混合冷媒在自動階層式冷凍系統之研究

Abstract

本論文為設計一個自動階層式冷凍系統，並研究適用此系統之混合冷媒R-32/R-134a（30/70 wt％），其溫度可達-40℃。此系統操作只需要一個壓縮機，其中增置一個相分離器以使混合冷媒的濃度改變。氣態冷媒流經階層熱交換器與膨脹後的低溫液態冷媒進行潛熱交換，然後較高濃度的高沸點成分冷媒流過蒸發器製冷，最後與較低濃度的低沸點成分冷媒匯流至壓縮機，完成一循環。 此系統利用兩個設計概念：第一，在熱交換器上利用非共沸混合冷媒溫度滑落的特性，以減少熱傳不可逆性；第二，利用混合冷媒的沸點差異以使冷媒流經相分離器之後，成分濃度明顯改變，進而提升冷凍能力與增加COP。 實驗結果顯示自重階層式冷凍系統之壓縮比明顯減低，而且所增加壓縮機消耗功率只有6％，其COP比傳統蒸氣壓縮循環只低落10％。 總括而言，此系統在一個壓縮機與特殊環路設計之下，可以增加蒸發壓力與減低壓縮比，達到所需低溫設計與節約能源。

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.