh air systems; Mechanical refrigeration.

3. Wheel system

This is a full heat exchange technology, which was used in Tokyo, Japan in the early days, so it is also called the Kyoto runner. It can not only exchange heat, but also exchange humidity. Its function is similar to that of plate heat exchange. Through the rotation of the runner, the outdoor air and indoor air can exchange heat and moisture at the runner. This method has high cooling efficiency and is suitable for environments with air pollution and is not suitable for installing fresh air systems. Due to the slight cross-contamination during use, the full heat exchange of the runner has no obvious advantages compared with the plate exchange.

4. Evaporative cooling system

In areas with dry climate, evaporative cooling technology can be used to further extend the time of natural cooling operation, so as to minimize the energy consumption of the data center air conditioning system. According to whether the water and air are in direct contact, evaporative cooling technology is divided into direct evaporative cooling and indirect evaporative cooling.

① Direct evaporation energy-saving technology

Direct evaporative cooling is the direct contact between air and water. After the water evaporates, the temperature of the air will drop. Its characteristic is to realize the isenthalpic humidification and cooling process for the air. The limit temperature of the air supply and cooling is the wet bulb temperature of the incoming air.

Use the humidified and cooled air to cool down the equipment room. In actual use, it can be used in conjunction with the fresh air system. When the outdoor temperature is low, the fresh air is directly used to cool down; when the outdoor temperature rises, the humidification system is turned on. After the water evaporates, the air temperature drops, and then enters the machine room to cool, which can prolong the life of the machine room. The time and efficiency of natural cooling are suitable for use when the air quality is good, as shown in Figure 20; it should be noted that the evaporation process will affect the humidity of the computer room.

② Indirect evaporation energy-saving technology

Indirect evaporative cooling refers to the process of transferring the cooling capacity of the humid air obtained by direct evaporative cooling to the circulating air in the computer room through a non-direct contact heat exchanger, so as to realize the process of equal humidity cooling of the air. In this process, the secondary air is treated The dry-bulb temperature and wet-bulb temperature both drop, while the moisture content remains unchanged, and the humidity cooling process such as enthalpy reduction is realized for the supply air flow, and the limit temperature of the supply air cooling is the dew bulb temperature of the inlet air.

The outdoor air is isolated by the evaporative heat exchanger, and the outdoor air cannot directly enter the computer room. It is suitable for use in an air-polluted environment. Although the cooling efficiency is lower than that of the direct evaporation technology, outdoor pollutants cannot enter the computer room. In addition, the evaporation process does not affect the humidity of the computer room. , the core device of indirect evaporation is the evaporation module, and its airflow organization includes two parts, namely, the airflow in the computer room and the airflow outside.