Answer / kangchuentat

FOOD ENGINEERING - ANSWER 23.2 : (a) The mass of dried material is constant throughout the drying process, or 100 kg x 0.2 = 20 kg. Mass of water = mass of dried material (% of water / % of dried material). Initial mass of water = 20 (80 / 20) = 80 kg. Final mass of water = 20 (10 / 90) = 2.222 kg. Mass of the water removed = Final mass - Initial mass = 80 - 2.222 = 77.778 kg. (b) Heat energy required to evaporate the water only = Mass of the water removed x Latent heat of vaporization = 77.778 kg x 2331 kJ / kg = 181300.518 kJ. (c) Minimum heat energy required to raise the temperature of the potatoes = Mass of potatoes of 80 % moisture x specific heat of potato x temperature changes = 100 kg x 3.43 kJ / (kg °C) x (71 - 24) °C = 16121 kJ. (d) Heat energy in steam = mass of steam x latent heat of steam = 250 kg x 2283 kJ / kg = 570750 kJ. (e) Efficiency of the dryer based heat input and output = (Ti - To) / (Ti - Ta) = (80 - 71) / (80 - 24) = 0.1607. The answer is given by Kang Chuen Tat; PO Box 6263, Dandenong, Victoria VIC 3175, Australia; SMS +61405421706; chuentat@hotmail.com; http://kangchuentat.wordpress.com.

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