Showing papers in "Heat Transfer Engineering in 1993"

Problems and Costs due to Heat Exchanger Fouling in New Zealand Industries

Abstract: Data are reported on the perception of fouling in New Zealand based on 200 companies and the application of nearly 2000 heat exchangers These data are discussed with respect to their occurrence for various heat exchanger applications, industries, and heat exchanger types The fouling mitigation strategies applied in New Zealand are depicted and compared with the state of the art in fouling mitigation as found from an extensive literature search Some comments are made as to the heat exchanger cleaning frequencies in New\Zealand industries Finally, an attempt is made to provide a reasonably reliable estimate of the annual fouling-related costs in New Zealand These results are then compared with those previously reported from the United States and England

Optimum Design of Longitudinal Rectangular Fins and Cylindrical Spines with Variable Heat Transfer Coefficient

Abstract: An integral approach is extended to determine the optimum dimensions for rectangular longitudinal fins and cylindrical pin fins by incorporating transverse heat conduction. The effects of variable heat transfer coefficient and heat transfer from the fin tip are also taken into account. The mathematical expression for optimum dimension ration thus obtained is found to be as simple as that of the classical one-dimensional approach, but the accuracy of numerical results is improved significantly. The same approach is then utilized for solving for the optimum dimensions of a cylindrical pin fin. To facilitate thermal designers, numerous design charts are presented according to the nature of the flow, the boundary condition at the fin tip, and the types of heat transfer under consideration.

Analysis of Performance of Steam Condensation Heat Transfer and Pressure Drop in Plate Condensers

Abstract: This article describes experimental study and calculation on steam condensation heat transfer and pressure drop in a plate condenser. Based on the results, it presents a formula expressing the relation of condensation heat transfer and having obuious physical meaning. Several important points affecting steam condensation heat transfer in plate condensers are analyzed. The Lockhart-Martinelli model is extended to two-phase nonadiabatic flow for calculation of frictional pressure drop in plate condensers.

Aerodynamics and Heat and Mass Transfer of Fluidized Particle Beds in Vortex Chambers

Abstract: This article is devoted to the results of studying the heat and mass transfer pr“particle-gas” systems proceeding in a bed fluidized in the field of centrifugal force. It has been shown that the processes are essentially intensified under these conditions as compared to those in fluidized beds. Under fluidization in the field of centrifugal force it is possible to obtain a steady uniform vortical layer of particles, which is uniformly blown through by gas with relative velocities exceeding the velocity of the particle swirling in a carrying medium. There exists a possibility of controlling this layer and the residence time of particles This article presents the results of experimental and computational studies of aerodynamics and heat and mass transfer of vortex chambers with a fluidized bed of particles. The peculiarities of the particle bed formation depending on the bed mass, dispersed composition, aerodynamics, and geometric characteristics of vortex chambers have been analyzed. The study of the heat ...

Comparison of River-Water Fouling Rates for Spirally Indented and Plain Tubes

Abstract: This article presents river-water fouling rates at 12 Tennessee Valley Authority (TVA) power plant condensers, nine of which were retubed with a commercially available enhanced tube. Four were located on the Cumberland River, six on the Ohio, and two on the Clinch River. The overall heat transfer coefficients were calculated from logged field data taken over periods extending from 1 to 10 years. The fouling resistances were next calculated with the separate resistance method and with a bundle correction factor to the condensing, single-tube Nusselt prediction. The bundle correction factor was determined for each condenser using the data taken within 1000 hours after each cleaning. With the use of this new bundle factor method, fouling rate data can be obtained even with variable operating conditions. The fouling rates with the enhanced tubes ranged from about the same as to about twice that of the plain tubes. However, the thermal performance with the enhanced tubes remained superior to that obta...

Effect of Height and Geometry on Local Heat Transfer and Pressure Drop in a Channel with Corrugated Walls

Abstract: Local heat transfer and pressure drop were measured from the corrugated portion of the bottom wall of a rectangular channel where the opposite top wall took one of the three following constructions: (1) plane; (2) couered with corrugations making an in-phase arrangement with the bottom wall; (3) couered with corrugations making an out-of-phase arrangement with the bottom wall. There were a total of seven corrugations, and the corrugation geometry was sinusoidal. Measurements were performed for three different aspect ratios (corrugation height / channel height), and Reynolds numbers ranged from 22,000 to 52,000 in the turbulent region. Heated-coating liquid crystal technique was used for temperature measurements. The highest local Nusselt number was detected on the second corrugation regardless of Reynolds number, aspect ratio, or geometry. Higher Reynolds numbers and higher aspect ratios had similar effects, increasing local heal transfer coefficient. At the higher Reynolds numbers and with highe...

The effects of internal heat generation, anisotropy, and base temperature nonuniformity on heat transfer from a two-dimensional rectangular fin

Abstract: A finite-element approach is used to study the two-dimensional performance of a straight fin of rectangular profile operating in a convecting environment. The analysis includes the effects of internal heat generation, anisotropy of fin material, base temperature elevation (depression), and unequal heat transfer coefficients for tip and main convection surfaces. Numerical results are presented showing the impact of these effects on the heat removal capability of the fin. The graphical information provided is believed to be useful for predicting the performance of a two-dimensional rectangular fin under more general and realistic conditions.

Sensible and Latent Heat Transfer to a Baffled Finned-Tube Heat Exchanger

Abstract: A straightforward heat exchanger thermal model is presented and verified experimentally. It may be used to predict the performance of a coil that has condensation occurring on the gas side. Experimentally determined heat and mass transfer coefficients are employed in the model Sample data are presented for a wide range of surface wetting conditions. Partially wet heat exchangers are analyzed by employing an area weighting scheme to the dry and wet portions of the coil with moderate success. It is shown that employing a heat / mass transfer analogy in predicting the performance of a condensing heal exchanger may lead to a considerably undersized coil design.

Flow Losses Through an Array of A-Frame Heat Exchangers

Abstract: The isothermal flow losses through an array of A-frame finned tube heat exchangers are determined experimentally. Correlations that describe these losses are derived from scale-model tests conducted on different V-bundle configurations, each simulating a section of a typical A-frame array. The experimental data are then compared with test data obtained from a full-scale heat exchanger installation. It is found that the downstream flow losses behind a V-bundle section are significantly influenced by the semiapex angle, the process stream duct diameter, and the distance between the A-frames.

Some Essential Principles for Adjustment of Seal Clearances in Rotary Regenerators

Abstract: In the present study, attention is paid to some indispensable principles for making a useful adjustment of seal clearances in a rotary regenerator, illustrated by means of examples. First, the effect of leakages and their distribution within such a heat exchanger on the efficiency of a steam power plant is presented. In this way it is shown that not only total leakage but also the distribution of the leakages within the rotary regenerator have an important influence on the efficiency. An interesting conclusion appears here—reduction in total leakage may even bring about a drop in power plant efficiency if distribution of leakages changes unfavorably. Second, in order to show in which rotor state seal clearances should be adjusted, appropriate examples of experimental results concerning radial seal clearances in the hot and cold rotor state are compared. Next, on the basis of a leakage network and some experimental data before and after reduction in radial seal clearances, the effect of reduction ...

Thermal Performance of Advanced Heat Exchangers for Ammonia Refrigeration Systems

Abstract: Current practice in the ammonia industry is to use plain tubes. This article demonstrates that the coefficient of performance (COP) can be increased by about 11% using tubes with ammonia-side enhancement and by about 26% using tubes with ammonia-side and water-side enhancement. Further improvements with respect to energy and cost are possible with the compact heat exchanger types—plate-frame and plate-fin—because they can be optimized for the system conditions for each application. These findings are based on experimentally determined overall heat transfer coefficients that were obtained for candidate heat exchangers for the Ocean Thermal Energy Conversion (OTEC) program.

Predicting the performance of multistage heat exchangers

Abstract: A simple method for predicting the thermal performance of multistage heat exchangers is proposed. The technique involves an area weighting scheme of the reseated Colburn J factors. The method is developed for, and probably limited to, applications with indirect contact, direct transfer heat exchangers operated in series. However, condensing heat exchangers are amenable to this technique, and thus it offers more flexibility and wider applicability than previously proposed methods. Experimental data for a two-stage, condensing heat exchanger are reported, and the behavior is compared to the predictive scheme with moderate success. While the behavior of multistage heat exchangers may be affected by interstage spacing, the data from the experiments do not allow quantification of the impact of this effect.

The Effect of Core Longitudinal Heat Conduction on the Transient Behavior of Multipass Shell-and-Tube Heat Exchangers

Abstract: This article presents a method for predicting the transient responses to arbitrary inlet temperature changes in multipass shell-and-tube heat exchangers, considering the longitudinal heat conduction in the wall core as well as the thermal capacities of both fluids and the wall. The final solution is obtained by means of numerical inversion of the Laplace transform. The effect of longitudinal heat conduction on transient behavior of heat exchangers is discussed.

The Influence of Height Above Sea Level on the COP of Air-Source Heat Pumps Used for Water Heating

Abstract: In some countries (e.g., South Africa), important market areas for air-source heat pumps are situated high above sea level, and it is therefore necessary to know what the influence of height is on the performance of heat pumps for water healing. Therefore, a study was conducted that predicts the influence of height on the coefficient of performance (COP) of air-source heat pumps. An expression was derived that gives the heat transfer to the evaporator of a heat pump as a junction of air density and enthalpy as well as the outside heat transfer coefficient. This expression was used to predict the influence of height above sea level on COP. The results were verified by means of a simulation study. It was concluded that height has little influence on COP if the evaporator of the heat pump is wet on the air side. A more significant weakening in the performance occurs, however, when the air side of the evaporator is dry.

Empirical Evaluation of Diving Wet Suit Material Heat Transfer and Thermal Conductivity

Abstract: This wet suit material testing program provides a quantitative thermal conductivity and heat transfer analysis, and comparison of various materials used in skin diving and SCUBA diving. Thermal resistance represents the primary subject examined, but due to compressibility of the baseline materials and its effect on heat transfer, this program also examines compression at simulated depth. This article reports the empirical heat transfer coefficients for both thermal conductivity and convection. Due to the limitations of the test apparatus, this analysis must restrict the convection evaluation to an approximately 20-cm-height, free-convection model. As a consequence, this model best simulates the overall heat transfer coefficient of a diver hovering in a horizontal position. This program also includes evaluations of some nonstandard materials in an effort to identify alternative wet suit materials.

Field Testing of a Probe to Measure Fouling in an Industrial Flue Gas Stream

Abstract: In order to design an efficient heat exchanger accurately, a better understanding is needed regarding the foulants that are deposited on the heat transfer surfaces of heat exchangers to recover energy from hot flue gases in industrial furnaces. This article briefly describes the design and fabrication of a gas-side fouling measuring device, and its testing in an industrial environment. It describes probe fabrication, instrumentation used for recording the data, field test location, tests performed, and data analysis. The conclusions of the tests performed are summarized. Although fouling deposits on the probe were minimal, the tests proved that the probe is capable of measuring fouling in a harsh industrial environment.

Parametric Effects on the Substrate Temperature Profile in Oxy-Acetylene Flames

Abstract: In the oxy-acetylene combustion method, the quality and morphology of the diamond film depends on a variety of factors, including the substrate surface temperature, gas flow rates and their ratio, and substrate position. Due to the nature of the oxy-acetylene flame, surface temperature of the substrate varies in the radial direction. The substrate temperature profile, in turn, is influenced by the coolant flow rate, nozzle size, substrate position on the heat sink, substrate position in the combustion flame, and the total flow rate of mixed gas (acetylene and oxygen). In this article, temperature measurements using fine-gauge thermocouples were made to characterize the influence of these parameters on the substrate temperature profile.

A Survey of Heat Exchanger Applications in New Zealand Industries

Abstract: To provide information on application and operational problems of heat exchangers in New Zealand, a comprehensive questionnaire was sent to 3500 potential users of heat transfer equipment. In this report, the method of investigation is described and heat exchanger data are presented. These data are then discussed with respect to their occurrence for various heat exchanger applications, industries, and heat exchanger types.

An Analysis of the Heat Transfer Characteristics of the Fin/Refractory System in a Cyclone Furnace

Abstract: Cyclone furnaces have been in use for steam generation for over 45 years. The tubes that form the rear wall of the cyclone furnace, called the reentrant throat, are prone to warping, which results in gaps between the tubes. These gaps should not be present, as they allow fuel to escape from the furnace without burning. The colloq“picket fence,” is used to describe these gaps because of their resemblance to a wooden picket fence. Many gap-filling techniques have been utilized over the years, but none has completely solved the problem of recurring gaps. An investigation, both theoretical and experimental, has been conducted to determine the heat transfer characteristics of the fin / refractory system used to fill these gaps. Ample evidence suggests that thermal stresses in the weld joint between the fin and the tubes contribute to the failure of the fins.