影響非結球型葉菜類壓差預冷降溫速率之因素

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論文名稱: 影響非結球型葉菜類壓差預冷降溫速率之因素
研究生姓名: 林棟樑
指導教授姓名: 王自存;李允中
出版年: 2000
學校名稱: 國立臺灣大學
系所名稱: 園藝學研究所
關鍵字: 壓差預冷;靜壓降;葉菜類;小白菜;芥藍;葉萵苣;預冷;半冷期;forced-air cooling;pressure drop;leafy vegetables;Pai-tsai;Chinese kale;leaf lettuce;precooling;half cooling time
摘要: 本研究探討小白菜、芥藍及葉萵苣三種非結球型葉菜類壓差預冷處理時,影響風阻之因素;並以小白菜為材料,探討通風量、靜壓降及紙箱開孔面積對壓差預冷降溫速率之影響。壓差預冷處理時,壓差風阻產生的靜壓降隨風量的增加而增加,但並不是呈線性相關,而是冪次方關係,次方值等於1.8。阻力關係式為ΔP=k×Q1.8,其中ΔP 靜壓降,Q為處理風量,k為阻力常數。k值愈大時通風阻力愈大。上述公式可用以比較不同處理間通風阻力之大小,及用以預估壓差處理時產生的靜壓降及處理風量大小,及預冷所需時間。 不同蔬菜種類之通風阻力以葉萵苣最大,小白菜和芥藍則差異不顯著。以15%紙箱開孔與塑膠籃包裝其通風阻力差異不大,低於15%開孔紙箱,靜壓降隨開孔面積減少而增大,紙箱開孔百分率小於5%以下則阻力快速增加。產品裝箱方式以葉柄平行通風方向阻力較小。 小白菜以5%開孔紙箱及塑膠籃包裝,每箱20公斤,壓差預冷處理時,紙箱長軸單箱通風處理,k值為270±8。紙箱長軸兩箱連接通風處理時,k值為482±20。塑膠籃單箱與兩箱排列處理時之k值則分別為186±9;354±8。不論紙箱包裝或塑膠籃包裝,兩箱處理之k值均約等於單箱處理的兩倍,預冷產生的靜壓降大小與冷風通過風產品距離成正比。 非結球型葉菜類壓差預冷時,入風口之降溫速度明顯較出風口快,加快風速可減少紙箱內溫度的差異。試驗中不同部位各測點之半冷期平均值在處理風量由1∼4 L/kg.sec 時分別為51、37、22及16分鐘。風量在3 L/kg.sec以下時,增加處理風量明顯加快降溫速率,而3 L/kg.sec 以上之風量處理時,則增加風量對降溫速率增加較不明顯。 包裝紙箱開孔比例1.25%時降溫速率較慢之外,開孔比例2.5%、5.0%及10%壓差預冷時之降溫速率沒有差異。紙箱開孔為1.25%、2.5%、5.0%及10%時靜壓降與風量之關係為二次曲線,且隨風量增加,靜壓降增加,試驗結果分別求得不同紙箱開孔比例下之二次迴歸方程式,信賴常數R2均在0.93以上,方程式分別為:1.25%開孔時Y1.25 = 0.9373-0.7276X+0.3972X2;2.5%時Y2.5 = 0.2909-0.5253X+0.4782X2;5%時Y5.0 = 0.0371-0.0359X+0.1729X2;及10%時Y 10.0 = 0.0605-0.0491X+0.1071X2,其中Y為靜壓降(mbar),X為風量(L/kg.sec)。 風量與半冷期之關係亦為二次曲線,求得之迴歸方程式紙箱開孔比例為1.25%時Y1.25=79.2-28.8X+3.1X2;2.5%時Y2.5=75.6-36.2X+5.3X2;5.0%時Y5.0=77.2-36.2X+5.3X2;10%時Y10=76.2-41.3X+7.3X2,信賴常數R2均在0.91以上,共中Y為半冷期(min),X為風量(L/kg.sec)。由以上風量與半冷期間求得之迴歸線來預估西螺壓差預冷設施實際操作時之半冷期,結果預估值與實測值略有差異,但此迴歸方程式仍具有實際的參考價值。This study investigated factors that affect the airflow resistance of three non-leafy vegetables : Pai-tsai (Brassica campestris L. Chinensis Group), Chinese kale (Brassica oleracea L. Alboglabra Group) and leaf lettuce (Lactuca sativa L. var. ''A-tsai'') during forced-air cooling. The effects of airflow, static pressure drop and percentage of box sidewall vent area on cooling rate of Pai-tsai during forced-air cooling were also studied. During forced-air cooling, the static pressure drop (△P) across a 20 kg container increases with the increase in airflow (Q). The relationship between △P and Q is not linear but exponential and can be expressed with the equation of △P = k x Qn . The power value "n" was found to be between 1.5 - 2.0 with an average value approximate to 1.8. By fixing the "n" value at 1.8, the term "k", which represents the resistant constant of each treatment, can be calculated from the equation. Airflow resistance (k) was found to vary with the patterns of vegetable packed inside container and kinds of vegetable. Among the three non-leafy vegetables tested, leaf lettuce, regardless of patterns of packing, always had a higher k value, while Pai-tsai and Chinese kale had similar resistance when packed in the same way. Fiberboard box with 15% venting had similar resistance with that of plastic basket. The static pressure drop increased as the venting area were less than 15%, and increased rapidly as the venting area was below 5%. The resistance was lower when the produce was packed in the direction parallel to the airflow than packed vertically. . When Pai-tsai was packed in either fiberboard box with 5% venting or plastic basket and cooled by forced-air method, the airflow resistance approximately doubled as the stack width increased from one row to two rows of container. This result indicates a linear relationship between pressure drop and the distance of cool air needed to pass through the product. During forced-air cooling of Pai-tsai, the rate of temperature drop was highest at the air inlet of the container and was much lower at the outlet. Such difference could be reduced by increasing the airflow. The average half-cooling time at airflow of 1, 2, 3 and 4 L/kg sec were 51, 37, 22 and 16 min respectively. Increasing the airflow strongly enhanced the rate of temperature drop only when the airflow was less than 3 L/kg sec and there was little enhancement when the airflow was above 3L/kg sec. Boxes with 2.5%, 5% or 10% venting had similar rate of temperature drop, while the box with 1.25% venting had lower rate of temperature drop. The relationship between airflow rate and pressure drop of Pai-tsai packed in boxes with 1.25%, 2.5%, 5% or 10% venting could be expressed in second order equations. Regression equations obtained from experimental data, with R2 values above 0.93, were as follow : 1.25% venting, Y1.25 = 0.3972 - 0.7276X + 0.3972X2; 2.5% venting, Y2.5 = 0.2909 - 0.5253X + 0.4782X2; 5% venting, Y5.0 = 0.0371 - 0.0359X + 0.1729X2; and 10% venting, Y10.0 = 0.0605 - 0.0491X + 0.1071X2; where Y represents static pressure drop (mbar) and X represents airflow rate(L/kg sec). By plotting the logarithm of the dimensionless temperature versus time, half-cooling time of each treatment could be obtained. The relationship between airflow rate and half-cooling time appeared also to be second order curve. The regression equations between airflow rate and half-cooling time for various box venting area were obtained, with R2 values above 0.91. They are : 1.25% venting, Y1.25 = 79.2 - 28.8X + 3.1X2; 2.5% venting, Y2.5 = 75.6 - 34.8X + 5.6X2; 5% venting, Y5.0 = 77.2 - 36.2X + 5.3X2; and 10% venting, Y10.0 = 76.2 - 41.3X + 7.3X2; where Y represents half-cooling time (min) and X represents airflow rate (L/kg sec). One of the above regression equation was used as model to predict the forced-air cooling operation of Pai-tsai using the cooling facility of Shiluo Farmer''s Association at Shiluo, Yunling County. Due to several pre-existed defects of the cooler, the average measured cooling time was 30% longer than the average predicted cooling time (as 3 half-cooling time). In conclusion, this study provides basic information for forced-air cooling of non-leafy vegetables, and regression equations obtained in this study are useful in designing forced-air cooler and in predicting cooling time during operation.
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