The heat and moisture exchange of the surface air cooler is carried out under the action of the temperature difference between the main air and the boundary layer air close to the outer surface of the heat exchanger and the water vapor partial pressure difference. It can realize two kinds of air treatment processes: when the temperature of boundary layer air is lower than the dew point temperature of main air, dehumidification cooling process or wet cooling process (wet condition) will occur; when the temperature of boundary layer air is lower than the temperature of main air, but higher than its dew point temperature, isowet cooling process (dry condition) will occur.
In the process of dehumidification and cooling, there is not only temperature difference but also water vapor partial pressure difference between the boundary layer air and the main air, so there is not only sensible heat exchange, but also latent heat exchange accompanied by moisture exchange through the surface of the heat exchanger. It can be seen that the surface air cooler under wet condition has greater heat exchange capacity than that under dry condition. In other words, for the same surface air cooler, when the dry bulb temperature and water temperature of the treated air remain unchanged, the higher the wet bulb temperature of the air, the greater the cooling and dehumidification capacity of the surface air cooler.
After long-term use, the heat transfer coefficient of the surface cooler will decrease because of the ash on the outer surface and the scaling on the inner surface. In order to ensure that the surface air cooler is still safe and reliable in this case, a certain safety factor should be considered in the selection and calculation. The safety factor can be considered by increasing the heat transfer area, such as increasing the number of rows or increasing the windward area. However, due to the limited product specifications, this method is often unable to achieve the right safety factor, or bring trouble to the selection and calculation.
(1) At the beginning of selection calculation, the calculated eg is multiplied by the safety factor a. Take a = 0.94 for the surface air cooler only used for cooling and a = 0.9 for the surface air cooler used for both cooling and heating.
(2) In the process of calculation, the safety factor is not considered, and the calculated initial temperature of water is further reduced after the specification of surface air cooler is selected. The decrease of initial water temperature can be considered as 10% ~ 20% of water temperature rise.
Precautions for surface air cooler
(1) When the air conditioner is not used in winter, the remaining water in the pipe should be drained to avoid freezing water and cracking the steel pipe in winter when the temperature is too low.
(2) The water in the surface cooler should not contain impurities, such as sand.
(3) Heat preservation should be done well in winter to reduce heat loss or avoid frosting.