超低温制冷系统中热力和电子膨胀阀性能研究

发布时间：2018-08-14 09:27

【摘要】：在食品加工业、工业生产和制药等领域内,超低温的应用广泛,如高商品附加值的海产品速冻储运、生物药品的生产与储运、低温生物制品的保存等都需要提供低温环境保障其顺利进行。制冷系统的正常而高效的运行是保证其能够提供稳定且达标的低温环境的重要因素。在蒸汽压缩式制冷系统中,除了压缩机、蒸发器和冷凝器,节流机构是制冷系统中一个重要的组成部分,其主要作用包括节流降压,调节制冷工质流量和保持合理的系统过热度。在制冷系统中,节流机构的运行状态以及与系统其他部件的匹配程度影响着制冷系统的性能和稳定性。目前超低温制冷系统中常用的节流机构有热力膨胀阀和电子膨胀阀,由于机械结构和工作原理的不同,电子膨胀阀具有热力膨胀阀不可比拟的优势,如反应迅速、调节精确等。本课题在搭建节流机构软硬件平台的基础上,研究对比了低温工况下对两种节流机构的性能,提出了电子膨胀阀的脉宽调制控制,并采用模糊控制算法进行优化,选择出低温工况下更适宜的控制方式,为今后各类超低温制冷系统电子膨胀阀的控制研究提供一定的参考依据。本文研究的主要内容及结论如下:(1)实验装置的硬件搭建。主要包括:复叠式制冷系统低温级热力膨胀阀和电子膨胀阀的选型;数据采集系统硬件平台的搭建,包括PLC、温度/压力传感器和质量流量计的选取,构建基于OPC协议的PLC与上位机程序LabVIEW通信;电子膨胀阀控制系统硬件平台的搭建,包括PLC和温度/压力传感器的选取,基于OPC协议的PLC与上位机程序LabVIEW通信;(2)实验装置控制系统的软件设计。主要包括:超低温冷库制冷监控系统的软件设计与开发,实现了对超低温复叠制冷系统的运行监控以及数据采集;电子膨胀阀控制器的软件设计与开发,完成了电磁式电子膨胀阀PWM控制的PLC程序开发,实现了对电磁式电子膨胀阀的PWM控制;电子膨胀阀控制系统的优化,针对制冷系统的非线性特点,使用模糊控制对电子膨胀阀控制系统进行了优化,以满足蒸发器出口过热度的控制要求。(3)低温工况下电子膨胀阀的实验研究。首先以系统稳定性和系统性能为标准,通过实验确定最适宜的占空比范围为30%~50%。使用设计开发后的两套电子膨胀阀控制系统,通过实验,测试和验证了两套控制器的控制性能。分别开展了对目标过热度的跟踪响应性能以及对外界扰动的抑制性能的实验研究。结果显示,在目标过热度跟踪性能方面,模糊逻辑控制器的超调量平均比PWM控制低5.1%,调节时间平均少220s,上升时间平均长80s,绝对误差积分平均低0.55,时间乘以绝对误差积分平均小1295.2;在扰动抑制性能方面,模糊逻辑控制器的超调量平均比PWM控制低1.2%,调节时间平均少150s,上升时间平均长75s,绝对误差积分平均小0.71,时间乘以绝对误差积分平均小1474.7。因此模糊控制器均有明显的优势。(4)复叠式制冷系统低温级电子膨胀阀与热力膨胀阀对比研究。使用优化后的模糊控制器与热力膨胀阀进行对比实验,测试低温工况下,电子膨胀阀与热力膨胀阀对过热度的控制性能。分别进行了-50℃、-55℃和-60℃三种不同低温工况以及变负荷工况下两种节流机构的控制性能对比。实验结果表明相较于热力膨胀阀,采用模糊控制器的电子膨胀阀在三种低温工况下,蒸发温度平均高5.8℃,但降温时间平均快1.73h,同时,过热度的变化更为稳定和平顺,压缩机排气温度和压力更低,最后,三种工况下的COP平均比采用热力膨胀阀高10.5%;在变负荷工况下,采用电子膨胀阀的系统过热度平均最大波动为1.75K,平均调节时间为510s,而采用热力膨胀阀为3.5K和735s,采用热力膨胀阀时,低温级压缩机排气温度和蒸发器进口温度变化更为剧烈,因此,采用模糊控制器的电子膨胀阀对系统稳定性和系统性能方面都有明显的优势,更适合本超低温冷库制冷系统。
[Abstract]:Ultra-low temperature is widely used in food processing industry, industrial production and pharmaceutical industry, such as high value-added seafood quick-freezing storage and transportation, bio-pharmaceutical production and storage and transportation, cryogenic bio-products preservation and so on. In a vapor compression refrigeration system, throttling mechanism is an important part of the refrigeration system besides compressor, evaporator and condenser. Its main functions include throttling and pressure reduction, regulating refrigerant flow and maintaining a reasonable system superheat. The performance and stability of the refrigeration system are affected by the running state and the matching degree with other parts of the system. Thermodynamic expansion valve and electronic expansion valve are commonly used in the ultra-low temperature refrigeration system. Because of the different mechanical structure and working principle, electronic expansion valve has incomparable advantages, such as reaction. On the basis of setting up the software and hardware platform of throttling mechanism, this paper studies and compares the performance of two kinds of throttling mechanism under low temperature condition, puts forward the pulse width modulation control of electronic expansion valve, and uses fuzzy control algorithm to optimize, chooses the more suitable control mode under low temperature condition, for all kinds of ultra-low in the future. The main contents and conclusions of this paper are as follows: (1) Hardware construction of the experimental device. It mainly includes: selection of low-temperature thermal expansion valve and electronic expansion valve of cascade refrigeration system; construction of hardware platform of data acquisition system, including PLC, temperature/pressure transmission. Selection of sensors and mass flowmeters, construction of PLC based on OPC protocol and host computer program LabVIEW communication; electronic expansion valve control system hardware platform, including the selection of PLC and temperature/pressure sensors, PLC based on OPC protocol and host computer program LabVIEW communication; (2) experimental device control system software design. Software design and development of refrigeration monitoring system for cryogenic cold storage have realized the operation monitoring and data acquisition of super-low temperature cascade refrigeration system; software design and development of electronic expansion valve controller have completed the development of PLC program for PWM control of electromagnetic electronic expansion valve, and realized the PWM control of electromagnetic electronic expansion valve; electronic expansion valve has been realized. According to the non-linearity of refrigeration system, fuzzy control is used to optimize the control system of electronic expansion valve to meet the control requirements of the superheat at the outlet of evaporator. The suitable duty cycle range is 30%~50%. Using the two sets of electronic expansion valve control systems designed and developed, the control performance of the two sets of controllers is tested and verified through experiments. In terms of performance, the overshoot of the fuzzy logic controller is 5.1% lower than that of the PWM controller, 220 seconds less than that of the PWM controller, 80 seconds longer than the rise time, 0.55 lower than that of the absolute error integral, 1295.2 smaller than that of the time multiplied by the absolute error integral. The average rise time is 75s, the average absolute error integral is 0.71, and the average absolute error integral is 1474.7. Therefore, the fuzzy controller has obvious advantages. (4) Comparing the low temperature electronic expansion valve and the thermal expansion valve of cascade refrigeration system. The control performance of the electronic expansion valve and the thermal expansion valve to the superheat was tested under the condition of low temperature. The control performance of the electronic expansion valve and the thermal expansion valve were compared under the condition of - 50, - 55, and - 60. The average evaporation temperature of the valve is 5.8, but the cooling time is 1.73 H. At the same time, the change of the superheat degree is more stable and smooth, and the exhaust temperature and pressure of the compressor are lower. Finally, the COP of the three working conditions is 10.5% higher than that of the thermal expansion valve. The average maximum fluctuation is 1.75K, the average adjustment time is 510s, and the thermal expansion valve is 3.5K and 735s. When the thermal expansion valve is used, the exhaust temperature and the inlet temperature of the evaporator of the cryogenic compressor change more dramatically. Therefore, the electronic expansion valve with fuzzy controller has obvious advantages in the stability and performance of the system. It is more suitable for the cryogenic cold storage refrigeration system.
【学位授予单位】：上海海洋大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TB657

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