Fresh water has become a very valued resource. At many locations around the world, industrial and commercial expansion is limited by the availability of fresh water. Only in a limited number of areas - for instance, the north coast of the United States, where about 20 percent of the world’s fresh water resides in the Great Lakes - is a relatively consistent recharge of the resource experienced.
Industrial expansion typically is focused around lakes or rivers that now have limited amounts of water for industrial or commercial expansion. This shortage can cause the owner to consider air-cooled systems, which have limits related to air temperature, capital and operating cost. An alternative to air or fresh-water cooling is sea-water cooling.
One environmental concern sometimes raised about sea-water cooling is that it becomes a thermal pollutant. While this is likely true for once-through cooling systems, by using an open recirculation cooling system, you change the heat sink from the ocean to the atmosphere. Open recirculation cooling towers discharge the heat and pure evaporative water to the atmosphere. It could be said that this evaporative water adds to the recharge of the ground water, even if only by a small amount.
This article will review issues related to sea-water open-recirculating cooling towers by looking at some real-world installations, provide suggestions in their design and answer some commonly occurring questions.
A Concrete Tower in Thailand
Southeast Asia has vast coastlines where industrial and commercial ventures have access to sea water. Padaeng Industries in Ma Ta Put, Thailand, is a zinc refinery that produces pigments and other specialty products.
Padaeng Industries has a 12-year-old, three-cell, forced-draft tower with a hot water temperature (HWT) of 96.8°F (36°C); a cold water temperature (CWT) of 87.8°F (31°C); a wet bulb temperature (WBT) of 82.4°F (28°C); and a 46,230 gal/min (10,500 m3/hr) water recirculation rate. Fluctuations in cold water temperature significantly affect production, yet they have been very satisfied with the system design. They operate the tower at 1.2 cycles of concentration and use screens and filters to keep suspended solids low. In addition, they use a low dosage biocide to control biological activity.
Following an accidental fire some time ago, Padaeng replaced the film fill in one cell with a low fouling, vertically offset fill. The company also replaced the OEM drift eliminators with high-efficiency, cellular, three-pass drift eliminators. Both the fill and drift eliminators are made of rigid PVC, meeting Cooling Technology Institute’s PVC material standard (CTI STD-136; see table 1). The drift eliminators have a low drift rate, and it can be expected that any contribution of salt in the environment from the cooling tower will be significantly lower than the naturally occurring salt in the air. The plant has been ISO 9001 and 14001 certified, plus an outside environmental management firm is contracted to sample and track air and particulate emissions.
Because of the low total suspended solids (less than 50 ppm) in the makeup and good biological control of the recirculating water, film fill fouling is minimized, keeping the tower’s thermal performance at design values.
Padaeng Industries’ cooling tower is a concrete structure, and care was taken to ensure high-quality concrete work to withstand the salt water. However, the fan shrouds, railings, ladders and fasteners were made of painted or galvanized carbon steel, and these components have seen significant corrosion. The forced-draft design is somewhat unusual for a field-erected tower, and the addition of a roof over the fan intakes indicates a shortcoming in its design. The roof was added to prevent the air intake fans from drawing in the hot exhaust air and reducing unit efficiency.
Cooling tower drift and salt contamination in the environment are considerations when operating open recirculating cooling towers with sea-water makeup. Small droplets of the circulation water can leave the tower as drift. Depending on the size of the droplet and the wind conditions, the range of dispersion can be less than a hundred meters to many kilometers. It is recommended that the distribution system be designed for low-pressure spray nozzles, which will reduce the quantity of small droplets impinging on the drift eliminators. Because all drift eliminators have a fractional efficiency limit (the droplet size where the drift eliminator is less than 100 percent efficient), more entrained droplets are captured by high-efficiency drift eliminators. Limiting drift to match or be less than coastal sea air salt content yields a drift rate of 0.001 to 0.0005 percent of the circulation water flow rate.
Some water treatment chemicals can affect drift rate by reducing the surface tension of the water, which should be taken into consideration when designing the water treatment program. Pending changes to CTI’s drift eliminator test code will limit surface tension to no less than 63 dynes/cm. When used to excess, some dispersants can reduce surface tension by a factor of two or more, producing droplet diameters much smaller for the same nozzle pressure. There has been no observed increase in water surface tension when adding oxidizing biocides such as chlorine, bromine or ozone. However, other water treatment chemicals such as methylene bisthiocyanate can significantly reduce the water’s surface tension and increase drift rate.
Counterflow Towers Near the Gulf
Latin America’s power-producing giant, Iberdrola of Bilbao, Spain, has a 2,000-plus MW, combined-cycle plant in Altamira, Mexico. Plant 3 and 4 are rated at 1,036 MW and have two 10-cell induced-draft counterflow cooling towers serving four gas and two steam turbines. The plant uses sea water from the Gulf of Mexico via two 24" intake lines located about one mile out in the Gulf. They use screens for trapping large debris; sodium hypochlorite as the biocide; H2SO4 for pH control; and water-management chemicals from Nalco for pretreatment. The towers run from 1.3 to 1.5 cycles of concentration and have been in operation for nearly seven years.
Iberdrola Units 3 and 4 operate efficiently, but the power plant wanted to improve the operation by reducing the pressure drop of the existing two layers of drift eliminators used to ensure the lowest drift rates. The existing drift eliminators were mounted just above the spray nozzles, so the OEM recommended the power plant use drift eliminators rated at 0.0005 percent drift loss, mounting them approximately one meter above the spray nozzle discharge. This modification will reduce pressure drop of the drift eliminators by 60 percent, which will increase tower airflow, thereby providing colder water. Also, the plant is considering whether installing replacement spray nozzles could improve water distribution.
Managing Sea-Water Issues
Biological control is a typical concern in all open, recirculating cooling towers. The biological activity will create waste secretions that act as glue for suspended solids, allowing them to adhere to the cooling tower fill and other heat transfer surfaces. Figure 2 show the impact of deposits, indicated by the fill weight gain, affecting cooling tower efficiency. It takes only a small amount of fill fouling to reduce airflow and create uneven water distribution, which negatively affects tower performance and increases cold water temperature.
Selecting the proper fill design and good water chemistry management are critical for good system operation. To help accomplish these objectives, one cooling tower fill manufacturer adopted an approach that has been used successfully for many years in the food processing industry, where biological control agent is incorporated into the materials of construction. In this case, the biological control agent is incorporated with the plastic resin. The material reduces biological activity by use of a silver-and-zinc compound. The compound does not leach from the plastic, nor is it harmful to the environment. The fill with biological control agent has been shown to reduce fouling and protect the cooling system’s thermal performance (figure 3).
Typical sea-water mineral content is quite consistent around the world at about 3.5 percent dissolved solids. However, some variations are observed due to local conditions such as fresh water dilutions from rivers or high evaporation rates in bays. The variation in total dissolved solids (TDS) is relatively easy to manage by modification to the tower’s cycles of concentration.
However, variations in total suspended solids (TSS) can cause significant fouling problems. Based on the quality of biological control and TSS, the selection of cooling tower fill can be made. Because the fill design plays the largest role in tower performance, it is suggested that seawater makeup water be limited to 20 ppm TSS, which allows for the use of high performance film fills like a vertical offset design. Typical seawater makeup can be run at up to 2.5 cycles of concentration using low fouling film fill.
Recommendations for Sea-Water Sites
Concentrated sea water, having high salt concentration, lowers the water’s vapor pressure and reduces the evaporative cooling rate by 5 percent to 8 percent, depending on salt concentration. Therefore, a typical sea-water cooling tower design will be 5 to 10 percent larger (plan area and/or power effects) than a similar capacity fresh-water system.
The smaller the approach temperature, the greater the demand (size and power) on the cooling system will be. Targeted approach temperatures must consider the effect salt water has on tower performance. For instance, if a 5.4°F (3°C) approach temperature is considered practical for fresh water, a 7.2°F (4°C) approach would be acceptable for a salt-water system.
Materials used should be stable in sea-water environments. Consult CTI Standard 136 for fill and drift eliminators. And, the structure should be pultruded fiberglass, treated wood, concrete or specialty materials formulated for sea-water applications.
For cooling towers constructed of flammable structural materials, a deluge fire protection system designed for exposure to sea water should be used. All piping, fasteners, railings and access stairways should be constructed of materials suitable for sea-water exposure as well.
To maximize drift eliminator effectiveness, they should be positioned approximately 1 meter above the spray nozzle discharge. Low pressure nozzles should be used with nozzle pressures not exceeded 13.8 kPa (2 psig). Drift eliminators should be rated at 0.0005 percent drift loss and be certified by the manufacturer’s testing.
Tower site selection is the same for saltwater towers as for fresh-water towers. It is suggested that the longitudinal direction (air inlet) be parallel to the normal wind direction and sensitive equipment be positioned away from the exhaust air path.