Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.     

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.     

Dynamics of a heat transformer working with the mixture NaOH---H2O

The model described in the paper is based on the balance equations for mass and energy for the different apparatus such as absorber, generator, evaporator, condenser and solution heat exchanger. It simulates a heat transformer process and ensures calculations not too time-consuming and much faster than in real time. With the aid of the model the transient behaviour of a heat transformer working with the mixture NaOH---H₂O is examined. For steady state operation the simulations agree well with experiments on a pilot plant. With NaOH---H₂O as working substances high COPS up to 0.49 were obtained with this heat transformer. For unsteady state operation the deviations between experiments and simulations were slightly higher than those for steady state. The model nevertheless predicts the transient behaviour with a satisfying accuracy. Also the startup and shutdown behaviour of a heat transformer was investigated under different conditions.     

Experimental studies on air-cooled NH3-H2O based modified gax absorption cooling system

An experimental investigation on the performance of an air-cooled modified generator absorber heat exchange (GAX) absorption cooling system has been carried out and presented in this paper. The conventional system is modified by incorporating high pressure GAX, low pressure GAX, a solution cooler and an additional solution heat exchanger to reduce the heat input to the system. The system is designed for a cooling capacity of 10.5 kW using ammonia-water (NH₃-H₂O) as the working fluid. The performance of the system in terms of the circulation ratio, internal heat recovery and coefficient of performance (COP) has been obtained. The system is capable of producing a low evaporator temperature of −5 °C, at a sink temperature of 35 °C, under no load conditions. The results indicate that at a generator and evaporator temperature of 120 °C and 2 °C respectively, the system delivers a maximum cooling capacity of about 9.5 kW with a fuel and total COP of 0.61 and 0.57 respectively.     

Optimization study of the compression/absorption cycle

For each external situation optimum working conditions for the compression/absorption cycle can be found. The improvement in cycle performance which is gained by optimizing the temperature gradient in the absorber is considerable, particularly for situations with small external temperature gradients. Theoretically, the external and internal temperature gradients should be equal to maximize the cycle performance. The introduction of a solution loop, however, changes this and the optimum internal temperature gradient is always larger than the external gradients. The optimum point of operation is found by studying the changes in the compressor and pump and the heat loss obtained in the solution heat exchanger with the working conditions. A comparison of a compression/absorption cycle, using NH₃-H₂O, and a compression cycle working with pure R12, always results in a higher coefficient of performance for the former. The capacity of the NH₃-H₂O system is also considerably higher.     

Dynamics of a heat transformer working with the mixture NaOHH2O

The model described in the paper is based on the balance equations for mass and energy for the different apparatus such as absorber, generator, evaporator, condenser and solution heat exchanger. It simulates a heat transformer process and ensures calculations not too time-consuming and much faster than in real time. With the aid of the model the transient behaviour of a heat transformer working with the mixture NaOHH₂O is examined. For steady state operation the simulations agree well with experiments on a pilot plant. With NaOHH₂O as working substances high COPS up to 0.49 were obtained with this heat transformer. For unsteady state operation the deviations between experiments and simulations were slightly higher than those for steady state. The model nevertheless predicts the transient behaviour with a satisfying accuracy. Also the startup and shutdown behaviour of a heat transformer was investigated under different conditions.     

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

Simulation study of a hybrid absorber–heat exchanger using hollow fiber membrane module for the ammonia–water absorption cycle

An innovative hybrid hollow fiber membrane absorber and heat exchanger (HFMAE) made of both porous and nonporous fibers is proposed and studied via mathematical simulation. The porous fibers allow both heat and mass transfers between absorption solution phase and vapor phase, while the nonporous fibers allow heat transfer between absorption solution phase and cooling fluid phase only. The application of HFMAE on an ammonia–water absorption heat pump system as a solution-cooled absorber is analyzed and compared to a plate heat exchanger falling film type absorber (PHEFFA). The substantially higher amount of absorption obtained by the HFMAE is made possible by the vast mass transfer interfacial area per unit device volume provided. The most dominant factor affecting the absorption performance of the HFMAE is the absorption solution phase mass transfer coefficient. The application of HFMAE as the solution-cooled absorber and the water-cooled absorber in a typical ammonia–water absorption chiller allows the increase of COP by 14.8% and the reduction of the overall system exergy loss by 26.7%.     

Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems

In this paper, the energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption systems is presented. A computational model has been developed for the parametric investigation of these systems. Newly developed computationally efficient property equations of water–lithium bromide solution have been used in the computer code. The analysis involves the determination of effects of generator, absorber and evaporator temperatures on the energetic and exergetic performance of these systems. The effects of pressure drop between evaporator and absorber, and effectiveness of heat exchangers are also investigated. The performance parameters computed are coefficient of performance, exergy destruction, efficiency defects and exergetic efficiency. The results indicate that coefficient of performance of the single effect system lies in range of 0.6–0.75 and the corresponding value of coefficient of performance for the series flow double effect system lies in the range of 1–1.28. The effect of parameters such as temperature difference between heat source and generator and evaporator and cold room have also been investigated. Irreversibility is highest in the absorber in both systems when compared to other system components.     

Performance enhancement of a diffusion-absorption refrigerator

An effort was undertaken to improve the current cycle performance of the diffusion-absorption refrigerator (DAR). The thermodynamic basis for the design changes is to reuse waste heat from the rectifier to heat the weak absorbent from the absorber. A new generator with heat exchanger (GWHX) was designed and fabricated. The test results of the DAR with GWHX are compared with baseline tests. The new generator design demonstrated a significant improvement in the cooling COP of as much as 50% compared to the baseline tests while the cooling capacity was unchanged.     

Thermodynamische Berechnungen für CO<Subscript>2</Subscript>-Kältemaschinenprozesse mit ein- oder zweistufiger Verdichtung in Klimaanlagen

For the design of an (automotive) air-conditioning system which employs R744 (carbon dioxide, CO₂) as a refrigerant, thermodynamic calculations are performed for vapour compression refrigeration processes with single-stage or double-stage compression and throttle-valve expansion. The process parameters are varied systematically for an evaporation temperature of 0?°C and a CO₂-cooler exit-temperature (lowest CO₂ temperature in heat transfer to the ambient) between 15?°C and 55?°C. Their influence upon the coefficient of performance, the specific and the volumetric refrigeration capacities is described. For the double-stage compression cycle, the ratio of the high-pressure and low-pressure mass flows is determined, too. The double-stage compression with interstage cooling and internal heat exchanger in the low- or high-pressure cycle shows the potential for a substantial increase in coefficient of performance relative to the single-stage compression cycle. Thermodynamically optimized intermediate pressure, high pressure and temperature change in the internal heat exchangers are presented for CO₂-cooler exit-temperatures of 25 and 40?°C. The increase in energy efficiency is paid for by additional invest and the more complex operation of the CO₂ refrigeration unit.The results are not only valid for air conditioning systems but can also be applied in the design and operation for other applications of CO₂ refrigeration systems with an evaporation temperature of 0?°C.     

The compression/absorption heat pump cycle—conceptual design improvements and comparisons with the compression cycle

Performance improvement of an industrial single-stage compression/absorption heat pump (CAHP) using an ammonia/water mixture as the working fluid has been studied theoretically. By allowing a higher absorber pressure (40 bar) than the highest design pressure of today's screw compressors (25 bar), higher COPs could be obtained. Longer falling-film tubes in the vertical shell-and-tube absorber and desorber also increased the COP. These two modifications together increased the COP of the CAHP by 10%. The improved design has a lower optimal absorber glide (temperature difference due to composition change in absorber) and reduced solution heat exchanger sizes. The study was performed with a constant total area. Furthermore, the CAHP performance was studied for five heating cases. Its performance was compared to that of a two-stage compression heat pump (CHP) using isobutane as working fluid, on the basis of approximately equal investment cost. It could be concluded that only heating cases where both the sink and the source temperature changes are high (>20 K) give superior performance for the CAHP.     

内压缩流程空分设备低温换热与精馏研究

详细分析和讨论了内压缩流程空分设备的氧/氮两组分、氧/氮/氩三组分及氧/氩两组分和氮/氩两组分的相平衡,上塔、下塔、粗氩塔、精氩塔的精馏特征以及高压换热器、低压换热器、过冷器和主冷的换热特点。     

小型太阳能气泡泵吸收式制冷机研究

本文确定了设计计算参数下小型太阳能气泡泵吸收式制冷机制冷循环中各状态点参数,进行了发生器、冷凝器、蒸发器、吸收器、溶液热交换器的热负荷计算和加热热水、冷水、冷却水、稀溶液等循环介质的流量计算.进行了发生器、冷凝器、蒸发器、吸收器、溶液热交换器等换热设备所需的传热面积计算.根据求得的传热面积确定了各换热设备的传热管数,计算了热水进出口配管、冷却水进口配管、冷媒水进口配管的内径尺寸,为开发小型吸收式制冷机提供了一定的理论和实践基础.     

The effect of heat transfer additive and surface roughness of micro-scale hatched tubes on absorption performance

The objectives of this paper are to investigate the effect of heat transfer additive and surface roughness of micro-scale hatched tubes on the absorption performance and to provide a guideline for the absorber design. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.39–6.97 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. Normal Octanol is used as the heat transfer additive with the concentration of 400 ppm. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. The effect of heat transfer additive on the heat transfer rate is found to be more significant in the bare tube than that in the micro-scale hatched tubes. It is found that the absorption performance for the micro-hatched tube with heat transfer additive becomes up to 4.5 times higher than that for the bare tube without heat transfer additive. It is concluded that the heat transfer enhancement by the heat transfer additive is more significant than that by the micro-scale surface treatment.     

内压缩流程中高压空气压力和流量的选取

利用BOC的GTC物性软件,在Excel电子表格中,建立了内压缩流程空分设备液氧汽化过程中,冷热流体间的温差分布及因传热温差而产生损失的计算模型。说明了计算模型建立过程中的一些问题,利用所建立的计算模型,计算出了内压缩液氧与高压空气单独换热时,不同的液氧压力,使损失最小的高压空气压力;计算了用组合式换热器汽化3MPa液氧时,膨胀空气量、膨胀空气的压力和温度、内压缩低压液氧量、内压缩液氮量等参数变化对高压空气压力和流量的影响,得出了一些结论。     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.     

20000m~3/h空分设备主换热器泄漏分析及处理

20000m3/h空分设备因增压机后冷却器泄漏,冷却水进入铝制板翅式换热器的低压膨胀空气通道,并产生冰堵,使其泄漏,产品氮的纯度由2×10-6O2下降到147×10-6O2。通过对空分设备的流程和换热器结构的分析,正确判断了泄漏情况;在此基础上对换热器进行了修复,空分设备产品氮纯度达到了设计值。     

Cycle analysis of an air-cooled LiBr/H2O absorption heat pump of parallel-flow type

A gas-fired absorption heat pump with cooling capacity of 2 RT was analysed as an air-conditioner for domestic use during the summer. The absorption heat pump considered was an air-cooled, double-effect, LiBr/H₂O system of parallel-flow type. The performance of the absorption heat pump in the cooling mode of operation was investigated through cycle simulation to obtain the system characteristics depending on the inlet temperature of air to the absorber, the working solution concentrations, the solution distribution ratio of the mass of solution into the first generator to the total mass of solution from the absorber, and the LTDs (leaving temperature differences) of the heat-exchanging components. When the predicted results were compared with the measured data for similar design conditions, reasonable agreement was observed. The optimum design and operating conditions of an air-cooled absorption system are suggested based on this cycle simulation analysis.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.