Standard Heat Exchanger Research Articles

HEAT EXCHANGER DESIGN METHODOLOGY FOR ELECTRONIC HEAT SINKS

This paper discusses the “inlet temperature difference” (ITD) based heat-exchanger (and its variants) design methodology frequently used by designers of electronic heat sinks. This is at variance with the accepted methodology recommended in standard heat-exchanger textbooks—the “log-mean temperature difference,” or the equivalent e-NTU design method. The purpose of this paper is to evaluate and discuss the ITD based design methodology. The paper shows that the ITD based method is an approximation at best. Variants of the method can lead to either under- or overprediction of the heat transfer rate. Its shortcomings are evaluated and designers are directed to the well established and accepted design methodology.

Optimization of heat exchanger network

Abstract In this paper, a new method is presented for optimization of heat exchanger networks making use of genetic algorithm and Sequential Quadratic Programming. The optimization problem is solved in the following two levels: 1- Structure of the optimized network is distinguished through genetic algorithm, and 2- The optimized thermal load of exchangers is determined through Sequential Quadratic Programming. Genetic algorithm uses these values for the determination of the fitness. For assuring the authenticity of the newly presented method, two standard heat exchanger networks are solved numerically. For representing the efficiency and applicability of this method for the industrial issues, an actual industrial optimization problem i.e. Aromatic Unit of Bandar Imam Petrochemistry in Iran is verified. The results indicate that the proposed multistage optimization algorithm of heat exchanger networks is better in all cases than those obtained using traditional optimization methods such as Pinch Analysis Method and Mathematical Optimization Method.

FIRST AND SECOND LAW ANALYSIS OF A LABORATORY AMMONIA/WATER ABSORPTION HEAT PUMP

At the Institute of Thermal Engineering, a small-capacity ammonia/water absorption heat pumping unit with about 5 kW cooling capacity has been constructed and tested in the laboratory. For all heat exchangers, standard plate heat exchangers have been used. The heat pump has been designed to operate in a wide operating range in order to enable different temperature levels for ice production, residential cooling, and heating applications. To identify potential for improvements, a thermodynamic analysis using both the first and the second law of thermodynamics has been carried out in order to locate irreversibilities associated with inefficient processes and to calculate the exergy loss of each component. Therefore the exergy losses of the main components have been separated into one part which is caused by the heat transfer and another part which originates from internal irreversibilities, e.g., from mixing processes. Furthermore, sensitivity calculations have been performed by varying the temperature levels of the heat sources and the heat sink in order to investigate the different component losses at different operating conditions. The results show that the component exergy losses depend particular on the temperature level of both the heat sources and the heat sink. At low temperature lifts, the exergy losses of the absorber and generator are dominant and with increasing temperature lift, the exergy losses of the dephlegmator and the rectification column become considerable.

Small dead space heat and moisture exchangers do not impede gas exchange during noninvasive ventilation: a comparison with a heated humidifier

Adverse respiratory and gasometrical effects have been described in patients with acute respiratory failure (ARF) undergoing noninvasive ventilation (NIV) with standard heat and moisture exchangers (HME). We decided to evaluate respiratory parameters and arterial blood gases (ABG) of patients during NIV with small dead space HME compared with heated humidifier (HH). Prospective randomized crossover study. A 16-bed medical intensive care unit (ICU). Fifty patients receiving NIV for ARF. The effects of HME and HH on respiratory rate, minute ventilation, EtCO(2), oxygen saturation, airway occlusion pressure at 0.1 s, ABG, and comfort perception were compared during two randomly determined NIV periods of 30 min. The relative impact of HME and HH on these parameters was successively compared with or without addition of a flex tube (40 and 10 patients, respectively). No difference was observed between HME and HH regarding any of the studied parameters, whether or not a flex tube was added. If one decides to humidify patients' airways during NIV, one may do so with small dead space HME or HH without altering respiratory parameters.

The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m2 and 17 kW/m2 with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance.

Design and performance study of the active magnetic refrigerator for room-temperature application

A room-temperature magnetic refrigerator, consisting of permanent magnet, active magnetic refrigeration (AMR) cycle bed, pumps, hydraulic circuit, active magnetic double regenerator cycle (AM2RC) and control subsystems, has been designed. The magnetic field is supplied by NdFeB permanent magnets. The AMR bed made by stainless steel 304 encloses gadolinium particles as the magnetic working substance. Each part of the refrigerator is controlled by the programmable controller. The different standard heat exchangers are employed to expel heat. The cycle performance of this self-designed facility is analyzed using Langevin theory. The results provide useful data for future design and development of room-temperature magnetic refrigeration.

Combined effect of chloride and pH on corrosion resistance of Cu-10Ni alloy

Cu-10Ni alloy is a standard heat exchanger material for saltwater (including seawater) applications owing to its excellent thermal conductivity and corrosion resistance. The excellent corrosion resistance is due to formation of Cu2O film, which accords protection. However, when varying amount of H+, OH− and Cl− ions are present, CuCl2− may also form. The CuCl2− is not protective as Cu2O and hence corrosion resistance may be affected. Present paper investigates combined effect of chloride ion and pH on corrosion resistance of Cu-10Ni alloy. Cathodic and anodic polarization test results are presented for saltwater containing various amounts of NaCl at pH 6 and 8. It is found that, icorr increases with increasing chloride concentration. The results are discussed using dissolution mechanisms, semi-conducting behaviour of Cu2O film and deterioration of the film in presence of chloride ions.

An Investigation of a Falling Film Desiccant Dehumidification/ Regeneration Cooling System

The present work outlines a liquid desiccant refrigeration cycle with a significantly higher efficiency, as compared to a traditional chiller cycle operating between the same two thermal reservoirs. The present investigation includes a general cooling design under nominal values. A parametric study enables the determination of the overall cooling and humidity levels as compared to the ambient conditions. Typical cooling achieved by this analysis is approximately 17°C below the ambient temperature, and the relative humidity of the inlet air can be reduced up to 22 percent. The proposed cycle yields a 18–23 percent higher coefficient of performance as compared to a chiller cycle operating between the same temperatures. The proposed dehumidifier displays close agreement when compared to a standard cross-flow heat exchanger and the general trends and performance of this cycle compare well with previous studies on falling film dehumidifiers.

Heat Exchanger Design Methodology for Electronic Heat Sinks

This paper discusses the “inlet temperature difference” (ITD) based heat-exchanger (and its variants) design methodology frequently used by designers of electronic heat sinks. This is at variance with the accepted methodology recommended in standard heat-exchanger textbooks—the “log-mean temperature difference,” or the equivalent ε-NTU design method. The purpose of this paper is to evaluate and discuss the ITD based design methodology. The paper shows that the ITD based method is an approximation at best. Variants of the method can lead to either under- or overprediction of the heat transfer rate. Its shortcomings are evaluated and designers are directed to the well established and accepted design methodology.

Innovative design of a magnetocaloric system

In the present paper we consider the problem of optimizing the cooling of a magnetocaloric refrigerator. In this work we first theoretically and then experimentally study the performance of a single material regenerator under different operating conditions. The basic principles of the design and implementation of our magnetic refrigerator prototype are presented as well as a new magnetic assembly of NdFeB permanent magnets. The design of the equipment uses a movement of relative displacement optimized for the phases of activation and inactivation of the magnetic field. Each part of the equipment is implemented in order to be controlled separately and to allow a large variety of the tests: gear pumps with individual control, sequence of programmable magnetocaloric cycle, unit control by programmable controller and application programming interface by color LCD touch screen, real-time processing data acquisition using a National Instruments System implemented on Independent PC, expelled heat using different standard heat exchangers.

Effects of processing and storage on juice colour and betacyanin stability of purple pitaya (Hylocereus polyrhizus) juice

Purple pitaya (Hylocereus polyrhizus) fruits have been recently proposed as a promising source of betalains. In the present study, purple pitaya juice processing at pilot-plant scale is reported for the first time. Each processing step was examined in terms of juice colour and betacyanin retention. Lightness (L *) and hue angle (h°) values were strongly influenced by juice filtration, while chroma (C *) and betacyanin retention were most strongly altered by pasteurisation. Three systems of juice pasteurisation were evaluated. Betacyanin loss and colour alteration were minimal upon pasteurisation in an HTST system and a standard tubular heat exchanger, respectively. Additionally, storage experiments using mucilage-free purple pitaya juice were conducted. Whereas light exposure of unsupplemented samples resulted in significant pigment degradation, detrimental effects of light exposition were completely prevented by the addition of 1% ascorbic acid prior to storage. After 6 months, about 70% of the initial betacyanin content was retained in the presence of ascorbic acid, irrespective of illumination.

Dynamic Performance Analysis of a Compressor Driven Metal Hydride Cooling System

Though several studies have been reported to show that compressor driven metal hydride cooling systems are competitive with conventional vapor compression systems, an elaborate computational model that takes into account the transient nature of the compressor and the conditioned space is still unreported. The results presented here are obtained for a room air conditioner with Zr0.9Ti0.1Cr0.55Fe1.45 as the hydrogen absorbing material and employing standard heat exchanger configurations. Though previous thermodynamic and transient studies predicted attainment of significant coefficients of performance, the present results indicate that even with the optimal design the maximum coefficient of performance and specific power will be around 2.38 and 750kJ∕kg-alloy.h, respectively. This indicates a need for better materials and effective control strategies so that these systems can become commercially viable.

Expectations of Closed-Brayton-Cycle Heat Exchangers in Nuclear Space Power Systems

Performance expectations of closed-Brayton-cycle heat exchangers to be used in 100-kWe nuclear space power systems were forecast. Proposed cycle state points for a system supporting a mission to three of Jupiter's moons required effectiveness values for the heat-source exchanger, recuperator, and rejection exchanger (gas cooler) of 0.98, 0.95, and 0.97, respectively. Performance parameters such as number of thermal units Ntu, equivalent thermal conductance UA, and entropy generation numbers Ns varied from 11 to 19, 23 to 39 kW/K, and 0.019 to 0.023 for some standard heat exchanger configurations. Pressure-loss contributions to entropy generation were significant; the largest frictional contribution was 114% of the heat-transfer irreversibility. Using conventional recuperator designs, the 0.95 effectiveness proved difficult to achieve without exceeding other performance targets; a metallic, plate-fin counterflow solution called for 15% more mass and 33% higher pressure loss than the target values. Two types of gas coolers showed promise. Single-pass counterflow and multipass cross-counterflow arrangements both met the 0.97 effectiveness requirement. Potential reliability-related advantages of the cross-counterflow design were noted. Cycle modifications, enhanced heat-transfer techniques, and incorporation of advanced materials were suggested options to reduce system development risk. Carbon-carbon sheeting or foam proved an attractive option to improve overall performance.

A Cost-Based Strategy to Design Multiple Shell and Tube Heat Exchangers

Process Integration has been applied in several industrial processes mainly using standard shell and tube heat exchangers (1-1 or 1-2). The flow arrangement in 1-2 multiple shell and tube heat exchangers involves part counter-current flow and part co-current flow. This fact is accounted for in the design by introducing a FT correction factor into the 1-1 heat exchanger design equation. To avoid some steep regions in the feasible space of heat exchangers design some authors introduce other parameters like XP or G. Until now it was not possible to have an overall map to give some guidelines of how to choose between the several XP approaches established in the literature. This paper summarizes the current existing criteria in a general design algorithm DeAl12 to show a path for the calculations of the main design variables of the heat exchanger. Also a new strategy design algorithm StratDeAl12 is introduced in this paper to allow the best choice between the existing XP approaches based on the heat exchanger cost minimisation. Several examples illustrate the advantage of using the developed algorithm and the deviations obtained in the heat exchanger cost if a wrong approach was chosen.

Flow-Induced Vibration in Heat Exchangers

Heat exchanger tube bundles may fail due to excessive vibration or noise. The main failure mechanisms are generated by the shellside fluid that passes around and between the tubes. This fluid may be a liquid, gas or multi-phase mixture. The most severe vibration mechanism is a fluidelastic instability, which may cause tube damage after only a few hours of operation. Clearly, such extreme causes of vibration must always be avoided. In contrast buffeting due to flow-turbulence causes very little vibration. However, after many years of service such remorseless low level vibration will produce tube wall thinning, due to fretting, which may be unacceptable in a high-integrity heat exchanger. Consequently, the issues of life cycle and integrity must frequently be included in heat exchanger specification. This paper reviews the various mechanisms that cause vibration and noise. Particular attention is given to methods for achieving good tube support arrangements that minimize vibration damage. References to the most recent sources of data are given and good working practice for the design and operation of standard and high-integrity heat exchangers is discussed.

Finite element solution of conjugate heat transfer problems with and without the use of gap elements

This paper describes the finite element solution of conjugate heat transfer problems with and without the use of gap elements. Direct and iterative methods to incorporate gap elements into a general finite element program are presented, along with their advantages and disadvantages of the two gap element treatments in the framework of finite elements. The numerical performance of the iterative gap element treatment is discussed in detail in comparison with analytical solutions for both 2‐ and 3‐D gap conductance problems. Numerical tests show that the number of iterations depends on the non‐dimensional number Bi = hL/k, and it increases approximately linearly with Bi for Bi≥0.6. Here, for gap heat transfer problems, h is taken to be the inverse of the contact resistance. This conclusion holds true for both 2‐ and 3‐D problems, for both linear and quadratic elements and for both transient and steady state calculations. Further numerical results for conjugate heat transfer problems encountered in heat exchanger and micro chemical reactors are computed using the gap element approach, the direct numerical simulations and analytical solutions whenever solvable. The results reveal that for the standard heat exchanger designs, an accurate prediction of temperature distribution in the moving streams must take into consideration the radial temperature distribution and the accuracy of the calculations depends on the non‐dimensional number Bi = hR/2k. From gap element calculations, it is found that classical analytical solutions are valid for a heat transfer analysis of an exchanger system, only when Bi<0.1. This important point so far has been neglected in virtually all the textbooks on heat transfer and must be included to complete the heat transfer theory for heat exchanger designs. Results also suggest that for thermal fluids systems with chemical reactions such as micro fuel cells, the gap element approach yields accurate results only when the heat transfer coefficient that accounts for the chemical reactions is used. However, when these heat transfer coefficients are not available, direct numerical simulations should be used for an accurate prediction of the thermal performance of these systems.

Charcoal as an airway isoflurane reflection filter

The isoflurane-saving and CO2-retaining effects of a charcoal filter were compared with a Siemens standard heat and moisture (HME) exchanger and an emptied specimen (dummy). Isoflurane was delivered during the inspiratory phase and consumption investigated at 10, 15 and 25 cycles min-1. The investigation was performed by ventilation with humidified air with a constant end-tidal CO2 and temperature. For a comparison, isoflurane was delivered in a conventional manner via the ventilator. The arrangement with a charcoal filter reduced the isoflurane consumption by a factor of 2.0-2.6, depending on ventilatory rate. Most of the saving was a result of the method of isoflurane delivery (factor 1.4-2.0), while adding the reflector gave a further reduction (factor 1.3-1.5). One circumstance that reduced the net efficiency of the charcoal filter was that it also reflected CO2; consequently, total minute ventilation had to be increased to maintain constant end-tidal CO2.

Charcoal as an airway isoflurane reflection filter

The isoflurane-saving and CO2-retaining effects of a charcoal filter were compared with a Siemens standard heat and moisture (HME) exchanger and an emptied specimen (dummy). Isoflurane was delivered during the inspiratory phase and consumption investigated at 10, 15 and 25 cycles min−1. The investigation was performed by ventilation with humidified air with a constant end-tidal CO2 and temperature. For a comparison, isoflurane was delivered in a conventional manner via the ventilator. The arrangement with a charcoal filter reduced the isoflurane consumption by a factor of 2.0−2.6, depending on ventilatory rate. Most of the saving was a result of the method of isoflurane delivery (factor 1.4−2.0), while adding the reflector gave a further reduction (factor 1.3−1.5). One circumstance that reduced the net efficiency of the charcoal filter was that it also reflected CO2; consequently, total minute ventilation had to be increased to maintain constant end-tidal CO2.

Contact resistance in air-cooled plate fin-tube air-conditioning condensers

Test equipment and procedures have been developed to measure the heat transfer performance of an industry-standard wavy plate fin-tube condenser, and to compare it with alternative designs. Two test rigs were employed: one used direct electric heating, and the other used an R22 thermosyphon. Analysis revealed that the standard production heat exchanger suffered from excessive fin-tube resistance. This was confirmed by sectioning and microscopic examination. Seven different designs were compared at the same Reynolds number. The results were analysed to isolate the air-side heat transfer coefficient.

Performance evaluation of a vortex generator heat transfer surface and comparison with different high performance surfaces

A comparative assessment of five different heat transfer configurations for operation in compact heat exchangers is presented. The configurations under consideration are four standard heat exchanger surfaces—two plain fin, an offset strip and a louvered fin geometry—and one surface with so called vortex generators for heat transfer augmentation. In the case of the standard surfaces, the basic performance characteristics in the form of heat transfer and friction data versus the Reynolds number have been taken from published experimental results. In the case of the vortex generator surface, the performance characteristics have been derived from a numerical prediction of the flow and temperature field in a closely spaced parallel plate channel with vortex generators in the form of delta wings mounted on the channel walls. In comparison to the plain fin surface with a rectangular cross section, the vortex generator surface shows best performance characteristics allowing a reduction in heat transfer surface area of 76%, for fixed heat duty and for fixed pumping power.