Integral-fin Tubes Research Articles

CONDENSATION PERFORMANCE OF SOME COMMERCIAL INTEGRAL FIN TUBES WITH STEAM AND CFC113

Integral fin tubes are used in some shell-side condensers to enhance the vapor-side heat transfer coefficient. Advanced manufacturing techniques have meant that tubes with complex fin profiles can be produced. In the present article, experimental heat transfer coefficients are presented for 12 commercially manufactured integral fin tubes, with a variety of fin profiles and dimensions, for condensation of CFC113 and steam. Best enhancement ratios of 3·3 for steam and 7·7 for CFC113 were found, for tubes with trapezoidal-cross-section fins and 26 and 50 fins per inch, respectively. The more complex Y-and T-profile finned tubes performed less well than the best of those with conventional approximately trapezoidal-cross-section fins. The results show good agreement with a recent model.

Shell-Side Boiling in Flooded Refrigerant Evaporators Part I: Integral Finned Tubes

Convective vaporization data were obtained on a staggered tube bank made of 18.9 mm (0.75 inch) outside diameter integral fin tubes (1024 fins/m; 26 fins/inch) with an equilateral pitch-to-diameter ratio of 1.25. Data are presented for pure Refrigerant-11 with saturation temperatures of 4.4°C (40°F) and 26.7°C (80°F). The data span vapor qualities of 0 to 0.9, mass velocities (based on minimum flow cross section area) of 7 to 18 kg/m2·s (5,200 to 13,300 lb/h·ft2), and heat fluxes of 15 to 45 kW/m2 (4,800 to 14,300 Btu/h·ft2). The data were taken in an apparatus that allows independent control of vapor quality, mass velocity, and heat flux. In contrast to the data presented on enhanced tubes in Part 2, the convective effects strongly influence the integral finned tube performance. The Chen superposition model (with or without using a suppression factor) largely under-predicted the data. The data were correlated in the form of an asymptotic model. The correlations fit 92% of the data within ±20%.

Effect of fin efficiency on a model for condensation heat transfer on a horizontal, integral-fin tube

A semi-empirical model for condensation on horizontal, integral-fin tubes has been adapted to account for ‘fin efficiency effects’. Specimen calculations have been made to investigate the effect of tube geometry and material on the enhancement ratio for condensation of steam and CFC11. The best fin spacing was found to be only weakly dependent on the other geometric variables and fin thermal conductivity. The best fin thickness was more strongly dependent on fin thermal conductivity. For the refrigerant the optimum fin thickness was smaller than presently used in practice. The model gave satisfactory agreement with experimental data for CFC113 and steam for typical fin geometries. In the case of CFC113 the enhancement ratio was almost independent of fin thermal conductivity for conductivities exceeding around 50 W m −1 K −1.

An approximate equation for the vapour-side heat-transfer coefficient for condensation on low-finned tubes

Simplifying approximations, together with dimensional analysis, have been used to obtain a formula for the relation between the heat flux and vapour-side temperature difference for condensation on low, integral-finned tubes. The final result involves two unknown (disposable) constants which have been determined from heat-transfer data. The resulting equation is found to be in satisfactory agreement with experimental data from 11 investigations with various condensing fluids and a range of fin and tube geometries.

Convective Vaporization of Pure Refrigerants in Enhanced and Integral-Fin Tube Banks

Convective Vaporization of Pure Refrigerants in Enhanced and Integral-Fin Tube Banks

Nucleate pool boiling data for five refrigerants on plain, integral-fin and enhanced tube geometries

Data are presented for nucleate pool boiling on five different horizontal tube geometries using five refrigerants at two saturation temperatures. The refrigerants tested are R-11, R-12, R-22, R-123 and R-134a at saturation temperatures of 4.44°C (40°F) and 26.7°C (80°F). The tube geometries tested are a plain tube, a 1024 fins/m integral-fin tube, and three commercially used enhanced tube geometries (GEWA TX19, GEWA SE, and Turbo-B). The wide range of data reported here is new to the literature. The ability of the Cooper and the Stephan-Abdelsalam correlations to predict the plain tube data is evaluated. The slopes of the boiling coefficient vs heat flux curve are found to be heat flux dependent. Of particular importance are the data on R-123 and R-134a, which are intended to replace R-11 and R-12, respectively. Except for the Turbo-B with R-11, it appears that the boiling coefficients for R-123 and R-134a are within 10% of the values for R-11 and R-12, respectively.

Accurate Heat Transfer Measurements for Condensation on Horizontal, Integral-Fin Tubes

In most earlier experimental investigations of condensation on low-fin tubes, vapor-side heat transfer coefficients have been found from overall (vapor-to-coolant) measurements using either predetermined coolant-side correlations or “Wilson plot” methods. When the outside resistance dominates, or is a significant proportion of the overall resistance, these procedures can give satisfactory accuracy. However, for externally enhanced tubes, and particularly with high-conductivity fluids such as water, significant uncertainties may be present. In order to provide reliable, high-accuracy data, to assist in the development of theoretical models, tests have been conducted using specially constructed plain and finned tubes fitted with thermocouples to measure the tube wall temperature, and hence the vapor-side heat transfer coefficient, directly. The paper describes the technique for manufacturing the tubes and gives results of systematic heat transfer measurements covering the effects of fin height, thickness, and spacing, tube diameter, and vapor velocity. The tests were carried out with steam, ethylene glycol, and R-113, with vertical vapor downflow. The heat flux was measured using an accurately calibrated 10-junction thermopile and paying particular attention to coolant mixing and isothermal immersion of thermocouple junctions. Care was taken to avoid errors due to the presence in the vapor of noncondensing gas and the occurrence of dropwise condensation. Smooth, consistent, and repeatable results were obtained in all cases. The data are presented in easily accessible form and are compared with the results of previous investigations, where indirect methods were used to determine the vapor-side data, and with theory.

PREDICTION OF CONDENSATION HEAT TRANSFER COEFFICIENT IN HORIZONTAL INTEGRAL-FIN TUBE BUNDLES

A method of predicting the condensation heat transfer coefficient in horizontal integral-fin tube bundles with negligible vapor shear is proposed and compared with experimental results. This method is based on an analytical model talcing the condensate inundation effect into account, where the condensation heat transfer coefficient decreases due to a decrease in effective fin area as condensate flows through the interfin spaces. Heat transfer measurements were carried out in eight-row tube bundles consisting of integral-fin tubes with 35, 26, and 16 fins/in. Refrigerants Rll and R114 were used as test fluids. The proposed method was able to predict the condensation heat transfer coefficient with enough accuracy to enable condensers to be designed without the need for complex calculations.

CONDENSATION HEAT TRANSFER ENHANCEMENT BY RIDGES ON INTEGRAL FIN SURFACES

A finned tube having ridges on the fin surface, called an R-tube, was investigated using R113 as a condensing medium. Heat transfer performance, including a local heat transfer coefficient along a circumferential direction, for the horizontal R-tube was compared with that of an integral finned tube and a commercial Thermoexcel-C tube produced by Hitachi, Ltd Because the ridges increase the thin condensate film areas on the fin surface, the mean condensation heat transfer coefficient of the R-tube was the highest among the tubes tested and was two times that of the integral finned tube. From measurements of the circumferential distribution of the heat flux, the local heat flux was highest at the top of the tube, but has a second peak approximately φ ≥ 110° from the top.

An Experimental Study of R-113 Film Condensation on Horizontal Integral-Fin Tubes

Heat transfer measurements were made at near-atmospheric pressure on a smooth tube, on 24 integral-fin tubes having machined, rectangular-shaped fins, and on a commercial integral-fin tube. All tubes were made of copper. The vapor flowed vertically downward with a nominal velocity of 0.4 m/s. Vapor-side heat transfer coefficients were determined with a typical uncertainty of ± 7 percent using a “modified Wilson plot” technique. The vapor-side heat transfer coefficient of the integral-fin tubes (based upon the outside surface area of the smooth tube) was enhanced considerably more than the surface area enhancement provided by the fins. Heat transfer enhancements (for the same vapor-to-wall temperature difference) up to around 7 were measured for a corresponding area enhancement of only 3.9. The optimum fin spacing was found to lie between 0.2 and 0.5 mm, depending upon fin thickness and height. The data were compared with those of other investigations and with several existing theoretical models. Visual observations of condensate drainage patterns from the finned tubes were also made.

Prediction of film condensation on horizontal integral fin tubes

This paper presents an analytical model for prediction of film condensation on horizontal integral fin tubes. The present model accounts for condensation on all surfaces in the flooded and unflooded regions, and includes the effect of fin efficiency. However, it is based on quite simple principles, and is applicable to hand or computer calculations. The model is applicable to fins of two different basic profile shapes: the special continuous profile shapes previously described by Gregorig, Adamek, or Webb, or fins having rectangular or trapezoidal cross-sections. The model is validated by showing its ability to predict a wide range of experimental data. The data include water, methanol, n-pentane, R-ll, R-12, R-22 and R-113. These data for 80 different tube geometries are predicted within ± 15%. Predictions by the present model are compared with models previously proposed by Webb et al., Honda and Nozu, and Beatty and Katz.

Film Condensation of R-11 Vapor on Single Horizontal Enhanced Condenser Tubes

The heat transfer performance of R-11 vapor condensing on single horizontal trapezodially shaped integral-fin tubes has been investigated by systematically varying the fin density, the semi-vertex angle, and the fin height. For the nine copper tubes tested, the best performance has been obtained with a tube having a fin density of 1417 fpm, a semi-vertex angle of 10 deg, and a fin height of 1.22 mm. This tube has yielded a maximum value of the heat transfer coefficient of 16,500 W/m2 K at a ΔT of about 3 K, corresponding to an enhancement ratio of 10.3. The performance of the tube has been further improved by fabricating from it “specially enhanced” tubes having axial grooves of varying height. An enhancement ratio of 12.3 has been obtained with this type of tube.

Experimental study on condensation heat transfer enhancement by various kinds of integral finned tubes.

Condensation on the outer surface of an integral finned tube, Thermoexcel-C, and a finned tube having ridges on the fin surface (called the R-tube) was investigated by using R 113 as a condensing medium. Heat transfer performances including a local heat-transfer coefficient in a circumferential direction were compared. The mean hcat-transfer coefficient of the R-tube was 30 %-40 % higher than that of Thermoexcel-C, and two times that of the integral finned tube. Since the ridges on the fin surface of the R-tube enlarged thin condensate film area in the upper part of the tube and promoted a convective heat transfer in the condensate film in the lower part : the heat-transfer performance of the R-tube was the best.

An Evaluation of Film Condensation on Horizontal Integral-Fin Tubes

This paper reviews nearly fifty years of progress that has been made in our understanding of film condensation on horizontal, integral-fin tubes. Both experimental and theoretical developments pertaining to single tubes and tube bundles are discussed. Special attention is given to the importance of surface tension, and appropriate research directions are provided.

Enhancement of film condensation

Since 1981, significant advances have been made in understanding the mechanism of enhanced film condensation on finned surfaces - for plates and horizontal tubes. The key advances involve understanding the role that surface tension force plays in draining the condensate from the fins, and in retaining condensate within the interfin region of finned tubes. This understanding has culminated in the development of analytical models for predicting the condensation rate on horizontal integral fin tubes and on banks of integral fin tubes. The models have been validated for both low and high surface tension fluids. In addition, data have been obtained to established the “row effect” on several high performance integral fin tube geometries. High performance fin geometries have been identified, as well as techniques to model currently used fin geometries. Models have been developed to predict condensate retention, and to account for the effect of drainage strips. This paper summarizes the key advances that have occurred since 1981.

A Prediction Method for Heat Transfer During Film Condensation on Horizontal Low Integral-Fin Tubes

A method for predicting the average heat transfer coefficient is presented for film condensation on horizontal low integral-fin tubes. Approximate equations based on the numerical analysis of surface tension drained condensate flow on the fin surface are developed for the heat transfer coefficients in the upper and lower portions of the flooding point below which the interfin space is flooded with condensate. For the unflooded region, the equation is modified to take account of the effect of gravity. These equations are used, along with the previously derived equation for the flooding point, to determine the wall temperature distribution, and in turn the average heat transfer coefficient. It is shown that the present model can predict the average heat transfer coefficient within ±20 percent for most of the available experimental data including 11 fluids and 22 tubes.

A generalized prediction method for heat transfer during film condensation on horizontal low integral-fin tubes.

A theoretical model for film condensation on a horizontal low integral-fin tube proposed in a p. revious p. ap. er is extended to include the effects of condensate flow and heat transfer at the fin root tube surface. Based on the numerical results, approximate expressions for the circumferential condensate flow rate and the heat transfer rate at the fin root tube surface are proposed. These expressions are incorporated into a set of algebraic equations which are solved simultaneously to obtain the local wall temperatures and the local heat transfer rates at the fin surface and at the fin root tube surface of the flooded and unflooded regions of the tube. The calculated values of the average heat transfer coefficient compared well with the available experimental data.

Horizontal Plain and Low-Finned Condenser Tubes—Effect of Fin Spacing and Drainage Strips on Heat Transfer and Condensate Retention

The paper reports a continuation of an experimental investigation of the effect of fin pitch on the heat transfer performance of horizontal, integral-fin tubes for condensation of steam at near-atmospheric pressure. The effects of “drainage strips” located along the lower edge of finned and plain tubes have been studied. These gave significant increases in the heat transfer coefficient for finned tubes but had only marginal effect for the plain tube. Condensate retention angles have also been measured for simulated condensation using water, ethylene glycol, and refrigerant-113 for finned tubes with and without drainage strips. In the latter case the data agreed satisfactorily with theory. Drainage strips were found to reduce the extent of holdup significantly.

Recent Progress in Enhancing Film Condensation Heat Transfer on Horizontal Tubes

An assessment of several promising techniques that use surface tension forces to enhance film condensation heat transfer on horizontal tubes is made. Recent progress on integral-fin tubes is stressed, including experimental findings on fin spacing, geometry, and material and the latest theoretical developments for predicting performance. Condensation heat transfer enhancement on wire-wrapped tubes is also examined, as well as the use of nonwetting strips and porous drainage devices. The effect of condensate inundation on plain and enhanced tubes is reviewed briefly, and future research directions are discussed.

An Analytical Model to Predict Condensate Retention on Horizontal Integral-Fin Tubes

In this paper, the authors develop a general analytical model to predict the amount of surface that is flooded during condensation on a horizontal, integral-fin tube. The model is based on capillary equations that predict the amount of liquid rise on a vertical u-shaped channel. The space between adjacent integral fins forms such a channel. The authors compare the model to test data taken during condensation on three integral-fin tubes (748-to-1378 fpm) and a range of fluid properties. The analytical model predicts the amount of liquid retention on a horizontal, integral-fin tube within ± 10 percent over most of the data. The analysis is performed for the case of negligible vapor shear.