Jörg Stahlhut, Balcke-Dürr, Oberhausen, Germany
In today’s competitive markets, electric utilities are constantly striving to maximize the efficiency and reliability of existing power plants. But although most might consider their equipment to be already optimized, there are still many parts of the world where power plants are operating with inefficient water-cooled turbine surface condensers.
The performance of a condenser is a significant factor in the overall operational efficiency of a power plant. Improper tube bundle layouts due to high tube densities or misplaced tubes and baffles cause needless pressure drops that limit condenser and turbine back pressure and thus the electrical output of the plant.
The modern condenser:
- Is characterized by a performance-optimized, erosion resistant and vibration safe tube bundle design with an almost ideal flow field that permits both steam and air to readily penetrate the tube bundle with minimum pressure drop
- Requires less heat transfer area for a given condenser pressure than the latest and most common condenser design standard prescribes
- Achieves lower condenser pressures for a given heat transfer area than the most common condenser design standard predicts
- Enables considerable efficiency rises in existing power plants and thus provides a valuable contribution to noticeable profit increases, CO2 emissions reduction and primary energy savings.
Three-dimensional numerical simulation is a reliable way to analyze and optimize the tube bundle design. This flexible method allows any type of tube bundle and the effect of specific changes to the design to be to be investigated. Such a simulation was used to compare the effectiveness of a traditional and a modern tube bundle design and illustrated the practical results achieved in a nuclear power plant, where traditional tube bundles were modularly replaced by a bundle type that was numerically performance-optimized.
Figure 1. 3-D meshes of condensers A and B. Condenser A is the traditional type; and Condenser B is the Tepee design
Analysis of tube bundle designs
During the design of a turbine surface condenser there are many factors that have to be considered. The driving force behind the condensation process is the temperature difference between the saturated steam coming from the turbine and the cooling water flowing in the heat exchanger tubes. In order to maximize the use of this potential, one has to achieve an ideal flow of steam around and between the tube bundles.
An ideal flow field can only be obtained by ensuring an even flow of steam to all sections of the tube bundles, preventing the formation of stagnation zones where non-condensable gases can accumulate and the heat exchanger area is not ideally exploited. The cumulative pressure losses on the shell-side must be minimized, as they affect the static pressure at the inlet of the condenser, which in turn affects the turbine back-pressure and thus the electrical output of the power plant. Furthermore the path of the condensate droplets between the tube bundles, and their effect on the heat transfer must also be considered.
Given the complex nature of the flow field on the shell-side of a condenser, computational fluid dynamics represents an ideal way to analyze and visualize the resulting flow path of steam around and through the tube bundles. It is an extremely flexible tool that can be used to compare the effectiveness of various design concepts.
As an example, two Condensers A and B have been considered for an existing 350 MW coal fired power plant in Asia. The shells of the condensers are equally dimensioned. Both condensers have the same basic structure, a divided, two-pass flow configuration and a total cooling surface of approximately 22 000 m2.
Each condenser comprises two tube bundles. While Condenser A represents a typical traditional bundle design, Condenser B is equipped with a state-of-the-art bundle, the so-called “Compact Tepee” bundle developed by Balcke-Dürr GmbH (see Figure 1). The major differences between the two condenser designs are the flow configuration; tube arrangement and support plates layout.
The boundary conditions of the two numerical models are identical. Air is used to simulate the non-condensable gases entering the condenser along with the steam at the inlet.
To analyse the fundamental influence of the two bundle design concepts on the effectiveness of the condensers, numerous flow fields, temperature and pressure distributions were evaluated.
Under the given boundary conditions the pressure losses in the tube bundles of Condenser B are generally lower than those of A. Whereas the minimum static pressure of Condenser A relative to the inlet pressure of 4900 Pa is -988 Pa, the minimum relative static pressure of Condenser B is -483 Pa. The resulting static inlet pressures are 4900 Pa and 4745 Pa.
Pressure losses are cumulative and depend not only on the velocity magnitude, but also on the path length through the tube bundles. An important factor which contributes to the lower static pressure losses of Condenser B, is the shape of the upper and lower bundle half.
The lower static inlet pressure of Condenser B can be considered as an equivalent decrease in the turbine back pressure. This would mean a power benefit due to the additional expansion in the turbine, whose magnitude depends on the efficiency in the last turbine stages.
In 1998 the operator of the 2 x 1350 MW nuclear power plant Gundremmingen, Germany (BWR) was faced with the task of refurbishing the four turbine surface condensers of power generating units B and C because of erosion and corrosion damage to the cooling tubes made of brass. The condensers belonging to the boiling water systems which have been in operation for about 14 years are fitted with approximately 4×43 000 tubes and have a heat exchange surface area of about 4×38 300 m2.
As detailed in Table 1, there was a choice between the following three refurbishment concepts which were subject of a pre-study:
- A: Retubing on job site using the previous tube material brass and previous tube field geometry
- B: Retubing on job site using the new tube material stainless steel and previous tube field geometry
- C: Installing of tube bundle modules completely prefabricated at works and made of the new tube material stainless steel with improved tube bundle design.
The comparison of the results obtained from the study shows that concept C involving the installation of new prefabricated tube bundles can be applied in comparatively short reconstruction time and also leads to a considerable increase in the output of the turboset, i.e. it constitutes the most economic and also the best technical solution.
Both variants which entail retubing on the job site (concepts A and B) require significantly longer outage periods for reconstruction. A further disadvantage of concept A is that the wet steam erosion resistance of the cooling tubes is not adequately improved by continuing to use brass as the tube material. This improvement is achieved by concept B through the use of stainless steel tubes but at the expense of a reduced turboset output. In view of the considerably lower thermal conductivity of stainless steel compared to brass, the retention of the existing tube bundle design means a reduction in the heat transfer coefficient and consequently a rise in the condenser pressure as well as a decrease in output. The tube bundle geometry must therefore be changed in order to avoid these undesirable forfeits in performance.
The decision was taken to completely replace the existing tube bundle with a modern ‘Tepee’ bundle in “high-performance” design as a modular construction. This bundle design whose performance-optimized shape is reminiscent of an Indian tent (Tepee) allows the steam-side pressure drop in the condenser to be kept to a minimum, the non-condensable gases to be extracted efficiently and the undesirable condensate sub-cooling to be virtually avoided.
An increase in the electrical turboset output of more than 20 MW is achieved at rated load point using the thin-walled (0.5 and 0.7 mm) cooling tubes made of stainless steel. For this particular application, the high efficiency and capacity of the Tepee bundle even permits the condensers to be provided with a heat exchange surface around nine per cent smaller than HEI’s “standard for steam surface condensers” requires.
Figure 2. Cross-section through ‘Tepee’ tube bundles
The replacement of the old condenser tube bundles by performance-optimized tube bundles has seen the power plant become more efficient by 2 x 13 MW. This corresponds to the power output of one to two barrage weirs on the river Danube. Although the old condenser had been equipped with 4000 more tubes, the new condenser with less “material” has a considerably better cooling capacity.
Furthermore the following has been achieved:
- Service life extension due to better arrangement and material properties (erosion and corrosion resistance) of new tubes
- Outage time reduction of 38 days due to installing shop-fabricated bundle modules instead of retubing
- Higher product quality due to pre-fabrication at workshop compared to retubing at job site
- Shortening of amortization period by being earlier back on the electricity grid than planned.
These results demonstrate the significant technical and commercial advantages of installing performance-optimized tube bundle modules compared to retubing.
By installing performance-optimized tube bundle modules, refurbishment measures in connection with turbine surface condensers of power plants can be carried out within an extremely short reconstruction period, and consequently the turboset can continue to function with high availability at a higher rated output.
These measures in conjunction with the high efficiency of the tube bundles lead to an increase in performance in particular in summer when the cooling water temperatures are high. Furthermore a profitability study taking account of all the operating and capital expenditure costs shows the power plant operator the significant cost advantages of this refurbishment concept compared to retubing.
Power plants are no longer subject to the, often out-dated, designs of existing equipment. The modular exchange of tube bundles in turbine surface condensers can offer a multitude of benefits, if sophisticated design is applied.