Experimental and numerical study of heat transfer characteristics using the heat balance in a linear compressor

A linear compressor was divided into several control volumes, and its thermal performance was analyzed by maintaining the whole energy balance throughout the heat transfer analysis for each control volume. During the steady state performance test of the compressor, basic measurements of the temperature and pressure were performed by using a calorimeter. First, the energy flow of each control volume was defined using the results of the measurements, and then heat transfer analysis was conducted. Next, the thermal network was defined by using a correlation for the energy balance. A software tool for compressor simulation was developed using a thermal network analysis in order to estimate the energy efficiency ratio (EER) according to the changes in thermal properties of the compressor. Experimental results show that the simulation tool is satisfied up to a 5% change in the EER according to variations in the ambient temperature and mixing ratio of the refrigerant.     

Performance of vapor compression systems with compressor oil flooding and regeneration

Vapor compression refrigeration technology has seen great improvement over the last several decades in terms of cycle efficiency through a concerted effort of manufacturers, regulators, and research engineers. As the standard vapor compression systems approach practical limits, cycle modifications should be investigated to increase system efficiency and capacity. One possible means of increasing cycle efficiency is to flood the compressor with a large quantity of oil to achieve a quasi-isothermal compression process, in addition to using a regenerator to increase refrigerant subcooling.In theory, compressor flooding and regeneration can provide a significant increase in system efficiency over the standard vapor compression system. The effectiveness of compressor flooding and regeneration increases as the temperature lift of the system increases. Therefore, this technology is particularly well suited towards lower evaporating temperatures and high ambient temperatures as seen in supermarket refrigeration applications. While predicted increases in cycle efficiency are over 40% for supermarket refrigeration applications, this technology is still very beneficial for typical air-conditioning applications, for which improvements in cycle efficiency greater than 5% are predicted. It has to be noted though that the beneficial effects of compressor flooding can only be realized if a regenerator is used to exchange heat between the refrigerant vapor exiting the evaporator and the liquid exiting the condenser.     

An exergy method for compressor performance analysis

An exergy method for compressor performance analysis is presented. The purpose of this is to identify and quantify defects in the use of a compressor's shaft power. This information can be used as the basis for compressor design improvements. The defects are attributed to friction, irreversible heat transfer, fluid throttling, and irreversible fluid mixing. They are described, on a common basis, as exergy destruction rates and their locations are identified. The method can be used with any type of positive displacement compressor. It is most readily applied where a detailed computer simulation program is available for the compressor. An analysis of an open reciprocating refrigeration compressor that used R12 refrigerant is given as an example. The results that are presented consist of graphs of the instantaneous rates of exergy destruction according to the mechanisms involved, a pie chart of the breakdown of the average shaft power wastage by mechanism, and a pie chart with a breakdown by location.     

气体压缩机中冷器的节能节材实验研究

中冷器的主要功能是排除气体被压缩过程产生的热量,是提高压缩机效率的重要设备。从传热阻力看,壳程空气的热阻占总热组的80%以上,壳程气体阻力大,风机或压缩机能耗较大。要提高中冷器的传热性能,关键是强化壳程空气的对流传热和减小壳程空气的流动阻力。着重介绍气冷侧强化传热技术产生的节能效益,建立扭曲管中冷器和传统弓形折流板中冷器并行对比实验测试平台,通过改变壳程空气质量流量、管内循环水温度和流量等参数以测试其热力性能和压降损失,实验结果表明,扭曲管中冷器的壳程气体压降小,综合传热性能明显优于传统弓形折流板中冷器35%-87%,低Re数条件下尤为显著。对压缩机冷却系统的优化设计有一定的指导作用。     

Capacity-controlled ground source heat pumps in hydronic heating systems

The objective of this study was to investigate the energy-saving potential of using variable-speed capacity control instead of the conventional intermittent operation mode for domestic ground source heat pumps. Variable-speed capacity control is commonly used in air-to-air heat pumps, but not in ground source heat pumps for hydronic heating systems, even though the energy-saving potential may be greater for this application. A theoretical analysis indicates how the energy efficiency is influenced by variable-speed capacity control of the compressor. The analysis shows that, to take full advantage of the capacity control, care should be taken to achieve the correct relationship between refrigerant flow and heat transfer media flows. Intermittent control and variable-speed capacity control were compared by laboratory tests on two capacity-controlled heat pumps and one standard heat pump with a single-speed compressor. Test data were then used for seasonal performance factor (SPF) calculations. The SPF calculations show that despite improved performance at part load the variable-speed controlled heat pump did not improve the annual efficiency compared to the intermittently operated heat pump. This is mainly due to inverter and compressor motor efficiencies and the need for improved efficiency and control of pumps used in the heating and ground collector systems.     

螺杆式空气源热泵机组压缩机油位试验研究

分析压缩机吸气回液、外置油分离器油分离效率对螺杆式空气源热泵机组压缩机油位的影响,提出改进翅片换热器分液、优化气液分离器结构、改善油分离器分离方式等改进措施,并对螺杆式空气源冷（热）水机组进行试验。结果表明,所提改进措施不仅能够提高压缩机运行过程中的油位,而且能够提高机组运行稳定性。     

A numerical model for thermal mapping in a hermetically sealed reciprocating refrigerant compressor

In a refrigerant compressor, improvement in performance such as reduction of various electrical and mechanical losses, reduction of gas leakage, better lubrication, reduction of suction gas heating etc. can be achieved by maintaining a low temperature rise inside the compressor. Proper selection and location of an internal over load protector relay, estimation of heat transfer coefficient and winding insulation coefficient are also vital in enhancing the performance. In this context it is necessary to understand the temperature distribution inside a compressor for an optimal design. In this paper, a numerical model has been created and a heat transfer analysis for a hermetically sealed reciprocating refrigerant compressor is presented. The temperature distribution inside the compressor has been obtained taking into consideration the various heat sources and sinks and compared with experimental results. The maximum temperature was observed at the rotor which was 427.5 K. The deviation of the predicted rotor temperature from that of experimental value is 5.5% only. A good agreement was found between experimental results and that predicted in the numerical analysis.     

Experimental assessment of energy storage via variable speed compressor

In this study, usage of a variable speed refrigeration system in latent heat thermal energy storage (LHTES) system is investigated to increase energy storage efficiency. Four different compressor speed control cases are compared to obtain a constant heat transfer fluid (HTF) temperature at the inlet of the energy storage tank. These control cases are (i) control with evaporation temperature, (ii) control with ethylene glycol temperature at the outlet section of evaporator, (iii) control with suction pressure of the compressor and (iv) on/off control. By means of the experimental analysis the best control strategy is obtained as control with Case (ii), in terms of stability of inlet temperature of heat transfer fluid, variations of energy efficiency of LHTES and coefficient of performance (COP) of the system. While the Case (ii) provides the most stabile inlet temperature of HTF, Case (iv) represents the worst stability. Besides, the highest energy efficiency (99.0%), exergy efficiency (87.0%) and COP (2.05) values observed in Case (ii). Additionally, the time period to reach the set value is nearly 50 min in Case (i), Case (iii) and Case (iv) on the other hand this unstable initial time period becomes nearly 25 min for Case (ii). As a result, variable speed compressor should be controlled with Case (ii) to stabilize ethylene glycol-water solution temperatures as well as increase efficiency and COP of the system.     

Maximum practical efficiency of helium temperature refrigerators

An ideal refrigerator using a perfect gas working fluid is defined which gives the efficiency of a refrigerator as a function of compressor and expander efficiency, heat exchanger temperature difference, and heat exchanger pressure drop. Although not suited to detailed hardware design, this approach clearly relates the overall cycle efficiency to component efficiencies. In contrast, computer studies of specific cycles using real fluid properties are usually such that the details tend to overshadow major trends. The results of the study show that in an efficient cycle the major losses are in the compressor and the cold and expansion device. For current compressor and expander efficiencies the maximum practical helium temperature refrigerator efficiency is about 37% of Carnot.     

A comprehensive model of a novel rotating spool compressor

A comprehensive simulation model of a novel rotating spool compressor is presented. The spool compressor provides a new rotary compression mechanism with easily manufactured components. A detailed analytical geometry model of the spool compressor is presented, which includes the geometry of the vane. This geometry model is included in an overall comprehensive compressor model that includes sub-models for friction, leakage, and heat transfer. The results of the comprehensive model were validated using experimental data from a prototype compressor. The prototype compressor has an overall displacement of 23.9 cm³, and was operated using R410A as the working fluid. The model predicts the volumetric efficiency, discharge temperature, and shaft power of the prototype compressor to within 3.13% MAE, 16.5 K and ?13.2 W average deviation, respectively. The trends and spread in the data indicate that additional effort should be focused on the operation of the active sealing elements within the compressor.     

MVR系统中离心式蒸汽压缩机与蒸发器的匹配特性研究

为了保证机械式蒸汽再压缩（mechanical vapor recompression, MVR）系统的运行经济性和稳定性,对MVR系统中离心式蒸汽压缩机与蒸发器的匹配特性进行研究。针对蒸发器换热系数在新投、工作和结垢工况下的变化,提出了蒸发器运行温阻特性线的概念,并将其与离心式蒸汽压缩机的温升特性线叠加,从而开展离心式蒸汽压缩机与蒸发器的匹配分析。通过分析发现,离心式蒸汽压缩机的设计流量偏大或蒸发器的换热面积过小会导致匹配不足,易发生喘振,从而影响MVR系统的运行稳定性。而离心式蒸汽压缩机的设计流量偏小或蒸发器的换热面积过大会导致匹配过度,致使MVR系统的运行经济性差,甚至可能造成MVR系统无法建立热力自循环。结果表明,离心式蒸汽压缩机在MVR系统启动过程中会出现不稳定的喘振现象,可以通过系统参数的临时调节或采取辅助措施来避开不稳定区。设计时应保证离心式蒸汽压缩机的喘振裕度大于20%,蒸发器换热面积的设计裕度为30%;MVR系统运行时实际蒸发温度与设计温度的偏差应控制在±5 ℃以内。研究结果可为MVR系统的设计和调试提供参考。     

Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump systemPerformance et caractéristiques de transfert de chaleur des frigorigènes hydrocarbures utilisés dans un système de pompe à chaleur

Performance of a heat pump system using hydrocarbon refrigerants has been investigated experimentally. Single component hydrocarbon refrigerants (propane, isobutane, butane and propylene) and binary mixtures of propane/isobutane and propane/butane are considered as working fluids in a heat pump system. The heat pump system consists of compressor, condenser, evaporator, and expansion device with auxiliary facilities such as evacuating and charging unit, the secondary heat transfer fluid circulation unit, and several measurement units. Performance of each refrigerant is compared at several compressor speeds and temperature levels of the secondary heat transfer fluid. Coefficient of performance (COP) and cooling/heating capacity of hydrocarbon refrigerants are presented. Experimental results show that some hydrocarbon refrigerants are comparable to R22. Condensation and evaporation heat transfer coefficients of selected refrigerants are obtained from overall conductance measurements for subsections of heat exchangers, and compared with those of R22. It is found that heat transfer is degraded for hydrocarbon refrigerant mixtures due to composition variation with phase change. Empirical correlations to estimate heat transfer coefficients for pure and mixed hydrocarbons are developed, and they show good agreement with experimental data. Some hydrocarbon refrigerants have better performance characteristics than R22.     

An investigation of the performance of a scroll compressor under liquid refrigerant injection

In this study, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the liquid refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection refrigerant. And furthermore, the performance of the liquid refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the refrigerant injection on the oil viscosity and refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.     

Potential of the single-screw compressor

The single-screw compressor has the potential to become the dominant form of compressor in the 50–150 kW power range. It is capable of high energy efficiency because losses due to leakage, frictional effects and heat transfer factors are capable of being reduced to a low level. A new gaterotor design for this purpose is described. Low cost is possible because the compressor is compact, the new gaterotor is less expensive to manufacture, the bearings can be simplified, and the casing can become a simple pressure vessel.     

Flash gas bypass for improving the performance of transcritical R744 systems that use microchannel evaporators

The performance of transcritical R744 systems with direct expansion (DX) can be significantly improved by implementing a Flash Gas Bypass (FGB). The idea behind the concept is to bypass refrigerant vapor, created during the isenthalpic expansion process, around the evaporator. By feeding the evaporator with liquid refrigerant, pressure drop is reduced and refrigerant distribution is improved. With R744 as the working fluid, increased refrigerant-side heat transfer coefficients are expected as well. In addition, the FGB concept proves to be beneficial in terms of system design, in particular for combined air-conditioning and heat pumping applications. An experimental comparison to a conventional DX-system reveals that FGB increases the cooling capacity and COP at the same time by up to 9 and 7%, respectively. Even larger improvements are possible in case a variable speed compressor is utilized to match the performance of the conventional DX-system. A simulation model helps to separate the individual improvement mechanisms. It was found that the reduction of refrigerant-side pressure drop is the dominant improvement mechanism of FGB.     

Experimental investigation on the drop-in performance of R407C as a substitute for R22 in a screw chiller with shell-and-tube heat exchangers

A drop-in test of a mixed refrigerant R407C is performed in a commercial screw chiller with shell-and-tube heat exchangers originally designed for R22. The test results show a severe performance reduction when substituting the refrigerant from R22 to R407C. The reason for the performance reduction is analyzed comprehensively, and the influence of thermodynamic properties, compressor efficiency, and heat transfer is evaluated quantitatively. The major factor causing the performance reduction is assessed as the degradation of the heat transfer in using the mixed refrigerant, R407C. The heat transfer degradation in the evaporator is found to be larger and influences more on the chiller performance reduction. The performance reduction caused by the evaporator is approximately two times compared with that of the condenser.     

Experimental investigation on convective heat transfer mechanism in a scroll compressor

This paper describes an experimental study on the convective heat transfer inside the scroll compressor. An experimental refrigeration system is composed with extensive instrumentations in the compressor that is operated at variable speeds. The 13 thermocouples installed inside the compressor monitor the temperatures of the scroll wrap during compression process of refrigerant. The temperature and the pressure of refrigerant at suction, and the pressure at discharge ports are measured, and applied to the numerical simulation as the operating condition parameters. The temperature measured at the discharge port is used to verify the simulation result with relevant heat transfer coefficient. This paper describes the effect of motion of the orbiting scroll on the convective heat transfer in the scroll wraps. Separate experiments are performed to investigate the heat transfer in such a peculiar physical condition. With this experimental result, the effect of the oscillation of the wall on the heat transfer is quantitatively analyzed and applied to the simulation of compression process in scroll compressor. The whole consecutive compression processes in the scroll compressor is simulated in detail by solving equations of mass and energy balance for the refrigerant. The modified heat transfer coefficient correlation considering the effect of motion of the orbiting scroll predicts the discharge temperature better than other typical heat transfer coefficients.     

城市原生污水源热泵机组的设计创新与实验研究

对现有城市原生污水源热泵机组当前的技术状况与存在的问题进行分析,并进行了改进,主要特色是:1)制冷剂侧切换污水源热泵系统;2)换热器采用蒸发、冷凝两用换热器;3)污水可直接进入换热器。同时,将变频螺杆压缩机及压缩机可变内容积比等重要技术应用到改进的机组中,并制作了污水源热泵机组样机。通过权威机构验证,污水源热泵机组处于名义制冷与制热工况下的满负荷运行性能卓越,产品优势明显。     

Theoretical study of R32 in an oil flooded compression cycle with a scroll machine

R32 is regarded as a potential alternative for R410A, but it has a low slope of isentropic line, high superheat inside a compressor and thus a high discharge temperature. These disadvantages limit its wider adoption. In order to improve the performance of R32 air conditioner, oil flooded compression with regenerator has been suggested. A single stage oil flooded compressor model is developed to obtain a more accurate system-level improvement. In the compressor model, the heat transfer losses between shell and ambient, suction gas and motor, and high-pressure and low-pressure cylinders are considered. By means of parametric studies, it was found that the novel cycle resulted to be beneficial to increase the compressor internal superheating, to decrease the compressor heat losses and to improve its overall isentropic efficiency while cooling capacity or heating capacity is degraded. COP_h improvement can reach up to 16.4% for an evaporating and condensing temperatures of −25 °C and 45 °C, respectively. The discharge temperature resulted to be lower than 110 °C. In addition, a thorough comparison between R32 and R410A with both novel and baseline systems has been carried out. The results indicate that the novel cycle has potential benefits for applications in R32 air conditioners.     

Thermodynamic analyses on an ejector enhanced CO2 transcritical heat pump cycle with vapor-injection

In this paper, an ejector enhanced vapor injection CO₂ transcritical heat pump cycle with sub-cooler (ESCVI) for heating application in cold regions is proposed. The thermodynamic analysis using energetic and exegetic methods is carried out to predict the performance characteristics of the ejector enhanced cycle, and then compared with those of the conventional vapor injection heat pump cycle with sub-cooler (SCVI). The simulation results demonstrate that the ejector enhanced cycle exhibits better performance than the conventional vapor injection cycle under the specified operating conditions. The improvements of the maximum system COP and volumetric heating capacity could reach up to 7.7% and 9.5%, respectively. Exergetic analysis indicates that the largest exergy destruction ratio is generated at the compressor followed by the evaporator and gas cooler. Additionally, the exergy efficiency of the ejector is introduced to quantify the effectiveness of the exergy recovery process, which may be a new criterion to evaluate the performance of the ejector enhanced vapor compression cycle.