Behavior of Inversion Point Temperature and New Applications Of Superheated Vapor Drying
This report summarizes new fields of applications of both superheated vapor drying and highly humid and high temperature air drying. And then it describes fundamental research in developing a controlled superheated vapor dryer.
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for home cooking and of washed clothing are illustrated. Through the fundamental research, by calculating from the well-known heat transmission formulae and by performing the experiment, the existence of the inversion point temperature has been confirmed, and the locus of its changing point with drying variables has been found. Moreover, for a vapor drying system controlled by humidity sensing, a direct measurement of the water content in a mixed gas of air and superheated vapor in the temperature range 373-573K has been also performed by using the Zr02-MgO porous ceramic sensor. 1. INTRODUCTION
Many studies of the evaporation of water in superheated vapor and of the use of its vapor as a drying agent have been done by various investigators[l-3, for example) for about thirty years. Successively, superheated vapor drying has been applied in many new drying fields in addition to industrial drying field, such as drying of instant foods for home cooking, and drying of washed clothing in our daily life, and so on. The most significant reason for this wide range of applications of superheated vapor is that more water evaporates in this vapor or highly humid air than in dry air above the inversion point temperature. This is because the evaporation rate curve of these agents in high temperature and in low temperature regions cross at one point, as was established experimentally by our previous study[4]. Moreover, superheated vapor has many unique drying merits which cannot be found in dry air, such as its effects on the drying materials of a porous condition for easy grinding and dissolution into liquid. On the other hand, as compared with these wide practical applications, theoretical research concerned with the inversion point temperature and fundamental research for determining controlled operating conditions or optimum design conditions for a superheated vapor drying system have not been sufficiently performed. From this viewpoint, by calculating from the well-known heat transmission formulae (using a horR. Toei et al. (eds.), Drying ’85 © Springer-Verlag Berlin Heidelberg 1985
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izontal plate surface by Pohlhausen) and in experimenting, by changing drying variables, such as the mass velocity or heat transfer of the vapor, the authors confirmed the existence of the inversion point temperature and found the locus of the temperature. The behavior of the locus enables one to specify the drying variables for a controlled drying system. Furthermore, for this system, which is controlled by humidity sensing, a new di-
rect measuring method of the vapor content in a mixed gas of air and superheated vapor over a high temperature range is also described using a ceramic sensor, which consists of a Zr02-MgO ceramic body with a surrounding heater. 2. RECENT SUPERHEATED VAPOR DRYING ADVANCES
Superheated vapor and highly humid and high temperature air are attractive drying agents, because of their merits which cannot be obtained with air drying , as stated above , but its applications have been limited to industrial purposes.
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