空分启动阶段主换热器热端温差过大的原因分析及处理

介绍了空分设备启动阶段主换热器热端温差过大的现象和原因 ,通过计算膨胀机的最大膨胀量 ,得出膨胀量偏大是导致热端温差过大的原因     

Thermoeconomic optimization of subcooled and superheated vapor compression refrigeration cycle

An exergy-based thermoeconomic optimization application is applied to a subcooled and superheated vapor compression refrigeration system. The advantage of using the exergy method of thermoeconomic optimization is that various elements of the system—i.e., condenser, evaporator, subcooling and superheating heat exchangers—can be optimized on their own. The application consists of determining the optimum heat exchanger areas with the corresponding optimum subcooling and superheating temperatures. A cost function is specified for the optimum conditions. All calculations are made for three refrigerants: R22, R134a, and R407c. Thermodynamic properties of refrigerants are formulated using the Artificial Neural Network methodology.     

Prediction of condensation in steam ejector for a refrigeration system

An experimental refrigeration system based on a two-stage steam ejector was set-up in the Thermodynamics and Heat Transfer Laboratory of our Department. The system optimization and realization have been described elsewhere ( [Grazzini and Mariani, 1998] and [Grazzini and Rocchetti, 2008] ). In both stages, primary flows are highly supersonic and reach low pressure and temperature levels. As usual in the literature, an ideal gas model was used during the design process. This paper is intended to check the validity of this assumption. In order to understand the actual working condition of our system, several models have been compared. The presence of high flow speed suggests the existence of metastable conditions. To set the border for the metastable region, the spinodal curve has been drawn. Isentropic expansion of vapour through the nozzle, modelled as ideal gas, seems well within the metastable zone. However, the Classic Nucleation Theory shows that the Wilson line is crossed at the nozzle throat. Condensation produces a marked difference in the conditions at the nozzle exit. Results coming from the present analysis will be used in further optimization of the experimental ejector design.     

压水堆核电厂过冷度显示的仿真

清华大学核电厂模拟培训中心模拟机的参考电厂是美国本屋公司设计的其模型中没有堆芯过冷度显示系统的模拟。根据三哩岛事故的教训，在该装置上增设过冷度显示的模拟是十分必要的。本文叙述了在原模拟机上增设过冷度显示所进行的软件设计、数据传输和显示线路设计。     

Experimental investigation of the effect of condenser subcooling in R134a and R1234yf air-conditioning systems with and without internal heat exchanger

This paper presents an experimental study about the effect of condenser subcooling on the performance of an air conditioning system operating with R134a and R1234yf, under the same operating conditions. For both refrigerants, it has been shown that the COP undergoes a maximum as a consequence of the trade-off between increasing refrigerating effect and increasing specific compression work. At a given operating condition, the system COP increased up to 18% for R1234yf and 9% for R134a. These results confirmed the trends obtained from a previous theoretical analysis, demonstrating that a system operating with R1234yf can benefit more from the condenser subcooling than that with R134a. The experimental results also showed that the presence of an internal heat exchanger significantly reduces the COP increase due to condenser subcooling, since both improvements compete towards reducing the throttling losses. Besides the interference between IHX and condenser subcooling, the use of both simultaneously still yields a more efficient air conditioning system, especially for R1234yf.     

垂直管氨水降膜吸收传质研究

吸收器是影响氨水吸收式制冷系统的关键部件。对常用的垂直管氨水降膜吸收器进行绝热吸收模拟分析,并将模拟分析结果与实验结果进行了对比。在此基础上,通过实验进一步研究了稀溶液进口过冷和外部吸收冷却对吸收传质的影响,得到稀溶液进口过冷能够促进吸收传质的结论。结果表明:氨水吸收式制冷传质率的模拟结果与实验结果的偏差在20%以内,在冷却吸收和进口过冷的实验测试工况下,吸收传质率提高了132%。     

A combined-cycle refrigeration system using ejector-cooling cycle as the bottom cycle

A combined-cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector-cooling cycle (EJC) is proposed and studied. The EJC is driven by the waste heat from the RAC/MC and acts as the bottom cycle of the RAC/MC. A system analysis shows that the COP of a CCRS is significantly higher than a single-stage refrigeration system. Improvement in COP can be as high as 18.4% for evaporating temperature of the RAC/MC T_e at −5°C. A prototype of the CCRS was built and tested in the present study. Experimental results show that at T_e=−4.5°C, COP is improved by 14% for a CCRS. For T_e at 5°C, COP can be improved by 24% for a CCRS with higher condensing temperature of the RAC/MC. The present study shows that the CCRS using the ejector-cooling cycle as the bottom cycle of the RAC/MC is viable. Further improvement in COP is possible since the prototype is not designed and operated at an optimal condition.     

Mechanical sub-cooling vapor compression systems: Current status and future directions

Using mechanical sub-cooling systems to increase COP of vapor compression cycles is a known method in literature to save energy and increase efficiency. Recently, much progress has been made with respect to investigation into its different aspects that can help to put it into practice. Numerical and experimental works are considered for the purpose of highlighting this progress. These can be categorized as: a) simulation of performance characteristics resulting from different refrigerant combinations in dedicated mechanical sub-cooling systems, b) variation in performance characteristics for a vapor compression cycle using integrated mechanical sub-cooling because of fouling, c) experimental study about consequences of employing a dedicated mechanical subcooling cycle with a simple vapor compression system, and d) experimental investigation about consequences of employing a subcooler in a two-stage refrigeration cycle. Some important results are discussed. Finally, some suggestions are made to provide direction into future research in this area to help put it into practice.     

Simulation of subcooled flow boiling in a micro-channel

Bubble behavior was simulated to analyze the mechanism of subcooled boiling in a micro-channel. Bubble growth, condensation, and collapse in subcooled boiling, as well as the function of the degree of subcooling, lift-off diameter, heat flux, and mass flux are discussed. The influence of surface tension on interfacial heat transfer is likewise presented. In the calculated results, both onset of nucleate boiling and boiling heat transfer are influenced by micro-channel size. The simulated flow pattern corresponds with some experimental results. Findings on the boiling heat transfer from the simulation were compared with experiments.     

Effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger

Liquid refrigerant injection technique can be a very effective method for controlling subcooling and the compressor discharge temperature of a refrigeration system at high ambient temperatures. In this study, the effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger were investigated by varying the liquid injection rate at the conditions of constant expansion valve opening in the evaporator and constant total flow rate. During the tests, the ambient temperature was maintained at 43 °C. With the increase of the liquid injection rate, the subcooling at the inner heat exchanger outlet increased and the superheat at the accumulator outlet decreased. However, unacceptable results such as the increase of the compressor discharge pressure and decrease of the system performance were also observed depending on the control method applied. To obtain high system performance and reliability, optimum control methods for liquid injection in the accumulator heat exchanger are suggested. The liquid injection technique for the refrigeration system with an accumulator heat exchanger was found to be an effective method for controlling adequate subcooling and the compressor discharge temperature of the refrigeration system at high ambient temperatures.