Which of the thermal pollution reduction methods is most expensive to…

Concept: Thermal-pollution control means stripping waste heat out of condenser cooling water before that water re-enters a river or lake. The heat can leave along two paths. On the latent-heat path a small fraction of the water evaporates, and every kilogram that vaporises carries away about 2260 kJ. On the sensible-heat path the water simply gives its heat to the surrounding air — by convection and radiation from an open surface, or by conduction and convection through the wall of a closed tube — and air stores only about 1 kJ per kilogram for each degree it warms. Because air is such a poor sensible-heat carrier, a design that gives up the latent-heat path must buy the same cooling duty with far more heat-exchange surface and far more air movement.

Application: Place each offered method on that heat-transfer axis:

• Large shallow pond — an open basin; evaporation from a wide free surface does most of the work, helped by convection and radiation, so the plant pays mainly for land and earthwork and for very little machinery.

• Cooling tower (wet) — water is spread over fill and contacted directly with air, so evaporating one to two per cent of the circulation removes the heat inside a compact structure; the air is moved either by an induced draught or by the tower's own natural draught, and make-up water replaces what is lost.

• Sprinkler tower — nozzles atomise the water so the same evaporative transfer happens from droplet surfaces; the extra cost over a pond is the pumping head at the nozzles.

• Dry tower — the water never meets the air; it stays inside finned tubes and the heat crosses the tube wall by conduction and convection. With the latent-heat path removed, the same duty needs a very large finned surface and a correspondingly large air-moving system: a mechanical-draught unit runs its fans for as long as the plant runs, and a natural-draught dry tower has to be built as a huge shell instead.

Cross-check: Weigh what each design saves against what it costs. The pond, the wet cooling tower and the sprinkler tower all move heat cheaply, but they do it by consuming water: the evaporated fraction is lost outright, and a recirculating tower loses more still to drift and to the blowdown needed to hold down dissolved solids. The dry tower's selling point is the opposite: it consumes no water and raises no vapour plume, which is why it is chosen at water-scarce sites, and that benefit is paid for entirely in metal surface and air-moving energy. A second check is the approach temperature. A dry tower can only approach the dry-bulb temperature of the ambient air, while every evaporative design approaches the lower wet-bulb temperature, so the dry tower must be built larger still to reach the same cold-water temperature.

Result: Of the four methods offered, the dry tower is the most expensive to construct and to operate.