Features
The heat and moisture exchange of the evaporative cooler occurs between the temperature difference of the main air and the boundary layer air adhering to the exterior surface of the heat exchanger.Vapor PressureIt operates under adverse conditions. It can achieve two air treatment processes: a dehumidifying cooling process, also known as a wet cooling process (wet condition), occurs when the air temperature in the boundary layer is below the dew point temperature of the main air; an isothermal cooling process (dry condition) occurs when the air temperature in the boundary layer is lower than the main air temperature but still above its dew point temperature.
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 associated with moisture transfer. Consequently, the surface cooler in a humid condition has a greater heat exchange capacity compared to a dry condition. Or, 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.
Thermal Calculation
Cooling coils are primarily used for air cooling and dehumidification, resulting in changes to the air's temperature and humidity levels.
1. Heat Exchanger Efficiency of the Condenser
(1) Total Heat Exchanger EfficiencyIn the formula:—Dry bulb temperature of air before and after treatment, ℃;Cold Water Temperature, °C.
(2) General Heat Exchange Efficiencyt3—Optimal air temperature after treatment, °C.
2. Heat Exchanger Thermal Calculation Type
There are two types: one is design-oriented, commonly used to select a standard air cooler to meet known initial and final air parameters.
Processing Requirements; The other is a verification type, commonly used to check how a certain model of air cooler can process air with certain air parameters to achieve specific final parameters.
3. Heat Transfer Calculation Method for the Heat Exchanger
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 RequirementsgShould 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 equal the heat absorbed by the cold water.
4. Consideration of safety factor in the thermal calculation of the evaporative cooler
After prolonged use, the heat exchanger's thermal conductivity may decrease due to reasons such as accumulated dust on the outer surface and scaling on the inner surface. To ensure the continued safety and reliability of the heat exchanger under such conditions, a certain safety factor should be considered during selection and calculation. The safety factor can be accounted for by increasing the heat transfer area, such as by adding rows or increasing the windward area. However, due to product specifications, this method often does not achieve an exact safety factor or may complicate the selection and calculation process. Therefore, in engineering, it can be considered that:
(1) At the outset of the selection calculation, the obtained EgMultiply by the safety factor a. For cooling-only heat exchangers, take a = 0.94; for dual-purpose chillers, take a = 0.9.
(2) The safety factor is not considered during the calculation process. After selecting the evaporator specifications, 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 the air conditioner during winter, it's essential to drain all remaining water from the pipes to prevent freezing due to low winter temperatures, which could crack the steel pipes.
(2) The water in the heat exchanger should be free of impurities such as sand.
(3) Ensure adequate insulation during winter to minimize heat loss or prevent pipe icing.
