在LiBr吸收式制冷机中使用索柯洛夫公式计算传热温差的探讨

目前在LiBr吸收式制冷机设计中，在计算发生器、冷凝器、蒸发器、吸收器、溶液热交换器的传热温差时都无一例外地使用索柯洛夫公式[1]。本文对索柯洛夫公式进行了分析和比较，认为目前该公式使用的意义已不大，直接用对数平均温差计算即可。     

用于温室农业的空调系统设计

为了满足温室夏季和冬季的温湿度要求,提出一种用于温室农业的空调系统。该系统由发生器、吸收器、溶液热交换器、冷凝器、截止阀、泵以及连接管路等组成,发生器内通入温度高的热水使稀溶液沸腾后成为浓溶液,冷凝器内通入冷却水用于冷却从发生器出来的水蒸气,并根据冬季和夏季条件的不同,通过控制截止阀和泵的启停,使系统稳定运行。同时冷凝器冷凝后的水被用于温室植物的灌溉,形成一个生态循环。并通过建立数学模型进行热力计算,结果显示该系统在制冷和制热工况下能效比分别为0.705和0.485。     

溶液热交换器对吸收式制冷机性能影响的研究

建立两级吸收式制冷循环中溶液热交换器的仿真模型，分析溶液热交换器浓溶液进出口温差对两级系统制冷系数、冷却水流量的影响，同时分析逆流、交叉流对溶液热交换器单位换热面积的影响。理论模拟结果得出溶液热交换器浓溶液进出口温差存在一个最佳值，对两级吸收式系统这个最佳温差大约为13℃，其对应的制冷系数增加12．3％，冷却水流量减小8．8％。最佳温差的确定对减小溶液热交换器体积，提高系统经济性和整体效率都有着重要的指导意义。     

扩散——吸收式制冷系统的热力计算

本文对扩散——吸收式制冷系统的工作机理及热力计算进行了探讨。对该系统发生器中热虹吸过程以及吸收器与蒸发器之间的气体循环过程作了热力分析,并提出扩散——吸收式制冷机选定循环参数和热力计算的方法。     

汽车发动机余热溴化锂吸收式制冷系统研究

根据现有汽车空调的制冷系统和发动机冷却水及排气系统的结构特点,结合溴化锂吸收式制冷系统的工作原理,提出将汽车排气管和发动机冷却水箱进行结构改造作为溴化锂吸收式制冷系统的发生器,代替传统的汽车空调的制冷和采暖系统及发动机冷却系统;应用热力学、传热学和流体力学的方法对该溴化锂制冷系统进行了热力计算和传热面积的计算,计算结果表明,溴化锂制冷系统充分利用了废气余热和冷却水余热,减少了汽车油耗,并且改造后的排气热交换器和冷却水箱传热面积小,结构简单紧凑.     

小型无泵溴化锂吸收式制冷系统的实验研究

建立了小型太阳能热水型无泵溴化锂吸收式制冷系统,系统主要采用降膜吸收器、降膜蒸发器、弦月型通道热虹吸提升管等新型设计。为了提高制冷系统的整体运行效果,首次设计了一套二次发生装置,使系统能在较低的初始溶液浓度范围（46％～54％）下运行,并保持较高的放气范围和吸收率,有助于提高吸收器性能;并使冷剂水产量较之不使用二次发生器的情况增大1．68倍,明显改善了蒸发效果;对冷凝器与蒸发器间压差的建立也起到一定的作用,改善了制冷系统的整体运行性能,平均制冷系数可达0．725。     

基于传质换热的LiBr吸收式制冷理论循环的热力学分析

本文以采用传质换热的溴化锂吸收式制冷系统为研究对象,在流出溶液热交换器溶液状态已定的基础上,采用EES软件对系统进行热力学分析及计算,研究传质量、系统的COP、系统所需总热量与发生器出口溶液浓度、冷却水温度之间的关系。经分析发现:传质量与发生器出口溶液浓度成反比,且与冷却水入口温度成正比的关系;而系统的COP、系统所需总热量与发生器出口溶液浓度、冷却水温度之间不是线性关系,存在着转折点。     

R245fa传热性能实验

蒸发器和冷凝器的传热性能是影响制冷机组和双工质发电机组做功效率的关键因素,为了提高中低温余热在热泵机组和地热双工质发电系统中的利用效率,本文选用R245fa循环工质,对满液式蒸发器和冷凝器进行实验研究,分别讨论热水温度进口温度对蒸发器和冷凝器的传热系数、蒸发压力和冷凝压力的影响。研究结果表明,在工质流量和冷却水流量保持不变的条件下,蒸发器传热系数随着热水进口温度和温差的增大而减小,冷凝器的传热系数随着热水进口温度的增加先增大后减小,蒸发器传热系数可以达到2500 W/(m2?℃);蒸发器压力和汽轮机前后的压差随着热水出口温度的增加而增加,热水流量对蒸发压力和冷凝压力的变化浮动较小。     

R245fa传热性能实验研究

蒸发器和冷凝器的传热性能是影响制冷机组和双工质发电机组做功效率的关键因素,为了提高中低温余热在热泵机组和地热双工质发电系统中的利用效率,本文选用R245fa循环工质,对满液式蒸发器和冷凝器进行实验研究,分别讨论热水温度进口温度对蒸发器和冷凝器的传热系数、蒸发压力和冷凝压力的影响。研究结果表明,在工质流量和冷却水流量保持不变的条件下,蒸发器传热系数随着热水进口温度和温差的增大而减小,冷凝器的传热系数随着热水进口温度的增加先增大后减小,蒸发器传热系数可以达到2500 W/(m2·℃);蒸发器压力和汽轮机前后的压差随着热水出口温度的增加而增加,热水流量对蒸发压力和冷凝压力的变化浮动较小。     

板翅式蒸发器液冷系统设计

为了研究板翅式换热器在沸腾相变换热情况下的传热及压降特性,设计一套采用氟里昂蒸气压缩循环的液体冷却试验系统,其中板翅式换热器作为蒸发器,R22作为制冷剂,热水作为被冷却液体提供制冷剂蒸发所需热量。对该系统的运行工况、参数范围及系统各部件（包括压缩机、冷凝器、毛细管、板翅式蒸发器）进行设计及选择,并介绍所搭建的试验台。     

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.     

Theoretical analysis of low-temperature hot source driven two-stage LiBr/H2O absorption refrigeration system

A detailed theoretical analysis is presented for a two-stage LiBr/H₂O absorption refrigeration system, which consists of an evaporator, a low-pressure absorber, a low-pressure generator, a high-pressure absorber, a high-pressure generator, a condenser, a low-pressure heat exchanger and a high-pressure heat exchanger, driven by a low-temperature hot source. A comparison of results from the theoretical analysis and preliminary experiment indicates that the theoretical analysis developed can represent a real system with a reasonable accuracy, and is useful for future development.     

分离式热管管排组合与传热

分离式热管在不改变蒸发段和冷凝段总换热面积的情况下通过管排组合可以改变热管换热器局部热管的面积比来改变传热。研究分离式热管换热器在不同管排组合下传热差异,总结大温差传热下采用何种管排组合取得废热回收过程中的热管安全、高压发生器中均匀传热以及传热能力的均衡。     

Economic biogas and cooling water rates in a lithium bromide—water absorption system

An economic analysis of the role of biogas and cooling water in a lithium bromide—water absorption system has been carried out to optimize the generator, condenser and absorber temperatures at a given evaporator temperature and solution pumping rate. The analysis has been repeated for different pumping rates (PR) to determine the optimum PR corresponding to the minimum over-all operating cost of the system. The study has also been carried out for the condition when biogas in the generator and cooling water in the absorber and condenser are supplied at equal flow-rates. It is found that the performance of the LiBr-H₂O system at equal biogas and cooling water flow-rates is about 5.988% higher than when operated at the minimum over-all operating cost, the latter being cheaper by only 2.71%. For low evaporation temperatures, use of a preheater in a LiBr-H₂O system creates a crystallization problem when operated at low pumping rates. The study has therefore been extended for a system without preheater. The parameters under study are illustrated graphically against the generator temperature. Equations to obtain the corresponding optimum condenser and absorber temperature are given. The functional relationship between crystallization limit and absorbent temperature has also been obtained. The optimum operating parameters are presented graphically.     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

Performance of an HCFC22-based vapour absorption refrigeration system

A single-stage vapour absorption refrigeration system (VARS) is tested with monochlorodifluoromethane (HCF22) as refrigerant and different absorbents: dimethylether of tetraethylene glycol (DMETEG) and dimethyl acetamide (DMA). The influence of generator temperatures in the range 75–95°C, which represents low-grade heat sources, is studied. Cooling water temperatures were varied between 20 and 30°C. Two cases of cooling water flow paths are considered, i.e. water entering either absorber or condenser, which are connected in series. For HCFC22-DMETEG, COP values in the range 0.2–0.36 and evaporator temperatures between 0 and 10°C are obtained. For HCF22-DMA, COP values in the range 0.3–0.45 and evaporator temperatures between −10 and 10°C are obtained. It is observed that HCFC22-DMETEG can work at lower heat source temperatures than HCFC22-DMA. However, at the same operating conditions HCFC22-DMA is better from the viewpoints of circulation ratio and COP. Experiments also show that at low heat source temperature, cooling water temperature has strong influence on circulation ratio but does not affect COP significantly. Preferably, cooling water should first flow through the condenser and then through the absorber in order to achieve improved overall performance.     

Modeling of an ammonia–water absorption heat pump water heater for residential applications

An investigation of a direct gas-fired single-effect ammonia–water absorption heat pump water heater for residential applications is presented. Combustion of natural gas provides heat to the desorber where refrigerant is generated. The absorber and condenser are hydronically coupled in parallel to a hot water storage tank, while the evaporator is hydronically coupled to an ambient air heat exchanger that extracts ambient heat. A thermodynamic model is developed and the system configuration is optimized to provide a baseline heating capacity of 2.79 kW at a coefficient of performance of 1.74. A detailed parametric study over a range of water and ambient temperatures is used to understand the variation in system performance as the water is heated from 14.5 to a minimum of 57.0 °C. The performance of the heat pump coupled to a 227-liter storage tank is also modeled for three different scenarios, a cold start response to a 76-liter draw, and response to stand-by losses. The absorption heat pump water heater is found to achieve coefficients of performance better than those of commercially available gas fired heaters and electric heat pump units.     

Investigation the effects of different heat inputs supplied to the generator on the energy performance in diffusion absorption refrigeration systems

This study aims to investigate experimentally the effects of three different heat inputs supplied to generator on the energy performance of the diffusion absorption refrigeration system. To achieve this goal, a conventional diffusion absorption refrigeration system, in which electrical resistance as heat input is employed, is taken a model, which is experimentally scrutinized under different heat inputs, 62, 80 and 115 W, but at the same ambient temperatures and the same filling rate of three-component working fluid. In the analyses, the energy losses rejected to ambient from rectifier, condenser, absorber, solution heat exchanger as well as other components such as solution tank and pipes, the energy gain by evaporator and also energy performance is investigated. While the highest energy performance is calculated for DAR-62 W system as 0.36, the lowest energy performance is calculated for DAR-115 W system as 0.30.     

Comparison of total energy systems using gas turbines and diesel engines for combined cooling

A gas turbine engine was used to drive the compressor of a vapour compression cycle so that the usually wasted energy in the exhaust gases was partially recovered and used in the generator of an absorption cycle. The cooling effect was therefore boosted. The degree of energy utilization was further enhanced when the energy released from the absorber and condenser of both cycles was recovered in the form of hot water, which could be used for different applications. The performance parameters for this combined system, such as the cooling effect, total heat recovered and performance effectiveness ratio, were calculated for various evaporator and condenser temperatures. It was found that a system driven by a gas turbine gives a better performance than a diesel engine system under similar operating conditions.     

An absorption based miniature heat pump system for electronics cooling

The development of an absorption based miniature heat pump system is motivated by the need for removal of increasing rates of heat from high performance electronic chips such as microprocessors. The goal of the present study is to keep the chip temperature near ambient temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid. A novel dual micro-channel array evaporator is adopted, which reduces both the mass flux through each micro-channel, as well as the channel length, thus reducing the pressure drop. Micro-channel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane. This acts as a common interface between the desorber and the condenser to separate the water vapor from LiBr solution. The escaped water vapor is immediately cooled and condensed at the condenser side. For direct air cooling of condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.