Seawater cooling

Use of evaporative cooling systems saves resources, protects the environment and provides economic benefits, writes Richard Aull.

One environmental concern sometimes raised about seawater 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.

Managing seawater 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. 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.

Typical seawater mineral content is quite consistent around the world at about 3.5 per cent 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 seawater sites
Concentrated sea water, having high salt concentration, lowers the water's vapour pressure and reduces the evaporative cooling rate by 5 per cent to 8 per cent, depending on salt concentration. Therefore, a typical seawater cooling tower design will be 5 to 10 per cent 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 seawater environments. Consult CTI Standard 136 for fill and drift eliminators. And, the structure should be pultruded fibreglass, treated wood, concrete or specialty materials formulated for seawater 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 seawater exposure as well.

To maximise drift eliminator effectiveness, they should be positioned approximately 1 m 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 per cent drift loss and be certified by the manufacturer's testing.