Immersed Heat Exchanger Research Articles

Visualization of the microscopic flow profile of state-of-the-art absorption heat pump working pairs under operational conditions

The research and development of novel working pairs and highly efficient heat exchangers for absorption heat pump applications require a detailed analysis of the microscopic flow profile at the absorber heat exchanger during the absorption process. A recently developed Single Aperture Defocussing Micro Particle Tracking method, based on a conventional Particle Image Velocimetry system, demonstrates the possibility of simultaneous measurements of velocity profiles and film thicknesses in falling film absorbers at steady-state flows. We show first results of velocity profile measurements of the state-of-the-art working pair LiBr/water, with and without a 2-Ethyl-1-Hexanol additive, during operation in a lab-scale absorption heat pump. Based on the results of the measurements, the advantages and disadvantages of the developed measurement technique and their potential application in the characterization of novel working pairs is discussed.

Design and Fabrication of Single Effect Absorption Cooling System of 5.25kW Cooling Capacity for Domestic Use <sup></sup>

The aim of this work is to design and fabricate solar assisted single effect absorption cooling system of 5.25 kW cooling capacity to cool a room having floor area of 15m2. The design of the major components of the cooling system like generator, condenser, evaporator, absorber and solution heat exchanger are described in this paper.

Simulation of a combined heating, cooling and domestic hot water system based on ground source absorption heat pump

The amount of energy used for heating and domestic hot water (DHW) is very high and will keep increasing. The conventional ground source electrical heat pump used in heating-dominated buildings has the problems of thermal imbalance, decrease of soil temperature, and deterioration of heating performance. Ground source absorption heat pump (GSAHP) is advantageous in both imbalance reduction and primary energy efficiency (PEE) improvement; however, the imbalance is still unacceptable in the warmer parts of cold regions. A combined heating/cooling/DHW (HCD) system based on GSAHP is proposed to overcome this problem. The GSAHPs using generator absorber heat exchange (GAX) and single-effect (SE) cycles are simulated to obtain the performance under various working conditions. Different HCD systems in Beijing and Shenyang are simulated comparatively in TRNSYS, based on which the thermal imbalance, soil temperature, heat recovery, and energy efficiency are analyzed. Results show that GSAHP–GAX–HCD is suitable for Beijing and GSAHP–SE–HCD is suitable for Shenyang. The imbalance ratio can be reduced to −14.8% in Beijing and to 6.0% in Shenyang with an annual soil temperature variation of only 0.5°C and 0.1°C. Furthermore, about 20% and 15% of the total condensation/absorption heat is recovered to produce DHW, and the PEE can reach 1.516 in Beijing and 1.163 in Shenyang. The combined HCD systems can achieve a PEE improvement of 23.6% and 44.4% compared with the normal heating/cooling systems.

Optimization design of the large temperature lift/drop multi-stage vertical absorption temperature transformer based on entransy dissipation method

In the district heating system, the AHE (Absorption Heat Exchanger) can transfer the heat of the primary network with a large temperature drop to the secondary network with a small temperature lift in a heating station. Triangular heat transfer processes exist in a traditional AHE, and limit the performance of the system. In order to eliminate the mismatched heat transfer processes, the new ATT (Absorption Temperature Transformer) is suggested, which can separate the condensation or evaporation pressure into several levels. A new method based on entransy dissipation analysis is applied to conduct an optimization design to the ATT. Simulation results show that the minimum total KA (multiplication of the heat transfer coefficient by the heat transfer area) is obtained when entransy dissipation per transferred heat is uniformly distributed in the four basic components. The flow path of the ATT is also optimized. The best different flow direction is obtained which has the lowest flow mismatched coefficient. The total KA reduction becomes not obvious when the stage number is over 3. And the total KA reduction of a 4-stage ATT reached 28.9% compared to a 1-stage AHE.

Design and operation of a tri-generation system for a station in China

Tri-generation usually refers to the simultaneous production of power, heating and cooling. Tri-generation can be regarded as a high-efficiency technology, provided that a large proportion of the energy rejected by the prime mover is used. In this paper, the design and operation of a tri-generation system for a railway station was investigated.The system is composed of the internal combustion engine (ICE), absorption heat pump (AHP), heat exchanger (HE), and other facilities. The system was built and operated in 2011, and the energy efficiency level was analyzed. The results indicated that the calculated maximum comprehensive energy efficiency (CEE) was 94.94% in the winter and 84.33% in the summer; with the calculated maximum exergy efficiency is 38.01% in the winter and 36.01% in the summer. The primary energy saving (PES) of the system was 32.2% in the winter and 4.9% in the summer. The system could therefore be regarded as high efficiency tri-generation. The recovered year of the increased investment was 5.47year compared with the reference system. These results could serve as a reference for designing or evaluating tri-generation systems.

Energy and exergy analysis of a novel multigeneration system producing cooling, heating, power and pure water

A novel multi-generation system producing power, pure water, cooling and heating is proposed and analysed in detail. The system comprises a hybrid generator–absorber heat exchanger (GAX), the Organic Rankine Cycle (ORC), the absorption heat transformer (AHT) and a desalination system. For comparison purposes, three working fluids, including R123, Isopentane and n-butane are considered in the ORC. Detailed energy and exergy analyses are carried out and then the system performance is optimised, thermodynamically. Results show that thermal and exergy efficiencies of the proposed system could be up to 89.2% and 43.05%, respectively. These efficiencies are considerably higher than the conventional values for single or cogeneration systems. It is also found that considering 1 kg/s of geothermal hot water stream, the produced pure water is enough for almost 36 residential units.

Investigation of Shroud Geometry to Passively Improve Heat Transfer in a Solar Thermal Storage Tank

A shroud and baffle configuration is used to passively increase heat transfer in a thermal store. The shroud and baffle are used to create a vena contracta near the surface of the heat exchanger, which will speed up the flow locally and thereby increasing heat transfer. The goal of this study is to investigate the geometry of the shroud in optimizing heat transfer by locally increasing the velocity near the surface of the heat exchanger. Two-dimensional transient simulations are conducted. The immersed heat exchanger is modeled as an isothermal cylinder, which is situated at the top of a solar thermal storage tank containing water (Pr = 3) with adiabatic walls. The shroud and baffle are modeled as adiabatic, and the geometry of the shroud and baffle are parametrically varied. Nusselt numbers and fractional energy discharge rates are obtained for a range of Rayleigh numbers, 105 ≤ RaD ≤ 107 in order to determine optimal shroud and baffle configurations. It was found that a baffle width of 75% of the width of the heat exchanger provided the best heat transfer performance.

Thermodynamic analysis and optimization of a novel dual-evaporator system powered by electrical and solar energy sources

A novel dual-evaporator system with dual-source (renewable and electrical energies) is proposed to provide negative and positive evaporator temperatures. The system is a combination of the generator–absorber heat exchange (GAX), ejector-expansion transcritical CO2 refrigeration (EETC), Organic Rankin Cycle (ORC) and supercritical CO2 power cycles. The system is analyzed and optimized thermodynamically in detail. It is found that allocating the lower temperatures (−25 to −45 °C) for EETC evaporator and higher temperatures (5–10 °C) for GAX evaporator is more suitable. Detailed exergy analyses reveal that 19.89% and 5.92% of total input exergy, are useful in EETC evaporator and GAX evaporator, respectively. The ejector is found to be the highest source of irreversibility in the system. Moreover, the system performs better than dual-evaporator systems recently reported in literature.

Performance Evaluation of R134a/DMF-Based Vapor Absorption Refrigeration System

This article describes an experimental investigation to measure performances of a vapor absorption refrigeration system of 1 ton of refrigeration capacity employing tetrafluoro ethane (R134a)/dimethyl formamide (DMF). Plate heat exchangers are used as system components for evaporator, condenser, absorber, generator, and solution heat exchanger. The bubble absorption principle is employed in the absorber. Hot water is used as a heat source to supply heat to the generator. Effects of operating parameters such as generator, condenser, and evaporator temperatures on system performance are investigated. System performance was compared with theoretically simulated performance. It was found that circulation ratio is lower at high generator and evaporator temperatures, whereas it is higher at higher condenser temperatures. The coefficient of performance is higher at high generator and evaporator temperatures, whereas it is lower at higher condenser temperatures. Experimental results indicate that with addition of a rectifier as well as improvement of vapor separation in the generator storage tank, the R134a/DMF-based vapor absorption refrigeration system with plate heat exchangers could be very competitive for applications ranging from –10°C to 10°C, with heat source temperature in the range of 80°C to 90°C and with cooling water as coolant for the absorber and condenser in a temperature range of 20°C to 35°C.

Heat and mass transfer during heating of a hybrid absorption/sensible storage tank

A numerical study of heat and mass transport in a tank of aqueous calcium chloride as heat is added to the tank is presented to evaluate the potential of storing water, diluted, and concentrated salt solutions in a single tank to provide sensible and thermochemical (sorption) storage for solar space heating and hot water. An immersed heat exchanger provides the means to sensibly charge the tank while a stable density interface between layers of different salt mass fraction minimizes mass transfer to permit long term storage. Transient simulations are presented over the wide range of conditions expected in practice, as represented in dimensionless form by the Rayleigh number (Ra), buoyancy ratio (N), geometric aspect ratio (a), Lewis number (Le) and Prandtl number (Pr). Correlations of Nusselt number (Nu) and Sherwood number (Sh) for 7×103<Ra<3.3×109, 1<N<26.4, 0.125<a<1, and 15<Le<1.5×104 are in the form Nu=0.45a0.5Ra0.28 and Sh=0.001Ra0.28N−0.69Le1.17.

Design improvement of central cooling plant for energy saving using cooling tower water

This paper investigates and recommends design improvement for a central cooling plant to enhance the energy efficiency of the system. A storage tank together with an immersed heat exchanger is installed in the liquid line between the condenser and expansion valve to utilize the make-up water of the cooling tower. The heat exchanger uses the make-up water to reduce the refrigerant temperature leaving the condenser and thus enhance the refrigerant sub-cooling. For this purpose, an existing central cooling plant of a commercial building, located in a hot and dry climate region, is used for experimentation and data collection. The energy consumption and relevant data of the existing central cooling plant were acquired over the course of a typical week in summer. First, theoretical-empirical system models including mechanistic relations between components are developed. A numerical algorithm together with monitored data and mathematical model implemented on a transient system simulation tool is used to predict the performance of the new design under transient loads. The integrated simulation tool was validated by comparing predicted and measured power consumption of the central cooling plant. Comparison of the proposed system with existing cooling plant is included in this paper to demonstrate the advantages of our new configuration. Results show that about 9.7% in average and up to 14% power saving can be obtained by using our design while maintaining the comfort condition inside the building.

Nanofibrous membrane-based absorption refrigeration system

This paper presents a study on the efficacy of highly porous nanofibrous membranes for application in membrane-based absorbers and desorbers. Permeability studies showed that membranes with a pore size greater than about one micron have a sufficient permeability for application in the absorber heat exchanger. Membranes with smaller pores were found to be adequate for the desorber heat exchanger. The membranes were implemented in experimental membrane-based absorber and desorber modules and successfully tested. Parametric studies were conducted on both absorber and desorber processes. Studies on the absorption process were focused on the effects of water vapor pressure, cooling water temperature, and the solution velocity on the absorption rate. Desorption studies were conducted on the effects of wall temperature, vapor and solution pressures, and the solution velocity on the desorption rate. Significantly higher absorption and desorption rates than in the falling film absorbers and desorbers were achieved.

Transient Modeling of a Lithium Bromide – Water Absorption Chiller

This article presents a thermodynamic framework for a lithium bromide – water absorption chiller, in which a transient model is developed to simulate the operation process. Local energy and mass balance within the main components like absorber, regenerator, condenser, evaporator and solution heat exchanger is respected to investigate the behavior of the chiller. Experimental correlations are used to predict heat transfer of the related working fluids. The cooling water is set to typical cooling tower conditions of tropical countries such as Singapore. The coefficient of performance (COP) is evaluated against a range of heat source temperatures from 75oC to 100oC. The results indicate the operation conditions of the chiller at its maximum COP is 95oC to 100oC.

Buoyancy Driven Mass Transfer in a Liquid Desiccant Storage Tank

A new concept for long-term solar thermal storage is based on the absorption properties of aqueous calcium chloride. Water, diluted and concentrated calcium chloride solutions are stored in a single tank. An immersed heat exchanger and stratification manifold are used to preserve long-term sorption storage, and to achieve thermal stratification. The feasibility of sensible heating the tank via large-scale natural convection without mixing salt solutions is demonstrated via measurements of velocity, CaCl2 mass fraction, and temperature in a 1500 l prototype tank. Experiments are conducted over a practical range of the relevant dimensionless parameters. For Rayleigh numbers from 3.4 × 108 to 5.6 × 1010 and buoyancy ratios from 0.8 to 46.2, measured Sherwood numbers are 11 ± 2 to 62 ± 9, and the tank is thermally stratified. Convective mixing between salt layers is inhibited by the presence of a steep density gradient at the interface between regions of differing mass fraction. The predicted storage time scales based on mixing via natural convection for the reported Sherwood numbers are 160–902 days.

Influence of stripper operating parameters on the performance of amine absorption systems for post-combustion carbon capture: Part I. High pressure strippers

Amine absorption is a mature technology that is widely applied on a commercial scale for the removal of acid gases from gas mixtures. It is viewed as the most promising technology to separate carbon dioxide (CO2) at point emission sources such as fossil fuel fired power plants as a part of a strategy called ‘carbon capture and storage’ being deployed to mitigate climate change. However, there are major challenges in advancing its use for this application; the most prominent one being – cost. It is estimated that the application of current technology to CO2 capture will result in a 70–100% increase in the cost of electricity (COE). As a part of this two-part study, we have examined the influence of stripper operating parameters on carbon capture at a 400MW pulverized coal-fired power plant retrofitted with amine absorption technology. We use the process simulation software, ProMax® to simulate the amine absorption process. Three commercial absorbents namely monoethanolamine (MEA), diethanolamine (DEA) and diglycolamine (DGA) are considered in this work. All the absorbents examined are studied at the typical working concentrations used in commercial operation. We also constrain the CO2 loading of rich amine solution to 0.4mol-CO2/mol-amine to closely resemble the chemical environment in commercial operation. The main pointer used to compare system performance is the parasitic power loss due to the carbon capture. Parasitic power loss comprises of the loss of plant output due to withdrawal of reboiler steam from the low pressure turbine; the electricity required for CO2 compression and by the plant auxiliaries such as blowers and pumps. We also examine the absorber and stripper column sizes and heat exchanger parameters which are key components of the system. On the basis of our findings in the first part of this study, we report that operating the stripper at higher pressures has significant advantages but also some disadvantages and operational challenges. We also report that DEA and DGA have a superior performance to MEA when used in high pressure strippers. We conclude that increasing the stripper pressure reduces the parasitic losses and the equipment sizing but may result in higher solvent losses and equipment corrosion.

Modelling and performance prediction of an integrated central cooling plant for HVAC energy efficiency improvement

The aim of this paper is to explore methods to reduce the energy consumption of a central cooling plant. To achieve this, we have developed a new design for heating, ventilating and air conditioning (HVAC) efficiency improvement. A storage tank together with an immersed heat exchanger is installed in the discharge line between the compressor and condenser. The heat exchanger uses the make-up water of the cooling tower to reduce the refrigerant temperature entering the condenser. To investigate the potential of energy savings, we used a real-world commercial building with a central cooling plant, located in a hot and dry climate, for our case study. The energy consumption and relevant data of the existing central cooling plant were acquired over the course of a typical week in summer. The integrated system has been modeled and analyzed to achieve energy conservation. The performance of the proposed cooling system was simulated using a transient simulation software package. Comparison of the proposed system with existing cooling plant is included in this paper to demonstrate the advantages of our new configuration. Results show that up to 18% power saving can be obtained by using our design.

A novel hybrid GAX-ejector absorption refrigeration cycle with an air-cooled absorber

A novel hybrid generator–absorber heat exchange (GAX)–ejector absorption refrigeration (hybrid GAX–E) cycle, with ammonia–water as working fluid, is proposed and analysed in detail. Thermodynamic models are developed for the hybrid GAX–E, hybrid GAX, simple GAX and single effect ammonia-water absorption refrigeration cycles. A comparison among the performances of these four cycles, at an absorber temperature of 70°C and a generator temperature of more than 170°C, shows that the coefficient of performance (COP) of the hybrid GAX–E cycle is higher than those of the other three cycles. The maximum COP for the hybrid GAX–E cycle is around 1.8 which is about 20%, 125% and 260% higher than the corresponding values for the hybrid GAX, the simple GAX and the single effect cycles, respectively. Results indicate that as the ejector expansion ratio increases from 6.1 to 8.8, the COP of the hybrid GAX–E cycle decreases by up to 53.3%.

Comparison of compact and tubular generators performance for R134a–DMF

Heat and mass transfer during R134a desorption in compact generator and tubular generator have been compared experimentally. An R134a–DMF based vapour absorption refrigeration system is built with brazed plate heat exchangers as condenser, absorber, evaporator and solution heat exchanger with a rated cooling capacity of 1kW. This kind of system can be operated with low grade thermal energy such as solar energy and waste heat. Generator temperature is chosen accordingly between 353K and 368K. Two types of generators in parallel, which are a brazed plate heat exchanger with chevron H-type plate channels and stainless steel concentric tubes, are investigated. Performances of the generators, at different operating conditions, have been compared, to explore the significance of compact generator for the improvement of absorption refrigeration technologies. Influence of solution flow rate, hot water inlet temperature, condensing temperature, evaporator temperature on generators is studied. Heat transfer rate, desorption rate, heat and mass transfer effectiveness increase with the hot water inlet temperature but decrease with increase in condensing temperature. Even though heat and mass transfer rates increase, heat transfer effectiveness and mass transfer effectiveness decrease with solution flow rate.

Experimental studies on heat and mass transfer in tubular generator for R134a-DMF absorption refrigeration system

Experimental investigations have been carried out to study heat and mass transfer during desorption of Tetrafluoro ethane (R134a) from R134a and Dimethyl formamide (DMF) solution in a tubular generator. Absorption refrigeration system has been built with brazed plate heat exchangers as condenser, absorber, evaporator and solution heat exchanger and with stainless steel concentric tubes as the generator. Effects of operational parameters viz., solution two phase Reynolds number, driving temperature ratio, driving pressure ratio, solution initial concentration on generator performance are analyzed. Desorption ratio, Sherwood number and Nusselt number increase as the solution Reynolds number, solution initial concentration, driving temperature ratio increase whereas these parameters decrease as the driving pressure ratio increases. Finally a correlation for Nusselt number and Sherwood number are proposed based on the experimental studies.

Dynamic Simulation of Sub-Scale ITER CS/STR Cooling Loop

The dynamic simulation of a sub-scale ITER Central Solenoid (CS)/STRucture (STR) cooling loop has been performed to investigate the cryogenic control strategies against pulsating heat loads. HELIOS facility has been modified for this particular study, which consists of a Liquid Helium (LHe) reservoir with two immersed heat exchangers, a circulation pump and an approximately 130 m long piping with three evenly distributed heated sections. The setup provides the equivalent thermo-hydraulic configuration of the ITER CS/STR, cooled by the forced-flow Supercritical Helium (SHe). The dynamic simulator, C-PREST, has been utilized to model the HELIOS test loop and to compare the results with the experimental data sets. This paper presents the dynamic simulation results and discusses the control strategy to minimize the cooling power requirements, to have high reliability.