二氧化碳板式熱交換器設計軟體開發(I)

Abstract

由於環保意識的逐漸成長，使用天然冷媒二氧化碳替代傳統冷媒作為冷凍、空調及 熱泵系統的工作流體已成為極受重視之議題。不過二氧化碳冷凍循環與傳統的冷媒操作 在次臨界流體區域有相當大的差異，由於二氧化碳系統操作點位於超臨界流體區，故其 熱力及輸送性質皆會產生極大變化，而這些物理性質的改變皆會強烈影響超臨界流體區 的熱傳及壓降性能。熱交換器（蒸發器或冷凝器）是冷凍系統中最重要的熱量傳送元件， 其主要工作是將室內側的熱負荷排放至室外側環境中。熱交換器熱傳效率的好壞，對整 個系統設備整體性能會有大的影響。過去以來，學術界及工業界一直致力於研究如何增 強熱交換器的熱傳性能，提高整個系統設備的能源效率；另外也要求熱交換器的尺寸縮 小，以降低成本，增加競爭力。空調系統的基本原理作用包括了四個基本元件，冷媒壓 縮機、冷凝器、膨脹設備、蒸發器等。而其中蒸發與冷凝的過程，都必須使用熱交換器， 因此板式熱交換器性能的好與壞，與整個空調系統有著密切關係。而由於二氧化碳的性 質變化及熱交換器的結構設計，二氧化碳在板式熱交換器內的流動特性是相當複雜的。 但是目前二氧化碳在熱交換器的性能分析方式對設計者的使用卻是相當困難的，經常會 出現設計過大或不足的現象，造成能源與材料的雙重浪費。因此本計畫預期進行開發超 臨界二氧化碳板式熱交換器設計軟體，讓設計者得以藉本軟體設計此類熱交換器。本計 畫第一年將進行氣體冷卻器設計軟體開發，第二年則進行兩相蒸發器設計軟體開發。

With the continuous rising needs for environmental awareness and concerns, exploiting natural refrigerants like carbon dioxide had attracted considerable attention as a working fluid in refrigeration, air-conditioning and heat pump systems. Unlike conventional refrigerants that normally operate in the subcritical region, the high pressure side of carbon dioxide is above critical point. Hence, tremendous variation in thermodynamic and transport properties are encountered. Moreover, all of these physical property effects strongly influence heat transfer and pressure drop characteristics at supercritical conditions. Heat exchanger is the major heat transport device applicable to heating/ventilation/air-conditioning and refrigeration systems (HVAC&R). Performance of the heat exchanger is directly in connection to the overall performance of the whole system. During the past, efforts were made in either academic or in industry to improve the performance of the system, especially in improving the energy efficiency and in reducing the material cost. There are four major components in HVAC&R system, namely compressor, condenser, evaporator, and expansion device. The process of condensing or evaporating requires heat exchangers. Hence, well designed heat exchanger gives rise to higher system performance. The heat exchanger using carbon dioxide, especially operated in supercritical state, is quite complex due to interactions amid thermodfluid, physical properties, and heat exchanger configurations. Until now, there is no well accepted design tool that is capable of designing the heat transfer performance of CO2 heat exchanger. In this regard, it is the objective of this project to develop design software of plate heat exchanger using carbon dioxide refrigerant. The first year’s project aims to develop the gas cooler design software that is operate din supercritical region whereas the second year will develop evaporator which is operated in two-phase condition.