Air-cooled, water-cooled, dual-cold source and free cooling. 
    For the refrigeration of high-precision constant temperature and humidity air conditioners, no matter which of the above, the state of the refrigerant in the high-precision constant temperature and humidity air conditioner is changed to absorb heat from the outside or release heat to the outside. Unless the components or copper tubes of the high-precision constant temperature and humidity air conditioner leak, the refrigerant is in a closed environment. The refrigerant circulates between the compressor, condenser, pressure expansion valve, fan and copper tube, and changes the state of the refrigerant itself, that is, the gas state becomes liquid state, and the liquid state becomes gas state. This is the so-called internal circulation. 
1. Air-cooled high-precision constant temperature
and humidity air conditioner air-cooled, that is, the refrigerant of the air conditioner is cooled by wind, and the condenser installed outdoors (high-precision constant temperature and humidity air conditioner outdoor unit) takes away the heat of the condenser and makes the refrigerant release heat. Air-cooled condensers are mainly composed of several groups of copper tubes and fans. Due to the poor heat transfer performance of air, fins are usually added outside the copper tubes to increase the heat transfer area on the air side. At the same time, fans are used to accelerate air flow and increase the heat dissipation effect. The speed of the fan can be constant, adjustable, or constant + adjustable. According to the requirements of equipment from different manufacturers, the distance between the copper tubes of the indoor unit and the outdoor unit, that is, the height difference between the indoor and outdoor units, is also required. When selecting, it should be combined with the actual situation. The air-cooled type generally uses downward air supply and upward return air. Since most data centers use anti-static floors, the space under the anti-static floor (generally 40~45cm from the floor) is used as an air static pressure box. An air outlet is set in the air intake direction of the equipment cabinet. The cold air becomes hot air after passing through the equipment cabinet, and then returns to the return air outlet of the precision air conditioner through the return air duct or the top ceiling (generally hot air is directly returned, so the placement of high-precision constant temperature and humidity air conditioners is very important). Therefore, air-cooled high-precision constant temperature and humidity air conditioners are widely used.  
There are some disadvantages of air-cooled high-precision constant temperature and humidity air conditioners for computer rooms:  
1. The outdoor unit has high noise and heat generation, which affects the surrounding environment.  
2. During the high temperature period in summer, the outdoor unit may cause a high-voltage alarm due to poor heat dissipation.  
3. The fins of the outdoor unit are prone to dust accumulation, and if they are not cleaned frequently, it will affect the cooling effect of the air conditioner.  
4. The cooling efficiency is low, lower than that of the water-cooled type.  
5. The application effect is not good in extremely cold areas.  
6. Requirements for the length of the copper tube between the indoor and outdoor units and the height difference between the indoor and outdoor units.
2. Water-cooled high-precision constant temperature and
    humidity air conditioner The structure of the water-cooled high-precision constant temperature and humidity air conditioner is not much different from that of the air-cooled type. The main difference is: the water-cooled high-precision constant temperature and humidity air conditioner is equipped with a water-cooled plate condenser in the indoor unit (compared with the air-cooled high-precision constant temperature and humidity air conditioner, the internal circulation of the refrigerant is only in the indoor unit, and the working pressure of the compressor is much smaller). The refrigerant releases heat when passing through the water-cooled plate condenser, and the cold water of the water-cooled plate condenser absorbs heat and is discharged to the building cooling tower through a water pump (generally installed outside the high-precision constant temperature and humidity air conditioner), and then the cooling tower discharges the heat into the air. Since the cooling tower is a heat and moisture exchange device (air and water are in direct contact), the cooling effect is good. Dry cooling type outdoor condenser is also selected.
    There are also some disadvantages of water-cooled computer room high-precision constant temperature and humidity air conditioners:  
1. Electronic information equipment must be kept away from water, so extremely high requirements are placed on the water pipes connected to the high-precision constant temperature and humidity air conditioners, including the quality of the water pipes themselves, the direction of the water pipes, the welding technology of the water pipes, etc. At the same   
time, water leakage must be closely monitored, so for customers, this is the most worrying point. 2. Since cooling water is circulating water, it will be very dirty after long-term contact with air, so a water filter must be installed on the water circulation pipeline.   
3. The single point failure of the cooling tower, water pump, and water pipe is increased, and backup equipment is required.  
4. The maintenance cost is high.  
5. The energy consumption is not much lower than that of air cooling. Therefore, water-cooled computer room high-precision constant temperature and humidity air conditioners are less used.
3. Free cooling (ethylene glycol cooling and refrigeration)
    This model is mainly used in cold northern regions and can also be regarded as a water-cooled high-precision constant temperature and humidity air conditioner. Once the outdoor temperature is below 0 degrees Celsius, add ethylene glycol solution to the water (to ensure that the water does not freeze) to ensure that the cooling water does not freeze. Unlike general water-cooled high-precision air conditioners, free cooling high-precision constant temperature and humidity air conditioners install dry cooling towers outdoors, which are similar to the outdoor units of air-cooled high-precision constant temperature and humidity air conditioners, except that the substances in the pipes are different. The former is hot water, and the latter is a high-temperature condensing agent.
4. Chilled water refrigeration   
    The premise of setting up a chilled water type high-precision constant temperature and humidity air conditioner is to have chilled water. You can use the chilled water of the central air conditioner, or build a new chilled water system. Chiller (air-cooled chiller, water-cooled chiller) Air-cooled condenser, cooling tower. The chiller (equivalent to the outdoor unit) includes four main components: compressor, evaporator, condenser, and expansion valve, thereby achieving the cooling and heating effect of the unit. The indoor unit of the chilled water high-precision constant temperature and humidity air conditioner is mainly composed of a chilled water coil, a fan, an electric heater, a humidifier, and a water valve. Generally, there is no compression refrigeration system. Therefore, the initial investment of this model is relatively small, but it relies on chilled water. If the chilled water cannot be supplied, the system will be paralyzed. Therefore, there is the following dual cold source system.
5. Dual cold source system  
The dual cold source system is to ensure the refrigeration security of high-precision constant temperature and humidity air conditioning, and make full use of the chilled water of the building's central air conditioning. Dual cold source high-precision constant temperature and humidity air conditioning can be used. There are two combinations:  
1. Air-cooled condensation + chilled water, with the chilled water system as the main use and the air-cooled system as the backup.  
2. Water-cooled condensation + chilled water, with the chilled water system as the main use and the water-cooled system as the backup.
Advantages: The dual systems are backed up for each other, with high safety and reliability. The cooling water of the building's central air conditioning can be fully utilized to achieve energy-saving effects. Disadvantages: Large initial investment. There are many pipelines, which take up a lot of space and bring trouble to installation.