塑膠熱交換器用於除濕輪式除濕機之性能改善研究

Abstract

塑膠熱交換器有重量輕、加工容易、不易與化學反應、成本低及可依不同長期操作溫度選擇不同種類的塑膠的優點。由於市售之板式塑膠熱交換器在冷凝熱傳性能的提升上，效果有限，因此本研究提出新的結構設計，以管陣排列的方式增加冷凝熱傳。本研究比較板式塑膠熱交換器和裸管式塑膠熱交換器之冷凝熱傳性能，實驗操作壓力為1 atm，固定熱側及冷側的溫濕度，並改變熱側與冷側的風量。研究結果顯示，以單位面積之冷凝量及熱傳量比較，板式熱交換器的性能較裸管式熱交換器差，由於裸管式熱交換器以濺灑模式排水，不易冷凝液堆積，故熱傳效果較好，且裸管式熱交換器之冷凝量及熱傳量，會隨著冷側風量的增加而增加，相對板式熱交換器的趨勢則是增加幅度較平緩。差壓的部分，熱側差壓因冷凝液的存在，空氣流動的阻抗變大，使的濕空氣的差壓較乾空氣的差壓高，而冷側差壓在高風量時，裸管式熱交換器壓降較板式熱交換器高，由於板式熱交換器的流道為同尺寸的長方形孔洞，而裸管式熱交換器非為同樣尺寸流道；以同樣體積的條件下，在熱側風量為4.4 CFM及冷側風量為42 CFM時，裸管式熱交換器較板式熱交換器之熱傳係數增加約45%、質傳係數增加34%和熱側壓降減少約26%。

Plastic heat exchangers feature some unique characteristics such as light weight, easier fabrication, ability to resist chemical attack, low cost and longevity. For dehumidification using adsorption, plastic heat exchangers are commonly employed to dehumidify the moisture air regenerated from the heater. In practice, the processed cold air is first used to cool the hot moisture air. As a consequence, the condensation heat transfer performance of the moisture air becomes the bottleneck of plastic plate heat exchangers. The existing plastic heat exchanger for this heat recovery take the form as plate type. Therefore, the present study propose a novel design to enhance the condensation heat transfer by using the tube bundle configuration. The condensation heat transfer performance of the moisture air for both conventional plate type and the proposed configuration are made and compared. The experiments had been conducted at 1 atm, and a dry bulb temperature of 55°C with a relative humidity of 100%. The effect of volumetric flow rate is also examined. The cold side temperatures ate maintained at a dry bulb temperature at 27°C and a relative humidity at 60% with various volumetric flow rate. The results show that the performance, in terms of heat flux (or heat transfer coefficient), of the bare plastic tube heat exchangers outperforms the plate heat exchangers. This is because addition augmentation splashing effects of during condensation for the proposed tube bundle design. For the pressure drops, the moisture air shows higher pressure drop as compared to dry air without condensation. The results show that the bare plastic tube heat exchangers can enhance the heat transfer coefficient about 45%, the mass transfer coefficient about 34% and pressure drops by 26% subject to a same volumetric flowrate (when hot side is operated at 4.4 CFM and the cold side of is running at 42 CFM). Keywords: plastic heat exchangers, condensation heat transfer performances, volumetric flow rate, pressure drops.