Views:0 Author:Site Editor Publish Time: 2020-10-29 Origin:Site
There are two ways of ventilation: blast and induced wind. ①Blowing type: The air first flows through the fan and then into the tube bundle. ②Induced wind type: the air flows through the tube bundle first and then into the ventilator. The operating cost of the former is more economical, the turbulence generated is beneficial to heat transfer, and it is used more. The latter has a uniform airflow distribution, which is conducive to precise temperature control and low noise, which is the direction of development. The outlet temperature of the hot fluid is mainly controlled by adjusting the air volume through the tube bundle, that is, adjusting the inclination angle of the blades, the fan speed and the opening degree of the shutters. For fluids that are easy to condense and freeze in winter, hot air circulation or steam heating can be used to adjust the fluid outlet temperature.
Classification and characteristics
Air coolers can be divided into power plant air coolers and petrochemical air coolers according to different occasions. According to the cooling method, it can be divided into dry, wet, and dry-wet combined air coolers.
Wet Air Cooler
Wet air coolers can be divided into three types: surface evaporation type, humidification type and spray type according to the spraying method. The latter two are the main ones in the petrochemical industry. The surface evaporative air cooler is an air-cooling device composed of light pipes that uses the water film evaporation outside the pipe to enhance heat transfer. Humidifying wet air coolers are only suitable for dry and hot areas where the relative humidity is lower than 50%, because the lower the relative humidity of dry air, the more the temperature drops after humidification, and the more significant the cooling effect. The spray type wet air cooler sprays water directly on the fin tube bundle, and uses the latent heat exchange of the evaporation of water and the air to be humidified and cooled to enhance heat transfer. At the same time, the presence of water mist makes the air cooler inlet air temperature close to the humidity of the environment. The bulb temperature increases the average temperature difference of heat transfer, and the heat transfer coefficient is 2 to 4 times higher than that of dry air coolers at 3% spray volume.
In short, compared to dry air coolers, it is more advantageous to use wet air coolers in the hot summer when the ambient temperature is higher. However, when the temperature of the fluid in the tube exceeds 70°C, the wet air cooler is prone to fouling, and the air resistance loss outside the tube is relatively large, which is about 1% of that of dry air cooling. 4 times. The tube bundle area cannot be too large, so the relative area of the unit device is small and the price is relatively high.
Dry air cooler
Dry air coolers only rely on the sensible heat of air temperature rise to exchange heat, and rely on the forced circulation of finned tubes and fans to enhance heat transfer. The operation is simple and easy to use, but because the cooling temperature depends on the dry bulb temperature of the air, generally the hot fluid in the tube can only be cooled to 15-20°C higher than the ambient temperature.
Therefore, for the hot and humid regions of southern my country, the wet air cooler has a poor evaporation effect, and dry air coolers are generally used. From the perspective of heat transfer, the specific heat of air is only 1/4 of that of water, and the density of air is much smaller than that of water. Therefore, if the same amount of heat is transferred, the temperature rise of the cooling medium is the same, and the amount of air required It will be 4 times that of water. Compared with water coolers, the volume of dry air coolers is very large. The most important thing is that the heat transfer coefficient on the air side is very low, about 50 ~ 60W/(m2·℃), resulting in a very low total heat transfer coefficient of the bare tube air cooler, which is about 10 lower than that of a water cooler. ～30 times. In order to offset the effect of lower heat transfer coefficient on the air side, air coolers generally use finned tubes with extended surfaces, and the finning ratio is roughly 10 to 24 times. There are also plate air coolers that use plate heat transfer elements. Because the cross-sectional shape of the flow channel formed by the plates changes continuously along the flow direction, the disturbance is enhanced, and it has high heat transfer efficiency and low pressure drop under low Reynolds number. It is especially suitable for Air coolers for large-scale equipment in the petrochemical industry (such as large-scale ethylene equipment, etc.), but due to the narrow flow channels of plate air coolers, in the cold winter in northern China, it is easy to cause the cooling medium in the flow channel to condense and block the flow channel, and easy to scale. As a result, the flow channel is blocked, and because the processing technology is mostly a fully welded structure, when part of it is damaged or blocked, the entire air cooler must be replaced, which causes a lot of waste. Therefore, the finned tube is still the mainstream heat transfer element of the air cooler. The essence of the air cooler can be regarded as an air-heat medium tube-fin heat exchanger. The key to enhancing the heat transfer performance of the air cooler is to develop low contact thermal resistance. , Finned tube with high heat transfer efficiency and low flow resistance. When the inside of the heat exchanger is fluid with higher pressure, adding ribs to the tube is equivalent to replacing the pressure-bearing high-quality tube with cheap ribs that are not pressure-bearing, and the economic effect is significant.
Dry-wet combined air cooler
Dry-wet combined air cooler is a combination of dry air cooler and wet air cooler. The general principle of the combination is to use a dry air cooler in the high temperature zone of the process fluid to condense the gas; use a wet air cooler in the low temperature zone to cool the condensate. In short, the choice of which form of air cooler should be determined based on the local atmospheric temperature, wind speed and relative humidity and other environmental and climatic conditions, combined with the heat exchange process requirements such as the final cooling temperature of the medium, and taking into account the economic efficiency.