The direct contact jet condenser: key to the Heller concept

As with any power plant air cooling concept, the Heller indirect dry cooling system uses ambient air as the ultimate heat sink. This feature determines the characteristics of the system: since the driving force for heat transfer at the cold end of the power cycle is the temperature difference between the condensing steam and the ambient air, the condenser temperature achievable with the Heller dry cooling system is a straight line function of the ambient air temperature over most of its operating range.

The condenser pressure achievable by a cooling system at a given ambient air temperature is very dependent on the physical constraints associated with that system as well as on the design of its individual components. The main factors contributing to the increasing interest in the Heller indirect dry cooling system (see also MPS, July 2006) are the following:

• it does not impose its own limit on the achievable condenser temperature (ie it does not “choke” earlier than the turbine does, as mainstream dry cooling systems do);

• the high efficiency of the water-to-air heat exchangers;

• the large volume (several thousand cubic metres) one-time circulating water fill of condensate quality;

• use of a circulating system machine group with pressure head recovery features; and,

• the condenser design, the main subject of this article.

Improvements in the system and its components over the years have been directed towards integration into modern power plants, and also to reducing investment and operating costs. Decades of development have polished the design of the water-to-air heat exchangers to increase their air-side performance. Though these highly efficient heat exchangers (the so-called Forgo-bundles) can also be used for air cooling systems employing surface condensers, the main thrust of our development work has been on systems using the direct contact jet condenser. The latter was developed as part of the original Heller system and it makes the system superior to alternative technologies by virtually eliminating the terminal temperature difference (TTD), which is the temperature difference between the warmed-up cooling water and the condensing steam. The TTD achieved is in the range 0.5-0.85°C (the value increasing with lower condenser temperature and higher steam load), which is considerably less than that with surface condensers, the latter achieving a TTD of about 5°C or even higher. This fact contributes to the performance of the Heller system, enabling it to make better use of the ambient air as heat sink than other dry cooling concepts.

For Professor Heller, who invented the system 45 years ago, a key goal was to achieve the lowest possible condenser temperature for a given temperature of incoming cooling water by keeping the TTD as low as possible. Apart from the presence of non-condensibles, two major factors determine the terminal temperature difference: the pressure drop of the condensing steam while travelling through the condenser and the extent of the heat transfer surface. Making the condenser airtight, having efficient deaeration, lowering the pressure drop and increasing the heat transfer area have remained the ultimate goals of condenser design since.

In the direct contact jet condenser, condensation of the turbine exhaust steam takes place on cooling water films, while in surface condensers the condensation takes place on the external surface of the cooling water tubes. The direct contact concept eliminates the temperature drop on across the condenser tubes and creates heat and mass transfer between the condensing steam and the cooling water stream. The latter has a flow rate about 50 times bigger than that of the steam. The temperature rise of the cooling water is similar to that experienced with surface condensers, so is the ratio between the two flow rates.

The direct contact jet condenser is also practically maintenance-free. The cumbersome task of condenser tube cleaning is eliminated, as are tube erosion and corrosion problems. And last but not least, not only is the condenser itself cheaper than any surface condenser of equivalent heat transfer capacity, but its features mean that the associated air cooling equipment can be smaller, and consequently cheaper, whether it is a natural draft cooling tower or a mechanical draft system.

The main components of a DCJC

The direct contact jet condenser has three distinct components: neckpiece; middle part; and hotwell.

The neckpiece design varies depending on the direction of turbine exhaust flow and the characteristics of the steam. Traditional (downward), lateral, axial or even upward exhaust flows can be accommodated. In earlier designs the neckpieces were welded to the LP turbine exhaust stub and supported by springs. In current designs they are connected to the turbine exhaust stub via flexible expansion joints and rest on the ground. The supports of the condenser body are designed to maintain the required fixed point and allow thermal expansion in two perpendicular directions.

Twin-body direct contact jet condensers are used for turbines with two LP casings. The two condenser bodies are interconnected on the steam side via ducts with expansion joints and connected in parallel on the cooling water side. This is the most common arrangement despite the thermodynamic advantages offered by series connection on the cooling water side. This is because series connection would result in different pressures and therefore different conden-sate levels in the two condenser bodies.

The neckpiece usually accommodates the first LP feedwater heater, which extends across the con-denser body.

Further LP turbine exhausts (bleeding points) cross the condenser neck-piece on their way to their respective feedwater heaters. This part of the condenser also accom-modates the spargers, the last stage of the steam throttle/cooler system through which the reduced and cooled bypass steam enters the condenser.

A curtain spray ensures that the bypass steam does not have a negative impact on the operation of the condenser. The main boiler feedwater pumps of large capacity steam turbines are usually turbine driven. These either have separate condensers, or their exhaust steam is led into the main condenser via vacuum steam ducts. This arrangement is also possible with the direct contact jet condensers.

The middle part of the direct contact jet condenser includes the parallel cooling water ducts to which the sprayer nozzles are attached. Selection of the right pitch and proper design of impact plates ensure formation of parallel cooling water films along which condensation of turbine exhaust steam takes place. Part of the cooling water is led into the last section of the “steam alley” where elimination of non-condensibles and also makeup water deaeration take place in counterflow. This is the point where air evacuation devices, steam jet hogging and holding ejectors or mechanical vacuum pumps, are tied in.

The lowest part of the condenser body is the hotwell from where the main cooling water circulating pumps take suction. To avoid cavitation when taking suction from vacuum, short, large diameter suction pipes and relatively low speed main CW pumps are employed.

The rectangular condenser body is made of carbon steel and is of modular design. Internal stiffening bars are fitted to make sure it can withstand vacuum.

The space under vacuum is similar to that of a surface condenser, so air evacuation takes the same time and vacuum holding is much the same as with surface condensers. The carbon steel material of the condenser contributes to the initial formation and maintenance of a double oxide layer (magnetite and haematite) that protects the internal surface of the aluminium water-to-air heat exchanger tubes. This important passivation process takes place during the commissioning phase of the cooling system.

Ongoing projects

Large coal fired power stations and also combined cycle plants – whether in operating in baseload or cycling mode – are possible candidates for the application of the Heller indirect dry cooling system, usually with direct contact jet condensers.

The technology is already in use at well over 20 000 MWe of installed generating capacity worldwide.

Notable applications include the world’s largest dry cooled CCGT plant (Gebze/ Adapazari, Turkey) as well as several ongoing projects in Russia, Italy and Syria.

Prospective candidates include planned 600-1000 MW coal-fired units in China and projects in the USA.