中央空调冷却塔英文文献翻译 第2页
Air in a counterflow induced draft cooling tower is drawn through the tower by a fan or fans located at the top of the tower. The air enters the tower at louvers in the base and then comes into contact with water that is distributed from basins at the top of the tower. Thus, the relative directions are counter (down for the water, up for the air) in this configuration. This arrangement is shown in Figure 4.2.15. In this configuration, the temperature of the water decreases as it falls down through the counterflowing air, and the air is heated and humidified. Droplets of water that might have been entrained in the air stream are caught at the drift eliminators and returned to the sump. Air and some carryover droplets are ejected through the fans and out the top of the tower. The water that has been cooled collects in the sump and is pumped back to the condenser.
FIGURE 4.2.15 Counterflow induced draft cooling tower.
Counterflow towers generally have better performance than crossflow types because of the even air distribution through the tower fill material. These towers also eject air at higher velocities which reduces problems with exhaust air recirculation into the tower. However, these towers are also somewhat taller than crossflow types and thus require more condenser pump head.
Crossflow Induced Draft
As in the counterflow cooling tower, the fan in the crossflow tower is located at the top of the unit (Figure 4.2.16). Air enters the tower at side or end louvers and moves horizontally through the tower fill. Water is distributed from the top of the tower where it is directed into the fill and is cooled by direct contact heat transfer with the air in crossflow (air horizontal and water down). Water collected in the sump is pumped back to the chiller condenser. The increased airflow possible with the crossflow tower allows these towers to have a much lower overall height. This results in lower pump head required on the condenser water pump compared to the counterflow tower. The reduced height also increases the possibility of recirculating the exhaust air from the top of the tower back into the side or end air intakes which can reduce the tower’s effectiveness.
Counterflow Forced Draft
Counterflow forced draft cooling towers have the fan mounted at or near the bottom of the unit near the air intakes (Figure 4.2.17). As in the other towers, water is distributed down through the tower and its fill, and through direct contact with atmospheric air it is cooled. Thermal operation of this tower is similar to the counterflow induced draft cooling tower. Fan vibration is not as severe for this arrangement compared to induced draft towers. There is also some additional evaporative cooling benefit because the fan discharges air directly across the sump which further cools the water.There are some disadvantages to this tower. First, the air distribution through the fill is uneven, which reduces tower effectiveness. Second, there is risk of exhaust air recirculation because of the high suction velocity at the fan inlets, which can reduce tower effectiveness. These towers find applications in smalland medium-sized systems.
Materials
Cooling towers operate in a continuously wet condition that requires construction materials to meet challenging criteria. Besides the wet conditions, recirculating water could have a high concentration of mineral salts due to the evaporation process. Cooling tower manufacturers build their units from a combination of materials that provide the best combination of corrosion resistance and cost. Wood is a traditional material used in cooling tower construction. Redwood or fir are often used and are usually pressure treated with preservative chemicals. Chemicals such as chromated copper arsenate or acid copper chromate help prevent decay due to fungi or destruction by termites.
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