lower the average wet-bulb temperature, the ―easier‖ it is for the tower to attain the desired range, typically 6°C (10°F) for HVAC applications. Thus, in a hot, dry climate towers can be 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 to within 3–6°C (5–10°F) 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 Designs
The ASHRAE 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, crossflow 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 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 thin 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 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 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
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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
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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.
FIGURE 4.2.16 Crossflow induced draft cooling tower.
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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 bronze. 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 extensive 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 acrylonitrile 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.
Performance
Rejection of the heat load produced at the chilling equipment is the primary goal of a cooling tower 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 analysis, 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 the shelf‖ unit from a cooling tower manufacturer. Manufacturer representatives are usually well informed about their products and their proper application. After the project design process has produced the information called for in Table 4.2.6, it is time to contact one or more cooling tower representatives and seek their input on correct tower selection. Control Scheme with Chillers
Most cooling towers are subject to large changes in load and ambient wet-bulb temperature during normal operations. For a typical cooling tower, the tower fan energy consumption is approximately 10% of the electric power used by the chiller compressor. The condenser pumps are about 2–5% of the compressor power. Controlling the capacity of a tower to supply adequately cooled water to the condenser while minimizing energy
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use is a desirable operational scheme. Probably the most common control scheme employed for towers serving an HVAC load is to maintain a fixed leaving water temperature, usually 27°C (80°F). Fan cycling is a common method to achieve this cooling tower control strategy and is applicable to multiunit and multicell tower installations. However, this control method does not minimize total energy consumed by the chiller/cooling tower system components.
Lowering the condensing water temperature increases a chiller’s efficiency. As long as the evaporator temperature is constant, a reduced condenser temperature will yield a lower pressure difference between the evaporator and condenser and reduce the load on the compressor. However, it is important to recognize that the efficiency improvements initially gained through lower condenser temperatures are limited. Improved chiller efficiency may be offset by increased tower fan and pumping costs. Maintaining a constant approach at some minimum temperature is desirable as long as the condensing temperature does not fall below the chiller manufacturer’s recommendations. Since most modern towers use two- or three-speed fans, a near optimal control scheme can be developed as follows (Braun and Diderrich, 1990):
? Tower fans should be sequenced to maintain a constant approach during part load operation to minimize chiller/cooling tower energy use.
? The product of range and condensing water flow rate, or the heat energy rejected, should be used to determine the sequencing of the tower fans.
? Develop a simple relationship between tower capacity and tower fan sequencing.
De Saulles and Pearson (1997) found that savings for a setpoint control versus the near optimal control for a cooling tower were very similar. Their control scheme called for the tower to produce water at the lowest setpoint possible, but not less than the chiller manufacturer would allow, and to compare that operation to the savings obtained using near optimal control as described above. They found that the level of savings that could be achieved was dependent on the load profile and the method of optimization. Their simulations showed 2.5 to 6.5% energy savings for the single setpoint method while the near optimal control yielded savings of 3 to 8%. Use of variable speed fans would increase the savings only in most tower installations. It is more economical to operate multiple cooling tower fans at the same speed than to operate one at maximum before starting the next fan. Variable speed fans should be used when possible in cooling towers.
The system designer should ensure that any newly installed cooling tower is tested according to ASME Standard PTC 23 (ASME 1986) or CTI Standard ATC-105. These field tests ensure that the tower is performing as designed and can meet the heat rejection
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