高速搅拌辅助对真空预冷纯水过程中防飞溅的影响

Effect of High-speed Agitation on Preventing the Excessive Boiling and Splashing during Vacuum Cooling Pure Water Process

研究真空预冷纯水下压强下降速率、冷媒温度及搅拌速率等操作参数,并通过对单位时间里的理论水分损失量、实际水分损失量、飞溅量及压强差等指标的分析来探讨对防飞溅的效果。结果表明:增加搅拌能够明显抑制真空预冷过程中纯水的飞溅。通过高清摄像头观察当转速增至900r/min及以上时,大量爆沸和飞溅的现象未被观察到。在0.48min-1压强下降速率和-10℃冷媒温度的条件下,当搅拌转速从0r/min增至1500r/min时,其纯水飞溅率分别为27.37%,16.39%,8.25%,3.00%,2.97%和2.70%,而其所对应的最大压差值分别是885,436,411,182,189和172Pa,表明预冷过程中最大压差值与水飞溅率存在相关性(R2=0.93),并且可将900r/min作为纯水防飞溅的临界转速。通过增加搅拌不仅不会对预冷速率产生负影响,反而会加快预冷速率。

Immersion vacuum cooling is regarded as the best method to displace the vacuum cooling due to its faster cooling rate compared to conventional cooling methods and lower weight loss compared to vacuum cooling for cooling cooked meat products. However, excessive solution boiling and splashing during the immersion vacuum cooling process is still the largest obstacle limiting its wide-spread application. Effective measures should be taken to control the excessive boiling and splashing but not compromising the cooling rate. Employing agitation during vacuum cooling process according to the previous pilot experiments results is considered the most effective method in preventing the excessive boiling and splashing of liquid water. In this paper, effects of different pressure reduction rate(0.21, 0.3, 0.48 min-1), agitation speed(0, 300, 600, 900, 1 200, 1 500 r/min) and cooling medium temperature (0,-5,-10 ℃) on those indexes such as the content of actual weight loss, content of theoretical weight loss, weight loss of liquid water droplets splashed out of beaker as well as different between saturated vapor pressure and vacuum chamber pressure in per interval time(1s) during vacuum cooling pure water process were studied. Results showed that by adding the agitation speed could obviously decrease the intensity of pure water boiling and splashing no matter what the pressure reduction rate and cooling medium temperature would be. It was attested that the typical morphological changes of liquid water cooled in vacuum cooling process in the course of vacuum chamber pressure step-down could be divided into four different stages (Ⅰ,Ⅱ,Ⅲ,Ⅳ). It was well known for us that the Ⅲ stage observed from the vacuum cooled pure water process could play a critical role in deciding the weight loss of liquid water droplets splashed out beaker in per interval time. The Ⅲ stage was non-existent or non-observed when the agitation speeds were equal or more than the 900 r/min. The values of total weight loss rate of liquid water droplets splashed out beaker were 27.37%, 16.39%, 8.25%, 3.00%, 2.97% and 2.70% as well as its responding values of maximum difference between saturated vapor pressure and vacuum chamber pressure were 885, 436, 369, 411, 182, 189 and 170 Pa when the agitation speeds were 0, 300, 600, 900, 1 200, 1 500 r/min respectively. The difference for the total weight loss rate of liquid water droplets splashed out beaker after vacuum cooling was not significant when the agitation speeds was equal or more than 900 r/min(P>0.05). when the agitation speed reached or surpassed a minimum agitation speed. The intensity of boiling and splashing of pure water could be controlled in a low level regarding of the pressure reduction rate and cooling medium temperature would be. It was attested that 900 r/min was very effective in avoiding the liquid water from splashing out of beaker could be regarded as the minimum agitation speed. The total weight loss rate of liquid water droplets splashed out beaker had a good positive correlation with the difference between saturated vapor pressure and vacuum chamber pressure (R2=0.93). Compared to the effect of cooling medium temperature on the result of preventing boiling and splashing of pure water, pressure reduction rate was more significant. however, all of them were worse compared to the agitation. Our results also showed that by adding the agitation function during vacuum cooling process hadn’t any reverse actions but increase on cooling rate. It seems that combining faster agitation speed and slower pressure reduction rate and higher cooling medium temperature may be the most effective method in controlling the intensity of boiling and splashing of liquid water. The reason for aforementioned results may be that by adding the agitation during vacuum cooled pure water process not only can increase the cooling rate which adversely decrease the difference between saturated vapor pressure and vacuum chamber pressure but also can destroy or disperse large bubbles developed or agglomerated from some flowing little bubbles produced in the bottom and wall of beaker during liquid water boiling stage. In word, the assistant function of agitation added during vacuum cooled liquid food may be a very good alternation in avoiding the excessive boiling and splashing of liquid food.