Ambient air acts as heat sink in indirect dry cooling system

Paul Boughton

András Balogh and Joseph Budik provide a brief overview of the Heller Indirect Dry Cooling System.

The Heller Indirect Dry Cooling System was invented in the early 1950s as the first dry cooling system for Rankin cycles and it has been applied with more than 20000MW generating capacity since. Its major objective is to use ambient air as heat sink via natural draft cooling tower – similarly to the hyperbolic wet cooling towers but without mass transfer thus water loss by evaporation taking place.

An intermediate water cycle carries away reject heat of the steam cycle to the place of where it is dumped into the ultimate heat sink – to the atmosphere. This intermediate cycle is usually not separated from the power cyclethe ‘meeting point’ is the direct contact (DC) jet condenser.

Cooled water of intermediate cycle is ejected into the condenser through nozzles forming thin water films that meet exhaust steam in coss flow. Condensation takes place on the surface of water filmsvirtually eliminating temperature difference between the two media.

The system is called ‘indirect’ thanks to this intermediate cycle that has a flow rate roughly 50-fold of the power cycle. Its circulating power demand is less than half of that of the fan power of air cooled condensers.

Warmed-up water of the intermediate cycle rejects heat to the atmosphere through mono-metallic (all-aluminum) plate-fin water-to-air heat exchangers arranged in vertical V-shape columns (deltas) around the circumference at the foot of natural draft cooling tower.

When selecting cooling option for a steam cyclea cost-benefit analysis is made that compares present value of the life cycle costs of the candidate options. Apart from investment costscontributing factors are difference in annual generation due to difference in condenser vacuumcost of non-generated power due to difference in availabilityand O&M costs including all-in-all makeup water cost and (water or air) circulating power. Summary of the comparison is best shown in the so-called envelope diagram that for a given economic environment defines break-even lines for the application of certain cooling option (wetindirect dry or ACC). Areas where the point defined by a selected electricity price and water price falls into provide answer which option is advisable to select.

The Heller Indirect Dry Cooling System offers a number of features for design engineers conceiving plant layout. There is no need to place the cooling tower close to the turbine hallthe circulating lines may extend to hundreds of metres to the place where the tower is best located. The pump pit is located close to the condenser to ensure ample suction head for the circulating pumps. Before returning to the condenser under vacuumthe intermediate water flow has considerable excess head that is usually regained by hydro turbine coupled in common shaft with the circulating pumpsaving about 30percent of the pumping power.

The space of DC jet condenser under vacuum is comparable with the surface condenserand it is incomparably smaller than that of the ACCs. Air evacuation does not take longer than usual and vacuum holding is as easy as with the surface condensers. The modular design condenser either rests on fixed supports and connected to the exhaust stub of LP casing via expansion joints or has spring supports and welded to the LP casing. It is made of carbon steelvirtually maintenance free as it does not have tubes and tube sheets. Versions of design are also made for lateral or axial exhaust. The Heller system fits to any modern plant water chemistry operating in the range pH 7- 8.5.

Cooling performance of the Heller dry cooling system is relatively insensitive to wind gusts as there is no space below the heat exchangers where the wind could blow through almost unrestricted distorting the draft or causing recirculation. The tower can accommodate the flue gas stack saving the costs of a tall chimneyor even the whole wet scrubber saving costs of reheating the flue-gastoo. Owing to the enormous thermal lift of the towerground level concentration of airborne pollutants emitted through a short stack inside the tower is a faction of that of a stand-alone stack.

Being a closed system with no water lossesin comparison with evaporative cooling of identical capacityit saves water equivalent needs of a 150000-strong town if applied with a single 600MW steam power unit.

András Balogh is President and CEOand Joseph Budik is Business Development DirectorEGI Contracting Engineering Co LtdBudapestHungary. www.egi.hu

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