使用環保冷媒R-290的串聯式雙蒸發器冷凍系統之研究

Abstract

本研究針對目前生活上最常使用的空調與食品冷凍系統，設計並建立一套串聯式雙蒸發器冷凍系統，並以丙烷作為冷媒，填充於串聯式雙蒸發器的冷凍系統，以進行實驗測試及數值模擬，並分析各種變因對系統性能的效應，其主要目的為符合環保需求與提高系統效率。 系統中的主要元件包括往復式壓縮機、氣冷式冷凝器、高溫毛細管、高溫蒸發器、低溫毛細管、低溫蒸發器等。本實驗的操縱變因包括系統的冷凝壓力、高溫毛細管的長度、低溫毛細管的長度、壓縮機頻率等。實驗所探討的系統性能則包括冷媒流率、高溫和低溫蒸發器內的冷媒熱傳係數、高溫和低溫蒸發器的冷凍能力、系統性能係數（COP）等。 實驗結果顯示，冷媒質量流率隨著冷凝壓力與過冷度的提高而變大，隨著高溫和低溫毛細管長度的增長而減小，而隨著壓縮機頻率的提高而稍微地變大。冷媒在高溫蒸發器內的熱傳係數會隨著冷凝壓力的提升而變小，隨著高溫毛細管長度的增長而稍微變大，隨著低溫毛細管長度的增長而變小，隨著壓縮機頻率的變大而變化不大。冷媒在低溫蒸發器的熱傳係數隨著冷凝壓力的提升而稍微變小，隨高溫毛細管長度的增長而變小，隨低溫毛細管長度的增長而變動不大，隨著壓縮機頻率的變大而變大。高溫蒸發器的冷凍能力隨著冷凝壓力的提升而下降，隨高溫毛細管長度的增長而變大，隨低溫毛細管長度的增長而變小，隨壓縮機頻率的變大而變化不大。低溫蒸發器的冷凍能力隨著冷凝壓力的提升而下降，隨著高溫毛細管長度的增長而變大，隨低溫毛細管長度的增長而變動不大，隨著壓縮機頻率的變大而變大。系統性能係數隨著冷凝壓力的提升而下降，隨著高溫毛細管長度的增長而變大，隨著低溫毛細管長度的增長而下降，隨著壓縮機頻率的變大而變大。 本研究利用Buckingham 理論來模擬串聯式雙蒸發器冷凍系統其性能的無因次方程式，所得的無因次方程式可用來模擬系統的冷媒質量流率、高溫蒸發器內的冷媒熱傳係數、低溫蒸發器內的冷媒熱傳係數、高溫蒸發器其冷凍能力的分配率、系統的性能係數。利用無因次方程式預測以上五個性統性能的結果分別與實驗值的相對誤差值範圍為-4∼+5%、-16∼+16%、-12∼+16%、-10∼+10%、-19∼+17%。 將文獻中所提出有關模擬冷媒質量流率與冷媒兩相熱傳係數的經驗方程式與本研究所得的結果作比較，結果發現，本研究的結果與文獻的結果有相近的趨勢，且本研究所提出模擬系統性能的無因次方程式在使用上較方便且準確度也不錯。

The performance of a refrigerating system with an environment-friendly refrigerant, propane（R-290）as the refrigerant, was experimentally studied. There were two evaporators connected in series within the system. The objective of the present study is to conform to the environmental protection and to increase the efficiency of the system. The system is mainly composed of a reciprocating compressor, a condenser, high- and low-temperature capillary tubes, and high- and low-temperature evaporators. The experimental results show that the mass flow rate of the refrigerant increases with the condensing pressure and the subcooling degree of the refrigerant, decreases with the lengths of the high- and low-temperature capillary tube, and increases slightly with the frequency of the compressor. The heat transfer coefficient of the refrigerant in the high-temperature evaporator decreases with the condensing pressure, increases slightly with the length of the high-temperature capillary tube, decreases with the length of the low-temperature capillary tube, and changes not apparently with the frequency of the compressor. The heat transfer coefficient of refrigerant in the low-temperature evaporator decreases slightly with the condensing pressure, decreases with the length of the high-temperature capillary tube, changes not apparently with the length of the low-temperature capillary tube, and increases with the frequency of the compressor. The cooling capacity of the high-temperature evaporator decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, decreases with the low-temperature capillary tube, changes not apparently with the frequency of the compressor. The cooling capacity of the low-temperature evaporator decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, changes not apparently with the length of the low-temperature capillary tube, and increases with the frequency of the compressor. The coefficient of performance（COP）of the system decreases with the condensing pressure, increases with the length of the high-temperature capillary tube, decreases with the length of the low-temperature capillary tube, increases with the frequency of the compressor. Based on Buckingham Pi theorem, this dissertation derives the dimensionless correlations for the characteristics of a series-connected two-evaporator refrigerating system with propane (R-290) as the refrigerant. Experimental data are substituted into the correlations to show the most relevant factors. Simplified correlations are then obtained. The results show that the mass flow rate of refrigerant ( ) is mainly affected by the condensing pressure, the length of the high-temperature capillary tube, and the subcooling of refrigerant, while the heat transfer coefficients of refrigerant in the evaporators (hr,He and hr,Le) are affected by the condensing pressure and the logarithmic-mean temperature difference of the specific evaporator. However, hr,He and hr,Le are also affected by the lengths of the low- and high-temperature capillary tube, respectively. Additionally, the ratio of the cooling capacity of the high-temperature evaporator to the total capacity ( ) is mainly affected by the condensing pressure and the logarithmic-mean temperature difference of both evaporators. The COP of the system is mainly affected by the condensing pressure, the frequency of the compressor, and the logarithmic-mean temperature difference of both evaporators. Compared with the results of the study, the predicted results by some correlations from literatures about the mass flow rate and the two-phase heat transfer coefficient of the refrigerant have the same consistency. However, the required variables of the correlations in this study are much fewer than those of the correlations from literatures.