外文文献及译文secret.docx

1、外文文献及译文secretCooling TowersIf a chiller is used to provide chilled water for building air conditioning, then the heat energy that is absorbed through that process must be rejected. The two most common ways to reject thermal energy from the vapor compression process are either directly to the air or

2、through a cooling tower. In a cooling tower, water is recirculated and evaporatively cooled through direct contact heat transfer with the ambient air. This cooled water can then be used to absorb and reject the thermal energy from the condenser of the chiller. The most common cooling tower used for

3、HVAC applications is the mechanical draft cooling tower (Figure 4.2.13). The mechanical draft tower uses one or more fans to force air through the tower, a heat transfer media or fill that brings the recirculated water into contact with the air, a water basin (sump) to collect the recirculated water

4、, and a water distribution system to ensure even dispersal of the water into the tower fill.Figure 4.2.14 shows the relationship between the recirculating water and air as they interact in a counterflow cooling tower. The evaporative cooling process involves simultaneous heat and mass transfer as th

5、e water comes into contact with the atmospheric air. Ideally, the water distribution system causes the water to splash or atomize into smaller droplets, increasing the surface area of water available for heat transfer. The approach to the wet-bulb is a commonly used indicator of tower size and perfo

6、rmance. It is defined as the temperature difference between the cooling water leaving the tower and the wet-bulb of the air entering the tower. Theoretically, the water being recirculated in a tower could reach the wetbulb temperature, but this does not occur in actual tower operations. FIGURE 4.2.1

7、4 Air/water temperature relationship in a counterflow cooling tower.The range for a chiller/tower combination is determined by the condenser thermal load and the cooling water flow rate, not by the capacity of the cooling tower. The range is defined as the temperature difference between the water en

8、tering the cooling tower and that leaving. The driver of tower performance is the ambient wet-bulb temperature. The lower the average wet-bulb temperature, the “easier” it is for the tower to attain the desired range, typically 6C (10F) for HVAC applications. Thus, in a hot, dry climate towers can b

9、e sized smaller than those in a hot and humid area for a given heat load.Cooling towers are widely used because they allow designers to avoid some common problems with rejection of heat from different processes. The primary advantage of the mechanical draft cooling tower is its ability to cool water

10、 to within 36C (510F) of the ambient wet-bulb temperature. This means more efficient operation of the connected chilling equipment because of improved (lower) head pressure operation which is a result of the lower condensing water temperatures supplied from the tower.Cooling Tower DesignsThe ASHRAE

11、Systems and Equipment Handbook (1996) describes over 10 types of cooling tower designs.Three basic cooling tower designs are used for most common HVAC applications. Based upon air and water flow direction and location of the fans, these towers can be classified as counterflow induced draft, crossflo

12、w induced draft, and counterflow forced draft. One component common to all cooling towers is the heat transfer packing material, or fill, installed below the water distribution system and in the air path. The two most common fills are splash and film.Splash fill tends to maximize the surface area of

13、 water available for heat transfer by forcing water to break apart into smaller droplets and remain entrained in the air stream for a longer time. Successive layers of staggered splash bars are arranged through which the water is directed. Film fill achieves this effect byforcing water to flow in th

14、in layers over densely packed fill sheets that are arranged for vertical flow. Towers using film type fill are usually more compact for a given thermal load, an advantage if space for the tower site is limited. Splash fill is not as sensitive to air or water distribution problems and performs better

15、 where water quality is so poor that excessive deposits in the fill material are a problem.Counterflow Induced Draft 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

16、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

17、 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

18、. 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

19、 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 DraftAs in the counterflow cooling tower, the fan in the crossflo

20、w 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 crossf

21、low (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

22、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 towers effectiveness.Counterflow Forced DraftCounterflow forced draft cooling towers have the fan mounted at

23、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 tow

24、er. 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 distrib

25、ution 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.MaterialsCooling tower

26、s 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 combinati

27、on 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 coppe

28、r chromate help prevent decay due to fungi or destruction by termites.FIGURE 4.2.16 Crossflow induced draft cooling tower.FIGURE 4.2.17 Counterflow forced draft cooling tower.Galvanized steel is commonly used for small- to mid-sized cooling tower structures. Hardware is usually made of brass or bron

29、ze. Critical components, such as drive shafts, hardware mounting points, etc., may be made from 302 or 304 stainless steel. Cast iron can be found in base castings, motor housings, and fan hubs. Metals coated with plastics are finding application for special components.Many manufacturers make extens

30、ive use of fiberglass-reinforced plastic (FRP) in their structure, pipe, fan blades, casing, inlet louvers, and connection components. Polyvinyl chloride (PVC) is used for fill media, drift eliminators, and louvers. Fill bars and flow orifices are commonly injection molded from polypropylene and acr

31、ylonitrile butadiene styrene (ABS). Concrete is normally used for the water basin or sump of field erected towers. Tiles or masonry are used in specialty towers when aesthetics are important.PerformanceRejection of the heat load produced at the chilling equipment is the primary goal of a cooling tow

32、er system. This heat rejection can be accomplished with an optimized system that minimizes the total compressor power requirements of the chiller and the tower loads such as the fans and condenser pumps. Several criteria must be determined before the designer can complete a thorough cooling tower an

33、alysis, including selection of tower range, water-to-air ratio, approach, fill type and configuration, and water distribution system. Table 4.2.6 lists some of the common design criteria and normally accepted ranges for cooling towers.Most common HVAC applications requiring a cooling tower will use an “off