Heat Transfer Area Of Evaporator Research Articles

Energetic performance evaluation of an automotive CO2 air conditioning system with a dual-evaporator configuration

Inefficient performance of CO2 refrigerant in hot climates has become a major obstacle to its wide application in electric vehicles. In this paper, the vacant heating core in cooling mode was exploited to increase the heat transfer area of evaporator and improve system performance. With the aid of a validated evaporator model and an experimental test rig, performance improvements brought by this added area were evaluated. Results indicated that this area is not suggested to be directly added to cooling core. This necessitates a dual-evaporator (D-E) configuration. For case I (small CO2 pressure drop), the growth rate in heat transfer rate brought by D-E configuration is 19∼20% and less sensitive to parallel or series operation. Nevertheless, this growth rate for case II (large CO2 pressure drop) is greatly affected, 30.6∼32.6% for parallel operation and 7.5∼17.6% for series operation. Specifically, it is around 17.5% and 7.5∼13.6% for series cross-parallel and cross-counter arrangements, respectively. By comprehensive consideration, the cross-parallel arrangement is recommended to be adopted. Experimental results indicated that the cross-parallel D-E configuration greatly increases the evaporation temperature and exhibits a slight impact on optimum high pressure within the transcritical cycle. The enhancement in COP brought by this configuration is around 6∼12% without additional cost, and it rises with the decreased air flowrate through evaporator and air velocity through gas cooler and the increased ambient temperature. This indicates that this configuration is an effective and economical technology to alleviate the efficiency degradation issue of transcritical CO2 automotive air-conditioners.

Power and efficiency optimizations of an irreversible regenerative organic Rankine cycle

Rational and efficient uses of low temperature waste heats have become an important research topic, and organic Rankine cycle is one of effectual methods for the recoveries of waste heats. The performances of simple and regenerative organic Rankine cycles have been analyzed and optimized by using classical thermodynamics and finite thermodynamics. Based on finite time thermodynamics, an irreversible regenerative organic Rankine cycle model is built in this paper. With the fixed total heat transfer area of all heat exchangers (evaporator, condenser and regenerator) in the cycle, the power output and thermal efficiency of the cycle are maximized with the changes of heat transfer area ratios of the heat exchangers, mass flow rate of the working fluid and superheat. The influences of the internal irreversible losses on the power output and thermal efficiency of the cycle are analyzed. The results illustrate that there exist an optimal evaporator heat transfer area, an optimal mass flow rate and an optimal superheat to make the power output reach its triple maximum. There also exist an optimal evaporator heat transfer area ratio, an optimal regenerator heat transfer area ratio and an optimal superheat to make the thermal efficiency reach its triple maximum. The power output and thermal efficiency after triple optimizations are increased by 25.22% and 12.16%, respectively. Moreover, the internal irreversible losses have similar effects on the power output and thermal efficiency. Useful guidelines for the designs of the practical regenerative organic Rankine cycles can be offered from the gained results of this paper.

Optimization of a separator assisted two-phase thermosyphon loop by using entropy generation analysis

In this paper, an optimization model is developed for a separator assisted two-phase thermosyphon loop and basic two-phase thermosyphon loop based on the entropy generation minimization method. The total entropy generation rate of the system is proposed as an optimization objective function and the total heat transfer area of evaporator and condenser is considered as a constraint. The model takes into account the irreversibility due to both irreversible heat transfer and pressure drop. The results show that there always exists minimum total entropy generation rate of the two systems when the heat transfer area ratio varies. The optimal heat transfer area ratios corresponding to the minimum total entropy generation rate also lead to the minimum total thermal resistance of the system and achieve the lowest temperature of the cooled object. Besides, the separator assisted two-phase thermosyphon loop can gain lower total entropy generation rate than the basic two-phase thermosyphon loop. Furthermore, the effects of the heat load, the total heat transfer area, the working fluid charge, the inner diameter of the separator and the height of two-phase tube on the minimum total entropy generation rate and the corresponding area ratio are also evaluated.

Performance evaluation and parametric analysis of AMTEC/TEG hybrid system

A general mathematical model of the AMTEC/TEG hybrid system consisting of alkali metal thermal electric converter (AMTEC) and thermoelectric generator (TEG) has been developed, in which the load parameters and main geometric parameters were considered. Instead of specifying some parameters, the evaporating temperature and condensing temperature of the AMTEC as well as the operating temperature of the TEG subsystem were determined by solving energy balance equations. Results show that electrode current density has a quite noticeable influence on both AMTEC subsystem and TEG subsystem. The maximum conversion efficiencies for AMTEC and AMTEC/TEG are 27.42% and 31.33%, respectively. Besides, it is interesting to note that the optimal value of the ratio of thermoelectric module area to electrode area is always near 0.2 regardless of the ratio of evaporator heat transfer area to electrode area. The length of thermoelectric arms l and the ratio of the thermoelectric couple area to the total area of the thermoelectric module b are critical factors, b ≥ 0.55 and l ≤ 11 mm should be satisfied to obtain a compact and low-cost system.

Thermodynamic analysis on an instantaneous water heating system of shower wastewater source heat pump

Abstract Water reuse and desalination systems are energy intensive processes, and their increasing use is leading energy consumption within water systems to be an increasingly important issue. Shower wastewater contains large amounts of heat, so there is an opportunity to recover energy from shower water to offset energy consumption elsewhere in water systems. This paper found ways to increase the output of hot water and lower the energy consumption by establishing a thermodynamic model of an instantaneous wastewater source heat pump. The system proved to be very effective, the heating COP (coefficient of performance) can reach 3.3 even in the winter. Under the conditions of limited heat transfer area, reducing the suction pressure of a compressor is a more feasible way to increase the hot water output to meet the needs of users rather than increasing the discharge pressure. Besides, increasing the heat transfer area of the evaporator is a more effective option. When the heat transfer area of evaporator varies from 0.5 to 1.0 square meters, a notable change is that the heating COP increases from 3.283 to 3.936. The heating COP in a system with a recuperator can reach 5.672, almost double that compared to the original systems.

Heat source temperature control investigation of a hydraulic retarder based organic Rankine cycle

In the paper, a hydraulic retarder based organic Rankine cycle (ORC) system is investigated. In order to achieve stable oil output temperature in the evaporator under different hydraulic retarder operating conditions, evaporating pressure is studied to match with a specified evaporator heat transfer area through iterative calculations. An experiment test is a setup to conduct the validation. The results show that regulating evaporating pressure can achieve a target evaporator oil output temperature. However, expander load moment and working fluid mass flow rate (MFR) need to be controlled precisely. With the increase of retarder rotor speed, required evaporating pressure generally decreases, while working fluid MFR increases dramatically. Effects of the changed evaporating pressures and MFR s on expander power output increases from 2.52 kW to 31.43 kW, while cycle thermal efficiency changes slightly in low filling rate condition, from 10.2% to 10.7% for 25% filling condition.

Study Analysis of Flue Gas Utilization as Alternative Power Generation in Cement Plant Using Organic Rankine Cycle System

Abstract—Cement plant produce large amount of heat source in cement making process, due to inefficiency of system there still waste heat available in form of flue gas that can be utilize. Flue gas in cement plant can be utilized as alternative power generation. With the 200-300 o C temperature output range of flue gas from suspension preheater and air quenching cooler (AQC) in cement plant, organic rankine cycle (ORC) can be suitable option for alternative power generation. ORC is development of rankine cycle, the different is the working fluid in ORC using refrigerant. In cement plant that produce 8466 TPD kiln production, used flue gas from suspension preheater to dry raw material and produce 163888 m3/h flue gas from AQC that still not utilized. Flue gas with 235 o C temperature from AQC can utilized for power generation purpose using ORC system. Waste heat recovery calculation carried out to know the potential recovery. Operating condition of the ORC system will determine power produced that can be generated and ORC components calculated and selected according to the operating condition of the system. Using R141b as working fluid with 8 bar pressure and 110 o C temperature inlet to turbine, power produced by turbine is 666 kW. For the components, evaporator and condenser use shell and tube heat exchanger, with evaporator heat transfer area is 676.49 m2 while condenser has 510 m2 of heat transfer area. And for working fluid pump it needs 16.235 Kw power to pump R141b back to evaporator.

Thermodynamic analysis and fluid selection for hydraulic retarder waste heat organic rankine cycle

An ORC (Organic Rankine Cycle) system driven by hydraulic retarder waste heat oil source is investigated in this study. The main purpose is to identify the appropriate organic working fluid in consideration of the thermodynamic properties and evaporator dimension requirements. In addition, six working fluids including dry and isentropic fluid are used as the candidates in this article. Based on a specified condition, thermal efficiency and expansion ratio flexibility in the operating condition of a wide evaporating pressure and condensing temperature range, exergy efficiency and exergy destruction rate, evaporator heat transfer, and evaporator heat transfer area are chosen as the evaluation criteria to select the optimal working fluid. The results indicate that R245ca, R265mfc, and R123 occupy a larger coverage area as for the high thermal efficiency region, meaning that the operating conditions are more flexible for these fluids in the real vehicle application. Also, a same behavior is presented by the expansion ratio results. Besides, the evaporator heat transfer areas for R245fa and R245ca are the smallest, the maximum areas for which are just 2.59 m2 and 3.22 m2, respectively. Consequently, the optimal working fluid for the hydraulic retarder ORC system turns out to be R245ca. © 2016 American Institute of Chemical Engineers Environ Prog, 36: 548–556, 2017

Analysis of a mechanical vapor recompression wastewater distillation system

This paper presents a comprehensive design model of single-effect mechanical vapor recompression (MVR) system. The analysis focuses on the prediction of the specific power consumption of the compressor and the heat transfer areas of the evaporator. The MVR unit, with a capacity of 20kg/h and horizontal tube falling film evaporation, is designed and installed in the laboratory. The performance of the system was investigated using high salinity wastewater containing Na2SO4. Except for the compressor consumption, experimental data shows good agreement for the predictions. The specific power consumption for the vapor compressor is 55.6kWh/m3. The experimental and theoretical results indicated that the evaporation rate increases by increasing the evaporate temperature from 70°C to 85°C. The evaporation rate, compressor consumption and heat transfer area of evaporator depend strongly on the temperature difference of heat transfer. The evaporation rate and specific power consumption increase linearly with temperature difference. On the other hand, the heat transfer area of evaporator goes down with the increase of temperature difference. This inferred that there is an optimum value of temperature difference that will lead to the optimum system with lower power consumption and smaller heat transfer area.

A model for study and optimization of real-operating refrigeration machines

A general model for the study and optimization of reversed cycle thermal machines with two heat reservoirs is presented. The model is based on the First and Second Laws of Thermodynamics, heat transfer equations at finite thermal source and sink capacities, and entropy generation terms in order to consider the internal and external irreversibilities of the cycle. The proposed model is applied to real-operating machines for which several constraints are imposed. Some results generated by the model when applied to refrigeration machines are presented. They point out interesting results regarding the limits of the variation range for the model variables under different operating conditions and also for different variation laws for the internal entropy generation term. Sensitivity studies with respect to the evaporator heat transfer area and other decision parameters show the dependence of the refrigerating machines performances on the model variables and decision parameters and what the investment concern is with respect to the gain in performances. Experimental data are used in this study, in comparison with the analytical ones. Copyright © 2008 John Wiley & Sons, Ltd.

Capacity modulation of an experimental motor-driven heat pump

Various methods of capacity modulation of electric motor driven heat pumps have been reviewed with a particular emphasis on variable-speed motors. The effect on the steady-state performance of the heat pump, of variation in the externally-controllable independent parameters, while the compressor speed or the evaporator heat transfer area was adjusted, has been studied experimentally. Based on these results some control algorithms have been proposed.