| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 98 === It is well known that drying of liquid-borne powders will create agglomerates, and the problem of agglomeration is particularly acute in the nanometer-size range. Since the liquid water will instantaneously flash into water vapor under the condition where the pressure is below its saturated state, and the outward expansion force during flashing is able to counteract the attracting force among fine particles, it is thus reasonable to use vacuum dry technique to prevent/mitigate agglomeration.
However, the flashing process will be terminated once the environment pressure risen above saturation state due to the tremendous volume expansion when liquid water converts to vapor. In order to maintain a low pressure environment for vacuum drying process, in this study, an absorption/ desorption system is successfully constructed to continuously absorb the flashed vapor in the vacuum chamber and desorb the absorbed water outside the vacuum chamber.
In the absorption/desorption tests, two techniques were applied to improve the absorption capability. The first kind is the replacement of cooling-water tube from bare tube to micro-pin-fin tube to enhance the heat transfer between aqueous lithium bromide solution and cooling water, while the second kind is the addition of 2-ethyl-1-hexanol(200ppm) to aqueous lithium bromide solution to increase the heat and mass transfers between solutions and vapors.
From the test results indicated that both the vapor absorption rate and the overall heat transfer rate were greatly promoted by each technique. Compared to the basic case(bare tube without surfactant), the effect of micro-pin fin tube can increase 13~38% on vapor absorption rate and 17~28% on overall heat transfer rate, the effect of additive can increase 11~65% on vapor absorption rate and 24~29% on overall heat transfer rate. With both micro-pin-fin tube and additive, the enhancement on vapor absorption is about 39~79% and on overall heat transfer rate is about26~55%.
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