Characteristics of a gravity-assisted heat pipe-based heat exchanger

An experimental study of the performance of an air-to-air thermosyphon-based heat exchanger utilizing R-22 as the working fluid has been carried out to investigate its behavior under different operating conditions. A test installation has been developed to model a variety of HVAC real life applications. The results reported in this article describe the influence of various parameters such as: supply and exhaust air stream mass flow rates, stream temperatures and exhaust stream moisture content on the effectiveness of the heat exchangers. Heat exchanger heat flow hysteresis has been recognized. Some optimization criteria are presented.     

Thermal modelling and energy conservation studies on freon rankine cycle cooling system with regenerative heat exchanger

This communication presents a thermodynamic analysis and assessment of a Freon fluid Rankine cycle cooling system. The system consists of two subsystems—Rankine engine (RE) power cycle and a vapour compression (V-C) refrigeration cycle. The heat engine subsystem consists of a boiler, turbine, condenser and a feed pump while the cooling subsystem consists of a mechanical compressor, condenser, evaporator and an expansion valve. A number of working fluid combinations for the RE cycle and V-C cycle subsystems have been chosen on the basis of their thermodynamic properties and their suitability judged in terms of the performance parameters, namely, the thermal efficiency of the power cycle and the coefficient of performance (COP) of the refrigeration cycle. A regenerative heat exchanger (RHE) is incorporated in the RE cycle to improve the cycle efficiency and achieve energy conservation.The effects of various operation parameters, namely, component temperatures, adiabatic expansion/compression efficiencies and effectiveness of the RHE on the overall COP have been assessed. It is found that R114 + R22 give the best overall system performance and the presence of the RHE improves the system COP significantly. The effect of V-C cycle condenser temperature is more pronounced as compared to that of the RE cycle condenser and similarly the effect of evaporator temperature in the V-C cycle is more pronounced as compared to that of the boiler in the Rankine cycle subsystem.     

Thermal—fluid modelling of an air—cooled condenser for refrigerants

A steady state thermodynamic model for an air—cooled finned—tube condenser, used in a typical deep freezer vapour compression system, operating with pure and refrigerant mixtures has been developed using finite difference method. The heat transfer aspects in the condenser are treated uniquely for superheated, two—phase and subcooled regimes and studied for various heat load, pressure, temperature, and mass flow rate. The condensation heat transfer coefficient, the tube length required for condensation, the degree of subcooling, and the temperature glide for different ambient temperatures are estimated. The simulation and the experimental results are in good agreement.     

Conceptual thermodynamic design of a dual mode absorption cycle with an auxiliary heat exchanger

This paper presents a conceptual thermodynamic design and feasibility study of a dual mode absorption cycle with auxiliary heat exchanger. The system consists of two single stage absorption cycles connected through the condenser of the first stage with the generator of the second stage. The purpose of the auxiliary heat exchanger is to maintain the condenser (1) temperature sufficiently high to operate the generator (2). A case study, using a single fluid pair H₂OLiBr in both the stages and two fluid pairs (H₂OLiBr at first stage and NH₃LINO₃ at second stage) has been considered. It is found that the COP for the two pair system in comparison to the single pair system. However, the former has the advantage of providing simultaneous refrigeration and air-conditioning.     

环路热管式空调机组传热特性的实验研究

采用环路热管式空调机组用于新风的预冷及再热,减少了表冷器的冷量及降低再热设备的能耗。文中研究了充液率、倾角对热管蒸发段和冷凝段温差及显热效率的影响。研究表明,环路热管式空调机组,充液率介于55%～75%时,热管蒸发段和冷凝段温差较大;随着倾角的增加,热管蒸发段和冷凝段温差也逐渐增大,显热效率也随之较高。因此,采用环路热管式空调机组可降低能耗,提高人体的热舒适性,可用于热带及亚热带地区,最大限度回收热量。     

Heat pipe heat exchanger optimization

An equation to determine the optimum position of the partition separating a heat pipe into evaporator and condenser regions in a heat pipe heat exchanger was formulated by minimizing the total thermal resistance of the heat path. The optimal position of the partition plate for various operating conditions was found and analysed. Some improvements of thermal efficiency could be achieved.     

Performance of a dehumidifying compression heat pump with auxiliary heat input

Experiments are carried out on a R114 dehumidifying heat pump installed in a closed circuit drying tunnel. The influence of auxiliary heat input before condenser and/or evaporator are studied. Actual coefficient of performance varied from about 2 to 3. Studies throw light on the significance of the location of the auxiliary heat input in the drying circuit. It is observed that the dryer temperature could be controlled within certain limits using the low grade heat input before the condenser or evaporator. This could substitute high temperature heat conventionally input after the condenser. This is also significant if the same dryer has to be used for drying different materials which require different drying temperatures.     

On the dynamics and control of a polymer film compact heat exchanger

Process intensification has the potential to change the state of the chemical and process industries. The polymer film compact heat exchanger (PFCHE) is a new type of intensified heat exchanger. The potential market is seen as being large [1] but as yet the unit has not been adopted by industry. The advantageous heat transfer characteristics of such a unit have been shown in previous work [2]. This work investigates the dynamic behaviour of the PFCHE and the process control problems that may arise.The PFCHE available at Newcastle University has been used to generate dynamic temperature data. The data have been employed to formulate and validate time series type models. These models were then used in simulated process control studies. The dynamic behaviour of the unit appeared to be linear and response times were quick. It was found that the responses of the model to disturbances in inlet temperatures could be controlled well using a digital form of PI control. There were, however, doubts as to the ability of the model to completely replicate the system.     

组合式热管换热器在热泵系统中的试验研究

采用组合式热管换热器作为生活浊水余热热泵热水器的热管蒸发器,并在试验台上进行了不同污水进口温度和不同污水流量的可视化试验,通过分析试验结果,发现组合式热管上升管和下降管的温差很大的现象,这与热管特性相矛盾。采用理论分析和实验相结合的方法,研究了热管中产生温差的原因。它主要是由于下降管压力损失较大,使热管工质在热管蒸发器中的滞留引起。     

Regenerative heat exchanger with a periodic change in the airflow direction for room ventilation

This paper presents the results of experimental research of heat transfer in air-to-air regenerative heat exchanger with periodic change of flow direction. The temperatures of the airflow and the material of regenerative packing in different sections have been obtained. The temperature efficiency of the heat exchanger has been determined at different flow rates. The developed mathematical model of the regenerative heat exchanger is described. It is shown that the model fairly well describes the experimental results. Based on numerical studies the dependence of thermal efficiency of the heat exchanger on the airflow rate is determined. It is shown that changing the ratio of the oppositely directed flow rates, it is possible to regulate the temperature of the air flowing into the room. The possibility of using the model for optimizing the operational and design parameters of heat exchanger is demonstrated.     

Experimental analysis of a diesel engine driven water-to-water heat pump

The paper describes the experimental analysis of a diesel driven water-to-water heat pump. The system was developed utilising commercially available equipment with, wherever possible, a minimum of modifications.Heat was extracted from water at ambient temperature by the evaporator and delivered to water at the condenser. Heat from the engine cooling water and exhaust system was also recovered, at a power less than that of the condenser, but at greater temperature. Therefore three distinct streams of heated water were available. The system was of medium size with a total output ranging from 7.5 to 13.5 kW. The system and component performance was determined over a range of compressor speeds, evaporating and condensing pressures. The heating coefficient of performance ranged between 2.7 and 7.3 while the ratio of the total heat output to the heating value of fuel supplied to the engine was found to be between 1.3 and 2.6.     

Influence of nucleation on the flow boiling heat transfer coefficient of a refrigerant mixture under varied heat flux conditions

The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers. The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s⁻¹ and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients are also compared with well known heat transfer correlations.     

Heat transfer and Marangoni flow in a circular heat pipe using self-rewetting fluids

In this study, Marangoni flow and heat transfer enhancement in a heat pipe have been investigated. The experiments were carried out at different heat inputs. A constant temperature water bath was used at the condenser section at three temperature levels. Heat transfer coefficients and thermal resistances of the heat pipe were measured for pure water and water/butanol solutions. The experimental results confirmed that the heat pipe filled with butanol solutions showed better thermal performance than the water-filled heat pipe. At maximum heat flux, 25% heat transfer improvement was obtained when 7 wt% butanol solution was used instead of pure water.     

地下水源热泵系统换热器匹配性研究

建立了地下水源热泵系统实验装置,针对夏季工况和冬季工况分别研究了换热器面积的匹配对系统性能的影响。实验结果分析表明,适当增大冷凝器面积既可提高机组性能,又可避免在冬季工况时冷凝压力过高,保证机组稳定可靠的运行。     

Theoretical studies on adsorption heat transformer using zeolite-water vapour pair

An adsorption heat transformer can raise the temperature level of a fraction of waste heat by rejecting the remaining heat to a low temperature level. In this work some alternatives in the design of an adsorption heat transformer, such as a 2-tank system, 3-tank system and 4-tank system, are evaluated using zeolite-water vapour as the adsorbent-adsorbate pair. The values of coefficient of performance (COP) are computed for each system for various temperatures of waste heat source at which the heat is available and heat sink at which the heat is delivered.It is found that an adsorption heat transformer can be used for a gross temperature lift as high as 50°C with a fairly good COP value. Moreover the 4-tank system gives a much improved COP value as compared to the 2-tank and 3-tank systems for the same operating conditions. It is also found that the effect of temperature driving force for heat transfer on the COP value is quite pronounced.     

Computer modeling of a direct contact condensing heat exchanger for gas furnaces

Experiments have been performed with a direct contact condensing heat exchanger for gas furnaces, in which a direct contact bubble distributor was used to heat and criculate water through a finned-tube heat exchanger which heats the return room air blown through. This paper presents the development of a computer model simulating the system, and the results of parametric studies. The heat transfer analysis of the system indicated possible further improvement of the system. A modified system was proposed and a model was also developed for the system. Comparisons of the heat transfer performance between the two models are given. The research activity concerning this work was supported by the Gas Research Institute (U.S.A.).     

适于低温的双级压缩热泵热水器动态仿真模型

建立了双级压缩空气源热泵的动态仿真模型。对压缩机、膨胀阀以及换热器分别建立部件模型。由于压缩机和膨胀阀的热惯性较小,所以采用稳态模型来描述。蒸发器和冷凝器部件建立动态模型时,针对制冷剂侧、管壁、以及载冷剂侧三个控制容积,分别建立控制方程。最后,通过参数耦合,建立系统仿真模型。     

Experimental study on the heat transfer characteristics of a new type flat micro heat pipe heat exchanger with latent heat thermal energy storage

An experimental energy storage system has been designed using a new type flat micro heat pipe heat exchanger that incorporates a moderate-temperature phase change material paraffin with a melting point of 58°C. The basic structure, working principles, and design concept are discussed. The heat transfer process during the charging and discharging of the heat exchanger under various operating conditions has been experimentally investigated. Results show that the performance of the new type flat micro heat pipe was steady and efficient during charging and discharging. The average thermal storage power and absorption efficiency have been determined to be approximately 537 W and 92.5%, respectively.     

Improvement of CANDU-1000 MW(e) power cycle by moderator heat recovery

Four different moderator heat recovery circuits are proposed for CANDU-1000 MW (e) reactors. The proposed circuits utilize all, or part, of the 155 MW(th) moderator heat load (at 70°C moderator outlet temperature from calandria) to the first stage of the water heating system. An economics study was carried out and indicated that the direct circulation of feed water through the moderator heat exchanger (with full heat recovery) is the most economical scheme. For this scheme the saved steam from the turbine extraction was found to produce additional electric power of 8 MW(e). This additional power represents a 0.7% increase in the plants nominal electric output. The outstanding features and advantages of the selected scheme are also presented.