Mechanical draught Heller indirect dry cooling systems
Heller indirect dry cooling system was invented by the founders of EGI Contracting Engineering Co Ltd, now a member of the GEA Energy Technology Division. This water saving cooling system has been serving coal-fired, gas fired and even nuclear power plants with an aggregate generating capacity over 20000MW built in arid areas worldwide in the past four decades.
The system and its components underwent considerable development in the past 20 years.
Induced mechanical draft version of Heller dry cooling system is applied when the task is to build low visual profile plants in densely populated areas. This version is also selected if it is an imperative to protect the dry power cooler from freezing in harsh winter climate. As the mechanical draft Heller indirect dry cooling tower offers the same flexibility in plant layout design than the natural draft version, ie, it can be located anywhere on the site unlike the mainstream mechanical draft dry cooling solutions; sites with constrained plot area are also candidates for its application.
The system applies either direct contact (DC) jet condenser or in some cases surface condenser, in spite of the thermodynamic disadvantage of the latter. BWR nuclear units are one of the examples of application of surface condensers, where the surface condenser comprises an important physical barrier between the power cycle and its cooling system. Combined heat and power generation plants may also apply mechanical draft version either with DC jet condenser or with surface condenser.
The mechanical draft systems apply induced draught cooling cells tied in parallel cooling sectors on cooling water side. The cells are sided by water-to-air heat exchangers. By outer appearance the tower is a row of cells or row of twin cells. As an alternative for smaller units, circular or rectangular tower can also be considered, with horizontal shaft fans located inside the tower (induced draft). For seasonal coolers the same water-to-air heat exchangers are applied, either in
all-dry, in dry/deluged or in combined versions, with forced draft or induced draft design.
The system is equipped with underground drain tank that can accommodates water fill of the all cooling sectors should the sectors be drained, and atmospheric expansion/surge tank that also serves for sector filling. If it applies DC jet condenser, its relatively large volume water fill is of condensate quality. Cooling water fill of systems having surface condenser is usually demineralised water, with alkaliser additive to maintain around pH8 in equilibrium state.
The dimensions, layout, outfit, total number and grouping of mechanical draft cells into sectors are defined according to the specific needs of each project. The vertical, integrated plate-fin water-to-air heat exchangers that side the cells are placed in pairs forming a ‘V’ with its open part turned outward, and enclosed in cold climate by extruded wing-profile aluminum louvers actuated electrically.
The cell-rows have frame structure, upper plenum and fan chambers made of galvanised steel. Unless the height of heat exchanger deltas (so-called Forgo coolers) demands other solution, heavy and vibration inducing parts of the drive train are placed at the ground level of the tower. As a result of that, the drive train (electric motor, right-angle gearbox, drive shaft) of the fans does not exert additional load to the tower steel skeleton and does not transmit vibration to it. Galvanised steel structure-enclosed shaft assemblies mounted in the middle of the cells transmit torque to the fans built into the fan chambers. Low speed–low noise fans may also be used to make the tower fit to the densely populated environment surrounding the plant. The result is a system that is silent, runs smoothly and easy to maintain.
Cooling capacity of the tower is controlled by variable frequency drives applied with each fan, and also by changing the number of operating sectors of the
cell-rows. Changing the circulating water flow rate by changing the number of operating circulating water pumps may also be used for coarse control, i.e. for switching over from summer to winter operation of the towers. Usually two 50percent circulating pumps (with DC jet condensers: two motor-pump-hydro turbine groups) are applied. One-pump operation can maintain the nominal heat rejection with approximately 5°C condenser temperature rise, or alternatively, nominal condenser temperature can be maintained in such operation at about 80percent heat rejection.
The Heller indirect dry cooling system – both its natural draught and mechanical draught versions – have space under vacuum comparable (if with DC jet condensers) or identical (if with surface condensers) with the once-through or wet cooling systems, so, it does not impose additional requirements to operators to create and maintain condenser vacuum by the air evacuation system. This cooling system makes plants specifically fit to operate in heavily dispatched mode with many startups and shutdowns, and owing to its single-metal, all aluminum heat exchangers, it does not require blow-down of corrosion product-contaminated condensate after standstills like mainstream air cooling systems.
Current projects with mechanical draft Heller indirect dry cooling system include an 800MW CCGT in Italy, a 240MW CCGT and a 120MW CHP in Moscow, Russia, with many more in the preparatory phase.
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András Balogh is President and CEO, Joseph Budik is Business Development Director of EGI Contracting/Engineering Co Ltd, a member of GEA Energy Technology Division, Budapest, Hungary www.egi.hu
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