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.     

全钛冷凝器主循环水系统的电偶腐蚀及其防护

由于钛合金在海水中的优良耐蚀性，现代舰船主循环水系统日益趋于采用全钛冷凝器。但采用全钛冷凝器后，对冷却水系统中其它结构材料将产生电偶腐蚀作用。本文通过电偶腐蚀试验和系统腐蚀试验，查明了这种影响，并提出了防止钛合金对其它电位较负材料产生的电偶腐蚀作用的措施。     

Assessment of R-438A as a retrofit refrigerant for R-22 in direct expansion water chiller

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h⁻¹). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants.     

基于实践应用的温湿度独立控制空调系统分析

温湿度独立控制空调系统成为近年研究的热点之一,但其在实践中的推广应用仍显缓慢。为了推动温湿度独立控制空调系统在实践中的应用,介绍该系统的去除显热和除湿系统部分,并对其中的冷水温度、供回水温差、系统节能性及优势等问题进行分析。研究表明：该系统具有节能性、安全性、可调性等优势,推荐采用直接蒸发式的新风机组与干式风机盘管组合模式的温湿度独立控制空调系统。     

Optimal supervisory control of a central chilled water plant with heuristic search sequential quadratic programming

A new methodology for adapting rigorous simulation programs to optimal supervisory control of a central chilled water plant is proposed in this article, which solves plant operation mode optimization and set points optimization by combining heuristic search with sequential quadratic programming. The mathematical basis of this algorithm is developed. A new derivative calculation strategy is introduced in set points optimization. This approach is applied to a central chilled water plant which consists of three chillers, two 3-cell cooling towers, three chilled water pumps and three condenser water pumps. Model verification study is performed. The optimal sequence of operation, set points of the decision variables at given load demand and weather condition are calculated. The plant performance and optimal control results are discussed.     

Development and performance of steam ejector refrigeration system operated in real application in Thailand

This paper presents the design and construction of a prototype steam ejector refrigeration system which can be operated under the actual condition of Thai environment, which is rather hot and humid. The prototype refrigerator was designed to produce a cooling capacity of approximately 3 kW. Water was selected to be used as the working fluid. The steam boiler used was a vertical fire tube type and it was designed to be used with LPG compact gas burner. The condenser was cooled by water obtained from a conventional cooling tower. The prototype refrigerator was used to produce chilled water which was used to cool a small tested room. It was observed that the room temperature of 24.2 °C was obtained at the cooling load of 3000 W. The cooling water was supplied to the condenser at about 30 °C. The COP obtained was 0.45. This prototype refrigerator is proven to be practical and can be used in actual environment of Thailand.     

Dew point refrigeration systems: Normalized sensitivity analysis and impact of fouling

Compressed natural gas is transmitted through a network of pipelines after its production from oil-gas wells. This gas is then cooled below the dew-point to remove the condensate that may accumulate in the transmission line, causing erosion and deposits. In this paper, the dew-point refrigeration system used with a gas compression system is investigated. The design conditions are obtained from the manufacturer data sheets, which are validated with a computer program to study both the design and rating of such systems. The coefficient of performance (COP) of the system at the design condition is 2.81, while the effectiveness of condenser is 0.84 and that of evaporator is 0.91. The system COP sensitivity with regard to inlet single-phase temperatures, superheat temperature and conductance of heat exchangers (UA), is examined. It is found that the system is more sensitive under design conditions as compared to performance operations. The effect of fouling that degrades the UA value of the condenser and evaporator is also studied. It is found that condenser fouling has a significant impact on the performance of the system. The impact of alternate refrigerants at optimized intermediate pressure, is also investigated.     

AHRI风冷冷水机组的测试要求

为了方便广大冷水机组厂商全面地了解AHRI风冷式冷水机组的测试要求,本文就冷水机组冷凝器进风温度测量方案,冷冻水测量系统配置及制冷量计算方法,不同大气压下测试的制冷量和能效比的修正方法等方面分别作了介绍。     

Steady-state model of centrifugal liquid chillers: Modèle pour des refroidisseurs de liquide centrifuges en régime permanent

A new steady-state model of vapour-compression type centrifugal liquid chillers is presented. The model has a number of advanced features and is capable of simulating both hermetic and open-drive centrifugal compressors. The model accounts for the real process phenomena such as superheating and subcooling in the heat exchangers as well as a capacity control formulation of the inlet guide vanes. The model algorithm is developed with the aim of requiring only those inputs that are readily known to the design engineer, e.g. the general parameters of the chiller, the chilled water flow temperature out of the evaporator and the return water temperature to the condenser inlet. The outputs include the condenser capacity, the refrigeration capacity (at the evaporator), the coefficient of performance, and also the mass flow rates and thermodynamic states of the refrigerant throughout the cycle. The model is validated with the experimental data on part load to full load performance of three different chillers operating at the University of Auckland and the agreement is found to be within ±10%. The model also demonstrates that the COP of the chillers increases with increasing cooling capacity.     

Development of a novel multi-capillary, multi-temperature commercial refrigerator cabinet with common low-pressure receiver

A multi-temperature 4 drawer catering cabinet was designed to operate using a low-pressure receiver with capillary expansion to the separate evaporator in each drawer. Low-pressure receivers have been shown to be an effective way of allowing evaporators to operate in a fully flooded mode thus enabling more efficient use of the evaporator surface for heat transfer. If a low-pressure receiver is used in a refrigeration circuit the control of refrigerant flow into the evaporator is less critical as the expansion device is not responsible for preventing liquid returning to the compressor. Therefore, a capillary expansion device can be used effectively over a range of operating pressures. The system was shown to be effective at maintaining temperatures in the storage drawers during chilled, frozen and mixed storage temperature tests carried out to the EN441 test standard. The cabinet operated successfully at all conditions except when the heat load in each drawer was excessive (>400 W above base level heat load). In this case, refrigerant was found to back up in the condenser and the low-pressure receiver was empty of liquid refrigerant. A solution to this would be to allow controlled flow of refrigerant from the condenser to the low-pressure receiver at high condensing pressures.     

串联式主机上游冰蓄冷系统中乙二醇泵变频分析

对串联式主机上游冰蓄冷系统中乙二醇泵的变频节能作定性分析，认为其实际节能效果不如常规的冷冻水变频系统，而且控制更复杂，如果在设计的时候将各个设备的压降都考虑周详，乙二醇泵不作变频也能取得很好的效果。     

The interaction between corrosion management and structural integrity of aging aircraft

Aircraft joints feature prominently in aircraft structural degradation; fatigue cracking and corrosion damage are major issues, which can reduce joint strength and degrade service life. Protecting the structure against corrosion usually involves use of highly developed protective coatings – paints and sealants – and, increasingly, the application of corrosion inhibiting compounds (CICs) which retard corrosion principally by penetrating into crevices and cracks, and displacing water. A combination of coatings and CIC use can provide effective corrosion protection, but both interact – in different ways – with joint structural performance and overall system durability. This paper discusses the interaction between these two corrosion protection measures and fatigue performance of joints. The first issue relates to the extent to which application of CICs (or other lubricants) can cause a reduction in the fatigue life of mechanically fastened joints. The CICs are lubricants which will reduce the friction at the faying surface of the joint, and change the load transfer characteristics of the joint. This paper discusses results from a test program assessing the fatigue life and failure mode of simple riveted lap joints; the results show a distinct reduction in fatigue life for joints containing CICs, and the paper discusses the changes thought to be responsible for the reduction. The second issue discussed is the degradation of protective coatings in service. Joints are key locations for coating cracking and failure, because areas such as sheet ends and fastener heads, where displacements are concentrated, may produce concentrated strain in coatings. So far, however, the potential influence of aircraft loading on coating degradation prognostics has received little attention. This paper discusses the role of joint displacement in service as a factor contributing to degradation in aircraft coatings at joints, and argues that this local strain effect, and indeed structural loading history, needs to be considered in predicting and assessing rates of coating degradation. It describes analyses of displacements in aircraft joints, to identify the levels of strain and to identify the roles and relative contributions of the various deflections in the joints. The results indicate the potential for very large strains in coatings.     

Thermodynamic performance assessment of carbon dioxide blends with low-global warming potential (GWP) working fluids for a heat pump water heater

Blends of CO₂ with ten low-global warming potential (GWP) working fluids are evaluated for use in a heat pump water heater. The effects that the discharge pressure, component ratio, hot-water outlet temperature and chilled water inlet temperature have on the coefficient of performance (COP) of heat pump are analyzed when the pinch point of the heat exchange is considered. It is found that temperature glide of zeotropic mixture has a good thermal match with the temperature change of water as two pinch points appear in the gas cooler/condenser or evaporator. The good thermal match in the heat exchangers promotes the system COP. Addition of low-GWP working fluids to pure CO₂ can reduce the high-side pressure. The results show that CO₂/R41 and CO₂/R32 are suitable candidates for heat pump water heaters because of their high COP and low high-side pressure in comparison with those of a pure CO₂ cycle.     

椭圆形套管-管翅式蒸发式冷凝器传热性能实验研究

本文搭建了蒸发式冷凝器性能测试系统,采用控制变量法实验研究了迎面风速、喷淋密度、湿球温度、循环水温度、冷却水流量各参数变化对椭圆形套管-管翅式蒸发式冷凝器传热性能的影响。实验结果表明：该冷凝器实验系统的最佳迎面风速和喷淋密度分别为3.1 m/s和0.005 6 kg/(m·s),冷凝器管外空气压降随迎面风速的增大而迅速增加;随着空气湿球温度升高,冷凝器外传热过程的热流密度(即外热流密度)降低67.5%,而内传热过程的热流密度(即内热流密度)增大47.5%,依靠内传热过程的增强,冷凝器性能良好;随着循环水温度升高,冷凝器的内热流密度降低率高达64.6%,传热性能急剧下降;随着冷却水流量增大,冷凝器的内热流密度大幅提高2.92倍,总热流密度增大21.1%,传热性能显著增强;该冷凝器在低湿球温度、低循环水温度、大冷却水流量的工况下传热性能较优。     

Transient simulation of vapour-compression packaged liquid chillers

This paper presents a transient simulation model that is useful for predicting the dynamic performance of vapour-compression liquid chillers over a wide range of operating conditions. The model employs a thermal capacitance approach for specific state variables to account for the dynamics of the chiller and ancillaries. The model accounts for the change in heat transfer coefficients throughout the heat exchangers thereby improving both physical realism and the accuracy of the simulation model. The model requires only a select few initial conditions (eg. the chilled water and condenser water temperatures). A simple compressor model based on empirical regression has been employed in the simulation. The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance (COP) as well as states of the refrigerant throughout the refrigeration cycle with respect to time. The model is validated with data from two in -situ screw chillers. Predictions are found to be within ±10%, although for one of the chillers a degree of empiricism was employed for the evaporator tube wall mass in order to give satisfactory results for the start-up process.     

Proposal of an eco-friendly high-performance air-conditioning system part 2. Application of evapo-transpiration condenser to residential air-conditioning system

Improving performance of existing air-conditioning system and minimizing its environmental effect are requested from global environmental issues. A new air-conditioning system using evapo-transpiration condenser has been proposed. Evapo-transpiration condenser is a condenser that uses transpiration by keeping condenser surface wet and water evaporation enhancing heat-transfer rate. In this paper, a prototype of the new system is tested and compared with the existing system in summer weather of Japan. Hourly-average condenser-temperature is achieved to be 5–10 °C reduction compared to that of the air-cooled condenser. Without any optimization in system operation, up to more than 30% hourly integral power consumption is saved at ambient temperature below 31 °C from this prototype. Besides, temperature of outlet-air from outdoor-unit is nearly the same as ambient temperature, while its relative humidity is slightly higher. Special maintenance is not necessary for the new evapo-transpiration condenser.     

蓄冷空调在大型公共建筑中的应用分析

介绍了水蓄冷、静态冰蓄冷、动态冰蓄冷系统的特点及对平衡电网负荷的作用,并以广州白云机场2号航站楼为例,对各种系统的应用进行了技术和经济分析,得出各种系统的特性、对电网削峰填谷的效果及存在电价差时水蓄冷的经济优势.     

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.     

Study on adsorption refrigeration cycle utilizing activated carbon fibers. Part 2. Cycle performance evaluation

Thermal heat driven adsorption systems have been gained considerable attention on the recent energy utilization trend. However, the drawbacks of these adsorption systems are their poor performance. It is urgently necessary to improve the system performance of the adsorption cycles. There are two major ways for the system performance improvement. One is to develop new adsorbent material well suited to low temperature heat regeneration. The other is to enhance heat and mass transfer in the adsorber/desorber heat exchanger. The objective of the paper is to investigate the system performance of an adsorption cycle. The cycle utilizes activated carbon fiber (ACF)/methanol as adsorbent/refrigerant pair. In this paper, specific cooling effect SCE and COP of the system are numerically evaluated from the adsorption equilibrium theory with different hot, cooling and chilled fluid inlet temperatures. It is confirmed that the influences of hot, cooling and chilled fluid inlet temperatures on the system performance are qualitatively similar to those of silica gel/water pair. Even though, the driving temperature levels of ACF/methanol and silica gel/water are different. There is an optimum condition for COP to reach at maximum for ACF/methanol pair. Particularly, the ACF/methanol system shows better performance with lower chilled fluid inlet temperature between −20 and 20 °C.     

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.