Features
The heat and moisture exchange of the evaporative cooler occurs between the temperature difference of the main air and the boundary layer air tightly adhering to the exterior surface of the heat exchanger.Vapor PressureIt operates under adverse conditions. It can achieve two air treatment processes: When the air temperature in the boundary layer is lower than the dew point temperature of the main air, a dehumidification cooling process, also known as a wet cooling process (wet condition), occurs; When the air temperature in the boundary layer is still lower than the main air temperature but higher than its dew point temperature, an isothermal cooling process occurs (dry condition).
During the dehumidification cooling process, there is not only a temperature difference but also a vapor pressure difference between the boundary layer air and the bulk air. Therefore, not only sensible heat exchange occurs at the heat exchanger surface, but also latent heat exchange accompanied by moisture exchange. Consequently, the surface cooler under humid conditions has a greater heat exchange capacity than under dry conditions. In other words, for the same surface cooler, when the dry bulb temperature of the treated air and the water temperature remain constant, the higher the wet bulb temperature of the air, the greater the cooling and dehumidifying capacity of the surface cooler.
Thermo-mechanical calculation
Cooling coils are primarily used for air cooling and dehumidification, resulting in changes to the air temperature and humidity.
1. Heat exchanger efficiency of the condenser
(1) Total Heat Exchange EfficiencyIn the formula:—Dry Bulb Temperature of Air Before and After Treatment, °C;Cold water temperature, ℃.
(2) General Heat Exchange Efficiencyt3—Ideal post-processing air temperature, ℃.
2. Heat Exchanger Thermal Calculation Type
There are two types: one is design-oriented, commonly used for selecting standard air coolers to meet known initial and final air parameters.
Processing requirements; another is for verification, commonly used to check how certain models of air coolers can process air with specific parameters to achieve certain final parameters.
3. Heat exchanger thermal calculation methods
Dry Bulb Temperature Efficiency Method — For a specific model of air cooler, the principles of thermodynamic calculation should meet the following conditions:
(1) Air treatment process required EgShould equal the E that the condenser can achieveg；
(2) The required E for the air treatment process should equal the E that the coil cooler can achieve.
(3) The heat released by the air should be equal to the heat absorbed by the cold water.
4. Consideration of safety factor in the thermodynamic calculation of evaporative coolers
The heat exchanger's thermal conductivity may decrease over time due to reasons such as accumulated dust on the outer surface and scaling on the inner surface. To ensure the safe and reliable operation of the heat exchanger under such conditions, a certain safety factor should be considered during selection calculations. This can be achieved by increasing the heat transfer area, for example, by adding rows or increasing the windward area. However, due to limited product specifications, this method often does not allow for an exact safety factor or can complicate the selection calculation. Therefore, in engineering, one may consider:
(1) At the outset of the selection calculation, the obtained EgMultiply by safety factor a. For coolers used solely for cooling, take a = 0.94; for those used for both cooling and heating, take a = 0.9.
(2) Safety coefficients are not considered during the calculation process. After the evaporator specifications are selected, the calculated initial water temperature should be further reduced. The reduction in initial water temperature can be considered as 10% to 20% of the water temperature rise.
Cautionary Notes
(1) When not using air conditioning during winter, it's crucial to drain the remaining water in the pipes to prevent freezing due to low winter temperatures, which could crack the steel pipes.
(2) The water in the evaporator should not contain impurities such as sand.
(3) Ensure adequate insulation during winter to minimize heat loss or prevent pipe icing.
