Showing papers on "NTU method published in 1995"
Book•
20 Oct 1995
TL;DR: In this paper, the three-part structure of Heat Exchanger Irreversibility has been studied in the context of solar-thermal power generation, showing that the three parts of the structure of a two-phase-flow heat exchanger can be used to estimate the entropy generation rate.
Abstract: List of Symbols Thermodynamics Concepts and Laws Definitions Closed Systems Open Systems The Momentum Theorem Useful Steps in Problem Solving The Temperature-Energy Interaction Diagram, and the Entropy Interaction-Energy Interaction Diagram Problems Entropy Generation and Exergy Destruction The Gouy-Stodola Theorem Systems Communicating with More than One Heat Reservoir Adiabatic Systems Exergy Analysis of Steady Flow Processes Exergy Analysis of Non-Flow Processes Characteristic Features of Irreversible Systems and Processes Problems Entropy Generation in Fluid Flow Relationship between Entropy Generation and Viscous Dissipation Laminar Flow Turbulent Flow The Transition Buckling Theory of Turbulent Flow Entropy Generation in "Isothermal" Turbulent Flow The Bernoulli Equation Entropy Generation in Heat Transfer The Local Rate of Entropy Generation in Convective Heat Transfer Fluid Friction vs. Heat Transfer Irreversibility Internal Flows External Flows Conduction Heat Transfer Convective Mass Transfer General Heat Exchanger Passage Heat Transfer Augmentation Techniques Problems Heat Exchangers Counterflow Heat Exchangers Heat Exchangers with Negligible Pressure Drop Irreversibility The Three-Part Structure of Heat Exchanger Irreversibility Two-Phase-Flow Heat Exchangers Other Heat Exchanger Entropy Generation Studies Distribution of Heat Exchanger Area on the Absolute Temperature Scale Distribution of Heat Transfer Area in Counterflow Heat Exchangers Problems Insulation Systems Power Plants and Refrigeration Plants as Insulation Systems The Generation of Entropy in an Insulation with Fixed Geometry Optimum Continuous Cooling Regime Counterflow Heat Exchangers as One-Dimensional Insulations Parallel Insulations Intermediate Cooling or Heating of Insulation Systems for Power and Refrigeration Plants Problems Storage Systems Sensible Heat Storage Optimum Heating and Cooling Processes Subject to Time Constraint Hot Storage vs. Cold Storage Latent Heat Storage Power Generation Model with Bypass Heat Leak and Two Finite-Size Heat Exchangers Power Plant Viewed as an Insulation Between Heat Source and Ambient Combined-Cycle Power Plant Optimal Combustion Chamber Temperature Other Power Plant Optimization Studies Why Maximum Power Means Minimum Entropy Generation Rate Maximum Power from Fluid Flow Problems Solar-Thermal Power Generation Models with Collector Heat Loss to the Ambient Collector-Ambient Heat Loss and Collector-Engine Heat Exchanger Collector-Ambient Heat Loss and Engine-Ambient Heat Exchanger Storage by Melting Extraterrestrial Solar Power Plant Nonisothermal Collectors Time-Varying Conditions Other Areas of Solar Power Conversion Study Problems Refrigeration Refrigeration Plant Model with Heat Transfer Irreversibilities Model with Heat Leak in Parallel with Reversible Compartment Model with Cold End Heat Exchanger and Room Temperature Heat Exchanger Minimization of the Heat-Leak Entropy Generation Problems Time-Dependent Operation Defrosting Refrigerators Cleaning the Heat Exchanger of a Power Plant Power Plants Driven by Heating from a Bed of Hot Dry Rock Maximum Rate of Ice Production Problems Appendices Local Entropy Generation Rate Variational Calculus Author Index Subject Index
848 citations
TL;DR: In this paper, the authors report the thermodynamic optimization of a heat-driven refrigeration plant, that is, a refrigerator without work input, which is driven by a heat source.
130 citations
TL;DR: In this paper, the thermal and hydraulic design theory of a direct-transfer-type two fluid heat exchanger (tubular, plate-type, and extended-surface exchangers) is well developed and is available in standard heat transfer literature.
Abstract: Publisher Summary The thermal and hydraulic design theory of a direct-transfer-type two fluid heat exchanger (tubular, plate-type, and extended-surface exchangers) is well developed and is available in standard heat transfer literature. The well-established algorithm for thermal design of a two-fluid heat exchanger, however, has no adequate equivalent when the physical situation implies more than one thermal communication. This chapter outlines only the effectiveness- NTU (number of heat transfer units) approach and corresponding rating and sizing problems for the determination of the effectiveness or NTU for a three-fluid heat exchanger. The thermal design theory of multifluid heat exchangers (having more than three-fluid streams) and multistream plate-fin heat exchangers and the study of nonlinear problems are not considered in this chapter.
91 citations
TL;DR: In this paper, an air-to-air heat exchanger using thermosyphon heat pipes was designed, constructed and tested under medium temperature (below 300°C) operating conditions.
Abstract: Using water as the working fluid, air-to-air heat exchangers using thermosyphon heat pipes were designed, constructed and tested under medium temperature (below 300°C) operating conditions. A heat exchanger test rig has been constructed and developed wherein the heated air is recycled to the counterflow heat exchanger. The lengths of both the evaporator section and the condenser section of the heat exchangers were 300 mm and the central adiabatic section was 150 mm. The heat exchangers which were tested used (1) continuous plate finned copper tubes, (2) circular, spirally-finned steel tubes and (3) bare copper tubes for their respective heat pipes. The working fluid was water with a fill ratio of 60% of the evaporator section length. The air face velocity range was from 1.5 to 5 m/s and the heat input into the evaporator section inlet was varied between 4 and 20 kW using electric heating elements. The heat exchangers showed high effectiveness compared with similar heat exchangers using other working fluids, such as Freon 22 (R22). The rectangular plate finned copper thermosyphon heat exchanger had the best performance but there was a limitation on testing this configuration that the adiabatic section temperature operating condition did not exceed 200°C, in order not to exceed the safe working pressure. A steel pipe heat exchanger will be used in the industrial application to which the project is directed. This heat exchanger has been designed, manufactured and tested for heat recovery in industry with medium temperatures (lower than 300°C), for example in bakeries to recover flue gas energy from the oven to heat up the proofing oven or other low temperature heating functions.
78 citations
Patent•
4 May 1995
TL;DR: In this paper, a method for maximizing the thermodynamic efficiency and the heat transfer capacity of a counter-current heat exchanger through the use of an optimized multi-component working fluid is presented.
Abstract: A method is disclosed for maximizing the thermodynamic efficiency and the heat transfer capacity of a counter-current heat exchanger through the use of an optimized multi-component working fluid. Given the operational temperatures and pressures of the heat exchanger, the disclosure teaches a method for selecting the components for the working fluid and for determining the molar fractions of the components that determine the optimal mixture for the working fluid. Because the effective specific heat of a high pressure stream of this mixture is equal to the effective specific heat of a low pressure stream throughout the entire temperature range of the heat exchanger, the thermodynamic efficiency of the heat exchange process is maximized. In addition, because the difference between the enthalpies per unit mass of the two streams are maximized throughout the temperature range of the exchanger, this mixture provides optimal capacity for heat transfer in the heat exchanger. Such an optimal working fluid can dramatically improve the performance of a heat engine or heat pump, and is especially effective in cryogenic refrigeration systems.
52 citations
TL;DR: In this paper, the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator was investigated, and the results indicated that the decrease seems to depend on the viscosity of the oil.
Abstract: The introduction of chlorine-free refrigerants to the market requires experimental investigations of their behaviour in heat pumps and refrigerators. One particular area of interest is the effect of the new oils on the heat transfer in evaporators and condensers. Oil can either increase or decrease the heat transfer coefficient. This paper presents the results from an experimental investigation of the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator. The tests were carried out at heat fluxes between 2 and 8 kW m−2 (corresponding to mass fluxes between approximately 40 and 170 kg s−1 m−2). The evaporation temperature was varied from−10 to +10°C. The global oil concentration ranged from 0 to 4.5 mass percentage based on the total liquid flow. The heat transfer coefficient decreased in most of the cases. The results indicate that the decrease seems to depend on the viscosity of the oil. The decrease can fairly well be estimated with the correlation for pure refrigerants by Shah if the viscosity of the mixture is used in the calculations. The data for the oil-contaminated refrigerant also agree well with data for pure refrigerants in a plot of α tp /α lo ∗ versus the inverse Martinelli-Lockhart parameter when α lo ∗ is calculated with a modified Dittus-Boelter correlation and the mixture viscosity is used in the calculations. The heat transfer is found to increase when introducing oil in the special cases where the flow rate is low and the viscosity is low (oil A, 2 and 4 kW m−2 oil B, 6kW m−2 at +10°C). This is most likely due to surface tension effects. It has been suggested that the increased surface tension leads to a better tube wetting and thus an increased heat transfer.
48 citations
TL;DR: In this paper, a computer-based design concept assuming from the calculation of optimum tubeside and shellside pressure drops allows to determine the optimum dimensions of segmentally baffled shell-and-tube heat exchangers.
39 citations
36 citations
TL;DR: In this paper, the optimal performance of an absorption heat transformer was investigated by using the cyclic model with continuous flow, and the effect of thermal resistances between the heat transformer and the heat reservoirs was considered in the model.
Abstract: The optimal performance of an absorption heat transformer, i.e. a type II absorption heat pump, is investigated by using the cyclic model with continuous flow. The effect of thermal resistances between the heat transformer and the heat reservoirs is considered in the model. A general expression related to the rate of heat-pumping, the coefficient of performance and the overall heat transfer area of the heat transformer is derived. The expression is used to optimise the main performance parameters of the heat transformer. The maximum rate of heat-pumping and the corresponding coefficient of performance are calculated. For a given overall heat transfer area of the heat transformer, the optimal relation of the heat transfer areas of the heat exchangers is obtained. The problems concerning the optimal choices of other performance parameters are discussed. The results obtained here can not only enrich the theory of finite time thermodynamics, but also provide some new theoretical bases for the optimal design and operation of real absorption heat transformers.
35 citations
TL;DR: In this paper, the effects of flow maldistribution within the channels and between channels were taken into account by introducing a dispersion term in the energy equation, and the phase lag due to different flow path lengths between inlet or outlet of the heat exchanger and inlet/outlet of the individual channels were also considered.
Abstract: Experiments on the transient behavior of two welded plate heat exchangers with identical construction but different numbers of plates have been carried out under different operating conditions. The temperature response on both sides following a step change in inlet temperature on one side has been compared to a theoretical model. The model takes the effects of flow maldistribution within the channels and between channels into account by introducing a dispersion term in the energy equation. The phase lag due to different flow path lengths between inlet or outlet of the heat exchanger and inlet or outlet of the individual channels are also taken into account. Heat conduction through the plates in the main flow direction of the fluids can be neglected for the exchangers under consideration. The model is validated by the experiments. It is found that the dispersion model considered gives a better simulation than the conventional plug flow model. From the experiments the effects of NTU, heat capacity rate ratio, and number of plates were also determined. This demonstrates the whole spectrum of dynamic behavior of plate heat exchangers. To suggest a proper control strategy for such heat exchangers, the parameters of conventional first and second-order systems with delay period have been determined from the results of the experiments and the theoretical model.
34 citations
TL;DR: In this article, the performance of an absorption heat pump affected by heat resistances was investigated and the optimal regions of the coefficient of performance and the specific heating load were determined in the cycle model of a heat-engine-driven heat pump.
Abstract: The cycle model of a heat-engine-driven heat pump is used to study the performance of an absorption heat pump affected by heat resistances. The coefficient of performance of the absorption heat pump is adopted to be the objective function for optimization. The optimal regions of the coefficient of performance and the specific heating load are determined. The optimal relations between the heat transfer areas of the four heat exchangers involved and the coefficient of performance, or the specific heating load of an absorption heat pump, are obtained. Problems concerning the optimal design of an absorption heat pump are also discussed.
31 Dec 1995
TL;DR: In this article, a plate heat exchanger with a propylene-glycol/water mixture was used as the working fluid in order to provide lower Reynolds numbers than those provided by water at similar test conditions.
Abstract: Plate heat exchangers are becoming increasingly important because of their potential applications in industrial processes, especially in terms of their thermal performance and their limited pressure drop. An experimental investigation to acquire both heat-transfer and pressure-drop data for a plate heat exchanger was conducted in order to respond to these interests. A propylene-glycol/water mixture was used as the working fluid in order to provide lower Reynolds numbers than those provided by water at similar test conditions. The plate heat exchanger was composed of 31 plates, each with a chevron angle of 30 degrees. The isothermal pressure drop data were taken in the fully laminar flow regime for Reynolds numbers from 10 to 80. The heat transfer data were taken in the fully laminar flow regime for Reynolds numbers of 80 to 720 with heat transfer rates of 1.1 {times} 10{sup 5} to 6.5 {times} 10{sup 5} W. The experimental data for the friction factor and Nusselt number were correlated using a standard power-law function. Other published heat-transfer and friction factor correlations for plate heat exchangers with similar plates at selected conditions are compared to the data.
TL;DR: In this article, the adsorption isotherms of a desiccant used in a commercially available heat and mass exchanger are measured using the program MOSHMX.
Abstract: Rotary regenerative heat and mass exchangers (enthalpy exchangers) can reduce air-conditioning costs in ventilated buildings by recovering energy from the exhaust air and transferring it to the supply air stream. In this study the adsorption isotherms of a desiccant used in a commercially available heat and mass exchanger are measured. The isotherms and other property data are incorporated into the program MOSHMX which numerically solves the governing equations for combined heat and mass transfer. The numerical results are then used to develop a computationally simple model for determining the performance of a specific enthalpy exchanger as a function of the air inlet conditions and the matrix rotation speed. The numerical results agree with the catalog information provided by the manufacturer. The enthalpy exchanger model is used in the transient simulation program TRNSYS to estimate the annual performance. Integrated energy savings (heating and cooling) are determined for a commercial application (a 200...
1 Nov 1995
TL;DR: In this paper, the authors describe a method used to compute the transient performances of assisted circulation heat recovery steam generators, which are composed of several heat exchangers, each of which is a bundle of tubes.
Abstract: This paper describes a method used to compute the transient performances of assisted circulation heat recovery steam generators. These heat recovery steam generators are composed of several heat exchangers, each of which is a bundle of tubes. The method presented here treats each heat exchanger in a similar way, replacing the bundle of tubes with an ‘equivalent’ linear heat exchanger.This equivalent linear heat exchanger is then discretized in as many slices as required by the accuracy. The mass and enthalpy equations on each of these control volumes are solved by a fully explicit numerical method, adapted for the special conditions encountered in this kind of problem, allowing a considerable reduction of the computation time compared to other methods.Some emphasis is put on the modifications required to solve the equations for the evaporators because they are two-phase heat exchangers. A model for the steam drums is also presented together with simple models for the main control loops used in such system...
TL;DR: In this paper, the performance of an endoreversible heat engine with heat leak was investigated and a comprehensive formulation and a general solution methodology for any mode of heat supply or release was presented.
Abstract: Finite-time thermodynamics are used for studying the performance of endoreversible heat engines with heat leak. A comprehensive formulation and a general solution methodology, valid for any mode of heat supply or release, are presented. Detailed analyses are conducted for several heat transfer modes and universal analytical and numerical results for the efficiency at maximum power are generated. Many established laws and major conclusions derived in several references are shown to represent very special cases of the new formulations. Furthermore, the nature of the leakage power law is found to deeply affect the efficiency at maximum power. Finally, for no leakage situations, if the heat to the engine is supplied and released via similar heat transfer modes, then the lowest efficiency at maximum power, when the only thermal resistance is between the working fluid and the hot reservoir, is found to be giben by 1/n, n being the power of the heat transfer law.
Patent•
1 Mar 1995
TL;DR: In this article, the authors characterized the relationship between the mass flow rate of the heat transfer fluid and the mass heat in liquid form, expressed in J/kg/°K, in order to satisfy, in operation, approximately the relationship: P Max =f m ×c×ΔT, in which: f m is the mass flux rate, c is the heat flow rate, and ΔT is the difference in temperature between the exit and the entry of the main exchanger.
Abstract: Unit for distribution and/or collection of cold and/or of heat, including: (a) a main exchanger (1) of heat between a refrigerant fluid (2, 21, 22) and a heat transfer fluid (3); (b) a means for producing cold with a means for heat exchange with the heat transfer fluid; (c) a closed main circuit (3) for continuous free circulation of the heat transfer fluid; (d) at least one heat transfer fluid drawing loop (7 to 11); characterized in that the flow cross-section of the main circuit (3), the maximum refrigerating power of the means for producing cold, Pmax, expressed in W, and the main heat exchanger (1) are sized relative to one another in order to satisfy, in operation, approximately the relationship: P Max =f m ×c×ΔT, in which: f m is the mass flow rate of the heat transfer fluid, expressed in kg/s; c is the mass heat of the heat transfer fluid, in liquid form, expressed in J/kg/°K.; ΔT is the difference in temperature of the heat transfer fluid between the exit and the entry of the main exchanger (1).
TL;DR: In this article, the Nusselt number was determined from the isotherms at the wall in the boundary layer of a plate-fin heat exchanger using Holographic interferometry.
TL;DR: In this paper, a set of experimental tests on the effect of polymeric additives in water are presented from the point of view of friction losses and heat transfer performance variations, in single-passage countercurrent straight-pipe heat exchangers.
Journal Article•
TL;DR: In this article, an extensively instrumented refrigerator was used to gather air and refrigerant-side temperature, pressure, power, and fluid flow measurements to provide in situ calorimetry for 95 operating conditions.
Abstract: Separate experiments with a condenser and an evaporator showed that changes in refrigerant-side heat transfer coefficients produced variations in overall heat transfer resistance of up to 20% over a wide range of operating conditions. An extensively instrumented refrigerator was used to gather air- and refrigerant-side temperature, pressure, power, and refrigerant mass flow measurements to provide in situ calorimetry for 95 operating conditions. These operating conditions included three different refrigerator charges that provide a variety of evaporator and condenser outlet conditions. Accounting for the variation in refrigerant-side heat transfer coefficients produced better results than assuming that the heat transfer coefficients were constant. The variable-conductance models developed here can be used to predict changes in performance resulting from the use of alternative refrigerants and different heat exchanger tube dimensions.
TL;DR: In this paper, the heat transfer characteristics of a low temperature latent heat storage system have been determined for the circular finned and unfinned tubes using sodium acetate trihydrate as a phase change material (PCM).
Abstract: Heat transfer characteristics of a low temperature latent heat storage system have been determined for the circular finned and unfinned tubes using sodium acetate trihydrate as a phase change material (PCM). In the heat storage stage (melting process). the PCM heat transfer coefficient in the unfinned-tube system showed good agreement with the calculated value by the heat conduction equation. The heat transfer between the tube wall and the PCM was not enhanced by the fins in the thin-finned-tube system, whereas 2 times higher heat-transfer coefficient for the thick-finned-tube system over the unfinned-tube system was obtained. The experimentally determined heattransfer coefficients in the unfinned-tube and thick-finned-tube systems are in the ranges of 40–170 and 80–320 W/m2-k, respectively. The amount of heat storage for three systems has been correlated in terms of Fourier, Stefan and Reynolds numbers. The thermal performance of heat storage systems are found to be strongly dependent on the
TL;DR: In this article, a mathematical model was developed utilizing the equation of state to calculate the properties of ammonia-water mixture, and the performance of the heat transformer is defined by COP and circulation ratio.
Abstract: In this paper, we study the performance of absorption heat transformers for upgrading low-level heat. A mathematical model was developed utilizing the equation of state to calculate the properties of ammonia-water mixture. The performance of the heat transformer is defined by COP and circulation ratio. The parameters that affect the performance are level of waste heat, condenser temperature, and effectiveness of heat exchangers. This study leads to identification of the field of the operating conditions.
31 Dec 1995
TL;DR: In this article, a three-dimensional unsteady numerical computation for a model of a two-row finned tube heat exchanger located in a uniform flow field has been performed to see the effect of tube diameter on the flow and thermal fields.
Abstract: Enhancement of air-side heat transfer in heat exchangers used in air-conditioning machines can be considered as a way to solve the problem caused by use of alternative refrigerants. Related with this, the present study aims to investigate the flow and thermal fields in finned tube heat exchangers. In previous papers (Torikoshi et al., 1994 and Xi et al., 1994, 1995), numerical schemes that used a compound grid system for finned tube heat exchangers were described. The schemes were validated with experimental data. In the present paper, a three-dimensional unsteady numerical computation for a model of a two-row finned tube heat exchanger located in a uniform flow field has been performed to see the effect of tube diameter on the flow and thermal fields. Several features were found in the study. One interesting finding was that increasing the tube diameter almost does not improve heat transfer performance but increases the resistance of the fluid flow inside the heat exchanger.
TL;DR: In this paper, a class of conceptual optimum refrigeration cycles is considered with a fixed overall heat conductance and a specified refrigerant operating temperature range to bound the optimization problems. And the resulting one degree of freedom problems are solved with a variable arithmetic mean temperature difference in the heat exchangers.
Abstract: A class of conceptual optimum refrigeration cycles is considered with a fixed overall heat conductance and a specified refrigerant operating temperature range to bound the optimization problems. These cycles deal with maximum refrigeration power, maximum refrigeration load, and maximum heat rejection load for the case of a heat pump. The resulting one degree of freedom problems are solved with a variable arithmetic mean temperature difference in the heat exchangers. The maximum refrigeration power solution yields an analytical closed form optimality rule which constitutes a close lower bound solution to the maximum refrigeration load and maximum heat rejection load problems.
31 Dec 1995
TL;DR: In this article, the authors applied the electrohydrodynamic (EHD) technique to the air side of a tube-and-plate-fin, cross-flow heat exchanger.
Abstract: Compound heat transfer augmentation refers to coupling of an active and a passive enhancement technique in an effort to benefit from the advantages of both techniques. In the present study, the electrohydrodynamic (EHD) technique was applied to the air side of a tube-and-plate-fin, cross-flow heat exchanger. The main objective of the study was to address the applicability/limitations of the EHD technique for air-side enhancement in compact heat exchangers. Experiments were conducted as a function of parameters that included the effect of electrode geometry, air temperature, and frost formation on the heat transfer surface. The general observation in all cases was that enhancements were maximum in the laminar flow regime, at the lowest temperature difference between the air and the heat transfer surface, and at the highest applied electric field potential. A maximum enhancement factor of 3.3 in the overall heat transfer coefficient was obtained utilizing a barbed parallel electrode at a temperature difference of 20 C and at a Reynolds number of 1,000.
Journal Article•
TL;DR: In this paper, the authors present equations for calculating the tube-side heat-transfer coefficient, calculating the shell-side temperature transfer coefficient, and calculating the heat-exchanger size.
Abstract: In recent years, chemical process plants are increasingly encountering processes that require heat exchange in three-phase fluids. A typical application, for example, is heating liquids containing solid catalyst particles and non-condensable gases. Heat exchangers designed for three-phase flow generally have tubes with large diameters (typically greater than two inches), because solids can build-up inside the tube and lead to plugging. At the same time, in order to keep heat-transfer coefficients high, the velocity of the process fluid within the tube should also be high. As a result, heat exchangers for three-phase flow may require less than five tubes -- each having a required linear length that could exceed several hundred feet. Given these limitations, it is obvious that a basic shell-and-tube heat exchanger is not the most practical solution for this purpose. An alternative for three-phase flow is a helical-coil heat exchanger. The helical-coil units offer a number of advantages, including perpendicular, counter-current flow and flexible overall dimensions for the exchanger itself. The paper presents equations for: calculating the tube-side heat-transfer coefficient; calculating the shell-side heat-transfer coefficient; calculating the heat-exchanger size; calculating the tube-side pressure drop; and calculating shell-side pressure-drop.
1 Jan 1995
TL;DR: In this paper, the effect of heat exchanger effectiveness on the liquid yield for a simple Joule-Thomson liquefaction process was investigated, and it was shown that the working fluid is nitrogen and the lower and upper operating pressures of the liquid cycle are 1 and 200 atm, respectively, for an exchanger that has an effectiveness below 85.5%.
Abstract: One of the most critical aspects of any low-temperature liquefaction and refrigeration system is that of efficient heat transfer. In fact, the industrial liquefaction of the “permanent gases” did not become possible until the principle of efficient recovery of the refrigeration from the cold, low-pressure return streams could be translated into engineering practice through the development of suitable heat exchange devices. This point readily becomes apparent when one considers the effect of heat exchanger effectiveness on the liquid yield for a simple Joule-Thomson liquefaction process. If the working fluid is nitrogen and the lower and upper operating pressures of the liquefaction cycle are 1 and 200 atm, respectively, it can be shown that the liquid yield under these conditions will be zero for an exchanger that has an effectiveness below 85.5%[1].(Heat exchanger effectiveness is defined here as the ratio of the actual heat transfer to the maximum heat transfer possible considering only the inlet temperatures.)
TL;DR: In this article, the authors use a transient state technique to measure the global heat exchange coefficient between a liquid and corrugated plates, which is based on the assumption that the fluid flow is similar to a plug flow.
TL;DR: In this article, a mathematical model for heat transfer with simultaneous mass transfer is constructed, based on heat and mass balances, and methods for solution are shown for two cases: 1. The water on the exhaust side of the heating surface is only condensate, whose heat content may be neglected.
Abstract: Designs of heat exchangers for heat recovery from humid air are described. A mathematical model for heat transfer with simultaneous mass transfer is constructed, based on heat and mass balances. Methods for solution are shown for two cases: 1. The water on the exhaust side of the heating surface is only condensate, whose heat content may be neglected. Also, the case of only partially condensing heat transfer surface is discussed. 2. The heating surface is flushed with abundant water. - Each case is illuminated with a calculation example.
31 Dec 1995
TL;DR: In this article, a study of the effect of all geometric parameters on heat exchanger size was conducted, and it was shown that the flat-tube, multi-louvered surface heat exchangers offer the design flexibility to change many geometric variables to achieve the desired heat duty and tube-side and air-side pressure drops.
Abstract: Highly compact flat-tube, multi-louvered surface heat exchangers are being used to replace conventional round-tube/flatplate fin geometries in automotive engine cooling and air-conditioning systems. These novel heat exchangers can benefit absorption systems, which are gaining increased attention as an environmentally friendly replacement for the CFC-based vapor compression cycles that are used in residential and commercial air-conditioning. A study of condensation of ammonia in air-cooled heat exchangers was conducted. Two-phase flow, heat-transfer and pressure drop in the novel flat tubes were modeled by adapting the available literature on round tubes. A computer program that enabled the detailed design of these heat exchangers was developed. A systematic study of the effect of all geometric parameters on heat exchanger size was conducted. The analysis showed that the flat-tube, multi-louvered fin heat exchanger geometry offers the design flexibility to change many geometric variables to achieve the desired heat duty and tube-side and air-side pressure drops. Tube and fin depth, and fin spacing were found to have a significant effect on the required heat exchanger mass. Louver geometry may be used to further refine heat exchanger designs. It was also shown that the available air flow rate has a very significant effect on heat exchanger performance.more » A decrease in air flow rate of just twenty percent could increase the heat exchanger mass requirements by a factor of 1.6.« less
TL;DR: In this paper, the effect of finite-rate heat transfer on the performance of a two-stage combined heat pump system in steady-state operation is investigated with the help of a combined cycle model.
Abstract: SYNOPSIS With the help of a combined cycle model, the effect of the irreversibility of finite-rate heat transfer on the performance of a two-stage combined heat pump system in steady-state operation is investigated. The optimal coefficient of performance of the combined heat pump system is derived for a given specific heating load. The optimal relations between the temperatures of the working fluid in heat exchangers and the coefficient of performance or specific heating load are found. The optimal distribution of heat exchanger areas is determined. Finally, a simple equivalent system is given.