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Improving heat rate in power plant condensers with enhanced tubes

Description: Argonne is working with the Tennessee Valley Authority (TVA) to study the effect of enhanced condenser tubing on the heat rate of fossil-fueled power plants. A computer model was developed to calculate the predicted heat rate improvement by employing enhanced tubes as against smooth tubes. Fourteen condensers on the TVA system were then fitted with enhanced tubes as a replacement for the conventional smooth-surface condenser tubes. The performance of these units was followed and the results analyzed and compared to the predictions obtained with the computer model. Findings from this research have been published in Power Engineering magazine and have been presented at the Electric Power Research Institute (EPRI) Power Plant Condenser Symposiums.
Date: March 1, 1995
Creator: Rabas, T.J.
Partner: UNT Libraries Government Documents Department

Heat exchanger performance calculations for enhanced-tube condenser applications

Description: The lack of a prediction method is sometimes used for the rejection of enhanced tubes for some condenser applications even though there is ample data from single-tube condensing experiments. Three methods are discussed that can be used to rate and/or size these multitube units based on the single-tube experimental results. The Kern vertical-number correction appears to be quite adequate for most operating conditions, the exceptions being large sizes and/or deep vacuum operation. The bundle-factor method is preferred for these applications; however, field test results are required to obtain this factor. If performance data are not available, pointwise or numerical methods are required but special care must be taken to insure that the adverse effects of noncondensable gas pockets and the saturation-temperature depression are properly addressed.
Date: January 1, 1992
Creator: Rabas, T.J.
Partner: UNT Libraries Government Documents Department

Heat exchanger performance calculations for enhanced-tube condenser applications

Description: The lack of a prediction method is sometimes used for the rejection of enhanced tubes for some condenser applications even though there is ample data from single-tube condensing experiments. Three methods are discussed that can be used to rate and/or size these multitube units based on the single-tube experimental results. The Kern vertical-number correction appears to be quite adequate for most operating conditions, the exceptions being large sizes and/or deep vacuum operation. The bundle-factor method is preferred for these applications; however, field test results are required to obtain this factor. If performance data are not available, pointwise or numerical methods are required but special care must be taken to insure that the adverse effects of noncondensable gas pockets and the saturation-temperature depression are properly addressed.
Date: July 1, 1992
Creator: Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Fouling characteristics of compact heat exchangers and enhanced tubes.

Description: Fouling is a complex phenomenon that (1) encompasses formation and transportation of precursors, and (2) attachment and possible removal of foulants. A basic understanding of fouling mechanisms should guide the development of effective mitigation techniques. The literature on fouling in complex flow passages of compact heat exchangers is limited; however, significant progress has been made with enhanced tubes.
Date: July 15, 1999
Creator: Panchal, C. B. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Condensation analysis for plate-frame heat exchangers

Description: A theoretical analysis is presented to predict single component and binary-mixture condensation in plate-frame heat exchangers. A thermodynamic property model based on the Peng-Robinson equation of state was developed for the binary-mixture equilibrium and formulated into a performance prediction program. A set of equations was formulated and a calculation algorithm was developed to predict the local rate of heat and mass transfer for binary mixtures. Friction-factor and heat-transfer-coefficient correlations were developed using experimental data obtained with ammonia condensation. The role of the mass-transfer resistance associated with the condensation process were analyzed for a propane/butane mixture using two limiting cases: (1) no liquid-phase mass-transfer resistance, and (2) infinite liquid-phase mass-transfer resistance. The results show that the vapor-phase mass-transfer resistance is the controlling mechanism for binary-mixture condensation.
Date: July 1, 1995
Creator: Arman, B. & Rabas, T.J.
Partner: UNT Libraries Government Documents Department

The effect of the exit condition on the performance of intube condensers

Description: Data collected from the open literature plus some new, unpublished data will be used to show that the exit condition can change the flow regimes, introduce certain types of instabilities, and alter flooding velocities with intube condensation. The major orientations will be considered: horizontal, vertical with vapor downflow, and vertical with vapor upflow (refluxing).
Date: July 1, 1995
Creator: Rabas, T.J. & Arman, B.
Partner: UNT Libraries Government Documents Department

The influence of the rib width on the performance of tubes with the separation and reattachment enhancement mechanism

Description: This paper presents numerically predicted turbulent heat-transfer and friction-factor results for tubes with transverse, rectangular ribs for different width-to-height ratios. The rib spacing was maintained at values where the separated flow over the rib reattached between adjacent ribs (i.e. the separation and reattachment enhancement mechanism). The mean Nusselt number was found to decrease slightly with an increase in the width to height ratio for low Prandtl number fluids (Pr = 0.71). However, the trend is more complex for higher Prandtl number fluids. The mean Nusselt number can either increase or decrease depending on the magnitude of the Prandtl number and rib spacing. The friction factors decreased with an increase in the width to height ratio and the magnitude of this decrease was somewhat Reynolds number dependent.
Date: January 1, 1992
Creator: Arman, B. & Rabas, T.J.
Partner: UNT Libraries Government Documents Department

The influence of the rib width on the performance of tubes with the separation and reattachment enhancement mechanism

Description: This paper presents numerically predicted turbulent heat-transfer and friction-factor results for tubes with transverse, rectangular ribs for different width-to-height ratios. The rib spacing was maintained at values where the separated flow over the rib reattached between adjacent ribs (i.e. the separation and reattachment enhancement mechanism). The mean Nusselt number was found to decrease slightly with an increase in the width to height ratio for low Prandtl number fluids (Pr = 0.71). However, the trend is more complex for higher Prandtl number fluids. The mean Nusselt number can either increase or decrease depending on the magnitude of the Prandtl number and rib spacing. The friction factors decreased with an increase in the width to height ratio and the magnitude of this decrease was somewhat Reynolds number dependent.
Date: December 1, 1992
Creator: Arman, B. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Heat-Rate Improvements Obtained by Retubing Condensers With New, Enhanced Tube Types

Description: Significant fuel savings can be achieved at power plants by retubing the condensers with enhanced tubes. Because of the higher overall heat-transfer coefficient, the exhaust steam is condensed at a lower pressure and the plant efficiency is therefore increased or plant heat rate is reduced. Only the spirally indented type of enhanced tube is currently being used in the U.S. and most other countries; however, different types of enhanced tubes have been proposed for power-plant condensers, each with their own set of attributes. This paper determines what attributes and their magnitudes of enhanced tubes lead to the most energy savings as measured by reduction of the plant heat rate. The particular attributes considered are the inside and outside enhancement levels, the inside efficiency index (inside enhancement level divided by pressure-drop increase), and the enhanced-tube fouling-rate multiplier. Two particular condensers were selected because all necessary information were known from previous heat-rate studies such as the condenser geometry, the circulating-water pump and system information, and the low-pressure turbine characteristics. These are {open_quotes}real-world{close_quotes} condensers and therefore the finding will be representative for many other condenser-retubing applications. However, the authors strongly recommend that an economic evaluation be performed at each site to determine the energy savings and payback time. This generic investigation showed that the outside enhancement level is the most important attribute, and a value of about 1.5 can lead to heat-rate savings of about 20 to 40 Btu/kW-hr. Increasing the inside enhancement is less effective because of the increased pressure drop that leads to a reduction of the coolant flow rate and velocity.
Date: January 1, 1995
Creator: Rabas, T. J. & Taborek, J.
Partner: UNT Libraries Government Documents Department

Fouling and thermal-performance characteristics of the Humboldt Bay Unit 2 power-plant condenser

Description: An experimental program was conducted at the Humboldt Bay condenser using eight clusters of four neighboring tubes with different conditions. In each cluster, there were (1) a new tube, the tubeside fluid being distilled water; (2) a new tube, the tubeside fluid being plant circulating water (seawater) and no cleaning; (3) an old tube, plant circulating water with no cleaning; and (4) an old tube, plant circulating water with normal periodic manual cleaning (blowing plugs or sponge balls). These tube clusters were located at four different locations within both the first and second passes of this two-pass condenser. Because of the different conditions, the fouling and cleaning characteristics can be obtained with measurements of the flow rate and inlet, outlet, and saturation temperatures. In addition to the fouling data, the thermal performance can be compared to that obtained with the standard rating method. There was a reduction in the thermal performance of the new, distilled-water tubes for about the first 80 days, and then the performance remained essentially constant. This performance change was most likely the result of the change from dropwise to filmwise condensation on the 7/8-in OD, 18 BWG Admiralty tubes. There was a continued increase of the tubeside fouling resistance with time (no asymptotic behavior) for the tubes that were not cleaned in each cluster using the plant circulating water. The fouling rates were somewhat larger for the first or lower temperature pass initially for the new tubes and after about 100 days for the old tubes. However, the fouling resistance values were substantially larger for the old tubes.
Date: July 1, 1993
Creator: Rabas, T. J. & Elliott, E. S.
Partner: UNT Libraries Government Documents Department

The influence of the Prandtl number on the thermal performance of tubes with the separation and reattachment enhancement mechanism

Description: This paper demonstrates that the heat-transfer performance of an enhanced tube with transverse, rectangular disruptions can be predicted with a numerical modeling method, an accomplishment not previously achieved. This computer code is then used to determine the influence of the Prandtl number. The numerical simulation demonstrated that six distinct regions exist: the three rib surfaces, the upstream and downstream recirculation regions, and the boundary-layer development zone. Three zones dominate the thermal performance: the rib top and downstream faces and the downstream recirculation zone. The thermal performance at the rib region begins to dominate tile overall performance as the Prandtl number becomes large. The contribution from the downstream recirculation zone becomes more important and dominates for low Prandtl number fluids such as air. The Reynolds number dependence at the rib region and the downstream recirculation zone is similar to that for reattaching flows with exponents in the 0.65 to 0.75 range. The location of the maximum in the recirculation moves closer to the rib with increasing Reynolds and Prandtl numbers and is bounded upstream by the location of the maximum wall shear stress and downstream by the reattachment length. The high turbulence level near the surface in this region is responsible for the heat-transfer enhancement.
Date: August 1, 1992
Creator: Rabas, T. J. & Arman, B.
Partner: UNT Libraries Government Documents Department

Disruption shape effects on the performance of enhanced tubes with the separation and reattachment mechanism

Description: A non-orthogonal, body-fitted numerical code is used to determine the thermo-hydraulic performance of enhanced tubes with transverse periodic sine-, semicircle-, arc-, and trapezoid-shaped disruptions. The turbulence closure was achieved with a two-layer turbulence model. It is shown that there is a tradeoff of the heat-transfer and pressure-drop performances when the disruption shape becomes more contoured; that is, both the heat transfer and the pressure drop increase. The local heat transfer is strongly dependent on the shape in the vicinity of the disruption but it is less dependent in the downstream recirculation region and in the boundary layer development zone. With increasing pitch, effect of the shape on the heat-transfer performance becomes less important. The pressure drop is more dependent on the disruption shape and it continues to decrease when the disruptions become less contoured because of the reduced form drag which is by far the major contribution to the total pressure drop. 27 refs.
Date: August 1, 1992
Creator: Arman, B. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Influence of Prandtl number and effects of disruption shape on the performance of enhanced tubes with the separation and reattachment mechanism

Description: The pressure-drop and heat-transfer performance of an enhanced tube with transverse disruptions can be predicted with a numerical modeling method, an accomplishment not previously achieved. Two computer codes were employed to achieve this goal - an orthogonal code and a nonorthogonal, body-fitted code. The turbulence closure was achieved with a two-layer turbulence model. The orthogonal computer code was used to determine the influence of the Prandti number. The numerical simulations demonstrated that six distinct regions exist and that three zones dominate the thermal performance. The nonorthogonal, body-fitted numerical code was used to determine the thermohydraulic performance of enhanced tubes with transverse, periodic sine-, semicircle-, arc-, and trapezoid-shaped disruptions. The research showed that there was a trade-off between the heat-transfer and pressure-drop performances when the disruption shape becomes more contoured, and that the local heat transfer is strongly dependent on the shape in the vicinity of the disruption, but it is less dependent in the downstream recirculation region and in the boundary-layer development zone.
Date: April 1, 1992
Creator: Arman, B. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Performance comparisons of enhanced tubes with discrete and wavy disruption shapes

Description: This paper presents comparisons of the friction factors and heat-transfer coefficients obtained with enhanced tubes with transverse discrete and almost transverse wavy two-dimensional disruptions. Both experimental data and numerical predictions were used for the comparisons. For the latter a two-layer turbulence model incorporated in a body-fitted, finite-volume method was used. The disruption shape, discrete or wavy, depends on the manufacturing process. If an extrusion process is used, discrete disruptions (ribs) of various profiles are obtained that are separated from each other by a flat or unaltered inside diameter. If a spirally indenting process is used, a wavy proflie is obtained with a continuously varying inside diameter between two adjacent disruption peaks. These disruptions are transverse or almost transverse to the tube axis and separated by a distance that exceeds the reattachment length. Based on these comparisons, the following conclusions are obtained: (1) the disruption shape is not an important correlating parameter for discrete disruptions, (2) only the friction factor is influenced by the shape for wavy disruptions, and (3) there are major differences between both the friction-factor and heat-transfer performance of discrete and wavy disruptions with the same maximum disruption height and spacing. However, the most important finding is that the groove radius of spirally indented tubes should be increased because of the substantial reduction of the friction factor but only a small decrease in the thermal performance. Additional comparisons of predicted results were made to obtain a fundamental understanding of the influence of these different shapes.
Date: August 1, 1993
Creator: Arman, B. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Smooth- and enhanced-tube heat transfer and pressure drop : Part I. Effect of Prandtl number with air, water, and glycol/water mixtures.

Description: An extensive experimental investigation was carried out to determine the pressure drop and heat transfer characteristics in laminar, transitional, and turbulent flow through one smooth tube and twenty-three enhanced tubes. The working fluids for the experiments were air, water, ethylene glycol, and ethylene glycol/water mixtures; Prandtl numbers (Pr) ranged from 0.7 to 125.3. The smooth-tube experiments were carried out with Pr values of 0.7, 6.8, 24.8, 39.1, and 125.3; Pr values of 0.7, 6.8, and 24.8 were tested with enhanced tubes. Reynolds number (Re) range (based on the maximum internal diameter of a tube) was 200 to 55,000, depending on Prandtl number and tube geometry. The results are presented and discussed in this paper.
Date: November 14, 2000
Creator: Obot, N. T.; Das, L. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Smooth- and enhanced-tube heat transfer and pressure drop : Part II. The role of transition to turbulent flow.

Description: The objectives of this presentation are two-fold: first, to demonstrate the connection between the attainable coefficients and transition to turbulent flow by using the transition-based corresponding states method to generalize results obtained with smooth tubes and enhanced tubes, and second, to provide guidelines on the calculation of heat transfer coefficients from pressure-drop data and vice versa by using the transition concept or the functional law of corresponding states.
Date: November 14, 2000
Creator: Obot, N. T.; Das, L. & Rabas, T. J.
Partner: UNT Libraries Government Documents Department

Reflux condensation of pure vapors with and without a noncondensable gas inside plain and enhanced tubes

Description: Estimates of the surface-area and vapor-release reductions are obtained when commercially available enhanced tubes (spirally ribbed) replace plain tubes in a reflux unit condensing pure organic vapors with different concentrations of a noncondensable gas. This investigation was undertaken because there are no existing data and/or prediction methods that are applicable for these shell-and-tube condensers commonly used in the process industries. To obtain these estimates, existing design methods published in the open literature were used. The major findings are that (1) surface-area reductions can almost approach the single-phase heat transfer enhancement level, and (2) vapor-release reductions can approach a factor of four. The important implication is that enhanced tubes appear to be very cost effective for addressing the recovery of volatile organic vapors (VOCs), and for a vast number of different reflux-condenser applications.
Date: June 1, 1997
Creator: Abdelmessih, A.N.; Rabas, T.J. & Panchal, C.B.
Partner: UNT Libraries Government Documents Department