Expected Future Developments
Beyond cooling tower designs that reduce water consumption, users should also expect increased regulation of siting and of cooling tower emissions such as plumes and noise.
Plume Abatement. A plume is heated air and water vapor that exits the cooling tower. Under certain ambient conditions, the plume becomes visible when moist, heated air is cooled past its dew point. Its presence is a concern for many reasons, including the fact that the plume can be viewed negatively when the public associates it with stack emissions or nuclear power plants. The plume can also create dangerous fogging and icing conditions and must often be abated if the towers are located near airports or roads.
To combat these problems, plume-abated (PA) towers have been developed (Figure 6). To avoid creating a visible plume under certain ambient conditions, saturated air from the wet section is mixed with hot and dry air in the dry section of the tower. Because some of the heat in the incoming water is dissipated in the dry section, not as much cooling has to take place via evaporation in the wet section; thus, water savings can be realized with the PA towers. However, given the special constraints on the dry section, the savings potential is limited. The main purpose of PA towers, where the dry and wet sections are integrated in the same system, is to avoid a visible plume, and water conservation is an incidental consequence.
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| 5. The cross-flow and counter-flow cooling tower. Source: Bechtel Power Corp. |
Site Area Restrictions. Cooling towers must be oriented properly to minimize factors that negatively impact tower thermal performance. One consideration is to ensure that cooling towers are adequately spaced so that hot discharge air from one tower is not drawn into the intake of another tower. Towers must also be located a sufficient distance from other structures in a power plant. Tall turbine and boiler buildings impede airflow into cooling towers, and tower drift can cause unacceptable salt deposits on switchyards. However, some sites simply do not have sufficient area available to address all of these concerns or to allow for enough linear cooling tower area to dissipate the required heat load. Unique layouts have been employed to ensure that towers incorporate all of the required features while being located within site boundaries.
Cooling towers must also be placed correctly with respect to the prevailing wind. The air inlets should be aligned parallel to the prevailing wind coincident with the highest ambient temperatures. This minimizes recirculation, where some of the saturated exiting air is induced back into the tower air inlets, raising the inlet WBT and resulting in a higher cold water temperature for a given ambient condition (see sidebar “Case Study 3”).
Resourceful Designs Required
Given impending water resource shortages, power plant designers will need to become more resourceful in designing cooling systems. As noted here, many innovative and hybrid technologies have been implemented already, and we expect many new applications will evolve in the coming years.
—Natasha Jones and Christopher Kaplan are mechanical engineers, and Ram Narula (rnarula@bechtel.com) is vice president, chief technology officer emeritus, and a Bechtel Fellow for Bechtel Power Corp.
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Comments (2)
In fact, this technology was featured in a recent Power magazine article: "Wet Surface Air Coolers Minimize Water Use by Maximizing Heat Transfer Efficiency" (September 2008).
The WSAC can reuse water from almost any source (cooling tower blowdown, gray water, etc) as spray system makeup, and operate at higher cycles of concentration. (This has been validated by an EPRI / NETL sponsored demonstration project in New Mexico.)
WSACs operate at numerous plants for not only cooling services, but also to reduce fresh water requirements and plant water discharge.