UPDATE: Cooling tower and spray pond technology
Worldwide cooling tower and spray cooling system developments, revealed during a European symposium, have value for U.S. power plants
By John A. Bartz, Consultant
The 9th Cooling Tower and Spray Pond Symposium, under the auspices of the International Association for Hydraulic Research, took place at the von Karman Institute for Fluid Dynamics, Belgium, in September 1994. Technical topics discussed included cooling system design, performance, operation, environmental effects, modeling and components. Symposium proceedings will not be published. However, information of primary interest to staffs of power plants in the United States is summarized as follows:
Fill material holds universal interest for wet cooling tower operators and some recent research proved valuable. Author M. Monjoie of Belgium presented a research summary of plastic material selection for cooling tower film fill. Motivation for the work came from one plastic`s (PVC) potential for producing toxic products when burned.
Studies of alternative plastic products considered mechanical properties, aging, forming, assembling, effects of fire, chemical and thermal properties, and recycling (Table 1). None of the investigated materials proved to be perfect. However, all are useable. PVC is the most adaptable film fill material and ABS and SB are good alternatives.
Freezup protection, a subject of constant study in the United States, enjoyed productive examination during the symposium. For example, L. Fabre of France reviewed a comprehensive program on cooling tower antifreeze systems in progress at Electricité de France.
Methods investigated included the use of antifreeze additives, air heating, mechanical air flow shutter, water screen in the air inlet, cold air introduction in a natural draft tower, hot water spray of sensitive zones, reduction of sprayed surface and bypass systems. Table 2 summarizes the most attractive systems. In all cases, bypass of hot water to the cold water basin is essential. Other systems may be installed alone or in combination, and hot water spraying is a useful supplement.
Plumes present plenty of fodder for discussion wherever cooling tower engineers gather. C. Winkler and G. Ernst of Germany described a three-dimensional mathematical model developed at the University of Karlsruhe to predict near-field flow patterns in cooling tower plumes. The mathematical model uses the k, e turbulence model. The numerical procedure employs a flux-corrected transport scheme and fast Fourier transformation method. Model predictions appear in Figure 1. This computer code is useful to study near-field plume problems, such as recirculation and interference in cooling towers.
Several papers on spray ponds and spray cooling towers were presented by Russian and Ukraine specialists. As an example, V. Goncharov of Russia described new, thermally efficient and economical spray cooling towers in use in that country. These systems have no packing. In field investigations of such towers, with water discharge capacity ranging from 1,500-30,000 m3/h (0.015-0.3 ft3/s), Goncharov found that the thermal efficiency practically equaled that of cooling towers containing packings with up to 20 percent less water discharge capacity needed for spray towers. Russians find spray towers attractive for avoiding fill freezup.
Dry cooling tower developments
P. Nagel of Belgium described two new developments in dry cooling technology for direct steam condensation: a flat, finned tube for the air-cooled condenser and a natural draft air-cooled condenser. The high thermal performance and compactness of the flat tube permits air-cooled condenser design with a single row of tubes. The single tube avoids steam backflow in a multi-tube design. High thermal performance of the flat tube greatly reduces the size of the tower needed for a natural draft tower application. This makes the design economically competitive with alternative designs, according to Nagel.
H. Schrey of Germany discussed operational considerations for air-cooled condensers with one or more tube rows. He described the mechanisms of inert gas accumulation for such designs. The tube row effect in multi-row condensers causing accumulation is well known. In addition, comparable problems can occur in a single tube. Longitudinal flow maldistribution in an improperly designed single row can happen and may result in dead zone buildup.
L. Ludvig of Hungary gave a status report on the Heller dry cooling system for the 2,000-MW Shahid Rajai oil- and gas-fired power plant in Iran. Heller`s indirect dry cooling system uses a direct contact condenser.
In Iran, the total capacity of dry-cooled power plants reached 2,700 MW in 1994 and will reach 5,400 MW when plants currently under commissioning or construction come on line.
The Shahid Rajai station, located west of Tehran, began operating the first unit in 1992 and now has four units. Site conditions are harsh. Ambient temperature varies from -2 F to 106 F (-19 C to 41 C) and maximum wind speed is 87 mph (42 m/s).
In the dry cooling system, turbine exhaust steam is condensed in a direct contact condenser. The mixed cooling water and condensate are cooled by mechanical draft in heat exchanger deltas comprised of aluminum Forgo (plate-fin) bundles. Data on the cooling system appear in Table 3. A water deluge auxiliary cooling system for the oil coolers and alternator coolers also is incorporated. Two years of operating experience has verified the design concepts. Further, performance requirements were met and reliability/availability are high, according to Ludvig.
R. Leitz of Germany described a recent combined-cycle installation where the dry cooling system has a low sound emission. The mechanical draft system has roof rows, each equipped with 10 fans of 20 feet (6.1 m) diameter. The fans have low blade tip speeds, hence the system needs no power-consuming, sound-attenuation baffles. The sound emission level is 100 dB (A) producing a sound pressure level of 52 dB (A) at a distance of 300 feet (100 m).
In a presentation applicable to both dry and wet cooling towers, H. van der Spek of the Netherlands described a research program on advanced low noise cooling fans. Results showed reductions in sound pressure level to 15 dB (A) by altering blade cord width and swept leading and trailing edge lines. In combination with reduction in pressure drop along the flow path, a sound power level decrease of 20 dB (A) and a 20 percent reduction in absorbed power can result. Hence, sound attenuators can be avoided. It also is possible to retrofit an existing tower and produce increased flowrate to boost performance, according to van der Spek.
Plumes also were a topic of D. Kröger of South Africa. He developed an empirical correlation to improve the prediction of tower performance from a one-dimensional analysis. It applies to wet and dry cooling towers. A numerical simulation revealed that sub-atmospheric pressure is induced on the tower outlet plane because of the acceleration of the buoyant plume immediately above the tower exit. The static pressure decrease (relative to atmospheric pressure) is a function of the densimetric Froude number and is almost independent of tower dimensions and heat exchanger characteristics. The effect of the pressure differential correction on a one-dimensional tower performance analysis equals approximately 1 percent, a figure worth considering as performance calculations continue to be refined.
Hybrid (wet/dry) systems
Power consumption levels can make hybrid cooling towers costly to operate. One solution came from W. Tesche of Germany. He described a method to minimize power needed by a process controlled mode of operation. The method calls for operating in a plume-free mode (defined by Tesche as maximum humidity not exceeding 95 percent) at minimum power for each set of conditions. Advantages gained by this method require only a slight increase in power consumption compared to a mechanical draft wet tower. Table 4 summarizes fan system economics for the planned Neckarwerke 410-MW hybrid tower in Germany. With a payback period of less than a year, the process-controlled mode is clearly worth considering.
Tesche also described physical modeling tests of dry and wet air mixing in the Neckarwerke tower. Tests showed that the plume in the center of the cooling tower is always invisible at full mass flowrate through the wet and dry sections. This avoids complicated special mixing elements. The final design resulted in a mix that exceeds the guarantee. It is possible to operate with no visible cooling tower plume at low energy consumption.
F. Bouton of Belgium described a detailed acceptance testing procedure for plume abatement towers. This method closely follows accepted thermal test procedures for other types of cooling towers. The paper concludes that plume visibility is not a good criterion for checking the quality of a plume abatement tower, unless all operating parameters (flowrate, range, air temperature and humidity) are exactly at the design point. Clearly, the choice of the design point is critical because it directly affects the tower`s size and price. Under the described procedure, fulfillment of the guarantee implies that the thermal guarantee (cold water temperature) is met, the plume abatement guarantee (average maximum humidity) is fulfilled, and the quality of air mixing is acceptable.
Continued advances in cooling system technology improve efficiency, cut overall costs, and reduce environmental impacts. After 16 years of biannual meetings, sufficient interest in cooling system improvement prompts plans for another symposium. A tentative schedule earmarks the next event for the fall of 1996 in Tehran, Iran. Plans are to visit the large dry cooling installation near Isfahan during that program. END
John A. Bartz is a consultant specializing in heat transfer and fluid mechanics. From 1978 until 1994, he managed projects at the Electric Power Research Institute, with emphasis on cooling systems. He has an MScME degree from Ohio State University.
Figure 1. Model predictions of cooling tower plume flow patterns. Note: Numerical model of the near field should be useful for tower recirculation and interference studies.