ARTICLE Year : 2014  Volume : 4  Issue : 2  Page : 110114 Forced Convection Drying of Khoa: A Heat Desiccated Milk Product Mahesh Kumar Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India Correspondence Address: In this research paper, the convective heat transfer coefficients for khoa drying under indoor forced convection mode by hot air are delineated. The convective heat transfer coefficients have been determined for khoa sample of size 0.09 Χ 0.06 Χ 0.015 m ^{3} . The khoa sample has been dried as single thin layer for three consecutive days with the help of laboratory dryer until no variation in its mass is recorded. The experimental data have been used to determine the values of the experimental constants in the Nusselt number expression by simple linear regression analysis and then the convective heat transfer coefficients have been evaluated. The convective heat transfer coefficients have been observed to decrease with the drying day progression from the 1 ^{st} day to the 3 ^{rd} day. The average value of convective heat transfer coefficients have been found to decrease from 2.49 to 1.94 W/m ^{2} °C. The experimental error in terms of percent uncertainty has also been evaluated.
1. Introduction India has shown impressive growth in the milk production, achieving an annual production of 112.5 million tonnes in the year 200910, which is 15% of the total milk production in the world [1] . The lack of cooling facilities to keep the liquid milk fresh led to the diversion of milk for preparation of indigenous milk products with comparatively longer shelf life. Khoa is a heat coagulated, partially dehydrated traditional value added Indian milk product. It is obtained by heat desiccation of whole milk to 6570% milk solids without the addition of any foreign ingredients. In India, due to its large scale consumption nearly 6 lakh tonnes of khoa is being manufactured for the preparation of variety of sweets. The khoa and khoa based sweets have high commercial significance because of their popularity throughout the country and longer shelf life. The total Indian sweet market is around rupees 520 billions in terms of annual sales [2],[3] . The shelf life of khoa is influenced by the moisture contents, which permit the growth of the molds within a few days of storage at room temperature. The presence of molds in khoa causes its fast deterioration by producing discoloration defects as well as disagreeable flavors. Bhat et al. [4] attempted to prevent the deterioration of khoa by its steaming for 1520 min. The shelf life of khoa samples was extended by the application of ultraviolet radiation [5] and by the addition of canesugar [6] . Various approaches like different packaging materials [7],[8],[9] , addition of preservatives and antioxidants [10],[11],[12],[13],[14],[15] were reported to increase the shelf life of khoa. A significant reduction of yeast and mold counts was reported by the use of solar radiation [16],[17] . The convective heat transfer coefficient is an experimentally determined parameter. It plays an important role in drying rate simulation, since the temperature difference between the air and the product varies with this coefficient. Kumar et al. [18] investigated the convective heat transfer coefficients of khoa for thin layer drying in an open sun and greenhouse drying for natural as well as forced convection modes. The convective heat transfer coefficient was observed to vary from 0.86 to 1.09 W/m 2 °C, 0.54 to 0.91 W/m 2 °C, and 0.54 to 1.03 W/m 2 °C for forced greenhouse, natural greenhouse and open sun drying modes respectively. The present study has been carried out to evaluate the convective heat transfer coefficients of khoa during its indoor hot air forced convection drying. This research work would be helpful in designing of a dryer for drying khoa to its optimum storage moisture level by which the storage period of khoa can be increased. The evaluated results would also be useful for predicting drying parameters. 2. Materials and Methods 2.1 Sample preparation and experimental observations The khoa sample was prepared by following the traditional khoa making process. The fresh milk obtained from a herd of cows was heated over a hot plate in an aluminum open pan. The heating was stopped when the desired consistency achieved, and the product was allowed to cool to the room temperature. Then a 100 g cuboids shaped khoa sample of size 0.09 × 0.06 × 0.015 m 3 was prepared with the help of a wooden mold. The photographic view of khoa sample is shown in [Figure 1]. Experiments were conducted during the month of October 2012 at Guru Jambheshwar University of Science and Technology Hisar (29°5'5" N 75°45'55" E). A digital hygrometer was used for measuring the relative humidity (RH) and temperature of the exit air. It was kept after the khoa tray, keeping its probe facing the exit air. Observations were recorded for khoa surface temperature, exit air temperature, exit air RH, and weight of khoa sample at every 15 min time interval. The air velocity over the khoa surface was measured to be 1.5 m/s with the help of the digital anemometer. The moisture evaporated was calculated by taking the difference of mass of khoa between two consecutive readings. The khoa sample was dried for three consecutive days until there is almost no variation in its mass observed.{Figure 1} 2.2 Experimental setup and instrumentation The schematic view of the experimental setup and the location of different measured parameter are shown in [Figure 2]. A khoa sample of thickness 1.5 cm for single layer drying was kept in a wire mesh tray of 0.09 m × 0.06 m size directly over the digital weighing balance of 6 kg capacity (model TJ6000, Scaletech, made in India) with a least count of 0.1 g. A heat convector (model FH812T, Usha Shriram, made in India) was used to blow hot air over the khoa surface. The khoa surface temperature (T k) and air temperature (Tα) were measured by calibrated copperconstantan thermocouples connected to a digital temperature indicator with a least count of 0.1°C (accuracy ± 0.1%). The thermocouples tend to deviate from the actual data after a long period. Hence, the thermocouples have been calibrated with respect to the zeal thermometer which gives accurate readings. The (γ or RH) and the temperature of the exit air (T e) were measured by a digital humidity/temperature meter (model LutronHT 3006, made in Taiwan). It had a least count of 0.1% RH ( accuracy of ± 3% on the full scale range of 1095% of RH) and 0.1°C temperature (an accuracy of ± 0.8°C on the full scale range of 50°C). The air velocity over the surface of khoa sample was measured with an electronic digital anemometer (model AM4201, made in Taiwan). It had a least count of 0.1 m/s with accuracy of ± 2% on the full scale range of 0.2 m/s to 30.0 m/s.{Figure 2} 2.3 Thermal modeling The convective heat transfer coefficient for evaporation under forced convection can be determined by using the following relations [19],[20] : [INLINE:1] The rate of heat utilized to evaporate moisture is given as [21] [INLINE:2] The moisture evaporated is determined by dividing above equation by the latent heat of vaporization (λ) and multiplying the area of khoa drying tray (A t) and time interval (t). [INLINE:3] Values of m and c in Eq. 5 are obtained by using the simple linear regression formulae. The expressions used for determining the values of the physical properties of humid air are given in AppendixA. [SUPPORTING:1] 2.4 Computation technique and experimental error The average value of khoa sample surface temperature (T k) and the exit air temperature after the khoa surface (T e) were calculated at 15 min time interval for corresponding moisture evaporated. The physical properties of humid air were evaluated for the mean temperature (T i) by using the equations given in AppendixA. Then these physical properties of humid air were used for determining the values of the Reynolds number (Re) and Prandtl number (Pr). The values of constant C and exponent n in the Nusselt number expression were determined by simple linear regression analysis. The evaluated values of experimental constants were considered further to determine the values of convective heat transfer coefficient (h c) from Eq. 1 at the increment of every 15 min time interval of observations. The experimental error was evaluated in terms of percent uncertainty (internal + external) for the moisture evaporated. The following equations were used for internal uncertainty [22],[23] : [INLINE:4] For external uncertainty, the least counts of all the instruments used in measuring the observation data were considered. 3. Results and Discussion The average values of experimental data recorded for khoa drying by hot air under indoor forced convection mode are presented in [Table 1].{Table 1} The experimental data given in [Table 1] were used to determine the values of constant C and exponent n in the Nusselt number expression by simple linear regression analysis. Then the values of the constant C and exponent n were considered further for determining the values of the convective heat transfer coefficient by Eq. 1. The evaluated values of experimental constants and the convective heat transfer coefficients for khoa drying by hot air under forced convection mode are summarized in [Table 2]. The range of Reynolds and Prandtl numbers have also been given. The product of Reynolds number and Prandtl number indicates that the entire khoa drying under the forced convection mode falls within a laminar flow, because Re Pr ≤ 10 5 [24] . It has been observed from [Table 1] that the rate of moisture removal increases initially and then decreases for a given day, also the total moisture removal decreases with drying day progression from the 1 st day to the 3 rd day.{Table 2} The variation in convective heat transfer coefficient with respect to drying time interval for each day of drying is illustrated in [Figure 3]. It can be seen that the convective heat transfer coefficient values for a particular day of drying do not change much with respect to drying time.{Figure 3} In order to compare the convective heat transfer coefficients for different drying days, the average values of convective heat transfer coefficients have been calculated for each day which is given in [Table 2]. These results are also illustrated in [Figure 4]. It can be seen from [Figure 4] that the convective heat transfer coefficients for khoa drying decreases as the day of drying progresses from the 1 st day to the 3 rd day. This is due to decrease in total moisture contents removal with the drying day progression from the 1 st to the 3 rd day. The trends of the evaluated results for the convective heat transfer coefficients are observed in accordance with those reported for khoa drying under greenhouse and open sun drying modes by Kumar et al. [19] . The average value of convective heat transfer coefficient is observed to decrease by 28.35% for the given drying days.{Figure 4} The percent uncertainty (internal + external) was found within 25.49% and the different values of the convective heat transfer coefficients were found to be within the given range. To show the variability of the convective heat transfer coefficients values from its true value (or error free value) Statistical Package for the Social Sciences, (version 16.0) software has been used, which provided error bars with 95% of the confidence interval. Error bars for the convective heat transfer coefficients are illustrated in [Figure 5]. The small length of error bars shows the smaller variability of the convective heat transfer coefficients from its true value.{Figure 5} 4. Conclusions The experimental data recorded for khoa drying by hot air under indoor forced convection modes were analyzed by using the Nusselt number expression with the help of simple linear regression method. The evaluated values of experimental constants C and n were further considered to determine the convective heat transfer coefficients. The convective heat transfer coefficients were observed to decrease as the day of drying progresses to the next day. The average values of convective heat transfer coefficients were observed to decrease from 2.49 to 1.94 W/m 2 °C. This is due to decrease in total moisture contents removal with the drying day progression from the 1 st day to the 3 rd day. The evaluated values of convective heat transfer coefficients would be useful in designing a dryer for drying khoa to its optimum storage moisture level. The experimental error in terms of percent uncertainty was found to be in the range of 25.49%. 5. Nomenclature [INLINE:5] References


