DEVELOPMENT OF A CABINET DRYER AND EVALUATION OF THIN LAYER DRYING CHARACTERISTICS OF COCOYAM

Abstract

A multipurpose mechanical convective hot air cabinet type dryer was designed and constructed with total dimension of (135cm height, and 60cm x 60 cm breadth. The dryer consists of the drying chamber unit (90cm x 60cm x 60cm) comprises of three stainless steel trays spaced 20cm apart; heat chamber unit which comprises of heating element of 3kw, heat exchanger and blower; contactor temperature sensor, temperature controller and switch, Other part includes control unit, heat divergent cone, chimney, vent and suction fan of 0.25hp. The design drawing and calculation was established, the machine was fabricated with well selected materials and components all sourced locally. Three species of cocoyam (Xanthosomasagitifolium (white and pink flesh) and Colocasia esculenta (Ghana) were dried with the fabricated dryer using a completely randomized design to study the effect of thin layer drying using three chip sizes (2x2, 3x3, 3x5cm), two shapes (slab and cylindrical) with three thicknesses (2, 3 and 5mm) pre treated with three methods (Sodium metabisulphite ((SM), Stem blanching (SB) and Control (CT) dried with four temperatures of 50, 65, 75 and 850C with a constant air velocity of 1.8m/s, and their drying characteristics such as dying rate, moisture diffusivity and activation energy were studied, each replicated two times the moisture content of three sample ranged from 1.48 to 1.61g moisture/g dry sample which reduced to 0.1 to 0.4 g moisture/g dry sample after oven drying for various chip sizes, shapes, thicknesses and temperatures. 2cm by 2mm thickness pre treated with sodium mtabishulphite dried at 850C dried better and faster than others and lease value obtained at blanching with longer drying time. The moisture diffusivity ranged from 1.0008 x 10-06 to 9.9369 x 10-07 m/s2 while the activation energy ranges from 10.58604-18.75731 kJmol-1. This was in range on similar materials. Drying patter was observed to be in the falling rate period.Non linear regression of the data using SPSS was employed for statistical modelling of drying curve. The experiment data was fitted in with ten thin layer drying models to find out the suitable model that would best describe the drying behaviour of the cocoyam varieties chips. Out of the ten thin layer model investigated, Logarithm and page best described the drying parameters of the cocoyam chips.