Rozprawy doktorskie na temat „CASCADE REFRIGERATION SYSTEM”

The investigations focus on load shifting, load clipping and energy efficiency through control strategies. Load shifting is achieved by increasing the amount of work done in the Eskom non-peak period. This then results in a decrease in the Eskom peak time work load. The mine refrigeration system is modelled and verified with the data. A simulation is made from the model and the simulation is used to develop the new control strategy and new operational parameters. Predicted results are verified to be within production operational constraints. A case study was carried out to prove the effectiveness of the newly developed control strategy and operational parameters. Firstly the cascade mine surface refrigeration system is automated to allow remote viewing and control of the system from a central point. The control strategy is tested through implementation on automated mine refrigeration systems. The real-time energy management system (REMS) is set up and the communication with the SCADA is tested through observing dam level temperatures and stopping and starting refrigeration machines. The decisions the controllers make are monitored until the system is fully automated. The results of the new control system on the flows, temperatures, dam levels, thermal energy and electrical energy are validated and verified. An assessment of the case study proved that DSM can be done on cascade mine refrigeration systems. A 4.2 MW load shift was predicted and research found an over performance of 0.3 MW. It is clear from the results that utilising the thermal storage in cascade mine surface refrigeration systems, will allow DSM load shifting. In general, this dissertation proved DSM can be done on refrigeration systems and it is recommended that further studies be done on underground mine refrigeration systems.
Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2008.

Nowadays traditional (direct expansion) supermarket refrigeration systems are mostly employed in supermarket establishments for refrigerating food products and beverages in the store. However, the installations of long piping system, fittings and joints in traditional systems are causing substantial refrigerant losses. The refrigerant losses in turn bring about cost and high environmental damage in terms of ozone layer depletion and global warming potential. Additionally, defrosting of air-coils is one of the most energy consuming processes in supermarket refrigeration systems due to susceptibility of the air-coils to moisture. Hence, the frost forming on air-coils as a result of moisture transfer should be removed to keep display cabinets under the required temperature. Various studies, though limited in scope, have been conducted both numerically and experimentally by several researchers in order to provide efficient and environmentally friendly supermarket refrigeration technologies. Empirical and mathematical expressions have also been continuously developed to quantify frost characteristics on flat plates and round tubes thereby determining the appropriate defrost periods. Cascade and secondary coolant refrigeration systems are the potential candidates to replace traditional ones due to the fact that the former can work on natural refrigerants and the latter essentially eliminates long connecting lines and environmentally damaging refrigerants. Development of empirical correlations for frost characteristics on real heat exchangers could also provide accurate prediction of defrost periods thereby eliminating unnecessary waste of energy and deterioration of food products. The current study, therefore, presents (a) mathematical expressions for carbon dioxide-ammonia (R744-R717) cascade refrigeration system; (b) mathematical expressions for frost property and air pressure drop; and (c) a numerical model for medium-temperature secondary coolant system incorporating the new frost property correlations. The thermodynamic analysis of the cascade refrigeration system is useful for the supermarket refrigeration industry to optimize the design and operating parameters of the system. The development of the frost property correlations from experiments on a lab-scale flat-finned-tube heat exchanger is also useful for the supermarket refrigeration industry for better prediction and control of defrost periods and duration for medium temperature air-coils. The medium-temperature secondary coolant model adopted the most appropriate heat transfer, mass transfer and pressure drop correlations obtained from the open literature. The system components such as air-coil, plate heat exchangers, distribution lines, coolant pump and compressor were modeled independently. Each component model was validated and linked to form a complete overall medium-temperature secondary coolant refrigeration system model. The experimental results showed that COP of the Monopropylene glycol/water based medium temperature secondary coolant refrigeration system could be 1.33, whereas the simulated results on a typical supermarket showed that COP of the system could be as high as 1.75. The fundamental difference between the existing secondary coolant models and the current one is that frost characteristics have been incorporated in the air-coil model. In addition to this, complete independent models have been developed for plate heat exchangers based on their respective applications. Hence, the main advantage of the current medium-temperature secondary coolant refrigeration system model is that defrost periods and time span required to defrost frosted air-coils can be accurately determined to achieve energy savings and prevent product deterioration in the display cabinets. The plate heat exchanger models can also enable one to reasonably determine pressure drops thereby leading to an appropriate coolant pump selection. Finally, a step-by-step exercise of the application of the secondary coolant model, which has been included in this project, can be used to completely design, select, evaluate and install such systems or retrofit the existing traditional direct expansion refrigeration systems in supermarkets.

Наведено аналіз останніх досліджень і приклади застосування газових гідратів двооксиду вуглецю. Наведено приклад можливості заміщення двооксидом вуглецю метану у природніх гідратах, організовуючи контрольований видобуток метану із субаквальних газогідратних покладів. Розглянуто питання про обмеження викиду парникових газів за рахунок їх уловлювання і зберігання за допомогою технології CCS (Carbon Capture and Storage). Наведено приклад каскадних систем на базі аміаку з двооксидом вуглецю із сумішами вуглеводнів як основу холодильних установок для одержання глибокого холоду. Розглянуто спосіб вибухового штампування з метою підвищення безпеки та збільшення економічної ефективності технологічного процесу за рахунок використання стабільних газових компонентів, застосування більш простого технологічного обладнання. Зроблено висновок, що використання двооксиду вуглецю в складі газових гідратів дозволяє на принципово нових основах істотно покращити технологічні процеси в різних галузях промисловості, а також ефективність енерго- та ресурсозбереження. The analysis of the latest studies and application examples of gas hydrates of carbon dioxide is given. An example of the possibility of replacing methane carbon dioxide in natural hydrates by organizing controlled methane production from subaquatic gas hydrate deposits is given. The issue of limiting the emission of greenhouse gases through their capture and storage using Carbon Capture and Storage technology is considered. An example of cascade systems based on ammonia with carbon dioxide and hydrocarbon mixtures as a basis for refrigeration facilities for obtaining deep cold is given. The method of explosive stamping is considered with the purpose of increasing safety and increasing the economic efficiency of the technological process due to the use of stable gas components, the use of more simple technological equipment. It is concluded that the use of carbon dioxide in gas hydrates allows essentially new ways to improve technological processes in various industries, as well as the efficiency of energy and resource saving.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
100
The Cryogenic systems use cascade system, thus reducing the compression ratio. As the volume of the compressors in two stages is decreased, the wasted work is reduced relatively, so that the energy can be saved, and the system operating temperature can be reached easily. Therefore, this study used the cascade refrigeration system available on the market for hardware design, control strategy and refrigerant collocation, in order to meet the appropriate operating conditions of this system design. Due to the faults of the original system compressor and control circuit, the compressor with similar discharge capacity was used instead, and the board circuit was replaced by traditional circuit. The hot section refrigerant A+B and cold section refrigerant A+B were filled in, so that the cascade system could meet the required temperature and stability. After the refrigeration system was completed. The chamber temperature was cooled to setting value, and then tested the performance of the system under the stable and maintained chamber temperature. The load was increased at the chamber temperature of -60℃, -70℃, -80℃ and -80℃ for testing. Afterwards, the temperature and pressure data in two hours stable operation were taken and the pressure-enthalpy chart was drawn to calculate the performance of refrigeration system. The results showed that the performance of cascade system in operation at -60℃ was 2.31, the performance at -70℃ was 1.07, the performance at -80℃ was 0.84 and the performance with additional load at -80℃ was 1.78, however, considering the time required in the process of cooling to -80℃ the cascade system performance was 0.33.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
97
Refrigeration system has a long history, and vacuum freeze-drying is a technology used for drying, color protection, fresh keeping, or nutrition preservation of food, biologics, biomaterials, or micro/nano materials, So Freeze-drying systems to be used for some time. But most operating temperature to the systems are -50℃, besides, old machines have only one cold trap chamber, if we want to remove the ice on cold trap, we will shutdown the machines, then Production process will be delayed. This thesis is focused on improving the cascade vacuum Freeze-drying Development, and this operating temperature can reach to -80℃,it is more favorable for specific high-tech products process and will not have a negative impact on production time. In experiments, freeze-drying system had a best evaporative temperature.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
98
Low-temperature storage refrigeration system is vital for much industry range. The traditional single-stage vapor compressor system is not suitable for low-temperature system; therefore, it should be used through cascade refrigeration system. For many years, as a result of environmental protection issue related global warning and depletion of ozone layer caused by the use of synthetic refrigerants (CFC’s, HCFC’s and HFC’s), the return to the use of harmless natural substances is a must to alternative refrigerants in refrigeration systems. Ammonia (R717) and Hydrocarbon refrigerant have excellent thermodynamic and thermo-physical properties; they can be used in a wide range of refrigeration system. In this paper, a cascade refrigeration system with Ammonia (R717) and Hydrocarbon refrigerant (HC) as working fluids in the low and high temperature stages, respectively, has been analysed and compared with R22 and R134a of traditional refrigerant. Further to research and analysis the relation of COP and mass flow ratio versus operating and design parameters of 6 group’s refrigerant in order to get optimization of parameter and analysed refrigerant in cascade low-temperature refrigeration system.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
102
The cascade refrigeration systems are widely applied in Taiwan, for use in air conditioning, refrigeration and food processing, pharmaceuticals, chemicals, machinery, etc. More attention recently, So this thesis refrigeration components easily available in the market to buy the dual refrigeration system developed for conducted to explore various expansion valve of the refrigerant flow. The system of high and low temperature circulation system individually in four A, B, C and D expansion valve testing, in order to best binary refrigerating system expansion valve and the coefficient of performance. Results in a high temperature due to the circulatory system A, B, C the expansion valve of the refrigerant flow rate is too small, resulting in low temperature startup cycle, temperature cycle system can,t be stabilized at the intercooler below -30 ℃, the system also due to temperature cycling and A the flow of refrigerant expansion valve B is too small, the evaporator can,t reach -80 ℃, and start C expansion valve of the evaporator inlet temperature difference is too large, said it could not achieve a good cooling effect, this thesis high binary refrigeration system, hypothermic circulatory system are available starting D expansion valve coefficient of 0.45 for the best performance.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
102
This paper examines how the common refrigerant R-134a and the non-azeotropic refrigerant R-404A perform in a binary refrigeration with the addition of a refrigerant precooler and a desuperheater and the changes in the system with the added refrigerants before and after subcooling through experiments and comparison. Inferences are made on the viability of installing a ‘refrigerant precooler’ in the refrigerant system through theoretical analysis of changes in the coefficient of performance (COP) of the system in the subcooled and superheated states. The purpose is to understand how the binary refrigeration system performs in terms of actual operation and system performance in the different models designed for this study and examine the possibility to optimize the ‘refrigerant precooler and desuperheater combination’ developed in this study based on the data obtained. Analysis of the data obtained from this experiment reveals that the addition of a heat receiver at the outlet of the compressor in a refrigerated vehicle can effectively control the degree of subcooling of the drainage pipe in the condenser at below 38℃ and the addition of a refrigerant precooler improves problems such as unstable refrigerant temperature rise in the refrigerated vehicle after evaporator pump-down and shutdown, thereby increasing the refrigeration speed. An analysis that compares the model in which both additional components are synchronized with the basic model shows that with the power consumption of the binary refrigeration system as the baseline, adding only the desuperheater, only the precooler, and the desuperheater and precooler combination achieves energy efficiency of 24%, 54.3% and 57.8%, respectively. Therefore, the addition of the desuperheater and precooler combination to the binary refrigeration system can effectively improve overall performance and reduce carbon emissions.

碩士
國立交通大學
機械工程系
89
The objective of this research is to design a auto-cascade refrigeration system with zeotropic refrigerant mixtures R-32/R-134a（30/70 wt％）, which might reach a low temperature —40℃. This system utilizes only one single compressor with a phase separator in order to shift the concentration contents of the refrigerant mixtures .The vapor phase flows through a cascade heat exchanger and exchanges latent heat with the low temperature liquid phase after expansion. Then, the mixture rich with higher boiling component flows into the evaporator to create the low temperature cooling, and merges with the mixture rich with lower boiling component, which leads finally back to the compressor and thus completes a cycle. This system have two advantages. First, utilization of the temperature glide of zeotropic refrigerant mixtures can reduce heat transfer irreversibility in the heat exchangers. Secondly, the phase separator shifts the concentration percentages of the components by the temperature difference of the boiling points. Thus, this system can raise the cooling capacity and can reduce the power consumption. Test results for a R-32/R-134a (30/70 wt %) cascade loop, compared with a baseline loop, indicates the increase of only 6％ in the power consumption and degradation of 10％ in the COP. In conclusion, this system can raise the evaporating pressure, can reduce the pressure ratio with one single compressor, and can achieve the designed low temperature.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
100
The study aims at providing convenient stores with the required energy for cooling, freezing, and supply of hot water and steam through acascade refrigeration system composed of HFC-134a and HFC-245fa refrigerants. In the experiment, the paper makes use of the feasibility of producing hot water and steam and raising the coefficient of performance (COP) of low temperature system by the high temperature feature of HFC-245fa refrigerant. Through planning of static and dynamic testing conditions, the paper designs the complete experimental methods and procedures, analyzes the effects of each experimental parameter on the performance of the system, and further studies and develops a refrigeration system with environmental protection concept and energy-saving effect. Experimental results show that when the evaporation temperature of HFC-245fa is 45oC and the effluent water temperature is 90oC, the COPH (COP as a heater) of heat-sensitive expansion valve is 1.81, being 4.6% more than the manual expansion valve; and the COPL (COP as a heating load) is 1.64, being 19.4% more than the manual expansion valve. Use of the heat pump system of binary refrigeration system for heating can save energy by 41% when compared to heating by natural gas, 92.5% when compared to heating by liquid petroleum gas, 98.5% when compared to heating by diesel boiler, and 122% when compared to heating by electric heater. Through cascade system, the study can offer compression again to condensing heat by the low-temperature side, achieving the effects of making high-temperature heat and saving energy.

碩士
國立臺北科技大學
冷凍空調工程系所
94
This study mainly discusses the optimum allocation of heat-exchanger thermal conductance in CO2/NH3 cascade refrigeration systems, such as evaporative, cascade condenser, condenser. Using cold-water temperature, chillier fluid temperature, cool capacity and total thermal conductance as conditional parameters, this research built three models of Reversed Carnot Cycle and Ideal Vapour Compressor Refrigeration Cycle and Real Vapour Compressor Refrigeration Cycle, which followed thermo dynamics analysis and Klein thermal conductance equation. The system produced Heat-exchanger thermal conductance optimum allocation by analyzing calculated maximum COP with different parameters and thermal conductance allocation. The result of this research articulated that the principle of three heat-exchangers optimum allocation in CO2/NH3 cascade refrigerator system: 1.Heat-exchanger thermal conductance optimum allocation in Reversed Carnot Cycle is 1：1：1. 2.Heat-exchanger thermal conductance optimum allocation would be 1：1.0625：1.0625 in Ideal Vapour Compressor Refrigeration Cycle and Real Vapour Compressor Refrigeration Cycle, of which restricted conditions are that average chiller fluid temperature is from -42℃ to -46℃, average cold water temperature range is from 31℃ to 32℃, total thermal conductance is from 120kW/K to 140kW/K, and cool capacity between 120kW to 160kW. If Total thermal conductance is over 170kW/K, then Heat-exchanger thermal conductance optimum allocation should approach 1：1：1.

碩士
國立臺北科技大學
冷凍與低溫科技研究所
91
Refrigerated display cabinets are widely used in 24-hr convenience store in Taiwan. In this research, we built a 5 in 1 closed type walk-in refrigerated display cabinet and its compression refrigeration system, which including traditional single stage compression system and cascade refrigeration system, to evaluate their system performance. The results of the performance test showed that the cascade refrigeration system adopting R134a and R507 combination has better system performance than the combination of R134a and R404A, and the power consumption test also showed 5 ~ 7% energy saving improved comparing with the single stage compression refrigeration system adopting R404A under the testing condition of 40℃ condensing temperature.

碩士
國立交通大學
機械工程系所
92
The objective of this research is to design a two auto-cascades refrigeration system with refrigerant mixture R-32/R-134a in which two phase separators and two heat exchangers are used to raise the concentration of the richer R-32 mixture flow through the evaporator in order to achieve the low temperature cooling , only utilize a 1.5HP compressor to support system power consumption. In comparison with the one-cascade system , test results for a R-32/R-134a(30/70 wt%) two cascades loop , showed an increase of only 2.14 % in the power consumption , and a cop valve 2.21 with an increment of 4.25% , as well as evaporator temperature of -51℃ . Also the condensation pressure and evaporation pressure in this two-cascade loop were much lower than those in one-cascade loop . The advantage of this system is that it needs only one 1.5 hp compressor to fulfil all the foregoing achievements .

碩士
國立交通大學
機械工程系
91
The objective of this research is to design and improve the efficiency of the cascade refrigeration system with zeotropic refrigerant mixtures. This system utilizes a single compressor with R-32/R-134a refrigerant mixtures. Phase separator is put in this system in order to change the concentration of refrigerant mixtures. The vapor phase flows through a cascade heat exchanger and exchanges latent heat with the low temperature liquid phase after expansion. Then, the higher-boiling-component mixture flows into the evaporator to create the low temperature cooling, and merges with the lower-boiling-component mixture, which leads finally back to the compressor and thus completes a cycle. Furthermore we design a Liquid Over Feeding Accumulator Heat Exchanger（LOF-AHX）between the outlet of the evaporator and condenser in order to make the high pressure liquid refrigerant exchange heat with the low pressure vapor refrigerant. This design enables the refrigerant: (1) to get super heat before sucking into compressor in order to protect compressor from liquid compression; (2) to get subcooling before entering expansion; (3) to increase the mass flow rate; (4) to raise the cooling capacity; (5) to reduce the pressure ratio and exit temperature. Due to the concentration shift effect of cascade loop of refrigerant mixtures, using LOF can raise cooling capacity substantially. Therefore, the optimal COP can reach 2.126, as well as the lowest temperature can reach —38.35oC. All of those results occur in cascade loop of refrigerant mixtures with LOF.

This study presents a comparative analysis of thermodynamic performance of cascade refrigeration systems (CRSs) for refrigerant couples R23/R290 and R23/R600A to discover whether R600A is a suitable substitute for R290. The discharge temperature, input power of the compressor, coefficient of performance (COP), exergy loss (X) and exergy efficiency (g) are chosen as the objective functions. The operating parameters considered in this thesis include condensing temperature, evaporating temperature in both high-temperature cycle i.e HTC and lower cycle i.e LTC and Temperature difference in the cascade heat exchanger. Under the same operation condition, the input power of R23/R600A CRS is lower than that of R23/R290 CRS, and COP opt is higher than that of R23/R290 CRS. The theoretical analysis indicates that R23/R600A is a more potential refrigerant couple than R23/R290 in Cascade Refrigeration System (CRS).

ABSTRACT Biomedical preservation requires the temperature for the storage of stem cell, blood, sperm & organs at a storage temperature of below -80°C.We can achieve effectively cooling of about -35°C with the help of simple vapour compression refrigeration system. With the help of compound vapour compression refrigeration system, temperature up to – 50 °C is achievable. The cascade refrigeration system is widely used where low temperatures are needed. The natural refrigerant CO2 makes it possible to achieve temperatures as low as -54°C in the low temperature stage. In this case propene is the refrigerant for the upper cascade stage.Using other refrigerants (e.g. ethane) in low temperature circuit temperatures as low as -88oC can be realized. So the required evaporating temperature below -80 °C can be achieved by cascade refrigeration system. Present work deals with thermodynamic analysis of cascade refrigeration system using ozone friendly hydrocarbon refrigerants Iso-butane (R-600a), Propane(R-290) & Ethane (R-170). Iso-butane (R-600a), Propane (R-290) & Ethane (R-170) are the natural refrigerants. These are hydrocarbons. These refrigerants are used as replacement to CFC refrigerant in low temperature applications. These refrigerants have zero ozone depletion potential and less global warming potential. Hydrocarbons are flammable but non toxic. System with charge mass sizes of 0.15 kg or less can be installed in any size of room. Since these refrigerants are pure natural gases, there is no problem of temperature glide as it occurs in azeotropic mixture. This study deals thermodynamically analysis of R-600a, R-290 & R-170, three stage cascade refrigeration system to optimize the operating parameters of the system. The operating parameters include: Condensing, evaporating, sub cooling and superheating temperatures in the high temperature circuit, temperature difference in the cascade heat exchanger, Condensing, evaporating, sub cooling and superheating temperatures in the low & medium temperature circuit. This article presents a thermodynamic analysis to optimize a cascade refrigeration system to be used for biomedical cold-storage application. It can also be used successfully for rapid freezing and storage of frozen food and liquidification of petroleum vapor. A combination of hydrocarbon refrigerants i.e. Isobutene, Propane and Ethane–(R600a, R290 & R170) cascade system has been promoted as a prospective alternative solution to the use of HFC refrigerants.

Biomedical preservation requires the temperature for the storage of stem cell, blood, sperm & organs at a storage temperature of below -80°C.We can achieve effectively cooling of about -35°C with the help of simple vapour compression refrigeration system. With the help of compound vapour compression refrigeration system, temperature up to – 50 °C is achievable. The cascade refrigeration system is widely used where low temperatures are needed. The natural refrigerant CO2 makes it possible to achieve temperatures as low as -54°C in the low temperature stage. In this case propene is the refrigerant for the upper cascade stage.Using other refrigerants (e.g. ethane) in low temperature circuit temperatures as low as -88oC can be realized. So the required evaporating temperature below -80 °C can be achieved by cascade refrigeration system. Present work deals with thermodynamic analysis of cascade refrigeration system using ozone friendly hydrocarbon refrigerants Iso-butane (R-600a), Propane(R-290) & Ethane (R-170). Iso-butane (R-600a), Propane (R-290) & Ethane (R-170) are the natural refrigerants. These are hydrocarbons. These refrigerants are used as replacement to CFC refrigerant in low temperature applications. These refrigerants have zero ozone depletion potential and less global warming potential. Hydrocarbons are flammable but non toxic. System with charge mass sizes of 0.15 kg or less can be installed in any size of room. Since these refrigerants are pure natural gases, there is no problem of temperature glide as it occurs in azeotropic mixture. This study deals thermodynamically analysis of R-600a, R-290 & R-170, three stage cascade refrigeration system to optimize the operating parameters of the system. The operating parameters include: Condensing, evaporating, sub cooling and superheating temperatures in the high temperature circuit, temperature difference in the cascade heat exchanger, Condensing, evaporating, sub cooling and superheating temperatures in the low & medium temperature circuit. This article presents a thermodynamic analysis to optimize a cascade refrigeration system to be used for biomedical cold-storage application. It can also be used successfully for rapid freezing and storage of frozen food and liquidification of petroleum vapor. A combination of hydrocarbon refrigerants i.e. Isobutene, Propane and Ethane–(R600a, R290 & R170) cascade system has been promoted as a prospective alternative solution to the use of HFC refrigerants.

碩士
中原大學
機械工程研究所
97
In general, the refrigeration system controls the temperature by turning on or turning off the fixed-frequency compressor, or installing the heating device, but these methods waste too much electricity. The main method of controlling low temperature is using compressor system. Compressor system has a high energy efficiency and large refrigeration capacity in the general environment. However, when the evaporation temperature is lower, the efficiency of refrigeration device will decrease significantly. Therefore, we try to work this compressor system with other devices under the range of ultra-low temperature (-40 ℃ below) to maintain the low temperature. In this study, we design a cascade system which is controlled by fuzzy, and operate the compressor system with Thermoelectric Cooler. We try to economize energy by" inputting different heat energy to change the speed of compressor." On the other hand, we also cool down the surface of the Thermoelectric Cooler by using of the cascade, and reducing the difference temperature of the hot and cold surface substantially. Therefore, it is cold enough to achieve the target output temperature, -40 ℃, in second times. According to the simulation results, the speeds of the compressor are different with operating the cascade refrigeration system at 65 watts, 30 watts and 10 watts respectively, and the power consumption is also reduced with lower load. Finally, the water temperature remains at -40 ℃ stably with our investigation.

碩士
國立臺北科技大學
冷凍空調工程系所
93
For the low-temperature applications, such as quick freezing and frozen storage of food, the required evaporating temperature of refrigeration system ranges from -40°C to -55°C, in which a single-stage vapor-compression refrigeration system is inadequate. Considering global environmental protection, the use of natural refrigerants in refrigeration systems has been revealed to be a complete solution to permanent alternative for fluorocarbon-based refrigerants. A cascade refrigeration system applying the natural refrigerants CO2 and NH3 fulfills these requirements for low-temperature applications of industry refrigeration systems. Although studies on carbon dioxide as a refrigerant are now gaining attention, the works on CO2/NH3 cascade refrigeration system are still lacking. This study applied thermodynamic analysis to find the optimum condensing temperature of the cascade-condenser in a cascade refrigeration system utilizing carbon dioxide and ammonia as refrigerants, in which the system attained the maximum COP. The design parameters for this paper is：High temperature circuit condensing temperature (Tc= 30, 35, 40℃), low temperature circuit evaporating temperature (TE= -45, -50, -55℃), cascade condenser difference temperature (△T= 3, 4, 5℃). The analysis result show：For condensing temperature is 35℃and evaporating temperature is -50℃of CO2/NH3 cascade system. The cascade condenser optimum condensing temperature TMC is -15℃. Under this condition, optimal TMC will increase by the raise of TE, Tc and △T.

The survey from US Department of Energy showed that about one-third of energy consumption in US is due to air conditioning and refrigeration systems. This significant usage of electricity in the HVAC industry has prompted researchers to develop dynamic models for the HVAC components, which leads to implementation of better control and optimization techniques. In this research, efforts are made to model a multi-evaporator system. A novel dynamic modeling technique is proposed based on moving boundary method, which can be generalized for any number of evaporators in a vapor compression cycle. The models were validated experimentally on a commercial supermarket refrigeration unit. Simulation results showed that the models capture the major dynamics of the system in both the steady state and transient external disturbances. Furthermore the use of MEMS (microelectromechanical) based Silicon Expansion Valves (SEVs) have reportedly shown power savings as compared to the Thermal Expansion Valves (TEVs). Experimental tests were conducted on a supermarket refrigeration unit fitted with the MEMS valves to explain the cause of these potential energy savings. In this study an advanced cascaded control algorithm was also designed to control the MEMS valves. The performance of the cascaded control architecture was compared with the standard Thermal Expansion Valves (TEVs) and a commercially available Microstaq (MS) Superheat Controller (SHC). The results reveal that the significant efficiency gains derived on the SEVs are due to better superheat regulation, tighter superheat control and superior cooling effects in shorter time period which reduces the total run-time of the compressor. It was also observed that the duty cycle was least for the cascaded control algorithm. The reduction in duty cycle indicates early shut-off for the compressor resulting in maximum power savings for the cascaded control, followed by the Microstaq controller and then the Thermal Expansion Valves.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
99
The temperature of the freezer ranging from -40 to -160℃ is widely used in cryomedicine, energy, biotechnology as cryopreservation. The system used to zeotropic refrigerant mixtures and single compressor, through zeotropic refrigerant mixture in the high boiling point and low boiling point in order to achieve the natural separation between the multi-level series with the stack method, to take the system from -40 to -160℃ temperature purposes. The feature of the auto-cascade refrigeration systems is affected by zeotropic refrigerant mixtures therefore the composition of the refrigerant and mass fraction are the key factors.The purpose of this study is to investigate auto-cascade refrigeration systems as the object of study, use simulation to look into the application of zeotropic HC refrigerant mixtures of R170/R290, R170/R1270, R170/R600 and 170/R600a to coefficient of performance (COP), and do a comprehensive appraisal of the composition of refrigerant. According to the research, in the same design conditions, R170/R290, R170/R1270, R170/R600, and R170/R600a zeotropic refrigerants mixture the best performance value and size of the sequence is: R170/R600(COP=1.34)>R170/R600a(0.82)>R170/R290(0.36)>R170/R1270(0.34), and the corresponding ratio of the best refrigerants composition were: 0.16,0.22,0.34,0.31. To R170/R1270 mixed refrigerant as a benchmark, the benchmark value of 1, R170/R600, R170/R600a, R170/R290 mixed refrigerant composition with the ratio of 3.92:2.41:1.06:1.

碩士
國立臺北科技大學
能源與冷凍空調工程系碩士班
100
With the development of technology, the requirement for cryogenic freezing is getting stricter in the scope of energy, military, bio-tech, and medical. Especially in medical, the cell preservation technique is the key of graft. Auto-cascade system uses zeotropic mixture as refrigerant to implement the natural separation and multi-stage cascade through the temperature glide for getting low temperature of -60°C to -160°C. The performance of Auto-cascade system is mainly affected by the zeotropic refrigerant, so the key point is to select the composition and fraction. The major issue of this study is to probe into the best fraction and COP of R1150/R290, R1150/R1270, R1150/R600, and R1150/R600a mixtures in Auto-cascade system with Regenerator through the theoretical analyzing. The study shows that 1). R1150/R600 mixture has the best COP 2.01 with R1150 mass fraction of 0.11, 2). The best COP 2.01 of R1150/R600 mixture is greater than the best COP 1.34 of R170/R600 mixture, 3). The best COP of R1150/R600 Auto-cascade with Regenerator has COP 2.21, which is greater than the value 2.01 of COP without Regenerator.