Today we systematically introduce the direct evaporative cooling air conditioning technology, indirect evaporative cooling air conditioning technology, evaporative cooling and mechanical refrigeration combined air conditioning technology and the application of evaporative cooling air conditioning technology related units and so on.
1. Direct evaporative cooling air conditioning technology
When air and water are in direct contact, air constantly transfers sensible heat to the water supply to reduce its own temperature. At the same time, water releases latent heat of vaporization due to endothermic heat. This process of using the sensible heat of air to exchange latent heat is a direct evaporative cooling process. Direct evaporative cooling air conditioners using this principle can be divided into cold mist type direct evaporative cooling air conditioners and cold air type direct evaporative cooling air conditioners.
1.1 Cold mist type direct evaporative cooling air conditioning
1.1.1 High pressure mist type
Figure 1 shows the high pressure micro mist type cold mist type direct evaporation cooling air conditioner. Its working principle is to use the high pressure plunger pump to increase the water pressure to 7 MPa, and then the pressurized water is transported to the professional nozzle through the high pressure resistant pipeline to atomize it, producing the micro mist particles with a diameter of 3~ 15μm, so that it can quickly absorb heat from the air to complete the vaporization and diffusion. So as to achieve the purpose of air humidification and cooling.
High pressure mist type cold mist type direct evaporative cooling air conditioner is widely used, which can be used not only indoors, but also in outdoor places. The spray tower on European Avenue of Seville World Exposition in Spain in 1992 was equipped with high pressure atomizing nozzle (Figure 2) for cooling tourists, and the design is simple and elegant, which is in harmony with the surrounding buildings.
1.1.2 Soda mixing
The humidification process of air conditioner with cold mist mixed with soft water can be divided into two parts: ejection and atomization. The compressed air with a certain pressure (0.1~ 1.0MPa) passes through the special nozzle chamber for reasonable configuration and diversion, forming a negative pressure area at the nozzle mouth. Due to the effect of negative pressure, the water in the water collector is continuously ejected into the nozzle chamber. The ejected water is stored in the water collector through the automatic water control device to provide a non-pressurized water source for the high-pressure air ejection spray. The velocity of compressed air and ejected water is different. The two streams of fluid flow orderly in the nozzle cavity with the set flow rate and direction.
In the process of atomization, the energy of compressed air at higher pressure is transferred to water at lower pressure, which increases the energy of water. The two streams of fluid are mixed and ejected at the outlet of the nozzle. In the mixing process, the high-pressure air and the water exchange momentum, and carry out intense friction and collision with the water. The cavitation effect is used to fully atomize the water into fine water droplets. When the soda water mixed fluid is sprayed from the nozzle at high speed, and the air in the atmosphere outside the friction contact, so as to tear the water droplets further, the diameter of the water droplets can reach 5~10 μm, so as to achieve good atomization effect.
1.2 Cold air direct evaporative cooling air conditioner
1.2.1 Evaporative Air Conditioner (Cold Fan)
Evaporative air conditioner is the use of water spray packing layer directly contact with the outdoor air to be treated, because the temperature of the spray water is generally lower than the temperature of the air to be treated (i.e. fresh air), the air continues to transfer its sensible heat to the water supply and to cool down; At the same time, part of the spray water (circulating water) will evaporate due to the continuous absorption of heat in the air, and the steam after evaporation is brought into the room by the air, so that the fresh air can be cooled and humidified. Therefore, this process of using the sensible heat of air to exchange latent heat can be called direct evaporative cooling of air, or adiabatic cooling and humidification of air. The air treatment process realized by direct evaporative cooling of air to be treated is equal enthalpy humidification and cooling process, and its limiting temperature is the wet bulb temperature of air.
Scope of application of evaporative air conditioner:
1) In the south, it is mainly used in textile, clothing, leather, chemical, metallurgy, food and other factories and some production places with high temperature heat source;
2) In the north, it is suitable for shopping malls, restaurants, hot pot restaurants, outpatient halls of hospitals, waiting halls of stations, airports, schools and other public places with dense population that need ventilation and cooling;
3) Places with polluting gases or emitting dust and harmful gases;
4) Places where traditional air conditioners have been installed but the fresh air volume or oxygen content is insufficient;
5) Energy saving transformation of communication base station and cooling equipment in computer room.
At present, evaporative air conditioners which adopt direct evaporative cooling air conditioning technology are mainly suitable for places where indoor temperature is too high and air velocity is too high. Direct evaporative cooling can also be combined with post air supply to send cold air directly to the staff's work area, which is conducive to improving work efficiency.
1.2.2 Window type evaporative cooling air conditioner
Figure 6 shows a window type evaporation-cooled air conditioner. With the continuous development and improvement, the appearance and function of the second generation of window type evaporative cooling air conditioner is improved continuously. Compared with the first generation, it is more beautiful. It can also remove the shell from inside the room, overhaul and replace the packing, which provides convenience for installation and maintenance.
Window type evaporative cooling air conditioner has the advantages of energy saving, environmental protection, economy, low carbon, comfortable and so on. It has been popularized and applied in northwest China in recent years, especially in student dormitories. Students' dormitories use hanging electric fans for cooling and a few use mechanical compression refrigeration air conditioning for cooling. Although the cooling effect of window evaporative cooled air conditioner is not as good as that of mechanical compression refrigerating air conditioner, it has the characteristics of large air supply and full fresh air operation, which can meet the comfort requirements of dormitory space. In the transition season, window evaporative cooled air conditioner is only used as ventilation equipment, which can meet the indoor comfort requirements. The use of window evaporative cooling air conditioner can effectively reduce the room temperature and improve the indoor thermal environment. The fresh air sent into the dormitory improves the indoor air quality, but at the same time, the exhaust system should be done well to achieve good ventilation and cooling and avoid excessive indoor humidity.
2. Indirect evaporative cooling air conditioning technology
Indirect evaporative cooling air conditioning technology, the output medium (air or water) and the working medium (air and water) indirect contact for heat and humidity exchange, there is no qualitative exchange between the output medium and the working medium, only sensible heat exchange. As a special form of indirect evaporative cooling air conditioning technology, dewpoint indirect evaporative cooling air conditioning technology uses the difference between the dry bulb temperature of air and the decreasing wet bulb temperature to complete heat transfer, which is different from the general indirect evaporative cooling air conditioning technology (using the difference between the dry bulb temperature of air and the fixed wet bulb temperature for heat transfer).
A small hole is made between the dry and wet channels in the dew-point indirect evaporating cooler. After the primary air entering the dry channel is pre-cooled, part of it enters the wet channel when it flows through the small hole. As a secondary air, heat and humidity exchange occurs with the water film of the wet channel, which reduces the temperature of the wet channel, enlarges the heat exchange difference between the dry and wet channels, and increases the primary air cooling range in the dry channel. The driving potential of dew-point indirect evaporative cooling technology is the difference between the dry bulb temperature of primary air and the dew point of secondary air, and the limit of air supply temperature is the dew point of primary air. Therefore, air whose temperature of air supply dry bulb is lower than that of outdoor wet bulb and close to the dew point can be provided with a larger temperature drop.
2.1 Dew-point indirect evaporative cooling air conditioning unit
The dew-point indirect evaporative cooling air conditioning technology breaks through the limitation of direct evaporative cooling air conditioning technology by wet bulb temperature. In dry areas, the dew-point indirect evaporative cooling air conditioning is very close to the air supply temperature of traditional mechanical compression refrigeration air conditioning, and compared with direct evaporative cooling air conditioning, the comfort is improved. The cooling mode of high temperature cold water + coil tube (AWA) is to make cold water first, and then use cold water to make cold air, and the dew point indirect evaporative cooling air conditioning unit (AA) is to make cold air directly with cold air, saving the unnecessary loss of energy conversion, the project cost is greatly reduced, no surface cooler, so there is no need to consider the problem of anti-freezing in winter. The unit uses the temperature difference between the dew point and the dry bulb temperature to continuously cool the air so that the supply air approaches the dew point.
The working air flows through the dry channel at the lower part of the core body, and is taken away by evaporative cooling of the wet side of the previous stage, and the enthalpy value decreases. After realizing precooling, the air flows through the throttle hole into the wet channel on the other side for equal enthalpy humidification. The output air flows through the dry channel at the upper part of the core body, and the absolute moisture content remains unchanged. The dry bulb temperature is successively reduced by the multiple wet channels arranged on the other side, and then sent into the room.
2.2 Dew-point indirect evaporative cooling chiller
The evaporative cooling chiller is different from the conventional evaporative cooling air conditioning technology in the form of cooling capacity, which is obtained by water side evaporative cooling technology. The unit consists of indirect evaporative cooling section, spray device, fan and so on. The outdoor fresh air is pre-cooled through the indirect evaporative cooling process, and the pre-cooled air enters the packing tower and the water film on the surface of the packing for sufficient heat and humidity exchange to prepare cold water, and then the air is discharged from the chiller under the action of the exhaust fan. Indirect evaporative cooling section can have many forms, such as surface cooling indirect section, plate fin indirect section, tube indirect section, dew point indirect section, etc.
The physical diagram and schematic diagram of the dew-point type indirect evaporative cooling chiller are shown in Figure 12. The water outlet temperature of evaporative cooling chiller is lower than that of conventional cooling tower, but higher than that of conventional chiller. Therefore, the application mode of evaporative cooling chiller is determined by its own performance characteristics, building functional requirements and the heat and humidity load characteristics of air conditioning objects.
3. Composite air conditioning technology of evaporative cooling and mechanical refrigeration
Evaporative cooling air conditioning technology has the unique advantages of energy saving, low carbon, economy and health, but the cold air or cold water parameters produced solely by this technology are closely related to the outdoor air state, which is not easy to control and has instability, and its own performance needs to be improved. While mechanical refrigeration technology can produce cold air or cold water with stable temperature, but relatively large energy consumption.
The organic combination of evaporative cooling and mechanical refrigeration can realize the collaborative operation and mutual coupling of the two technologies, make up for the shortcomings of simply using one method, is conducive to the optimization of evaporative cooling equipment, broaden its scope of use, improve the performance of mechanical refrigeration, promote energy saving and emission reduction, and realize the complementary advantages of the two. FIG. 13 shows the physical diagram and working principle diagram of evaporative cooling + mechanical refrigeration (direct expansion) combined air conditioning unit.
The combined air conditioning unit has two operating modes: (1) In view of the drying condition, natural cooling is carried out by giving full play to the advantages of evaporative cooling air conditioning technology. The design requirements can be met by opening only the direct evaporative cooling section. The air to be treated enters the direct evaporative cooling section through the filtration section, is humidified and cooled by equal enthalpy and then sent indoors by the fan; ② For the high temperature and high humidity conditions, the direct evaporation cooling section is closed, the mechanical refrigeration section is opened, the outdoor air is mixed with the indoor return air, and then the mechanical refrigeration circulation evaporator is used to reduce the humidity and cool the air before it is sent into the room by the fan.
4. Double cold source evaporative air conditioners for data centers
Due to the pursuit of high energy efficiency, people's acceptance of evaporative cooling, which is an energy-saving and environmental protection technology, is increasing. Evaporative cooling air conditioning technology is widely used in the field of data centers. Data centers use evaporative air conditioners with double cold sources.
And working principle diagram and working principle diagram, it can not only use its own circulating water to spray the filler to cool the air to be treated, but also use deep well water to effectively use the rich cold source contained in the natural environment, so as to effectively reduce the power consumption and cost of operation.
5. Closed cooling tower
5.1 Multi-condition composite closed cooling tower
The cooling tower is a set of cooling equipment with open cooling system added on the basis of the traditional closed tower. The coil and the packing adopt different circuits, which have the high efficiency of water cooling for the packing and the high air volume characteristics of air cooling. In late autumn, the water supply for the packing is stopped to effectively avoid the packing being frozen. A four-stage operation strategy is designed according to the climate characteristics of extremely hot summer and extremely cold winter in North China and Northeast China.
1) Summer: open the closed and open cycles at the same time, and the cold water produced by the open cycle is used in the closed cycle to meet the demand of extremely high heat load, as shown in Figure 16 (a);
2) Early autumn: closed tower does not turn on, open tower is fully open, compared with summer mode power saving;
3) Late autumn: closed tower fully open, open tower fully closed, can prevent packing ice and tube row freezing at low temperature;
4) In winter: closed tower spray water is closed, only the fan is turned on, and the open tower is completely closed, which is conducive to energy saving and water saving, as shown in Figure 16 (b).
5.2 Closed type mixed cooling tower
FIG. 17 shows the closed mixed cooling tower. The cooling tower is equipped with a finned tube in series with the condensate coil, which has three operation modes, namely dry/wet combined mode, adiabatic mode and dry mode (see Figure 18). Through the selection of different operation modes, it can be applied to areas requiring continuous and reliable operation, high water price, and occasions where water supply is limited or white fog needs to be eliminated.
1) Dry/wet combined operation mode (see Figure 18 (a))
The fluid first enters the finned tube for cooling and then enters the condensate coil for further cooling. The spray water is sent from the collecting pan by the circulating pump to the spray unit above the condensate coil. The spray water wets the surface of the condensate coil, takes away the heat of the fluid in the tube and falls to the surface of the packing, which is further cooled in the packing and then falls into the water collecting pan for recycling. The air flows through the packing and condensing coil respectively, absorbing heat to reach saturation state and then discharged by axial flow fan. At this time, the exhaust temperature is relatively low, and the fluid in the finned tube installed above the axial flow fan can be cooled significantly by the exhaust air.
The combined dry/wet operation mode utilizes sensible heat and latent heat of evaporation. Compared with traditional cooling towers, the tendency of white fog is greatly reduced in extreme weather, and a lot of water can be saved.
2) Adiabatic operation mode (see Figure 18 (b))
In adiabatic operation mode, all the fluid cooled by the finned tube is discharged by bypass. There is no heat exchange in the condensate coil, and the spray water is only used to pre-cool the air entering from the outside. Under most climate conditions, the surrounding air still has considerable potential to absorb water, the temperature of the adiabatic cooling air is significantly reduced, and the fluid inside the finned tube is pre-cooled when discharged by the axial flow fan to ensure the temperature required by the design fluid, so as to maximize the system efficiency.
3) Dry operation mode (see Figure 18 (c))
In dry operation mode, the spray water system is closed, saving pump energy consumption. The fluid to be cooled flows through the finned tube into the condensate coil. The flow control valve is kept fully open to ensure that the fluid flows through the two coils in series to maximize the heat transfer area.
Post time: Jun-12-2023