Judul: Kajian Pengering Rotari Tipe Co-Current untuk Pengeringan Sawut Ubijalar
Abstrak: Rotary drying is a very complicated process that implies not only thermal drying but also movement of wet material within the dryer. The objective of this research were to evaluate performance of rotary dryer, to identify amount of energy consumption which are used during the drying process, to develop model of rotary drying and to identify drying cost per kg of sweet potato grates. Drying characterisctics of product need to be established because of important information for the design, prediction, and modelling. The model of rotary drying was constructed based on energy and mass balance. The differential equations were solved by simultaneous-numerically. This model applied to predict temperature dryer chamber, temperature product, moisture content and RH. The result showed that the performance of rotary dryer depends on feed rate sweet potato grates into drying chamber. High feed rate could decrease temperature in drying chamber. The residence time of all experiments were 18 minutes. Hold-up of all experiments in this study were relatively low that ranges 9-36 kg. The specific energy consumption in all of the experiments was between 5.51-14.26 MJ/kg H2O. The high feed rate (3 kg/1 min) had the lowest specific energy consumption. Conversely, the lowest feed rate (3 kg/4 min) had the highest specific energy consumption for all experiments. The high total efficiency could be found from high feed rate, the feed rate (3 kg/1 min) is highest total efficiency for all experiments. The model can be used to obtain temperature profiles of air and the product in dryer chamber. Using the model, change of air temperature and product during drying were successfully predicted. Coefficient of determination (COD) between measured and calculated ranges 0.819-0.992, respectively. However, the model could not predict moisture content and RH accurately. The drying cost of grates is Rp 1 494 per kg wet grates. It is relatively expensive for drying.
Keyword: Co-current rotary dyer, Specific energy consumption, Feed rate, Sweet potato

Judul: Uji Performansi Pengering Efek Rumah Kaca (ERK)-Hybrid Tipe Rak Berputar untuk Pengeringan Sawut Ubi Jalar (Ipomoea batatas L.)
Abstrak: Rotating rack green house effect (GHE) solar dryer is one type of dryer with the source of energy from solar and biomass energy for drying of agricultural products on the racks that can be rotated verticaly. The capacity of the drying chamber is 48 kg of chopped sweet potatoes. The objective of this study is to test the performance of the hybrid green house effect dryer for drying chopped sweet potatoes and to define modeling of heat transfer in the drying chamber. There are three experiments of drying: the first is without products, the second and the third are drying products without rotating rack and implement the rotating rack 45° every 60 minutes, respectively. The results of chopped sweet potatoes drying show that drying temperature on the dryer chamber ranged between 31.6-61.5°C. To reduce moisture content from around 71.3 %wb to 9.96 %wb for Experiment 2 and to reduce moisture content of 72.76 %wb to 9.5 %wb for Experiment 3, it is needed 14.5 hours and 13.5 hour, respectively. The specific energy consumption of both experiments were 42.83 MJ/kg of moisture evaporated and 35.15 MJ/kg of moisture evaporated and the drying efficiency of 5.78 % and 7.47%, respectively. Based on the performance test, it can be concluded that rotating rack dryer shows better performance than that of without rotating rack.
Keyword: solar dryer, biomass, Chopped sweet potatoes

Judul: Approximation of the Median of Poisson Distribution Using the Relation of Poisson-Gamma Distribution
Abstrak: A major drawback of median is that median cannot usually be defined in closed form, even in the cases where their distribution functions are known. This study gives an approximation of the closed form for the median of the Poisson distribution by using the relation of Poisson-Gamma distribution. The approximation uses elementary techniques based on the monotonicity of certain sequences involving tail probabilities of the Poisson distribution and the Central Limit Theorem. Furthermore, a closed form expression for the mean absolute deviation 􀜧|􀜰􀰒 􀵆 􀜯􀝁􀝀􁈺􀜰􀰒􁈻| is also studied. The median is relevant because of the fact that it minimizes the mean absolute deviation of a Poisson random variable (􀜰􀰒􁈻 by considering median as central tendency.
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