Natural draught tower is vital to indirect dry cooling system

Paul Boughton

Natural draught dry cooling tower is a vital part of the traditional Heller dry cooling system. As an air moving devicetowers are passive elements as opposed to the large number of fan-gear-motor units of air cooled condensers.

Their inherent reliability and availability largely contribute to the general operational features of the Heller indirect dry cooling system.

Construction of the towers begins with the foundations of the tower structure and placement of tower internals.

The latter are cylindrical steel drain tanks buried inside the towerand are used to accommodate the water volume of water-to-air heat exchangers.

The tower has reinforced concrete circular ring plate foundation that usually does not need to be supported by piles. Concrete towers rest on so-called X-legs.

These reinforced concrete legs are cast in situlifted and tilted into position and held together by the lower concrete ring. This ring serves as a starting point of the laser positioned slip form that raises the superstructure of the tower. Though the tower shell does not require coatingexternal finish may be used for decorative purposes. Pre-cast concrete collar slabs held by appropriate members form transition piece between tower shell and the horizontal concrete roofing of heat exchanger circle.

Since the upright all-aluminium water-to air heat exchangers are placed around the tower base circumferenceerection of heat exchangers and raising the tower shell may be performed in parallel. The heat exchangers are mounted on V-frames turned outside with their open part. This open part accommodates the louvres. The V-frames (deltas) rest on steel legs anchored to the tower foundation. The all-aluminium heat exchangers are usually two-pass cross-counter flow design having a lower in-and outlet header and an upper return header. The in- and outlet nozzles are connected to the forward and return sector ring-pipes by steel-treaded flexible joints. Circular vent-pipes with standpipes protruding inside the tower connect the heat exchangers of each tower sector.

The tower shells are designed to withstand the ground-level acceleration serving as a basis for the plant structural design. In areas most exposed to earthquakesteel structure towers with corrugated aluminium cladding are customary.

Strange as it may sound; the amount of steel in their structure is roughly equivalent to the amount of re-bars applied with the concrete-shell towers.

These towers have a conical lower part and a cylindrical upper structure as opposed to the hyperbolic shape of the reinforced concrete cooling towers.

Erection of the steel towers is an elaborate process assisted by indigenous auxiliary means. One such equipment is a platform tilted in the angle of the lowerconical part of the tower travelling on circular rails inside the tower. This travelling platform serves as a positioning device for the 24 panels that make up a row of the conical part of the tower steel skeleton.

Components of the cylindrical part are put in place in self-building modewithout using a central tower crane. Stiffening rings of the tower are erected on top of each other on the flat concrete groundand then lifted to the position of the lowest one which is welded to the end of the conical part of the tower.

Structural parts (beam triangles) forming the tower skeleton and their cover panels are put in place up to the level of next stiffening ringand the rest of the stiffening rings travel upwards to this position by jacks built in combination of hydraulic cylinderspylonsratchets and chains. This waythe whole tower is erected with the assistance of mobile cranes only.

Once builtowners and operators surely enjoy the features of Heller dry cooling towers. Due to their mere heightdry cooling towers help avoid hot air recirculation or suction by GT air inlet as it may happen with CCGTs with ACCs.

Wind gusts have far less distorting impact on their draft owing to their louvred vertical water-to air heat exchangers than on towers with horizontal heat exchangers where wind can pass across almost freely below the tower shell.

As heat exchangers placed around the tower are grouped into parallel connected sectorsone tower shell can serve two power units. Besides saving investment coststhe solution provides better vacuum for the operating unit if one unit stands still for any reason.

Flue gas can also be led into the dry cooling towers thus saving construction cost of a tall chimney. The short metal stack inside the tower provides better dispersion of airborne pollutants by atmospheric diffusion than a tall chimney due to the large ‘incremental stack height’ of the vast stream of warm dry air leaving the tower at a velocity close to 6 m/s surrounding the central flue gas stream. Standard atmospheric diffusion calculations confirm that maximum ground level concentrations of SO2NOx and particulates remain a fraction of those resulted in case of a stand-alone chimney.
The most widespread FGD technologythe wet scrubber cools down flue gases close to 65¢ªC. If discharged through a tall chimneyscrubbed wet flue gas needs to be reheated before discharge to bring it well above its dew point.

Both investment and O&M costs of reheat can be saved if the entire wet scrubber is located inside the Heller dry cooling tower and it may also contribute to resolve space constraints of the site.

András Balogh is President and CEOand Joseph Budik is Business Development Director of EGI Contracting Engineering Co LtdBudapestHungaryMember of the GEA Group.
www.egi.hu

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