Evaporative cooling technology analysis

At present, in the comfort air conditioning system, the dehumidification air conditioning system consisting of dehumidification technology, evaporative cooling technology and mechanical refrigeration technology has received widespread attention at home and abroad. Therefore, the application of evaporative cooling technology is not limited to dry climate, but can be applied to any climatic environment conditions, and meets the energy saving and health requirements of modern people.

Evaporative cooling technology principle
1. Direct evaporative cooling

Direct evaporative cooling (referred to as DEC) refers to the direct contact between air and water. The evaporation of water causes the temperature of air and water to decrease. During this process, the moisture content of the air increases, and the sensible heat of the air is converted into Latent heat, this is an adiabatic humidification process. The entire evaporative cooling process is carried out in a cooling tower, a water spray chamber or other adiabatic humidification equipment. The principle of the device is shown in Figure 1. The corresponding evaporative cooling process can be represented as Figure 2 on the Id diagram. As can be seen from the figure, after the outdoor air in the state 1 is subjected to heat and moisture exchange with water in the contact heat exchanger, the temperature is lowered, the moisture content is increased, and the state is changed along the adiabatic line to the state 2, and the water temperature is lowered from tw2 to tw1.

2. Indirect evaporative cooling

Indirect evaporative cooling (referred to as IEC) refers to the air and water cooled by direct evaporative cooling process through the non-contact heat exchanger to cool the air to be treated, then the air supply with reduced temperature and constant moisture content can be obtained. This process is an isotope cooling process. If the air used for direct evaporative cooling is referred to as secondary air, and the air to be treated is referred to as primary air, a schematic diagram of the indirect evaporative cooling device can be obtained, as shown in FIG.

The indirect evaporative cooling process can be expressed as Figure 4 on the id diagram. If the primary air and the secondary air are both outdoor air, their initial state point w is in the same position in the figure, when the secondary air flows through direct evaporative cooling. When the device HUM, the air state changes from w to 1, and the primary air exchanges heat with the secondary air of the state 1 in the heat exchanger HX, the state changes from w to 2, and the secondary air changes from state 1 to state E. Then drained. The primary air coming out of the HX1 device is again cooled by the cooling water (water temperature tw1) flowing out of the HUM device in the heat exchanger HX2, and then sent to the room; after the heat exchange, the cooling water returns to the HUM device, and the direct evaporative cooling is performed again. The process cools down and then returns to the HX2 unit to exchange heat with the secondary air, thus circulating. Therefore, in the indirect evaporative cooling process, the primary air cooling process is iso-wet cooling, the temperature is reduced from tw to to, and the moisture content is unchanged.

3. Advantages of evaporative air conditioners

Compared with traditional compressor-type air conditioners, evaporative air conditioners have the following advantages [5], which is the main reason for us to continuously study the evaporative air-conditioning technology.

· The initial investment cost is low; about 1/2 of the traditional mechanical refrigeration, the cost of the mechanical refrigeration system is about 400 yuan / m2, and the evaporative refrigeration system is about 250 yuan / m2, usually the initial investment can be recovered in three years.

·The power consumption is 1/8 of the traditional air conditioner. The main power required in the evaporative air conditioner (evaporative air conditioner) is the fan and pump, no refrigeration compressor, and the energy efficiency ratio is very high. Usually the mechanical refrigeration system The power consumption is about 50 W/m2, while the evaporative air conditioning system is about 10 W/m2, and the power saving is about 80%.

· Evaporative air conditioner (evaporative air conditioner) operates in a new wind operation with air filtration and humidification function, continuously inputting 100% fresh cold air, and effective positive pressure air supply can discharge harmful air out of the room and keep indoor clean. Greatly improve the indoor air quality.

·Protect the environment, zero pollution; because the evaporative air conditioner (evaporative air conditioner) equipment uses water as the refrigerant (not freon), it has no pollution to the atmosphere.

·Simple maintenance and low maintenance cost; traditional compression air conditioning units generally require complex complete sets of equipment such as chillers, cooling towers, cooling water pumps, and end devices. They are troublesome to operate and maintain, and require professional operation, which is expensive and evaporates. The air conditioning system is easy to operate and manage, and generally does not require professionals.

4. There are still misunderstandings in evaporative cooling technology

One of the misunderstandings: Evaporative cooling technology is limited to arid climates
Evaporative cooling technology was applied early in some arid climates and was promoted in hot and dry areas such as the southwestern United States, Australia, India, Pakistan, South Africa, Israel, and northwestern China. However, this does not mean that evaporative cooling technology is limited to use in arid climates. Evaporative cooling systems with direct evaporative cooling (DEC) and indirect evaporative cooling (IEC) and a combination of the two evaporative cooling systems can be used in almost all weather conditions; in addition, combined with dehumidification technology and evaporative cooling technology The dehumidification cooling system is an environmentally-friendly cooling method based on the evaporative cooling research. The dehumidification cooling air conditioner is usually composed of a desiccant moisture absorption, a desiccant regeneration, an evaporative cooling and a heat exchange process. The latent heat treatment of the treated air is accomplished by moisture absorption by a desiccant, and the sensible heat treatment is accomplished by evaporative cooling.

Misunderstanding 2: - Evaporative cooling technology will replace mechanical refrigeration
Evaporative cooling has many shortcomings compared to mechanical refrigeration. If the dehumidification function is lacking, the ability to cool the air is seriously affected by the external climate environment, and the multi-stage evaporative cooling system is more complicated to control. When the cooling requirement cannot be achieved by evaporative cooling alone, mechanical refrigeration can be started to compensate; if the indoor air quality is to be improved, only the new air volume can be increased, but the mechanical refrigeration energy consumption will be greatly increased, while the evaporative cooling air conditioner can achieve a new wind. Energy efficient operation. Therefore, the combination of mechanical refrigeration and evaporative cooling can meet a variety of different needs and reduce energy consumption. Therefore, evaporative cooling technology and mechanical refrigeration have their own advantages, and combined use can exert unexpected results. At present, in the comfort air conditioning system, the dehumidification air conditioning system consisting of dehumidification technology, evaporative cooling technology and mechanical refrigeration technology has received widespread attention at home and abroad.

5. Characteristics of evaporative air conditioning technology and its technical difficulties

·Technical characteristics
The main function of the evaporative air conditioner is to heat and humidify the air to meet the air supply requirements, and to meet the requirements of temperature, humidity, wind speed and cleanliness of the air conditioning space; the core problem is the heat and mass exchange of air and water. From the perspective of thermodynamics, whether it is direct evaporative cooling or indirect evaporative cooling, it is a complex irreversible process in which flow, heat transfer and mass transfer occur simultaneously, and multiple heat transfer processes are coupled to each other and cross each other. The heat and mass transfer driving force in the evaporative cooling process is relatively small, so the evaporative air conditioner technology is a low-grade energy utilization technology. Direct evaporative air conditioners can directly use the dry and wet bulb temperature difference of the air to obtain the cooling capacity, which is also the main means of cooling the evaporative air conditioner, which consumes only a small amount of primary energy and has no destructive effect on the environment.

·Technical Difficulties
The problem of heat and mass exchange in evaporative air conditioning systems is a major research direction in the heat transfer industry. In recent years, many scholars at home and abroad are conducting more in-depth theoretical and experimental research on this issue, but the thermal mass exchange characteristics of various components have not been comprehensively considered from the system as a whole; the heat and mass exchange process between water and air shows a high degree. The time-varying and nonlinear characteristics make it very difficult to establish both the physical model and the mathematical model, which also hinders the in-depth study of this topic.

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